PA overcurrent protection

High current/voltage PA protection

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 warm 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 PFET I use in my design has a maximum internal resistance of 1.05 mOhm.

17 mOhm (BTS device) at 50 Amp is approx 0.85V over the device, so 0.85V x 50A => 42.5 Watt !

At 1.05 mOhm (my device) at 50 Amp is approx 0.053V, 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.


The board

The board 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 PFET (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 PFET 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 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 overcurrent board needs +12V to operate.

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

Click on the picture above to open the schematic as a PDF file

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

BOM file can be downloaded here