Linear Technology’s LT8315 monolithic flyback regulator

With an output power of 15W and a wide input voltage range from 18V to 560V, Linear Technology’s LT8315 high voltage monolithic flyback regulator is targeting electric vehicles and battery stacks as well as general automotive, industrial and medical applications. By sampling the reflected isolated output voltage across the third winding on the power transformer, the part requires no opto-isolator or LT1431 for regulation, this simplifying the design of isolated DC/DC converters.

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Seen here:
http://www.power-eetimes.com/news/15w-high-voltage-flyback-regulator-does-it-without-opto-isolator

The high level of integration and the use of low ripple Burst Mode operation result in a simple to use, low component count and high efficiency application solution for isolated power delivery.

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What sort of voodoo are they using to see what the output voltage is if it isn't electrically or photo coupled to the IC in some way?

Also that would be pretty nice to have regardless of your ev project, though shame the output current isn't higher.

The principle I believe is similar to their LT3799 offline LED driver. They monitor the aux winding on the primary side for feedback data.

The voltage on the various windings will be related by their turns ratio. If the forward drop in the output diode is assumed constant (and it should be within 0.1V or so) then sampling the voltage on a different winding while the diode is turned on gives a reliable indication of the output voltage. There will be a spike just before the diode turns on that the chip needs to ignore, but otherwise the voltage will be almost flat (just the ripple across output cap) until the core discharges.