SG2540M Ver la hoja de datos (PDF) - Microsemi Corporation

Número de pieza

componentes Descripción

Fabricante

SG2540M

OFF-LINE START-UP CONTROLLER

Microsemi Corporation 

APPLICATION INFORMATION

SG1540/SG2540/SG3540

FIGURE 3 - EFFICIENT PRIMARY SIDE START-UP

PRIMARY SIDE START-UP

When the design goal is efficient start-up for a control PWM referenced to the primary side of the power transformer, the configuration

in Figure 3 is recommended. An energy storage capacitor CSTART is trickle-charged from the 300-400 Volt DC bus by resistor RSTART.

The value of RSTART is chosen to provide a constant 1mA charging current, allowing the use of a ½ watt resistor. As the voltage on

CSTART ramps up from zero, the only load current is the standby current of the SG1540 and that of the divider network R1-R3. (Connecting

the TIMING pin to +VIN disables the internal power oscillator and forces the circuitry into a micropower standby model. Since the input

bias current at the START pin is 1µA maximum, a divider current of 100µA is adequate).

When the voltage at the START pin reaches +2.5 Volts, the hysteresis transistor turns off, overdriving the START pin. The VOUT pin

is switched to the HIGH state, providing power to the PWM control circuit. As energy flows out of the START capacitor, its voltage

decays; but it remains connected to the PWM circuit until the dropout voltage is reached (VSTART - VHYSTERESIS). The bootstrap winding

on the power transformer and rectifier diode D5 prevent this from happening. As the PWM control circuit becomes active, the power

transistor begins to switch, providing operating current to the PWM circuit through the SG1540.

RESISTOR CALCULATIONS

Given that VSTART and VDROPOUT have been chosen, and that the divider current at start-up is 100µA, then the values for R1 through R3

are calculated as follows:

1. For simplification, let X = VSTART - 2.5

2.5

and Y = VDROPOUT -2.5

2.5

2. Then,R1 = 2.5 x 104 * X

[1]

R2 = R1/Y

[2]

and R3 = R1 * R2

[3]

X * R2 - R1

DESIGN EXAMPLE

Suppose we have a power MOSFET device, and so want to start at +18 volts and drop out at +12 volts.

Then

X = 6.20

and Y = 3.80

Therefore

R1 = 2.5 x 104 * 6.2 = 155K

R2 = 1.5 x 105/3.8 = 39.5K

R3 = 1.5 x 105 * 3.9 x 104 = 63.7K

6.2 * 3.9 x 104 - 1.5 x 105

(Choose 150K)

(Choose 39K)

(Choose 62K)

4/90 Rev 1.1 2/94

Copyright © 1994

LINFINITY Microelectronics Inc.

4

11861 Western Avenue ∞ Garden Grove, CA 92841

(714) 898-8121 ∞ FAX: (714) 893-2570

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