EMRFD Message Archive 14150

Message Date From Subject
14150 2017-07-10 09:08:05 martinfilbertjarv... Capacitors for tx output lpf and bisaing Mosfets?

Hi all and thanks for the add to the group. I am home brewing a triband transceiver10-15watt output (started as qrp but bumping up power a bit). Im using IRF530 as ouput device running it on 13.8vdc. I was using simple 500vdc disc ceramc caps in output lpf but noticed they were getting warm (surprising for low power?) and am wondering what the best capacitors to use would be?

Also, i read conflicting advice on the need (or not) to bias mosfets used in a tx pa - any thoughts appreciated - thanks.


73


Martin

14151 2017-07-10 18:57:47 kerrypwr Re: Capacitors for tx output lpf and bisaing Mosfets?
All capacitors have loss, expressed as dissipation factor (DF), tan delta or equivalent series resistance (ESR).  This loss creates heat, just as a resistance does.

This is from a Johansen note and shows the DF of three kinds of dielectric;

https://s3.postimg.org/fyckofscz/Dielectrics.gif

 

High dielectric constants allow capacitors to be made small but the trade-off is higher DF; more power is dissipated as heat in the capacitor.

You possibly have X7R ceramics; they seem to be the most common although there are many others.

High power requires low-DF capacitors; mica is often used and, in critical applications, porcelain.

Wes' paper The Two Faces of Q is a wonderful read that discusses capacitor Q; since Q is the ratio of energy stored to energy lost it's another measure of loss.

It's on his website.

Bias level determines the amplifier class at which a Mosfet (or BJT or valve/tube) operates; in very broad terms, low/zero bias = class C, high bias = class A.

This home-made Mosfet amplifier can be switched between the two;

https://s11.postimg.org/y368m724j/image.jpg

 


Kerry VK2TIL.
14152 2017-07-10 20:05:35 martin jarvis Re: Capacitors for tx output lpf and bisaing Mosfets?
Thanks Kerry - very useful, the caps im using are np0 dielectric

14153 2017-07-10 21:04:38 kerrypwr Re: Capacitors for tx output lpf and bisaing Mosfets?
NP0 have low DF but it's still DF I suppose; 10-15 watts might be enough to warm them, esp. if they are at the high end of their specification.

Parallel combinations of capacitors tend to be able to dissipate more power than a single component because of increased surface area.

Kerry VK2TIL.
14154 2017-07-10 21:58:43 victorkoren Re: Capacitors for tx output lpf and bisaing Mosfets?
I am using regular 50V NPO capacitors in the LPF of my 25W 2xIRF510 PA without warming or any problems.
Usually the real breakdown voltage of 50V NPO capacitors is higher so they still work fine even when SWR is up to 1:3.
Victor - 4Z4ME
14155 2017-07-11 04:22:12 Steve Dick Re: Capacitors for tx output lpf and bisaing Mosfets?
Note that 25W into a 50 ohm load corresponds to about 50 volts peak voltage and about 1/2 amp peak current in any of the series legs of the lowpass filter.  Not a good idea using a capacitor right at its rated voltage. There should be some de-rating used in the design. Look at the manufacturer’s data sheets. Also, peak currents can be fairly high, which many small NPO ceramic caps are not really rated for.  If the LPF has parallel resonant traps in the series legs, voltages can rise considerably above the 50 volt capacitor voltage rating. Best to simulate the filter in a program like LT-SPICE to see what the peak voltages and currents will look like under normal load and high SWR conditions.

Steve K1RF
 
14156 2017-07-11 06:03:05 Bill Carver Re: Capacitors for tx output lpf and bisaing Mosfets?
NPO is the correct choice for ceramic capacitors in power RF circuits.
Their "Q" is higher. Amusing, the letter "Q" was chosen because of the
word "quality", and the quality and RF losses may vary from one vendor
to another. Barring measurement of every capacitor, choosing a well
known US capacitor vendor might make a difference in RF loss in the
capacitors. For example look at W5PQL's website: he sells a solid state
HF KW amp kit. Jim used Cornell Dubilier SMD MICA capacitors before,
which didn't need parallelling but their cost was about five bucks a
capacitor. I was told his current kit uses multiple ceramic capacitors
in parallel to share the RF current and share the heating.

Measuring the Q, or ESR, of capacitors at RF is pretty straightforward
with modest instrumentation. Read Wes Hayward's paper and read all the
talk of capacitor losses that was on here a month or two ago. But still
need instrumentation.........and still have the capacitors you already
have! So you could just bypass the measurements and hip-shooting: where
you have one capacitor, put two capacitors, each half the value, in
parallel. This divides the RF current, and the heat, equally between
them with double the surface area to dissipate the heat so the two will
run cooler everything else being equal. And if their individual ESR is
lower than twice the ESR of the capacitor they replace, power loss and
heating will drop even more.

W7AAZ
14157 2017-07-11 09:05:30 martin jarvis Re: Capacitors for tx output lpf and bisaing Mosfets?
Thanks for the replies, i'll try paralleling smaller values - Victor ca you share the schematic of your 
​
25W 2xIRF510 PA? thanks

14158 2017-07-11 22:02:29 victorkoren Re: Capacitors for tx output lpf and bisaing Mosfets?
Its the WA2EBY PA. I had some iterations on winding the output transformer and the parallel capacitor on its primary to get acceptable gain at 10m band.
About using 50V NPO capacitors at the PA output LPF: I did paralleled capacitors to increase the current capability but about the voltage breakdown, from my long experience NPO capacitors from reputable manufacturers do have actual breakdown voltage much higher than its specifications. The manufacturer specification for capacitor breakdown voltage is that the DC current through the capacitor should be below the specification limit at the specified voltage. It does not imply where is the real breakdown voltage.
Victor - 4Z4ME
14159 2017-07-16 13:43:18 kb1gmx Re: Capacitors for tx output lpf and bisaing Mosfets?
side note...

Capacitor voltages are working voltage and not a fail voltage.  To get the latter you must 
consult the makers datasheet.  General rule is never exceed the working voltage on 
DC peaks (blocking and bypassing) and AC voltage peaks (P-P) in resonant circuits.

However depending on the circuit the carry current (and ESR) may be more an issue.

Some ceramics are poor for ESR/Loss and will heat terribly.  An example are 
common Z5R, good enough for bypassing only.

Allison