UM0670 datasheet(32/43 Pages) STMICROELECTRONICS

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UM0670

Design features

Doc ID 15327 Rev 2

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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 VCC 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 VCC 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

VCC 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 VCC 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

C107

MULT

VCC

COMP

INV

C110

D104

GND

L6562A

ZCD

AM00412

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