SMPS design for power amps (Class D in particular)

[Jaka Racman]

Hi Bruno,

I am still trying to figure if you are intending to use a clamped forward or flyback converter in your design. In any case I think you will need a series diode in line with your clamp switch.

Best regards,

Jaka Racman

[Robin]

Further to the dc protection....

Regarding your friends speakers catching fire....

With the CE approvals (and indeed UL) I think it makes sense for all amplifiers which can deliver enough dc to be hazardous to incorporate some sort of dc protection but I also think that loudspeaker manufacturers knowing that this can be a common fault condition should also ensure that loudspeakers can not physically catch fire. I think many loudspeaker makers think that because their products contain no active electronics they are somehow exempt from having to meet any safety requirements, but they too should test for this...just my opinion i am afraid - i am not sure how many do test

Robin

[Bruno Putzeys]

Originally posted by Jaka Racman
Hi Bruno,

I am still trying to figure if you are intending to use a clamped forward or flyback converter in your design. In any case I think you will need a series diode in line with your clamp switch.

Best regards,

Jaka Racman

You'll have to be patient with me because I am largely ignorant of official (=academically sanctioned) terminogy in any of the fields I'm active in. I find it easier and even quicker to "reinvent the wheel" than to try studying from paper how the wheel is supposed to work. After that I understand what the text books are saying, but not before. The advantage is that while most of the time I'm redoing something that already exists, sometimes I get something which didn't pre-exist.
So far this approach has worked for me in class D, AD/DA, deltasigma modulators and linear (opamp) amps. Now I'm just turning to SMPS.

The clamp is intended to recover energy from the leakage inductance, store it in the cap and deliver it in forward mode. Adding a diode would defeat that purpose.
So how do you call such a topology?

[jcarr]

Robin:

>There is always the problem of how you tackle it without putting something nasty in the audio path (typically a relay at the amplifier output).<

I consider this to be a questionable approach, and not only because of the effect on the sonic path. When a relay in the output pops open, this will frequently be accompanied by a transient spike which can spell instant death to the tweeters - sometime more.

>I have seen other schemes that crowbar the supply fuses or trip out a relay in the mains feed that forms part of the soft start.<

I find methods like this to be preferable (or cut out both rails in the case of transformers with dual secondaries), as the low impedance of the speaker load will pull down the power supply to the output stage fairly quickly, without any transients.

>Using an SMPS actually simplifies this as it is easy to shut down the supply without having to put extra components in the audio path.<

Agreed.

regards, jonathan carr

[Bruno Putzeys]

Originally posted by Robin
Further to the dc protection....

Regarding your friends speakers catching fire....

With the CE approvals (and indeed UL) I think it makes sense for all amplifiers which can deliver enough dc to be hazardous to incorporate some sort of dc protection but I also think that loudspeaker manufacturers knowing that this can be a common fault condition should also ensure that loudspeakers can not physically catch fire. I think many loudspeaker makers think that because their products contain no active electronics they are somehow exempt from having to meet any safety requirements, but they too should test for this...just my opinion i am afraid - i am not sure how many do test

Robin

I would never consider putting an amplifier in use (apart from experimental testing when you're always present) that doesn't have DC protection on board.

DC protection is sometimes omitted in low-power systems where amps and speakers are co-packaged and inseparable, and where safety testing has demonstrated that there is no fire hazard.

I am certain that no UL approval is granted to separate amplifiers without dc protection. If a house burns down due to a failed amplifier, and the amp has no UL listing, the manufacturer is required to foot the bill all by himself.
(note to non-US readers: UL means Underwriters Laboratory and an underwriter is an insurance company. No UL approval means no insurance).

It is legal to sell non-UL approved equipment (at your own risk!) in most of the US except California. Similar safety requirements hold in Europe, and are squarely mandatory (no no personal risk taking is allowed).

[Jaka Racman]

Hi Bruno,

I am not trying to criticisize your your design, but understand it and maybe give some helpful advice, if it is necessary at all (you seem to have things under control at all times).

From your description I think your design is close to this one. I think that you transform coupled inductor into separate transformer and series inductor on the secondary side and connect clamping capacitor to ground.

Regarding series diode you are correct, it is not required. I reverse oriented clamp transistor in my drawing.

Here is a short list of alternative single switch PFC topologies. I found that reading is sometimes advantageous to prevent reinventing the wheel, especially when you find that the wheel has already been patented. The last and most frustrating experience I had in this area was UcD patent, but there were others, like coupled inductor in forward converters or Peavey ampliverter. On the other hand, too much reading makes you difficult to think out of the box.

Best regards,

Jaka Racman

BTW, if the first link is really similar to your design, it was the first hit while googling for "active clamp flyback single stage pfc".

[Bruno Putzeys]

Hi Jaka,

I was't seeing your comment as criticism, only explaining why I can't answer to seemingly simple questions like "are you using a clamped forward converter"...

Originally posted by Jaka Racman
From your description I think your design is close to this one. I think that you transform coupled inductor into separate transformer and series inductor on the secondary side and connect clamping capacitor to ground.

Sounds correct. I do have a full wave rectifier on the output.
It looks like the converter in this paper doesn't have the zero-load problem that my circuit does.

[Robin]

Hi Bruno,

I know this is slightly off thread and should probably be on the UcD thread but it has been mentioned on here so I wanted to ask.

I agree with your comments on dc protection and wonder what is incorporated on the Hypex UcD modules, and assume it is probably left up to the user. I assume that shutting down the amplifier enable is insufficient in the case of output mosfet damage.

Robin

[Bruno Putzeys]

The hypex modules do not incorporate DC protection because usually you have just one per box, instead of 1 per channel. So it's left to the user to make such a centralised protection. Maybe it would be a good idea to incorporate a detection on the module (status pin).

I hear that JP is making a psu tailored for the modules that incorporates a dc protection.

Simply turning off the amplifier will no longer do when it's blown. Molten silicon is hard to turn off

[Robin]

Thanks Bruno,

I'll mention this on the UcD thread to get comment from JP on latest status

Robin