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I'm building a half bridge for my mini DRSSTC, and I was wondering if I can use 270uF caps on the bridge. Everyone uses large capacitances but with being a mini coil im VERY limited on space...and 270uF is the largest I can go. Would it still function properly?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The rule of thumb for 230V mains (~320 Vdc bus voltage) is to use around 1uF for every watt of input power. For 120 volt mains fullwave rectified, that might be 3-4 times the value.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Well.... Instead of all this guessing that seem to be going on you could go and do some math and know exactly how much bus capacitance you need.
I find that a really good way to do this is just create a simple spice model with just your bridge L and C and you bus cap and charge it with an AC source. This will show you with rather high accuracy how much buss capacitance you need. The Bridge will pull current from the bus at the same amplitude and phase as the primary LC current and also on both cycles (negative and positive half cycles). So lets say you have 100Apk primary current going to be pulling 100Apk pulses from the bus cap.
What you have to do if figure in your max PW and off time and figure out if - You have enough power (joules) in your bank to supply the required power during a burst. How do you calculate the amount of power in joules for one burst you may ask? Well this can be complex without running the system first so this is why I suggest a spice model. It will get you very close to the amount of power you need per burst. Then you need to calculate the the potential of the cap bank. This is very strait foreword - knowing the voltage and capacitance of the bank:
Now that number there is ALL of the energy in that bank ie - if you discharged it to 0VDC. So in order to have minimal voltage sag you have ask your self how much sag you are willing to deal with and back solve to the voltage you want minus the sag and get the required value of the cap.
Now This math won't help much without knowing how much how much energy your coil needs per burst in the first place. So that is a good place to start. BUT going on pure gut like every one else has so far I would say that is not enough C and you will be seeing a lot of sag. A 500uF bank would probably be the minimum you would want to go with. This is based on an "estimate" of your coil specs.
Okay, so I'll be using a voltage doubler to get 340 volts across the bridge. My tank cap is going to very small, only about 4 - 12nF to achieve resonance, so I'm guessing that the overall bang energy wont be too high? I also decided to go ahead and replace the old spacers with 30mm ones, so I can now use 560uF caps. So with the new 560uF caps, I would guess I could just go ahead with it and assume for now I'm safe?
EDIT: When I say 340V across the bridge, I mean it will be setup like Steve does in his coil here
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Half Bridge Caps
