June 2010

Doc ID 8317 Rev 5

1/10

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

TM: ACS is a trademark of STMicroelectronics

OUT

G

COM

Integrated

driver

Over voltage

protection

( Trisil™ -Like)

AC power

switch

TM: Trisil is a trademark of STMicroelectronics

www.st.com