First, some background:I made a really neat flyback plasma speaker of my own design utilizing a LM339 quad comparator. One comparator makes up a oscilator, another one is used as a proper comparator, and the third one is part of a fan control circuit. I'm using a FDP33N25 MOSFET for it's really good ratings compared to it's price. Very comparable to the classic IRFP250N, at less than half the cost. I am driving it really hard with a NPN PNP emitter follower totem poll config. I noticed that the MOSFET does periodically fail and analyzing it with the scope, I see these HUGE high voltage 400V (!) transient spikes!
A dissipative snubber would need to dissipate close to 90W just to reduce that to closer to a safe range. At first I could not believe I was dissipating that much power until I used a 100W 120V lamp as the resistor and it was glowing pretty bright!
Luckily, I discovered adding a large capacitor across the drain and source effectively eliminated the overvoltage spikes and now the maximum peak voltage observed is only 150V.
It seems like the addition of the capacitor affected the switching speed of the MOSFET without affecting the switching losses within the MOSFET, and recycled that high frequency high power flyback energy into lower frequency lower voltage ringing which is more easily "digested" by the flyback transformer. However only a select few junk bin capacitors proved good enough for the job, most smaller film caps overheat, and on the scope, I can verify the MOSFET is switching on and off with a duty cycle of approx. 40%, but during the off time, the LC network between 2 or 3 old 0.47uF MMC film capacitors and my flyback transformer primary is some really strong damped ringing that maxes out at a peek of ~100V. I am unsure of the RMS current due to the non-sinusoidal nature of this waveform but it must be pretty high (given Xc and the high amplitude ringing), explaining why my 3, 1cm diameter 3cm long 0.47uF MMC film capacitors get pretty warm. The frequency of the damped ringing was measured with 3 0.47uF capacitors and was about 75KHz.
However, I also had these really tiny unknown 0.47uF capacitors, but they seem to be VERY good considering their size, most capacitors even 10 times bigger than them seem to want to bloat, smoke and die. with a 2S3P arrangement they still seem to overheat after several seconds to a minute or so but still pretty damn impressed they are handling that reasonably well. The overheating might simply be attributed to their low working voltage and no balancing resistors.
All my questions: So I am looking to make this design into a kit for people to buy, but I want a decent small capacitor that is suitable for this, to save PCB realestate and cut production costs, but I am also looking to learn more about capacitors too.
- So why do some film capacitor datasheets only give dissipation factor ratings at a few test frequencies (like 10KHz and 100KHz) while others give ESR ratings as well? Are the ratings used to determine the same things? Q factor seems to be simply the inverse of the dissipation factor or tan delta.
- Based on what I googled, I figured out dissipation factor is the ratio of ESR to the capacitive reactance. And since the frequency and capacitance is given it is not hard to calculate the ESR. So then why is this not conveniently already listed on the datasheet? Is it because the dissipation factor is what is directly measured? (like the total measured impedance of the capacitor at a certain test frequency)
- So far, every time I try to calculate ESR from tan delta at the 10KHz and 100KHz figures, I always find that the calculated ESR is LOWER at higher frequencies, even if tan delta gets worse. Am I doing something wrong or is that normal?
[li]Should I choose "self-healing" Polypropylene MKP capacitors or the (smaller) MLCC capacitors? What dissipation factor should I look for for?
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Sorry this post is long, it is my first one. I have a video of the circuit working below. Also you can see the pictures on my old question over at instructables.
http://www.instructables.com/answers/Dissipation-factor-vs-ESR/[/list]