Carsten Groen's (OZ9AAR) personal place

High current/voltage PA protection
Revision 2 - Single/Dual/Quad outputs

If you don't want to source and build the board(s) yourself, I can deliver ready made and tested boards:

Single channel (as described here)  Euro 125,- each.

Dual channel (see below) Euro 160,- each.

Quad channel (see below) Euro 195,- each.

Email me for details, info on the My CV/Contact page.

NOTE: "Version 2" of the board adds the Vin and Vout monitor outputs. These measures the INPUT and OUTPUT voltages, each with a resistive divider, both separately adjustable. This means that you can for example, get 4.0V out when the IN/OUPUT voltage is 60V etc. this makes it easy to monitor by a processor with 0..5V analog input, sd for example my REPAM module.

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 the board in my own 500W 70cm Power Amplifier for EME.

As always, USE AT YOUR OWN RISK!

Quad and dual channel versions

Boards as delivered assembled from me, included (M3) screws, nuts and spacers.

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.

Below is the configuration used in the PA Monitor application used together with REPAM module for the schematic shown above.

The board

Dual and Quad channels

The original design has a single output. I was in the need for both a dual and quad output board, so I designed two boards based on the original design. The dual and quad versions will switch power off as soon as just one of the outputs exceeds the preset maximum current allowed (on the trimmer).

The dual and quad boards will handle 50 Amp combined for all outputs.

The board is a 4 layer board (73 x 60 mm), 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 !

To use the board:

1. Apply 12V to board
2. Apply the HV input power (f.ex 50V)
3. Connect the ON input BRIEFLY to GND (for one or two seconds)
4. The OUTPUT will now be active.
5. Connect OFF input to GND for a few seconds
6. Once the OUTPUT has sunk below approx. 25V, you can remove the GND connection to OFF input

The load on the OUTPUT terminal must NOT be too high. When powering on (ON to GND briefly), any capacitors etc on the load will be charged thru R14/R15 (100R/2W resistors), and only when the voltage on OUTPUT gets beyond approx. 25V, the "self-holding" circuit will kick in, and you can remove the GND connection to ON. If for some reason the OUTPUT is NOT able to pass above 25V (for example because of a high idle load from the SSPA), the self-holding circuit will NOT be able to keep the power FET active and all the current will try to flow thru R14/R15 (which are not able to survive this for an extended period!).

When driving a load with large capacitors, you can get into problems! When the circuit detects overload (too high current) it will switch off FET Q1. This immediately removes the power from the output terminals. If there is a large capacitance on the output (for example large electrolytic capacitor(s) on a PA module), the voltage will only slowly ramp down. When ramping down, the current drawn by the PA will also get lower and lower, getting closer or below the max current allowed using VR1. When the current gets below the setpoint AND the voltage at the same time is NOT below approx. 24V, the "selfhold circuit" will kick in again and turn on Q1 FET. The current will then rise again, once it exceeds the setpoint set with VR1, it switches off again. This will continue "forever". This "self oscillation" is not as such catastrophic in any way, but it is of course NOT desired! The "fix" is to make sure that there is not "too much" capacity on the PA module(s), typically max is around 200 to 330 uF (+/-). You can change the behavior a bit by changing the resistor R10 from 2.7 KOhm to one that is a little bit lower in resistance, this will raise the "selfhold voltage" from approx. 24V (22 to 25V) which then could minimize the chance of this "oscillation".

Note, as described here (and finished boards are delivered) the minimum voltage that must be present on the OUTPUT is around 22 to 25V for the "self hold" to keep the switch in the "ON" position. If/when the voltage is lower than this, the switch will switch off. By changing R10 (from 2.7KOhm) to 10 KOhm, this threshold is lowered to around 7 or 8 Volt. This can be used f.ex if you want to drive a PA module at only 12V.

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.