start-up    circuit in  smps   question - Page 1
Forum section: electronics / general

author: libyantiger
author_rank: Newbie
timestamp: July 31, 2013, 12:23:10 am
content: hiwhy the start up voltage do not charge up all way to the high voltage rail in a smpswhat prevent this from happening?

author: c4757p
author_rank: Super Contributor
timestamp: July 31, 2013, 12:25:22 am
content: A Zener diode or other shunt regulator pulling against the startup resistor.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 31, 2013, 02:47:22 am
content: Another trick I have seen (and personally used) is to use a RC network operating from one side of the line to supply the startup current. The capacitor limits the current with next to no power dissipation and the resistor, used to limit peak current, also dissipates very little power.

author: libyantiger
author_rank: Newbie
timestamp: August 04, 2013, 05:41:30 pm
content: impressive answer thanks for all

author: libyantiger
author_rank: Newbie
timestamp: August 04, 2013, 06:06:13 pm
content: hito AcHmed99 ...now after the aux takes over.... cap get rids of what discharging it constantlyand should be able to charge up to the high voltage rail ....i think the only thing that will prevent itis the fact that start up resistor is still providing some current to the pwm chipthe rate at which it provides current will prevent the cap to charge up to hv rail?? i wonder if this is rightbut since i am electronics technician many times when repairing smp s that uses the famous uc3842 and when the smps is not working due to bad chip i can actually see the vcc at16vdc yet now the ic is not working and the cap should actually go on and charge up to hv rail since nothing is working and nothing is demanding any current out of the capacitor...or may be the uc3842 based power supply uses a resistor divider rather than resistor capacitorciruit ..sorry for asking and replying my own questioni JUST want to involve you in the contradicting ideas that runs inside my mindthanks for you answers

author: libyantiger
author_rank: Newbie
timestamp: August 04, 2013, 08:57:26 pm
content: very logicali think that is why i face from time to time a very fancy start up circuit...some start-up circuits use additional components to literally ..disconnect the start-up resistoronce the aux winding takes over ..this as they say to save some power that is beingdisspated from the main line

author: libyantiger
author_rank: Newbie
timestamp: August 19, 2013, 10:01:40 am
content: well i am gonna divert alittle from the subject and ask you new questionin uc3842 based power supply if a short happen in secondery side and ocp kicked inand while the ocp is doing its job ..we suddenly remove the short do you think the smps willwork immediately or we have to remove the 220 power cord for a while and reinstert it again ..if so then why

author: libyantiger
author_rank: Newbie
timestamp: August 19, 2013, 10:15:00 am
content: well by the way all 3842 smps are using simple resistor divider for start up so if we assume thatthe ocp needed resting the smps to go away due to fact that voltage of the aux winding fall(due to short at the secondry) and the start-up cap is not able to dliver enough curret to the thirsty us3842 then our assumption is worngbecause no cap in startup just resistor divider and they will deliver all current needed just will be wastful way driving it from hot side ...that is why the aux exist not because the rsistors are enable to do it alone but for effeciency purposes not wasting power over the resistors while we can get it from the auxthanks