General Assist Technical Reference

LATT 3 Phase Inverter PCB Check

Main relay, DC supply path, capacitor board and heatsink thermistor checks
This reference explains the main DC power path through the LATT 3 phase inverter board and identifies the key areas to check when diagnosing inverter or compressor drive faults.
Electrical safety: The inverter circuit contains hazardous high-voltage DC. Testing must only be carried out by appropriately qualified personnel using suitable test equipment and safe electrical work practices. Stored DC voltage may remain after power is isolated.

1. Inverter board overview

The LATT 3 phase inverter board receives DC power after power factor correction and distributes this supply through the main relay and capacitor board to the inverter power module.

AreaFunction
PFC outputProvides corrected DC positive supply to the main relay.
Main relaySwitches the DC positive supply toward the DC P / capacitor board when compressor operation is commanded.
Capacitor boardForms part of the DC link supplying P and N to the IPM.
IPMUses the P and N DC supply to generate the compressor inverter output.
Heatsink thermistorMonitors the heatsink temperature from the heatsink plate behind the inverter PCB.

2. DC power path

Supply sequence:
PFC DC positive output → Main relay → DC P / capacitor board → P and N DC supply → IPM → compressor inverter output.

The main relay is supplied with DC positive from the PFC circuit after power factor correction. The relay output then feeds the DC P connection to the capacitor board.

3. Main relay operation

The main relay is switched to the ON position when the compressor cycles on. This allows the corrected DC positive supply to pass toward the capacitor board.

Diagnostic point: DC test points are provided at the main relay so the technician can confirm whether the expected DC supply is present on the relay circuit.

4. IPM supply

The IPM is supplied from the capacitor board through the P and N DC connections. This DC supply is then switched by the inverter stage to drive the compressor.

CheckWhat it confirms
DC present at main relay inputPFC stage is supplying the relay circuit.
DC present at relay output when compressor is commandedMain relay has switched and is passing the DC positive supply.
P and N supply present at IPMDC link is reaching the inverter power module through the capacitor board.
Important: The supplied training material identifies the test locations and circuit path but does not specify a test voltage. Use the applicable service manual for the exact model when confirming expected voltage values.

5. Heatsink thermistor

The heatsink thermistor is installed on the heatsink plate behind the inverter PCB. When investigating inverter temperature or protection-related faults, include the thermistor, its connection and the heatsink condition in the inspection.

6. Suggested check sequence

1Confirm the inverter PCB and surrounding components have no obvious signs of damage, overheating or contamination.
2Confirm the PFC circuit is providing DC positive supply to the main relay.
3With a compressor demand present, confirm the main relay switches to the ON position.
4Use the main relay DC test points to confirm supply through the relay circuit.
5Confirm the capacitor board is transferring the DC link supply to P and N at the IPM.
6Inspect the heatsink thermistor and its connection if temperature-related inverter protection is suspected.

7. Key points

  • The main relay receives DC positive from the PFC circuit after power factor correction.
  • The relay output supplies DC P to the capacitor board.
  • The main relay switches on when the compressor cycles on.
  • The IPM receives its DC supply through P and N from the capacitor board.
  • The heatsink thermistor is fitted to the heatsink plate behind the inverter PCB.
General Assist technical reference – LATT 3 Phase Inverter PCB.