DER-58 3-Watt Flyback: LNK304 vs LNK306

Posted by: DDC on 07/09/2018

I work at a magnetics company and one of our customers is designing a power supply based on the "DER-58" 3-Watt Wide Input Flyback reference design. The circuit is laid out according to the reference design, the only difference is that they are using an LNK306 instead of LNK304. We made transformer samples based on the DER-58 specifications, but the output current the customer could draw was lower than the 225mA they desire. At Vin of up to 120Vac, they can draw 225mA @12V. At Vin = 280Vac they can only draw 100mA @12V. We want 12V, 225mA out with Vin = 480Vac max. The DER-58 app note claims that the Vin can be up to 580Vac. Is this app note still valid for the LNK306? Do we need to make any circuit adjustments? Thanks in advance!

Comments

Does no one at Power Integrations know what the issue is? Is anything unclear? Thanks,

Hi DDC,

Based from your email, the designer just changed the IC from LNK304 to LNK306. These 2 devices have different peak current limit, 275mA for LNK304 and 515mA for LNK306. Our IC is an ON-OFF controller and require to reach this peak current within the required duty cycle and frequency for power delivery.

But in your case, you did use a higher peak current limit IC without modifying the transformer. The way we see it, the transformer design is saturating. This is consistent with your statements where you can only load 100mA at 280Vac, where current di/dt is much higher at this input condition. And because the transformer is saturating, it reaches the peak current limit very fast turning off the primary FET at this rate. This would lower the power delivery.

Anyway, you need to redesign the transformer. This is the first thing that you can verify. With the same winding, changed the inductance to 1.4mH only. If this turns okay and if you don't want to optimize the transformer design, then go ahead. If you still need optimization you can actually reduced the number of turns for both the primary and secondary turns but you need to double check the EMI and thermals. Thank you.

Best Regards,
PI-SevenofNine

Thanks for the response SevenOfNine, we will substitute the LNK304 today and see what happens.

Best regards,

Thank you for your support. Changing to the LNK304 unfortunately did not help. We will try a transformer with a larger core. Is the DER-58 circuit tested and proven? I'm surprised it wasn't working with the recommended circuit and transformer design.

Hi DDC,
Yes, DER-58 was proven working. All the test data in the report was based on actual reading gathered on the working unit.
Regards,
Jedidiah

Hi DDC,

Yes, DER-58 should be working. Can you please send to us the schematic and transformer design. We will try to check if something is missed.

And when you changed to LNK304, at 280Vac you can only load 100mA? Thanks.

Best Regards,
PI-SevenofNine

We are building a new transformer with a larger core to see if it helps. I'll see if I can get the schematic, but the customer said it was according to the DER-58 schematic. We built the original transformer exactly per the schematic as well.

Hi DDC,

Thank you. Let us know what happen after changing the transformer to a larger core. Thank you.

Best Regards,
PI-SevenofNine