Functional description

STCC08

6/13

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:

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.

Table 8.

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

10

AC switch OK

1

Open collector

Open AC switch