Method · Diagnostic

Track down a missing voltage rail
step by step

April 26, 2026 Level: Technician Power Delivery · Multimeter · Schematic

When voltage disappears

A missing voltage rail signals a failure in the power delivery network. The rail may show 0V, intermediate voltage, or oscillation instead of its nominal value. Before replacing ICs, establish where the chain breaks: supply, controller, switching stage, or load circuit.

Typical missing-rail scenarios: TPS51125 or ISL6259 PWM controller not oscillating, MOSFET gate drive absent, output inductor shorted, or feedback divider open. Each requires a different repair strategy.

Do not blindly apply voltage at the output node. Current-limiting 5V supply only. Test for shorts before powering the rail.

Step 1: Verify source rail
integrity

All voltage controllers need input power. Measure the supply rail (typically 12V, 3.3V, or 5V) feeding the PWM IC and gate driver.

Multimeter approach

  • Set DMM to DC voltage mode, range 20V.
  • Probe the PWM controller VIN and VREF pins simultaneously against ground.
  • Record exact voltage: should match schematic spec (±5% tolerance normal).
  • If supply voltage is low or absent, investigate the upstream regulator or connector before proceeding.

If source voltage is present and within spec, move to the controller output.

Use a second DMM or scope channel to monitor supply ripple while probing controller pins. Excessive ripple (>200mV) can cause controller lockup.

Step 2: Check PWM oscillation
and boot signal

The PWM IC must toggle at a stable frequency. Controllers like TPS51125, ISL6259, and LP8550 generate a clock output or drive gate signals directly. A silent controller outputs no drive signal.

Oscilloscope diagnosis (1 MHz–2 MHz typical)

  • Set CH1 to PWM output or gate-drive pin (check schematic for pinout).
  • Trigger on rising edge, 1 V/div scale, 1 µs/div timebase.
  • Expected waveform: 0V to 3.3V or 5V squarewave, duty cycle 20–80%.
  • If flat 0V or rail voltage: controller not running.
  • If sawtooth <100mV: oscillator may run, but output driver dead.

PWM disabled? Check EN pins and PGOOD feedback

  • Locate ENABLE or EN_L (active-low enable) pin on PWM IC datasheet.
  • Measure voltage: should be high (near VIN) for enabled, low (<0.8V) for disabled.
  • Check PGOOD or FAULT feedback loops—if asserted (low), controller shuts down the rail to protect downstream load.
  • Trace the enable net back to the main control MCU or enable source. An external latch can hold the rail off if a prior fault was logged.
If oscillation exists but gate drive is weak or absent, the driver stage IC may be faulty. Measure the gate pin voltage at the MOSFET—should see sharp 0V–3.3V transitions at the PWM frequency.

Step 3: Probe gate and switching
node

Once you confirm PWM oscillation, measure the high-side and low-side MOSFET gate signals. If present, check the switching node (between the high and low FETs) for activity.

Gate voltage measurement

  • High-side gate (relative to source, typically 3.3V–5V above source node).
  • Low-side gate (relative to ground, typically 0V–3.3V).
  • Use 1 M input impedance probe to avoid loading the gate charge pump.

Switching node diagnosis

The switching node (output of the high/low FET totem pole) should show a high-frequency triangle or stepped waveform at the input voltage and ground, with rise/fall transitions.

Observation Likely Root Cause Next Step
Switching node stuck at VIN Low-side FET shorted or high-side gate missing Check MOSFET gate drive continuity; measure low-side source/drain with ohmmeter
Switching node stuck at GND High-side FET shorted or low-side gate missing Verify high-side bootstrap voltage; check low-side gate driver
Switching node ringing >0.5V overshoot Inductor ESR low; parasitic oscillation Inductor and output capacitor OK; continue to output filtering
No switching node activity Both FET drivers dead or boot supply failed Inspect gate driver IC; measure bootstrap capacitor voltage

Step 4: Measure output filter
and rails

The output filter (inductor and capacitors) smooths the switching node into a DC rail. If the switching node toggles but output is dead, the filter stage has failed.

Output inductor test (DC resistance)

  • Power off. Set DMM to ohms, 200 Ω range.
  • Measure between inductor input and output. Typical value: 0.5–10 mΩ for modern DC-DC inductors.
  • If open (>10 Ω) or shorted (<0.1 mΩ): replace inductor.
  • If reading correct but output voltage absent, inductor may be internally shorted. Measure voltage across inductor with multimeter at 20V DC range: should see V_IN − V_OUT during low-side FET on, then V_OUT during high-side FET on.

Output capacitor ESR and leakage

  • Bulk capacitor (typically 100–330 µF ceramic or electrolytic): measure DCR with ohmmeter (0.01–0.1 Ω expected).
  • If ESR >1 Ω or capacitor reads open: replace.
  • Check for leakage: if capacitor charged to target voltage with power off, touching probes should not cause discharge in <1 second.

Final output rail voltage

With switching node and filter confirmed good, measure the final output node (after inductor and filters). Should read nominal voltage per schematic.

If output sits at ~50% of nominal (e.g., 2.5V instead of 5V), the feedback divider may be open or the sense node disconnected from the PWM control loop.

Step 5: Inspect feedback divider
and compensation

The voltage regulator compares the output sense signal to an internal reference (typically 0.6V or 0.8V for most PWM ICs). If the sense net is open or the divider resistor is burned, the controller cannot stabilize the output.

Feedback divider network

  • Locate the VOUT_SENSE or FB pin on the PWM IC.
  • Trace back to the output node: you should find two resistors in series (voltage divider) connected between VOUT and GND, with the FB tap at the midpoint.
  • Measure DC resistance of each resistor (power off): typical values 10 kΩ–100 kΩ. If either reads open or shorted, replace immediately.
  • Measure voltage at the FB pin with power on: should read 0.5–0.9V (the internal reference). If FB is at GND or rail voltage, the divider is disconnected or shorted.

Compensation network

Most regulators include a small capacitor and resistor between the error amp output and the frequency compensation network. If these burn or disconnect, the loop becomes unstable.

  • Locate the COMP pin or compensation node (check IC datasheet).
  • Measure DC voltage: typically 0.5–4V, depending on load and control loop.
  • Inspect nearby resistors and capacitors for blackening, cracking, or disconnection. Replace if damaged.
If the output voltage is present and stable but off by a fixed percentage, the sense resistors may have drifted due to thermal aging. Measure their exact values and calculate the expected output voltage manually.

Diagnostic checklist

Use this sequence to isolate the fault node systematically:

  1. Source voltage present? Measure VIN at PWM IC. If low or absent, repair upstream supply.
  2. PWM oscillating? Probe gate drive or clock output with scope. If flat, check EN pin and PGOOD feedback.
  3. Gate drive present? Measure high-side and low-side gate pins. If one is missing, gate driver IC or bootstrap circuit has failed.
  4. Switching node toggling? Probe between FET drains. If stuck at rail or GND, a MOSFET or driver is shorted.
  5. Output filter intact? Measure inductor DCR and capacitor ESR. If either is out of spec, replace.
  6. Output voltage present after filter? If yes, continue. If no, inductor may be shorted internally.
  7. Feedback divider connected? Measure FB pin voltage. If at GND or rail, divider is open or shorted. Replace resistors.

Most missing-rail faults resolve by step 4 or 5. If the output sits at intermediate voltage or oscillates, focus on the feedback and compensation networks—these control the final regulation accuracy.

Never leave a disabled rail unpowered without identifying the root cause. Many systems have cross-rail dependencies; a missing secondary rail can prevent the main supply from turning on due to power sequencing logic.