HF-PA-v3

VU3CER's Robust Class-C / Class-D HF PA for https://github.com/kholia/Easy-Digital-Beacons-v1 project for the 20m band.

This design aspires to be the standard mW-to-QRP-Gallon HF PA in the ~600 INR (8 USD) design space.

Design Constraints:

• PCB is to be homebrewed.

• PCB is single-sided - only one layer (side) is available for routing.

• IRF510 MOSFET needs to be on an edge to allow usage of heatsinks.

• Typical external linears (e.g. PA150 HF Linear Amp) need 2W minimum to function. So we need a minimum of 2W of RF output from our design.

• Minimal part count is strongly preferred. Low cost, and ease of availability are strong driving and deciding factors.

  The cost of this HF-PA-v3 system is less than one RD16HHF1 MOSFET ;)

• Reproducibility is a MUST! We focus a lot on this aspect by documenting each, and every detail possible.

• Repairability in the field is a MUST!

  Dhiru's finding: Repairing homebrewed PCBs is so much easier, and fun than reworking commercial lead-free PTH PCBs.

• No moving mechanical parts are allowed in the design - this rules out usage of (failure-prone) active cooling options like fans.

Input: Few milliwatts is fine. Note: Si5351's output is around 8-10mW.

Output: 5W+ @ 28v for the 20m band (QRP Gallon achieved!) with around ~42% efficiency. 4.25W+ @ 24v for 20m band (14 MHz) using a single IRF510 with ~33% to ~43% efficiency. Solid 3W+ at 19v with less than 500mA of total current consumption. Solid 4W+ at 22v with less than 500mA of total current consumption. More output is possible via a bit more gate biasing voltage + increased PA voltages (>= 29v). The Easy-Digital-Beacons-v1 system itself consumes 2.49W when operating in the Beacon Mode. In comparison to this v3 HF PA, https://github.com/kholia/HF-PA-v2/ is more efficient (@ 14 MHz) but is not as robust. In comparison, ICOM IC-705 efficiency is around 24% - It consumes around 1.5A @ 13.8v to output 5 watts - Of course, ICOM's PA is a linear one (with known low efficiencies) whereas ours is a switching one.

Robustness check: The PA works fine with 100% duty cycle for more than 8 hours! In real-life, WSPR defaults to using ~2-minute TX intervals every 10 minutes. FT8 is naturally gapped every ~15 seconds, and so on. We have also run FT8 on this HF PA for more than a day at a time. The whole HF PA v3 system is thermally super stable.

Note 1: The output power is software controllable (via PWM grounding) for flexibility.

Note 2: The idea behind making these (and my other) design notes publicly available is to deliver REPRODUCIBLE, flexible, home-brewable, standardized, reasonable, and cost-effective solutions for some common problems. I include many references, simulations, and notes in my projects to enable you to do your own (better) designs - do share them ;)

(Image Credit: Wanting book by Luke Burgis)

Note 3: All screenshots are clickable and zoomable.

Note 4: Use the HF-PA-v3-No-Regulator PCB version included in this repository. This version doesn't need the 7805 linear regulator.

Please see the older https://github.com/kholia/HF-PA-v2 project for the power-supply (over-current protected), and LPF designs.

(26-September-2021 UPDATE): A multi-band (40-20-17-15) version of this PA is now available!

Schematic:

PCB renderings:

Pics (or it didn't happen):

This HF PA is capable of winning some nice awards (e.g. QRP-ARCI 1000 Mile per Watt award).

The HF PA is connected to a 20m dipole over 55 meters of HLF-200 coax.

PA tuning process

• Remove the MOSFET from the TO-220 socket.

• Set gate bias to 3v. This ensures that IRF510 isn't turned ON - double-check this by monitoring the power consumption.

• Connect the MOSFET back, and measure RF output. Happy with the output power? - If yes, you are done else follow the next steps.

• Remove the MOSFET, increase the gate bias voltage in very small increments (0.1v types).

• Connect the MOSFET back, and measure power consumption without any RF input. Ensure that MOSFET remains OFF without any RF input.

• Apply RF input, and measure RF output.

• Repeat these steps as needed.

The QRP Labs' procedure for bias adjustment seems to be better.

PA BOM

• 1 x SN74ACT00N (TI) - 65 INR

• 1 x IRF510 (Genuine Vishay parts are tried-and-tested) - 35 to 50 INR

  • The SEC (SEC Electronics Inc) branded IRF510(s) also work great!

  • The SEC IRF510 performs even better than Vishay's version. This particular result will vary between different product batches.

  • IRF510 with International Rectifier branding are most likely fake or "new old stock" if you are very lucky!

    Note: IR and Infineon have stopped manufacturing IRF510 MOSFETs.

  • AliExpress IRF510(s) are likely to be fake

  • The IR IRF510 gave ~3W versus 5.2W+ from SEC IRF510 versus ~5W from Vishay's IRF510!

    Note: The gate threshold voltage can vary even between two samples picked from the same manufacturer!

  • The Cgs ("gate capacitance") of a fake IRF510 is typically more than 1000pF on a LCR-T4 meter. This value is around ~400pF for genuine IRF510 parts on a LCR-T4 meter.

  • Ensure proper sourcing (trusted supply chain, etc) to get genuine parts

• 10K Preset (Potentiometer) Bourns-3386P - 15 to 20 INR

• 4 x 10k Ohm - 0.25W Metal Film Resistor - 10 INR

• 1 x 4.7 Ohm Resistor - 5 INR

• A very large heat sink - 50 to 200 INR - larger heatsink is better!

• 1 x 7805 (TO-220 package from CDIL / ST) Voltage Regulator (Optional but makes PA voltage experiments easier) - 10 INR

• MT3608 2A Max DC-DC Step Up Power Module (Robu) - 60 INR

• 2 x SMA connectors (PCB mount) - 75 INR

• 5 x 100nF (104) Ceramic Capacitor - 15 INR

• 1 x 10nF (103) Ceramic Capacitor - 5 INR

• 1 x uF (105) Ceramic Capacitor - 5 INR

• 1 x FT37-43 (Black Toroid) - 50 INR

  • With 5T of 22 SWG wire, I see 25uH of inductance on my LCR-T4 meter

  • This 5T idea was also found (later) in the NB6M's Miniboots design!

• Misc: Relimate Connectors, Copper Clad PCB, Various Consumables (~100 INR)

• Tools: LCR-T4 Meter (for measuring things), RF Power Meter

• Optional power supply: LM50-20B24 Mornsun SMPS - 24V 2.2A - 52.8W AC/DC SMPS - 700 INR

• PA Total Cost: ~600 INR

LPF BOM

• 3 x T37-6 - Yellow Toroid, 1 x T37-2 - Red Toroid (LPF)

• 2 x 180pf, 2 x 390pf, 1 x 30pF NP0 Ceramic Capacitors (LPF caps, Kemet / Vishay / Multicomp Pro)

  Note: The C0G (NP0) property is important for filters. Don't doubt this ;)

  Note 2: Get >= 100v rated caps - we are going to output more power!

• 3 meters - Copper Magnetic Winding Wire (22 SWG, ~0.7mm) - 25 INR

• LPF PCB or roll your own

Note: LPF design and values are borrowed from GQRP technical pages (W3NQN), QCX manual, and uSDX WB2CBA PCB documentation.

Power Supply BOM

See https://github.com/kholia/HF-PA-v2.

BOM + Equipment Sources

• https://www.semikart.com/ (IRF510, NP0 LPF caps - Kemet and others)

  • Multicomp Pro 100v C0G caps from Element14 are recommended

• https://www.semikart.com/search/SN74ACT00N

• https://www.electronicscomp.com/

  • IRF510 (Vishay), regular caps, ST voltage regulators

• https://www.onlinetps.com/shop/ (IR IRF510, new old stock perhaps?)

• https://www.electroncomponents.com/ (transistors, connectors, consumables, misc)

• https://www.sunrom.com/ (regular ceramic caps, SMA connectors)

• https://projectpoint.in/

  • Transistors, regular caps, voltage regulators, pots / trimmers, MFR resistors, misc

  • 2-Pin High Quality Relimate Connectors (all those 2-pin headers are actually Relimate Connectors)

  • Copper Magnetic Winding Wire

  • 3-Pin Relimate Cable Female to Female (High Quality 2500mA) - 'TO-220 socket'

• Semikart, eBay (gr_makis) - Toroids

• https://www.ktron.in/ (SMA connectors)

• LCR-T4 LCR Meter

• [AVOID] https://www.fabtolab.com/78l05-voltage-regulator (https://www.ventor.co.in also works)

• https://robu.in/ (voltage boost module)

Note for international buyers: The BOM can be carefully sourced via Mouser and DigiKey.

O(Current consumption)

The ESP8266 usually draws a peak current of about 250mA after it wakes up and then more or less constantly draws around 70mA before it goes back to sleep.

DS3231 -> 5mA

Si5351 -> ~50 to 100mA

PA -> Less than 1 A (rms)

Upper bound -> 1.4A maximum.

Tips

MOSFETs are sensitive with regards to ESD and high soldering temperatures. Use best practices when dealing with MOSFETs and perhaps always ;)

By using these TO-220 Sockets, we can easily support any MOSFET pinout. This technique allows usage of off-pcb heat sinks with a lot of flexibility. It also avoids common ESD + MOSFET problems as well. The gate bias can be easily checked by safely removing the MOSFET from the TO-220 socket. Also, this socket idea allows us to safely eliminate the LDO from the design.

SN74ACT00N can be driven at 6v - this is the secret LU2HES sauce! RF input drive strength matters the most.

Note: Juan (WJ6C, uSDXn project) reports that IRF530 works better (produces more power) than IRF510 at higher voltages like 19.5v or 24v. Dhiru's finding IRF510 works better at 19v (3.25-Watt VS 3-Watt for IRF530). And it should, right (with its less gate capacitance)?

A burnt 78(L)05 regulator can easily eat up 4-5W of power by itself. If there is no RF output but power is being consumed then check your linear regulator.

Ideas (for v4)

• Be a bit bold, and try Class-D biasing (IRF510 operates in saturation in Class-D)!

  Update: Done in https://github.com/kholia/HF-PA-v4 design.

• Try adding a 2N2222 driver stage for output boost to 10W (idea from VU2ASH).

  Update: Done in https://github.com/kholia/HF-PA-v4 design.

• Add support for 15m band. Is this even possible? Keep all leads short (no more TO-220 socket idea then it seems).

  Update: Done in https://github.com/kholia/HF-PA-v4 design.

References

See References.md.

Credits

This design uses ideas from NA5N, Sandeep VU3SXT, PY2OHH, NB6M, LU2HES, Hans Summers, and VU2ESE (Farhan).

Also see https://github.com/kholia/HF-PA-v2#credits.