Hello,

Converter: Dual Active Bridge
Input bus voltage: 600VDC, output bus voltage: 500VDC
To protect the transistors (Mosfet) on the primary and secondary sides, I will use snubber capacitors (on the DC bus) and suppressor arrays.



I believe pulse are about 1.2kV, limit them to 800 - 700V (transistors are rated at 950V).
The snubber capacitance will be 0.22uF.


Question: under what voltage to choose a capacitor?
I would like to use two 0.47uF 400V capacitors in series to add up to 800V, which is above the DC bus.
There are capacitors with higher voltages, but I'm trying to balance the cost, so it's preferable to connect them in series.
Is the voltage of the snubber capacitors selected based on bus voltage or maximum surge voltage?

Thank you!

 

Your diagram shows the DC bus capacitor.
Also it does not show anything such as an inductive load that could possibly make a high-voltage pulse.

 

@Ian0, Thank you for the answer!



I saw an example where the snubber was not on each transistor, but on the DC bus.



On the primary side, the leakage inductance is 43uH.

How is the voltage of snubber capacitors selected? By DC bus voltage or expected surge voltage?

 

If it is on the DC bus, then it is a decoupling capacitor.
In a bridge circuit, the voltage on the MOSFETs is clamped by their reverse diodes, so cannot exceed the bus voltage. You might need snubbers to suppress ringing.

 

Well.
What voltage is chosen for the snubber capacitor?
According to the operating voltage of the transistor or according to the expected surge?

 

Well.
What voltage is chosen for the snubber capacitor?
According to the operating voltage of the transistor or according to the expected surge?

A little above the bus voltage. The bus would clamp any voltages higher than that. For a 400V bus, the next voltage would be 630V. You will probably only need a small value of capacitance, so you could use 1000V, which tends to be the next available value above 630V.

 

@Ian0,
Thank you very much for your response!

Problem solved.

 

Hi,
i'd like to recommend you to use a 1000V capacitor. Two 400V capacitors in series will result a higher parasitic inductance. Use a special grade capacitors. General purpose capacitors lose capacity quite quickly .
And more. Did you evaluated resistor value and resistor power rating? A relatively high power rating thick film resistors are usually used in RC snubbers to withstand high current pulses

 

Polypropylene capacitor make good snubbers.
The peak current is easy to calculate as it is just the supply voltage divided by the resistance. Check that both the capacitor and the resistors will withstand the current.
The resistor power dissipation depends only on the frequency and the value of the capacitance. The value of the resistor doesn’t make any difference - the power it dissipates will be the same (unless it is so large it doesn’t do any good)

 

Resistor in RC snubber must withstand average power and pulse power. If a typical RC snubber- 10 Ohm and 10 nF is used at 500VDC , the power during swithing pulse is :
U^2 / R = 500^2 / 10 = 25 kW
Pulse duration is about :
T = RC = 10 Ohm * 10 nF = 0.1 usec

Both capacitor and resistor must withstand that pulse

 

Hi @Pyrex, I will consider a 1kV capacitor option if I can find one. Thank you!