Dear support member,
My customer used TPS54A20.
I have two question about Series Capacitor.
1.
Please tell me the merits / disadvantages when adding Ct with larger capacity than the calculated value in (13)?
2.
Also, if the input voltage drops and the capacitance of Ct becomes insufficient,
what kind of trouble will occur please tell me?
Best regard.
Bob Lee.
Hi Bob,
1) A larger value of Ct will reduce the voltage ripple on the series capacitor. This is generally a good thing because the circuit is looking for the capacitor to act as a voltage source. The largest voltage ripple is seen at full load and the lowest input voltage. Typically, I suggest keeping the voltage ripple in the series capacitor to about 5% of the nominal voltage. The main tradeoff with a larger value of capacitance is that it will take longer to precharge the series capacitor before switching begins. This will add some delay from EN pin going high to the PGOOD signal going high after the soft start is complete.
2) If the value of Ct becomes too low and there is large voltage ripple on Ct, it could trigger a fault. The converter has a series capacitor monitoring circuit that continually checks to see if the capacitor voltage is greater than 35% of VIN and less than 65% of VIN (see page 19/20 of the datasheet). I suggest you select Ct to be large enough to avoid this condition.
Note: It's important to keep in mind derating of the capacitance with temperature and dc bias. There is a video training series we put together at 
You can find more tips on how to design the converter and a 5 minute section specifically dedicated to series capacitor selection.
Sincerely,
Pradeep
Hi Bob,
The fault is a series capacitor undervoltage / overvoltage protection fault. The series capacitor monitoring circuit keeps track of the series capacitor voltage to ensure it is within the correct bounds. If not, the converter will follow the fault response as outlined in pages 19 and 20 of the datasheet (copied below).
Does that answer your question?
Pradeep
