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. On libdrone both regulated rails come from the flight controller: the Matek H7A3-SLIM carries a 5 V BEC (Battery Eliminator Circuit) for logic and peripherals and a separate, switchable 9 V BEC for the video transmitter, each rated 2 A continuous. No external converter is fitted. Each rail has noise characteristics that must be managed.

Concept¶

Linear regulation (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.

Linear regulators have one major advantage: their output is inherently quiet. There is no switching, no oscillation, no harmonics. That is why they are used for small, low-current rails close to sensitive ICs. They cannot be the main BEC on a 6S flight controller: at the H7A3-SLIM's 2 A rating the same 17 V drop would dissipate 34 W. A 2 A BEC fed from 6S is a switching regulator, and its output needs the switching-noise discipline below — capacitors at the point of use (→ capacitor-placement-emc) and separation from sensitive wiring.

The H7A3-SLIM 5 V BEC is rated 2 A continuous, 3 A maximum (Matek specification). 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)
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Total typical:   ~750 mA (within 2 A continuous rating)
Peak:            ~900 mA

Switching regulation (onboard switchable BEC)¶

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.

On the Cinematic build, this function is built into the Matek H7A3-SLIM itself: a 9 V / 2 A switchable BEC feeds the VTX directly. There is no separate XL4015 buck converter module on this build — that was the earlier Pro-generation design; it's dropped here because the FC already provides it. The efficiency argument is the same as any buck vs. LDO comparison (roughly 90% efficient switching vs. ~25% efficient linear at this voltage drop), and the EMC argument is the same too: the switching still generates noise at its switching frequency and harmonics, and it needs the same discipline — short VTX power wiring, star grounding, and physical separation from GPS and receiver wiring. See star-grounding.

Because the switchable BEC is onboard the FC, the only wire its noise can travel along is the VTX power lead. Ferrite beads on that lead, at the FC pad, are a remedy for high-frequency hash if a VTX-on/VTX-off test shows a problem — not a default fitting. See ferrite-beads.

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¶

Onboard switchable BEC operating parameters¶

BEC load limits and consequences of overload¶

If the total load exceeds the 5 V BEC's rating, the rail sags or the regulator protects itself. 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. Verify the battery voltage shown in the OSD or ground station (Betaflight or ArduPilot) matches a handheld meter at the XT60 (within 0.1 V).
3. Check the 5 V BEC with a multimeter at a known 5 V pad (receiver or GPS power).
4. Check the 9 V VTX rail with a multimeter at the 9 V pad before the VTX is ever plugged in: ~9 V with the rail switched on, never pack voltage.
5. With the VTX connected, its menu typically shows input voltage — it should hold near 9 V at full throttle. A sag points at the wiring or ground path, not the setting.

Rationale¶

Why the VTX has its own regulator rather than sharing the FC's logic BEC¶

The HDZero Freestyle V2 VTX draws up to 600 mA at 800 mW output power. Adding 600 mA to the 5V logic BEC load would bring total consumption close to its continuous limit, leaving no margin for payload sensors, thermal derating on hot days, or transient demand spikes. Using the FC's separate switchable 9V rail for the VTX keeps the two loads isolated — and prevents VTX RF switching noise from coupling into the 5V rail and reaching the receiver or GPS.

Why the external XL4015 buck was dropped¶

Earlier builds fed the VTX from a separate XL4015 buck module. The H7A3-SLIM's onboard, switchable 9 V BEC does the same job with one fewer module, one fewer wire run to keep away from the GPS, and FC control of VTX power.

Why the VTX supply runs at 9V¶

The HDZero VTX accepts 7–25V. At 9V the internal power amplifier operates in a reasonable efficiency region for the 200–800 mW range used in typical flight, without the extra step-up a higher voltage would otherwise need.

Connections¶

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