Class D frequency?

[zenmasterbrian]

My own interest in class D is for subwoofer only in a multiamped system. So the class D amp would only have to go to 100hz, nothing more.

So, it could sample as low as 200hz, but of course that is audible.

So, maybe have a low sampling frequency and filter it out.

Or, maybe has a higher sampling freq, but not as high as 40khz. So it is inaudible.

What is the concensus on these topology and architecture issues.

I've looked but if another thread deals with this, I'll go there.

[SpittinLLama]

I'm not fully sure what you are asking but I don't know why you would want to sample the incoming signal so slowly. If you want to use that as a way to filter then you'd be better off using a simple op amp filter then sample at the higher frequency to get excellent resolution. If you're talking about the PWM oscillation frequency switching slower improves efficiency and helps on EMI issues but you only want to go so low. I liked 75kHz for a sub woofer application with a 100Hz type frequency repsonse. I set my LC filter in the low kHz range just because going lower meant a bigger physical size inductor and more cost but there is no reason you couldn't set the filter point at like 1kHz. You don't want to set the LC filter at 100Hz or too close as there will be phase shift as the filter begins to affect the signal. Use an input filter to nicely filter out the audio frequencies you want to filter. The LC filter is for filtering the PWM waveform. Or I don't know what I am talking about and ignore the above.

-SL

[zenmasterbrian]

Class D design is complex.

Slower sampling does give higer efficiency. I'd like to try and keep it bipolar. That was why I suggeste 25khz. Then I could put the filter in the khz range, as you said, because going lower means larger components.

But still, some 25khz would beed through to the driver.

[BWRX]

Use a higher switching frequency to reduce the size and cost of the filter components. 100kHz would be a better starting point.

Why stick with bipolars when you can use mosfets?

[zenmasterbrian]

I'd like to stay with bipolars, taking advantage of the fact that this amp doesn't have to cover the full audio range.

I'd like to stay with bipolars because the same number of them handles more power than MOS.

I know that such designs are complex and there are many trade off issues raised.

Is it OK to let a significant amount of say 25khz, hit the driver?

[BWRX]

The next question would be what sort of power output are you looking for from the amp?

You can always parallel a couple mosfets if you need more current capability.

Higher frequencies going to the sub will just be wasted as heat in the voice coil. If the driver can handle it then it won't matter too much.

[zenmasterbrian]

Maybe 1000W. I'd like to try and stay bipolar.

It seems impossible to totally filter out the switching frequency.

So it has to be ultrasonic. Is there any agreed upon standard as to how high is high enough?

[BWRX]

1kW is an awful lot of power to be aiming for if you don't really know what you're doing. (I'm only assuming this since you asked what a good switching frequency would be in the first place). At 1kW you'd need to supply a lot of base current to get bipolars conducting. You'd need a lot of current to charge up the gates of mosfets too, but it would only be for a short period of time. You'd have less losses too.

It is impossible to totally filter out the switching frequency, but you can certainly reduce it to an acceptable level. There isn't any standard, but like I said before, most try to make the switching frequency reasonably high to reduce the size and cost of the filter components.

[kartino]

1000W? At 4 ohm or 8 ohm?

For new class D DIYer this is not an easy job. Further more you deal with high voltage switching, expensive parts and lot of of trial.

On my brain the amps will need material such as mosfet like advances device ATP18XXXXXXX, with snubering, disable body diode (expensive diode too...) triangle generator... imho you could not successfull with self oscillate, gate driver ic.

You can use half bridge topology for that power level. Don't use bipolar, you really need high grade mosfet.

Hi zenmasterbrian,

What bipolar for 1000W switching amps?

Best regards,
kartino

[zenmasterbrian]

Many switching amps go at high freq because they want the full audio range.

I've decided that this is the lowest part of a multi amp system. So it doesn't have to be that high. But I do believe it needs to be ultrasonic, because it will not be possible to totally filter it out.

Any good books that deal with class D. Usually it is considered beyond the scope of DIY, but still any thing good to read?