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Old 13th January 2012, 11:41 PM   #31
Enzo is offline Enzo  United States
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Why are we obsessed over this detail? It is a small guitar amp for heaven's sake. How good does the cross regulation have to be? How efficient does the power supply have to be? All it really needs to do is pass the electrical requirements of the government. One style might be "better" than another, but that is a rule of thumb. Rules of thumb are not laws of physics.

And a custom wound transformer? I don't think Fender has any problem at all having custom specified transformers made for their SMPS. They have been having custom made linear transformers in their products for as long as I have been alive, and I am in my 60s. Designer says I need THIS, and they have it made for him.
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Old 14th January 2012, 12:02 AM   #32
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Theres nothing wrong with custom wound gate-drive transformers, ...-but its worth NOT using one if you don't need to.

The basic question here is:

Is it possible to provide a satisfactory SMPS power supply for a Class D audio amplifier of peak power <500W, by simply just using a single switch flyback?

And if not, then is it possible with a two-switch flyback?

And if the answer to the above two is "NO", then why so?

(Vin = 90-265VAC and voltage doubler rectifier is used)



"How good does the cross regulation have to be?"
...answer = good enough to not make the amplifier sound bad

"how efficient does the power supply have to be?"
...Efficient enough such that a fan is not needed for cooling



"Why are we obsessed over this detail?"
.....because this question has never been fully answered on this forum before, and many of the sub 500W smps's on this website appear to be well over-engineered, meaning that audio engineers are spending valuable audio electronics time on smps electronics.

So its worth finding a really simple , cheap solution for Class D power supply.
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Old 14th January 2012, 04:35 AM   #33
dtproff is offline dtproff  United States
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You can do a flyback at any power level as long as you can store the energy in the transformer and deliver it to the secondary.

As to you question of the RCD snubber, if it is a 115Vac am and you use a 600V MOSFET you probably don't necessarily have to have hte snubber. Especially if is you design your transformer as a low leakage transformer.

I have used many SMD snubbers on the back of the board but I always design for low leakage so my efficiency is better as is my EMI.

Tony
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Old 14th January 2012, 08:42 AM   #34
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I see what you mean dtproff, but i doubt most winding places will be able to consistently wind to less than 1% leakage............


At continuous max load (330W) and 90VAC mains input, the flyback RCD clamp dissipation is 13.5W for a worst-case 2% leakage transformer.
-Thus 4 pieces of MVM5 (5W) resistors would be needed in the clamp……

..This is a big clamp.......however, of course, that dissipation is only when on maximum power.
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Old 14th January 2012, 09:51 AM   #35
dtproff is offline dtproff  United States
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Quote:
Originally Posted by eem2am View Post
I see what you mean dtproff, but i doubt most winding places will be able to consistently wind to less than 1% leakage............


At continuous max load (330W) and 90VAC mains input, the flyback RCD clamp dissipation is 13.5W for a worst-case 2% leakage transformer.
-Thus 4 pieces of MVM5 (5W) resistors would be needed in the clamp……

..This is a big clamp.......however, of course, that dissipation is only when on maximum power.
Did I mention that I don't do flybacks at 330W continuous power? Just kidding. One of the reasons I really like the resonant topology is that I can use that leakage for something useful.

As to the 1% flyback leakage. We consistently use <1% as the leakage target because I can use a 600V Mosfet @ 264Vac for a 65W design. This allows me to use the 650V avalanche of the MOSFET to clip the top off the leakage spike (I still have a small snubber). The avalanche energy is approximately 100X below the rated avalanche energy point so the self heating is minimal. I will tell you that you only do this when you are fightiing for the last few fractions of a penny in cost on the adapter.

As to winding consistance, we use a leakage spec max target that is about 15% higher than what I wind by hand in the lab. Then we run a couple hundred units and measure the CP/CPK of the transformer and then we set the final leakage spec. The I wind a couple units using the worst case specs for the transformer bulk cap, EMI filters and output capacitors and I rerun EMI.

If you want a few tricks on winding consistency and how we spec everything, let me know. I run a million or so a month on some of these.

Tony
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