0
\$\begingroup\$

Is it feasible to use one flyback controller with two transformers, yet only one feedback? The primary winding of the transformers would be connected in parallel, isolated feedback would be taken from the most loaded output, then others would be less regulated, but yet in required range.

\$\endgroup\$
1
\$\begingroup\$

If this is about two flyback cores and windings, then the load on the (#1 core) secondary winding may mismatch the load on the (#2 core) secondary winding, leading to different voltage/time during the 'flyback' phase. So, there's no way to prevent DC currents in one core from changing its inductance, and ruining the match (even with the primaries connected together, i.e. in parallel).

It's feasible, but may drift unpredictably from 'nominal' behavior.

Matching is improved by winding multiple secondaries on the SAME core, which no longer requires multiple primary windings.

\$\endgroup\$
1
\$\begingroup\$

The energy per cycle delivered to the transformer that has the biggest load is determined by the on-time of the switching transistor. During the on-time, the current ramps up (fairly linearly) thus storing energy in that transformer's primary winding. When the transistor turns off, that stored energy is released to the secondary load and therefore joules (stored) x F (switching frequency) becomes power delivered to the load.

If you have a second identical transformer with paralleled primary winding, and the load on this transformer is much lighter than the first transformer, you have a problem because the primary energy it stores will be identical to the first transformer and if this is dumped into a much lighter load every switching cycle then the load will be over-powered and have too much voltage placed on it.

In fact, the output voltage on the 2nd transformer will rise until the power dissipated in the 2nd load is about the same as the first load.

You might be able to play tunes with more primary inductance on the 2nd transformer so that it stores less energy per cycle but this will rely on the two loads being at some fixed ratio.

It doesn't sound very feasible but maybe someone has a cunning plan?

\$\endgroup\$
  • \$\begingroup\$ Just checking that the plan is cunning enough to not even start it... \$\endgroup\$ – Gregory Kornblum Mar 8 '17 at 9:08
  • \$\begingroup\$ I'm not sure this analysis is correct since the primary wave-forms are the same so to some extent simple transformer should in the short term provide some control of the less loaded output. The current in the less loaded primary will not decay to zero however so I suspect it will saturate blowing up your primary switch. It is certainly not recommended though two or more secondaries on the same core is common and works well. \$\endgroup\$ – Warren Hill Mar 8 '17 at 15:10
  • \$\begingroup\$ @WarrenHill it'll either saturate or you'll push too much voltage to the lightly loaded transformer's load. Either way it's not good news. \$\endgroup\$ – Andy aka Mar 8 '17 at 15:21
  • \$\begingroup\$ Agreed definitely not good news. \$\endgroup\$ – Warren Hill Mar 8 '17 at 15:23
  • \$\begingroup\$ Guys, so what should I do? I need like eight secondaries, and i really don't want to use two feedback circuits and two controllers... And the transformer for 8 secondaries is too exotic for all suppliers i know. \$\endgroup\$ – Gregory Kornblum Mar 8 '17 at 19:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.