What is the most elegant way to use the LTC4269-1 with an 30 V auxiliary supply?

Hello,

Sounds like you are trying to use a 30V secondary-side auxiliary supply without a Schottky diode in series with the LTC4269-1’s 12V output or without using the SHDN pin. This configuration expects an opto-coupler pulling the SHDN pin high by using a series diode from the auxiliary supply to the output. I don’t think you can get away from the higher VDS voltage without using the SHDN pin. You could also look into using the auxiliary on the primary-side, with either this design or by using an LTC4278 or LT4276.

Q2’s gate is rated for +/- 20V, but it should be insulated from the output by the secondary-side gate driver, Q6 and Q7, as the gate cannot exceed the voltage at the bases of the driver, which is roughly 7Vpk from the isolated gate drive transformer. I don't think you need a zener here, unless I am misunderstanding your question.

The LTC4269-1 will keep trying to regulate the output in this scenario where a 30V auxiliary is connected directly to the output. The converter allows the secondary current to go negative to bleed off the output voltage, and it’s not expecting a secondary-side auxiliary supply that will keep sourcing current. If the MOSFETs can handle the higher VDS voltage you can investigate using a NPN to pull down on the bases of the secondary-side gate driver. This could be done with a series auxiliary diode, or if your auxiliary voltage is significantly higher than the output voltage a Zener diode could be used to drive the NPN that pulls down on the bases of the gate driver. This will prevent the negative secondary current as only the body diode of the secondary MOSFET will conduct. I haven't tested this idea but it might be worth checking out.

Best Regards,

Eric

My simulation indicates that the NPN will not be effective to pull the secondary gate low when 30 V is applied to the output.

I will shift my attention to designing in an ideal diode and an opto to use the SHDN pin. Thank you for your support.

Using the SHDN pin and ideal diodes is a good way to go about this application. However, in your simulation the NPN should pull down on Q5/Q6's bases, otherwise it will be fighting against the gate drive buffer and not work.

The VCC pin has an abx max of 18V as a low impedance source, or 30mA as a voltage clamp. You can rely on the 20.5V typical clamp up to 30mA. This is likely not strong enough to handle the reflected voltage from your 30V auxiliary, so you may need to use an additional 18V zener at the VCC pin. Increasing R7 will help but you should assure that under normal operation the VCC stays above 11V, so you can't increase it by much.

Best Regards,

Eric

Yes, you're right. My mistake. When I change the connection from the gate to the bases, it does shut off the secondary MOSFET.

Thanks for your input on the Vcc pin. I was thinking of a putting a zener there too.

Yes, you're right. My mistake. When I change the connection from the gate to the bases, it does shut off the secondary MOSFET.

Thanks for your input on the Vcc pin. I was thinking of a putting a zener there too.