ST AN1172 Application note

ACS™

AN1172

Application note

A logic-level transient-voltage protected AC switch

Introduction

Home appliances such as washing machines, refrigerators and dishwashers employ a lot of low power loads such as valves, door lock systems, dispensers or drain pumps. Since these loads are powered by the mains in ON / OFF mode, they were initially controlled by relays. Recently, relays have been replaced by triacs, due to their smaller size and lower driving energy. Nevertheless triacs don't fulfill alone the new requirements that users now need and are used with others components.

Power switches must now be directly driven by a microcontroller unit (MCU) and must be robust to withstand the A.C. line transients so that systems may fall into line with electromagnetic compatibility (EMC) standards. ACSs (for Alternating Current Switches) have been designed with this goal mind, i.e. to offer logic level and more robust semiconductor devices.

On the other hand, ACSs have been developed adopting a functional integration approach. They can be used directly between a MCU and the load. An external protection or a buffer circuit are not required since these are already integrated on the die. This considerably reduces the overall electronic board size. Moreover, the array of ACSs allows one device to control the various loads typically required in a washer appliance.

Table 1. gives the RMS current of loads that can be controlled by ACS402-5SB4 or

ACS108-5SA/N, in ON / OFF control mode.

Table 1. ACS108 and ACS402 targeted loads

(dI

Load

Door Lock <0.3 1 0.15 0.15 <10

Lamp <0.8 1 0.4 0.15 <20

Relay, Valve, Dispenser, Micromotor

Pump <0.2 >0.2 <0.1 <10 <10

Fan <0.6 >0.2 <0.3 <10 <20

RMS

(A)

<0.1 >0.7 <0.05 <2 <10

Power Factor

/dt)c

out

(A/ms)

(dV

/dt)c

out

(V/µs)

Turn-off delay

(ms)

May 2006 Rev 2 1/23

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Contents AN1172

1 ACS triggering mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.1 Negative gate current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2 New layout possibilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Inductive loads on/off control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 Valves and relays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Pumps and Fans ON / OFF control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Resistive loads on/off control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.1 Inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2 Transient junction temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 Light bulb flashover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 Electromagnetic compatibility standards . . . . . . . . . . . . . . . . . . . . . . . 13

4.1 IEC 61000-4-5 standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2 IEC 61000-4-4 standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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AN1172 ACS triggering mode

1 ACS triggering mode

1.1 Negative gate current

The ACS silicon structure is different from the triac one. For instance, the gate embeds a diode junction. Then the gate current can only circulate in one direction, from the COM pin to the Gate one. A peak reverse voltage (VGM) of this junction is also defined in the ACS data sheet.

In order to sink a current from the gate by a microcontroller output port, the supply voltage positive terminal must be connected to the drive reference, i.e. the COM pin of ACSs (see

Figure 1.).

An interesting benefit of such a connection is that the ACS is not fired when the MCU is at reset state. Indeed, in this case, all the MCU port pins are at high level. This means that the gate resistors are all connected to the COM terminal. No spurious triggering can then occur.

It should be noticed that for a direct switch / MCU connection, the MCU current capability is not the only point to check to decide if the buffer circuit can be removed. Actually, the transistor, used to amplify the MCU current in order to control the gate, also play an overvoltage protection role. Annex B gives the gate voltage limits between which the MCU output port will be not stressed. It is also shown that with ACSs, the gate voltage remains inside these limits even with worst cases of dI/dt gradients at turn-on.

Figure 1. Gate / MCU connection

AC MAINS

Iout

Com

Valve / PUMP

etc.

Vdd

Vss

MCU

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ACS triggering mode AN1172

1.2 New layout possibilities

It has already been said that ACS silicon structure is different from the triac, according to the gate operation. A second difference is that ACS have been developed in an integration goal.

To allow different cells to be associated in one single package or controlled by one single drive die, the common drive reference voltage must be connected to the back of the die. Indeed, each die bottom is electrically linked to the other ones by the frame. This is achieved by the ACS silicon structure, where an integrated level shifter allows both thyristors to be controlled by means of a gate voltage referenced at the back of the die (COM pin). (See

References, 1.).

Thanks to ACSs arrays, the copper tracks count is reduced since the different COM pins are connected together inside the package. This also allows smaller gate / MCU copper tracks loop areas, and so increases the EMI immunity of the overall electronic board. Figure 2. shows an example of connection between an ACS402-5SB4 and an ST62xx, both in DIL20 packages.

Figure 2. Reduction of gate / MCU loop areas

ACS402

OUT1

OUT2

OUT3

OUT4

COM

PA3

PA2

PA1

PA0

ST6

Vdd

A particular benefit of such a pin out appears with Surface Mount Devices (SMD). In this case, the tab pin is the COM one. The copper surface used to perform a heat-sink can then be used as a supply voltage bus. It allows new layout possibilities and, above all, a miniaturization of the Printed Circuit Board (PCB). Indeed, unlike triacs, the heat-sink areas are at the same voltage and so can be regrouped (see Figure 3.). The heatsink area therefore depends on the maximum amount of dissipated power at the same time, by all the switches put on it. So, the number of switches which will conduct at the same time and their conduction time should be known.

Figure 3. Printed circuit area reduction thanks to ACSs in SOT223 packages

LOAD

MCU Ref.

PCB required for TriacsPCB required for Triacs

Copper heatsink

LOAD

COPPER HEATSINK

OUT

COM

OUT

COM

PCB required for ACSs

MCU Ref.

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AN1172 Inductive loads on/off control

2 Inductive loads on/off control

2.1 Valves and relays

2.1.1 Turn-off overvoltages are clamped by ACSs

Valves and relays are both electromagnetic systems. In the case of AC high voltage operation, their windings present a high series resistance (a few kΩ) and a high series inductance (tens of Henry). Hence, they absorb a low RMS current (typically, 10 to 50 mA). In this case, the current rate of decrease is low and an automatic switch turn-off may result, when its current becomes lower than the holding level (see References, 2.). There may be an over-voltage due to the fact that there is still some current through the inductive load. The inductive energy thus creates a back electromotive voltage which tends to force the switch to conduct. If this over-voltage is not clamped, it can exceed the device breakdown level and damage it.

ACSs are over-voltage self-protected. They can sustain their holding current in such an operating mode, as shown in Figure 4.

Figure 4. ACS voltage and current waveforms at turn-off (230 V 35 mA RMS valve)

Iout (10 mA/div)

Vout (200 V/div)

During clamping periods, the inductive energy is dissipated both in the silicon die and the series resistance of the load. The worst case appears when the load inductance is the highest, i.e. for electromagnet loads.

In annex C, a theoretical analysis is performed with a 0.1 power factor load and an RMS current lower than 40 mA (value which never appears in practice where, for such RMS currents, the power factor is always higher than 0.7). Then, it is demonstrated that, even in this worst case scenario, the transient junction temperature remains below 160°C. And the clamping period time (t ACSs dies thanks to their reliable planar technology.

) always lasts less than 1 ms. Such a thermal stress is suitable for

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Inductive loads on/off control AN1172

2.1.2 Maximum switching frequency

As far as thermal management involving clamping phases is concerned, a maximum load commutation frequency must be defined to avoid excessive device heating. Figure 5. gives the maximum supplementary temperature rise due to recurrent clampings, versus the ACS switching period (see Appendix C). This value is given for a 230 V - 50 Hz mains voltage (110 V mains is less stressing), for the worst case of load (power factor = 0.1, peak load current = i this case, the energy absorbed by the die equals 25 mJ.

The chosen package is the TO92 one (ACS108-5SA device) because it presents the highest Rth value, among ACS packages on offer (DIL20, TO92, SOT223, DIL8).

It can be seen that this temperature elevation can be neglected (< 4° C) as long as the control frequency is less than one Hertz. Such a value is suitable for most appliance applications where loads are at most controlled once per second. For that reason, in ACS data-sheets, the maximum allowed current is given for a 1 Hertz maximum frequency 0.1 minimum load power factor. Turn-off dissipated power is then reviewed for a wide range of application needs.

This enables us to conclude that no varistor is needed across ACSs to clamp the loads inductive energy at turn-off, even with electromagnets which are the highest inductive loads in Appliances.

max) and for the maximum VCL and iH values (800 V and 60 mA respectively). In

Figure 5. Supplementary temperature elevation due to repetitive clampings (@

clamping energy = 25 mJ, package: TO92)

Appliances

∆ T

rep

(° C)

0 0.5 1.0 1.5 2.0

ACS switching period (T) (s)

operation field

2.2 Pumps and Fans ON / OFF control

2.2.1 Application requirements for (dI/dt)c and (dV/dt)c

There is a higher risk that a triac or an ACS will fail to turn-off when both the load current rate of decrease and the reapplied voltage rate across the device are steep (see

References, 3.). This risk increases as the junction temperature increases. The maximum

current decreasing rate that ACS can switch off, called (dI/dt)c is defined for a maximum reapplied voltage rate, called (dV/dt)c, and for its maximum T

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AN1172 Inductive loads on/off control

Pumps and Fans are, for the most part, induction or permanent magnet motors. Their series inductance is in the range of one Henry, and their winding resistance equals a few hundred Ohm. Their power factor is low. Hence, after switch turn-off, the reapplied voltage across it is high and appears with a high rate of increase (as described in Equation 1 where L and cosϕ are the inductance and power factor of the load, V the mains RMS voltage and C is the ACS capacitance value).

Equation 1

−

()

Figure 6. shows that the (dV/dt)c rate for an ACS402-5 die without any snubber, controlling a

230 V 220 mA pump, is lower than 10 V/µs. The measure will be similar with an ACS108-5 die because it presents the same capacitance value as an ACS402-5.

Equation 2 shows that the current rate of decrease is almost half the RMS current (0.44 ratio

for a 50 Hz mains frequency and 0.53 for 60 Hz).

≅ϕ

sin2Vcdt/dV

)V()µs/V(

)F()H(

Equation 2

≅π

10f2I2cdt/dI

)Hz()A(RMS)ms/A(

To summarize, it can be said that the worst case commutation appears with pumps or fans. In this case, the stress that ACSs must withstand is:

Equation 3

≅

⎧ ⎨

()

⎩

Figure 6. 230 V 220 mA RMS pump switch-off

≤

I5.0cdt/dI

)A(RMS)ms/A(

µs/V10cdt/dV

Vout (50 V/div)

dV/dt = 8,7 V/µs

Iout (10 mA/div)

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