Voltage regulation

Summary¶

A 6S LiPo varies from 25.2V (full) to 21.0V (minimum), and voltage fluctuates by 3–5V during aggressive flight as motor currents change. Every sensitive electronic component needs stable, clean voltage to function correctly. Two regulation approaches are used on libdrone: the BEC (Battery Eliminator Circuit) inside the flight controller provides regulated 5V for logic electronics using a linear topology; the XL4015 switching buck converter provides 9–12V for the video transmitter at much higher efficiency. Each has specific noise characteristics that must be managed.

Concept¶

Linear regulation (BEC/LDO)¶

A linear regulator passes current through a series transistor that drops the excess voltage as heat. Input 22V, output 5V: the transistor drops 17V. At 0.5A load, the heat dissipated is 17V × 0.5A = 8.5W — substantial.

The internal BEC in the H7A3-SLIM uses a linear architecture. Linear regulators have one major advantage: their output is inherently quiet. There is no switching, no oscillation, no harmonics. The output noise is low and broadband — easily filtered by small capacitors at the point of use. For sensitive digital logic (gyroscope, receiver, GPS) this clean supply is essential.

The H7A3-SLIM BEC is rated 5V / 1A continuous, 2A peak. The total load budget:

FC logic: ~200 mA ELRS receiver: ~150 mA GPS module: ~100 mA Buzzer: ~30 mA (intermittent) Fan: ~70 mA (always-on) GX12 payload: ~200 mA (typical SEN66 mast) ───────────────────────────── Total typical: ~750 mA (within 1A continuous rating) Peak: ~900 mA (within 2A peak rating)

Switching regulation (buck converter)¶

A buck converter (step-down switching regulator) rapidly switches a MOSFET on and off, storing energy in an inductor during the on phase and releasing it during the off phase. The output capacitor smooths the result. Output voltage is controlled by the duty cycle.

The XL4015 switching at 180 kHz achieves ~90% efficiency from 22V to 12V at 400 mA — dissipating ~0.5W versus ~4W for an equivalent LDO. This makes the switcher practical for the VTX supply where an LDO would need a heatsink and reduce system endurance.

The switching creates electromagnetic interference at 180 kHz and its harmonics. These harmonics propagate along the output wire and radiate from it as an antenna. Three to four clip-on ferrite beads (mix 31 or mix 43 material) on the output wire immediately at the converter output add approximately 250–500 Ω of impedance at 180 kHz, reducing harmonic propagation by 20–30 dB. Without ferrites, the 180 kHz switching noise can couple onto the GPS antenna feed line and degrade satellite reception.

Decoupling capacitors¶

Every integrated circuit has a power supply rejection ratio (PSRR) — the ability to reject noise on its supply voltage. For digital ICs, PSRR falls rapidly with frequency: good at DC, poor at MHz. A 100 nF ceramic capacitor directly on the IC's supply pin provides a local energy reservoir that supplies transient current demand locally (preventing voltage droops that propagate back to the supply) and bypasses high-frequency noise to ground before it enters the IC.

The placement rule is absolute: the bypass capacitor must be on the same PCB as the IC, as close to the supply pin as possible. A capacitor 10 mm away on a different board, connected by a wire, provides almost no high-frequency bypassing — the wire inductance dominates above a few MHz.

This is why the ESC's 1000 µF capacitor must be soldered directly to the ESC power pads. → See electronic-speed-controllers.

Reference¶

Regulation comparison¶

XL4015 operating parameters¶

BEC load limits and consequences of overload¶

If the total BEC load exceeds 1A continuous (or 2A peak), the H7A3-SLIM's internal BEC thermal protection activates — the 5V rail folds back to protect the IC. The immediate effect: the ELRS receiver drops connection (RC link lost), GPS stops, FC may reset. This is effectively a failsafe trigger. Heavy payloads requiring more than 200 mA from the GX12 5V rail should use a dedicated payload power converter from battery voltage rather than loading the BEC.

Procedure¶

Verifying power rail voltages¶

1. Before first flight: connect battery, allow full power-up sequence.
2. In Betaflight: verify battery voltage shown in OSD matches physical cell meter reading (within 0.1V).
3. Check 5V BEC: Betaflight Configurator → Power tab shows BEC output voltage if the FC has a 5V ADC channel. Alternatively, measure with multimeter at a known 5V pad (receiver power, GPS power).
4. Check VTX supply: VTX menu typically shows input voltage. Should read 9–12V stable during flight. If VTX shows lower voltage during full throttle, check XL4015 output capacitors.

Rationale¶

Why the VTX has its own converter rather than sharing the BEC¶

The HDZero Freestyle V2 VTX draws up to 600 mA at 800 mW output power. Adding 600 mA to the BEC load would bring total BEC consumption close to the 1A continuous limit. This leaves no margin for payload sensors, BEC thermal derating on hot days, or transient demand spikes. Isolating the VTX on its own dedicated converter also prevents VTX RF switching noise from coupling into the BEC output and reaching the receiver or GPS.

Why the VTX supply is set to 12V and not 9V¶

The HDZero VTX accepts 7–25V. At lower supply voltage, the VTX's internal power amplifier runs at lower efficiency for the same RF output power — dissipating more heat. At 12V, the internal amplifier operates in its optimal efficiency region for the 200–800 mW range used in typical field deployment. The efficiency difference is small (~5%) but relevant for thermal management during extended mapping missions.

Connections¶

requires: - lipo-batteries - power-rail-architecture related: - electronic-speed-controllers - emc-signal-integrity leads_to: - power-sequencing