General Assist Technical Reference

LATT 3 Phase Circuit Diagram

Outdoor unit power flow and PCB circuit relationship
Technical reference for Filter PCB, PFC/Active Filter PCB, Transistor PCB, Capacitor Board and compressor power circuits
The LATT three-phase outdoor unit receives approximately 400–415 VAC mains power. The supply passes through the Filter PCB and PFC/Active Filter PCB before being converted to a DC bus used by the inverter circuit and compressor.

1. Circuit Overview

The LATT outdoor unit separates the mains power, DC conversion, inverter and control circuits across several PCBs.

Circuit SectionFunction
AC Power SupplyProvides approximately 400–415 VAC three-phase power to the outdoor unit.
Filter PCBReceives the incoming three-phase mains supply and feeds the downstream power circuit.
PFC / Active Filter PCB / Diode BridgeConverts the AC input to approximately 560–600 VDC between the positive and negative DC bus.
Capacitor BoardSmooths the DC output voltage.
Transistor PCB / IPMUses the DC bus to generate the controlled output to the compressor.
Main PCBProvides control functions and communicates with the Transistor PCB.

2. Main Power Flow

Three-Phase AC Path
AC Power Supply
↓
Filter PCB
↓
PFC / Active Filter PCB & Diode Bridge
↓
DC Bus
↓
Transistor PCB / IPM
↓
Compressor

AC Input

The incoming mains supply to the Filter PCB is approximately 400–415 VAC.

The three supply phases are shown as:

  • L1
  • L2
  • L3

DC Conversion

The PFC / Active Filter PCB and diode bridge convert the incoming AC power to high-voltage DC.

Expected DC Bus
560–600 VDC
Measured between + and N
High-voltage warning: The DC bus operates at approximately 560–600 VDC. Capacitors may retain hazardous voltage after mains power has been disconnected.

3. PCB and Circuit Functions

Filter PCB

The Filter PCB receives the three-phase mains supply and feeds it to the PFC / Active Filter circuit.

PFC / Active Filter PCB and Diode Bridge

The PFC circuit converts the approximately 400–415 VAC mains supply into approximately 560–600 VDC.

The diagram identifies the positive DC output as + and the negative reference as N.

Reactors

Three reactors are installed in the power circuit between the PFC / Active Filter section and the Transistor PCB circuit.

Capacitor Board

The Capacitor Board is connected across the DC circuit and smooths the DC output voltage.

The capacitor board is part of the high-voltage DC circuit. Do not assume the circuit is discharged immediately after isolating power.

Transistor PCB / IPM

The Transistor PCB contains the IPM and controls the power delivered to the DC compressor.

The diagram also identifies a Current Transformer (CT) circuit between the Transistor PCB and Filter PCB.

Main Relay Circuit

The main relay supply is shown as being supplied via the positive DC circuit and providing positive supply to the Capacitor Board.

Main PCB

The Main PCB is supplied separately from the main compressor inverter circuit.

Main PCB supply: approximately 330–370 VDCis created independently on the PCB to power:
  • Outdoor fans
  • Low-voltage thermistors

Control harnesses connect the PFC PCB and Transistor Board back to the Main PCB.

4. Compressor Circuit

The Transistor PCB / IPM provides the compressor output through the three inverter output terminals:

Transistor PCB OutputCompressor Terminal
UR
VS
WC
The compressor is an inverter-driven DC compressor. The U, V and W connections are controlled outputs from the IPM and should not be treated as a fixed-frequency mains three-phase supply.

5. Diagnostic Reference

1Confirm approximately 400–415 VAC three-phase mains supply is present at the input to the Filter PCB.
2Confirm the Filter PCB is passing the mains supply towards the PFC / Active Filter PCB.
3Check the PFC DC bus. The reference indicates approximately 560–600 VDC between + and N.
4Inspect the three reactors and their associated wiring and connections.
5Check the Capacitor Board connections and DC bus circuit.
6Check the control harnesses between the PFC PCB, Transistor PCB and Main PCB.
7If the DC bus is correct but the compressor is not being driven, continue diagnosis through the Transistor PCB / IPM and compressor circuit.

6. Quick Reference

CircuitReference
Incoming AC SupplyApproximately 400–415 VAC
PFC DC OutputApproximately 560–600 VDC between + and N
Main PCB Internal DC SupplyApproximately 330–370 VDC
Capacitor BoardSmooths DC output voltage
Transistor PCBIPM / compressor inverter control
Compressor OutputU / V / W from Transistor PCB
CT CircuitBetween Transistor PCB and Filter PCB
Electrical safety: This circuit contains approximately 400–415 VAC mains voltage and a high-voltage DC bus of approximately 560–600 VDC. Testing must only be undertaken by appropriately qualified personnel using suitable test equipment and safe electrical work practices. Confirm capacitor discharge before touching or disconnecting high-voltage components.