STCC08 Datasheet, PDF (6/13 Pages) STMicroelectronics – AC switch failure mode detector

Functional description
STCC08
Failure mode detection
This function detects AC switch failure modes required by the IEC 60335 standard:
● The AC switch diode mode failure in both negative and positive AC line polarities
This failure is simulated with a diode placed in parallel with the power switch according
to the IEC 60335-1 standard.
● The AC switch short-circuit failure mode, any external shorting and any spurious power
switch turn-on
● The open AC switch failure mode
This function improves the system safety. A diode mode failure, a short circuit or an AC
switch open can lead to dangerous situations for the system or the user.
The AVF block monitors the state of the AC switch in both AC polarities. Its output AVF is
transmitted to the MCU for failure detection.
The MCU detects the failure according to the following truth table:
Table 8. STCC08 status truth table with R_SHUNT in parallel with the load
IN (MCU generated)
AVF
MCU DIAGNOSTIC
0
Open collector
AC switch OK
0
Open collector to 0 toggle
Diode mode
0
0
Short AC switch
1
0
AC switch OK
1
Open collector
Open AC switch
For a diode mode, the AVF output toggles from open collector to GND each the half of the
AC mains. Therefore, the AVF output is a pulsed signal during a diode mode failure.
The MCU can put the system in a safe configuration, switching off the front end relay
already existing in home appliance designs.
Status reading
The AVF output is an open collector, active at low level. It can either be connected to a MCU
input, in pull up input configuration, or through an opto-coupler in insulated designs.
For inductive loads, there is a phase shift between load current and AC line voltage. When
the ACS control is removed:
● The AC switch - latch structure - still conducts and the AVF is at low level until the next
zero current crossing.
● The AVF is at low level until next zero current crossing.
● In the worst case, the load current crosses zero close to peak mains voltage. Then the
AVF signal should be read at peak mains voltage.
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