ST AN2716 APPLICATION NOTE

AN2716

STCC08 application guidelines

Introduction

The purpose of this document is to:

■Describe the STCC08 device features

■Give technical recommendations to:

–Implement the STCC08 in the appliance

–Achieve robust STCC08 design regarding EMI tests (IEC 61000-4-4)

June 2010

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Contents	AN2716

Contents

1	STCC08 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		4
	1.1	Main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	4
	1.2	STCC08 non-insulated and insulated application diagram . . . . . . . . . . . .	5
	1.3	AC switch control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	1.4	AC switch failure mode detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6

1.4.1 STCC08 AVF output configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.4.2 AC switch state detection principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4.3 AC switch failures detection during normal operation . . . . . . . . . . . . . . . 9

1.5 AVF signal reading synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.5.1 Detection window width definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 1.5.2 AC switch state detection after IN signal removal . . . . . . . . . . . . . . . . . 13

2	STCC08 consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
3	Recommended component values . . . . . . . . . . . . . . . . . . . . . . . . . . . .		16
4	ZVS detection application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
5	Electromagnetic compatibility (EMC) tests . . . . . . . . . . . . . . . . . . . . .		18
	5.1	Demonstration board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
	5.2	IEC 61000-4-4 burst immunity test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18

5.2.1 Test conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2.2 Demonstration board immunity test . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.2.3 Advice to improve the application immunity . . . . . . . . . . . . . . . . . . . . . . 19

6	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		20
Appendix A Timing definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		21
Appendix B Demonstration board component layout. . . . . . . . . . . . . . . . . . . . .		24
Appendix C Demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . .		25

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Appendix D Demonstration board circuit layout view. . . . . . . . . . . . . . . . . . . . . 26

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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STCC08 description	AN2716

1 STCC08 description

1.1Main features

The STCC08 was designed to improve the safety of home appliances (see Reference 1.). This new device can drive up to 10 mA IGT AC switches (TRIAC, ACST and ACS - see Reference 2.) and send back to the microcontroller (MCU) a signal image of the voltage across the controlled AC switch. The STCC08 has three functional blocks (see Figure 1 and

Table 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

●An “AVF driver” block used to give an image of the AC switch voltage to the MCU (digital information)

Figure 1. STCC08 block diagram

Gate driver

G (6)	+	IN (1)
	-
		RIG(7)	IN
	STCC08		AVF
			N/C
VCC (5)	AVF driver		AC
		AVF (2)

18 GND

27 RIG

36 G

45 VCC

AC (4)		SO-8 package
	Power switch	GND (8)
	signal shaping

Table 1.	STCC08 pin descriptions
Pin	Symbol	Type	Description
1	IN	SIGNAL	AC switch drive
2	AVF	SIGNAL	Alternating voltage feedback: AC switch state output
3	NC		Not connected
4	AC	SIGNAL	AC switch state sense input
5	VCC	POWER	Positive power supply
6	G	SIGNAL	AC switch gate driver output
7	RIG	SIGNAL	AC switch gate current setting
8	GND	POWER	Power supply reference

Knowing the STCC08 IN input state (used to turn on or off the AC switch) and the voltage across the controlled AC switch (given by the STCC08 AVF output signal state), the MCU is able to identify all AC switch failures (diode mode, short circuit, or open circuit) and to take the appropriate actions to put the appliance in a secure state (see Reference 3.) by switching off an appliance front-end relay (see Section 1.4.3).

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1.2STCC08 non-insulated and insulated application diagram

The STCC08 is dedicated to applications with MCU. Figure 2 and Figure 3 show respectively the STCC08 non-insolated and insolated application diagrams. The STCC08 is compatible with 3.3 V and 5 V power supplies. The DC power supply must be a negative one. This means the Vcc terminal has to be connected to neutral. The GND voltage is then VCC (3.3 V or 5 V) below neutral. Such a connection is mandatory to drive ACS and most ACST and TRIAC devices (AC switches can be triggered only by a current taken from the gate).

Figure 2. Un-insulated application diagram

			VCC = 3.3V to 5V
Neutral
	ACS	Gate driver	VCC
		G	IN
		+
		-	RIG
		STCC08	R
			IG
			VCC
		VCC
RShunt		RAC	AVF
	Load	AVF driver
		AC
		Power switch	GND
		signal shaping
Line			MCU
Front end relay

Figure 3. Insulated application diagram

					Uninsulatated	Insulated
				VCC1	5 V or 3.3 V
						Power supply
					Power supply
Neutral
	ACS	G	Gate driver	IN	R2	VCC2
			+
			-
				RIG
			STCC08	R	OPTO1
		VCC		IG	R1
					VCC1	V
RShunt		RAC			RAVF	CC2
			AVF driver	AVF
	Load	AC
			Power switch	GND
Line			signal shaping
						MCU
Front end relay					OPTO2

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STCC08 description	AN2716

1.3AC switch control

The STCC08 controls up to 10 mA IGTTRIAC, ACST and ACS through the “gate driver” block designed to:

●Drive the AC switch according to the IN control input state (CMOS compatible)

–For IN = 1 = VCC Ö the AC switch is turned on

–For IN = 0 = GND Ö the AC switch is turned off

●Regulate the gate current of the AC switch thanks to the internal current controller

The gate current value (Pin G - to turn on the AC switch) is defined by the external resistor RIG value (resistor connected between the STCC08 RIG input and ground). Figure 4 gives the maximum value of this resistor (RIG_max) according to the minimum ambient

temperature (Tamb_min) of the appliance for two AC switch IGT values. Note that as the AC switch junction temperature increases when the AC switch is on (Tj > Tamb), the IG gate

current required to turn on the AC switch decreases (IG < IGT). So only the minimum ambient temperature has to be considered.

Figure 4. Resistor RIG value according to the minimum ambient temperature and AC switch IGT

RIG_max (Ω)
130
120
110
100
90
80
70
60
50
40						IGT AC Switch:
30
							IGT 10mA @ 25 °C
20							IGT 5mA @ 25 °C
10
0
-20	-15	-10	-5	0	5	10	15	20	25	30
				Tamb_min(°C)

For example, with a 10 mA IGT AC switch and a -20 °C minimum ambient temperature, a maximum 30 Ω RIG resistor can be used to turn on the AC switch throughout the whole appliance temperature range.

1.4AC switch failure mode detection

1.4.1STCC08 AVF output configuration

The STCC08 AVF driver block is used to send the AC switch state to the MCU (see Section 1.4.2). The STCC08 AVF output is an open collector and can be loaded with an external resistor (RAVF) or connected directly to the MCU, in pull-up input configuration (see

Figure 5).

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AN2716	STCC08 description

Figure 5. STCC08 AVF output configuration

VCC

STCC08 VCC STCC08

RAVF

AVF driver	AVF	AVF driver	AVF
	IAVF		IAVF

GND

GND

VCC

VCC

UP-PULL

R

MCU	MCU
(a) AVF output loaded with an external resistor (RAVF)	(b) AVF output loaded with the MCU pull-up resistor (RPULL-UP)

The IAVF current in the STCC08 AVF pin must be lower than 5 mA (IAVF_max). Equation 1 defines the minimal RAVF or pull-up resistor value to use. For example, with VCC_max = 5.5 V, the RAVF or RPULL-UP resistor value must be higher than 1.1 kΩ to fulfill this condition.

Equation 1

R AVF _min or RPULL-UP_min >

VCC_max

I AVF _max (5 mA )

The AVF signal toggles from VCC to GND according to the AC switch state and the AC line

(see Section 1.4.2). The RAVF or RPULL-UP resistor values modify the AVF signal rise time (tR_AVF). This rise time increases with the MCU I/O pin capacitance and RAVF or

RPULL-UP resistors. To limit the influence of this delay on the AC switch state detection (see Section 1.4.2) and on the ZVS detection application (see Section 4), a 300 kΩ maximum

RAVF or RPULL-UP resistor value is recommended. With a 47 pF CL load capacitor and a 300 kΩ maximum RAVF resistor, the AVF rise time is typically about 50 µs (see Figure 6).

Figure 6. AVF signal rising time measurement information

STCC08		VCC		R AVF= 300 KΩ
				C L= 47 pF
		RAVF
AVF driver	AVF			90 % VCC
	GND	CL	VAVF	10 % VCC
				tR_AVF

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STCC08 description	AN2716

1.4.2AC switch state detection principle

The AC switch state detection is achieved thanks to the STCC08 “power switch signal shaping” block. This block measures the current (IAC) through the STCC08 AC input, which

is the image of the AC switch voltage (VT) during both AC line cycle (VLine). As illustrated in Figure 7, three cases can be considered.

●Case 1: If the AC switch is off (VT = VLine), a low IAC current, in phase shift with the AC line (see Section 1.5.1), flows through resistor RAC for the both AC line polarities. In this case, the STCC08 AVF output signal (VAVF) remains at high level state (+VCC).

●Case 2: If the AC switch is on (VT ≈ 0), no IAC current flows through the resistor RAC. In this case, the AVF output signal (VAVF) remains at low level state (GND).

●Case 3: If the AC switch is off and the AC line voltage falls to zero voltage, no IAC current flows through the resistor RAC. In this case, the AVF output signal (VAVF) goes to zero level (GND).

Knowing the STCC08 IN input state, the MCU is then able to determine the AC switch state by analyzing the AVF signal (see Section 1.4.3).

Figure 7. AC switch failure detection principle

VT

ILoad

Neutral

t (s)

G

STCC08

VT

Line

ACS

VCC

IAC

V

AC

IACT

Shunt

V

RAVF

-IACT

R

CC

t (s)

Load

R

IAC

AC

Power switch

AVF

1

3

1

3

1

3

1

2

Line

ILOAD

signal shaping

VAVF

VAVF

t (s)

AC switch off

AC switch on

The IAC current follows the waveform of the voltage across the AC switch. The AVF signal toggles between VCC and zero level (GND) at a certain IAC current value noted as IACT (see Figure 7). The IACT electrical parameter dispersion is shown in Table 2.

Table 2.	IACT electrical parameter dispersion
		Maximum (IACT_max)	Minimum (IACT_min)
	IACT	236 µA	82 µA

The IAC peak current value (IAC-peak) must be lower than 2.2 mA (maximum IAC current allowed through STCC08 AC input). Equation 2 defines the minimum resistor RAC value

(RAC_Min) to limit this IAC current in the worst case (if the AC load fails in short circuit). For example, with VLine_rms_max = 230 V + 10%, the RAC resistor value must be higher than 163 kΩ.

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Equation 2

	R AC _ min	>	2 X V Line_ rms_ max
			I AC _ peak _ max ( 2 . 2 mA )

The resistor RShunt is used to detect the AC switch state whatever the AC load state is (connected or disconnected). Without this resistor, the STCC08 AVF signal remains at low

level (GND) whatever the AC switch state when the AC load is disconnected (IAC = 0). To

detect the AC switch state when the AC load is disconnected (with the resistor RShunt), the IAC-peak current remains higher than the IACT_Max current (IAC current threshold to ensure the AC switch state detection - see Table 2). Equation 3 defines the condition on RAC and

RShunt resistors to ensure a right AC switch states detection in the worst case (if the AC load is disconnected). For example, with VLine_RMS_Min = 230 V - 10%, the RAC resistor value plus the RShunt resistor value must be lower than 1.24 MΩ.

Equation 3

RShunt_max + R AC_ max	2 X V Line_ rms_ min
	<
	I ACT_ max (236 µA)

To limit the power dissipation by resistors RAC and RShunt lower than ¼ W and their influences on the AC load, a 300 kΩ and 100 kΩ minimum value is recommended

respectively under 230 V rms and 100 V rms AC line. Equation 4 and Equation 5 give the power dissipation for RAC and RShunt in the worst case.

Equation 4

(V Line_ rms_ max )2

(PRAC) max =

R AC _ min

Equation 5

(V Line_ rms_ max )2

(PRShunt ) max =

RShunt _ min

1.4.3AC switch failures detection during normal operation

Knowing the STCC08 IN input state, the MCU is able to determine the AC switch state by analyzing the AVF signal. Figure 8 andTable 3 give the AC switch failure modes according to the AVF signal state and the STCC08 IN signal state. In case of AC switch failure, the MCU can put the appliance in a safe configuration by switching off an appliance front-end relay.

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