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Class D Switching Power Amplifiers and Power D/A conversion

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Old 22nd September 2004, 07:46 PM   #11
Robin is offline Robin  United Kingdom
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Default Alternative Configurations

Hi Bruno,

Your understanding of the topology I proposed is correct. You can also use a bridge instead of the p-p (even a zero voltage switched full bridge).

The topology you propose is very interesting. However, it sounds like you would still need a forward inductor for each output so I guess you would still have minimum load concerns at low currents for multiple outputs without some clever mutual windings. I understand that you will be able to achieve better secondary regulation with a faster dynaic response but I was particularly interested in multiple output supplies in which case the current fed converter behaves very much like a normal multi tapped transformer except with the addition of pfc and regulation against changes in line input voltage.

It is at last refreshing to discuss some of these topologies with someone who has an open mind and the knowledge to consider something different to the norm. You would not believe the number of psu designers that look at you with a blank face if you propose anything that isn't a normal forward or flyback converter :-)

Robin
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Old 22nd September 2004, 08:51 PM   #12
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Hi Robin,

have you considered what happens in current fed topology when reflected secondary voltage is below peak of the line voltage? IMHO this topology only works in boost mode.

Bruno,

is this roughly what are you considering? Do you intend to achieve continious input current with this configuration?

Best regards,

Jaka Racman


edit: after rereading Bruno's post, I think there is a mistake in the rectifier configuration in my schematic. But the question about continious input current still remains.
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Old 22nd September 2004, 08:59 PM   #13
Robin is offline Robin  United Kingdom
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Hi Jaka,

Absolutely correct, this topology does work in boost mode as a normal pfc front end would, the difference being that the power switch configuration is different to allow both control of the pfc loop as well as the modulation for controlling the output voltage through a transformer.

Robin
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Old 22nd September 2004, 09:09 PM   #14
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Hi Robin,

my main concern is the startup transient. I once considered this kind of topology for charging a rather large bank of capacitors (several kJ of stored energy) and found no simple way to control inrush current when reflected secondary voltage was below peak of the mains voltage.

Best regards,

Jaka Racman
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Old 22nd September 2004, 09:14 PM   #15
Robin is offline Robin  United Kingdom
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Hi Jaka,

By using a kind of active clamp on the primary after the inductor and across the transformer and power switch you can get it to operate in flyback mode during startup and fault conditions which controls this phenomenon until the output voltage reaches it's normal level. I worked the required logic out once driven from a pfc ic. I'll have to try and dig it out to post for you.

During normal operation the active clamp then acts to aid zero voltage switching across the bridge as well as act as a clamp.

I hope all this makes sense.

Robin
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Old 22nd September 2004, 10:00 PM   #16
Robin is offline Robin  United Kingdom
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Following what I mentioned earlier, until I dig out some of my old material on this, have a look at this datasheet for a unitrode device (TI). Although this device is now discontinued it shows the idea of a current fed topology and the logic is not at all hard to work out. I, in fact worked on a slightly different configuration but this is nevertheless interesting.

www.bmh.nu/pdf/Switch/UCC3857.pdf

Robin
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Old 22nd September 2004, 10:35 PM   #17
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Hi Robin,

makes perfect sense, thank you.

Here is my opinion about various topopogies in SMPS. In spite of myriad of topologies, you can not bend the laws of physics. There is no transformer isolated topology that can process the same amount of power as non isolated one with switch element of the same rating. Because you have to maintain volt seconds balance on the transformer, you either have to use single switch with higher voltage rating or two or more switches with the same voltage rating. I have seen many articles proposing various single switch PFC topologies, but the added complexity usually results in the same amount of components as the two stage approach.

My choice for amplifier power supply would be probably simple self oscillating flyback, maybe even with bipolar transistor instead of mosfet. I would not care about PFC, if it is not specifically required for audio. PFC does not solve flicker problem, which may also be problematic with non class A audio amplifiers.

BTW, does any one know what are current agency regulations regarding audio amplifiers? Are they considered class1( protectively earthed) or class2 (double isolated) equipment. How is output handled where it can exceed Extra Safe Low Voltage? What about DC protection at the output? I have a friend that had an accident with his amplifier. Because of cold solder joint on amplifier PCB, DC offset literary burned one of the loudspeakers down. Fire has caught woofer membrane and then grille cloth. If he was not at home, his house would have burned down.

Best regards,

Jaka Racman
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Old 23rd September 2004, 06:59 AM   #18
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Hi Jaka,

Barring the rectifier, your schematic reflects what I'm up to, except that in my drawing the clamp switch and cap are swapped so you get a ground-referred feedback takeoff point.

The input needs a filter, since the primary current is not a neat triangle.

I do intend to achieve zvs on both switches.

Cheers,

Bruno
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Old 23rd September 2004, 07:05 AM   #19
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Default Re: Alternative Configurations

Quote:
Originally posted by Robin
However, it sounds like you would still need a forward inductor for each output so I guess you would still have minimum load concerns at low currents for multiple outputs without some clever mutual windings.
Dammit you're right...

It's most easily solved by converting to synchronous rectification, but that would undo the principal aim, which was simplicity. On the upside, it would automatically couple the rails, completely ridding us of the class d psu pumping problem that folks are making such a lot of.

Well as I might have said this is pretty much my first smps project... let's see where it goes.
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Old 23rd September 2004, 08:22 AM   #20
Robin is offline Robin  United Kingdom
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Hi Jaka.

I'll check on the class I/II thing and let you know. Regarding the output voltage of the amplifier, higher power amplifiers are indeed considered a hazard and as such they changed the spec for agency approvals in europe on the speaker connectors. Old style binding posts were not allowed to have exposed contacts anymore which meant the all metal type could not be used and even the plastic ones had to have exposed parts shrouded. What people did to get round this was plug the end cap where you would insert a banana plug with a plastic bung that was easily removed so that if someone took it out to use banana plugs they did it at their own risk....

I don't think DC protection is mandatory and it is really at the discretion of the manufacturer. Lower power amps generally never have and some higher power amps do. In professional audio I would say that most quality amplifiers have DC protection but in hifi many manufacturers don't seem to bother. 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 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. 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
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