I have an internship coming up working with comparators and amplifiers, so I wanted to take on a side project involving both of those components. Designing a D-Class amplifier seemed like a great way to get some experience making a more involved pcb layout, but before that, I wanted to verify that my schematic is ok. Below is the circuit I came up with.

LTspice Schematic Screenshot LTspice Schematic

Link to LTspice schematic (google drive)

Inputs are a 1Vp-max audio signal from an audio jack and a 200kHz triangle wave. For the triangle wave I am planning on using a SN54LS624 VCO. Audio inputs are offset ~.1V in in order to cause dead time for minimization of shoot through current of the amplifier FETs.

For my PWM I'm using a pair of LT1819s, chosen for high slew rate and ability to handle 200kHz without issue.

For the gate drivers I'm using LTC4440-5, chosen for its voltage output range. I've never worked or learned about a gate driver prior to this, so it's possible that I'm grossly misusing it. High side driver ranges from -20 to 42V, while low side ranges from -20V to 4V. Is there a way to change this? I don't think it's necessary to bia my NMOS at 42/-20V, and doing so might damage them at worst or slow them down at best. Is this something I'm missing or is my design valid thus far? I know that dual high side and low side drivers exist, and can have built in dead time, should I consider using one of those instead?

Power MOSFETs are IPB136N08N3, chosen for Vds max of 80V, and Vgs max of +-20V. My high side gate driver biases with a Vgs max of 22V, yet another reason I think I'm messing that bit up.

Filter is a 3rd order low pass with a cutoff of 23kHz and attenuation of -45dB at 200kHz. Initially I had a 2nd order but was noticing a low signal to noise ratio at the lower input voltages (Input = 0.5Vp).

Project is planned to use two 20V 3.25A DC power supplies, using diodes for 5V and ~1V (1V for offset and potentially for tri-gen). I've never had any experience with power supplies, so this is another shaky area for me.

Other concerns are current spikes in the push-pull NMOS show below. I don't think it's a big deal as each spike is ~25-50ns long, so maybe that's just the simulation being really precise and wouldn't happen in reality but I'm not sure.

Power MOS current spikes

^Power dissipation on high side NMOS

Zoomed In ^Zoomed into spike, lasts approx 50ns

  • \$\begingroup\$ LTC4440 are high side drivers, and TS can only go to -5v wrt GND, so your -20v low side FET supply won't work. Might be better to use a dedicated low side driver for M2. LT1819 is an opamp, not a comparator. They are designed to do different things, so read the data sheet carefully and compare to a similarly fast comparator. \$\endgroup\$ – Neil_UK Feb 24 '19 at 20:44
  • \$\begingroup\$ Your audio input impedance is only 100uF||487 ohms. Why such a low impedance? It is not to have ultra-low Johnson noise, as your source baseline noise level could be much higher, unless you use a 24 bit or 32 bit audio source. \$\endgroup\$ – user105652 Feb 24 '19 at 21:36
  • \$\begingroup\$ Your LTC4440 inputs cannot go below ground, yet your op-amps are driving the inputs with signals that swing below ground by several volts. You need to re-think your entire design. \$\endgroup\$ – user105652 Feb 24 '19 at 22:12
  • \$\begingroup\$ @Neil_UK Thanks for the catch on the op-amp, annoyingly it's listed on LT under comparators, but now I see the bold text in the data sheet say "don't use as comparator". Replaced that with a LTC6752-2 dedicated comparator. I see the problem with the gate driver, but I'm having trouble fixing it. I want there to be zero DC offset on the output, but every gate driver seems to work for a Vdd to GND. Would a low side driver be to swing my output from +20 to -20V \$\endgroup\$ – Michael Feb 25 '19 at 3:55
  • \$\begingroup\$ No lowside drivers I've come across use a voltage swing like that, you might be able to get away with a 20v swing between -20 and GND, though a dedicated 12-15v supply wrt to the -20v would enable almost any lowside to work. A fully isolated driver would also work, but they too require dedicated rails to power their outputs. \$\endgroup\$ – Neil_UK Feb 25 '19 at 7:31

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