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RL circuit on inverter

franticEP

Apr 15, 2015
4
Joined
Apr 15, 2015
Messages
4
Hi,
i've found an old inverter schematic with an inductor and resistor on high mosfets' drain as figure below
hbridgerl.jpg
I try to reproduce it without RL circuit and mosfets become hot (80° over ambient) for conduction and switching losses as in my calculations.

Then i tried to mount the RL circuit and temperature rise on mosfets was very low (30° over ambient).

I did not understand how this circuit (RL red circle in figure) acts and how you can calculate component values.

Could you help me? I don't found anything (tutorial or application note on this subject).

This RL also allow me to increase switching frequency...

Parameters:
Fswitching=50Khz
Pout=150Wrms
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
The inductors limit the rate of increase in current . It is possible that this has been used to limit crowbar currents as the mosfets are switching, or even during the recovery time of the body diodes.

Do all 4 mosfets get hot without the RL in place? If all 4 get hot, then the problem is crowbar currents (which is better handled by dead time in the switching). If it is just the top 2 then the problem is probably related to the speed at which the mosfet is turned on (i.e. it's slow). The purpose of the inductor in this case is to limit the rate of increase in current as the mosfet switches on. This gives the mosfet time to turn on before current rises to a peak. It is interesting that there is a resistor across the inductor, it suggests that the peak currents are not very high.

edit yeah, 150W. Even at 1A, the voltage across the resistor would only be 100V as the mosfet switches off. Presumably the mosfets have a Vds > 350V.
 
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