May be I didn't use the word well, my rate current-20A here is the max current (or the peak ) that motor need to carry a heavy load. And under my normal test condition this current is just arround 7-10A. My bad

 

I hope this tip is still of some use to someone.

The reason the power elements get fried is deceptively simple. All motors generate a counter EMF when being switched. The voltage generated that way can be many times that of the power supply. The addition of an inductor in series to the motor will avoid the high voltage spikes from reaching the switching elements. Also remember to use inversely polarized diodes to provide a discharge path to the currents generated at switching.

I don´t know how to calculate the series inductor but I use secondaries of transformers rated at least half the operating current of the motor (rating at the same value is better) and that has proved to work fine.

If anyone uses this idea please let us know.

Best of luck.