Flyback poor regulation

Feb 25, 2010 2 Replies
H

I built a 10W multi-output flyback all of my outputs are reasonable except one my -12V output, it varies widely over line and load from



-12.5V full load to -18.7 no load.



I've been tinkering in spice trying to come up with a low parts solution but I'm not having any luck. I know I can buy an LDO but I don't want to place an order just for that.



Here is the Flyback schematic. U10 being fed off the +5Vdc output is a



3.3 V LDO. The -7Vdc output feeds a MC7905 on another PCB which is bolted to the front of the enclosure. Oddly enough the -7Vdc hardly moves over line load variations go figure. The negative outputs are for 0.150A full load.

formatting link



If anyone has any suggestion fire away please:


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J

Could be a leakage inductance spike. If so, it will be very "soft" and a modest load/bleeder resistor on -12 might pull it down a bunch. Or a series R-C damper from out- to ground.

John

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H

It takes about 20mA to get the output to -13 Vdc. I guess that will have to do for now.I'll try your other RC suggestion tommorow.If not I guess next order I'll get a negative LDO I guess.

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