AN827 Datasheet, PDF (13/20 Pages) STMicroelectronics – A 500W HIGH POWER FACTOR WITH THE L4981A

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:
f_sw = _R-----_o---_s-²--_c--^.-⁴-_â‹---_C4----_o---_s---_c- = ₃----₀---_.-²-₁--^.--_k4---⁴-_â‹---₁----_n-- = 80 kHz approx.
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 220µF 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 15Ω 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
AC POWER
SOURCE
LARCET 3KW
PM1200
AC POWER
ANALYSER
EMI
FILTER
PFC
L4981
DEMO
LOAD
Table 1. 500W Demoboard Evaluation Results
V_in
f
P_i
PF
THD
H3
H5
H7
V_out
P_o
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
13/20