How to spec flyback diodes for H-bridge

[DamoRC]

If I expect to deliver 12V and 10A through an H-bridge for driving a brushed DC motor, how do I decide what flyback diodes to use. Apart from the fact that they should be Schottky diodes, should they simply be rated for 12V and 10A or is there more to it than that?

As always, thanks in advance.

 

[dknguyen]

It should be low forward voltage and fast recovery to be able to work fast enough to clamp the voltage spike before it gets too high (takes a finite time to turn on and off) and to clamp the forward voltage as close as possible to the rail voltage (low forward drop). This is important because if you look at the operation of rail clamp diodes (which is what the diodes really are in an H-bridge) you will find that unless the diodes have a negative forward voltage drop (which is never) they will only turn on once the voltage spike has EXCEEDED the rail voltage. THe higher the forward voltage drop the more the voltage spike must exceed the rail voltages before the diode turns on to clamp the voltage.

Like a Schotky diode. It doesn't have to be 10A (though it's probably safer), since the load is intermittent but you have to go through the power dissipation calculations. You can google around for this. SOme of it involves the switching frequency and assuming the current decays linearily with time reaching zero by the end of the "off" period and assuming the current at the beginning of the off time is equal to the operating current (the current at the time the transistors are opened)

You can take off the load by changing the way "method" of how you switch the transistors in the H-bridge on or off.
https://www.modularcircuits.com/h-bridge_secrets1.htm
https://www.modularcircuits.com/h-bridge_secrets2.htm
https://www.modularcircuits.com/h-bridge_secrets3.htm[/QUOTE]

YOu can also add an resistor and capacitor in series to be in parallel with the inductive load to help supress it more since the "RC snubber" is faster than a diode (since it doesn't have to "turn on") but it won't clamp the voltage. This can take the load off your diodes too.
http://www.cde.com/tech/design.pdf

 

[Bob Scott]

If you are using power MOSFETs for your H Bridge, you may not need external diodes. It's just a coincidence of manufacturing, but MOSFETs have diodes built-in. (substrate diode)

Take a look at all the application notes at IRF.com. There is a cautionary note about using these internal diodes with a fast changing voltage (high dv/dt). I personally have had no problems driving motors with an H bridge but I tend to use MOSFETS with very low "on" resistance and therefore rediculously high current capability.

 

[dknguyen]

The parasitic diodes are very powerful but slow. So if you decide to rely on them for a high power application, an RC snubber will go a long way to catching the initial burst in the spike that the parasitic diodes are not fast enough to catch.

 

[Speakerguy]

Internal (body) diodes can blow. Much safer to use an external Schottky.

 

[DamoRC]

As always, thanks guys. Lots of good info and advice here. I have been to the H-Bridge secrets pages a number of times as they are a good reference.

I'll also check out IRF.com as advised.

Thanks again.