AN938 Datasheet, PDF (28/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Applications
AN938
Figure 38. Output voltage vs. R4
Vo
(V)
21
18
15
12
9
6
3
0
3456789
R4(KΩ)
AM03520v1
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
Pd = li ⋅ Vce = li ⋅ (Vi – 50)
In the buck converter, the average input current is:
Equation 23
li
=
Io ⋅ ^T-----^o---ⁿ--
T
=
Io
⋅
V_-----o_--
Vi
● 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)
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