1
\$\begingroup\$

I'm looking for an SMT capacitor that can couple 300W at 144MHz on the output side of an LDMOS bias.

This answer talks about wattage dissipation based on ESR, so I can figure that out; my questions for choosing an RF power cap are about sizing:

  • What are the typical power ratings for capacitors in the 0402 -- 1206 range?

    • There is plenty of documentation for resistor power ratings by SMT size, but couldn't find any for capacitors---are those the same ratings for caps?
  • Is the DC rating voltage for the capacitor a peak or RMS rating for RF pass-through?

    • One side of the cap will have a 50V positive bias, does that affect the DC rating that I need to choose (ie, 50V + V_rms or some such thing)?
  • Mica vs Ceramic vs other?

  • Are there other considerations when choosing an RF power cap?

\$\endgroup\$
3
  • \$\begingroup\$ You need to focus on the current requirements rather than the power requirements and you also need to define the capacitance value before you can move on with this problem. There are plenty of capacitors that might do but without current and operating frequency, there can be no direct help. You might also need to take into account the ESL of the capacitor. \$\endgroup\$
    – Andy aka
    Jun 2, 2021 at 11:49
  • \$\begingroup\$ @Andyaka, assume 1nF for now. RMS current at 50-ohm for 300W is ~2.45A, ~122.5V, and operating freq is 144MHz. \$\endgroup\$
    – KJ7LNW
    Jun 2, 2021 at 20:15
  • \$\begingroup\$ Just go to a reputable cap supplier site and look at the interactive graphs for ESR, ESL And operating frequency. Choose C0G dielectric of course and you’ll find that less than 3 amps should suit a 1206 capacitor of suitably rated voltage. Kemerovo and TDK both spring to mind. I’ve recently designed a circuit that needs 11 amps and 1 nF so you should easily find one. C0G has lowest dissipation factor and is the dielectric of choice here. X7R won’t cut the mustard. \$\endgroup\$
    – Andy aka
    Jun 2, 2021 at 20:51

1 Answer 1

1
\$\begingroup\$

This is an excellent question that I have not had to consider for many decades so my answer is what I know. Some may be shocked that the ideal part is an $800 door-knob porcelain cap with gold-plated electrodes over palladium.

Here are my rules of thumb for dielectrics for RF high power

  • 1210 ~1/4W derate 50% or more if ESR>=50m and derate voltage 50% if using >=50% of rated power
  • 603 1/8 W no go
  • Any non piezo dielectric is ok but depends on ESR , ESL due to metallurgy (gold/palladium) and construction
  • Value 1nF is about 1 Ohm which is marginal for return loss at 144 MHz so bigger is desireable.
  • ESR needs to dissipate possible antenna faults of reflected power and be self-healing.
  • ESL needs to be lower for a high SRF >200 MHz for aging
  • MTBF reduces sharply when not derating as above
  • Consider up 1 uF polypropylene for <20 mOhm
\$\endgroup\$
3
  • \$\begingroup\$ Thanks Tony. Where did you get the 1210=1/4W and 0603=1/8W numbers? I've not seen any datasheets that list max capacitor wattage by component size---but resistors do! Do capacitor SMT sizes generally have a similar dissipation wattage as resistors for the same size? \$\endgroup\$
    – KJ7LNW
    Jun 3, 2021 at 2:58
  • \$\begingroup\$ Not exactly..... I recall reading it online somewhere \$\endgroup\$ Jun 3, 2021 at 6:08
  • 1
    \$\begingroup\$ @KJ7LNW A reasonable assumption is to take the dissipation rating of the respective resistor (Al2O3 base, 1/2W, ~ 25 W/m/K) and reduce it by the ratio of thermal conductivity (BaTiO3 (~type 2) or CaZrO3 (~type 1 / C0G) dielectrics are ~3 W/m/K). Which, is pretty bad actually, nevermind the thickness may be greater. There is some improvement when considering the metal layers in the chip, but who knows how much they put in there. Preferably: check the data/char sheet for current ratings. \$\endgroup\$ Sep 4 at 16:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.