AN4205

Input stage of the AC/DC converter

Doc ID 023929 Rev 1

5/23

2

Input stage of the AC/DC converter

Figure 2.

Input stage of AC/DC converter

The input stage consists of a flyback topology where the main active elements are the

standard STD3NK80Z Power MOSFET device and the reliable UC3842 PWM driver. As a

power conversion topology, the flyback has been chosen because the control is easier, it's

cheap and doesn't need an inductive input filter. In fact the main transformer works as an

inductive filter itself. Therefore, the properties of the flyback meet the cost saving target of

this project. Next, the specifications and the design of the input stage will be shown.

The first step is to define the specifications that the user must set for the AC/DC converter.

The flyback operation, with reference to the basic circuit in Figure 2, is a two-step process.

The first step is the on-time of the switch, where the energy is taken from the input and

stored in the primary winding of the flyback transformer. At the secondary stage, the diode is

reverse-biased and the load is supplied by the energy of the bulk capacitor. Then the switch

turns off (off-time) and the energy stored in the primary is transferred to the transformer

secondary. So through the diode, forward-biased, the stored energy is delivered to the out

Table 1.

Input stage specifications data

Name

Symbol

Value

Minimum DC input voltage

250 V

Maximum DC input voltage

370 V

DC output voltage

~19 V

Output power

~7 W

Electrical efficiency

η

80%

Switching frequency

100 kHz

AC

3

6

5

4

8

1

2

7

+

-

Fly-back topology

PWM driver

Input stage

C1

R1

C2

C3

C4

C5

C6

C8

C7

C9

C10

C11

R2

R3

R4

R5

R6

R8

R9

R7

D3

DZ1

D1

D2

D4

D5

L1

T1

M1

AM16368v1