ST AN2859 APPLICATION NOTE

AN2859

Application note

Multiplexed diagnostics of AC switches using two STCC08s

Introduction

The aim of this application note is to present opportunities to reduce the number of input pins used on a microcontroller unit (MCU) to diagnose failures of several AC switches with the STCC08. This document deals with the multiplexed diagnostics of two STCC08 and gives technical recommendations on the implementation of this solution.

STCC08 overview

The STCC08 has been designed to improve home appliance safety. This new device can drive an AC switch (Triac, ACST and ACS) with a gate current IGT up to 10 mA and to send back to the microcontroller unit a signal image of the voltage across the controlled AC switch (this signal defines the AC switch state). The STCC08 has three functional blocks (see

Figure 1).

■A "gate driver" block used to drive an AC switch and to interface directly the STCC08 with the MCU (CMOS compatible)

■A "power switch signal shaping" block used to measure the AC switch voltage in both AC line cycles

■An "AVF driver" block used to give an image of the AC switch voltage to the MCU (digital information)

Figure 1. STCC08 block diagram

G	Gate driver
	IN
	+
	-
	RIG	1	8	GND
	IN

	STCC08	AVF	2	7	RIG
	VCC	N/C	3	6	G
	AVF driver	AVF			VCC
		AC	4	5
	AC
	Power switch	GND		SO -8
	signal shaping

For more information about the STCC08, please refer to the ST Application note AN2716.

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Contents	AN2859

Contents

1	Multiplexed diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 3
	1.1	Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	3
	1.2	Failure mode detection of two AC switches . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.3	VAVF signal reading synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	9
2	VSTATE level definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		11
3	Resistance settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
4	Detection windows digital value setting . . . . . . . . . . . . . . . . . . . . . . . .		15
5	Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
6	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		18
Appendix A AC switch state deduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			19
Appendix B VSTATE signal voltage definition . . . . . . . . . . . . . . . . . . . . . . . . . . . .			21

Appendix C Resistance settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

C.1 First case: V1_Min > V0_Max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 C.2 Second case: V2_Min > V1_Max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 C.3 Third case: V2_Max < V3_Min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

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AN2859	Multiplexed diagnostics

1 Multiplexed diagnostics

1.1Principle

The multiplexed diagnostic allows the detection of the state of several AC switches independently using only one MCU input. In this case, an analog/digital converter input (ADC) of the MCU should be used and must be configured with no pull-up resistor. In this document, only the multiplexed diagnostic of two STCC08 (STCC081 and STCC082) is described (see Figure 2). Note that two output pins of an MCU should be used to control each STCC08 (IN1 and IN2).

Figure 2. Multiplexed diagnostic schematic of two STCC08

Neutral
	ACS2	G		IN2
				RIG
			STCC082	VCC
		VCC
				R1
		RAC			MCU
RShunt				AVF
	Load2	AC		R2	VCC
Line				VAVF2
Neutral
	ACS1	G		IN1
				RIG
				VCC
		VCC	STCC081	R3
RShunt		RAC		AVF
	Load1	AC		R4
Line				GND VAVF1	VSTATE

To distinguish the state of each AC switch (ACS1 and ACS2) a divider bridge is used. Resistors R1, R2, R3 and R4 are designed to convert the VAVF digital signal given by each

STCC08 (VAVF1 and VAVF2) into an analog signal (VSTATE). Knowing the control state of each STCC08 (IN1 and IN2), the MCU is able to identify the state of each AC switch by

analyzing the VSTATE signal (see Section 1.2).

Note:	The STCC08 AVF output is an open collector output. Resistors R1 and R3 bias the
	STCC08 AVF output and limit the collector current to 5 mA. For further information, and in
	particular, resistor values for RAC, Rshunt, and RIG, refer to the ST Application note AN2716.

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1.2Failure mode detection of two AC switches

Figure 3 to Figure 12 give the VSTATE signal level according to the state of each AC switch. V0, V1, V2 and V3 are levels reached by the parameter VSTATE and depends on R1, R2, R3, and R4 resitor values.Table 1 shows that we only need four different levels to define the

state of each AC switch.

Figure 3. Case 1: VSTATE = V3 (except at each zero crossing of the AC line)

VCC

			VAC
VCC/COM		R1	ILoad_2
	ILoad_2	STCC082	R2
		AVF	VAC
VAC
	ACS2	G	ILoad_1
		VAVF2

Line	Load2	RAC	AC	IN2	VCC
	RShunt				VSTATE

VCC/COM	R3	V3

	ILoad_1	STCC081	R4
		AVF	V2
VAC	ACS1	G	VSTATE
		VAVF1
			V1

Line	Load1	RAC	AC
	RShunt		IN1	V0

ACS1 and ACS2 are not in conducting state

Figure 4. Case 2: VSTATE = V0

VCC

			VAC
VCC/COM		R1	ILoad_2
		STCC082
	ILoad_2		R2
		AVF	VAC
V
		G	ILoad_1
AC	ACS2
		VAVF2

Line
Load2	RAC	AC	IN2	VCC
RShunt				VSTATE

VCC/COM	R3	V3

		STCC08	R4
	ILoad_1	1
		AVF	V2
V AC	ACS1	G	VSTATE
		VAVF1
			V1

Line	Load1	RAC	AC
	RShunt		IN1	V0

ACS1 and ACS2 are in conducting state

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Figure 5.	Case 3: VSTATE = V1 (except at each zero crossing of the AC line)
			VCC	VAC
VCC/COM			R1	ILoad_2
	ILoad_2	STCC082	R2
		AVF		VAC
VAC
	ACS2	G	VAVF2	ILoad_1

Line	Load2	RAC	AC	VCC
	RShunt		IN2			VSTATE
VCC/COM				R3		V3
				R4
	ILoad_1		STCC081
			AVF			V2
VAC		ACS1	G		VSTATE
				VAVF1		V1
Line
	Load1	RAC	AC
						V0
	RShunt		IN1

ACS1 is in conducting state and ACS2 is not in conducting state

Figure 6. Case 4: VSTATE = V2 (except at each zero crossing of the AC line)

VCC

		VAC
VCC/COM	R1	ILoad_2
STCC082	R2
ILoad_2		VAC
AVF

VAC	ACS2	G	VAVF2	ILoad_1
Line Load2
	RAC	AC	VCC
RShunt		IN2		VSTATE
VCC/COM			R3	V3
		STCC081	R4
ILoad_1
		AVF		V2

VAC	ACS1	G
		VAVF1 VSTATE V1

Line Load1	RAC	AC
RShunt		IN1	V0

ACS1 is not in conducting state and ACS2 is in conducting state

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Figure 7.	Case 5: VSTATE toggles between V1 and V3 at each AC line cycle
	(except at each zero crossing of the AC line)
				VCC	VAC
VCC/COM				R1	ILoad_2
			STCC082
ILoad_2				R2
VAC			AVF		VAC
		ACS2	G	VAVF2	ILoad_1

Line	Load2	RAC	AC	VCC
	RShunt		IN2		VSTATE
VCC/COM				R3	V3
				R4
			STCC081

	ILoad_1	AVF		V2
VAC	ACS1	G	VAVF1	VSTATE
				V1

Line Load1	RAC	AC
RShunt		IN1	V0

ACS1 is failed in diode mode and ACS2 is not in conducting state

Figure 8. Case 6: VSTATE toggles between V2 and V3 at each AC line cycle (except at each zero crossing of the AC line)

			VCC		VAC
VCC/COM			R1		ILoad_2
		STCC082
ILoad_2			R2
		AVF			VAC
VAC
	ACS2	G	VAVF2		ILoad_1
Line Load2	RAC	AC	VCC
RShunt		IN2			VSTATE
VCC/COM		STCC081	R3		V3
ILoad_1			R4
		AVF			V2
VAC
	ACS1	G	VAVF1	VSTATE
					V1
Line Load1
	RAC	AC
RShunt		IN1			V0

ACS1 is not in conducting state and ACS2 is failed in diode mode

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Figure 9. Case 7: VSTATE toggles between V1 and V2 at each AC line cycle (except at each zero crossing of the AC line)

VCC

				VAC
VCC/COM			R1	ILoad_2
ILoad_2		STCC082	R2
		AVF		VAC
VAC
	ACS2	G	VAVF2	ILoad_1

Line	Load2	RAC	AC	VCC
	RShunt		IN2		VSTATE
VCC/COM				R3	V3
			STCC081	R4
	ILoad_1
			AVF		V2

VAC	ACS1	G	VSTATE
		VAVF1		V1

Line	Load1	RAC	AC
	RShunt		IN1	V0

ACS1 and ACS2 are failed in diode mode not on the same AC line polarities

Figure 10. Case 8: VSTATE toggles between V0 and V3 at each AC line cycle (except at each zero crossing of the AC line)

VCC

		VAC
VCC/COM	R1	ILoad_2
STCC082	R2
ILoad_2
AVF		VAC

VAC	ACS2	G	VAVF2	ILoad_1
Line Load2	RAC	AC	VCC
RShunt		IN2		VSTATE
VCC/COM		STCC081	R3	V3
ILoad_1			R4
		AVF		V2
VAC
	ACS1	G	VAVF1	VSTATE
Line Load1				V1
	RAC	AC
RShunt		IN1		V0

ACS1 and ACS2 are failed in diode mode on the same AC line polarities

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Figure 11. Case 9: VSTATE toggles between V2 and V0 at each AC line cycle
			VCC	VAC
VCC/COM			R1	ILoad_2
ILoad_2		STCC082	R2
		AVF		VAC
VAC
	ACS2	G	VAVF2	ILoad_1

Line	Load2	RAC	AC	VCC
	RShunt		IN2		VSTATE
VCC/COM				R3	V3
			STCC081	R4
	ILoad_1
			AVF		V2

VAC	ACS1	G	VSTATE
		VAVF1		V1

Line Load1	RAC	AC
RShunt		IN1			V0
ACS1 is failed in diode mode and ACS2 is failed in short circuit
Figure 12. Case 10: VSTATE toggles between V1 and V0 at each AC line cycle
			VCC		VAC
VCC/COM			R1		ILoad_2
		STCC082
ILoad_2			R2
		AVF			VAC
VAC
	ACS2	G	VAVF2		ILoad_1
Line Load2
	RAC	AC	VCC
RShunt		IN2			VSTATE
VCC/COM		STCC081	R3		V3
ILoad_1			R4
		AVF			V2
VAC
	ACS1	G	VAVF1	VSTATE
					V1
Line Load1
	RAC	AC
RShunt		IN1			V0

ACS1 is failed in shot circuit and ACS2 is failed in diode mode

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	Table 1.	Variation of the VSTATE signal according to the AC switch states
		ACS1 state	ACS2 state	VSTATE status
		ON	ON	VSTATE = V0
		ON	OFF	VSTATE = V1
		OFF	ON	VSTATE = V2
		OFF	OFF	VSTATE = V3

Knowing the control state of each STCC08 (IN1 and IN2) and according to Table 1, the MCU

is able to detect the AC switch state by analyzing VSTATE signal. Appendix A defines the states of each ACS according to the VSTATE signal level (V0, V1, V2 and V3) and the control state of each STCC08. In the case of failure of one of the AC switches, the MCU can place

the application in a safe configuration by switching off an appliance front-end relay.

1.3VAVF signal reading synchronization

The STCC08 AVF output signal is an image of the AC switch voltage. This signal toggles between VCC and zero level (GND) according to whether the STCC08 AC input current (IAC) is higher or not than IACT (see AN2716). In case of multiplexed diagnostics the slight IACT electrical variation between ICs may result in the state of the AVF signal of each STCC08 (either VCC or zero level) not changing at exactly the same time. This has an impact on the

VSTATE signal and on the AC switches state detection (see Figure 13). Note that IACT1 and IACT2 define respectively the STTCO8 IAC input current for STCC081 and STCC082 to allow

VAVF signal to toggle between VCC and GND. For example, if the two STCC08 are not controlled (IN1 = IN2 = 0) and AC1 and AC2 are not in conducting state the AC1 and AC2

can be interpreted (see Table 1) as failed in short circuit if VSTATE is read between t0 and t1 (VSTATE = V0).

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