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20 W Universal Supply using 8 Lead PDIP

Figure 8 shows a 12 V, 20 W secondary regulated flyback power

supply using the TOP224P in an eight lead PDIP package and

operating from universal 85 to 265 VAC input voltage. This

example demonstrates the advantage of the higher power 8 pin

leadframe used with the TOPSwitch-II family. This low cost

package transfers heat directly to the board through six source

pins, eliminating the heatsink and the associated cost. Efficiency

is typically 80% at low line input. Output voltage is directly

sensed by optocoupler U2 and Zener diode VR2. The output

voltage is determined by the Zener diode (VR2) voltage and the

voltage drops across the optocoupler (U2) LED and resistor R1.

Other output voltages are possible by adjusting the transformer

turns ratio and value of Zener diode VR2.

AC power is rectified and filtered by BR1 and C1 to create the

high voltage DC bus applied to the primary winding of T1. The

other side of the transformer primary is driven by the integrated

TOPSwitch-II high-voltage MOSFET. D1 and VR1 clamp

leading-edge voltage spikes caused by transformer leakage

inductance. The power secondary winding is rectified and

filtered by D2, C2, L1, and C3 to create the 12 V output voltage.

R2 and VR2 provide a slight pre-load on the 12 V output to

improve load regulation at light loads. The bias winding is

rectified and filtered by D3 and C4 to create a TOPSwitch bias

voltage. L2 and Y1-safety capacitor C7 attenuate common

mode emission currents caused by high voltage switching

waveforms on the DRAIN side of the primary winding and the

primary to secondary capacitance. Leakage inductance of L2

with C1 and C6 attenuates differential-mode emission currents

caused by the fundamental and harmonics of the trapezoidal or

triangular primary current waveform. C5 filters internal

MOSFET gate drive charge current spikes on the CONTROL

pin, determines the auto-restart frequency, and together with

R1 and R3, compensates the control loop.

Figure 8. Schematic Diagram of a 20 W Universal Input TOPSwitch-II Power Supply using an 8 lead PDIP.

PI-2019-033197

D2

MUR420

D3

1N4148

C2

330

µF

35 V

C3

220

µF

35 V

T1

D1

BYV26C

VR1

P6KE200

VR2

1N5241B

11 V

R2

220

Ω

BR1

400 V

C1

47

µF

400 V

F1

3.15 A

J1

C6

0.1

µF

250 VAC

L2

22 mH

L

N

C5

47

µF

U1

TOP224P

D

S

C

CONTROL

TOPSwitch-II

R3

6.8

Ω

L1

3.3

µH

C4

0.1

µF

U2

PC817A

R1

100

Ω

C7

1 nF

250 VAC

Y1

+12 V

RTN