Carsten Groen's (OZ9AAR) personal place

High current/voltage PA protection
Revision 2

If you don't want to source and build the board yourself, I can deliver readymade and tested boards at Euro 110,- each. Email me for details, info on the My CV/Contact page.

NOTE: "Version 2" of the board adds the Vin and Vout sense outputs (measures the INPUT and OUTPUT voltages, with a resistive divider for each)

When constructing various power amplifiers for Ham radio usage it is often desireable to make some kind of overcurrent protection. Modern PA modules are usually made from (expensive) LDMOS devices, and it is a VERY good idea to protect these from excessive current!

There are a number of circuits floating around, many uses the BT50085 device. This is, in my opinion, not an optimal solution. If you use this device in a high current circuit, the device gets VERY hot as it has a high internal resistance (17 mOhm). Also the maximum voltage is only 58V, and some of the modern LDMOS devices (for HF) likes to run at 65VDC. The N-FET I use in my design has a maximum internal resistance of 1.05 mOhm.

17 mOhm (BTS device) @ 50 Amp will have 0.85V across the device, so 0.85V x 50A => 42.5 Watt !

At 1.05 mOhm @ 50 Amp, the device I use will only see 0.053V across it, so 0.053V x 50A => 2.63Watt, quite a difference...

So, I decided to design a switch that would remove some of these shortcomings, and that would also include a few "extras" that I would like. On this page you will find Gerber files (for producing board with JLCPCB), BOM file, schematics etc. 

I currently use "Version 1" of the board (did not have Vin/Vout monitoring outputs) in my own 500W 70cm Power Amplifier.

As always, USE AT YOUR OWN RISK!

Below is a block schematic of a possible PA module using the overcurrent/switch module and some of my other modules.

The REPAM device can be found here.

The Dual RF Head can be found here.

The board

The board is a 4 layer board, and as designed, are able to handle 80VDC at a maximum of 60 Amp. It includes "softstart" and a Iout voltage output that is proportional (50 mV/Amp) with the current being drawn thru the switch. It also has terminals for ON and OFF signals, pulling either of these to ground will switch on/off. The ON/OFF only needs to be momentary, once the switch is ON it will keep itself in that state until OFF input it connected momentarily to ground, an overcurrent situation occurs, or the power input is removed. The switch always starts in the "OFF" position.

The overcurrent trigger point can be set with the blue trimmer at the lower left of the board. There is a small testpoint to the left of it, connect a multimeter there and adjust the trimmer. The voltage at the setpoint will be 50 mV/Amp. If you adjust the trimmer so you see 1000 mV, the overcurrent trip point it set to 1000/50 => 20 Amp.

The power N-FET (Q1) has an maximum allowed power dissipation of 3.8W at 25 degC if mounted to a 6 cm2 copper area on the PCB. The polygon the tab of the N-FET is mounted to on this PCB is a bit smaller, around 4 cm2. We dissipate around 2.6W at 50 Amp, and around 3W at 60 Amp. As the FET also has a large area for the source connections, there is plenty of area for the device. However, if running at full power (60 Amp) for a prolonged time, please make sure there are some airflow over the PCB, just to be on the safe side.

Also note that the Iout voltage is measured using a high quality (and somewhat expensive) current shunt resistor (with Kelvin connections) and amplified by a current-sense amplifier (U3) with 50 mV/mV amplification factor. The current shunt resistor has a resistance of 1 mOhm, so 1A will give you 1 mV over the resistor, this is then amplified with a factor of 50 in U3. The result is that Iout has 50 mV/Amp, so 60 Amp will give you 3.0V on Iout.

The "INPUT" and "OUTPUT" terminals (where the HV is switched) are each sampled with resistive dividers and and brought out to the edge of the PCB (Vout and Vin terminals). Using the two trimmers VR2 and VR3 can be used to set the division factor for the INPUT and OUTPUT voltages. As the REPAM device has and input range of 0 to 5V on its analog inputs, it makes sense to divide these voltages so they can be read by (f.ex) the REPAM module. 

The overcurrent board needs +12V to operate.

REMEMBER the main power (up to 80V) and the 12V supply MUST HAVE COMMON GROUND !

You can download the schematic file as a PDF here.

Default if you order the finished board from me, the VR2 and VR3 will be set so that 60V on INPUT/OUTPUT will give 4V on the Vin and Vout terminals.

The VR1 trimmer that sets the overcurrent trip point is set to 200 mV on the small circular measuring point next to the trimmer. This means that the switch will open if more than 4Amp are flowing thru it (200 mV / 50 mV/Amp gives 4 Amp trip point).

You need to adjust VR1, VR2 and VR3 to your specific usage!

Gerber files (can be sent to JLCPCB) can be downloaded here

BOM file can be downloaded here

Below is a list of the actual prices for all the components, to this you need just to add the PCB (around Euro 4,-). Shipping from Mouser is free for the components.

Calibration of the board

Calibration consists of adjusting the 3 trimmers on the board, VR1, V2 and VR3.

VR1 is located at the lower left corner. VR1 sets the maximum current allowed thru the switch before it automatically switches off. If you got an assembled board from me, the trip point is set to 4 Amp. This means that the "Trip point" circular measuring pad, will have 200 mV if you measure it. The trip point is 50 mV/Ampere at this point. So if you adjust VR1 so the point reads 1V, the trip point will be 1000 mV/50mV = 20 Ampere.

VR2 and VR3 sets the factor for the Vout and Vin outputs. VR2 (the top one of the trimmers) sets the scaling for the voltage on the "INPUT" terminal and VR3 sets the scaling for the "OUTPUT" terminal.

Boards delivered assembled will have both trimmers set so that 60V on either terminal will give 4V out on the Vout/Vin terminal at the edge of the board.