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The diagram shows the totem pole driver circuit for the Hi Side P channel Mosfet driven from PIC pin PWM output. This is part of a buck boost circuit running at 250KHz. The biased totem pole transistors drive the gate of the P channel mosfet approx 6.5 volts below Vin to turn it on. The Mosfet gate threshold is 2-4V. This works well if Vin is above about 8 volts but when Vin is only 5V then the gate is driven at 3V only 2 V below Vin. Instead of using the level shifter (or as well as) is there a way to drive the P-MOSFET gate to ground that allows the gate to charge up and then discharge quickly when connection to ground is removed. Thanksenter image description here

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    \$\begingroup\$ is there a way to drive the P-MOSFET gate... Yes, the easiest and most reliable way is to use a gate driver IC. \$\endgroup\$ – Bimpelrekkie Feb 13 at 21:20
  • \$\begingroup\$ Thank you. I was trying to learn about how the discrete components work and construct a P channel drive circuit using transistors, just wanting to learn, I think I know that driver IC is easist. \$\endgroup\$ – Steve Y Feb 14 at 9:04
  • \$\begingroup\$ OK! So removing the emitter resitor allows it to drive to zero! That's great and also I've replaced the 180Ohm resistor from the PIC pin with 1.8K. \$\endgroup\$ – Steve Y Feb 15 at 9:38
  • \$\begingroup\$ Will half bridge gate drivers work with P channel on high side or do they have to be N channel? \$\endgroup\$ – Steve Y Feb 15 at 9:43
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Didn't do the math but I think that your first transistor is not biased correctly to pull the driver pair.

Why did you put the emitter resistor? You need all the strength available and you are in straight switching mode, there is no need to limit the collector current (more than the collector resistor). 180 ohm into base also seem a little low to me. Did you forget a zero?

Anyway. This kind of discrete driver is only used for huge mosfet/igbt modules (when you have maybe DPAK transistors as gate drivers). It's an interesting exercise but the market is full of dedicate gate drivers ICs (with all the nice protections as a bonus).

Also consider using an N-channel mosfet, they are cheaper and more efficient (from the schematic it seems some kind of buck converter or bridge leg). They are difficult to drive because you need to bootstrap the gate but it's done in the driver IC.

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  • \$\begingroup\$ Thanks for your reply, my understanding was that Ie was approx Vb-Vbe/Re, this sets the \$\endgroup\$ – Steve Y Feb 14 at 9:19
  • \$\begingroup\$ Thanks for your reply, my understanding was that the collector current in active mode was approx to the emitter current so setting the emmitter current with Re=Vb-Vbe/Re=5-0.7/180 gives 0.024A so V across Rc=270X0.024=6.45V. So the DC bias is always 6.45V below Vin. Yes you are right it is a DC DC buck boost converter running at 250KHz. I wanted to make this one using transistors and using P Mosfets on the Hi Side to start with as cutrrent will be upto 5 Amps. N channel hi side Mosfets and boostrapping can come later for higher current circuit. So, how to drive the gate of P Mosfet to ground? \$\endgroup\$ – Steve Y Feb 14 at 9:38

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