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LF: RE: Guard Circuit

To: [email protected]
Subject: LF: RE: Guard Circuit
From: "Talbot Andrew" <[email protected]>
Date: Mon, 18 Dec 2000 16:01:46 -0000
Reply-to: [email protected]
Sender: <[email protected]>
Tnx for that Jim, think I can see how it works.  Now have to think how to translate into a 340V supply half bridge version - hopefully link and L values will stay realistic.
-----Original Message-----
From: James Moritz [mailto:[email protected]]
Sent: 2000-12-18 14:44
To: [email protected]
Subject: LF: Guard Circuit

Dear Andy, LF Group,

Aha... wondered why you hadn't mentioned it before - unfortunately, the guard circuit is not shown in the simplified diagram in the LF handbook.

What it is - the tank coil has a link winding on it. This feeds the 'AC' side of a bridge rectifier made up of 4 fast diodes, via a series leakage inductance tuning capacitor. the 'DC' side is connected to some big plastic film reservoir capacitors, and then across the PA supply rail. In fig 3.20 in the handbook, it is assembled on the 3 heatsinks largely concealed by the tank capacitor boards. I think it works like this:

When the tank circuit current (determined by the load

impedance) exceeds a certain ratio with the supply voltage, the

voltage across the link winding forward biases the guard circuit

diodes, and returns power to the DC supply. This effectively

clamps the voltage across the tank circuit, and due to the

impedance transforming properties of the LC circuit, appears to the

PA as a non-linear resistance in series with the load which increases rapidly when the output current exceeds a certain point. If you monitor the 'guard', PA input, and supply currents as the load is decreased, the guard and PA currents increase, but the difference between them (ie. the supply current) decreases. With a dead short on the output, the PA current is increased by about 50%, eg, after the unit was re-tuned:

With 50ohm load, 60V supply -

Total PA supply current for 3 PA's = 21.8A = PSU supply current

Total guard current = 0.1A

With short circuit load -

Total PA current = 34.9A

Total guard current = 28.3A

Supply current from PSU = 6.5A

With the full supply voltage and a short circuit load, the PA modules run quite a bit warmer than normal, but not dangerously so, so this is an impressively rugged design!

I can send you a photocopy of the full diagrams if you like - also the details of how I tuned it up for a different frequency and load impedance, if anyone's interested.

Cheers, Jim Moritz

73 de M0BMU

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