On-Board Charger OBC

Recall that On-Board Charger (OBC) of a Battery Electric Vehicle (BEV) converts AC to DC.

There are two types of OBC: fan-cooled and water-cooled:

Outside OBC, there are three ports: DC(out), AC(in) and Signal.

The inside of the OBJ may look like the picture shown below. You can see many capacitors and inductors.

The PCBs may be organised as: Filter board, Power Supply Board, Control Board, and Driver Board.

The relationship between the boards is shown in the diagram below:

1) Filter Board

Filter board (also known as EMI filter board) filters out 2 types of EMI noise: Differential & Common Mode Noise.

At the AC input, we can experience 2 Types of EMI (Electromagnetic interference) noise: Differential Mode (DM) & Common Mode (CM) Noise. DM noise is conducted on the line and neutral, in opposite directions. The basic DM filter uses a single-winding inductor inserted into the line path, along with a capacitor from line to neutral, thus blocking noise from propagating through the system. CM noise is noise conducted on both the line and neutral (ground) but in the same direction. The basic CM filter uses a dual-winding inductor in both line and neutral paths, plus a capacitor from line to ground.

A typical EMI Filter Circuit may look like the diagram shown below. You can see inductor 'L' and many capacitors 'C'.

2) Power Supply Board

The power supply board consists of two parts: AC-DC and DC-DC.

The first part AC-DC is to convert AC to DC.

We often call it a rectifier. Rectifier can be half or full wave rectifier. A Full wave rectifier is often used in BEV's OBC. A capacitor is often added to ‘smoothen’ the DC ripple output.

Beside AC-DC conversion, OBC’s Rectifier also includes the function of PFC (Power Factor Correction).

PF (Power Factor) = Ratio of Real (useful) Power (in Watts W), to Apparent (total) Power (in Volt*Amps VA).

It is a measure of how efficiently electrical power is being used (converted into useful work output). PF range from 0 to 1. Anything less than one means that extra power is required to achieve the actual task at hand.

What causes low PF?

#1 Voltage & Current not in phase. When using AC power, the Voltage & Current may not always be in phase. If the load is resistive, Voltage & Current is in phase. But if the load is inductive/capacitive (the circuit include inductors/capacitors/transformer), then Voltage & Current may not be in phase.

#2 ‘Peaky’ Current. Non-linear load (eg: rectifier) sometimes draw ‘peaky’ current.

How does PFC work?

PFC can be passive or active. PFC in OBC is mostly active. The diagram below show an example of an active PFC. By changing the time for the Gate S1 to stay Open, and the time taken to Open/Close the Gate S1, we can change the current, to be in shape/phase as the voltage.

The second part DC-DC is to convert DC to DC (step up or down).

In OBC, DC-DC is usually an isolated full bridge DC-DC converter.

How does DC-DC work?

Microcontroller MCU (or Pulse-Width Modulation PWM Controller) senses Vout, Iout from the AC-DC. MCU then compares Vout with Vref. The difference in value is known as error. The error is used to adjust the duty cycle of the gate (switch), thus affecting their turn-on and turn-off. The end-result is to maintain a constant Vout (regardless of the load current or variations in the supply voltage).

After getting a constant Vout, the later part of the DC-DC circuit is to 'step up' or 'step down'. It is mostly 'step down' for BEV's OBC.

3) Control Board and Driver Board

The control board consists of microcontroller (MCU). MCU is also a pulse-width modulation (PWM) controller. It acts like a 'brain' and sends PWM signals to the driver board.

The driver board is also known as the gate (switch) driver board. It acts as an interface between the MCU (on control board) and the gates/switches (on the power supply board). The driver board will 'drive' the gates/switches to open or to close.

I hope you've learned something about OBC. Don't worry about the complicated circuitry. Just remember the diagram shown below and you will be able to understand the basic function of the OBC. In fact, once you understand OBC, you'll understand other components such as MCU and DC-DC converter. This is because they share the same functional diagram shown below. The only difference is on the power supply board, whether it is DC-AC (for MCU) or DC-DC (for DC-DC converter).