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AN1132: 产品描述90W SMPS FOR MONITORS WITH STANDBY FUNCTION; 文件大小190KB，共12页; 制造商ST(意法半导体); 官网地址http://www.st.com/

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AN1132概述

90W SMPS FOR MONITORS WITH STANDBY FUNCTION

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AN1132

APPLICATION NOTE

90W SMPS FOR MONITORS WITH STANDBY FUNCTION

by Claudio Adragna

Purpose of this note is to provide a brief summary of the specifications and the functionality of the ap-

plication board implementing a 90W multioutput SMPS for monitors, based on the L5991, current

mode PWM controller.

Evaluation results are also presented so as to underline the benefits offered by the L5991 in such a

new generation of SMPS that requires a superior efficiency in standby conditions, aiming at compli-

ance with energy saving standards.

Design Specifications

Table 1 summarises the electrical specification of the application. The complete electrical schematic is

shown in fig. 1 and the bill of material is listed in Table 2.

Table 1. Design Specification

Input Voltage Range (V

)

Mains Frequency (f

)

Maximum Output Power (P

out

)

Horizontal Deflection

88 to 264 Vac

50/60 Hz

90W

out

= 200V

out

= 0.325A

Full load ripple = 1%

out

= 80V

Video Amplifier

Outputs

Vertical Deflection

out

= 0.125A

Full load ripple = 1%

out

±15V

out

= 0.33A

Full load ripple = 1%

out

= 6.3V

Heater

out

= 0.8A

Full load ripple = 2%

Switching Frequency in Normal Mode (f

osc

)

Switching Frequency in Suspend / OFF mode (f

)

Target Efficiency (@ Pout =90W, Vin =88

÷264

Vac) (η)

Maximum Input Power (@ Pout = 0.5 W, Vin =88

264 Vac)

40kHz

18kHz

> 80%

≤

The selected topology is flyback. The operation mode (@ Pout = 90W ) is CCM (Continuous Conduction

Mode) at low mains voltage, DCM (Discontinuous Conduction Mode) at high mains voltage. This design

choice relieves the stress on the power components at low mains voltage, compared with a full DCM so-

lution.

The application will benefit from the features of the L5991 PWM controller in order to minimise the power

drawn from the mains under light load conditions: low start-up and quiescent currents, and Standby

function.

September 2000

1/12

AN1132 APPLICATION NOTE

Application Board Functionality

The outstanding feature of this application board is the so-called Standby Function, directly available

from the L5991. When the power demanded by the load is roughly included between 40 and 90 W (Nor-

mal mode) the switching frequency of the converter is set at 40 kHz. When the monitor enters in low-

consumption mode (Suspend or OFF mode), the power demanded by the load will be much lower, few

Watts. The L5991 will automatically recognise this new operating condition and change the oscillator fre-

quency to 18 kHz. The capacitor C6, along with the parallel of R19 and R20, sets f

osc

; f

is set by C6

and R20.

If the user would like to fine tune the power level that causes the switching frequency to be moved from

osc

to f

inSB

), he or she can add a fixed DC offset (typically in the range 0-200 mV) on the current

sense pin (13, ISEN). This can be accomplished by means of R17, currently not used. The offset will be

the partition of the reference voltage (pin 4, VREF) through R17 and R13.

To change the power level that causes the switching frequency to be moved from f

to f

osc

inNW

), the

ratio f

osc

/ f

should be changed.

R10 and R11 allow to provide an additional DC offset on the current sense which depends on the supply

input voltage. This can be used for compensating L5991’s delay to output. In the present case the delay

is not compensated (R10 and R11 are not assembled) and the effect is a slight dependence of P

inSB

and P

inNW

on the mains voltage (see table 7). This is reinforced by the slope compensation circuit (Q3

and R33), which adds a little offset (variable with the duty cycle) on the current sense pin.

Additionally, the board includes some protection functions tipically required, not only in monitor applica-

tions, such as overvoltage (OVP) and overcurrent protection (OCP).

OCP is inherent in the functionality of the L5991: the controller provides both pulse-by-pulse and "hic-

cup" mode current limitation (see Application Information in the datasheet), which fully protect the con-

verter in case of overload or short circuit.

The OVP, in this specific case, is realised by sensing the supply voltage of the L5991 (generated by the

auxiliary winding) through the divider R7-R8 and feeding this partition into pin 14 (DIS). The divider ratio

is such that the OVP is tripped when the supply voltage exceeds 20V. This protection is particularly ef-

fective in case of feedback disconnection.

At maximum load and minimum mains voltage the converter operates at about 55% duty cycle (this is

why slope compensation is required) but no limitation is imposed on its maximum value: L5991’s pin3

(DC) is shorted to pin 4 (VREF). If desired, it is possible to set the maximum duty cycle by adding the di-

vider R34-R35. Please refer to Application Information in L5991 datasheet for calculation of the voltage

divider.

The application board is supplied with a start-up circuit simply made of a dropping resistor (R2+R3) that

draws current from upstream the bridge rectifier.

Figure 3. Low-consumption start-up circuit (not currently implemented)

88 to 264

VAC

R05 33K

R04

2.2M

Q01

STK2N50

R06

10K

D02

20V

Q02

BC337

D04 1N4148

L5991

5/12

AN1132

AN1132概述

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