I have been working on a H-bridge design that controls a 12V motor to rotate forwards or backwards at an adjustable speed using PWM from an Arduino. The motion starts slow and increases to max speed for 10s then decreases back to slow. The entire motion is approximately 40s. I am using Q2 and Q4 for switching direction and applying PWM to Q1 and Q3 for speed control. This motion is initiated by inputs from two relays. The Arduino UNO Mini has a default PWM frequency of 980Hz for pin D5 and my duty cycle "flow" is: 10% -> 90% -> 10%. My previous schematic:

enter image description here

I updated the circuit from my previous post. Thanks to the help from this amazing community! I managed to get my power dissipation down to approx. 3.2W (100% duty) for the PMOS and 700mW (100% duty) for the NMOS. New schematic:

enter image description here

I figured I could go with the SUM110P06-08L and SUM40012EL since the both have low Rdson and a manageable Qg. I stuck with the TC4420EOA to drive Q1 and Q3 since I am familiar with this chip and I no longer can operate at 100% duty due to the charging/discharging gate resistors. The resistors on Q1 and Q2 were specifically selected for these MOSFETs. Q1 charges fast through the 2W resistor R4 (R9 in LTspice) and discharges slow through the 1/2W resistor R5 (R3 in LTspice). Q2s gate charges slow through the 1/2W resistor R7 (R4 in LTspice) and discharges fast through the 2W resistor R6 (R10 in LTspice). The diodes, 1N5818-TP, direct current depending on charging or discharging. I know doing this methods puts unnecessary strain on the MOSFETs which generates heat but I refuse to use a NMOS for high side-switching. The resistors R3 (R1 in LTspice), R9 (R2 in LTspice), R7 (R8 in LTspice) and R12 (R5 in LTspice) are meant to prevent latching. I also noticed R3, R9, R7 and R12 create a voltage offset on the gate if they are too small. I had to adjust these to ensure I was within spec on the gate threshold voltages for each MOS. I went with the FAN3268T since this uses TTL and I don't have to worry about adding a charge pump to hold Q2 or Q4 "ON". I will drive the FAN3268T with two inputs from the \$\mu\$C. R10 and R11 in KiCad are simply gate resistors. I added a fan circuit as well to help manage the generated heat. I plan to use heatsinks too.

I simulated this in LTspice using the MOSFETs with the closest Rdson and Qg compared to SUM110P06-08L and SUM40012EL. This circuit is shown below:

enter image description here

V3 and V4 are supposed to be simulating FAN3268T. Where V2 is simulating TC4420EOA The circuit characteristics are shown below for V3 and V4 being 12V (Reverse operation at 10% Duty input (90% duty load)):

enter image description here

The circuit characteristics are shown below for V3 and V4 being 0V (Forward operation at 10% Duty input (10% duty load)):

enter image description here


Should I be worried about the voltage spike on the gate of Q1 during "power on"?

I was going to add TVS diodes across drain and source and between gate and source on each MOS but I was worried about leakage. Are TVS diodes, or any diodes, in these places necessary? I know they help with ESD but modern MOSFETs have this protection built into them now. I know the MOSFETs I chose do not have ESD protection between the gate and source, but the do have protection between drain and source.

In regards to the FAN3268T, when reviewing the block diagram, there is a gate that has 3 inputs and 1 output. Is this a NAND or AND gate? It has a very small bubble on the output.

Also if anyone sees any issues that may arise with this setup please comment! ANY feedback is much appreciated!

For future viewers, if anyone plans to use this circuit I am NOT responsible for failed designs, ANY damage, harm to self or others, etc.

  • \$\begingroup\$ The diodes and resistors on the gates of the MOSFETs actually turn the devices ON with a delay, and quickly turn them OFF. This is how I suggested as a way to minimize cross-conduction at the expense of a little more power dissipation during turn-on. I don't see a spike on Q1 gate, and TVS is usually not needed. \$\endgroup\$
    – PStechPaul
    Jul 9, 2022 at 19:15
  • \$\begingroup\$ Regarding your gate driver: I would not purchase any gate driver that doesn’t provide a minimum dead time in this era, unless I could guarantee dead time with my upstream pulse generator. Trying to do it in discrete component analog domain is a respectable challenge but I trust the chips more. \$\endgroup\$
    – Bryan
    Jul 9, 2022 at 20:06

1 Answer 1


It is less complicated to select the direction with the P-MOSFETs on the high side used as slow static switches and control the motor current with fast low side PWM only. There is no trouble with cross conduction and no switching loss on the high side.

C1 and C2 suppress the miller effect in the P-MOSFETs because this high impedance resistor drive would suffer from that without them.

Changing the direction needs some time here. The software must insert a quiet period without PWM to settle the high side FET state. With both high side FETs off, a controlled brake using the low side FETs is possible.

Schottky diodes, snubber networks and supply capacitors are not shown here.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Using a BTS7960 bridge would save you a lot of work and they can be gotten for about 7.00 US, will do 40A and even have a heat sink. It has all types of protection and you can monitor the load. \$\endgroup\$
    – Gil
    Jul 12, 2022 at 4:34
  • \$\begingroup\$ @Gil Yes, I know them, but BTS7960 is an obsolete part, hard to get, you need two of them and it is a bit slow (5us Ton). In the current market situation I would not bind a design to parts with unclear availability. If this trend continues, we probably must fall back to pre 2000 designs like this (cheap) one. I just wanted to show, that the OP's solution is needlessly complex while staying in the chosen discrete component style. \$\endgroup\$
    – Jens
    Jul 12, 2022 at 14:06
  • \$\begingroup\$ I was speaking of the module, not the parts. I also had no clue this was a commercial venture. They were originally designed and made by Siemens now Infineon makes them. Check Infineon Automotive parts. \$\endgroup\$
    – Gil
    Jul 12, 2022 at 20:40

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