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I have some doubts about the design of this DC/DC converter circuit/PCB circuit:

DC/DC converter, eFuse circuit

On the left side there is a 400V to 12V DC/DC converter (Vicorpower DC/DC Mini, 300W). The maximum output capacitance of the converter is calculated with 833.3µF. (Datasheet: "The capacitance value is not the absolute maximum value, but the value for which general application of the converter can be deemed appropriate. Testing will be required to ensure that the module is stable if this value is exceeded. Approximately 10X the value calculated will cause the converter to go into current limit at turn-on.")

On the 12V output side of the converter there are several PCBs. Each of them has some DC/DC converters and some capacitors at different voltage levels. Also, each PCB has an eFuse of the type TI TPS2592AL (programmable SoftStart control). The DC/DC converters on the right hand PCBs are of the type TI TPS6213X (buck converter, programmable Soft Start).

Now I have two questions about this case:

  1. The Vicorpower DC/DC converter has a maximum output capacitance of 833.3µF. Does this only mean I have to add the capacitor values on the 12 V line in front of the eFuse? Or do I also have to add the capacitor values on the 5V/3.3V line of the PCBs for the total output capacitance?

  2. For the circuit calculation of the eFuses I have to figure out the "Load capacitance" of the eFuses. Similar question: Do the capacitors behind all DC/DC converters count? Or just on the 12V line?

Thanks!

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  1. Add the recommended ~830uF capacitance as close as you can, on the output of the Vicor power module. Make sure the ESR and ripple current ratings etc are all correct. Make sure the type of capacitor (Electrolytic, Cermaic, Tantalum etc.) chosen is correct and has the correct ratings and ESR as mentioned before.

Add smallish 47 or 100uF capacitors on the inputs of each of these separate PCB which are the 12->5V Buck converters. The datasheet for the converters will say what they want as input capacitance, otherwise as I said go for some kind of local supply. This will prevent current-lag related voltage sag/ripple when the 5V output goes under load, yet the 12V input has no power supply decoupling available.

  1. The load capacitance will be the capacitors you put after the efuses, because remember a discharged cap acts almost like a short circuit during power-up stage, so that will greatly affect the way the efuse works for "soft start" operations.

Therefore if you put a 100uF cap after the efuse, just before the 12->5V DC/DC converter, then use 100uF as the load capacitance for the efuse calcs.

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