How do I test an inverter AC outdoor PCB IPM module and IGBTs using a digital multimeter?
To test an IPM (Intelligent Power Module) and its internal IGBTs on an outdoor inverter AC PCB, you perform static Diode Tests using a digital multimeter.
Inside an IPM, six internal IGBT switches form a 3-phase inverter bridge connected to the compressor terminals (U, V, W). Each IGBT has an anti-parallel freewheeling diode built across its Collector and Emitter. Testing these body diodes measures whether the internal IGBT junctions are intact, shorted, or open circuit.
Step 1: Safety & Preparation (Power Off)
- Disconnect Power: Cut main electrical power to the outdoor unit.
- Discharge DC Bus Capacitors: Wait at least 5 to 10 minutes. Set your multimeter to DC Volts and measure across the large electrolytic DC filter capacitors (or between the P/+ and N/- terminals of the IPM). Ensure the voltage drops below 5V DC before touching the board.
- Unplug Compressor Wires: Disconnect the three compressor wires (U, V, W) from the PCB harness headers to eliminate motor winding resistance from your readings.
- Set Meter Mode: Switch your digital multimeter to Diode Mode (→∣− ).
Step 2: Identify the IPM Test Terminals
Locate the following main terminals on the IPM chip pins or PCB traces:
- P (or DC+): Positive DC bus rail terminal.
- N (or DC− / GND): Negative DC bus rail terminal.
- U, V, W: The 3-phase output terminals going to the compressor.
Step 3: Perform the Diode Test Matrix
1
Test the Upper Arm (Positive Rail P to U, V, W)
Place the RED probe (+) on the P terminal and touch the BLACK probe (-) sequentially to U, then V, then W.
- Expected Result: Open Circuit (OL / infinite resistance).
Reverse the probes: Place the BLACK probe (-) on the P terminal and touch the RED probe (+) to U, then V, then W.
- Expected Result: Standard diode forward voltage drop between 0.4V to 0.7V DC across all three legs. Readings must be balanced within ±0.05V.
2
Test the Lower Arm (Negative Rail N to U, V, W)
Place the BLACK probe (-) on the N terminal and touch the RED probe (+) sequentially to U, then V, then W.
- Expected Result: Open Circuit (OL / infinite resistance).
Reverse the probes: Place the RED probe (+) on the N terminal and touch the BLACK probe (-) to U, then V, then W.
- Expected Result: Standard diode forward voltage drop between 0.4V to 0.7V DC across all three legs, fully balanced.
3
Test Output Phase-to-Phase (U-V, V-W, W-U)
Measure between phase outputs directly: U-V, V-W, and W-U in both probe directions.
- Expected Result: Open Circuit (OL) in both directions for all combinations.
Summary Reference Table
| Red Probe (+) | Black Probe (-) | Expected Voltage Drop | Fault Condition |
|---|---|---|---|
| P | U / V / W | OL (Open) | If 0.00V, upper IGBT is shorted. |
| U / V / W | P | 0.4V – 0.7V DC | If OL, diode/IGBT junction is open. |
| U / V / W | N | OL (Open) | If 0.00V, lower IGBT is shorted. |
| N | U / V / W | 0.4V – 0.7V DC | If unbalance >0.05V, module is degraded. |
| U | V / W | OL (Open) | Any continuity (0.00V) indicates internal failure. |
Diagnostic Verdicts
- Pass: All six forward-bias readings fall within 0.4V to 0.7V DC, all reverse readings show OL, and output phases show no short circuits.
- Fail (Shorted IPM): Any reading showing 0.00V (beeping continuity) indicates a blown, shorted IGBT. This will immediately trip the main circuit breaker or blow the outdoor PCB fuse upon startup.
- Fail (Open / Degraded IPM): Any reading showing OL in both directions or showing an imbalance (e.g., U=0.45V, V=0.45V, but W=0.21V) means an internal gate driver or silicon junction has broken down.