3

Rev D, July 2000

Performance in a Flyback Converter

Figure 3 shows the design of a commercially available 60 watt flyback converter with two outputs

(+160V, +15V), operating at a switching frequency of 80kHz and an input voltage of 220VAC. This

type of switching power supply is used for applications, such as monitors, TVs, and miscellaneous

instruments, requiring multiple output voltages . This discontinuous mode flyback converter, using

a KA3882 current mode controller, features good voltage tracking with the use of pulse by pulse

current sensing on the primary side, and an isolated secondary feedback loop. The PWM IC

KA3882 directly drives the power MOSFET.

As the power MOSFET sequentially turns on and off, energy is stored in the transformer core dur-

ing the on time, and is then transferred to the output capacitor during the off time. When the power

MOSFET turns off, the energy stored in the leakage inductance causes a voltage spike across the

drain-to-source terminal of the power MOSFET, which amounts to at least twice the input voltage

voltage spike from exceeding the BVdss rating of a MOSFET. A power MOSFET must have high

voltage capability with lower on-resistance and smaller gate charge for higher efficiency.

Figure 3: Flyback Converter Circuit Diagram

1. 'n' indicates a turns ratio of the transformer windings. The voltage of Vin + nVo + leakage inductance voltage of the transformer appears at

the primary side.

7

8

6

3

5

2

1

4

L1

D1

T1

Fuse

U1

U2

U3

Q1

Vout1

Vout2

QFET(FQP7N60)

or Conventional

MOSFET

Vin=220VAC

1

2

3

C1

1

2

C4

C2

C3

C5

R2

R1

1

2

R3

1

R5

C6

R14

D3

1

2

C10

C9

R11

1

2

D4

1

2

R11

1

2

R13

D5

C8

R10

C7

R9

R6

R7

R8

R19

R20

2

D7

1

2

C13

R14

R16

R17

1

2

C14

D2

1

2

C11

R4

C12

R18

1

5

4

8

D6

1

2

5V 0.8A

160V 0.3A