DRSSTC RF bypass capacitor type
Thomas, Thu Oct 19 2006, 06:28AMI have a very simple and quick question on the RF blocking capacitor in the DRSSTCs. Say I have 2 20uF 250Vdc electrolytic capacitors. Is it wise for me to series them to get 500Vdc @ 10uF? I'm only asking this because I see people using poly capacitors. Would the electrolytic capacitors over heat from the large amount of current found is a DRSSTC?
Thanks
Re: DRSSTC RF bypass capacitor type
Steve Conner, Thu Oct 19 2006, 12:09PM
They very probably would overheat. The ripple current can be in the 10-100A RMS range (depending on coil size and power) which is hard on any kind of capacitor. For my last DRSSTC, I got best results with:
One 3300uF 420V inverter grade electrolytic
Two 4.7uF 400V plastic film
Two 0.33uF 630V plastic film
all hooked up in parallel. I had to experiment to find the combination of plastic film caps that gave the lowest RF ripple voltage on the bus, as measured with a scope and a x100 HV probe. Some combinations actually made the ripple worse than with just the electrolytic on its own, because of self-resonances.
I think the easiest option is to get a good quality inverter grade electrolytic, and build your bridge as compact as possible and bolt it straight to the capacitor with short thick copper straps. If I had built mine neater, I probably wouldn't have needed bypass caps at all.
Steve Conner, Thu Oct 19 2006, 12:09PM
They very probably would overheat. The ripple current can be in the 10-100A RMS range (depending on coil size and power) which is hard on any kind of capacitor. For my last DRSSTC, I got best results with:
One 3300uF 420V inverter grade electrolytic
Two 4.7uF 400V plastic film
Two 0.33uF 630V plastic film
all hooked up in parallel. I had to experiment to find the combination of plastic film caps that gave the lowest RF ripple voltage on the bus, as measured with a scope and a x100 HV probe. Some combinations actually made the ripple worse than with just the electrolytic on its own, because of self-resonances.
I think the easiest option is to get a good quality inverter grade electrolytic, and build your bridge as compact as possible and bolt it straight to the capacitor with short thick copper straps. If I had built mine neater, I probably wouldn't have needed bypass caps at all.
Re: DRSSTC RF bypass capacitor type
Thomas, Fri Oct 20 2006, 03:18PM
Thanks, I'm going to use 1 4.7uF capacitor from GE (#41L4471) and 1 .47uF capacitor from GE (#40L3472) both are rated at 400Vdc at about 235A, significant difference from electrolytics current wise. Also to note this is a half bridge so I only feel the need to use 1 of each to put next to my IGBTs.
Thomas, Fri Oct 20 2006, 03:18PM
Thanks, I'm going to use 1 4.7uF capacitor from GE (#41L4471) and 1 .47uF capacitor from GE (#40L3472) both are rated at 400Vdc at about 235A, significant difference from electrolytics current wise. Also to note this is a half bridge so I only feel the need to use 1 of each to put next to my IGBTs.
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