varistor as snubber

[artem_bond]

Hello all,

I'm working on SMPS and I can't understand why people didn't used
varistors in snubbers. I try to used varistor parallel to transformer
primary coil and It's working well. I have no idea how I can check
surge voltage on FET but I think varistor can decrease this value by
volt-amps diagram and big spurious capacitance. But I can't find any
mentions about using varistors in SMPS in snubber cell.

 

[Spehro Pefhany]

On 26 Apr 2004 18:47:50 -0700, the renowned artem_bond@mail.ru
(artem_bond) wrote:

Hello all,

I'm working on SMPS and I can't understand why people didn't used
varistors in snubbers. I try to used varistor parallel to transformer
primary coil and It's working well. I have no idea how I can check
surge voltage on FET but I think varistor can decrease this value by
volt-amps diagram and big spurious capacitance. But I can't find any
mentions about using varistors in SMPS in snubber cell.

Varistors are a bad idea for repetitive spikes, they have a "wear out"
mechanism. Use silicon, not semiconductors that are made by grinding
stuff up and baking it.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Terry Given]

"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:49gr80dpler9unughd8d0rk6l68pke8fv7@4ax.com...

On 26 Apr 2004 18:47:50 -0700, the renowned artem_bond@mail.ru
(artem_bond) wrote:

Hello all,

I'm working on SMPS and I can't understand why people didn't used
varistors in snubbers. I try to used varistor parallel to transformer
primary coil and It's working well. I have no idea how I can check
surge voltage on FET but I think varistor can decrease this value by
volt-amps diagram and big spurious capacitance. But I can't find any
mentions about using varistors in SMPS in snubber cell.

It is probably the varistor capacitance that is doing most of the work -
look at the flyback waveform, if the peak is rounded the MOV isnt clamping;
if its flat (or at least straight) the MOV is clamping. A simple test would
be to replace the MOV with its equivalent (ie measured) capacitance, and see
how your circuit behaves.

A capacitive snubber presents a nasty turn-on load for the main power switch
though.

Varistors are a bad idea for repetitive spikes, they have a "wear out"
mechanism. Use silicon, not semiconductors that are made by grinding
stuff up and baking it.

very true. IIRC Its more like a cumulative pulse energy rating - many little
pulses, few short pulses etc. Each time a MOV clamps a signal, it is
damaged; enough damage and it fails, initially short-circuit - so MOVs
should ALWAYS have a protective fuse (somewhere in the circuit). Given
enough energy, it will fail open-circuit (ie component gone)

actual clamping voltage is fairly poorly controlled too, and the slope
resistance tends to be quite a bit higher than a good tranzorb


cheers
terry

 

[Jan Panteltje]

On a sunny day (Tue, 27 Apr 2004 02:03:45 GMT) it happened Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote in
<49gr80dpler9unughd8d0rk6l68pke8fv7@4ax.com>:

On 26 Apr 2004 18:47:50 -0700, the renowned artem_bond@mail.ru
(artem_bond) wrote:

Hello all,

I'm working on SMPS and I can't understand why people didn't used
varistors in snubbers. I try to used varistor parallel to transformer
primary coil and It's working well. I have no idea how I can check
surge voltage on FET but I think varistor can decrease this value by
volt-amps diagram and big spurious capacitance. But I can't find any
mentions about using varistors in SMPS in snubber cell.

Varistors are a bad idea for repetitive spikes, they have a "wear out"
mechanism. Use silicon, not semiconductors that are made by grinding
stuff up and baking it.


Best regards,
Spehro Pefhany
Well, in the old TV (vacuum tube time) you would see HV varistors (VDR)

across the vertical output a lot.
Of all the hundreds / thausands I repaired I have -perhasp- seen one
defective one.
I have used VDR extensively to protect inductive switched loads, across
mains as peak limiter, etc.., never a problem (used Siemens).
Do you have data on failure rates?
As to the OP
In a switch mode, these things will dissipate some power.
Better design you do not need them.
But why should it not work...
Have not used these in a switchmode.
JP

 

[John Fields]

On Tue, 27 Apr 2004 22:08:12 GMT, Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote:

Well, in the old TV (vacuum tube time) you would see HV varistors (VDR)
across the vertical output a lot.
Of all the hundreds / thausands I repaired I have -perhasp- seen one
defective one.
I have used VDR extensively to protect inductive switched loads, across
mains as peak limiter, etc.., never a problem (used Siemens).
Do you have data on failure rates?
As to the OP
In a switch mode, these things will dissipate some power.
Better design you do not need them.
But why should it not work...
Have not used these in a switchmode.

---
MOVs are made by sintering conductive material in a non-conductive
binder in such a way that when a the voltage across the MOV rises to a
point which is high enough to arc between the conductive grains, the
MOV presents a low impedance to the voltage, (much like when a gas
tube ionizes) which then causes current to flow through the MOV until
the voltage falls to a point where the internal arcing is quenched.
However, because of the high local temperatures reached within the MOV
because of the energy released within it, some of the conductive
material will melt and will fuse together, forming conductive channels
through the MOV. This is a phenomenon which is inherent in the device
and the damage caused with each hit is cumulative in that with every
hit the MOV takes, it becomes quiescently more and more conductive,
until the point is reached where, with normal operating voltage across
it its power dissipation capabilities are exceeded and it fails. It
_always_ fails shorted, usually with spectacular results, and has been
known to have started fires when it melts or explodes before finally
clearing open.

--
John Fields

 

[Spehro Pefhany]

On Tue, 27 Apr 2004 19:01:31 -0500, the renowned John Fields
<jfields@austininstruments.com> wrote:

On Tue, 27 Apr 2004 22:08:12 GMT, Jan Panteltje
pNaonStpealmtje@yahoo.com> wrote:


Well, in the old TV (vacuum tube time) you would see HV varistors (VDR)
across the vertical output a lot.
Of all the hundreds / thausands I repaired I have -perhasp- seen one
defective one.
I have used VDR extensively to protect inductive switched loads, across
mains as peak limiter, etc.., never a problem (used Siemens).
Do you have data on failure rates?
As to the OP
In a switch mode, these things will dissipate some power.
Better design you do not need them.
But why should it not work...
Have not used these in a switchmode.

---
MOVs are made by sintering conductive material in a non-conductive
binder in such a way that when a the voltage across the MOV rises to a
point which is high enough to arc between the conductive grains, the
MOV presents a low impedance to the voltage, (much like when a gas
tube ionizes) which then causes current to flow through the MOV until
the voltage falls to a point where the internal arcing is quenched.
However, because of the high local temperatures reached within the MOV
because of the energy released within it, some of the conductive
material will melt and will fuse together, forming conductive channels
through the MOV. This is a phenomenon which is inherent in the device
and the damage caused with each hit is cumulative in that with every
hit the MOV takes, it becomes quiescently more and more conductive,
until the point is reached where, with normal operating voltage across
it its power dissipation capabilities are exceeded and it fails. It
_always_ fails shorted, usually with spectacular results, and has been
known to have started fires when it melts or explodes before finally
clearing open.

Yes, they recently revised UL standards to require a current limiting
device in series with varistors, because of fires caused by thermal
runaway in the nasty little things, just as you say. I've seen plenty
of failures in ZnO varistors- Japanese, American and Taiwanese types.
Their clamping characteristics are also pretty crummy. They certainly
have their uses, but..

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Jan Panteltje]

On a sunny day (Tue, 27 Apr 2004 19:01:31 -0500) it happened John Fields
<jfields@austininstruments.com> wrote in
<drqt8013cip921ghl4qv0qjr01pkbo7vp1@4ax.com>:

clearing open.
All very well, but in a vertical output stage the thing gets hits 50 times

per second, that say 4 hours per day and for 10 years, and still working!
The theoretical way *I* understood these things worked was certainly not
'arcing'.
I was under the impression that these things were semiconductor based.
So I just looked up some papers.
There is really ONLY significant degradation when used outside the specified
peak current range.
Have a read here perhaps:
http://www.iaei.org/magazine/04_b/04_b_brown.htm

When used within range (specs) no 'arcing' or such will happen.
It is good practice to measure the current (if repetitive) to see what
the thing is actually (periodically) exposed to.
The kA devices, when used as a 'snubber' in electronics will only get A or mA
pulses so are way way below the rated kA surge.
In such a case, no degradation occurs.
In case of -main protection- from one time surges such as lightning, you will
(very likely) have to replace... and a lot of other stuff too.
In such a case teh series fuse is needed to break the circuit.
But this was about snubbers (OP) and here I think these devices have proved
reliable, at least in V output in TV.
JP

 

[Jan Panteltje]

Also want to add, regarding to the OP, that in case of using
VDR in switch mode, we need not only look at the heat dissipated,
(that sets the lifetime in hours MTBF), but also at the capacitance.
Thermal run-away is the most dominant failure mode
(according to http://www.iaei.org/magazine/04_b/04_b_brown.htm, read conclusion
and see graph in figure 3 )
But:
for frequencies of 10 kHz or more I think it is very
possible the capacitance of these devices will short / have some effect on
the operation of the switch mode
I have no real values here at hand, maybe someone knows the values.
It does not seem to be in the data sheets.
There are pspice models available for at least some varistors.

Maybe that is why I never felt like using these in a 100kHz switcher...
But perhaps this capacitance could be used to your advantage too.
JP