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Old 28th July 2008, 04:50 PM   #1
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Default grounding problem..need help

hi all,

Attached is a AC to DC circuit employing a PFC. Based on the circuit is it possible if i connect a direct DC voltage from a power supply as a supply to the UCC3817 IC... will there be any GROUNDING problem. because when i connect a direct DC supply from a power supply to power up the UCC3817 IC , the AC mains trips off.. please advise.
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Old 28th July 2008, 05:00 PM   #2
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Hi,
This is a classic S.M.P.S.U. THIS CIRCUIT DOES NOT PROVIDE MAINS ISOLATION The IC is at half mains potential at all times. Dont even think about connecting anything at all to this.
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Old 28th July 2008, 05:06 PM   #3
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And having said all that is it faulty- is that why you want to run the IC up. Start up resistors are favourite failure points, R15 and R18.
C12 could be charged to 340 volts - beware
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Old 29th July 2008, 01:58 AM   #4
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Yesterday I tried to connect the circuit as attached, where i utlized the secondary part of the inductor in order to get 16V to supply to the IC..but i noticed that using the secondary connection as attached the Vcc that im getting is only 6V, this is not enough to start up the IC. I even tried to change the resistance value of R15 and R18 but no changes in the Vcc, perhaps R15 and R18 is just to reduce the current. any suggestion ?
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Old 29th July 2008, 06:25 AM   #5
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Hi,
First -- safety-- The secondary side of the inductor is NOT isolated. Everything on the circuit has a potential low impedance path to the mains via the bridge rectifier.
Can I ask first whether this is a circuit that has in the past worked O.K. for example is it out of a specific piece of equipment, or is it a circuit you are building from a design from somewhere.
A few aspects of the design look strange to be honest.
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Old 29th July 2008, 08:48 AM   #6
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i got this circuit from the UCC3817 datasheet...perhaps i myself wonder the purpose of diode D2 in the circuit...
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Old 29th July 2008, 08:57 AM   #7
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now im getting 16 V for the Vcc..its by changing the value of of the capacitor C10 and C11
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Old 29th July 2008, 10:59 AM   #8
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Hi,
I don't know what to advise with this ! Found the full application note where you got the circuit from. I know it's easy to say that if it's built exactly to the design it should work. Some of the components seem critical - L1 for instance. Does it depend on having a load attached in order to start up properly ?
Has anything been damaged when you said at first it tripped the supply when you connected a PSU to it. The whole thing is always "live" and connecting anything to it must draw a "spike" of current at the very least due to differences in voltage between the equipment , and if your PSU is grounded then that puts a short on it of sorts.
Normally working on anything like this requires the use of a mains isolation transformer running both this and the test equipment so you can connect 'scopes etc to it to fault find on it, but all this is very much SAFETY RELATED and beyond the scope of this forum.
Sorry can't be any more help.
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Old 29th July 2008, 05:41 PM   #9
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hi,

Basically a load is attached in order to start up properly. Actually what happened was earlier a PSU is connected to the Vcc of the power factor corrector I( ucc3817), when the mains supply is given there is when it tripped.. but no damage was caused to the components. so i believe there must be a direct short from the live to neutral or due to grounding problem..
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Old 29th July 2008, 06:10 PM   #10
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Hi,
It's difficult without actually working on the unit.
I am not quite sure I understand your last post. Are you saying that when you power this circuit up with a known good load connected ( psu connected to v-out ) that the mains feed then trips out.
Do you mean an earth leakage breaker or a current overload trip.
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