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Electric Vehicles Demand the Safety of High-Reliability Circuit Protection

Next-generation electric vehicles incorporate an array of complex and highly compact circuitry, and engineering design that can deliver both innovation and human safety remains a major challenge. It’s not simply a question of sophisticated microprocessor technology. These vehicles also implement the latest in EV onboard charging techniques. Designers need to ensure that the possibility of overloads, transients, and electrostatic discharge is taken into account, with safety and reliability the over-arching criteria, all the way through from concept to timing closure and sign-off. Designers must plan for contingency, and this whitepaper addresses seven mission-critical onboard charging circuits that provide recommendations for both circuit protection and efficient power control.

hybrid
Figure 1, above, is an overview of the electronic systems typical of a hybrid vehicle where the combination of internal combustion engine (ICE) and electric drive effectively multiplies potential transients by an order of magnitude, presenting the designer with a special challenge.

Moreover, the deployment of both an onboard charger and AC power line can result in the generation of both transients and overloads. The charger’s circuits demand adequate protection, and communications circuits must also be able to survive ESD transients, in order to avoid data corruption. Add to the design engineers will need to minimize power consumption, allowing the battery charge time to be as short as possible.

The AC line voltage, converted to DC, charges the main battery pack to within an output range of of 300–500 volts. Key challenge for designers is meeting consumer demand for faster charging, which requires high-power charging circuitry, including three-phase designs. A single-phase circuit is shown in Figure 2. Each circuit block indicates the suggested protection components plus any components needed to optimize the charger’s efficiency.

can bus

technology
Figure 2. Onboard Charger Block Diagram

MOV: Metal Oxide Varistor
TVS: Transient-Voltage Suppression

Input susceptibility

Lightning strikes and surges on the AC lineare among the transients threatening the input voltage, and an overload fuse provides the first line of defense — a high voltage rating will cover a worst-case current scenario.

A MOV (metal oxide varistor) positioned downstream will absorb the transient energy and help prevent damage to other downstream circuits. If the charger uses three-phase power, MOVs for both phase-phase transient protection and phase-neutral transient protection could also be added.

Better yet for protection of downstream circuits, place a bipolar thyristor in series with the MOV. The thyristor will have a very low clamping voltage, typically around 5V, and also enables use of an MOV with a lower standoff voltage. The net effect of this combination is the reduction of the peak downstream transient voltage.

A gas discharge tube (GDT) can provide further circuit protection, offering high-resistance, plus electrical isolation between the hot and neutral lines and the vehicle’s chassis ground. GDTs also provide an additional level of protection against fast-rising transients provoked by lightning.

Thyristors – the advantages

The benefits of thyistors include the capacity to supply the necessary power for fast, high-power charging. Compared to rectifier diodes, they provide a “softer” start (lower in-rush current) and reduce electrical stress on the PFC block. Thyristors also suppress surge currents that may have passed through the input voltage and EMI filter stages.

PFC circuit — controlling inductance

The power factor correction circuit reduces the total power drawn from the AC power line, while a gate driver and an insulated gate bipolar transistor (IGBT) can control the amount of inductance in the circuit (ensure the gate driver operates within a voltage range suitable for control of the IGBT). Also assess the gate driver’s immunity to latch-up since fast rise and fall times are need to switch the IGBT. Combined with a low supply current, these fast times improve the circuit’s power efficiency. Also ensure the gate driver has internal ESD protection. If not, add an external ESD diode. Bi-directional or uni-directional ESD diodes can withstand ESD transients as high as 30kV.

Protecting the DC Link

The DC link comprises the capacitor bank that stabilizes the ripple effect of the high-power DC/DC converter. Designers can use a high voltage TVS diode to protect the capacitor bank from large-voltage transients.

DC/DC Converter

The DC/DC section steps up the charge voltage and smilarly to the Power Factor Correction circuit, requires a robust gate driver. If a gate driver selection does not include internal ESD protection, be sure to add an ESD diode for protection — this will not degrade the performance of the gate driver.

Importantly, the power IGBTs must be protected from voltage transients. In addition to protection from external transients, the IGBT creates turn-off switching transients due to L•di/dt effects from internal parasitic inductance. Place a TVS diode between the collector and gate of each IGBT to eliminate the potential damage to an IGBT from this transient.

The TVS diode reduces the di/dt of the current transient by raising 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. Known as active clamping, this use of a TVS diode as a collector-gate feedback element ensures the IGBT remains stable. Some IGBTs that have built-in active clamping TVS diodes; otherwise add TVS diodes to the circuit.

Output Voltage

Current overloads and in-vehicle voltage transients can occur when motors power on or off or when current is instantaneously interrupted — by a break in a cable, for example. For these reasons, the output voltage needs specific protection. Possibly use a fuse to protect against a possible over-current resulting from a short within the battery pack or its cabling. An MOV or a TVS diode protects against any potentially damaging voltage transients.

Control Unit

The charger’s control unit communicates with the data network via the CAN bus. To avoid damage to the communication circuits, which could result in data corruption, provide ESD and transient protection. This can be achieved with a dual-line TVS diode array, for example, designed for CAN bus signalline protection. This type of array for the protection of signal lines has a minimal capacitance and will not degrade the transmitter/receiver I/O states.

TVS Diode
Figure 3. TVS Diode Array For CAN Bus Lines Protection

By following these recommendations for protection and control, design engineers can be assured their new onboard charging systems will have robust, reliable, and safe circuits for their electric vehicle consumers. Whenever possible, remember to use AEC-Q qualified components that have been certified for use in hazardous automotive environments (i.e., AEC-Q101 covers discrete semiconductors and AEC-Q200 covers passive components such as varistors). It’s important to remember that you can also take advantage of the manufacturers’ expertise and wealth of application knowledge for assistance when selecting appropriate protection and power control components.

(The article is an original piece written by Littelfuse.)

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