why osciallating in small current

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shalongbasi

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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
 

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.
 

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
 

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
 

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