QRP Power Board
Overview
This board helps the amateur radio enthusiast power their QRP outdoor adventures.
The design’s primary function is negotiating power from a USB Power Delivery (USB-PD) power bank on behalf of the radio - but it can also provide a regulated output voltage from 5V USB ports, solar panels, or other DC power sources. The USB-PD configuration typically provides enough power for 10W transmit, and 5V USB (or other DC inputs) can support RX or low duty cycle TX. With the addition of a supercapacitor bank, 10W transmit is possible with a solar panel the size of a clipboard.
The buck-boost converter’s switching frequency is nominally 1.2MHz. This high switching frequency was selected to minimize the number of harmonics that line up with the amateur radio HF bands. At most one harmonic lies within each amateur radio HF band between 80m-12m, and a couple of harmonics may be in the 10m band. The converter will fully shut down if the “DISABLE” connection is asserted above ~2V.
USB-PD power bank supplies 12V to the KX-2. No battery inside the KX-2.
SOTA chasing with power from a laptop. The supercapacitor bank buffers energy for TX. No battery inside the KX-2.
Powering the KX-2 with a solar panel + supercapacitor. No battery inside the KX-2.
Solar panel -> buck-boost -> QCX-Mini. (No supercapacitor bank.) The full 5W output is possible in good sunlight.
QCX-Mini powered by a 5V phone charger. The onboard buck-boost current limits during TX, but RF output power is still 1-2W with a clean CW envelope.
VX-3R powered from 5V USB-PD. No battery inside VX-3R. Gives full 3W transmit power on 2m.
Powering the KX-2 with spare AAA batteries and a supercapacitor bank. No battery inside the KX-2.
USB power banks tested in the field.
Board mounted between relay board and back panel of original case.
Re-assembled (tr)uSDX after adding USB-C power capability.
USB-C powered (tr)uSDX on GM/ES-008, with 12V from PowerAdd bank.
Buck-boost efficiency measured across load, with multiple input voltages. Output Voltage is 12.3V.
KX-2 power consumption vs. output power. For a fixed output power, the input current does not decrease for Vin>11V; therefore, the KX2 becomes less efficient as input voltage increases above 11V. A 12V external source allows 10W TX power, minimizes wasted power, and does not fully drain the internal battery if it is present. 12W TX is possible with a 15V USB-PD source.
Output Voltage is 12.3V. Curve shows maximum output current before output droops by 5%.
RF spectrum measured with 5” diameter loop of wire around board and power bank.
Red: buck-boost operating at 200mA output, powered by Energizer 5V power bank
Green: ZMI Pro 20k USB-PD power bank sourcing 200mA.
Blue: noise floor
sota-pwr_2.1.PDFDisclaimers
This documentation and any related physical hardware are provided in the spirit of amateur radio experimentation. Any use of this documentation or hardware is at the operator's risk, and should comply with all safety best practices.
This board can consume more than 500mA from a 5V USB port, and has no protections against consuming more current than was negotiated via USB-PD. If the board malfunctions, it could produce output voltages that cause damage to the radio - or a short circuit on the power supply feeding the board. Plug this board into your equipment at your own risk.
Never operate this device unattended, and always monitor temperatures to prevent overheating. Do not assume that thermal protections will prevent permanent damage. Do not use the CC/CV behavior to charge a battery without sufficient battery protection features.
This board was developed after measuring the power needs of the Elecraft KX2. Please note that this board is neither an Elecraft product nor endorsed by Elecraft.
Configuration 1
Powers RX: Yes
Powers TX: Yes (KX2: 4W at 9V, 10W at 12V, 12W at 15V)
Battery Life: limited only by power bank capacity
In this configuration, a USB Power Delivery (USB-PD) power bank supplies the KX2. No internal battery is required inside the KX2 in this configuration; however, if an internal battery is present, it will power the KX2 when the USB-PD output disappears or droops to a lower voltage than the battery. A 12V or 15V capable USB-PD power bank is most appropriate when the internal battery is present: the external power bank will fully deplete itself before the internal battery begins to drain.
Most field testing with a KX2 has occurred with the ZMI Pro 20K USB PD Backup Battery & Hub supplying 15V. Its USB-A connectors are useful for charging a phone or tablet at the same time as powering a radio via USB-PD at 15V (12W maximum RF output). The PowerAdd Pilot 2GS 18W Power Bank and Nimble CHAMP 10K mAh are more compact power banks that have worked in the field with the KX2 at 12V (10W maximum RF output). The author has plugged the board into every available power bank, laptop charger, dock, and computer and has not found any failure to negotiate.
The board negotiates for a 9V-15V output, at 1.5A or greater. The current draw during TX may exceed 1.5A, so it is the user’s responsibility to understand whether the power bank has sufficient output capability for the TX power setting. 18W power banks typically output 12V/1.5A or 9V/2A, so the 1.5A negotiated current level is the recommended minimum if 12V is required. Change the minimum current to 2.5A to guarantee that the KX2 cannot draw too much current.
If the board and power bank cannot negotiate a “contract” for 9V, 12V, or 15V at 1.5A or greater, the board’s USB-PD controller will fall back to 5V and provide power through the buck-boost converter (Configuration 2).
TX operation using a USB-PD source is the most stressful condition for the power switching MOSFETs, protection diodes, and copper traces on this small board. 10W CW and SSB QSOs with SOTA-style operation are not typically thermally-limited, but digital modes at high power can present a problem.
Configuration 2
Powers RX: Yes (suggest backlight off)
Powers TX: No
Battery Life (25% TX @ 10W, 75% RX): approximately 6 hours w/ internal battery
In this configuration, a 5V USB power bank supplies a regulated 12.3V output to the Elecraft KX2. The draw on the 5V power bank is limited at approximately 600mA, so the USB power bank is insufficient for KX2 transmit operation (but will reduce the energy used from the internal battery during TX). Under transmit the board output will droop due to the input current limit and the internal battery will instantly supply the KX2 the required power for transmitting. The internal battery is therefore only drained during TX and operating time is significantly extended.
This configuration can be thermally stressful on the buck-boost converter if the radio is transmitting often (e.g. a SOTA activation). The board may thermally limit on a hot day, so Configuration 2 is only recommended for RX operation with the occasional transmission.
Adding a supercapacitor on the 12.3V output of the buck-boost converter enables low duty cycle TX operation. However, this high continuous power is thermally stressful and board temperature should be carefully monitored.
Configuration 3
Powers RX: Yes
Powers Tx: Yes (though TX/RX duty cycle may be limited)
Battery Life: unlimited, if the sun is shining
The majority of testing with this board has occurred with this solar panel and a homebrew supercapacitor bank built with 5x 2.3V/100F capacitors: Renogy 10W 12V Portable Solar Panel Battery Maintainer Trickle Charger with Lighter Plug, Alligator Clips, and Battery Cables. The solar panel’s maximum rated power should significantly exceed the average power consumption of the radio in order to account for non-optimal solar panel orientation and buck-boost inefficiency. The buck-boost regulator switches between CC/CV modes depending on the supercapacitor voltage, but always respects the ~600mA input current limit.
Note that the 5x 2.3V/100F supercapacitor bank takes a few minutes to charge in full sunlight with the Renogy 10W panel. During the initial charging event, the buck-boost converter operates in constant-current mode (or limits the solar panel to 18V if the panel cannot produce enough power to reach the buck-boost converter’s output current limit). The initial charging event is thermally stressful on the buck-boost converter, due to the long duration and constant operation at the maximum power of the buck-boost converter. The buck-boost converter may shut down and restart repeatedly if its internal die temperature is too high. Back-to-back SOTA-style CW contacts at 10W are possible in direct sunlight.
The amber colored LED turns on when the buck-boost output is not within 5% of its CV regulation threshold. This gives a visual indication of when the buck-boost is working to charge the supercap.
The following oscilloscope trace shows the supercapacitor voltage during a simulated solar-powered QSO. A 18V/500mA limited power supply simulates a solar panel, and a programmable load draws 1.7A/180mA TX/RX current from the supercapacitor (corresponding to 5W CW TX and standby RX current). The programmable load’s TX/RX current toggles with Morse code at approximately 10-15WPM. The trace is annotated with the SOTA contact messages controlling the load. 10W TX is possible with more time between each QSO.
Other Configuration Ideas
The 12.3V regulated output can act as a CC/CV charger for an external 3S lithium polymer (LiPo) battery. The LiPo battery should implement its own internal balancing and temperature protection, and the user should understand all risks associated with batteries. Also, be mindful that the buck-boost may thermally limit if operated at its CC limit for extended periods of time.
The 18V solar input regulation point can be modified to support other panels.
The 18V solar input regulation feature can be deleted to turn the input connector into a general purpose DC input with a 3-24V input range. (Jumper across R9 to remove the input regulation feature. ) These power sources could be used for an exciting SOTA or POTA activation. Add a supercapacitor bank if the buck-boost output current is not sufficient for continuous TX operation.
Alkaline batteries (e.g. 9V or AAA in series). A regulated 12.3V gives more output power for radios like the QCX-mini
RC plane 2s-4s LiPo. Many radios will work directly with these voltage ranges, but the buck-boost will provide a consistent output voltage to the radio.
Your HT’s battery
Extra batteries from your head lamp (not recommended if you still might need the head lamp on your trip!)
Power tool battery
Car battery or 12V accessory outlet
Power harvested from your neighbor’s QRO operation on field day
Bicycle dynamo
Thermoelectric generator (on top of camp stove, or sandwiched between snow and a human)
The 12.3V regulated output can be modified to support radios other than the Elecraft KX2. The QCX-Mini (verified) or MTR-3b (not verified, but should work) can be fully powered from a solar panel. No supercapacitor needed.
The buck-boost converter can be disabled, and the USB-PD voltage can be set to 5V only. This allows operation with radios accepting 5V, such as some HTs. (Desoldering inductor L2 will ensure that the buck-boost cannot produce its 12.3V output.)
The 9V, 12V, and 15V USB-PD voltages can be modified. The >1.5A USB-PD request can also be modified to ensure the expected power draw does not exceed the negotiation.
Multiple boards’ outputs can be connected in parallel. For example, one board can accept 5V from a power bank, while the other board is connected to a solar panel. Or multiple solar panels and boards might be useful in a moving vehicle, where both solar panels may not have optimal sunlight.
The USB-PD source will not be damaged if the board output is higher than the USB-PD voltage, due to an internal protection diode (D2). Note that the USB-PD source will still effectively “see” a short circuit on its output if there is a low impedance source on the output of the board, and this may damage the board or power bank.
Specs, Capability, Limitations
Specs (Buck-Boost)
Maximum Solar Input: 27.5V
Minimum Solar Input (reverse polarity): -40V
Maximum USB VBUS: 20V
Maximum Output Voltage: 16V
Maximum Output Current: see curves
Maximum Continuous Output Current @ 5V input: 160mA (corresponds to KX2 with backlight off, RX mode)
Ambient Temperature: -20C to +50C
Tested thermal Capability (USB-C pathway)
Digital operation (50% TX/RX, 15 second intervals): limit to 1.75A current draw (approx 5W TX on KX2)
CW operation (typical SOTA QSO with 10 seconds between contacts): 2.25A current draw (approx 10W TX on KX2)
SSB operation (typical SOTA QSO): TBD, 10W TX on KX2 has not caused issues.
Continuous: TBD
Limitations
When connected to a USB-PD source, do not unplug input power while transferring significant power (e.g. transmitting). Unplugging under load may exceed the Safe Operating Area (SOA) of the power switching MOSFETs on the board. Unplug the board from the radio before unplugging the power input to the board.
When connecting to a USB-PD source, ensure that there is no short circuit on the output of the board. The USB-PD power bank may have a current limit greater than the capability of the board, causing power switching MOSFETs to overheat and become permanently damaged. A discharged capacitor looks like a short circuit, as well.
Be careful when hot plugging a connector into the solar input. The maximum solar input rating is a firm limit, and oscillation due to hot-plugging through long cables can generate significant overshoot. Consider adding a TVS diode or damping network if it is not possible to apply a gentle voltage ramp.
When the buck-boost converter runs at its maximum output voltage and output current for multiple minutes, an overtemperature protection feature causes the buck-boost converter to shut down every ~250ms. The board must cool down and cycle input power in order to stop the repetitive overtemperature cycling. Removing load from the output is not sufficient. Using a 5V USB source to charge a supercapacitor bank or battery is not recommended.
The output of the board is lower than the USB-PD voltage (due to the forward voltage drop of a Schottky diode). There is an additional diode inside the KX2, so the measured input voltage tends towards 8.5V when using a 9V USB-PD source. There appears to be an issue with reliably tuning the KX2 ATU at low input voltages that are technically within spec.
Mechanical Packaging
This design is tiny and delicate! Usage in the field requires care. Consider adding heatshrink tubing for a basic level of protection against moisture or dirt, and route wires through slots for strain relief.
A right-angle 0.1" header can mate with a common RC plane JST connector.
Further Reading on USB-C
USB-C and USB-PD
USB Power Delivery (USB-PD) is a distinct concept from the physical USB-C connector. After a hardware negotiation on the USB-C connector’s “CC” lines, USB-PD capable sources are capable of providing higher voltages and currents than available with legacy 5V bus voltage.
The following references may be helpful in understanding USB-PD:
Please note that this design has no formal certifications whatsoever. The board’s design and testing show no major issues functioning properly in the USB ecosystem.
The USB specification does not technically permit the same device to implement both USB-PD and proprietary high current protocols. This board implements only USB-PD, so special protocols (e.g. by Qualcomm or Apple) are not supported.
USB A-to-C Cables
USB Type A-to-C cables (example) need special attention and a brief USB history.
USB-A is what most people consider the “original” USB plug/receptacle, and typically provides 5V and at least 500mA on modern devices. Some USB-A receptacles are capable of providing higher voltages and currents than 5V/500mA, and this negotiation for increased power occurs on the USB D+/D- lines. This is different from the more modern USB-PD negotiation that occurs on the “CC” lines in the USB-C connector.
In order to allow a legacy USB-A receptacle (which may not be capable of providing >500mA, and does not contain “CC” lines) to provide power to a device with a USB-C connector (which is 3A capable as of USB-C version 1.2), USB A-to-C cables must handle this capability mismatch. Otherwise, a USB-C device might draw 3A from a USB-A receptacle only capable of supplying 500mA. The cable itself signals a 500mA capability to the USB-C device through a pair of 56kOhm resistors on the CC lines.
The USB-C specification is clear on how legacy A-to-C cables shall be implemented. See table 4-24: Universal Serial Bus Type-C Cable and Connector Specification. Non-compliant cables are commonly produced and are often branded as supporting “fast charging.” These non-compliant cables present a 10kOhm resistor (rather than the correct 56kOhm resistor) to signal to a sink that the source is capable of providing 3.0A at 5V. Such cables are dangerous because they can be plugged into USB-A receptacles and cabling that are only capable of 500mA.
This design will not produce power with a noncompliant 10kOhm A-to-C cable. The buck-boost converter has an input current limit and should always consume <600mA from a 5V source, but it is always the user’s responsibility to understand the USB-A source’s behavior. Carefully plan the current drawn from the 12.3V output, if you are concerned about asking 600mA from your USB-A source.
The following USB A-to-C cables bear no endorsement but are confirmed to work with this board at the time of writing. Cables marketed as “5A” or with purple plastic plugs are almost certainly noncompliant with the USB A-to-C cable specification.
The USB A-to-C cables from the NanoVNA or TinySA devices do not work, due to missing 56kOhm resistors internally. Some Micro USB to USB-C adaptors are OK.