I have a few questions regarding an H-Bridge design that I like. I am having a little hard time understanding some parts of it.

So this is the design that I found online here. enter image description here

I changed the motor voltage and the resistor values to the transistors from 10k to 3.3k. So the questions I have are

1) Are the 10k pullup resistors too high for the P-Channel Mosfets? Should they be lower?

2) For the N-Channel Mosfets, why is the 9V voltage going to a 10k resistor then a transistor to ground? Can't I switch the N-Channel side with something like this? enter image description here

3) Are the resistor values going to make switching slow for the N-Channel Mosfets?

I plan on running the H-Bridge using an arduino. The N-Channel Mosfets are FQP30N06L and P-Channel Mosfets are FQP27P06. I plan on using the 2N2222a transistor.

Are there any problems with this design that I am missing or misunderstanding or should everything work as expected? Thank you.

Also when connecting the ground of the 9V source do I connect the ground of the arduino to the ground of the mosfets and transistors? Ground is always the same everywhere, correct?

  • \$\begingroup\$ What is the motivation to use 5V instead of 9V for the pull-ups? \$\endgroup\$
    – Andy aka
    Jan 30 '16 at 11:05
  • \$\begingroup\$ The pull-ups on the P-Channel Mosfets are 9V. For the N-Channel Mosfets I just didn't know if there would be a difference. \$\endgroup\$
    – maxned
    Jan 31 '16 at 2:58

It was on the Internet, it must be good?! The high value resisters make the drive only suitable for low frequencies and small high on resistance mosfets. Lowering resister values will help this somewhat but it will waste more power. Turning a mosfet on fast and off slow is a recipe for shoot through unless you are really careful. The P-Chanel drive circuit should be changed. The second diagram will be more unreliable than the first.

  • \$\begingroup\$ Alright. Would I just be better off controlling each mosfet individually? If so, then what value resistors should I use for pullup and pulldown? Also how fast PWM is considered fast and slow? \$\endgroup\$
    – maxned
    Jan 31 '16 at 3:00

It depends on how fast you want to drive the H-bridge.

The phase of the N-ch drivers is inverted in the 2nd drawing -- you need to exchange the input 1A and -1B on the bottom.

You are likely to get some shoot-through currents when the circuit switches -- is it possible that the P-ch turns on before the N-ch turns off. Your slow drivers may mitigate this somewhat, but it depends on the details of the FETs (on-resistance, capacitance etc.).

The 4 zeners across the supply are useless: a) the MOSFETS will clamp any inductive spikes the motor generates; b) the zeners would have to be very large to clamp any spikes on the supply itself. Just remove them.

  • \$\begingroup\$ I am not using any zener diodes. Those are Schottky diodes. I read in many places that it is better to have them just in case so if anything I didn't ruin the mosfets. \$\endgroup\$
    – maxned
    Jan 31 '16 at 3:02
  • \$\begingroup\$ They are not necessary in this application, and wouldn't protect the FETs anyway. \$\endgroup\$
    – jp314
    Jan 31 '16 at 5:08
  • \$\begingroup\$ Really? So why do many H-Bridge designs that I look at include them? \$\endgroup\$
    – maxned
    Jan 31 '16 at 16:03
  • \$\begingroup\$ Don't believe everything you see on the internet ! Diodes are necessary if the driver is single-ended (to clamp inductive spikes); if some components of the circuit are not connected directly to the FETs (e.g. there are long lengths of wire to the FET's D & S) -- but in this case, you need the diodes across the circuit, not the FETs. Also if you have IGBTs (as might be used in a high power 600 V inverter), they need diodes. \$\endgroup\$
    – jp314
    Jan 31 '16 at 17:17
  • \$\begingroup\$ So I don't need diodes at all in the circuit because the Mosfets already include them? Alright is this H-Bridge design better? i.stack.imgur.com/Ywvbc.png \$\endgroup\$
    – maxned
    Jan 31 '16 at 22:40

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