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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567
For a half-bridge inverter I have two 0.22uf capacitors in parallel for each leg (0.44uf total). The capacitors filter and DC signal. They also offer some impedance at a given frequency. Is there a way to predict how my inverter's performance will change if I swap 0.15uf capacitors (0.3uf total) in their place? I can see that there will be a little more impedance on the line, which would reduce my available excitation voltage, but it may also filter out more DC, reducing my current draw. Or, is the difference insignificant?
Well...I waited for an answer. I decided to do the experiment myself. I removed my old blocking capacitors and replaced them with some CDE capacitors with the different value. I then ran the inverter, measuring the current needed to levitate a specific piece of aluminum.
Registered Member #2478
Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47
I think that it shouldn't make a noticeable difference. Blocking capacitors should not have significant ac voltage across them. Unless your blocking caps are much too small already, it will be the difference between 2 volts of ripple and 3 volts of ripple. Note that larger ripple voltage will mean more dissipation in the capacitors. It is generally better to have an oversized blocking cap than an undersized one, as there is more room for error in that direction.
If you are using a blocking capacitor, /all/ DC is filtered out. Depending on the design of your circuit, your blocking capacitors may be serving double duty as RF bypass. Operation with RF bypass caps alone, without a large electrolytic DC bus stiffening capacitor, will provide a substantially improved power factor, close to 1. However, output power will be reduced (by at most a factor of sqrt(2), if my math is correct) with the same voltage and instantaneous current stresses on the transistors.
By the way, what was the result of your experiment?
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What should I expect by changing the capacitor?
