Hi All,
I am looking at overvoltage protection for a 24vnominal system that must continue to operate normally with up to 200v being applied to it for 500ms!
Previsouly we have used clamps/crowbars and disconnects, but the regs have just changed and we can no longer either simply absorb / clamp the overvoltage pulse and we must now continue to operate normally during the pulse.
Now, this overvoltage event actually never really happens, so most of the time (99.9999% or more actually) i need a device that just sits their, passes through 50A continuous whilst dropping the least amount of power and making the least amount of heat. But, for the test itself, and perhaps once every 25 years in service, it may actually need to step in and limit outputvoltage to under say 35ish volts whilst still passing 50A to the load and whilst up to 200V is present at the input.
This is one of those really stupid things that is more a sideeffect of out-of-date regs than something that has much use in the real world, but it's a test we have to "pass" ie to demonstrate that our system can handle this event.
During the event, i'm really not to concerned about the buck efficiency, the current ripple or to a large degree, the voltage ripple on either side of the converter either,all things that normally you'd want to be minimised, but here, it's simply not very useful to minimise those things. As a result i'm wondering about how little inductance i can use, how far i can push that inductance into saturation during the event, and what sort of control architecture to implement.
50A continuous is a reasonable level of current to have to deal with on a continous basis, and the application demands automotive environmental level design (ie -45 to 105degC ambient). A quick estimate suggests we'd want something around 10mOhm total resistance, and with the Switching elements having to be 250 V rated that means a fair number of parallel FETs. It also means a fairly bulky inductor, hence the need to run it into saturation as much as we dare?
So, i'd like any suggestions as to how to achieve this. I don't really want to have complex "Bypassing" eg a contactor or relay that helps reduce the normal series resistance when the system is not bucking around as it were, an dideally we'd get continuous reverse polarity protection included "for free" in the architecture, ie when connected backwards zero current is allowed to flow.
I've used Wurth WE-HCF inductors previous for an automotive rated DC/DC previously, but that actually operated allthe time, whereas here we basically never operate
https://www.we-online.com/en/components/products/WE-HCFI've considered some form of linear dropper ie pass transistor, but the powers are really quite large (200 to 35v is a 165V drop, which at 50A is 8.25kW, so even for 250ms that's around 2kJ of energy) Not sure if even some massive series cascade of To-3 cased monsters can deal with this level of energy?