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Design of Class D audio amplifiers.

Ankit Sahay
Prodigy 100 points

During the design of Class D audio amplifier the output stage is a power stage implemented using CMOS. As this CMOS is operated with PWM signal and as it switches from NMOS to PMOS and vice-versa, a very large current is being drawn from my power source. This is perhaps due to very low Rds(on) of the power MOSFETs. I tried the simulations with POWER MOSFETs which have larger Rds(on) and the problem got solved but obviously now power dissipation in the MOSFETs have increased.

Is there any way I can do away with this power surge that happens at the time of switching?

  • 0
    Prodigy 225 points

    Have you made sure that you have the MOSFET gate drive signals arranged so that one MOSFET turns off before the other turns on?  If not you will get transient 'shoot-through' currents that can easily destroy the MOSFETs.  Most gate driver chips will have the required delays done internally, but if not it is simple to use a diode with parallel resistor in the track to the MOSFET gates to modify the MOSFET turn-on and turn-off behaviour. There is plenty of information on the web about this.

  • 0 in reply to Chris Morriss
    Prodigy 100 points
    Thank you so much for your advice.
    I am driving the PMOS and NMOS directly from the PWM output coming from an op-amp. I am using IRF3205 as NMOS and IRF4905 as PMOS. Does these two power mosfets have driver circuit already or do I have to arrange for the dead time from external circuit?
  • 0 in reply to Ankit Sahay
    Prodigy 225 points

    Normally a class-D amplifier would use two N-channel MOSFETs, with suitable gate drivers as shown in the left hand schematic attached.  Are you using an N-channel and a P-channel device as shown in the right hand side?  This is not likely to work very well.

    Have a look at the data sheet for the IRS20957 class-D gate driver on the IR web site to get a good idea how a dedicated gate driver connects between the PWM source and the output MOSFETs.

  • 0 in reply to Chris Morriss
    Prodigy 100 points
    Thank you.
    Yes, I am using the schematic in the right. I will try to change to the left one.
  • 0 in reply to Chris Morriss
    Prodigy 100 points
    Can you please elaborate on how to use a diode with parallel resistor to do it? It will be very helpful.
    Can you post the rough circuit diagram for the power stage?