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AN827 APPLICATION NOTE

Pin 18 COSC (Oscillator capacitor)

These pins determine the oscillator frequency of the circuit. A resistor, R17, is connected from pin 17 to ground.

A capacitor, C4, is connected from pin 18 to ground. The operating frequency is calculated as follows:

Pin 19 VCC (Supply voltage input)

The IC must be supplied with a very low current, 0.3mA typical, during start-up. The turn-on threshold is 15.5V

typical with 5.5 Volts typical of hysteresis. The start-up current is provided by the resistor/capacitor network driv-

en off the rectified line voltage. A fast start-up circuit is employed to quickly turn on the IC and reduce power

consumption in the start-up resistor, R28. The capacitor, C12, has a value of 220

µF to ensure sufficient hold-

up time to allow the auxiliary winding to provide voltage after initial start-up. The fast start-up is realized with Q2,

Q3, R25, R26, R27, R28, D5 and C12. The fast start-up circuit is turned-off when the controller turn-on threshold

is reached and Vref forward biases Q2, pulling the gate of Q3 to ground.

The auxiliary winding on the main boost inductor provides the normal operating voltage for the controller. The

voltage induced on this winding is rectified by diodes D7-D10. Resistor R29 provides current limiting and zener

D6 regulates the supply voltage to 18 Volts.

Pin 20 GDRV (Gate driver output)

The output of this pin is internally clamped at 15V to prevent breakdown of the power mosfet gate oxide. A re-

sistor, R18, of 15

Ω is placed in series with the gate of the power mosfet to avoid overshoot and limit the di/dt of

the switch. A 1N4148 diode, D3, is connected to the gate to provide fast turn-off of the power mosfet.

EVALUATION RESULTS

The 500W demoboard has been evaluated for the following parameters: PF (power factor), % THD (percent

total harmonic distortion), H3..H7 (percentage of current’s nth harmonic amplitude), Vout (output voltage) and

efficiency (n). The test configuration and test results are shown below:

Test Set-Up and Equipment

Table 1. 500W Demoboard Evaluation Results

f

PF

THD

H3

H5

H7

η

Vrms

(Hz)

(W)

(%)

(%)

(%)

(%)

(V)

(W)

(%)

88

60

560

99.9

2.9

1.3

1.7

1.2

402

490

87.5

110

60

543

99.9

2.8

1.4

1.8

1.3

403

492

90.6

220

50

525

99.8

3.3

1

2.4

1.1

406

499

95.1

270

50

523

99.8

3.4

1

2.6

1.1

408

504

96.3

f

sw

2.44

R

os c

C

osc

⋅

-----------------------------

2.44

30.1k 1n

⋅

--------------------------

80 kHz approx.

==

=

AC POWER

SOURCE

LARCET 3KW

PM1200

AC POWER

ANALYSER

EMI

FILTER

PFC

L4981

DEMO

LOAD