Unusual Parallel/Bridged Class D Design. Is it copasetic?

Buckeye · 15th March 2012, 04:35 PM

I just ran across an unusual (maybe oddball) Class D design that uses two TDA8927 in a parallel/bridge configuration. On paper, this thing cranks out around 250 W. I am somewhat skeptical about actually making this thing work. I've attached a schematic of one side. The other side is identical, but obviously runs out of phase to get the bridge mode. But looking at this half, there are a couple things that make me queasy. First is the summing of the two outputs through some chokes in the LP filter. Usually I like to see a 0.1-Ohm resistor in series with each output to keep the amps from fighting each other. But assuming the chokes have at least 0.1-Ohms resistance, my next concern is the feedback loop to the controller. Instead of summing the two outputs through a resistor network, only one output is fed back. Seems like this design assumes the two amps are absolutely identical. We all know this is never the case. Bottom line, I think this design is just asking for trouble. Any opinions?

As always, thanks in advance.

Eva · 15th March 2012, 09:28 PM

Consider that output transistors are 0.2ohm typ. MOSFET working as switches and with some dead time.

Buckeye · 15th March 2012, 09:41 PM

Thanks for your reply, Eva. Unfortunately, I don't understand your point. What's the bottom line? You think this will work? Thanks.

15th March 2012, 04:35 PM	#1
Buckeye diyAudio Member Join Date: Jan 2011	Unusual Parallel/Bridged Class D Design. Is it copasetic? I just ran across an unusual (maybe oddball) Class D design that uses two TDA8927 in a parallel/bridge configuration. On paper, this thing cranks out around 250 W. I am somewhat skeptical about actually making this thing work. I've attached a schematic of one side. The other side is identical, but obviously runs out of phase to get the bridge mode. But looking at this half, there are a couple things that make me queasy. First is the summing of the two outputs through some chokes in the LP filter. Usually I like to see a 0.1-Ohm resistor in series with each output to keep the amps from fighting each other. But assuming the chokes have at least 0.1-Ohms resistance, my next concern is the feedback loop to the controller. Instead of summing the two outputs through a resistor network, only one output is fed back. Seems like this design assumes the two amps are absolutely identical. We all know this is never the case. Bottom line, I think this design is just asking for trouble. Any opinions? As always, thanks in advance.

15th March 2012, 09:28 PM	#2
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	Consider that output transistors are 0.2ohm typ. MOSFET working as switches and with some dead time. __________________ I use to feel like the small child in The Emperor's New Clothes tale

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15th March 2012, 09:41 PM	#3
Buckeye diyAudio Member Join Date: Jan 2011	Thanks for your reply, Eva. Unfortunately, I don't understand your point. What's the bottom line? You think this will work? Thanks.