AN993 Datasheet, PDF (20/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

Description of the demonstration application
AN993
4.6
Safety circuitry
In normal operation the inductive load ensures a zero voltage switching mode, but if the
lamp is disconnected the switching losses in the power MOSFETs will increase
considerably. To prevent this occurrence, a safety circuitry has been designed. When the
lamp is connected the EN1 input (pin 8) of the L6574 is held close to ground by the series of
R₂₉, the lamp filament and R_S2. If the lamp is not present, EN1 is pulled up to VS by R₂₄,
forcing the L6574 into a latched shutdown state. To resume normal operation, the ballast
must be turned off, and then on again.
A second alarm has been designed to protect the application against any extra voltages
which can arise if the lamp does not strike after the ignition sequence, for instance if the
lamp is old. A partition of this extra voltage is rectified and delivered to the EN2 input (pin 9)
of the L6574, restarting the start-up procedure (preheating and ignition sequences).
Figure 17. Open load safety circuit
+V_S
LAMP
C19
8.2nF
1600V
R29
6.8K
D98IN816A
R24
390K
EN1 (PIN8)
C16
R25
.47uF 20K
Figure 18. Extra voltage safety circuit
L2 2.1mH
C17
100nF
250V
EN2 (PIN9)
R20
6.8K
R26
750K
R23 1k D5 1N4148
R27
750K
C15
330nF
R28
3.9K
LAMP
C18
8.2nF
1600V
AM01325v1
AM01326v1
20/27
Doc ID 5656 Rev 10