why osciallating in small current

[shalongbasi]

I am using Pspice simulating a H bridge mosfet circuit which has 1mH inductance load and resistor load. But I found that voltage across the load oscillating with a small load current(1A 50ohm resistor), and it is almost stable with a large current (10A with 5ohm resistor) when it is switching from one side to another side and all MosFET are off. I do not why this happened and how to eliminate the oscillation. thanks

 

[dick_freebird]

You have a "tank" composed of L1, R1 and the Cds/Cdg/Cgs
of all the "off" FETs, that rings down as R1 scrubs off the energy
(slowly). You might put a snubber shunt across the load (RC
in parallel with L, and ditch R1 unless you are using it for a
hard current limit) to try and kill the Q better.

 

[shalongbasi]

R1 is load and L1 is introduced because we need to connect to the R1 with about several KiloMeters wire. So there is no way to put a RC parallel with L1. Is there any other way make the ring go down faster? thanks

 

[keith1200rs]

Does it really matter? It is just residual energy decaying. One way to stop it, depending on the H-bridge is to drive both sides of the load to ground when off.

Keith