Helle, Queen of Dissociation Mar 11

hmz, if I have a buck regulator with external FET, with external power available, how hard would it be to have it work with higher input voltages than it itself is rated for

I presume moderately as the FET driver is usually for an N channel with charge pump so that that can't cleanly be an input for an external charge pump

Show threadHelle, Queen of Dissociation 
but idk I feel like there must be some route other than the cursed idea of get a stronger charge pump, a voltage divider for measurement and a high side (or mmm, may be able to do lowside) output amperage measurement and write some tight code to make a MCU pretend it is a buck regulator
Mar 11 at 7:06amWeb
Show threadHelle, Queen of Dissociation Mar 11
yes, I could by the like exotic high input voltage buck regulator, but they charge exotic bucks for the buck
Show threadZ̈oéMar 11
@helle you can search for flyback or „off-line“ converters those usually go up to 400 V in their regular application
Show threadHelle, Queen of Dissociation Mar 11
@uint8_t yeah, but iirc the ones that otherwise have the features I need there go above the cost of just higher input voltage ones
Show threadZ̈oéMar 11
@helle gate drive transformers! but those come with their own caveats and you can’t just drop in.
the second issue is the analog bias supply for the buck itself.
Show threadHelle, Queen of Dissociation Mar 11
@uint8_t yeah, I was hoping to just run the buck reg off of the logic supply, but I am seeing the issue with that more closely rereading the datasheet
Show threadZ̈oéMar 11
@helle any modern regulator also has feedforward and slope compensation that is not going to play well with stability if you decouple it from the actual supply voltage
Show threadHelle, Queen of Dissociation Mar 11
@uint8_t urgh, *sigh* yep, this is fitting my worries and an MCU is not going to run more than the basic algorithms before slowly running into trouble
Show threadZ̈oéMar 11
@helle no, you can reasonably implement digital control on commonplace MCUs but it takes some control theory knowledge to make it stable
Show threadceemosMar 11

@helle if you know you will always convert down to less than 50% of input, you could go the cursed route of putting a gate drive transformer in between

i have thought about that for way too long but never tried it, it is also very cursed, but it should just work

the only dangerous operating mode would be very long on times, where the positive output might not turn on the mosfet properly any more while the negative gets too close to the max gate voltage

Show threadHelle, Queen of Dissociation Mar 11
@ceemos so Vout in current situations is at most 60% Vin
Show threadceemosMar 11

@helle that might still work but it makes it more cursed -- instead of nice +-15V drive levels you might end up with +10/-20, but that would still be ok

but also you need to be sure the controller does not do anything stupid at startup/shutdown, or at least give the mosfet some thermal mass in case it goes linear for a bit... generally i'd only considered that for high power low efficiency stuff, not sure how it would scale down