LF: RE: Guard Circuit

["Talbot Andrew" <ACTALBOT@dera.gov.uk>]

Another way of looking at the guard circuit operation is this :

 

As the load resistance decreases, so the loaded Q of the Tank circuit goes up and the voltage across the L / C rises, (cf. more current flowing)  By clamping a proportion of this voltage to the supply then as the load R progressivelyd ecreases, the output devices are then switching more of their current into an out of phase circulatory current being returned to the supply until, with Rl = zero, all the current is circulating.

 

The difficult part of implementing this is to match the link on the tank coil to the supply.  As the loaded Q when operating normally is around 6, then ideally a voltage division of 6  would be needed.  But as this is an air cored coil, getting precise transformation ratios is not easy hence the series capacitor mentioned, needed to act as some sort of trimming element.

 

I did wonder about monitoring tank voltage directly via a high voltage diode ( 2000V diodes at low current eg. BY584) and using this to kill drive to the output devices. This is the same as monitoring RF current, but it may not be quick enough.  The only way is to try it out - and blow up moreI RF840s.  I've only got 6 left now out of an original stock of 24.  The final amp will use IRF462s and probably be set at 800W, but these cost money so testing will be done with the cheap surplus devices in the meantime.

 

Andy G4JNT

 

-----Original Message-----
From: James Moritz [mailto:j.r.moritz@herts.ac.uk]
Sent: 2000-12-18 14:44
To: rsgb_lf_group@blacksheep.org
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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