TOP233P datasheet(16/36 Pages) POWERINT

16 / 36 page

TOP232-234

7/01

17 W PC Standby Power Supply

Figure 26 shows a 17 W PC standby application with 3.3 V and

5 V secondary outputs and a 15 V primary output. The supply

uses the TOP232 operating from 230 VAC or 100/115 VAC

with doubler input. This design takes advantage of the soft-

start, line under-voltage detect, tighter current limit variation

and higher switching frequency features of TOPSwitch-FX. For

example, the higher switching frequency with tighter current

limit variation allows use of an EE19 transformer core.

Furthermore, the spacing between high voltage DRAIN pin and

low voltage pins of the TOPSwitch-FX packages provides large

creepage distance which is a significant advantage in high

pollution environments such as fan cooled PC power supplies.

Capacitor C1 provides high frequency decoupling of the high

voltage DC supply, and is necessary only if there is a long trace

length from the source of the DC supply to the inputs of this

standby circuit. The line sense resistor R1 senses the DC input

voltage for line under-voltage. When AC is turned off, the

under-voltage detect feature of the TOPSwitch-FX prevents

auto-restart glitches at the output caused by the slow discharge

of large storage capacitance in the main converter. This is

achieved by turning the power supply off when the input

voltage goes below a level needed to maintain output regulation

and keeping it off until the input voltage goes above the

under-voltage threshold (V

UV), when the AC is turned on again.

The under voltage threshold is set at 200VDC, slightly below

the required lowest operating DC input voltage, for start-up at

170VAC. This feature saves several components needed to

implement the glitch free turn off with discrete or

TOPSwitch-II based designs.

The bias winding is rectified and filtered by D2 and C6 to create

a bias voltage for the TOP232 and to provide a 15V primary

bias output voltage for the main power supply primary control

circuitry. Both 3.3V and 5V output voltages are sensed by R9,

R10 and R11 using a TL431 (U3) circuit shown. Resistor R6

limits current through optocoupler U2 and sets overall AC

control loop gain. Resistor R7 assures that there is sufficient

bias current for the TL431 when the optocoupler is at a minimum

current. Capacitor C8 provides a soft-finish function to eliminate

turn-on overshoot. The no load regulation (cycle-skipping) of

TOPSwitch-FX permits the circuit to meet the low standby

power requirement of the Blue Angel specification for PCs.

Figure 26. 17 W PC Standby Supply.

3.3 V

@ 2 A

200-375

VDC

BAV21

µF

35 V

C11

100

µF

10 V

µF

SFH615-2

TL431CLP

0.1

µF

301

Ω

16.2 k

Ω

UF4005

VR1

BZY97C-200

TOP232Y

TOPSwitch-FX

510

Ω

0.01

µF

1 kV

(optional)

(Primary

Referenced)

35 V

SB540

5 V

@ 2 A

C12

1000

µF

10 V

C10

1000

µF

10 V

CY1

1 nF

PI-2537-040401

RTN

15 V

@ 30 mA

6.8

Ω

3.9 M

Ω

3.3

µH

3.3

µH

R10

12.1 k

Ω

R11

10 k

Ω

C13

100

µF

10 V

CONTROL

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