Reliable, durable EV charging infrastructure by Murata

There are a number of challenges involved in designing electric vehicle fast-charging solutions (EVSE). For this type of application, AC-DC power conversion happens inside the charger, rather than through the vehicle’s on-board charger (OBC) as it does with lower-rated equipment. Equally, the DC-charging voltage can range anywhere from 300V to over 900V, while currents can be as high as 500A.

A DC fast charger is generally comprised of an AC-DC rectifier and an isolated DC-DC converter. For optimal performance and reliability, careful consideration should be given to the characteristics of the isolated DC-DC charger, including ensuring bipolar voltage output, sufficient isolation and the ability to withstand continuously high voltages.

Here are the most important factors to consider when selecting an isolated DC-DC converter for EVSE applications.

1. Enabling significant power for rapid switching
To meet the high requirements of fast-charging applications and the need for fast-switching, the gate driver must be carefully designed and sufficiently powered, which normally requires a DC-DC converter.

Fig. 1. DC-DC converter supplies the gate-circuit for a high-side power switch

The DC-DC converter must deliver enough power to charge and discharge the gate capacitance of the power switch. This can be calculated as the product of gate charge, PWM frequency, and gate-voltage swing, with values depending on the chosen power-switch.

NOTE: To mitigate against the negative effects of transients and ensure that the turn-off characteristic is well-defined, designers should generally apply a negative turn-off voltage.

2. Selecting the appropriate power converter
Table 1 compares the gate-drive requirements for typical version of high-voltage power switches on the market: silicon IGBTs or MOSFETs, silicon-carbide (SiC) MOSFETs, and gallium-nitride (GaN) MOSFETs.

Table 1. Comparison of gate-drive requirements for typical high-voltage power switches

The voltage and current must also be considered, since power transistors have a large margin between threshold switching voltage (VGTH) and maximum voltage. A larger voltage swing demands higher gate-drive power, so voltage must be chosen to maximize efficiency and EMC performance.

3. Creating a bipolar voltage output
There are several ways a DC-DC converter may provide a bipolar voltage output. Murata’s 1 W and 2-Watt MGJ1 * MGJ2 series support fixed output voltages.

Thanks to a patented technology, 3-Watt MGJ3 and 6-Watt MGJ6 converters provide the necessary voltages required for silicon IGBTs, silicon MOSFETS, or SiC MOSFETS from a single output configuration. By powering up or down with an on/off control, these devices reduce the risk of shoot-through.

Murata offers a range of products offering combinations of positive and negative voltages and special outputs.

Figure 2. A single output configuration can supply various positive and negative gate-drive voltages.

NOTE: If a high voltage is applied to the switch when the charger resumes operation after a period of disuse, the gate could be put under duress before the operating levels stabilize. To prevent this issue, Murata’s MGJ DC-DC converters feature clamped outputs to limit the no-load voltage to a safe value.

4. Minimizing voltage spikes
To minimize the risk of disruptive voltage spikes across connection resistances and inductances, it is important to select a DC-DC converter with low coupling capacitance. Murata’s MGJ converters have low coupling capacitance, typically 3-4pF.

5. Sufficient isolation to withstand high voltages
Isolated DC-DC converters for EV fast chargers should be able to cope with continuous high voltages across the isolation barrier while also maintaining performance. Further, they should show no signs of degradation due to discharge effects. In testing, the Murata’s range of MGJ series solutions are rated for continuous operation up to 3KV.

Summary
Due to high side voltages and complex gate-driver design required for EVSE, designers should look for an isolated DC-DC converter that offers:

    • Bipolar output capable of producing a turn-off signal below 0V
    • Clamped output voltage or very low minimum-load requirement
    • Low isolation capacitance
    • Isolation capable of withstanding continuous high voltage

With all of these characteristics in mind, Murata’s MGJ DC-DC converters are ideally suited to electric-vehicle fast-charging solutions.

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