Help with H bridge

[Dan0]

Hi. I need to control a 24V DC (35A) motor using H bridge, but I don't know wich flyback diode I need. Can anyone help me?

And how can I know the switching time limit?

Thank u very much!

 

[Dan0]

forgot to say

Its controlled by PIC

 

[dknguyen]

Diode

YOu need a flyback diode with a particularily fast recovery time.

You can know the switching time limit of the MOSFET by looking that the datasheets of the power mosfet you are using. Look at the rise and fall times then you can decide how "ideal" you want the square wave to be. Determine you own minimum switching time- the longer the longer the PWM pulse stays on or off, the less effect the rise and fall times will have on making the PWM waveform deviate away from a square wave.

If you want to determine the switching limit as a whole, just use 20kHz if you arent sure. Too slow makes the motor throb, and too fast makes a lot of energy wasted because the MOSFET has to move between the lossy intermediate stage between full conduction and no conduction every time it switches. The more often this happens, the more time is spent in this lossy state and the more power is dissipated.

 

[motion]

Let me see. If a 35A passes through a silicon diode the power dissipation would 0.6V X 35A = 25W. If your H-bridge is ON only 25% of the time, the average dissipation is 75% of 25W = 18.75W. That is a lot of power being lost in the diode. How come those H-bridges run much cooler than that?

The secret is that much of reverse current is carried by the MOSFETs themselves. The advantage of using MOSFETs is they can also conduct current the other way while the gate turn ON voltage is applied. In fact, with a properly designed circuit, you can do away with an external flyback diode and just make sure the right MOSFETs in the H-bridge are turned ON to carry the reverse current.