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Re: LF: Class E PA

To: [email protected]
Subject: Re: LF: Class E PA
From: Johan Bodin <[email protected]>
Date: Thu, 11 Oct 2018 21:09:29 +0200
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Hi Tom,

the keyed PSU sounds like a good way to go considering your very slow keying speeds!

I'm also attracted by the combination of simplicity and ultimate efficiency of single ended class E but keep in mind that the advertised impressive efficiency of class E only occurs at the "sweet spot" where the load impedance equals what the circuit was designed for. I did some experiments (x years ago) and my personal opinion is/was that a good voltage switching Class D is more forgiving and better suited for casual experiments as it tolerates just about any abuse except too much load current (if properly designed with catch diodes that takes care of any out-of-phase load current).

Have  a look at the old Decca TX. It's a very robust design and Andy G4JNT has written an excellent article about its overcurrent protection circuit.

Johan SM6LKM

P.S. I just love the word "unkaputtbar" :-)

DK1IS wrote:
Am 09.10.2018 um 19:20 schrieb DK7FC:
Hi Tom,

Why do you decide for a class-E at all? Do you see any actual advantage over a class D?

73, Stefan

Hi Stefan,

A very good question! Knowing you are a dedicated class D fan I have to choose my words carefully ;-) ! - Well, I think class E would fit my personal needs best:
  • single ended circuit, that means no timing problems at square wave drive with 50% duty cycle
  • no fiddling with dead times or double frequency drive
  • 28 V DC power supply is well sufficient for 180 W output bringing about 18 kV rms to my antenna at a current of 0.6 A (and that´s the voltage limit of the antenna)
  • omission of all transformers between PA and the feeding point of the antenna: matching is part of the output network and so is the regular cable capacity
  • fine tuning can be done in the shack
  • perhaps even a little bit more efficiency than class D - but that´s not so important

Meanwhile I found out that my nice commercial power supply (Ascom Frako, 24/26.8/28.2 V, 20 A, short-circuit proof, "unkaputtbar") has a control input at logic level for a DC softstart with a time constant about 100 ms which won´t affect the long lasting signals at vlf and avoids any key clicks. I´ll use this input for keying thus omitting the problem with the first positive gate pulse. Drive will be there all the time with constant phase and asynchronous soft keying is done via the power supply.

Tom, DK1IS

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