PPBUS_G3H
Primary 5V supply in G3 mechanical off

What it is

PPBUS_G3H is the primary 5V power rail on Apple board-level power distribution, active during G3 mechanical off state (device disconnected or in hard shutdown). The _G3H suffix denotes "G3 host" — always-on power that persists when AC is present but the device is powered down.

This rail supplies core logic subsystems including DRAM, storage controllers, SMC (System Management Controller), and low-level reset logic. Unlike PP5V_S5 (which requires S5 soft-off state to exist), PPBUS_G3H is independent of power sequencing and remains available as long as the PSU can deliver current and the main power FET is not disabled by firmware.

Typical current draw ranges from 80 mA to 400 mA depending on board revision and peripheral attachment. On boards with aggressive sleep states, PPBUS_G3H may be the last rail to drop before complete power removal.

In practice

Technicians encounter PPBUS_G3H faults in three primary scenarios:

• No-power boards: Device shows no signs of life. Multimeter reads 0V on PPBUS_G3H at expected test points (commonly near SMC or DRAM power pins). Indicates failed PSU, main FET, or catastrophic short on this rail.
• Partial boot: LEDs flicker, fans briefly spin, then shutdown. PPBUS_G3H voltage drops from nominal 5.0V to 2.8V within 200 ms. Suggests excessive inrush current or a shorted component draining the rail during power-up.
• Sleep/wake failure: Board powers on but SMC does not respond to wake signals. PPBUS_G3H may be present but SMC is not executing firmware. This indicates either a cold solder joint on SMC power pins or a logic-level fault on the SMC itself.

To diagnose PPBUS_G3H, probe with the device powered off and AC connected. Use a digital multimeter in voltage mode; expected reading is 4.8V–5.2V with less than 50 mV ripple. If ripple exceeds 200 mV, suspect a failed decoupling capacitor or EMI filter.

How to measure

Test point selection

PPBUS_G3H is typically available at:

• SMC power pins (usually VDD_SMCPWR or similar labeling)
• DRAM power rail test point (sometimes labeled TP_5V or TP_PPBUS)
• Any large decoupling capacitor anode connected to 5V (far end from ground)
• The source of the main 5V switching regulator

Proper technique

Use a quality multimeter with at least 10 MΩ input impedance. Set to DC voltage mode. Place red probe on the rail, black on chassis ground or a known ground plane. Record the steady-state voltage after 5 seconds. For ripple measurement, use an oscilloscope set to 100 mV/div, AC coupling, 1 MHz bandwidth limit enabled to reject switching noise. Peak-to-peak ripple should be under 100 mV under typical load.

Common failure modes

PSU output FET failure

The main switching FET that gates PPBUS_G3H fails shorted or open. If shorted, voltage remains high but current becomes uncontrolled, triggering over-current shutdown. If open, voltage drops to zero immediately after PSU attempts to power the board.

Bulk capacitor ESR rise or failure

Large electrolytic capacitors on PPBUS_G3H lose capacitance or develop high ESR (equivalent series resistance). Result: excessive ripple (200–500 mV) and slow voltage recovery during load transients. Causes intermittent reset of SMC and memory corruption.

Short under decoupling network

A shorted capacitor, trace nick, or failed IC power pin creates a DC short path to ground. PPBUS_G3H collapses to 0V–1V. Use DC injection or diode mode to locate the short; measure resistance from the rail to ground (should be >1 kΩ).

SMC firmware halt with rail present

PPBUS_G3H is present and stable at 5.0V, but SMC does not boot. Usually indicates a cold solder joint on an SMC power pin, failed EEPROM connection, or a logic-level fault in the SMC itself (requires reflowing or chip replacement).

See also

Related terms and power rails in this glossary:

• PP3V3_S3 — 3.3 V S3 suspend rail — Secondary 5V rail active during S5 soft-off