Flyback poor regulation

[Hammy]

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:

 

[John Larkin]

On Wed, 24 Feb 2010 20:05:56 -0500, Hammy <spam@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

 

[Hammy]

On Wed, 24 Feb 2010 17:25:09 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 24 Feb 2010 20:05:56 -0500, Hammy <spam@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.