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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
