D

7/96

TOP200-4/14

7

Simple Optocoupler Feedback

The circuit shown in Figure 8 is a 7.5 V,

15 W secondary regulated flyback power

supply using the TOP202 that will

operate from 85 to 265 VAC input

voltage.

Improved output voltage

accuracy and regulation over the circuit

of Figure 7 is achieved by using an

optocoupler and secondary referenced

Zener diode. The general operation of

the power stage of this circuit is the same

as that described for Figure 7.

The input voltage is rectified and filtered

by BR1 and C1. L2, C6 and C7 reduce

conducted emission currents. The bias

winding is rectified and filtered by D3

and C4 to create a typical 11 V bias

voltage.

Zener diode (VR2) voltage

together with the forward voltage of the

LED in the optocoupler U2 determine

the output voltage. R1, the optocoupler

current transfer ratio, and the TOPSwitch

control current to duty cycle transfer

function set the DC control loop gain.

C5 together with the control pin dynamic

impedance and capacitor ESR establish

a control loop pole-zero pair. C5 also

determines the auto-restart frequency

and filters internal gate drive switching

currents. R2 and VR2 provide minimum

current loading when output current is

low. See DN-11 for more information

regarding the use of the TOP202 in a

low-cost, 15 W universal power supply.

Accurate Optocoupler Feedback

The circuit shown in Figure 9 is a highly

accurate, 15 V, 30 W secondary-

regulated flyback power supply that will

operate from 85 to 265 VAC input

voltage.

A TL431 shunt regulator

directly senses and accurately regulates

the output voltage. The effective output

voltage can be fine tuned by adjusting

the resistor divider formed by R4 and

R5. Other output voltages are possible

by adjusting the transformer turns ratios

as well as the divider ratio.

The general operation of the input and

power stages of this circuit are the same

as that described for Figures 7 and 8. R3

and C5 tailor frequency response. The

TL431 (U2) regulates the output voltage

by controlling optocoupler LED current

(and TOPSwitch duty cycle) to maintain

an average voltage of 2.5 V at the TL431

input pin. Divider R4 and R5 determine

the actual output voltage. C9 rolls off

the high frequency gain of the TL431 for

stable operation. R1 limits optocoupler

LED current and determines high

frequency loop gain.

For more

information, refer to application note

AN-14.

Figure 8. Schematic Diagram of a 15 W Universal Input Power Supply Utilizing the TOP202 and Simple Optocoupler Feedback.

PI-1695-112895

7.5 V

RTN

C5

47

µF

D2

UG8BT

D3

1N4148

R2

68

Ω

VR2

1N5234B

6.2 V

C3

120

µF

25 V

T1

D1

UF4005

C2

680

µF

25 V

VR1

P6KE150

CIRCUIT PERFORMANCE:

Line Regulation - ±0.5%

(85-265 VAC)

Load Regulation - ±1%

(10 -100%)

Ripple Voltage ± 50 mV

Meets CISPR-22 Class B

BR1

400 V

C1

33

µF

400 V

R1

39

Ω

U2

NEC2501

U1

TOP202YAI

DRAIN

SOURCE

CONTROL

C4

0.1

µF

C7

1 nF

Y1

L1

3.3

µH

F1

3.15 A

J1

C6

0.1

µF

L2

22 mH

L

N

ST202A REFERENCE DESIGN BOARD