Searching for schematics: any high power class D subwoofer amp

[J.R.Freeman]

Greetings all,

I have been researching the design of a class D mono subwoofer amplifier, capable of high power output and stable with low impedance loads. A car audio subwoofer amplifier seems to fit the bill. I have been reading through threads over in the class D section, but most over there are after full range fidelity as a primary goal. My intentions are not so pure: frequency response and THD aren't so much of a concern as are lots of work-horse reliable watts.

Clocked or self-oscillating, how to drive multiple output devices, these are things I've been wondering about. To that end, if anyone knows of high-powered schematics that they are allowed to post, I would love to see them. I would be interested in how, for example, a Hifonics rig drives those massive banks of fets all together.

Does anyone out there have such references (as long as it isn't copyright protected that is)?
I should add that this project is just for fun and daydreaming, and is not a commercial endeavor.

Jim

[Perry Babin]

It's difficult to find schematic diagrams for the big class D amps. When you do find one, the driver boards are generally omitted. Many of the big amps use the IRS21844S driver ICs to drive the FETs. The larger amps use 4 of the ICS to drive 24 output FETs. If you're going to build a class D amp without a full schematic diagram (for a proven design), I'd recommend a clocked drive circuit. They're easier to troubleshoot when something doesn't work as expected.

[J.R.Freeman]

Originally Posted by Perry Babin

It's difficult to find schematic diagrams for the big class D amps. When you do find one, the driver boards are generally omitted. Many of the big amps use the IRS21844S driver ICs to drive the FETs. The larger amps use 4 of the ICS to drive 24 output FETs. If you're going to build a class D amp without a full schematic diagram (for a proven design), I'd recommend a clocked drive circuit. They're easier to troubleshoot when something doesn't work as expected.

Hi Mr. Babin,

Thank you for your reply. I agree - I think clocked might be the way to go. I have heard good things about self-oscillating amps, but I think it might be a bit outside of my skill level to consider such a design. Although clocked tends to have more components, it seems a bit more simplistic, conceptually.

Wow, 24 output FETs! That's massive. So each IRS21844S would be driving 3 pairs each - do you need to pay special attention to the surge current as required by each gate? I was considering IRS2011 also as a gate driver, but I will certainly take a closer look at IRS21844S.

Anyway, thanks for your help. This gives me more to think about!

Jim

[Perry Babin]

If you're going to build an amp of your own design, I think it's much better to have a clocked design because you can troubleshoot problems step by step. For example, if it's drawing excessive current, you can remove the outputs and examine the drive signals right up to the gates to determine what's causing the excessive current draw.

I didn't understand the question about the surge current.

[J.R.Freeman]

Hello again,

Sorry, that questions was a little vague. I suppose the real question is: in order for a single IRS21844S to drive 6 FETs, are resistors required between the outputs of IRS21844S and the gates of the FETs?

Anyway, thanks again for your comments. I agree with your suggestion of a clocked design - that makes a lot of sense.

Jim

[Perry Babin]

Resistors are almost always used between the FETs and the drivers. Since the driver ICs are virtually always destroyed when the FETs fail, you may want to use the DIP version and a socket on the PC board.

[J.R.Freeman]

Hi Mr. Babin,

Thank you for all of your comments. Good suggestion with the socket! Please don't feel obliged to respond any further! However, if I haven't over-stayed my welcome yet: I've been looking at the data-sheet for IRS21844S (found here) and it leaves me with some questions about how to use this chip.

Would this IC normally be used in pairs, making a full bridge? It calls itself a half bridge driver, but there are a few things that make me think it can't be used with a a +/- rail voltage. COM seems to expect 0v for example, and the input seems to provide for only 2 states: high side driver on, low side driver on. So no input means low side driver on?

Anyway, to me this IC looks like it needs to be used with a single supply, and in pairs to make a full bridge. Is that the normal configuration for those big amps you mentioned?

Jim

[Perry Babin]

It can be used in half bridge but is generally used in full bridge configuration.

It can be used with a split (plus/minus) supply. The IC is connected to the negative rail and is supplied by a 12v regulator that's referenced to the negative rail.

Logic low input will turn the low side FET on.

[J.R.Freeman]

I see what you mean about the split supply - COM could be the negative side of that supply. However, if a logic zero at the input turns on the low side driver, wouldn't the low side be on when there is no signal at the input? If that's the case, the negative side of the split supply would be applied to the output of the amplifier. That makes me think this IC can't be used with a split supply, but maybe I am still missing something?

Jim

[Perry Babin]

The chip receives a shutdown signal until the drive signal is available or until the power supplies are all up and running. When the IC is in SD, it will not turn on either bank of FETs.