I have to protect a switching DCDC converter for MIL STD 704A standard especially overvoltage 70V 20ms and 80V 50ms. TVS diodes does not protect because they protect for voltages <1ms. Do you have any suggestion about surge protection circuit?
For military and space applications we use surge suppressors just downstream of our power connectors to provide over voltage, under voltage, reverse voltage, and current limiting protection. An example of one is the Analog Devices LTC4364 (https://www.analog.com/media/en/technical-documentation/data-sheets/ltc4364-1-4364-2.pdf):
With the right mosfets, the LTC4364 can block up to 100 V continuously, and higher voltages for a short amount of time. It can be configured to either shut down the load in the event of a surge, or the gate voltage of the mosfet can be modulated to allow some voltage though to keep the load powered (typically for a short period of time). As an added bonus, you can set the timer capacitor (CTMR in the figure) to slowly ramp up the gate voltage. This allows you to control the inrush for large capacitance loads.
One thing to note, the LTC4364 seems to have a boost converter inside it to provide voltage to the mosfet gates (it's not mentioned in the data sheet). We've failed conductive emission tests because of this converter, and I'd recommend putting a ferrite bead between the TVS diodes and Vcc of the surge suppressor.