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

What is the most elegant way to use the LTC4269-1 with an 30 V auxiliary supply? Take the Demonstration circuit 1335B-C, (DC1335B-C) as your starting point This is a high-power supply featuring the LTC4269IDKD-1, LTC4269-1. I need to plan for the scenario that 30 V could be applied by an auxiliary supply across +VOUT and -VOUT. I know a Schottky diode works, but I can't accept the power loss and heat generated when PoE is in use. I know an ideal diode is an alternative (P-channel MOSFET) but I'd rather not incur the cost. Is there a more elegant way to solve this problem? Is there a condition with this potentially higher voltage on the output that will shut down the part or stop switching the FETs without using the SHDN pin?

I've noticed that the gate for Q2 is max 20 V. It's something to keep in mind for this scenario. What if I replace D14 with a 12 V Zener diode? Would this protect the gate on the secondary MOSFET Q2? Will the LTC4269-1 output regulation stop or go to a very low duty cycle on the SG pin when the 30 V is applied to what would otherwise be a 12 V output?

The data sheet reads, "Auxiliary power can be injected into an LTC4269-1-based PD at the input of the LTC4269-1 VPORTN, at VNEG, or even the power supply output." What did you have in mind for the case when auxiliary power injected at the power supply output? Do we need to use the shutdown (SHDN) pin or can we get away without it?

My calculations and simulations suggest that I can expect a higher Vds across the primary and secondary MOSFETs when I connect 32 V to the +VOUT. On the primary I'm seeing 114 V in simulation and 121 V in calculation. I have selected a Vds of 150 V for both primary and secondary.

Vds1 is the primary MOSFET

Vds2 is the secondary MOSFET



Added question about possibility of shut down without using SHDN pin.
[edited by: current view at 11:45 PM (GMT 0) on 8 Apr 2020]
  • +1
    •  Analog Employees 
    on Apr 18, 2020 12:13 AM

    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

  • Vcc has an absolute maximum voltage of 18 V. It also has an internal 19.5V clamp with respect to GND. Can rely on the clamp to limit this voltage, even though it's higher than the abs max?

  • 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.

  • 0
    •  Analog Employees 
    on Apr 22, 2020 9:49 PM in reply to current view

    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.