AN2716 datasheet(12/28 Pages) STMICROELECTRONICS

12 / 28 page

STCC08 description

AN2716

12/28

Doc ID 14460 Rev 2

The t1 and t2 values are defined in Appendix A. For example, with RAC = RShunt = 300 kΩ,

F = 50 Hz, VAC_Line = 230 V, a minimum rms current and a phase shift of the AC load

respectively equal to 0.1 A and 90°, t1 and t2 are respectively 1.7 ms and 8.3ms.

When the AC switch is failed in diode mode, the AC switch is on during only half of the AC

line cycle for resistive load (the load current is in phase with the AC line voltage). In this

case, the AVF signal falls to GND at each cycle of the AC line when the AC switch is on.

However, with an inductive load, the load current (ILoad) falls to zero after the zero crossing

of the AC line voltage (See Figure 10). This has an impact on the AC switch state detection.

If the AC switch has failed in diode mode and the STCC08 AVF signal is read just after each

zero crossing of the AC line voltage, the AC switch failed in diode mode will not be

discriminated from the AC switch short circuit failure.

Figure 10.

Load current and STCC08 AVF variation when the AC switch is failed in

diode mode

The STCC08 AVF signal should be read after the AC line peak voltage (tAVF_READ) (See

Equation 6) and before the time t2 to detect all AC switch states for most home

appliance loads.

Equation 6

Note:

Note, if the STCC08 AVF signal is read between times t1 and t2 (see Figure 9), all the AC

switch failures will be detected but the AC switch failed in diode mode will be confused with

the AC switch failed in short-circuit.

To detect the AC switch state when the STCC08 is controlled by a pulsed gate current, the

MCU should store the IN signal state in a RAM register. This RAM register could be

initialized at each zero crossing of the AC line voltage.

AVF

VLine

ILoad

t (s)

READ

AVF

Detection

FMin

AVF_READ

AN2716 Datasheet(PDF) 12 Page - STMicroelectronics