A

8/97

6

TOP209/210

2

General Circuit Description

Figure 7 shows a low-cost, DC input, flyback switching power

supply using the TOP210 integrated circuit. This 5 V, 4 W

power supply operates from a DC voltage derived from rectified

and filtered AC mains voltage of 85 to 265 VAC. The 5 V

output is indirectly sensed via the primary bias winding. The

output voltage is determined by the TOPSwitch CONTROL pin

shunt regulator voltage (V

and D3, and the turns ratio between the bias winding and output

winding of T1. Other output voltages are also possible by

adjusting the transformer turns ratios.

The high voltage DC bus is applied to the primary winding of

T1. Capacitor C1 filters the high voltage supply, and is only

necessary if the connections between the high voltage DC

supply and the TOP210 are long.

The other side of the

transformer primary is driven by the integrated high-voltage

MOSFET within the TOP210. D1 and VR1 clamp the voltage

spike caused by transformer leakage inductance to a safe value

and reduce ringing at the DRAIN of U1. The power secondary

winding is rectified and filtered by D2, C2, L1, and C3 to create

the 5V output voltage. The bias winding is rectified and filtered

by D3, R1 and C5 to create a bias voltage to the TOP210. C5

also filters internal MOSFET gate drive charge current spikes

on the CONTROL pin, determines the auto-restart frequency,

and together with R1, compensates the control loop.

PI-2045-041798

+5 V

RTN

C1

10 nF

400 V

C5

47

µF

10 V

D2

1N5822

D3

1N4148

C2

330

µF

10 V

T1

D1

UF4005

C3

100

µF

10 V

R1

15

Ω

VR1

BZY97-

C120

120 V

L1

3.3

µH

CIRCUIT PERFORMANCE:

Line Regulation - –1.5%

(104-370 VDC)

Load Regulation - –5%

(10-100%)

Ripple Voltage –25 mV

+

-

U1

TOP210

2

1

3

4

5

8

DC

INPUT

TRD1

D

S

C

CONTROL

TOPSwitch

Figure 7. Schematic Diagram of a Minimum Parts Count 5 V, 4 W Bias Supply Using the TOP210.