Hi,

When using a H bridge to control a dc motor, 4 diodes across the output rails are often used to protect the outputs against emf.

When the bridge is rated at say 3A the diodes are often shown as 3A types, but is this really necessary - cannot these diodes be a much lower value ?

Would be interested to know how the amperage and voltage for these diodes should be calculated ?

Thanks.

 

Have an easy explanation for this.

A motor (wich you are controling) act much the same as an inductor. And for an inductor, the current through it cannot change instantly. So that means if the motor for some reason draw 3A, when you cut the power (all transistors in H-bridge stop conducting) instantly, the motor will feed a current with same amount back through the diodes for a short time.

In the real world the current won't get that high because of losses, but it's good to have a secure current rating for the diodes anyway.

 

Thanks Grossel

 

The current was mentioned, but not the voltage. When the magnetic field in any inductor (like a motor winding) collapses after power is removed, The reverse-voltage generated can be many times higher than the power supply voltage. You may also have a spike in the forward direction after the initial magnetic field drops out, depending on the specific motor and the circuit in which it is used. You should make sure you have a diode with a PIV rating high enough to handle spikes in either direction. Using diodes with a PIV of 20-30 times your supply voltage is not a bad idea for safety.

 

What about diodes speed?
Should I use standard diodes or schottky diodes?
How to calculate speed requirements?

 

Don't use Schottky diodes unless you have to (the exception to this is SiC since the only thing in mass-production is Schottky diodes but that is an aside).

Ideally you would want the switching speed of the diodes to match that of your main switching device. If the diode is too fast the snappyness can cause Vce (or Vds...) to ring alot due to the sudden change in current flow


as to the original question rating of the diodes w.r.t. the rating of the switches...

Depends.
Just like the switches, the diodes must be rated for the PEAK current (ie no point putting something like a bas16 down into a system that will see say.. 10Amps).
However... RMS current, just like with the main switch, is what is important

In some instances you can use lower-rated diodes then switches, in others you can use lowe-rated switches then diodes (I have a small aux converter to a 150KW synchronous machine drive which controls the excitation current now the DPF of this is around 0.3 and thus the diodes do most of the work)

you either drop down over-rated parts or you do some sims/cals to figure out over your operating loadcycle what quadrant of the power CAST diagram your converter will be operating mostly in

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Is it some PWM control? If yes, the diodes must be faster than regular rectifying diodes. 100 ns or faster will be good.

Slow diodes will get hot and/or break down.

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If the motor appears to be loaded or even stops the current will be the same as short circuit. Even the diodes are pretty robust devices..