AN4205

Input stage of the AC/DC converter

Doc ID 023929 Rev 1

13/23

2.4

UC3842 driver

A simple PWM driver has been chosen to drive the gate of the STD3NK80Z device and

direct the flyback topology requested by the converter. The driver is the common UC3842

and it provides all the essential features necessary to the operation of the basic current

mode controller. The UC3842 needs 15 V for its correct biasing and this voltage reference is

supplied by the auxiliary output of the transformer.

Figure 6.

UC3842 PWM driver and network

The next step is to design the network for the PWM driver pins.

The first step is to determine the required circuit dead time in order to select the oscillator

components. Once obtained, Figure 7 is used to pinpoint the nearest standard value of C9.

Next, the appropriate R5 value is interpolated using the parameters for C9 and oscillator

frequency. Figure 8 illustrates the R5/C9 combinations versus oscillator frequency.

3

6

5

4

8

1

2

7

+

-

PWM driver

R1

C2

C3

C4

C8

C7

C9

C10

C11

R2

R3

R4

R5

R6

R8

R9

R7

D3

D2

D4

M1

AM16372v1

Figure 7.

Dead time vs. C9 (R5>5k)

Figure 8.

Time resistance vs. frequency

0.3

1

3

10

30

12.2

4.7

10

22

47

100

C9=(nF)

AM16373v1

3

10

30

100

100

1k

10k

100k

10M

Frequency (Hz)

AM16374v1