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Class D Switching Power Amplifiers and Power D/A conversion

Bridging Class-D Amplifiers- The Old Fashion Way
Bridging Class-D Amplifiers- The Old Fashion Way
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Old 19th July 2016, 08:58 AM   #1
bimbla is offline bimbla
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Default Bridging Class-D Amplifiers- The Old Fashion Way

Hello all,

It seems that bridging Class-D amplifiers is on the list of several members here.
There are some posts where members have said that the amplifier works well as a mono channel but in bridge mode it does not work as well.

White papers on UCD also mention that there are no patents on the bridge topology as it is a complex subject.

I thought of an old fashioned approach that could sort things out...and if it does, will help in a lot of a applications.

What if we employ an HF transformer at the output stage and follow that with the LPF?
Transformer output will do two things: 1. Impedance matching 2. Isolation
Unlike linear amplifiers, the output transformer can be small in Class-D topology.

Not only will the isolated output ease bridging but the output voltage can be adjusted to give 100/70V line in applications that require it.

I am not an expert in power switching or magnetics and so would like to invite the members here to give an opinion. I hope that I have not missed anything obvious, if so, my apologies in advance.

regards,

bimbla.
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Old 19th July 2016, 09:08 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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One very obvious error in your thought. Class D is a rail to rail switching PWM amplifier. A transformer will only see the rail to rail frequency and level that doesn't vary. The PWM will not cause any difference in the flux of the core and therefore will not transform any audio frequency modulation. If you wish to bridge the output from two amplifiers, ensure the frequencies are 100% locked to avoid distortion.
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Old 19th July 2016, 09:46 AM   #3
Pafi is offline Pafi  Hungary
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Quote:
Unlike linear amplifiers, the output transformer can be small in Class-D topology.
Unfortunately this is not true. PWM output contains the audio signal also. You must transmit every frequency here, not only the switching frequency.
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Old 19th July 2016, 10:02 AM   #4
Pafi is offline Pafi  Hungary
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Quote:
A transformer will only see the rail to rail frequency and level that doesn't vary. The PWM will not cause any difference in the flux of the core and therefore will not transform any audio frequency modulation.
The perfect opposite is true. The PWM definitely changes the flux, since flux is the integral of the voltage. Flux changes so much that a small core will saturate.

Quote:
If you wish to bridge the output from two amplifiers, ensure the frequencies are 100% locked to avoid distortion.
Well, actually the interference caused by cross-coupling 2 ClassD amps has 2 forms: when frequencies are different, then it's whistling, and if the frequencies are locked, then noise and/or harmonic distortion. This is the real challenge in bridging 2 ClassD amps. The interference should be kept very low even without synchronisation, this way THD will also remain low when synchronised.
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Old 19th July 2016, 10:06 AM   #5
Pafi is offline Pafi  Hungary
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bimbla,

just read what you wrote:
Quote:
What if we employ an HF transformer at the output stage and follow that with the LPF?
HF transformer will transmitt HF. LPF will filter out HF. What remains: nothing.
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Old 19th July 2016, 11:26 AM   #6
bimbla is offline bimbla
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Default Class-D Bridge

Yup.

I was too hasty in my thoughts and got carried away and missed these basics.
Sorry and thanks!!

regards,

bimbla.
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Old 8th September 2016, 05:03 AM   #7
spare_parts is offline spare_parts  United States
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what about using a transformer to step up the output voltage of a (for instance) 12v powered amplifier? Further, is it not possible to modulate a switching power supply PWM driver (like sg3525, tl494, etc.) with an audio signal? How about a single circuit power supply/ amp?
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Old 8th September 2016, 10:56 AM   #8
DUG is offline DUG  Canada
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There are many class D bridge amp IC's.

bimbla, you seem to have dismissed them.

I and others seem to think the TPA3116/8 IC's work very well, not just in BTL but PBTL as well.
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Old 9th September 2016, 01:03 PM   #9
bimbla is offline bimbla
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I was looking for a way to reduce the size of the output transformer for 100V line output application. But then that is not possible.

Theoretically I could eliminate it completely if I have a bridge amplifier operating on single supply of say 142Vdc. I then land up with an amplifier that should be able to deliver around 1250W/ 8E
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Old 9th September 2016, 08:31 PM   #10
ChocoHolic is offline ChocoHolic  Germany
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Quote:
Originally Posted by bimbla View Post
... single supply of say 142Vdc...
More popular and easily available are unbridged designs running an 8R load from rails in the category of +/-65V...+/-85V.
In bridged mode they could serve 16R with voltage levels of a 100V system.
Some of these types are even fine for 4R unbridged - allowing 8R bridged.
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