----- Original Message -----
Sent: Monday, January 10, 2011 2:22
PM
Subject: LF: Capacitor warning and First
500kHz Class E breadboard
A warning to anyone contemplating making power amps - especially class E
types.
Don't even think about using the small high voltage ceramic caps
availalble at low cost from the suppliers - and sold as "suitable for SMPSUs
etc" in resonant circuits
They are appalling, terrible, revolting things. You only have to
breath on them and they change their value.wildly
I've just made up a first version of a breadboard for a 500kHz
Class E amplifier, (target design 500W from 50 V supply),
calculated the L/C values as per Alan's spreadsheet, combined with an L
matched output for 50 ohm. For the capacitors I aimed to get within 5%
of the calculated values using many of these small ceramic objects in
parallel - lots in || should share the current, shouldn't they, which would be
good?
So I soldered them up - used the G4HUP L/C meter to measure the values
- at which point I should have been a bit suspicious as the measured
C was sometimes a fair bit lower than it should have been. At the time I
just put it down to "finger trouble" - one does tend to ignore little things
like that sometimes.
Anyway, made up the amp, used a 12V supply rail and monitored the
switching waveforms, expecting to see something vaguely approaching the half
sine on drains etc.
Results, terrible, nowhere near what they should have been - clearly
summat was very wrong I remeasured the || combination of the caps -
after unsoldering they were sometimes as much as half the value they
should have been - until they cooled right down and approached room
temp. Occasionally a cap value approaching correct would show up on the meter
once in a while - but that was was probably just coincidence
:-)
And yes, I did double, triple and quadruple check the meter against good
caps - it was OK every time.
These little blue things appear to have a huge temperature coefficent -
are completely useless, designed to do nothing of value, and all have now gone
into the waste bin.
In fact, I was wondering if they were
random-capacitors.
I did have a just two trusted 22nF caps in the junk box that
were within 20 - 30% of the right values for the amp and an awful lot of 3.3nF
1700V things of a more respectable heritage, so just put these into the
circuit, tested at 12V and got waveforms almost correct. Increased
drive freq to 515kHz (no real scope for changing cap values at this stage, so
freq change would be easier to try with) where the waveform looked best,
and increased supply volts. Got to 30V and
it was delivering 150 Watts; 40V and was getting around 350 Watts at which
point I assumed it was going to work, so switched off for a
break. Wasn't going to tempt fate by gong to the full 50V
having not even bothered to tune things up properly
The yellow cap shunting the device was running quite warm - which is
hardly surprising just a single poly[whatever] carrying all those amps - and
the one in series with the tank was slightly warm. So, now
need to browse the Farnell catalogue (the cubic printed paper version is
easier) for some caps that will do the job properly - like more of those
yellow ones. Actually, may have enough of the 3.3nF ones, but that would
end up with lots in parallel
So, for anyone who is contemplating a class E amplifier design:
Get close to the calculated component values and it will probably
mostly-work immediately.
Play with the drive frequency while monitoring waveforms to see
which way tuning needs to go
Don't use cheap high voltage ceramic capacitors
Don't use cheap high voltage ceramic capacitors
They would work well in a random number generator tho.....
Andy
