AN938 datasheet(28/33 Pages) STMICROELECTRONICS

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Applications

AN938

28/33

Doc ID 5410 Rev 10

Figure 38.

Output voltage vs. R4

Particular care has to be taken for input filter capacitors because they have to support high

ripple current at mains frequency plus the high frequency current.

Total RMS current is typically heavy, and very low ESR is requested for system reliability.

Input caps can also affect system efficiency.

The transformer could be a single secondary winding with four diodes for rectification or

center-tapped with two diodes only, but higher reverse voltage.

Considering a maximum output power close to 100 W, equivalent system efficiency of 93-

95%, the mains transformer has to be designed around 200 VA.

A low voltage secondary PFC can reduce the VA need close to the delivered watts (110 W),

reducing also the value of the electrolytic capacitors.

Weight and volume of the complete application are also significantly reduced.

4.2

Higher input voltage

Since the max. operating input voltage is 55 V, an input line conditioner is requested. Fixing,

for example, the device supply voltage at 50 V, the power dissipation of the pre-regulator is:

Equation 22

In the buck converter, the average input current is:

Equation 23

●

Vo = 5.1 V; Io = 3.5 A; Po = 17.85 W; Ii = 0.357A; Pd = 3.57 W; (Vi = 60 V)

●

Vo = 12 V; Io = 3.5 A; Po = 42 W; Ii = 0.84 A; Pd = 8.4 W (Vi = 60 V)

AM03520v1

4567

9R4(K

Ω)

(V)

li Vce

–

()

⋅

Ton

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-------

⋅

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