Voltage and current derating of MLCC capacitors for RF PA - Page 1
Forum section: electronics / projects

author: bd139
author_rank: Super Contributor
timestamp: April 05, 2018, 12:50:27 pm
content: Using some "Suntan" branded leaded MLCC capacitors as coupling capacitors between the switching MOSFET and the output matching network in an RF PA. These appear to be just SMD packages with legs bonded and dipped. X7R 100V rated.However they keep exploding. So I tried a TDK branded SMD 100nF rated the same. This just exploded as well.Unfortunately when they explode it causes a mismatch due to the sudden lack of coupling and this takes the MOSFET switch with it. I put a 45V zener across drain/source and a fuse inline with the supply this appears to have fixed that without causing too much of a problem. The fuses cost nearly as much as the MOSFET though.The max differential voltage is 15V according to LTspice simulation of the amplifier. Current delta is half an amp approx.Am I hammering these too hard? If so what should I chuck in there instead? Should I be derating them? I have thought about using a few smaller ones in parallel.

author: dmills
author_rank: Super Contributor
timestamp: April 05, 2018, 03:27:50 pm
content: What operating frequency?C0G is often better then X7R at HF, and sometimes you find you need to switch to something like an ATC part (Usually at VHF and on up).Regards, Dan.

author: bd139
author_rank: Super Contributor
timestamp: April 05, 2018, 04:39:41 pm
content: 3.5-30MHz (HF). Nothing particularly quick. I've used C0G for frequency dependent parts of the circuits and X7R for coupling so far. C0G caps with high enough capacitance are quite expensive as the voltage rating goes up.I stuffed an older style ceramic plate in from my surplus stock and that hasn't exploded (yet)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 05, 2018, 05:16:31 pm
content: 1. Capacitors do not carry power dissipation ratings. Assume a power rating of 0.5 to 1 times the power rating of a resistor of the same size.2. Read the datasheet or supporting information carefully. Determine the ESR at the frequency(ies) of interest.3. Derive an estimated current rating, based on #1 and #2.4. Use as many capacitors in parallel, as are necessary to achieve the required capacity.100nF seems awfully large for a 3MHz PA. You'll probably find it's better to use 10 x 10nF, or 1nF even, in which case C0G won't be so painful.You may also be able to use a transmission line transformer for coupling, instead of (or in addition to) capacitors.Tim

author: bd139
author_rank: Super Contributor
timestamp: April 05, 2018, 05:26:30 pm
content: Thanks for the tips. I will look at that this evening.I used 100n so reactance was negligible at the lowest frequency.I could use a transformer. Not a bad option. Was trying to avoid that though as it means ordering more toroids.This is a one off amp so reproducibility is not end of world.

author: dmills
author_rank: Super Contributor
timestamp: April 06, 2018, 10:45:25 am
content: One other tip, if doing the TLT thing, place the caps at the high Z end of the transformer, significantly less RF current at the higher impedance....And yea, large banks of 10nF or such are fairly commonplace above 10W or so.Regards, Dan.

author: bd139
author_rank: Super Contributor
timestamp: April 06, 2018, 12:10:01 pm
content: Will bear that in mind. Thanks. Appreciated.Think I got to the bottom of this. These aren’t X7R’s. Vendor fired! I compared temp drift of one in a tuned circuit on an oscillator and it was much worse than a Vishay/BC X7R. Wonder if they had very high dissipation factor. I’ve just ordered some 10nF C0G that are SMD which weren’t too expensive. Going to re-layout the amp as surface mount and stop being a curmudgeon. Now to find a suitable alternative for the 2n3866 driver though yet.