TOP242 datasheet(21/48 Pages) POWERINT

21 / 48 page

TOP242-249

August 8, 2000

7/01

Figure 41. 30 W Power Supply using External Current Limit Programming and Line Sensing for UV and OV.

Application Examples

A High Efficiency, 30 W, Universal Input Power Supply

The circuit shown in Figure 41 takes advantage of several of the

TOPSwitch-GX 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

°C, in an open frame configuration. A nominal efficiency of

80% at full load is achieved using TOP244Y.

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

value just above the low line operating peak DRAIN 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-GX power dissipation, while at the same time

avoiding transformer core saturation during startup and output

transient conditions. The resistors R1 & R2 provide a signal

that reduces the current limit with increasing line voltage,

which in turn limits the maximum overload power at high input

line voltage. This function in combination with the built-in

soft-start feature of TOPSwitch-GX, allows the use of a low cost

RCD clamp (R3, C3 and D1) with a higher reflected voltage, by

safely limiting the TOPSwitch-GX drain voltage, with adequate

margin under worst case conditions. Resistor R4 provides line

sensing, setting UV at 100 VDC and OV at 450 VDC. The

extended maximum duty cycle feature of TOPSwitch-GX

(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 frequency

reduction feature of the TOPSwitch-GX 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.

Output regulation is achieved by using a simple Zener sense

circuit 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

2 W

4.7 nF

1 kV

CY1

2.2 nF

TOP244Y

150

Ω

µF

400 V

0.1

µF

MBR1060

C10

560

µF

35 V

C12

220

µF

35 V

C11

560

µF

35 V

RTN

6.8

Ω

4.7 M

Ω

1/2 W

2 M

Ω

1/2 W

9.09 k

Ω

PI-2657-040501

3.3

µH

BR1

600 V

3.15 A

20 mH

CX1

100 nF

250 VAC

CONTROL

TOPSwitch-GX

PERFORMANCE SUMMARY

Output Power:

30 W

Regulation:

± 4%

Efficiency:

≥ 79%

Ripple:

≤ 50 mV pk-pk

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