Electrical – the role of including bulk input capacitor and startup resistors in the flyback dc to dc buck converter

buckdc/dc converter

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In the above figure \$Cin\$ is the bulk capacitor and \$Rstarup1\$ and \$Rstartup2\$ are the startup resistors (Startup circuit).For the given circuit I am using UCC28C40.In it's datasheet it is said that \$Cin\$ is used to maintain an acceptable minimum bulk voltage level (Vbulk(min)).

But what is purpose of setting bulk voltage to minimum value? What will happen if these components (Cin and startup resistors) are not included?

Best Answer

Without the start-up circuit the chip will never receive any initial power to begin driving the MOSFET and hence it won't receive full normal power (needed) from the auxiliary winding on the transformer.

The start-up resistors are only present to ensure that the chip gets enough voltage to start driving the MOSFET - then it can receive normal operating power through the extra winding.

A bulk capacitor is needed on virtually any power supply regulator or indeed any chip. It provides a local circulatory path for high currents taken by those chips and, from the perspective of the power feed makes life easier. Ultimately it reduces EMI emissions and also susceptibility - it is a fundamental component in virtually every circuit ever developed.

In it's datasheet it is said that Cin is used to maintain an acceptable minimum bulk voltage level (Vbulk(min))

Absolutely!! When the MOSFET turns on it draws a significant surge of current from the power feed lines and, if those lines have inductance and resistance (which they all do have) then the surge will cause a fall in line voltage for the period of that surge current. The capacitor is there to supply that surge current and reduce the effect of surge volt drop down the wires.