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How to discharge a capacitor quickly…(filter of an H-bridge/inverter)

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En021

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I have used four IGBTs to build an H-bridge. The inputs of the h-bridge are SPWM and are programmed in a way as shown below:
1.png
The output of the H-bridge is a three level pwm(as expected), and then its passed through a filter.
But the output from the filter isn’t sine wave (shown in red)
2.png
It follows the sine curve till it reaches the positive peak value.. It holds the value even when the width of the duty cycle starts to narrow; and doesn’t begin to discharge until the ‘B’ switches start to operate; i.e. until the second half cycle…
3.png
Now since the ‘B’ switches are open during the first half cycle, the capacitor(c2) experiences an open loop, and it holds the value till it gets a closed loop(till the 'B's are on)…
Is there any way for the capacitor to discharge itself through during the first half cycle?
(p.s. 1. It yields the same result when the inductors are replaced with resistors;
2. The same goes for c1 during the first half cycle while the ‘A’ switches are open)
 
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FvM

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You are essentially using a wrong modulation scheme. The IGBTs must be switched alternatingly on both half bridges, at least if driving a reactive load.

To reduce switching losses and inductor current ripple, you'll preferably use an unipolar (three-level) modulation scheme. A good explanation can be found in this project report https://www.wpi.edu/Pubs/E-project/...-190851/unrestricted/PWM_Techniques_final.pdf
 
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KlausST

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Hi,

both halfbridges must not be "high Z" (besides of dead time). Either low side is ON or high side is ON.

Klaus
 
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Looks like a typical NO LOAD waveform (the capacitor is only discharging when the low-side switch short it to GND).

If you have any load then it's a wrong modulation scheme.
 

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