I was testing this H bridge on wire wound high power resistors. I have noticed a high drop in the output voltage. The measurements taken are as follow:

Vin=13V; Vout=9V; Iout=2.7A =>Rds=1.48 Ohms

Vin=12.8V; Vout=8.7V; Iout=2.46 =>Rds=1.6 Ohms

Vin=15.2V; Vout=10.8V; Iout=3A =>Rds=1.46 Ohms

Vin=17V; Vout=11V; Iout=3.3A =>Rds=1.8 Ohms

Vin=20V; Vout-19V; Iout=0.56A =>Rds=1.78 Ohms

Ofcource some of the test where done where the Driver was really Hot (near 140 C as measured by an infrared Thermometer) so here is the temperature Derating:

Rds on as a function of temperature

Also There is another curve that describes the variation of the supply voltage :

Rds on as a function of supply voltage

The curves implies that Rds on should never reach higher 0.8 Ohms (suppose derating due to temperature = 2 @ 150C)

Tests I also tried:

  • Use of a demo board that uses the same chip but SMD footprint. Almost identical results
  • Use of 99% duty cycle signal instead of a constant DC signal at the PWM inputs of the driver (maybe charge pump needs that). Same results
  • Use of 5V logic instead of 3.3V logic for PWM and enable signal... Same results.
  • Trying to drive a motor instead of series wound resistors. Same results.
  • Removed snubber/schottcky diodes at output. Same results
  • Buy drivers from a different supplier to make sure that the driver isn't a counterfeit component(I didn't trust the supplier).Same results.

All measurements are taken using a true RMS multimeter. Vin is the voltage between GND and VS. Vout is between OUT1 and OUT2. I am measuring exactly at the pins so no drops are present because of the PCB/wiring.



Question is obvious: WHAT THE HELL IS GOING ON? The efficiency is crap and the driver is overheating quickly. I'm using a very large heatsink, still, the driver is unable to drive more than 2A continuous!! What am I doing wrong??

  • 2
    \$\begingroup\$ What is the voltage? \$\endgroup\$
    – user76844
    Nov 8, 2016 at 18:50
  • 3
    \$\begingroup\$ By the way, the input capacitor is very small. Should be tens or hundreds of microfarads \$\endgroup\$
    – user76844
    Nov 8, 2016 at 18:53
  • 2
    \$\begingroup\$ What Cboot capacitors are you using? \$\endgroup\$ Nov 8, 2016 at 18:58
  • 1
    \$\begingroup\$ voltage is Vin of each experiment. That is 12 .8V,13V,15.2V...etc \$\endgroup\$
    – fhlb
    Nov 9, 2016 at 7:57
  • 1
    \$\begingroup\$ I placed 200uF-50V very close to the IC (not seen in schematic) along with a 220uF at the input of the switching regulator I am using to power the Logic and microcontroller. so total capacitance is 420uF. I used aluminum caps \$\endgroup\$
    – fhlb
    Nov 9, 2016 at 8:00

1 Answer 1


I have a feeling that Rds-on here is (per leg) so you need to multiply it by 2 to compare it with delta(V)/I. Many integrated H-bridge IC specify the Rds-on for the high side FET and for the low side FET. Take for example this datasheet.

Returning to the definition for Rds-on, it is easy to see why and to expect this.


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