LATT 3 Phase Inverter PCB Check

Purpose
This check explains the DC power path through the LATT 3 Phase inverter board and identifies the main relay, capacitor board supply, IPM supply path and heatsink thermistor location.

How the inverter DC supply is routed

  1. PFC output: The main relay receives the positive DC supply from the PFC circuit after power factor correction.
  2. Main relay: The relay switches to the ON position when the compressor is commanded to run.
  3. Capacitor board: The main relay output supplies DC P to the capacitor board.
  4. IPM supply: The inverter IPM is supplied from the capacitor board through the P and N DC supply rails.
  5. Compressor output: The IPM then controls the inverter compressor from this DC supply.

Main relay check

Check pointWhat to confirm
Relay inputConfirm the positive DC supply from the PFC circuit is present at the main relay.
Relay operationWhen the compressor cycles ON, confirm that the main relay switches to the ON position.
Relay outputUse the designated DC test points at the main relay to confirm the output to the capacitor board.
Electrical safety
The inverter section contains high-voltage DC. Testing must only be carried out by suitably qualified service personnel using appropriate electrical safety procedures and test equipment.

IPM and capacitor board

The IPM receives its DC supply via the capacitor board through the P and N rails. When checking a no-compressor or inverter-related fault, confirm that the DC supply path is established through the following sequence:

PFC Circuit → Main Relay → Capacitor Board → P / N DC Supply → IPM → Inverter Compressor

Heatsink thermistor

A heatsink thermistor is fitted to the heatsink plate behind the inverter PCB. This should be included in the inspection when investigating inverter temperature or protection-related concerns.

Quick diagnostic sequence

  1. Confirm the PFC circuit is supplying the positive DC feed to the main relay.
  2. Command compressor operation and confirm the main relay changes to the ON position.
  3. Check the DC test points at the main relay and confirm the output continues to the capacitor board.
  4. Confirm the P and N DC supply path from the capacitor board to the IPM.
  5. Inspect the heatsink thermistor and its installation behind the inverter PCB where relevant to the fault being investigated.
Important: This information identifies the circuit path and test locations shown in the LATT 3 Phase inverter board training material. Exact terminal markings, expected readings and component layouts may vary by model; always refer to the applicable service documentation before testing.