Power Supply Problem with Class E Amps

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Power Supply Problem with Class E Amps

Postby WA2WVL » Mon Jun 12, 2006 9:28 pm

I recently tested a 1500 Watt class E amplifier and foumd a problem to be aware of.

Capacitor input power supplys have poor regulation and can have a NO LOAD rise in DC voltage of up to 40% from the FULL LOAD voltage.
(I measured 39%) The result of this was that each time the amp was keyed it delivered 3KW decaying down to 1.5KW. This causes undue stress on the FETs and had to be fixed.

A simple fix was found. A Wirewound resistor was added in series with the primary of the 60Hz transformer and shorted each time that amp was keyed by a relay controled by the keyline. By adjusting the resistor value the NO LOAD and FULL LOAD voltages can be the same. In my case the value was 150 ohms but depends on bleeder current and other factors.

This resistor also provides a "Soft Start" for the turnon of the power supply.

Floyd WA2WVL
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Postby steve_qix » Wed Jun 14, 2006 6:28 am

hi Floyd,

Yes, this is a well-known issue. If you examine the schematics for any of the rigs on the class E web site http://www.classeradio.org , you'll see that the primary voltage is disconnected from the AC mains during receive. This is to prevent the exact problem you're encountering.

I've only seen about a 20% rise in voltage with my supplies, but that's 20% too much for me!!! The other reason for disconnecting the AC from the supply is it saves energy, and is safer.


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Postby W1DAN » Thu Jun 15, 2006 12:26 pm


I ramp my modulator up from 0 to carrier level DC out. That way the high DC voltage at the PS starts to reduce as the voltage at the RF deck ramps up.

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