Switcher steps in with 12 V supply

Power Integrations has unveiled a flyback switcher with an integrated 750 V MOSFET and secondary-side sensing that can be used to provide an alternative 12 V power supply for an EV if its onboard 12 V battery fails (writes Nick Flaherty).

The InnoSwitch3-AQ can be used for traction inverters, onboard charger, DCDC bus converters and BMSs.

The 12 V battery is used to provide the gate drive voltage in the traction inverter, so if the battery fails or is disconnected there is no 12 V input to the gate drivers and traction controllers.

The functional safety requirements of the ISO 26262 standard require an emergency power supply to provide the output voltage for the inverter controller system and the gate drivers to move the vehicle safely.

“This will be taking power from the HV bus, which can be a wide range – as low as 30 V and as high as 550 V,” said Edward Ong at Power Integrations.

The InnoSwitch3-AQ uses Power Integrations’ high-speed FluxLink coupling to achieve ±3% accuracy for combined line and load regulation while eliminating dedicated isolated transformer sense windings and optocouplers. The link allows the controller to maintain output voltage regulation even under the transient stress test.

“You need a feedback loop on the secondary side – in automotive applications opto-isolators can’t be used as they degrade with time, and that shortens the life of the power supply system, so the challenge is that you need the isolation to feed the data back to primary side,” said Ong.

“However, the FluxLink generates a magnetic coupling without the use of opto-isolators. The advantage of inductive coupling is that it allows a faster response time, so if a vehicle is at low traction the power supply can easily respond. The bandwidth of the feedback loop can go up to more than 20 kHz. “For a consumer vehicle the 400 V bus can go up to 550 V, so the AQ is rated for 750 V and it can scale up to an emergency power supply for 800 V buses.”

The MOSFET meets stringent automotive derating requirements, and an on-chip synchronous rectifier controller delivers above 90% efficiency at the nominal 400 VDC input voltage with less than 10 mW no-load energy usage across the input voltage range.

The InnoSwitch3-AQ comes in a surface-mount package with 11 mm primary-to-secondary creepage, which exceeds the requirements for high-altitude (5000 m) isolation.

A reference design uses the switch to demonstrate start-up and shutdown from a 30 VDC to 550 VDC input. It has a fast transient response and a variety of safety and protection features.