DFS06M datasheet(6/42 Pages) STMICROELECTRONICS

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AN1262 APPLICATION NOTE

6/42

INPUT BULK CAPACITOR SELECTION

The input bulk capacitor Cin, along with the bridge rectifier, converts the AC mains voltage to an unregulated

DC bus, Vin, which is the input voltage for the downstream flyback converter. Cin must be large enough to have

a relatively low ripple superimposed on top of the DC level, as shown in fig. 3. At minimum specified mains volt-

age, the value of Cin determines the absolute minimum, Vinmin, of the DC input voltage of the converter. The

maximum duty cycle and the maximum peak current allowed by the IC must not be exceeded at this voltage.

However, as to thermal consideration, the bus DC voltage (VDCmin @ VACmin) should be considered.

Figure 3. Input voltage waveforms:

a) without holdup capability;

b) with holdup capability.

Large values of Cin result in higher VDCmin and Vinmin, lower peak and RMS current through the power MOSFET

(i.e. less power dissipation in the device) and less duty cycle range to achieve regulation but, on the other hand,

also in bigger capacitor size, higher peak and RMS current drawn from the mains (i.e. more power dissipation

in the bridge rectifier). Small values of Cin give origin to the opposite situation.

Experience shows that a good compromise between these contrasting requirements is a Cin value that causes

the peak-to-peak ripple amplitude to be 25-30% of the peak mains voltage (@ VACmin), which means that Vinmin

will be 70-75% of the peak value. Anyway, if holdup capability is required, a much larger capacitance values will

be needed: the voltage ripple across Cin is expected to be 25-30% of the peak value, after 1 mains cycle miss-

ing, which means that in normal operation the ripple will be much less.

Table 5 summarizes the required capacitance per watt of input power for a given value of Vinmin, with and with-

out holdup requirement, and shows the resulting values of VDCmin. This allows to calculate the minimum capac-

itance needed, by multiplying the value taken from the table times Pin. Then a standard value will be selected,

taking also the tolerance into account.

Vin

VPKmin

Vinmin

VDCmin

Vin

VPKmin

Vinmin

after fail

VDCmin

after fail

1 missing cycle

Vinmin

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