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munky 16th March 2008 10:02 PM

Need Help: Class D Cross Over
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Hello, I'm building a class D amp that can be used in a car as a school project. So, the input is 12Vdc, output is 20Hz-20kHz, and I'm trying to get as much power coming out of it (hopefully 30W). For amplification, I'm using the H-bridge topology. Right now, I'm getting cross over distortion, and I don't know how to get the MOSFETs to get some dead time, so both MOSFETs will never be on at the same time. Attached is how the H-bridge looks like, any help is appreciated. Thanks.

luka 16th March 2008 11:03 PM


What is you fets driver and circuit before it, coz there would be best to implement dead time

munky 17th March 2008 02:22 AM

2 Attachment(s)
Hello, the FETS that I'm using are IRF9540 and IRF540.

luka 17th March 2008 07:02 AM

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I don't know how I would do it, but see how I have it done...
4070 is my inverter to get + and - signal for IR. Dead time is made with RCD circuit between 4070 and IR... but in your case I have no idea how to add dead is too early in the morning :cannotbe:

Pafi 17th March 2008 07:18 AM


With the smallest modification: much higher base resistance, faster transistors with lower beta!

Pafi 17th March 2008 09:45 AM

With 12V DC supply voltage theoretical undistorted maximum output power is Vdd*Vdd/(2*Rl)=144/8=18W (@ 4 ohm). Do you use 2 ohm speaker?

lumanauw 17th March 2008 10:35 AM

Hi, Munky,

You can try put R on emitors of Q3-4-5-9 to define voltage drop accross 100ohm R (RC/RE=voltage gain = Vgs) Usually about 10V.

mag 17th March 2008 10:46 AM

Hi Munky
if you drive direcly the P-mos with the same signal as the N-mos you will always have cross conduction.
The only way to avoid it is to introduce some dead time, ie a small amount of time where neither the P-mos nor the N-mos will conduct.

In an implementation like yours the easiest way is to use two comparator in place of U2E.
The inverting inputs of the two comparators must be set to two slightly different DC levels. The voltage difference between the two levels will set the amount of dead time.

The output of the first comparator will drive Q2 and Q8 (at the same time) the output of the second comparator will drive Q1 and Q7.

What is your PWM frequency? The output filter sounds strange with only 2uH of inductance. To do this filter design a 2nd order low pass filter with a cutoff of around 25-30Khz when loaded with 2ohm (you are running in full bridge so every side of the bridge sees the half of the total load impedance).

If you are doing this project just to learn something about class-D it is ok but I suggest you to use a dedicated mosfet driver IC to simplify the things and make it more reliable.

Eva 17th March 2008 12:15 PM

Your discrete gate driver scheme is not suitable for audio class D. Consider hi/low side driver ICs.

phase_accurate 17th March 2008 12:28 PM

Since it is only 12 V AND complementary you could also use low-side-only drivers.



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