Author Topic: Lightly Toasted MOV in SMPS  (Read 562 times)

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Offline SmokeDetector

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Lightly Toasted MOV in SMPS
« on: August 13, 2017, 01:24:50 PM »
Fixing up a Carvin EM900 in-ear monitor for a friend.

The only visible issue is an MOV that has burned up a little, which seems odd, as I thought they failed open-circuit? This one is showing about 28K ohms across it.

The MOV looks to be part of a flyback/snubber - it's in series with a fast diode across the primary coil of the transformer, which I assume is to increase efficiency by not clamping until the voltage goes above the 170V DC rating of this MOV, but it also makes me think the MOV is going to have a limited life if it's sitting there clamping all those spikes? Clearly I'm missing something.

I'm going to replace the MOV as a first step, but am wondering if there's some other issue that could cause this component to fail this way?
 

Offline BradC

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Re: Lightly Toasted MOV in SMPS
« Reply #1 on: August 13, 2017, 03:08:12 PM »
No they generally gradually fail short. They then open up by burning out or a massive surge.
 

Offline SmokeDetector

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Re: Lightly Toasted MOV in SMPS
« Reply #2 on: August 13, 2017, 06:56:06 PM »

Ahh,that makes more sense.

I checked the fast recovery diode in series with the MOV and that looks to have failed, so it's conducting all the time, exposing the MOV to the full DC rail voltage (300V+) whenever the MOSFET turns on, leading to its failure.

Will replace diode and MOV and see where that gets me.

Russell.
 

Online Armadillo

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Re: Lightly Toasted MOV in SMPS
« Reply #3 on: August 13, 2017, 11:26:27 PM »
If only the flyback snubber components failed, then you would have not sensed a problem with the power supply failure unless the mosfet went with it because of larger voltage peak impressed onto it.

Hence better turn on using light bulb in series as caution.
 

Offline T3sl4co1l

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Re: Lightly Toasted MOV in SMPS
« Reply #4 on: August 14, 2017, 12:34:51 AM »
I would think the MOSFET has failed as well, and controller, and...

It may've saved itself by switching very short pulses, but it needs to be checked.  That sort of thing can be a cascading failure.

MOVs aren't a good design idea in that application.  Normally a P6KE200 or so is used (avalanche diodes / TVS are good for ~infinite pulses, as long as ratings are observed).  Alternately, an R || C network, which charges up to some peak voltage and burns the excess power in the resistor (which can be much larger than a diode or MOV: 5W or more).

Maybe they needed more power dissipation?  Also a result of bad design...

Tim
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Offline SmokeDetector

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Re: Lightly Toasted MOV in SMPS
« Reply #5 on: August 14, 2017, 10:14:42 AM »
My half-baked theory (a bit like the MOV) is that once the diode started conducting all the time, the MOV pulled down the voltage across the primary of the transformer, dropping its output enough to stop the unit working, my friend noticed this, then smelled something burning, then turned the unit off.

I've attached an attempt at a schematic showing what I think are the relevant bits, including the component values. Also pics of the board in question. Toasted MOV is the charred black thing tucked up against the transformer, D1 is the diode.

I pulled out the MOSFET and did a basic test with the diode setting on my meter, and it seems to be behaving properly. The 1A fuse on the mains input also hadn't blown, so I'm feeling hopeful, but there's also the control bits, which are SMDs under the board. I will take the advice above and sort out a light bulb before I fire it up (hopefully not literally)...

Russell.
« Last Edit: August 14, 2017, 03:27:16 PM by SmokeDetector »
 

Online Armadillo

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Re: Lightly Toasted MOV in SMPS
« Reply #6 on: August 14, 2017, 02:35:59 PM »
Is the labelling a "C7" ?
 

Offline SmokeDetector

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Re: Lightly Toasted MOV in SMPS
« Reply #7 on: August 14, 2017, 03:26:57 PM »
Is the labelling a "C7" ?

Yep, that's the one.
 

Online Armadillo

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Re: Lightly Toasted MOV in SMPS
« Reply #8 on: August 14, 2017, 04:36:06 PM »
Is the labelling a "C7" ?

Yep, that's the one.

I meant could that be a capacitor instead of "MOV".
 

Offline T3sl4co1l

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Re: Lightly Toasted MOV in SMPS
« Reply #9 on: August 14, 2017, 04:57:59 PM »
Oh, and there is, in fact, a resistor R2 right next to it (under shrink wrap, as is often done for whatever reason).  So it IS actually an RCD clamp, but they put an MOV in there? ??? !? :-//

Tim
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Online Armadillo

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Re: Lightly Toasted MOV in SMPS
« Reply #10 on: August 14, 2017, 05:26:40 PM »
Oh, and there is, in fact, a resistor R2 right next to it (under shrink wrap, as is often done for whatever reason).  So it IS actually an RCD clamp, but they put an MOV in there? ??? !? :-//

Tim

There should instead put a Zener diode, all this is for code compliance. Put a capacitor to ring back the energy for saving instead of compliance? LOL.   ;D
 

Offline T3sl4co1l

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Re: Lightly Toasted MOV in SMPS
« Reply #11 on: August 14, 2017, 05:28:24 PM »
There should instead put a Zener diode, all this is for code compliance. Put a capacitor to ring back the energy for saving instead of compliance? LOL.   ;D

Code doesn't care about peak voltage snubbers..?

No ringing, the R||C is behind the clamp diode.

Tim
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Online Armadillo

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Re: Lightly Toasted MOV in SMPS
« Reply #12 on: August 14, 2017, 05:45:04 PM »
There should instead put a Zener diode, all this is for code compliance. Put a capacitor to ring back the energy for saving instead of compliance? LOL.   ;D

Code doesn't care about peak voltage snubbers..?

No ringing, the R||C is behind the clamp diode.

Tim

How about after design "EMI"?
That mod, I think is a leakage induction transformer.
 

Offline carl_lab

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Re: Lightly Toasted MOV in SMPS
« Reply #13 on: August 14, 2017, 09:37:29 PM »
Is the labelling a "C7" ?
Most likely it's a ceramic disk capacitor not a MOV.
 

Offline SmokeDetector

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Re: Lightly Toasted MOV in SMPS
« Reply #14 on: August 14, 2017, 10:35:35 PM »
Is the labelling a "C7" ?

Yep, that's the one.
I meant could that be a capacitor instead of "MOV".

Well, it's blue where it isn't discoloured from the heat, it has the distinctive shape of a MOV, not a ceramic cap, and the part number on the package (as per my schematic) is for a MOV with a 170V DC rating.
 

Online Armadillo

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Re: Lightly Toasted MOV in SMPS
« Reply #15 on: August 14, 2017, 10:43:19 PM »

Well, it's blue where it isn't discoloured from the heat, it has the distinctive shape of a MOV, not a ceramic cap, and the part number on the package (as per my schematic) is for a MOV with a 170V DC rating.

Yes, they could have installed a MOV instead, and your identification is not wrong.
Like I said, likely for code compliance because the transformer is quite leaky.
I would not challenge the manufacturer, the have all the tests done.

 

Offline SmokeDetector

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Re: Lightly Toasted MOV in SMPS
« Reply #16 on: August 14, 2017, 10:57:26 PM »
Oh, and there is, in fact, a resistor R2 right next to it (under shrink wrap, as is often done for whatever reason).  So it IS actually an RCD clamp, but they put an MOV in there? ??? !? :-//

That shrouded resistor is connected to the positive DC bus, but doesn't connect to anything on the high side of the MOSFET, rather the trace runs down under the transformer to a diode and electrolytic cap, and looks to be feeding VCC on the controller chip (a UC3843).  However, I'll have a closer look tomorrow with the datasheet for that chip and see  if I can confirm what it's doing.
 

Offline T3sl4co1l

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Re: Lightly Toasted MOV in SMPS
« Reply #17 on: August 15, 2017, 12:37:25 AM »
Oh, so that's the startup resistor?  Guh...

Tim
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Offline schmitt trigger

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Re: Lightly Toasted MOV in SMPS
« Reply #18 on: August 15, 2017, 01:39:41 AM »

The MOV looks to be part of a flyback/snubber - it's in series with a fast diode across the primary coil of the transformer, which I assume is to increase efficiency by not clamping until the voltage goes above the 170V DC rating of this MOV, but it also makes me think the MOV is going to have a limited life if it's sitting there clamping all those spikes? Clearly I'm missing something.


I fully agree with your assessment; I also believe this is a snubber.

I remember that Power Integrations had app notes where they showed P6KE-series Tranzorbs for the exact same purpose. The company I worked for in the 1990s copied that idea, which went into a high volume product.
The Tranzorb became the highest field failure for this product.

An MOV would be an ever worse idea.
 


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