Andy,
hams around here use multi-layer ceramic caps for high power
class-E service, specifically the ATC series caps from American
Technical Ceramics:
http://www.atceramics.com/products/multilayer_capacitors.asp
Very stable, high current (single ATC100C will handle 12 amps), and
small in size!
--
73 Warren K2ORS
WD2XGJ
WD2XSH/23
WE2XEB/2
WE2XGR/1
On Mon, Jan 10, 2011 at 9:22 AM, Andy Talbot <[email protected]> wrote:
> 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
>
> These things http://www.g4jnt.com/Download/UselessCaps.jpg
>
> 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
> A picture of that breadboard can be seen at
> http://www.g4jnt.com/Download/ClassE_BrdBd01.jpg
>
> 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
> www.g4jnt.com
>
>
>
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