Synchronous rectification self destruct when power is applied to output ?

[int11]

Do all conventional buck converters with synchronous rectification self destruct if a voltage greater than regulated output voltage is applied to the output ?
For example an adjustable synchronous buck converter is used to charges a car battery. Everything works fine, until you adjust supply output lower than battery voltage. The duty cycle attempts to decreases to 0% because of feedback loop.
The low side MOSFET stays on and goes up in a a puff of smoke. Alternatively the duty cycle merely reduces, and you have an unregulated boost converter with the converters original input terminal voltage going sky high and similarly releasing smoke.

I confirmed this with a 3A synchronous buck converter based on integrated switch IC (AP6503), the converter input terminals shot to 40V soon as I applied above 10mv over the regulated output voltage.

What confuses me is there seems to be extremely little on the subject. With number of rechargeable batteries existing in the world and popularity of power supplies with synchronous rectification, there hasn't been anyone who has connected the two together.

Adam

 

[blasphemy000]

This is normal operation of these types of converters, at least the ones that I have seen. You cannot apply power backwards into the output terminal because it will kill the MOSFETs or possibly fry the controller as well.

When you build one of these converters, if there is any chance of a reverse polarity condition occurring at the output, then you must incorporate some sort of protection to prevent voltage being fed backwards into the output terminal.
Every battery charger that I've seen uses some method of this. The most simple method is to just put a diode(one large enough to handle the output current) at the output terminal. If the output voltage rises above the output of the converter, the diode shuts off due to being reversed biased, and prevents any current from flowing backwards into the output terminal. There are also active circuits, using MOSFETs usually, that can accomplish the same thing. Which method you choose to implement will depend on output current and how simple or complex you want it to be.

- Brad