[chbot] H bridge warning!

[Robin Gilks]

Greetings all

I was getting a bit tired of the whistle from the motors when using a PWM
frequency of 2KHz so having checked several web sites, they indicated that
the BTS7960B as used in these cheapy bridges was good for 25KHz.

I cranked the PWM frequency up to 15KHz and at 10amps the heatsink got too
hot to touch. I've not looked at the circuit values to see how thay have
set up the slew rate control**  but it looks like the devices are in the
linear portion for too long at the higher frequencies.

Back down to 2KHz and it whistles and is cool :)


** On checking, they use a 1K resistor which sets the slew rate at
approximately 8V/us. I reckon that means that out of every 64us at the
high rate, at least 6us is in the linear region i.e. about 10% of the 240W
going through the circuit is lost in the bridge components. No wonder its
hot.

-- 
Robin Gilks
Internet: robin at gilks.org    http://www.gilks.org
Tel:      (+64) (3) 347 4370