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Old 13th June 2019, 05:15 PM   #1
bogdan2011 is offline bogdan2011
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Default Preamp SMPS build

Greetings,
I'm building a preamp flyback that outputs +-15V and +12V. It's based on UC3843, and uses a salvaged transformer.
The first of the issues that I have is the snubber resistor (R10) burns out when powering up the PSU and the current draw is high (I tested it with a lightbulb in series and it lights up). The diode and zener are intact. I'm not sure if it needs to be a power resistor, some schematics even omit it (it's used to dampen the ringing).
Without it, the PSU seems to be working fine, but the MOSFET heats up.
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Old 13th June 2019, 07:28 PM   #2
FauxFrench is offline FauxFrench  France
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Hi Bogdan,

Where did you get this snubber circuit from?

As I see it, it is more a return diode arrangement, returning the excess energy to the primary. However, return diode arrangements normally involve an auxiliary winding dedicated for that return diode arrangement. Here there are no such auxiliary winding. The UF4007 (D6) is a return diode. Perhaps D5 is meant to be a zener-diode that gives a voltage drop for the return energy. R10 (100 Ohm) is a damping resistor but my impression is that it has a too high value. My guess is that 10 Ohm will do better.
If you want to use a traditional RC-snubber instead (my recommendation), you choose R10 to 10 Ohm (2W) and replace D5 and D6 with a 470pF capacitor for a start. Then, using an oscilloscope, you try to increase and decrease R10 and the value of the new capacitor until you have a well damped ringing.
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Old 13th June 2019, 07:42 PM   #3
bogdan2011 is offline bogdan2011
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The snubber circuit is taken from here:
https://www.eetimes.com/author.asp?s...doc_id=1280601

It clamps the flyback voltage at whatever the value of the zener and returns it to the bulk capacitor via the diode. At least that's what I think it does. I'll try replacing it and see what I get.
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Old 14th June 2019, 09:40 AM   #4
Elvee is online now Elvee  Belgium
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A SMPS for a preamp? Bizarre idea IMHO.

Anyway, the first thing to do is to check the reflected flyback voltage at the primary.
Since the TVS is 120V, it has to be <100V, which is unusually low for a system working from 230V (I suppose it is the case because of the 400V main cap).

If it is higher, use an adapted TVS voltage.

If it isn't, I would use a more conventional "clipper" (one calls this circuit a snubber, but in reality, a snubber is more like Fauxfrench's description).
That is, I would short R10 and replace the TVS with a // RC, 47nF/400V and a resistor of 10K to 100K: the value depends on the leakage inductance.
Start with a low value, even <10K, and increase it as long as the peak voltage on the MOS remains acceptable.

A snubber could also be used, but in practice a clipper is sufficient and more effective.
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Old 14th June 2019, 09:51 AM   #5
bogdan2011 is offline bogdan2011
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Quote:
Originally Posted by Elvee View Post
A SMPS for a preamp? Bizarre idea IMHO.



Anyway, the first thing to do is to check the reflected flyback voltage at the primary.

Since the TVS is 120V, it has to be <100V, which is unusually low for a system working from 230V (I suppose it is the case because of the 400V main cap).



If it is higher, use an adapted TVS voltage.



If it isn't, I would use a more conventional "clipper" (one calls this circuit a snubber, but in reality, a snubber is more like Fauxfrench's description).

That is, I would short R10 and replace the TVS with a // RC, 47nF/400V and a resistor of 10K to 100K: the value depends on the leakage inductance.

Start with a low value, even <10K, and increase it as long as the peak voltage on the MOS remains acceptable.



A snubber could also be used, but in practice a clipper is sufficient and more effective.
I know an SMPS is an overkill for a preamp, but I want to see how feasible it is. It also serves as a learning process.
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Old 14th June 2019, 10:01 AM   #6
FauxFrench is offline FauxFrench  France
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Quote:
Originally Posted by bogdan2011 View Post
The snubber circuit is taken from here:
https://www.eetimes.com/author.asp?s...doc_id=1280601

It clamps the flyback voltage at whatever the value of the zener and returns it to the bulk capacitor via the diode. At least that's what I think it does. I'll try replacing it and see what I get.
Thanks. You are right with your assumption on the functioning.
Following the advises of Elvee is almost for certain a good idea. If you cannot make that work, we can then try with an ordinary snubber.

Last edited by FauxFrench; 14th June 2019 at 10:03 AM.
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Old 14th June 2019, 10:16 PM   #7
Elvee is online now Elvee  Belgium
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Do you know the primary reflected voltage?
It is determined by the turns ratio, and it governs the snubber or clipper design choices.

Once you know for sure what voltage you have to manage, you can make informed decisions.

You can use resistors, varistors or TVS to dissipate the excess power, but if you opt for non-linear devices, you cannot afford to make the wrong choice: if the knee voltage is too low, you will see the symptoms you currently observe, and if it is too high, the MOS will go into avalanche regime.

Anyway, it is better to use a capacitor, even if it is paralleled by a TVS or varistor, and maybe surprisingly, a fast diode is not required: a slow diode greatly improves matters.....
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Old 14th June 2019, 10:32 PM   #8
bogdan2011 is offline bogdan2011
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I always calculate the number of primary turns using 310V nominal voltage, which is the average voltage after rectifying mains AC.
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Old 24th June 2019, 08:41 PM   #9
bogdan2011 is offline bogdan2011
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Quote:
Originally Posted by Elvee View Post
A SMPS for a preamp? Bizarre idea IMHO.

Anyway, the first thing to do is to check the reflected flyback voltage at the primary.
Since the TVS is 120V, it has to be <100V, which is unusually low for a system working from 230V (I suppose it is the case because of the 400V main cap).

If it is higher, use an adapted TVS voltage.

If it isn't, I would use a more conventional "clipper" (one calls this circuit a snubber, but in reality, a snubber is more like Fauxfrench's description).
That is, I would short R10 and replace the TVS with a // RC, 47nF/400V and a resistor of 10K to 100K: the value depends on the leakage inductance.
Start with a low value, even <10K, and increase it as long as the peak voltage on the MOS remains acceptable.

A snubber could also be used, but in practice a clipper is sufficient and more effective.
I forgot to ask - what power rating should the RC resistor be?
A standard 1/4W resistor blew up instantly.
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Old 24th June 2019, 10:05 PM   #10
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by bogdan2011 View Post
A standard 1/4W resistor blew up instantly.
1W as a starting choice would be better, but if a 0.25W one blows instantly, there is a probably something wrong somewhere: it should be able to withstand more than 1W for at least a few seconds.

If the secondary output voltage is OK, the leakage inductance must be way too high.

It is difficult to diagnose anything without hard, quantitative data: schematic, voltages, inductances, turn ratio, etc.

If both the "improved" and the conventional snubber/clipper fail, there has to be something wrong with what you try to snubber/clip
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Last edited by Elvee; 24th June 2019 at 10:08 PM.
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