STEVAL-ILL013V1 Datasheet, PDF (32/43 Pages) STMicroelectronics – 80 W offline LED driver with PFC

UM0670
10 Design features
Design features
10.1
Proper startup circuit design
High PF boost converter design is described in the EVL6562A-TM-80W (Section 11:
References and related materials: 1.) and this type of design typically includes a single
electrolytic capacitor on the V_CC pin of the L6562A to ensure proper startup. The situation
for the STEVAL-ILL013V1 is different, as this input capacitor also supplies the second
L6562A driver, used for controlling the modified buck converter, and also provides a supply
voltage for the PWM generator. Generally, this means that there is always some additional
current discharging the input capacitor during startup, and therefore the high PF boost
converter does not start properly if the low input AC voltage is applied to the board. This is
due to there being insufficient energy in the input capacitor to guarantee proper startup. The
waweforms which illustrate this situation are given in Figure 41 (circuit without capacitor
C110 and diode D104). As soon as the input voltage reaches the turn ON threshold, the
L6562A starts operating and the input voltage on the V_CC pin is decreased. As soon as it
reaches the turn OFF threshold, the capacitor is charged again and the L6562A stops
operating. This behavior is repeated and so after a short period the output voltage should
reach 400 V. However, this is not possible due to low energy in the input capacitor on the
V_CC pin. This problem is solved by adding capacitor C110 and diode D104 to the original
schematic, as shown in Figure 39. Thanks to this configuration the input capacitor C107 on
the V_CC pin is not discharged because it is supplying the PWM generator and second
converter. Capacitor C110 is used to provide supply voltage to the PWM generator and the
second converter. Capacitor C110 is charged via a capacitive supply source connected to
the ZCD when the L6562A operates. The voltage on capacitor C110 is added to the voltage
on capacitor C107 through diode D104, and as soon as the voltage on capacitor C110 also
reaches the turn ON threshold, the L6562A starts operating continuously and the output
voltage reaches 400 V, as shown in Figure 40.
Figure 39. Proper startup circuit design
D104
C107
VCC ^{ZCD COMP INV}
L6562A ^GD
MULT
GND
CS
C110
AM00412
Doc ID 15327 Rev 2
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