SMPS design ...

[Arfa Daily]

I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type, single
12 volt output at, I would guess, 3 - 4 amps. It appears to be a very simple
design, in that the chopper drive circuit is discrete, employing two bipolar
transistors as an astable. The output of this is fed pretty much directly to
the gate of a single FET. In the drain of the FET, is a single primary
winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic cap,
and two 3 watt cement-body resistors, all in series. On every one of the
examples sent to me, the two resistors are chalky and very discoloured to
the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go around
150 ohms, so I'm taking that to be the original value, based on the fact
that this type of resistor doesn't usually go low, and some of them have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has crystalised, and
on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where one or
both of the resistors has gone open, the supply works just fine. Loaded up
to a couple of amps, it runs cool and the regulation is good. The switching
FET is barely breaking a sweat, as you would expect. So I went ahead and
replaced the resistors with a pair of 150 ohm 3 watt types that look pretty
much identical to the originals - even down to the blue body colour. The cap
checks ok for value and leakage. With the resistors in place, the supply
still works just fine, except that it now runs pretty hot, even when
unloaded. The FET is a lot hotter than it was before. The resistors get well
hot, as I was expecting, given the condition of the originals, but with the
supply loaded up to a couple of amps, they get very hot, and the FET becomes
uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design, this
network across the transformer primary, is a simple snubber (as opposed to a
clamp or combination clamp and snubber as is also sometimes found in this
position). Texts suggest that its purpose is to limit the level of voltage
spikes at the switching point, to keep the switching device operating within
its SOA and reduce dissipation, which seems a fair enough comment. However,
quite the reverse appears to be true. The whole supply seems a lot happier
with that network 'not there' as it effectively is, when the resistors are
open.

So has anyone got any good thoughts as to what is going on here ? I've done
a great deal of repairs to switchers over the years, and am well versed with
the principles of operation and repair, but I freely admit that I am not a
designer in this field, so I'm at a bit of a loss as to whether it's just
generally a poor design, or whether there's something else wrong that I'm
missing. As those components were originally designed in, and are clearly
faulty now, they need to be replaced, but the fact that the supply seems to
run less efficiently when they are in place, feels altogether
counter-intuitive

Arfa

 

[Jeff Liebermann]

On Tue, 18 Oct 2011 01:47:17 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> wrote:

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic cap,
and two 3 watt cement-body resistors, all in series.

1. A what frequency is the SMPS supply running?
2. Look at the xformer or inductor with a loosly coupled pickup, and
see if there is any RF or spurious oscillations happening. 330PF at
perhaps 40KHz is 12K ohms, which isn't going to conduct enough current
to get the 150 ohm resistor even slightly warm. However, 330pf at
some higher RF frequency just might do the trick.
3. The design sounds like it might be belching some switching noise
back out the power line as conducted EMI/RFI. The 330pf and 150 ohm
resistor networks seem like they're there to reduce this EMI to
regulatory limits.

Hint: The most efficient switching power supply, is full of fast rise
time waveforms, and is therefore also an EMI/RFI noisy supply.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

 

[Phil Allison]

"Arfa Daily"

I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single 12 volt output at, I would guess, 3 - 4 amps. It appears to be a
very simple design, in that the chopper drive circuit is discrete,
employing two bipolar transistors as an astable. The output of this is fed
pretty much directly to the gate of a single FET. In the drain of the FET,
is a single primary winding up to the raw rail from the input bridge.

** What sort of SMPS is this ??

One that puts out regulated DC ???

Or is it for driving 12V halogen lights with raw high frequency energy.




..... Phil

 

[N_Cook]

Arfa Daily <arfa.daily@ntlworld.com> wrote in message
news04nq.10308$tk5.3106@newsfe13.ams2...

I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single
12 volt output at, I would guess, 3 - 4 amps. It appears to be a very
simple
design, in that the chopper drive circuit is discrete, employing two
bipolar
transistors as an astable. The output of this is fed pretty much directly
to
the gate of a single FET. In the drain of the FET, is a single primary
winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic
cap,
and two 3 watt cement-body resistors, all in series. On every one of the
examples sent to me, the two resistors are chalky and very discoloured to
the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go around
150 ohms, so I'm taking that to be the original value, based on the fact
that this type of resistor doesn't usually go low, and some of them have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has crystalised,
and
on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where one
or
both of the resistors has gone open, the supply works just fine. Loaded up
to a couple of amps, it runs cool and the regulation is good. The
switching
FET is barely breaking a sweat, as you would expect. So I went ahead and
replaced the resistors with a pair of 150 ohm 3 watt types that look
pretty
much identical to the originals - even down to the blue body colour. The
cap
checks ok for value and leakage. With the resistors in place, the supply
still works just fine, except that it now runs pretty hot, even when
unloaded. The FET is a lot hotter than it was before. The resistors get
well
hot, as I was expecting, given the condition of the originals, but with
the
supply loaded up to a couple of amps, they get very hot, and the FET
becomes
uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design,
this
network across the transformer primary, is a simple snubber (as opposed to
a
clamp or combination clamp and snubber as is also sometimes found in this
position). Texts suggest that its purpose is to limit the level of voltage
spikes at the switching point, to keep the switching device operating
within
its SOA and reduce dissipation, which seems a fair enough comment.
However,
quite the reverse appears to be true. The whole supply seems a lot happier
with that network 'not there' as it effectively is, when the resistors are
open.

So has anyone got any good thoughts as to what is going on here ? I've
done
a great deal of repairs to switchers over the years, and am well versed
with
the principles of operation and repair, but I freely admit that I am not a
designer in this field, so I'm at a bit of a loss as to whether it's just
generally a poor design, or whether there's something else wrong that I'm
missing. As those components were originally designed in, and are clearly
faulty now, they need to be replaced, but the fact that the supply seems
to
run less efficiently when they are in place, feels altogether
counter-intuitive

Arfa

What happens if you double the R and halve the C ?

 

[Phil Allison]

"Nutcase Kook"

What happens if you double the R and halve the C ?

** The C gets jealous - silly .............




.... Phil

 

[petrus bitbyter]

"Arfa Daily" <arfa.daily@ntlworld.com> schreef in bericht
news04nq.10308$tk5.3106@newsfe13.ams2...

I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single 12 volt output at, I would guess, 3 - 4 amps. It appears to be a
very simple design, in that the chopper drive circuit is discrete,
employing two bipolar transistors as an astable. The output of this is fed
pretty much directly to the gate of a single FET. In the drain of the FET,
is a single primary winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic
cap, and two 3 watt cement-body resistors, all in series. On every one of
the examples sent to me, the two resistors are chalky and very discoloured
to the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go around
150 ohms, so I'm taking that to be the original value, based on the fact
that this type of resistor doesn't usually go low, and some of them have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has crystalised,
and on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where one
or both of the resistors has gone open, the supply works just fine. Loaded
up to a couple of amps, it runs cool and the regulation is good. The
switching FET is barely breaking a sweat, as you would expect. So I went
ahead and replaced the resistors with a pair of 150 ohm 3 watt types that
look pretty much identical to the originals - even down to the blue body
colour. The cap checks ok for value and leakage. With the resistors in
place, the supply still works just fine, except that it now runs pretty
hot, even when unloaded. The FET is a lot hotter than it was before. The
resistors get well hot, as I was expecting, given the condition of the
originals, but with the supply loaded up to a couple of amps, they get
very hot, and the FET becomes uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design,
this network across the transformer primary, is a simple snubber (as
opposed to a clamp or combination clamp and snubber as is also sometimes
found in this position). Texts suggest that its purpose is to limit the
level of voltage spikes at the switching point, to keep the switching
device operating within its SOA and reduce dissipation, which seems a fair
enough comment. However, quite the reverse appears to be true. The whole
supply seems a lot happier with that network 'not there' as it effectively
is, when the resistors are open.

So has anyone got any good thoughts as to what is going on here ? I've
done a great deal of repairs to switchers over the years, and am well
versed with the principles of operation and repair, but I freely admit
that I am not a designer in this field, so I'm at a bit of a loss as to
whether it's just generally a poor design, or whether there's something
else wrong that I'm missing. As those components were originally designed
in, and are clearly faulty now, they need to be replaced, but the fact
that the supply seems to run less efficiently when they are in place,
feels altogether counter-intuitive

Arfa

Makes me think of the old audio tube output amplifiers. The Miller capacitor
could make them yell like an HF-transmiter. Usually a stop resistor in the
grid circuit prevented this oscillation. So adding (or increasing) the
resistance in the gate circuit may solve the problem.

petrus bitbyter

 

[Gareth Magennis]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news04nq.10308$tk5.3106@newsfe13.ams2...

I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single 12 volt output at, I would guess, 3 - 4 amps. It appears to be a
very simple design, in that the chopper drive circuit is discrete,
employing two bipolar transistors as an astable. The output of this is fed
pretty much directly to the gate of a single FET. In the drain of the FET,
is a single primary winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic
cap, and two 3 watt cement-body resistors, all in series. On every one of
the examples sent to me, the two resistors are chalky and very discoloured
to the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go around
150 ohms, so I'm taking that to be the original value, based on the fact
that this type of resistor doesn't usually go low, and some of them have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has crystalised,
and on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where one
or both of the resistors has gone open, the supply works just fine. Loaded
up to a couple of amps, it runs cool and the regulation is good. The
switching FET is barely breaking a sweat, as you would expect. So I went
ahead and replaced the resistors with a pair of 150 ohm 3 watt types that
look pretty much identical to the originals - even down to the blue body
colour. The cap checks ok for value and leakage. With the resistors in
place, the supply still works just fine, except that it now runs pretty
hot, even when unloaded. The FET is a lot hotter than it was before. The
resistors get well hot, as I was expecting, given the condition of the
originals, but with the supply loaded up to a couple of amps, they get
very hot, and the FET becomes uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design,
this network across the transformer primary, is a simple snubber (as
opposed to a clamp or combination clamp and snubber as is also sometimes
found in this position). Texts suggest that its purpose is to limit the
level of voltage spikes at the switching point, to keep the switching
device operating within its SOA and reduce dissipation, which seems a fair
enough comment. However, quite the reverse appears to be true. The whole
supply seems a lot happier with that network 'not there' as it effectively
is, when the resistors are open.

So has anyone got any good thoughts as to what is going on here ? I've
done a great deal of repairs to switchers over the years, and am well
versed with the principles of operation and repair, but I freely admit
that I am not a designer in this field, so I'm at a bit of a loss as to
whether it's just generally a poor design, or whether there's something
else wrong that I'm missing. As those components were originally designed
in, and are clearly faulty now, they need to be replaced, but the fact
that the supply seems to run less efficiently when they are in place,
feels altogether counter-intuitive

Arfa

Hi Arfa,

sorry to hijack the thread, but you don't have any experience with
Wharfedale SMPS's do you? The ones in the powered subs (Titan and EVP
series). Maybe you know likely failure points?

I'm rubbish at SMPS repair, can't stand the things, but its Ł100 to have
Wharfedale repair it (who just put in another PCB).
This one does tick into life but immediately shuts down and stays that way.


Cheers,



Gareth.

 

[Ian Field]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news04nq.10308$tk5.3106@newsfe13.ams2...

I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single 12 volt output at, I would guess, 3 - 4 amps. It appears to be a
very simple design, in that the chopper drive circuit is discrete,
employing two bipolar transistors as an astable. The output of this is fed
pretty much directly to the gate of a single FET. In the drain of the FET,
is a single primary winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic
cap, and two 3 watt cement-body resistors, all in series. On every one of
the examples sent to me, the two resistors are chalky and very discoloured
to the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go around
150 ohms, so I'm taking that to be the original value, based on the fact
that this type of resistor doesn't usually go low, and some of them have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has crystalised,
and on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where one
or both of the resistors has gone open, the supply works just fine. Loaded
up to a couple of amps, it runs cool and the regulation is good. The
switching FET is barely breaking a sweat, as you would expect. So I went
ahead and replaced the resistors with a pair of 150 ohm 3 watt types that
look pretty much identical to the originals - even down to the blue body
colour. The cap checks ok for value and leakage. With the resistors in
place, the supply still works just fine, except that it now runs pretty
hot, even when unloaded. The FET is a lot hotter than it was before. The
resistors get well hot, as I was expecting, given the condition of the
originals, but with the supply loaded up to a couple of amps, they get
very hot, and the FET becomes uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design,
this network across the transformer primary, is a simple snubber (as
opposed to a clamp or combination clamp and snubber as is also sometimes
found in this position). Texts suggest that its purpose is to limit the
level of voltage spikes at the switching point, to keep the switching
device operating within its SOA and reduce dissipation, which seems a fair
enough comment. However, quite the reverse appears to be true. The whole
supply seems a lot happier with that network 'not there' as it effectively
is, when the resistors are open.

So has anyone got any good thoughts as to what is going on here ? I've
done a great deal of repairs to switchers over the years, and am well
versed with the principles of operation and repair, but I freely admit
that I am not a designer in this field, so I'm at a bit of a loss as to
whether it's just generally a poor design, or whether there's something
else wrong that I'm missing. As those components were originally designed
in, and are clearly faulty now, they need to be replaced, but the fact
that the supply seems to run less efficiently when they are in place,
feels altogether counter-intuitive

All the snubbers of this type I've seen were a fast diode feeding the tops
of the flyback pulses to a capacitor that was shunted by a bleed resistor -
if the diode in such a snubber were to fail S/C I would expect the chopper
transistor to warm up a bit.

 

[John Stumbles]

On Tue, 18 Oct 2011 19:52:01 +0200, petrus bitbyter wrote:

---snip--- (hint! )

Makes me think of the old audio tube output amplifiers. The Miller
capacitor could make them yell like an HF-transmiter. Usually a stop
resistor in the grid circuit prevented this oscillation. So adding (or
increasing) the resistance in the gate circuit may solve the problem.

Some ancient neurons in my brain stirred. I think you mean Miller
capacitance? Not an external component but the effective capacitance at
the grid of a valve (or gate of a FET) due to the actual capacitance
between grid (gate) and anode within the device amplified by the voltage
gain of the circuit it forms.
http://en.wikipedia.org/wiki/Miller_effect

--
John Stumbles

I don't want to be part of a club that would have someone like me as a
member.

 

[petrus bitbyter]

"John Stumbles" <john.stumbles@ntlworld.com> schreef in bericht
news:9g8q6uFcblU4@mid.individual.net...

On Tue, 18 Oct 2011 19:52:01 +0200, petrus bitbyter wrote:

---snip--- (hint! )

Makes me think of the old audio tube output amplifiers. The Miller
capacitor could make them yell like an HF-transmiter. Usually a stop
resistor in the grid circuit prevented this oscillation. So adding (or
increasing) the resistance in the gate circuit may solve the problem.

Some ancient neurons in my brain stirred. I think you mean Miller
capacitance? Not an external component but the effective capacitance at
the grid of a valve (or gate of a FET) due to the actual capacitance
between grid (gate) and anode within the device amplified by the voltage
gain of the circuit it forms.
http://en.wikipedia.org/wiki/Miller_effect

--
John Stumbles

I don't want to be part of a club that would have someone like me as a
member.\

Sure. I should have written capacitance to make things more clear. This
capacitance was - and still is - made up by the grid and the anode of the
tube, especially in triodes. This *is* a capacitor as it consists of to
conductors separated by an insulator. Even though the remedy - a stop
resistor in the grid circuit - was known, it was sometimes left out for cost
reduction. It is known that many FETs act in the same way triodes do.

There are other things that can make an audio amplifier oscillate. Last year
I got a modern tube amplifier. The ultra linear amplifier oscillated due to
too low a resistance in the screen grid circuit.

Modern semiconducters suffer from even more capacitances then the old tubes.
I once was told that SMPS-designers consumate their components not by the
number but by the bucket as heavy duty switchers oscillate very easily and
than blow themselfs.

petrus bitbyter

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9g4kevF2n3U1@mid.individual.net...

"Arfa Daily"

I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single 12 volt output at, I would guess, 3 - 4 amps. It appears to be a
very simple design, in that the chopper drive circuit is discrete,
employing two bipolar transistors as an astable. The output of this is
fed pretty much directly to the gate of a single FET. In the drain of the
FET, is a single primary winding up to the raw rail from the input
bridge.


** What sort of SMPS is this ??

One that puts out regulated DC ???

Or is it for driving 12V halogen lights with raw high frequency energy.




.... Phil

No, it's regulated DC. It is used in a vending machine

Arfa

 

[Phil Allison]

"Arfa Daily"

"Phil Allison"
"Arfa Daily"

I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single 12 volt output at, I would guess, 3 - 4 amps. It appears to be a
very simple design, in that the chopper drive circuit is discrete,
employing two bipolar transistors as an astable. The output of this is
fed pretty much directly to the gate of a single FET. In the drain of
the FET, is a single primary winding up to the raw rail from the input
bridge.


** What sort of SMPS is this ??

One that puts out regulated DC ???

Or is it for driving 12V halogen lights with raw high frequency energy.



No, it's regulated DC.

** With just two transistors and one mosfet ?

That is what you have told us.

And SFA else.


.... Phil

 

[Arfa Daily]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:j7jb95$j6l$1@dont-email.me...

Arfa Daily <arfa.daily@ntlworld.com> wrote in message
news04nq.10308$tk5.3106@newsfe13.ams2...
I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single
12 volt output at, I would guess, 3 - 4 amps. It appears to be a very
simple
design, in that the chopper drive circuit is discrete, employing two
bipolar
transistors as an astable. The output of this is fed pretty much directly
to
the gate of a single FET. In the drain of the FET, is a single primary
winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic
cap,
and two 3 watt cement-body resistors, all in series. On every one of the
examples sent to me, the two resistors are chalky and very discoloured to
the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go around
150 ohms, so I'm taking that to be the original value, based on the fact
that this type of resistor doesn't usually go low, and some of them have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has crystalised,
and
on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where one
or
both of the resistors has gone open, the supply works just fine. Loaded
up
to a couple of amps, it runs cool and the regulation is good. The
switching
FET is barely breaking a sweat, as you would expect. So I went ahead and
replaced the resistors with a pair of 150 ohm 3 watt types that look
pretty
much identical to the originals - even down to the blue body colour. The
cap
checks ok for value and leakage. With the resistors in place, the supply
still works just fine, except that it now runs pretty hot, even when
unloaded. The FET is a lot hotter than it was before. The resistors get
well
hot, as I was expecting, given the condition of the originals, but with
the
supply loaded up to a couple of amps, they get very hot, and the FET
becomes
uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design,
this
network across the transformer primary, is a simple snubber (as opposed
to
a
clamp or combination clamp and snubber as is also sometimes found in this
position). Texts suggest that its purpose is to limit the level of
voltage
spikes at the switching point, to keep the switching device operating
within
its SOA and reduce dissipation, which seems a fair enough comment.
However,
quite the reverse appears to be true. The whole supply seems a lot
happier
with that network 'not there' as it effectively is, when the resistors
are
open.

So has anyone got any good thoughts as to what is going on here ? I've
done
a great deal of repairs to switchers over the years, and am well versed
with
the principles of operation and repair, but I freely admit that I am not
a
designer in this field, so I'm at a bit of a loss as to whether it's just
generally a poor design, or whether there's something else wrong that I'm
missing. As those components were originally designed in, and are clearly
faulty now, they need to be replaced, but the fact that the supply seems
to
run less efficiently when they are in place, feels altogether
counter-intuitive

Arfa



What happens if you double the R and halve the C ?

Don't know. However, these are a commercial item that I am repairing for a
company, and as they are, is as they were designed, and I guess the company
that wants me to mend them, would want them left as designed and approved.

As it happens, today I got back to doing some more work on them, and this
time, the situation didn't seem half as bad, which I also can't figure. I
used the same mains isolation transformer to run them, and exactly the same
load - a couple of low voltage halogen lamps totaling 40 watts. Today, the
FET got no hotter under these conditions, than it did with no load. In fact,
it stayed quite cool. Replacement resistors still ran hot, as I'm sure that
they must be expected to, given that they are rated at 3 watts each, but not
so hot that you would feel uncomfortable about them over dissipating. This
has left me a bit non-plussed. Something must be different between what I
was doing Monday, and what I did today, but I can't figure what.

As to them generating high levels of RF, there is certainly no evidence on a
'scope, of any RF on the switching waveform. There are a couple of radios on
in the workshop all the time, one of which is an HF radio usually on 10
metres, and the other is a weather sat VHF one. Neither showed any signs of
picking up anything nasty whilst any of the supplies was running.

The nominal switching frequency is around 50 kHz but on the rising edge,
there is a very tall very narrow spike when the resistors are burnt out.
When they are replaced, the spike is still there, but quite a lot smaller,
so I guess that the purpose of the network is to reduce the level of that
spike to get it down within the ratings of the switching FET. Because the
spike is very fast and narrow, I guess that the 330pF will have a much
smaller Xc to that component of the waveform.

Arfa

 

[Arfa Daily]

"Ian Field" <gangprobing.alien@ntlworld.com> wrote in message
news:mrDnq.515$M61.43@newsfe09.ams2...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news04nq.10308$tk5.3106@newsfe13.ams2...
I've been given a number of switchers to look at, by a company that I do
other work for. The one that concerns me here, is an open frame type,
single 12 volt output at, I would guess, 3 - 4 amps. It appears to be a
very simple design, in that the chopper drive circuit is discrete,
employing two bipolar transistors as an astable. The output of this is
fed pretty much directly to the gate of a single FET. In the drain of the
FET, is a single primary winding up to the raw rail from the input
bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic
cap, and two 3 watt cement-body resistors, all in series. On every one of
the examples sent to me, the two resistors are chalky and very
discoloured to the point where you can't read the bands. On some of them,
one of the resistors is open. Of the remaining resistors, they all seem
to go around 150 ohms, so I'm taking that to be the original value, based
on the fact that this type of resistor doesn't usually go low, and some
of them have gone open. Make no mistake, these resistors look like they
run very hot normally, to the point where the solder on their joints has
crystalised, and on some, scorch damage has been done to the print, and
the substrate.

Now here's the bit that I am finding puzzling. If you take one where one
or both of the resistors has gone open, the supply works just fine.
Loaded up to a couple of amps, it runs cool and the regulation is good.
The switching FET is barely breaking a sweat, as you would expect. So I
went ahead and replaced the resistors with a pair of 150 ohm 3 watt types
that look pretty much identical to the originals - even down to the blue
body colour. The cap checks ok for value and leakage. With the resistors
in place, the supply still works just fine, except that it now runs
pretty hot, even when unloaded. The FET is a lot hotter than it was
before. The resistors get well hot, as I was expecting, given the
condition of the originals, but with the supply loaded up to a couple of
amps, they get very hot, and the FET becomes uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design,
this network across the transformer primary, is a simple snubber (as
opposed to a clamp or combination clamp and snubber as is also sometimes
found in this position). Texts suggest that its purpose is to limit the
level of voltage spikes at the switching point, to keep the switching
device operating within its SOA and reduce dissipation, which seems a
fair enough comment. However, quite the reverse appears to be true. The
whole supply seems a lot happier with that network 'not there' as it
effectively is, when the resistors are open.

So has anyone got any good thoughts as to what is going on here ? I've
done a great deal of repairs to switchers over the years, and am well
versed with the principles of operation and repair, but I freely admit
that I am not a designer in this field, so I'm at a bit of a loss as to
whether it's just generally a poor design, or whether there's something
else wrong that I'm missing. As those components were originally designed
in, and are clearly faulty now, they need to be replaced, but the fact
that the supply seems to run less efficiently when they are in place,
feels altogether counter-intuitive


All the snubbers of this type I've seen were a fast diode feeding the tops
of the flyback pulses to a capacitor that was shunted by a bleed
resistor - if the diode in such a snubber were to fail S/C I would expect
the chopper transistor to warm up a bit.

According to the write-ups, that is a combinational snubber and clamping
circuit, Some designs have just a snubber - like the one I'm working on
here - some have just clamping diodes, and some have a network of R, C and
D, as you say.

Arfa

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9g98iuFrf0U1@mid.individual.net...

"Arfa Daily"
"Phil Allison"
"Arfa Daily"

I've been given a number of switchers to look at, by a company that I
do other work for. The one that concerns me here, is an open frame
type, single 12 volt output at, I would guess, 3 - 4 amps. It appears
to be a very simple design, in that the chopper drive circuit is
discrete, employing two bipolar transistors as an astable. The output
of this is fed pretty much directly to the gate of a single FET. In the
drain of the FET, is a single primary winding up to the raw rail from
the input bridge.


** What sort of SMPS is this ??

One that puts out regulated DC ???

Or is it for driving 12V halogen lights with raw high frequency energy.



No, it's regulated DC.


** With just two transistors and one mosfet ?

That is what you have told us.

And SFA else.


... Phil


You shouldn't take things quite so literally, Phil. I was talking about the

primary side only when I said that, but yes, it pretty much has a pair of
small TO92 transistors, and a TO220 mosfet. Obviously, it also has input
filtering, a bridge, made of 4 discrete diodes, a main filter cap, and
assorted R and C to make those two little transistors into an oscillator.
About 20 components altogether. The secondary side is exactly like any other
fixed voltage typical design, and there is, of course, a perfectly normal 6
pin opto for regulation feedback. Better ?

Arfa

 

[Phil Allison]

"Arfa Daily"

No, it's regulated DC.


** With just two transistors and one mosfet ?

That is what you have told us.

And SFA else.


You shouldn't take things quite so literally, Phil. I was talking about
the primary side only when I said that, but yes, it pretty much has a pair
of small TO92 transistors, and a TO220 mosfet. Obviously, it also has
input filtering, a bridge, made of 4 discrete diodes, a main filter cap,
and assorted R and C to make those two little transistors into an
oscillator. About 20 components altogether. The secondary side is exactly
like any other fixed voltage typical design, and there is, of course, a
perfectly normal 6 pin opto for regulation feedback. Better ?

** All that needed to be in the first post, plus the operating frequency.

No it seems the whole story was BS anyhow.



..... Phil

 

[N_Cook]

Arfa Daily <arfa.daily@ntlworld.com> wrote in message
news:l5Knq.8102$Qv2.4777@newsfe29.ams2...

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:j7jb95$j6l$1@dont-email.me...
Arfa Daily <arfa.daily@ntlworld.com> wrote in message
news04nq.10308$tk5.3106@newsfe13.ams2...
I've been given a number of switchers to look at, by a company that I
do
other work for. The one that concerns me here, is an open frame type,
single
12 volt output at, I would guess, 3 - 4 amps. It appears to be a very
simple
design, in that the chopper drive circuit is discrete, employing two
bipolar
transistors as an astable. The output of this is fed pretty much
directly
to
the gate of a single FET. In the drain of the FET, is a single primary
winding up to the raw rail from the input bridge.

Across that primary, is a network comprising a 330pF 2 Kv disc ceramic
cap,
and two 3 watt cement-body resistors, all in series. On every one of
the
examples sent to me, the two resistors are chalky and very discoloured
to
the point where you can't read the bands. On some of them, one of the
resistors is open. Of the remaining resistors, they all seem to go
around
150 ohms, so I'm taking that to be the original value, based on the
fact
that this type of resistor doesn't usually go low, and some of them
have
gone open. Make no mistake, these resistors look like they run very hot
normally, to the point where the solder on their joints has
crystalised,
and
on some, scorch damage has been done to the print, and the substrate.

Now here's the bit that I am finding puzzling. If you take one where
one
or
both of the resistors has gone open, the supply works just fine. Loaded
up
to a couple of amps, it runs cool and the regulation is good. The
switching
FET is barely breaking a sweat, as you would expect. So I went ahead
and
replaced the resistors with a pair of 150 ohm 3 watt types that look
pretty
much identical to the originals - even down to the blue body colour.
The
cap
checks ok for value and leakage. With the resistors in place, the
supply
still works just fine, except that it now runs pretty hot, even when
unloaded. The FET is a lot hotter than it was before. The resistors get
well
hot, as I was expecting, given the condition of the originals, but with
the
supply loaded up to a couple of amps, they get very hot, and the FET
becomes
uncomfortably hot as well.

As far as I can make out, doing some on-line reading about SMPS design,
this
network across the transformer primary, is a simple snubber (as opposed
to
a
clamp or combination clamp and snubber as is also sometimes found in
this
position). Texts suggest that its purpose is to limit the level of
voltage
spikes at the switching point, to keep the switching device operating
within
its SOA and reduce dissipation, which seems a fair enough comment.
However,
quite the reverse appears to be true. The whole supply seems a lot
happier
with that network 'not there' as it effectively is, when the resistors
are
open.

So has anyone got any good thoughts as to what is going on here ? I've
done
a great deal of repairs to switchers over the years, and am well versed
with
the principles of operation and repair, but I freely admit that I am
not
a
designer in this field, so I'm at a bit of a loss as to whether it's
just
generally a poor design, or whether there's something else wrong that
I'm
missing. As those components were originally designed in, and are
clearly
faulty now, they need to be replaced, but the fact that the supply
seems
to
run less efficiently when they are in place, feels altogether
counter-intuitive

Arfa



What happens if you double the R and halve the C ?

Don't know. However, these are a commercial item that I am repairing for a
company, and as they are, is as they were designed, and I guess the
company
that wants me to mend them, would want them left as designed and approved.

As it happens, today I got back to doing some more work on them, and this
time, the situation didn't seem half as bad, which I also can't figure. I
used the same mains isolation transformer to run them, and exactly the
same
load - a couple of low voltage halogen lamps totaling 40 watts. Today, the
FET got no hotter under these conditions, than it did with no load. In
fact,
it stayed quite cool. Replacement resistors still ran hot, as I'm sure
that
they must be expected to, given that they are rated at 3 watts each, but
not
so hot that you would feel uncomfortable about them over dissipating. This
has left me a bit non-plussed. Something must be different between what I
was doing Monday, and what I did today, but I can't figure what.

As to them generating high levels of RF, there is certainly no evidence on
a
'scope, of any RF on the switching waveform. There are a couple of radios
on
in the workshop all the time, one of which is an HF radio usually on 10
metres, and the other is a weather sat VHF one. Neither showed any signs
of
picking up anything nasty whilst any of the supplies was running.

The nominal switching frequency is around 50 kHz but on the rising edge,
there is a very tall very narrow spike when the resistors are burnt out.
When they are replaced, the spike is still there, but quite a lot smaller,
so I guess that the purpose of the network is to reduce the level of that
spike to get it down within the ratings of the switching FET. Because the
spike is very fast and narrow, I guess that the 330pF will have a much
smaller Xc to that component of the waveform.

Arfa

Are the 330pF multilayer ceramic that could have metalisation creep /
cracks/ unreliable /leaky from humidity. I would replace them with a
different brand/construction

 

[Jim Yanik]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:JKKnq.3161$V_3.67@newsfe06.ams2:

You shouldn't take things quite so literally, Phil. I was talking
about the primary side only when I said that, but yes, it pretty much
has a pair of small TO92 transistors, and a TO220 mosfet. Obviously,
it also has input filtering, a bridge, made of 4 discrete diodes, a
main filter cap, and assorted R and C to make those two little
transistors into an oscillator. About 20 components altogether. The
secondary side is exactly like any other fixed voltage typical design,
and there is, of course, a perfectly normal 6 pin opto for regulation
feedback. Better ?

Arfa

I've been wondering about how the regulating feedback loop works.
How does it modify the multivibrator's output?


--
Jim Yanik
jyanik
at
localnet
dot com

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9g9bm5FgjuU1@mid.individual.net...

"Arfa Daily"
No, it's regulated DC.


** With just two transistors and one mosfet ?

That is what you have told us.

And SFA else.


You shouldn't take things quite so literally, Phil. I was talking about
the primary side only when I said that, but yes, it pretty much has a
pair of small TO92 transistors, and a TO220 mosfet. Obviously, it also
has input filtering, a bridge, made of 4 discrete diodes, a main filter
cap, and assorted R and C to make those two little transistors into an
oscillator. About 20 components altogether. The secondary side is exactly
like any other fixed voltage typical design, and there is, of course, a
perfectly normal 6 pin opto for regulation feedback. Better ?


** All that needed to be in the first post, plus the operating frequency.

No it seems the whole story was BS anyhow.



.... Phil

Well, actually, it didn't, as it was irrelevant to the situation. How the
mosfet receives its drive is neither here nor there in regards to what's
going on at its drain. As long as it has a pulse width modulated 'square'
wave of sufficient amplitude to fully drive the gate, how that waveform is
produced is of no consequence. It actually looks, on this supply, pretty
much exactly the same as the drive waveform on some of the others that the
company have sent, and which use a dedicated PWM chip to produce the drive.
As to the operating frequency, yes, I probably should have stated this, but
most supplies of this sort of size, operate between 40 kHz and 80 KHz, as
would be understood by anyone who works with them regularly.

And why do you think it's all bullshit ? Do you think that I just sit here,
and think to myself "Hmmm. I wonder what dumb-arsed story I can come up with
to start a thread with ?" No, of course not. It's just you, as ever,
spoiling for a fight. Well listen up pal. If you've got anything interesting
to add, as you sometimes actually do, then go ahead and say it. Otherwise,
no one is interested in *your* bullshit. Ok ?

Arfa

 

[Arfa Daily]

"Jim Yanik" <jyanik@abuse.gov> wrote in message
news:Xns9F8477002E528jyaniklocalnetcom@216.168.3.44...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:JKKnq.3161$V_3.67@newsfe06.ams2:



You shouldn't take things quite so literally, Phil. I was talking
about the primary side only when I said that, but yes, it pretty much
has a pair of small TO92 transistors, and a TO220 mosfet. Obviously,
it also has input filtering, a bridge, made of 4 discrete diodes, a
main filter cap, and assorted R and C to make those two little
transistors into an oscillator. About 20 components altogether. The
secondary side is exactly like any other fixed voltage typical design,
and there is, of course, a perfectly normal 6 pin opto for regulation
feedback. Better ?

Arfa



I've been wondering about how the regulating feedback loop works.
How does it modify the multivibrator's output?


--
Jim Yanik

Don't know, Jim. I don't have any schematics for it, as is ever the case,
and I have not at this point bothered to trace out the circuit in that area,
as none of the examples that were sent to me, had problems in that area.
However, I don't suppose it would be too difficult to patch the transistor
side of the opto into the CR network on one side of the multivibrator. It
would just be the equivalent of putting a pot in there. If I get time
tomorrow, I'll have a look, and see if I can figure how that bit is hooked
up. Probably no more than 10 or so components in that area.

Arfa