ST AN1379 Application note

AN1379

Application note

Z01 and ACS behavior compared under fast voltage transients

Introduction

Home appliances such as washing machines, refrigerators and dishwashers integrate a lot of low power loads such as valves, door lock systems, dispensers and drain pumps. These loads are mains-powered in on / off mode, and are mostly controlled by Triacs or relays.

In most cases, the AC switching function now needs to be directly driven by a microcontroller unit (MCU) and it must withstand the AC line transients to make the system compliant with the new European Electromagnetic Compatibility (EMC) standards. STMicroelectronics ACS™ (alternating current switches) have been designed to meet these needs, as shown in this application note. Compared to Triacs, they offer high robustness and dV/dt capability, while contributing to a substantial reduction of the overall electronic board size.

The application specific discrete (ASD) concept, developed by STMicroelectronics, allows several devices, such as diodes, thyristors, transistors and some passive components used to make a complete function, to be integrated on the same silicon die. This technology has been used to develop the new ACS structures.

An ACS embeds an integrated driver, a clamping structure, and a bidirectional, thyristor-type switch (see Figure 1). The primary loads to be targeted by these new devices are high inductive loads like electromagnets, where the serial inductance can reach teens of Henry and the turn-off operation can thus cause many problems. The second section explains how the clamping feature of ACSs enables them to directly drive any inductive load without any external clamping device, such as metal oxide varistors, and how ACSs can also sustain overvoltages coming from the mains.

Silicon devices are subjected not only to surge voltages but also to fast transient voltages, as described in the IEC 61000-4-4 standard. They must not only present clamping ability but also high immunity to high dV/dt rates. The results of tests reported in this application note show the maximum levels withstood by ACSs and Triacs, for different gate sensitivities.

Figure 1. ACS symbol description

AC power		OUT
switch

G	Over voltage
	protection
	(Trisil™-Like)

Integrated

driver

COM

TM: Trisil is a trademark of STMicroelectronics

TM: ACS is a trademark of STMicroelectronics

June 2010

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ACS: an overvoltage protected AC device	AN1379

1 ACS: an overvoltage protected AC device

1.1Inductive load switch-off

Valves and relays are electromagnetic systems. In the case of AC high voltage operation, their windings show 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 rate of decrease of the current is low and an automatic switch turn-off may result when its current becomes lower than the holding level [1].

There may be an overvoltage due to the fact that there is still some current through the inductive load. The inductive energy thus creates a back electromotive force. If this overvoltage is not clamped, it can reach the device breakdown level and damage it.

ACSs are self-protected against overvoltage. They can sustain their holding current in such an operating mode, as shown in Figure 2.

Figure 2. Valve turn-off - typical oscillogram with the ACS108-5TA device

Iout (10mA/div)

Ih = 20 mA

Vout (200V/div)

Time: 400 µs/div

1.2IEC 61000-4-5 standard

The IEC 61000-4-5 standard has been established to check whether systems can continue to work after there has been a voltage surge superimposed on the mains. A standard voltage waveform has been chosen which embodies typical overvoltages due to lightning or the disconnection of running inductive loads from the line.

As the line to neutral surge can appear at peak mains voltage, the overall voltage can reach 2.4 kV (2 kV surge + peak mains voltage for 240 V rms line). This will be higher than the breakdown level of the silicon devices used in appliances. To prevent the destruction of components, designers use a varistor connected across silicon devices.

When a surge occurs and the ACS is off, the mains overvoltage is first clamped by the device. But an excessive energy surge can raise the ACS current above its breakover level.

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AN1379	ACS: an overvoltage protected AC device

Then, the switch turns on in breakover mode [2] [3]. Such an event is particularly stressful on the semiconductor especially so if the current and its rate of increase are both high. The worst case occurs when ACSs are driving low resistance, non inductive loads (only a few tens of µH as a series parasitic inductance).

For example, Figure 3 and Figure 4 have been recorded with a thermal active door lock system at a low temperature controlled by an ACS108-5TA device. The 2 kV surge is superimposed on the 230 V, 50 Hz mains and synchronized with its peak value, as shown in Figure 3. Figure 4 shows the device turn on in this mode. As the load was previously off, its resistance is cold and equals 150 ohm. In this case, the current rises at a rate of 100 A/µs and reaches 15 A. Such transient surges would damage Triacs, but not ACSs, as they are designed to turn on in breakover mode. The varistor is then no longer needed in parallel across ACSs, unlike Triacs. The difference between ACS and Triac + varistor is that, with the ACS, the load is switched on during a half or one mains cycle. This can be accepted as such events happen a few times in the system's life.

Figure 3. 2 kV surge on the mains

VAC (500 V/div)

Iout (5 A/div)

Figure 4. ACS breakover

VAC (200 V/div)

Iout (5 A/div)

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