YO3BN - 2xIRF510 50W QRO for 80m Band

Posted at - 25th August 2017

2xIRF510 50W QRO for 80m Band

Being a QRP station most of the time (4W on 80m), during summer time the noise floor became too loud and ionospheric propagation very poor on 80m band, so I need to do something to counteract these poor conditions. A good antenna was a valid option, but it is too complicated for the moment, therefore I decided to make a push-pull amplifier with two IRF510 found in my junkbox.

Prototyping

First, I set up a super simple push-pull amplifier circuit as ugly style prototyping. The IRF510 performed very well on 80m, it worked at the first attempt, shortly after finishing the working prototype I have integrated this amplifier to radio traffic boosting me out over noise. Friends gave me 59+20 reports on the air, so there was a no-brainer decision to build a solid amplifier with these cheap mosfets.

The main objectives were:

VOX operation
integrated power supply
temperature controled cooling system

Unfortunately, I did not have too much time to draw schematics in *CAD programs for this project. So, here they are in a pen and paper format :)

Amplifier Board

On the amplifier board, we have basically two IRF510 in push-pull configuration. Begining with input, we have two resistors of 100 ohms @5W in parallel seen as 50ohms by our transceiver, a 150ohms attenuator, then 1:1 voltage transformer with 4 turns as primary and 2x4 turns as secondary wound on a binocular core, 43 type from FAIR-RITE (2843000302). Because 4W means 21Vp over 50ohms and IRF510 gate max voltage is approx. 20V input transformer was chosen as 1:1 ratio, so its main function here is to split fullwave into positive halfwave voltages for mosfets. For a halfwave cycle the transformer gives positive voltage to the gate of one mosfet triggering conduction and negative voltage to gate of the other mosfet locking it, for next halfwave cycle voltage polarity is reversed. The two trimmers set the bias voltage individually to each mosfet.

The drains are connected to +25V through primary winding of the output transformer which acts as a 1:4 turn ratio transformer, formed from 2 turns center tapped in primary and 4 turns as secondary winding. The role of the primary winding is to reassemble the halfwaves into a fullwave form while maximizing current through. The primary wires are twisted together to equalize waveform amplitude for both half cycles. The secondary winding transforms impedance from low to medium in order to deliver most of the power to 50ohm load. The core is a 43 type, FAIR-RITE (2643540002). A 2200uF low ESR capacitor and a RFC wound on a toroid (KEMET ESD-R-25D-8) suppress the RF going to power supply path. A tuned circuit formed by 1.8uH and 750pF set the output impedance to 50ohms for 80m band. The 1.5nF @10KV capacitor between mosfet drains dramatically improves the output sine wave for lower frequencies. That's it, maybe an improvement would be to increase the two bias resistors from 2k2 to 10k or so in order to increase input impedance.

VOX and Cooling Board

The regulator circuit can be seen in up left corner, regulating voltage from 37-25V to 15V to supply fixed voltage to precision circuits, temperature sensing and vox timer. It consits from a zener diode, several filter capacitors and a common base darlington combination of two transistors, 2N3904 and BD139, working as current amplifier.

The cooling circuit, up right corner. I used an unknown temperature sensor for this circuit, measuring it with ohm-meter gave about 8k for 23° C and 6k for 35° C. Basically the temperature threshold is set by the 500 potentiometer opening the 2N3409 transistor which in turn open the BD136 transistor which is a voltage amplifier for fan. Note the positive feedback through 82k, which set the fan as totally ON or OFF, no variable turation.

The vox circuit on the bottom of the picture, works as follows. From the input, the two BAT41 diodes detects RF from transceiver, opening the 2N3904 transistor which will charge 100uF capacitor from its collector. The capacitor then discharges through 5k potetiometer and 82k resistor. While discharging the capacitor next transistor 2N3906 is left open which in turn open the next one 2N3906 acting as an ON/OFF switch, controlling the next transistor BD136 which works as a voltage amplifier. The 26V is supplied to TX relay and mosfet bias. The vox circuit is very effective, switching to TX blazing fast and the timer is set by 5k potentiometer. The relay circuitry surrounded by dots in schematic switch the relay violently ON and slowly releasing it OFF, acting as a relay wear protector not as a timer. The next zener diode trim the voltage from 25V to 10-15V preparing voltage for LM317 which outputs approx 5V for mosfet bias. The last potentiometer set the gate bias for both mosfets.

Output Filter

The filter is formed by two PI filters for 80m band formed by two 2uH inductors on iron powder toroids and 4x750pF. The output voltage is 150Vpp on a 50ohm load.

Other Notes

The PCB was made for 4xIRF510 but during testing I have experienced some problems with MOSFET biasing, therefore only two were left on the board.

Ceramic capacitors on output become hot during long CW tests even if they are rated at 10kV, maybe mica capacitors should be used instead

Power is supplied by a 2x24V 100VA toroidal transformer, filtered by 3x10000uF @35V. Power supply is not regulated, so voltage drops from 35V to 26V during high power transmission. Without filter capacitor the sinewave becomes noisy and thicker

The IRF510 seem to be a rugged device, I tested it with a shorted load, without load for several seconds or accidentally and nothing happens

My IRF510 mosfets are manufactured by Vishay

Current consumption is about 4.2A/25V at maximum power, so efficiency is around 50%

Weighs approx. 3kg

On Air

From 4W to 50W is a huge difference, testing the RF with my EMF meter indicate an increase about 12dB, on SDR my signal increased about 11.5dB too. On air friends gave me in general 58 - 59 for 4W and 59+10dB - 59+20dB for 50W.

Some spikes can be seen left-right, they seem to be from transceiver not from amplifier. They are more visible with amplifier because they are amplified of course, probably I have a bad stage somewhere in tx chain.

All Photos

73 de YO3BN