Description of the demonstration application

AN993

20/27

Doc ID 5656 Rev 10

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

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

Figure 18.

Extra voltage safety circuit

AM01325v1

R24

390K

R25

20K

R29

6.8K

C19

8.2nF

1600V

LAMP

C16

.47uF

EN1 (PIN8)

D98IN816A

AM01326v1

C17

100nF

250V

LAMP

R28

3.9K

R27

750K

R26

750K

C18

8.2nF

1600V

R20

6.8K

EN2 (PIN9)

D5 1N4148

C15

330nF

L2 2.1mH

R23 1k