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LTC4366-2 output regulation problem

Category: Hardware
Product Number: LTC4366-2

We are designing surge protection circuit for meet MIL-STD-1275 standards. In this design we used LTC4366 with example circuit at datasheet page 15. We are adjust timer capacitor to hold on surge time during test.

We have some test cases.
1. With no load, we run MIL-STD-1275 test procedure voltages with power supply (28V-100V-40V), Outpu voltage clamping works fine.
2. While 1A or more loaded, @28V we can shutdown circuit via SHDN pin with no problem.
3.While 1A or more loaded, we increase the input voltage above 45 V, output voltage clamping does not work properly and output voltage oscillates as figure below.

Our observation is at high input voltage (above 45V) and load 1A and more circuit does not properly works. I added osiloscope visuals.
Could you help me with this?

No load or light loaded case

1 A or more loaded case

  • We have some updates for test cases.

    We put 100uF alum cap. at input and output. As a result of this we take the following ossiloscope results. As you can see output is clamped at the beginnig but after a while it start to oscilate. When we look at the oscilatiion at lower time step in the second figure, output voltage rises to 36V which is the clamping voltage but it can not hold at that point. 

    Green : Output

    Yellow: Input

  • Hi

    Could you measure the voltage across VOUT to VSS and OUT to FB pin?  Is R4 (10 ohm) resistor placed as close possible to the gate of the power MOSFET? 

    You can't use two power MOSFETs in parallel expecting to share the load and stress to share equally during the startup and surge events.  My recommendation would to try the surge with just one power MOSFET and see if the problem goes away.