Design considerations for establishing reliable circuit protection in more electrified and automated vehicles.

figure 1. In a hybrid electric vehicle electrical architecture, the on-board charger must be matched to the AC power line and its possible overloads and transient overvoltages. (Littelfuse)

figure 2. On-board charger block diagram and recommended protection and power control devices. (Littelfuse)

image 3. TVS diode array for protection of CAN bus. (Littelfuse)

Designing circuits for electric vehicles is extremely challenging. To ensure a reliable and safe design that can withstand overloads, transients, and electrostatic discharge (ESD), designers need to ensure their circuits have the necessary components to prevent damage. Taking the on-board charger as an example, this paper presents recommendations for circuit protection and efficient power control.

As shown in the schematic diagram in Figure 1, HEVs represent the worst-case scenario for designers who must develop circuits that can withstand transients from internal combustion engines and high-power electric motors. In this environment, the on-board charger must match the AC power cord and the overloads and transients it can create.

Designers should protect on-board chargers as they would any line-powered product. They also wanted to protect the communication circuitry from data corruption, while minimizing internal power consumption so that the battery charge time was as short as possible.

The on-board charger converts the AC line voltage to the DC voltage required to charge the main battery pack, which ranges from 300 to 500 V when the battery is fully charged. Consumers expect EVs to charge faster, so higher-power charging circuits, including three-phase power sources, are required.

Figure 2 shows the block diagram of the on-board charger. In this example, a single-phase circuit is represented. Protection components are required for each circuit subframe, and power control components are required for both circuit subframes to optimize the efficiency of the charger.

go to transient protection

The input voltage section is susceptible to transients such as lightning strikes and surges on the AC line. The first type of line protection is a fuse that provides overload protection. Designers should consider fuses with high interrupting current ratings and high voltage ratings to ensure that the fuse opens under worst-case current overload conditions. To prevent surge transients or lightning strikes, designers should place metal oxide varistors (MOVs) as close as possible to the charger input connections. The MOV will absorb the transient energy and prevent it from damaging downstream circuits.

If the on-board charger uses three-phase power, designers should consider adding MOVs for phase-to-phase transient protection as well as phase-to-neutral transient protection. To better protect downstream circuits, designers can place semiconductor discharge tubes in series with MOVs. The clamping voltage of the semiconductor discharge tube is very low, about 5 V. The use of semiconductor discharge tubes allows designers to choose MOVs with lower operating voltages. The benefit is that it can reduce the peak transient voltage that the downstream circuit is subjected to momentarily.

A fourth protection element for advanced circuit protection is the gas discharge tube. Gas discharge tubes provide high resistance electrical isolation between the live and neutral conductors and the vehicle chassis ground. Gas discharge tubes provide a higher level of protection against fast rising transients caused by lightning disturbances.

IGBT protection and control

For fast, high-power charging, designers should choose rectifier thyristors with sufficient current handling capability to provide the necessary power. Thyristors can also safely absorb inrush current transients that may pass through the input voltage and EMI filter stages.

Power Factor Correction (PFC) circuits improve charging efficiency by reducing the total power drawn from the AC power line. Designers can use gate drivers and insulated gate bipolar transistors (IGBTs) to control the amount of inductance in a circuit. Designers should ensure that gate drivers with sufficient operating voltage range are selected to control the IGBTs. Designers should also consider choosing a gate driver with high noise immunity to lock in and fast rise and fall times to quickly switch the IGBT.

Fast rise and fall times combined with low supply current minimize power dissipation in this block. The gate driver should provide ESD protection. Designers should choose gate drivers with built-in ESD protection, or add an external ESD diode. ESD diodes can be bidirectional or unidirectional and can withstand ESD transients up to 30 kV.

Understanding Key Diodes

The DC link consists of a capacitor bank to stabilize the ripple generated by the high power DC/DC converter. Designers concerned about large voltage transients reaching the DC link can use high voltage TVS diodes to protect capacitor banks.

The DC/DC section boosts the output charging voltage and generates charging current for the battery. This circuit block requires a reliable gate driver similar to the PFC circuit block. If the gate driver selection does not include internal ESD protection, the designer can choose an ESD diode to protect the gate driver. Adding external ESD diodes does not degrade gate driver performance.

Designers should also ensure that their power IGBTs are protected from voltage transients. In addition to preventing external transients, IGBTs also generate turn-off switching transients due to the L·di/dt effect of the internal parasitic inductance (L is the inductance and di/dt is the current rate of change). To eliminate the potential damage to the IGBT from this transient, designers should place a TVS diode between the collector and gate of each IGBT.

TVS diodes reduce the di/dt of current transients by increasing the gate voltage. When the collector-emitter voltage exceeds the breakdown voltage of the TVS diode, current flows through the TVS diode into the gate to raise its potential. The TVS diode continues to conduct until the transient is eliminated.

Using a TVS diode as a collector-gate feedback element is called active clamping and keeps the IGBT in a stable state. For more information on Active Clamping, see Reference Application Note 1. Some IGBTs have built-in active clamp TVS diodes. Designers should choose this type of IGBT or add TVS diodes to their circuit.

Protect CAN bus signal

When the motor is switched on and off or the current is momentarily interrupted by a cable break, the output voltage side needs to be protected against current overload and voltage transients in the vehicle. Designers should consider using fuses to prevent overcurrent from shorting the battery pack or cables carrying the battery voltage. Use MOV or TVS diodes to prevent potentially damaging voltage transients.

The control unit of the charger communicates with the data network via the CAN bus. To avoid communication block and data corruption, designers should provide ESD and transient protection. They can use a single space-saving element for protection. Figure 3 shows a two-wire TVS diode array designed to protect CAN bus signal lines. Diode arrays designed to protect communication lines contain small capacitance and do not degrade the I/O state of the transmitter/receiver.

Designers who follow protection and power control recommendations will provide reliable and safe circuits for their company’s electric vehicle customers. When possible, designers should use qualified components that are AEC-Q certified for use in the automotive environment. For example, AEC-Q101 covers discrete semiconductors, and AEC-Q200 covers passive components such as varistors. In addition, designers should consider utilizing the help of the manufacturer’s experts to select appropriate protective components.