Hello,

I designed a Multi-Winding/Multi-Output Flyback to generate +40V and -225V. In my application, the outputs need to work in a flip-flop load, where only one output handles a variable load of 0 to 50W at a time, while the other remains lightly loaded (just for stability). I used the LT8316 with no-opto (PSR) feedback, as shown in the attached file (some components are equivalent).

For the transformer, I used the TDK B66229G1000X187, with approximate inductances shown in the attached LTspice circuit. The winding specifications are:

• Primary: 46 turns of AWG 22 (bottom layer)
• Aux: 7 turns of AWG 28
• Sec (+40V): 12 turns of AWG 22
• Sec (-225V): 68 turns of AWG 22 (top layer)

I measured the leakage inductance at 13uH by shorting all the secondaries, including aux. This will make the K=0.98 (317/(317+6.5)).

Problem: In simulation with K=0.98, the outputs regulate well under any load. However, in the real circuit, the +40V rail drops to about +5V with even a 10W load, while the -225V rail performs well up to 45W.
As a test, removing the rectifier diode from the +40v rail resulted in a decent -225V across various loads, but by removing the rectifier diode from the -225v rail, the +40V rail still dropped to +5V even under light load (1w).

In summary, the +40V rail has poor load regulation, while the -225V rail performs well. How this is possible and how to resolve? If the design is bad, how come -225v is performing well?

I've also attached my Schematic and PCB layout, which may not be ideal; however, even shortening paths (e.g., from aux to feedback resistor) has not improved performance.

 

• Primary: 46 turns of AWG 22 (bottom layer)
• Aux: 7 turns of AWG 28
• Sec (+40V): 12 turns of AWG 22
• Sec (-225V): 68 turns of AWG 22 (top layer)

The cross regulation is a function of how the transformer was wound.
If you were making +40 and -40 I would say to not wind one on top of the other but to pull both wires at the same time. That way the two windings will in the same place in the Universe.
By having two very different turns and having one on top of the other there will be bad cross regulation.

Because regulation looks at the primary not the secondaries you will not have good regulation.

Try increasing the watts burned in Rload1 and Rload2.

 

Thank you @ronsimpson for your response. From what I'm hearing, there is no simple solution to fix this. Will there be a better topology or controller, or this is common among all topologies or controllers when there are different rail voltages? Another question is why the regulation is always good on -225v rail, since I wound another bobbin with different layer set, which resulted the same?