TOP233P datasheet(14/36 Pages) POWERINT

14 / 36 page

TOP232-234

7/01

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-2525-040401

3.3

µH

BR1

600 V

3.15 A

20 mH

CX1

100 nF

250 VAC

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