about H-Bridge FET Driver conrol

[bosskardo]

Hi
I came across a H-Bridge FET Driver design where both high side outputs were set high (both high side inputs connect to the same microcontroller pin) when the motor was run in either side. The direction and speed was controlled by giving a PWM to low side input corresponding to the direction.
Why would both high side outputs be set high together, is there a benefit to this?

Thanks

 

[chuckey]

This sounds like a stepper motor drive drive where the high side switches just provide a current limited PWM source. Stepper motors need lots of current, and a high voltage to get them to work quickly due to the time constant (L/R) of their coils, the old way was to put a high power resistor in series with them, so you would use a 24V supply to feed a 5V motor, now people use PWM to get the same result.
Frank

 

[KlausST]

Hi,

can you show us the schematic?

It seems it needs one connection for both high sides, and two conections for the two low sides. In total three....
I see no benefit in this.

Klaus

 

[chuckey]

With a stepper motor putting repetitive pulses on one coil causes no movement, putting a pulse on an adjacent coil causes the rotor to step to that next position. hence the direction of rotation is set by the phase of the driving pulses.
Frank

 

[bosskardo]

Actually it a DC motor but I get it now. This way you can apply more power to the motor.
I have another question about the image below. Why are there the marked connections to power supply and ground?

 

[chuckey]

For some reason you need Um, which is 1/10 of the output voltage. The inclusion of a 36K leakage path to earth is then off set by the 33K to +24V, so as to keep the mean DC output to 1/2 of the 24V.
Frank