Buck converter load disconnection

Question

We use a buck converter to drive a resistive load with varying voltage/power. Now we detected a problem with load disconnection at high duty cycles where the driving mosfets and the flyback diode burn and the buck converter gets shorted entirely. I made a simplified simulation of our circuit to demonstrate the problem, the switch S1 simulates load disconnection after 5ms:

Here switching frequency is 50kHz and duty cycle is 90% and very high voltage spikes occur. When the duty cycle drops below ca 60% the overvoltage spikes do not show up anymore.

I know already from Buck converters- No Load condition that a synchronous buck converter would not have this problem and I've already simulated this and can confirm this. But synchronous buck converter cannot be used here anymore, we need to quickly fix this present solution.

The schematic shows some solutions we've tried:

1. Putting a high voltage TVS diode (VAR1), i.e. changing R3 to e.g. 1mOhm. This helps a bit but still we get spikes over 800V which would damage the components.

2. Adding a bypass (bleeder) resistor R4. But this does not help either until it is set below ca 20 Ohm, which is of course unusable in real application.

Of course in real application the load current is measured and when there is underload or overload the mosfet driving gets disconnected (not simulated here). But this takes at least 20 ms and cannot be made shorter. Here the damage happens within max 100 us from the point the load gets disconnected.

The question is of course, how to avoid this damage with minimum effort, cheap, low part count, space saving etc.

Simulation file in Simetrix Elements 8.1 (free evaluation)

Answer 1

You could provide a RC snubber circuit parallel to the diode (google for “RC snubber DCDC”) This slows down the sharp edges of the switching waveform and of course reduces efficiency a bit. This is often done to reduce EMI problems, and it might also help reducing the harmful voltage spikes.

Answer 2

It seems, at least for the presented simulation, that the solution with TVS diode is the only sufficiently working solution.

But other type of TVS diode has to be used.

SMAJ types are not suitable for clamping such high currents, again only when simulated with model files downloaded from Littelfuse model library. For used SMAJ160CA the current flowing through it exceeds its Ipp more than 5 times.

When I use the AK10 type, again simulated with model file from Littelfuse, the clamping works as expected:

Now we have to buy samples of those TVS diodes and check them in real application.

Because this question was only about simulation this answer can be IMO accepted.