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LR745

High Input Voltage SMPS Start-up Circuit

Supertex Inc 

Block Diagram Detailed

Description

The Supertex LR7 is a high voltage switch mode power supply

start-up circuit, which has 3 terminals: VIN, GND, and VOUT. An

input voltage range of 25VDC to 450VDC can be applied directly

at the input VIN pin. The output voltage, VOUT, is monitored by

the 2 comparators, comp1 and comp2. An internal reference,

VREF, and resistor divider R1, R2, and R3 set the nominal VOUT

trip points of 7.0V for comp1 and 13.25V for comp2.

When a voltage is applied on VIN, VOUT will start to ramp up from

0V. When VOUT is less than 7.0V, the output of comp1 will be at

a logic high state keeping the D flip flop in a reset state. The

output of the D flip flop, Q, will be at logic low keeping transistor

M2 off. The data input for the D flip flop, D, is internally connected

to a logic high. As VOUT becomes greater than 7.0V, comp1 will

change to a logic low state. VOUT will continue to increase, and

the constant current source of typically 3mA output will charge an

external storage capacitor. As VOUT reaches above 13.25V, the

output of comp2, will then switch from a logic high to a logic low

state. The D flip flop’s output does not change state since its

clock input is designed to trigger only on a rising edge, logic low

to logic high transition. When there is no load connected to the

output, the output voltage will continue to increase until it

reaches 21.5V which is the zener voltage minus the threshold

voltage of transistor M1. The zener voltage is typically 23V and

the threshold voltage of M1 is typically 1.5V. The zener diode is

biased by resistor R4.

VOUT will start to decrease when it is connected to an external

load greater than the internal constant current source, which is

the case when the PWM IC starts up. When VOUT falls below

13.25V, the output of comp2 will switch from a logic low to a logic

high. The output of comp2 will clock in a logic 1 into the D flip flop

causing the D flip flop’s output, Q, to switch from a logic low to a

logic high. Transistor M2 will then be turned on pulling the gate

of transistor M1 to ground thereby turning transistor M1 off.

Transistor M1 will remain off as long as VOUT is greater than 7.0V.

Once VOUT decreases below 7.0V, comp1 will reset the D flip flop,

thereby turning transistor M2 off and transistor M1 back on.

LR745

Typical Application

Figure 1 shows a simplified typical configuration of a switch

mode power supply, SMPS, using the Supertex LR7 in the start-

up circuit.

The LR7’s VOUT terminal is connected to the VCC line of a PWM

IC, Unitrode part #UC3844. An auxiliary winding on the trans-

former is used to generate a VCC voltage to power the PWM IC

after start-up. The LR7 is used to supply power for the PWM IC

only during start-up. After start-up, the LR7 turns off and the

auxiliary winding is used to supply power for the PWM IC. Figure

2 shows the typical current and voltage waveforms at various

stages from power up to operation powered by the auxiliary

winding.

Stage I

Once a voltage is applied on VIN, the LR7 will start to charge the

VCC capacitor, C1. The VCC voltage will start to increase at a rate

limited by the internal current limiter of 3.0mA. The PWM IC is

in its start-up condition and will typically draw 0.5mA from the VCC

line. The VCC voltage will continue to increase until it reaches the

PWM IC’s start threshold voltage of typically 16V.

Stage II

Once VCC reaches 16V, the PWM IC is in its operating condition

and will draw typically 20mA depending on the operating fre-

quency and size of the switching MOSFET. The output of LR7,

VOUT, is internally current limited to 3.0mA. The remaining 17mA

will be supplied by C1 causing the VCC voltage decrease. When

VCC decreases to 13.25V, the LR7 will turn off its output thereby

reducing its input current from 3.0mA to 10’s of microamperes.

At this point, all 20mA will be supplied by C1. The PWM IC can

now operate to a minimum VCC voltage of typically 10V.

Once the switching MOSFET starts operating, the energy in the

primary winding is transferred to the secondary outputs and the

auxiliary winding, thereby building up VAUX. It is necessary to

size the VCC storage capacitor, C1, such that VAUX increases to

a voltage greater than 10V before VCC decreases to 10V. This

allows VAUX to supply the required operating current for the PWM IC.

(Continued on page 14-9)

VIN

IIN

High Voltage

IAUX VAUX D2

C2

LR7

VOUT

VCC

GND

C1

PWM IC

UC3844

Figure 1: Simplified SMPS using LR7

3

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