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AN1317 - APPLICATION NOTE

Figure 5: 2W typical single output not isolated power supply with Inverter topology

3. DESIGN METHODOLOGY

The schematic of either fig. 1 or fig. 5 can be separated into six blocks:

• The oscillator network composed by R1 and C3.

• The Buck or inverter structure, which is composed by the on chip MOSFET, the inductor L1, the free

wheeling diode D3 and the output filtering capacitor C5.

• The VIPer20 supply circuit, composed by D2 and C2.

• The front rectifier and filter.

• The error amplifier compensation network composed by R2 and C4.

All these functions will be detailed in the next paragraphs.

3.1 Switching Frequency And Duty Cycle

Sections 1 and 2 showed that the input voltage transformation is entirely managed by the VIPer20 which

controls the switching duty cycle. Whatever the topology is, the goal is to look for the widest load

output load. For maximum load, although the VIPer20 is perfectly compatible from the continuous mode,

it must be avoided because the power dissipation in the free wheeling diode D3 would be too high and

the inductor size and price would increase. For all the above reasons, these topologies are operated at

13V

OSC

COMP

SOURCE

DRAIN

VDD

-

+

VIPer20

R2

3.9k

C4

100nF

R1

10k

C3

10nF

D1

1N4007

C1

22uF

400V

L1

470uH

D3

BYT01-400V

C5

33uF

16V

D2

BYT01-400V

C2

10uF

16V

AC IN

AC IN

-13V

GND OUT