Low cost H-Bridge circuit for PWM

Question

I try to design a (relatively) low cost H-Bridge on N channel MOSFETs. I need to drive 8.5A peltier device with 12V and (roughly) 20kHz PWM.

(after reading some of the comments I changed this question to use P-MOSFETS for high side)

I plan to drive only low side MOSFETs with PWM, so I don't care too much about capacitance of high side MOSFETs. I was able to find quite cheap P-MOS transistors with an Rds value of about 0.02-0.06R.

My circuit would look like this:

I could probably use bipolar transistors for driving high side MOSFETs, but I have some spare N channel, I hope that using MOSFETs there is fine too.

Now the question is:

1. Is it a good circuit design?
2. What resistance values I should use for driving high side?

Answer 1

I think your circuit looks good. Although you made the common mistake of placing the PMOS symbols upside-down. The source of the PMOS should be on top for this kind of circuit. Also, put a bypass capacitor on the VDD pin of the IC.

If you won't be switching Q1 and Q3 very fast, R6 and R8 can be fairly large. 10k is probably fine. They'll turn on fast, but turn off slowly. The gate resistors on Q5 and Q6 aren't strictly necessary, but don't hurt and may prevent large current spikes from the uC pins. A small value (100 or less) is sufficient.

A few other suggestions:

1. Assuming you're controlling everything with GPIO pins from a microcontroller, Q5 and Q6 will need to be logic-level FETs compatible with your logic voltage (5V, 3.3V, whatever).

2. Put pull-downs at HI-A and HI-B. That'll prevent the gates of Q5 and Q6 from floating during initial power-up and accidentally turning on Q1 and Q3 momentarily.

3. Similarly, put pull-downs on the gates of Q2 and Q4 in case the TC427 floats its pins momentarily on power-up.

4. It's a good idea to put a capacitor (0.1uF is enough) between the 12V rail and the gates of Q1 and Q3. When the 12V first comes up, the gate capacitance of Q1 and Q3 will hold the voltage on the gates low until enough current has flowed through the resistors and into the gate. That may cause Q1 and Q3 to turn on momentarily. The capacitor will force the gate voltage to follow the 12V rail as it comes online and prevent that from happening.

5. MOSFETs do not turn off instantaneously. So if you, for example, turn on Q1 and turn off Q2 in the same instruction cycle in your firmware, Q2 may stay on for a few extra milliseconds and cause a shoot-through condition. Therefore, write your code with small delays between switching instructions. Better yet, use a micocontroller that has an H-bridge driver peripheral with built-in delays.