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Registered Member #2655
Joined: Wed Jan 27 2010, 04:09PM
Location: Czech Republic
Posts: 18
Hello, I'm not sure I've took in account everything in designing primary circuit of DRSSTC. Capacitance is clear - you need to have same (or little bit lower) resonant frequency as res. freq. of secondary circuit. But I'm not so sure about number of capacitors in series. I've calculated with power dissipation, dU/dt, and max voltage across cap determined by estimated primary current and capacitive reactance of MMC. Have I missed something? Thanks for reply!
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
There are a number of ways to calculate the required voltage of the cap bank. Probably the simplest is to calculate V=L*dI/dt for your desired frequency and maximum primary current. This is the voltage across the inductor, but this is the same as the voltage across the capacitor since they cancel each other out. Then just series as many as you need to reach that voltage (and probably exceed it by about 1.5 to give a safety margin since running caps at their peak voltage rating in tesla coil abuse is not the best idea).
Registered Member #2655
Joined: Wed Jan 27 2010, 04:09PM
Location: Czech Republic
Posts: 18
Dr. Kilovolt wrote ...
Capacitor current I=2pi*f*C*V. Therefore, V=I/(2*pi*f*C).
I know, I've used this equation. But I'm more curious about other factors that can further derate caps. You certainly shouldn't exceed dU/dt and most likely neither power dissipation on capacitor (okay, with interruped the power dissipation is usually not such a big deal).
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
So this depends on the type of the caps you want to use. Basically not exceeding the maximum voltage and dV/dt for low duty factors (such as in DRSSTC duty) will suffice, as long as the capacitors don't heat up. For continuous operation however, the voltage must be derated, there might be a voltage vs. frequency derating curve in the datasheet.
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Use CDE942's if you are unsure; they are tried and tested, and people know how many to use to get reliable operation. Build a MMC that stays within spec'd ratings is very expensive.
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DRSSTC tank cap params
