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 I 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)

up to 10 mA and to send

GT

Figure 1. STCC08 block diagram

G

V

CC

AC

Power switch

signal shaping

Gate driver

+

-

STCC08

AVF driver

IN

R

IG

AVF

GND

IN

AVF

N/C

AC

1

2

3

4

SO -8

8

7

6

5

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

December 2009 Doc ID 15255 Rev 1 1/29

GND

R

IG

G

V

CC

www.st.com

Contents AN2859

1 Multiplexed diagnostics

1.1 Principle

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 (STCC08 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

R

Shunt

Line

Load2

ACS2

R

AC

V

AC

G

CC

STCC08

2

IN2

R

AVF

and STCC082) is

1

IG

V

CC

R1

R2

V

AVF2

MCU

V

CC

Neutral

R

Shunt

Line

Load1

ACS1

R

AC

V

AC

G

CC

STCC08

1

To distinguish the state of each AC switch (ACS1 and ACS2) a divider bridge is used. Resistors R STCC08 (V

, R2, R3 and R4 are designed to convert the V

1

AVF 1

and V

) into an analog signal (V

AVF 2

STATE

each STCC08 (IN1 and IN2), the MCU is able to identify the state of each AC switch by analyzing the V

signal (see Section 1.2).

STATE

Note: The STCC08 AVF output is an open collector output. Resistors R

STCC08 AVF output and limit the collector current to 5 mA. For further information, and in particular, resistor values for R

AC

, R

, and RIG, refer to the ST Application note AN2716.

shunt

IN1

R

IG

V

CC

R3

AVF

R4

GND

V

digital signal given by each

AVF

AVF1

V

STATE

). Knowing the control state of

and R3 bias the

1

Doc ID 15255 Rev 1 3/29

Multiplexed diagnostics AN2859

1.2 Failure mode detection of two AC switches

Figure 3 to Figure 12 give the V

V

, V1, V2 and V3 are levels reached by the parameter V

0

signal level according to the state of each AC switch.

STATE

and depends on R1, R2, R3,

STATE

and R4 resitor values.Ta ble 1 shows that we only need four different levels to define the state of each AC switch.

Figure 3. Case 1: V

VCC/COM

I

Load_2

V

AC

Line

VCC/COM

V

AC

Line

RShunt

Load2

RShunt

Load1

I

Load_1

ACS2

RAC

ACS1

RAC

= V3 (except at each zero crossing of the AC line)

STATE

VCC

VS TATE

V3

V2

V1

V0

I

Load_2

I

Load_1

STCC08

G

AC

STCC08

G

AC

AVF

IN2

AVF

IN1

R1

2

R2

V

AVF2

VCC

R3

1

R4

V

AVF1

STAT E

V

AC

V

AC

ACS1 and ACS2 are not in conducting state

Figure 4. Case 2: V

VCC/COM

I

Load_2

V

AC

Line

Load2

RShunt

VCC/COM

V

AC

Line

Load1

RShunt

I

Load_1

ACS2

RAC

ACS1

RAC

STATE

= V

STCC08

G

AC

STCC08

G

AC

0

AVF

IN2

AVF

IN1

VCC

V

AC

R1

2

R2

V

AVF2

I

Load_2

I

Load_1

V

AC

VCC

VSTATE

R3

1

R4

V

AVF1

STAT E

V3

V2

V1

V0

ACS1 and ACS2 are in conducting state

4/29 Doc ID 15255 Rev 1

AN2859 Multiplexed diagnostics

Figure 5. Case 3: V

V /COM

CC

I

Load_2

V

AC

Line

VCC/COM

V

AC

Line

RShunt

Load2

RShunt

Load1

I

Load_1

ACS2

RAC

ACS1

RAC

= V1 (except at each zero crossing of the AC line)

STATE

V

CC

R1

STCC08

G

AC

STCC08

G

AC

2

AVF

IN2

1

AVF

IN1

R2

V

AVF2

V

CC

R3

V3

R4

V2

V

AVF1

STATE

V1

V0

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

Figure 6. Case 4: V

= V2 (except at each zero crossing of the AC line)

STATE

VCC

V

AVF2

V

AVF1

V

R1

R3

CC

R2

R4

V

STAT E

V3

V2

V1

V0

V /COM

CC

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

Load_2

I

Load_1

ACS2

RAC

ACS1

RAC

STCC08

AVF

G

AC

STCC08

AVF

G

AC

2

IN2

1

IN1

VSTATE

V

STAT E

I

Load_2

I

Load_1

I

Load_2

I

Load_1

V

AC

V

AC

V

AC

V

AC

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

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Multiplexed diagnostics AN2859

Figure 7. Case 5: V

toggles between V1 and V3 at each AC line cycle

STATE

(except at each zero crossing of the AC line)

V

CC

V /COM

CC

I

Load_2

STCC08

2

R1

R2

AVF

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

Load_1

ACS2

RAC

ACS1

RAC

G

AC

STCC08

AVF

G

AC

IN2

IN1

V

AVF2

V

CC

V

R3

1

R4

V3

V2

V

AVF1

STATE

V1

V0

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

Figure 8. Case 6: V

toggles between V2 and V3 at each AC line cycle

STATE

(except at each zero crossing of the AC line)

V

CC

V

AVF2

AVF1

V

R1

R3

CC

R2

R4

V

STAT E

V3

V2

V1

V0

V /COM

CC

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

Load_2

I

Load_1

ACS2

RAC

ACS1

RAC

STCC08

AVF

G

AC

IN2

STCC08

AVF

G

AC

IN1

2

1

STATE

V

STATE

I

Load_2

I

Load_1

I

Load_2

I

Load_1

V

AC

V

AC

V

AC

V

AC

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

6/29 Doc ID 15255 Rev 1

AN2859 Multiplexed diagnostics

Figure 9. Case 7: V

toggles between V1 and V2 at each AC line cycle

STATE

(except at each zero crossing of the AC line)

V

CC

V

AVF2

V

AVF1

R1

R3

CC

R2

R4

V

STAT E

V3

V2

V1

V0

V /COM

CC

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

Load_2

I

Load_1

ACS2

RAC

ACS1

RAC

STCC08

AVF

G

AC

STCC08

AVF

G

AC

2

IN2

1

IN1

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

V

STAT E

I

Load_2

I

Load_1

V

AC

Figure 10. Case 8: V

toggles between V0 and V3 at each AC line cycle

STATE

(except at each zero crossing of the AC line)

VCC

V /COMCC

ILoad_2

STCC08

R1

2

R2

ILoad_2

AVF

V

AC

Line

V /COMCC

V

AC

Line

Load2

RShunt

ILoad_1

Load1

RShunt

ACS2

RAC

ACS1

RAC

G

AC

STCC08

AVF

G

AC

IN2

IN1

VAVF2

ILoad_1

VCC

VSTAT E

R3

1

R4

V3

V2

VAVF1

VSTAT E

V1

V0

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

V

AC

V

AC

Doc ID 15255 Rev 1 7/29

Multiplexed diagnostics AN2859

Figure 11. Case 9: V

V /COM

CC

I

Load_2

V

AC

Line

Load2

RShunt

V /COM

CC

V

AC

Line

Load1

RShunt

I

ACS2

RAC

Load_1

ACS1

RAC

toggles between V2 and V0 at each AC line cycle

STATE

V

CC

R1

STCC08

2

R2

AVF

G

V

AVF2

V

AC

IN2

CC

R3

STCC08

1

R4

AVF

G

V

AVF1

V

STAT E

AC

IN1

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

Figure 12. Case 10: V

V /COM

CC

I

Load_2

V

AC

Line

Load2

RShunt

V /COM

CC

I

V

AC

Line

Load1

RShunt

Load_1

ACS2

RAC

ACS1

RAC

toggles between V1 and V0 at each AC line cycle

STATE

V

CC

R1

STCC08

2

R2

AVF

G

V

AVF2

V

AC

IN2

CC

R3

STCC08

1

R4

AVF

G

V

AVF1

V

STATE

AC

IN1

V3

V2

V1

V0

V3

V2

V1

V0

V

STAT E

STATE

I

Load_2

I

Load_1

I

Load_2

I

Load_1

V

AC

V

AC

V

AC

V

AC

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

8/29 Doc ID 15255 Rev 1

AN2859 Multiplexed diagnostics

Table 1. Variation of the V

Knowing the control state of each STCC08 (IN1 and IN2) and according to Ta bl e 1, the MCU is able to detect the AC switch state by analyzing V states of each ACS according to the V 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.

1.3 V

The STCC08 AVF output signal is an image of the AC switch voltage. This signal toggles between V is higher or not than I electrical variation between ICs may result in the state of the AVF signal of each STCC08 (either V V I V 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 V (V

signal according to the AC switch states

STATE

ACS1 state ACS2 state V

ON ON V

ON OFF V

OFF ON V

OFF OFF V

signal. Appendix A defines the

signal level (V0, V1, V2 and V3) and the control

STATE

signal reading synchronization

AVF

and zero level (GND) according to whether the STCC08 AC input current (IAC)

CC

or zero level) not changing at exactly the same time. This has an impact on the

CC

signal and on the AC switches state detection (see Figure 13). Note that I

STATE

define respectively the STTCO8 IAC input current for STCC081 and STCC082 to allow

ACT2

signal to toggle between VCC and GND. For example, if the two STCC08 are not

AVF

= V0).

STATE

(see AN2716). In case of multiplexed diagnostics the slight I

ACT

STATE

is read between t0 and t

STATE

status

= V

0

1

2

3

ACT1

ACT

and

1

Doc ID 15255 Rev 1 9/29