Flyback poor regulation
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From: Hammy on 24 Feb 2010 20:05 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. http://i49.tinypic.com/2dloh81.png If anyone has any suggestion fire away please:
From: John Larkin on 24 Feb 2010 20:25 On Wed, 24 Feb 2010 20:05:56 -0500, Hammy <spam(a)spam.com> wrote: >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. > >http://i49.tinypic.com/2dloh81.png > >If anyone has any suggestion fire away please: 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
From: Hammy on 24 Feb 2010 20:57
On Wed, 24 Feb 2010 17:25:09 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 24 Feb 2010 20:05:56 -0500, Hammy <spam(a)spam.com> wrote: > >>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. >> >>http://i49.tinypic.com/2dloh81.png >> >>If anyone has any suggestion fire away please: > >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 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. |