Need help with switching power supply repair

[Jim Yanik]

Kripton wrote in news:4e7c30a0$0$16475$426a34cc@news.free.fr:

I've never seen a chip 3842 dead...
but the small cap 10uF beside it has really often been solving the
case !!!!!

I've had to replace a lot of 3842's.

it depends on the PS design,some designs leave the IC
vulnerable,particularly if the switcher FET/XSTR blows.

but the 3842 makes a nice PS circuit.

--
Jim Yanik
jyanik
at
localnet
dot com

 

[Arfa Daily]

"Kripton" wrote in message news:4e7c30a0$0$16475$426a34cc@news.free.fr...

On 2011-09-23 02:56:37 +0200, "Arfa Daily" <arfa.daily@ntlworld.com> said:



"sci.electronics.repair" <rnewman36@gmail.com> wrote in message
news:26b39f80-37e4-4dd2-bdae-46c118a3c01c@1g2000vbu.googlegroups.com...
On Sep 21, 8:08 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
"senator richards" <rnewma...@gmail.com> wrote in message

news:dd3e3ab9-1f98-41b8-a7ec-08431af9a33d@u20g2000yqj.googlegroups.com...

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the
first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy

The cap that filters the supply to the pwm chip on the primary side,
maybe ?
It's pretty common on most designs of switcher. Work on the thing on an
isolation transformer if at all possible. They are potentially very
very
dangerous if you are not fully competent with them

Arfa

Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5 which
appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R

A brand new cap might be a little better than that, but certainly right
ball park at 0.5 ohms, and would not be an issue at that figure. About
the only other thing that you could try, assuming that it uses a startup
resistor from the 170v, is to disconnect the self-feed diode from the
switching transformer. That way, you will force it to run from the
startup supply only, just in case the self-run voltage is fluctuating,
and interfering with your reading on the pwm chip's supply. Bear in mind
though, that you can't run it for too long on the startup resistor, as it
will get quite hot, being normally intended to supply current to the
chip, only for as long as it takes the supply to fully start up and
settle. If the voltage supply to the chip still jiggles around when it is
only being fed by the resistor, then after the 47uF cap, which may yet be
faulty, but not in an ESR way, the next prime suspect would have to be
the chip itself.

Arfa

I've never seen a chip 3842 dead...
but the small cap 10uF beside it has really often been solving the case
!!!!!

--
----------
Kripton

No ? There are a number of chips in the 35 and 38 series pwm controllers,
which are all pretty similar in function, and whilst I'm not certain that I
have specifically had a 3842 faulty, I have certainly replaced many of those
series over the years, usually for a dead symptom, due to the internal
voltage reference having failed. I agree that the caps decoupling the supply
and reference voltages are by far the commonest problem though, and that was
what I first suggested to the OP as a possibility for his problem. The
symptoms that he has, are pretty weird for a switcher. I, like others on
here, would initially have suspected the filter cap for the rail that the
regulation feedback is derived from, but the fact that he replaced all of
the secondary caps right off, seems to knock that one on the head.

Arfa

 

[Arfa Daily]

A brand new cap might be a little better than that, but certainly
right ball park at 0.5 ohms, and would not be an issue at that figure.

IIRC,the values on the DSE meter are MAX values,so IMO,if a cap is reading
what's listed,it's marginal.

"Approx worst case ESR values for new capacitors at 20deg C"

is what it actually says. Bob once told me that when he put that chart on
the front, he took it directly from a manufacturer's data sheet - Pan IIRC

I guess that there are two ways you can take that. I have always taken it on
the positive side, rather than the negative view that you take. As I'm sure
you well know from your own experience, using an ESR meter is a black art
anyway, as much magic as it is science, but given that, in the many years
that I have owned and used daily, a Bob Parker (DSE) meter, I have always
found the chart to be an excellent guide. From many years of experience with
SMPS's, and having found and replaced many pwm chip supply decoupling caps,
I would not feel the immediate need to replace one of that value and voltage
rating, that went 0.5 ohms. I would not consider it 'marginal' and
potentially the source of the problem, until it had got a lot closer to 1
ohm. Typically, caps that have failed in that position, will be up around
the 10 ohm mark, or open circuit.

About the only other thing that you could try, assuming that it uses a
startup resistor from the 170v, is to disconnect the self-feed diode
from the switching transformer. That way, you will force it to run
from the startup supply only, just in case the self-run voltage is
fluctuating, and interfering with your reading on the pwm chip's
supply. Bear in mind though, that you can't run it for too long on the
startup resistor, as it will get quite hot, being normally intended to
supply current to the chip, only for as long as it takes the supply to
fully start up and settle. If the voltage supply to the chip still
jiggles around when it is only being fed by the resistor, then after
the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa



usually,the startup resistor doesn't supply enough current to keep the IC
running,just enough to start,that's why they have the housekeeping supply
from the XFMR. Otherwise,you get a "burst" or "chirp" mode of
operation;the
PS starts,then dies because the IC isn't getting enough power.
Often,it's the housekeeping supply electrolytic cap that's gone bad,as you
said.

I've also seen where the start resistor goes high in value,even
open,preventing starting.(but not this guy's problem)

--
Jim Yanik

Yes, agreed, but you've got to admit that the supply to the chip fluctuating
between 7 and 15 volts is a pretty bizarre state of affairs, and there's not
actually too much bar the startup R and it's decoupling cap, connected to
that pin. Of course, the self-powering supply, also connected to this point,
might be interfering with the reading, which is why I suggested
disconnecting it. As to whether the supply will run in this condition,
depends on the value of the startup resistor, but running or not, it might
at least give an indication of where next to look, depending on whether the
voltage on the chip side of the resistor is now steady, indicating that the
fluctuating voltage is coming back via the self-powering supply, and in
which case is a red herring, or still varying as a result of the chip
drawing current erratically. I might have felt inclined to think that the
fluctuation was the startup bleed via the startup resistor at 7 volts,
followed by the self-power coming on line at 15 volts as the supply gets
going, followed by the supply shutting back down and so on, due to some
secondary side problem. However, that then doesn't stack up with the
secondary side voltages being steady, but wrong, and worse than wrong,
unbalanced.

It is an odd state of affairs to be sure, but of course, as we all know,
some problems on switchers can be very obscure.

Arfa

 

[Jamie]

Jim Yanik wrote:

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:bPQeq.11661$fR1.8387@newsfe24.ams2:



"sci.electronics.repair" <rnewman36@gmail.com> wrote in message
news:26b39f80-37e4-4dd2-bdae-46c118a3c01c@1g2000vbu.googlegroups.com...

On Sep 21, 8:08 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:

"senator richards" <rnewma...@gmail.com> wrote in message

news:dd3e3ab9-1f98-41b8-a7ec-08431af9a33d@u20g2000yqj.googlegroups.co
m...


I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and -
15vdc at .8A. Currently it is outputting +17 on one output and
somewhere between +22 and +30 on the other output. My experience
with SMPS has usually been shorted rectifiers or bad output filter
caps so the first thing I did was check all the diodes and replace
the output caps. Obviously that didn't fix the problem. The high
voltage is about 170vdc. The supply to the pwm chip is fluctuating
between 7-15v, so i'm thinking this might be the problem, but
maybe its something else. In case its not obvious, i'm fairly new
to tinkering with these things. Thanks in advance for any help.

Randy

The cap that filters the supply to the pwm chip on the primary side,
maybe ?
It's pretty common on most designs of switcher. Work on the thing on
an isolation transformer if at all possible. They are potentially
very very dangerous if you are not fully competent with them

Arfa

Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5
which appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R

A brand new cap might be a little better than that, but certainly
right ball park at 0.5 ohms, and would not be an issue at that figure.


IIRC,the values on the DSE meter are MAX values,so IMO,if a cap is reading
what's listed,it's marginal.


About the only other thing that you could try, assuming that it uses a
startup resistor from the 170v, is to disconnect the self-feed diode
from the switching transformer. That way, you will force it to run
from the startup supply only, just in case the self-run voltage is
fluctuating, and interfering with your reading on the pwm chip's
supply. Bear in mind though, that you can't run it for too long on the
startup resistor, as it will get quite hot, being normally intended to
supply current to the chip, only for as long as it takes the supply to
fully start up and settle. If the voltage supply to the chip still
jiggles around when it is only being fed by the resistor, then after
the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa




usually,the startup resistor doesn't supply enough current to keep the IC
running,just enough to start,that's why they have the housekeeping supply
from the XFMR. Otherwise,you get a "burst" or "chirp" mode of operation;the
PS starts,then dies because the IC isn't getting enough power.
Often,it's the housekeeping supply electrolytic cap that's gone bad,as you
said.

I've also seen where the start resistor goes high in value,even
open,preventing starting.(but not this guy's problem)

Talking about switching supplies. Today I decided to look at a supply

removed from an allen Bradley Panel computer, this is the second time
the supply has quick, we were able to buy exact replacement supplies so it
was cheaper to do that at the time.

This time I decided to investigate the issue on one of the broken
supplies. I found the main cap on the HV side to be shorted, it was
something like 230Uf 450VDC. This cap did not cause any components to
open that I could see other than a varistor in series to it for the
inrush currents. Any way, I could not get that damn cap out of the
board, I don't know what type of solder they used on that cap but the
only thing I had that would melt the solder was a 140 watt soldering
pistol, and that didn't do me a lot of good since I needed to get to the
other side of it.

My regular station set to 850F would not melt this solder but items
around it had no issues breaking down with that temp.. I guess If I
really want to fix this thing, I will have to use my air iron and some
added heat with another iron on the area to blow the solder out the back
side.

Oh well.

Jamie

 

[Archon]

On 9/23/2011 7:22 PM, Jamie wrote:

Jim Yanik wrote:

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:bPQeq.11661$fR1.8387@newsfe24.ams2:


"sci.electronics.repair" <rnewman36@gmail.com> wrote in message
news:26b39f80-37e4-4dd2-bdae-46c118a3c01c@1g2000vbu.googlegroups.com...

On Sep 21, 8:08 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:

"senator richards" <rnewma...@gmail.com> wrote in message

news:dd3e3ab9-1f98-41b8-a7ec-08431af9a33d@u20g2000yqj.googlegroups.co
m...

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and -
15vdc at .8A. Currently it is outputting +17 on one output and
somewhere between +22 and +30 on the other output. My experience
with SMPS has usually been shorted rectifiers or bad output filter
caps so the first thing I did was check all the diodes and replace
the output caps. Obviously that didn't fix the problem. The high
voltage is about 170vdc. The supply to the pwm chip is fluctuating
between 7-15v, so i'm thinking this might be the problem, but
maybe its something else. In case its not obvious, i'm fairly new
to tinkering with these things. Thanks in advance for any help.

Randy

The cap that filters the supply to the pwm chip on the primary side,
maybe ?
It's pretty common on most designs of switcher. Work on the thing on
an isolation transformer if at all possible. They are potentially
very very dangerous if you are not fully competent with them

Arfa

Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5
which appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R

A brand new cap might be a little better than that, but certainly
right ball park at 0.5 ohms, and would not be an issue at that figure.


IIRC,the values on the DSE meter are MAX values,so IMO,if a cap is
reading what's listed,it's marginal.


About the only other thing that you could try, assuming that it uses a
startup resistor from the 170v, is to disconnect the self-feed diode
from the switching transformer. That way, you will force it to run
from the startup supply only, just in case the self-run voltage is
fluctuating, and interfering with your reading on the pwm chip's
supply. Bear in mind though, that you can't run it for too long on the
startup resistor, as it will get quite hot, being normally intended to
supply current to the chip, only for as long as it takes the supply to
fully start up and settle. If the voltage supply to the chip still
jiggles around when it is only being fed by the resistor, then after
the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa



usually,the startup resistor doesn't supply enough current to keep the
IC running,just enough to start,that's why they have the housekeeping
supply from the XFMR. Otherwise,you get a "burst" or "chirp" mode of
operation;the PS starts,then dies because the IC isn't getting enough
power.
Often,it's the housekeeping supply electrolytic cap that's gone bad,as
you said.
I've also seen where the start resistor goes high in value,even
open,preventing starting.(but not this guy's problem)

Talking about switching supplies. Today I decided to look at a supply
removed from an allen Bradley Panel computer, this is the second time
the supply has quick, we were able to buy exact replacement supplies so it
was cheaper to do that at the time.

This time I decided to investigate the issue on one of the broken
supplies. I found the main cap on the HV side to be shorted, it was
something like 230Uf 450VDC. This cap did not cause any components to
open that I could see other than a varistor in series to it for the
inrush currents. Any way, I could not get that damn cap out of the
board, I don't know what type of solder they used on that cap but the
only thing I had that would melt the solder was a 140 watt soldering
pistol, and that didn't do me a lot of good since I needed to get to the
other side of it.

My regular station set to 850F would not melt this solder but items
around it had no issues breaking down with that temp.. I guess If I
really want to fix this thing, I will have to use my air iron and some
added heat with another iron on the area to blow the solder out the back
side.

Oh well.

Jamie


I find adding fresh solder to difficult melts and sucking it out

repetitively eventually will fill the joint with regular solder. Of
course if theres a heavy plane of copper, its a problem.
JC

 

[Franc Zabkar]

On Fri, 23 Sep 2011 16:52:09 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> put finger to keyboard and composed:

However, that then doesn't stack up with the
secondary side voltages being steady, but wrong, and worse than wrong,
unbalanced.

As has already been mentioned, there could be something wrong with the
OP's measurements. He states that "currently it is outputting +17 on
one output and somewhere between +22 and +30 on the other output."

One would expect that the +/-15V supplies would have identical
windings. Moreover, since they would be wound over the same core, then
one would expect that the two outputs should track each other
reasonably well, allowing for slight load differences.

Maybe there is a clue in the fact that the OP's measurements are both
positive. Could it be that he is uing the wrong 0V reference?

For example, if he is mistakenly using the -15V rail as the 0V
reference, then he would be measuring +15V for ground, and +30V for
the +15V rail, if the supply were working correctly.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Arfa Daily]

"Franc Zabkar" <fzabkar@iinternode.on.net> wrote in message
newsbeq77t7ek0ovdfbsu9mud8no2e1js8nul@4ax.com...

On Fri, 23 Sep 2011 16:52:09 +0100, "Arfa Daily"
arfa.daily@ntlworld.com> put finger to keyboard and composed:

However, that then doesn't stack up with the
secondary side voltages being steady, but wrong, and worse than wrong,
unbalanced.

As has already been mentioned, there could be something wrong with the
OP's measurements. He states that "currently it is outputting +17 on
one output and somewhere between +22 and +30 on the other output."

One would expect that the +/-15V supplies would have identical
windings. Moreover, since they would be wound over the same core, then
one would expect that the two outputs should track each other
reasonably well, allowing for slight load differences.

Maybe there is a clue in the fact that the OP's measurements are both
positive. Could it be that he is uing the wrong 0V reference?

For example, if he is mistakenly using the -15V rail as the 0V
reference, then he would be measuring +15V for ground, and +30V for
the +15V rail, if the supply were working correctly.

- Franc Zabkar
--

Hmmm. That's a very valid point, Franc. Although possibly not specifically
in switchmode power supplies, I got the impression that the OP was
reasonably savvy in electronic repairs in general, so did not even consider
that the measurement technique could be in error. To be honest, I never even
noticed that he was declaring both to be positive ... :-\

Arfa

 

[Geo]

On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
<rnewman36@gmail.com> wrote:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

I don't see that a fluctuating supply would cause a polarity reversal

on the -15v rail.

 

[Franc Zabkar]

On Fri, 23 Sep 2011 01:56:37 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> put finger to keyboard and composed:

About the only
other thing that you could try, assuming that it uses a startup resistor
from the 170v, is to disconnect the self-feed diode from the switching
transformer. That way, you will force it to run from the startup supply
only, just in case the self-run voltage is fluctuating, and interfering with
your reading on the pwm chip's supply. Bear in mind though, that you can't
run it for too long on the startup resistor, as it will get quite hot, being
normally intended to supply current to the chip, only for as long as it
takes the supply to fully start up and settle.

If the feed is coming directly from the +170V rail, then I can't see
that there will be much difference in the power dissipation of the
startup resistor after the power supply has settled into run mode.

For example, if we assume that the chip's supply voltages are 5V at
startup and 15V in run mode, the the voltage across the resistor will
vary between 165V and 155V.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Arfa Daily]

"Franc Zabkar" <fzabkar@iinternode.on.net> wrote in message
news:ma7s775bkq5h737pgblve5n8tksndqf4tk@4ax.com...

On Fri, 23 Sep 2011 01:56:37 +0100, "Arfa Daily"
arfa.daily@ntlworld.com> put finger to keyboard and composed:

About the only
other thing that you could try, assuming that it uses a startup resistor
from the 170v, is to disconnect the self-feed diode from the switching
transformer. That way, you will force it to run from the startup supply
only, just in case the self-run voltage is fluctuating, and interfering
with
your reading on the pwm chip's supply. Bear in mind though, that you can't
run it for too long on the startup resistor, as it will get quite hot,
being
normally intended to supply current to the chip, only for as long as it
takes the supply to fully start up and settle.

If the feed is coming directly from the +170V rail, then I can't see
that there will be much difference in the power dissipation of the
startup resistor after the power supply has settled into run mode.

For example, if we assume that the chip's supply voltages are 5V at
startup and 15V in run mode, the the voltage across the resistor will
vary between 165V and 155V.

- Franc Zabkar
--

Typically, the startup supply doesn't come *directly* from the high voltage
rail, It is normally fed via a diode, and the self-powering rail is arranged
to be a higher voltage than the startup supply. This results in the diode
feeding the startup supply, to become reverse biased once the supply fully
starts, which then results in the current draw from that supply reducing to
as good as zero, and thus likewise dropping the power dissipation in the
startup resistor to virtually zero.

Arfa

 

[Arfa Daily]

"Geo" <nhhu-o3hu@dea.spamcon.org> wrote in message
news:vp2s77dq27rf3h4is6td7n8mov44rsbqt5@4ax.com...

On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
rnewman36@gmail.com> wrote:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

I don't see that a fluctuating supply would cause a polarity reversal
on the -15v rail.

No, you're right, it wouldn't, which is why Franc and I elsewhere in the
thread, have now entered into the discussion of how exactly the OP is taking
his measurements i.e. what point he is using as his ground, for a start.

Arfa

 

[Franc Zabkar]

On Sun, 25 Sep 2011 01:29:05 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> put finger to keyboard and composed:

Typically, the startup supply doesn't come *directly* from the high voltage
rail, It is normally fed via a diode, and the self-powering rail is arranged
to be a higher voltage than the startup supply. This results in the diode
feeding the startup supply, to become reverse biased once the supply fully
starts, which then results in the current draw from that supply reducing to
as good as zero, and thus likewise dropping the power dissipation in the
startup resistor to virtually zero.

Arfa

Your comment took me by surprise. Just to make sure I wasn't having a
brain fart, I consulted an application note:
http://www.nalanda.nitc.ac.in/industry/AppNotes/Unitrode/slua143.pdf

I confess that I haven't really thought about this subject before, but
according to page 4 of the PDF, there is no back-biased diode. AISI, a
diode that is fed from a 170VDC supply cannot become reverse biased by
a much lower bootstrap voltage.

The app note suggests that the UC3842 will remain in the off state
until the capacitor on its Vcc pin charges up to the UVLO (under
voltage lockout) turn-on voltage of 16V. During this time the IC draws
only 1mA.

After the UVLO turn-on threshold is reached, the IC turns on and
pulses the chopper. The bootstrap winding then generates the Vcc
supply for the IC and prevents the capacitor from discharging below
the IC's UVLO turn-off threshold of 10V.

Fig 31 on page 13 of the same document has an application circuit for
an isolated +/-12V supply. The 56K resistor (R2) can pass about 3mA.
This means that 2mA goes toward charging the capacitor. The UC3842
datasheet specifies a typical operating current of 14mA.

http://www.elektronik.sk/datasheet/UC3842.pdf

If you were to disconnect the bootstrap winding, then the output
voltage will only remain alive for as long as it takes the Vcc
capacitor to discharge from 16V to 10V.

I = C . dV/dt

so ...

dt = C x dV / I

= 100uF x (16V - 10V) / (14 - 3)mA
= 55 msec


- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Arfa Daily]

"Franc Zabkar" <fzabkar@iinternode.on.net> wrote in message
news:ehet77tj8mdnnuam8c1houuf8lh9s945u3@4ax.com...

On Sun, 25 Sep 2011 01:29:05 +0100, "Arfa Daily"
arfa.daily@ntlworld.com> put finger to keyboard and composed:

Typically, the startup supply doesn't come *directly* from the high
voltage
rail, It is normally fed via a diode, and the self-powering rail is
arranged
to be a higher voltage than the startup supply. This results in the diode
feeding the startup supply, to become reverse biased once the supply fully
starts, which then results in the current draw from that supply reducing
to
as good as zero, and thus likewise dropping the power dissipation in the
startup resistor to virtually zero.

Arfa

Your comment took me by surprise. Just to make sure I wasn't having a
brain fart, I consulted an application note:
http://www.nalanda.nitc.ac.in/industry/AppNotes/Unitrode/slua143.pdf

I confess that I haven't really thought about this subject before, but
according to page 4 of the PDF, there is no back-biased diode. AISI, a
diode that is fed from a 170VDC supply cannot become reverse biased by
a much lower bootstrap voltage.

The app note suggests that the UC3842 will remain in the off state
until the capacitor on its Vcc pin charges up to the UVLO (under
voltage lockout) turn-on voltage of 16V. During this time the IC draws
only 1mA.

After the UVLO turn-on threshold is reached, the IC turns on and
pulses the chopper. The bootstrap winding then generates the Vcc
supply for the IC and prevents the capacitor from discharging below
the IC's UVLO turn-off threshold of 10V.

Fig 31 on page 13 of the same document has an application circuit for
an isolated +/-12V supply. The 56K resistor (R2) can pass about 3mA.
This means that 2mA goes toward charging the capacitor. The UC3842
datasheet specifies a typical operating current of 14mA.

http://www.elektronik.sk/datasheet/UC3842.pdf

If you were to disconnect the bootstrap winding, then the output
voltage will only remain alive for as long as it takes the Vcc
capacitor to discharge from 16V to 10V.

I = C . dV/dt

so ...

dt = C x dV / I

= 100uF x (16V - 10V) / (14 - 3)mA
= 55 msec


- Franc Zabkar

Hmmm. OK. I'll buy that. I must admit that I haven't taken too much notice
of how that bit of the circuitry works in recent years. Perhaps I'm just
going back to the early days when I was first taught about these things on
manufacturers' service courses. Maybe there used to be a zener on the end of
the startup resistor, and then a diode. The diode could then be reverse
biased by the self-powering supply, and the current in the startup resistor
would decrease to that of the zener's draw only. I'll have to have a look
back on some of the older schematics for VCRs and DVDs etc

Arfa