Saturday, 15 April 2017

pwm - Voltage Spikes in MOSFET H Bridge


As a part of my effort to build a pure sine inverter I made the following MOSFET H Bridge. This was connected to a load through a LC filter where PWM signals are given as in the diagram below.PWM frequency is 16kHz with Q1 & Q3 switching at 50Hz.



PWM signals for Q1,Q2,Q3,Q4


While testing after working for few minutes the fuse blew, Q1 and Q2 failed short circuiting Drain & Source.


For debugging I removed the Filter, connected a resistive load directly and reduced the supply voltage to 170V.


Circuit after modifying


I checked the gate signals of individual MOSFETs. which seems fine. However I got the following waveform for Vds of Q2. There are few spikes reaching up to 380V when supply voltage is 170V. I guess that a supply voltage of 325V these spikes will be much larger damaging the MOSFETs.


enter image description here


What is the reason for these spikes?


Can they be minimized by a RC snubber?


If so how can i calculate values for R & C ?


Any help is much appreciated. Thanks in advance.



Update


Further zooming in i found that the spikes occur when the opposite MOSFET is turned on. Following is the Vds of Q2(Yellow) and Vds of Q4(Cyan). I further reduced supply voltage to 90V.


enter image description here




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