cupoftea
Advanced Member level 6
Hi,
Customer has sent us a 24V,1A offline flyback (90-265vac) to re-do
with a different controller (theirs went obsolete).
Its noteable that the FSGM0565 that they had been using has
a current limit of some 3A...so in effect they had no
realistic overload protection, but just short cct output protection.
(3A is well above their transformer's peak current capability, which is 300mT at 1.25A)
FSGM0565
Anyway, they are also using a 50 Ohm NTC...
B57236S0500M00 (NTC)
This has a resistance of 400 Ohms at minus 25 degrees C. (!!!)
The customer is reluctant to tell us what is the lowest ambient temperature where this flyback may operate.
Our proposed alternative controller has a startup voltage of 110VDC (ie the rectified mains, as sensed by the controller
must be at >110Vdc for at least some time within any 60ms time period.)
The thing is, that this wont be possible at minus 25degC, as the 400 Ohm NTC in the line wont allow the HVDC bus
to rise above 110Vdc for long enough. They insist on using the same 50R NTC. (they also insist on us using their same
flyback transformer which is E25/13/7 and ns/np = 19/84)
Anyway, would you agree that start-up testing at minus 25degc is going to be essential for this case? Also, are there any cheap places where
you can go and use a thermal chamber in UK? (sorry to speak of UK alone, please also tell for other countries as it
may be useful for people there).
This job has a sting in the tail IMHO, and this is the low temperature startup. We did wonder why they were using a FSGM0565 with
a ridiculously high primary current limit (3A) and a ridiculously low startup voltage of 28V. We reckon this is deliberate due to some customers having low ambeint temperatures.
_______________________ _________________________
Unfortunately, there are no offtheshelf offline flyback controllers which have all of ...
1...no UVLO
2...Output short circuit protection
3...No overload protection
...and this is what we need.
We cant use UCC38C42 because it doesnt have output short cct protection, and we have no room on the PCB to do it with discrete circuitry.
Customer has sent us a 24V,1A offline flyback (90-265vac) to re-do
with a different controller (theirs went obsolete).
Its noteable that the FSGM0565 that they had been using has
a current limit of some 3A...so in effect they had no
realistic overload protection, but just short cct output protection.
(3A is well above their transformer's peak current capability, which is 300mT at 1.25A)
FSGM0565
Anyway, they are also using a 50 Ohm NTC...
B57236S0500M00 (NTC)
This has a resistance of 400 Ohms at minus 25 degrees C. (!!!)
The customer is reluctant to tell us what is the lowest ambient temperature where this flyback may operate.
Our proposed alternative controller has a startup voltage of 110VDC (ie the rectified mains, as sensed by the controller
must be at >110Vdc for at least some time within any 60ms time period.)
The thing is, that this wont be possible at minus 25degC, as the 400 Ohm NTC in the line wont allow the HVDC bus
to rise above 110Vdc for long enough. They insist on using the same 50R NTC. (they also insist on us using their same
flyback transformer which is E25/13/7 and ns/np = 19/84)
Anyway, would you agree that start-up testing at minus 25degc is going to be essential for this case? Also, are there any cheap places where
you can go and use a thermal chamber in UK? (sorry to speak of UK alone, please also tell for other countries as it
may be useful for people there).
This job has a sting in the tail IMHO, and this is the low temperature startup. We did wonder why they were using a FSGM0565 with
a ridiculously high primary current limit (3A) and a ridiculously low startup voltage of 28V. We reckon this is deliberate due to some customers having low ambeint temperatures.
_______________________ _________________________
Unfortunately, there are no offtheshelf offline flyback controllers which have all of ...
1...no UVLO
2...Output short circuit protection
3...No overload protection
...and this is what we need.
We cant use UCC38C42 because it doesnt have output short cct protection, and we have no room on the PCB to do it with discrete circuitry.
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