TOP232P datasheet(14/38 Pages) POWERINT

14 / 38 page

Rev. D 10/20

TOP232-234

www.power.com

Figure 24. 30 W Power Supply using External Current Limit.

Application Examples

A High Efficiency, 30 W, Universal Input Power Supply

The circuit shown in Figure 24 takes advantage of several of

the TOPSwitch-FX features to reduce system cost and power

supply size and to improve efficiency. This design delivers 30 W

at 12 V, from an 85 to 265 VAC input, at an ambient of 50 ˚C,

in an open frame configuration. A nominal efficiency of 80% at

full load is achieved using TOP234.

The current limit is externally set by resistors R1 and R2 to a

value just above the low line operating peak current of

approximately 70% of the default current limit. This allows use

of a smaller transformer core size and/or higher transformer

primary inductance for a given output power, reducing

TOPSwitch-FX power dissipation, while at the same time

avoiding transformer core saturation during startup and output

transient conditions. The resistor R1 provides a feed-forward

signal that reduces the current limit with increasing line

voltage, which, in turn, limits the maximum overload power at

high input line voltage. The feed-forward function in

combination with the built-in soft-start feature of TOPSwitch-

FX, allows the use of a low cost RCD clamp (R3, C3 and D1)

with a higher reflected voltage, by safely limiting the

TOPSwitch-FX drain voltage, with adequate margin, under

worst case conditions. The extended maximum duty cycle

feature of TOPSwitch-FX (guaranteed minimum value of 75%

vs. 64% for TOPSwitch-II) allows the use of a smaller input

capacitor (C1). The extended maximum duty cycle and the

higher reflected voltage possible with the RCD clamp also

permit the use of a higher primary to secondary turns ratio for

T1 which reduces the peak reverse voltage experienced by the

secondary rectifier D8. As a result, a 60 V Schottky rectifier

can be used for up to 15 V outputs, which greatly improves

power supply efficiency. The cycle skipping feature of the

TOPSwitch-FX eliminates the need for any dummy loading for

regulation at no load and reduces the no load/standby

consumption of the power supply. Frequency jitter provides

improved margin for conducted EMI meeting the CISPR 22

(FCC B) specification.

A simple Zener sense circuit is used for low cost. The output

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

the voltage drops across the optocoupler (U2) LED and

resistor R6. Resistor R8 provides bias current to Zener VR2 for

typical regulation of ±5% at the 12 V output level, over line and

load and component variations.

12 V

@ 2.5 A

1N4148

µF

10 V

LTV817A

VR2

1N5240C

10 V, 2%

150

Ω

R15

150

Ω

C14

1 nF

UF4005

68 k

Ω

4.7 nF

1KV

CY1

2.2 nF

TOP234Y

TOPSwitch-FX

150

Ω

µF

400 V

100 nF

MBR1060

C10

560

µF

35 V

C12

220

µF

35 V

C11

560

µF

35 V

CONTROL

RTN

6.8

Ω

4.7 M

Ω

1/2 W

9.09 k

Ω

PI-2525-062515

3.3

µH

BR1

600 V

3.15 A

20 mH

CX1

100 nF

250 VAC

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