With the ACST8-8CFP, STMicroelectronics is extending its AC switch family by introducing a new high voltage switch for induction motor drives in washing machines and other power applications.
Most vertical axis washing machines are now electronically controlled, and their control module uses from five to eight AC switches to drive the various loads. Robustness and reliability are therefore key features for the selection of these power switches and like the other ACS/ACST devices, the ACST8-8CFP includes these essential features.
Induction motor drive application
Vertical axis washer drums are driven through a gearbox by a bi-directional single-phase induction motor. This two coil motor uses a phase-shifting capacitor that increases both the motor torque and the electrical power factor. Two switches drive the motor, alternately controlling the rotation direction. Since the two AC switches are connected to the motor capacitor, they have to withstand its peak voltage that reaches 750 V and to resist any spurious discharge that generates a stressing current rise exceeding 100 A/ms.
The ACST8-8CFP is an 8 A, 800 V a.c. switch that has been designed to drive 230 V a.c. 50-60 Hz single-phase induction motors up to 450 VA. The 8 A current rating allows the induction motor to be switched off without any snubber aid circuit when the rotor is stalled. Its 800 V voltage rating matches with the capacitor operation at all the -40°C to 125°C junction temperatures range.
The device's turn-on speed has been reinforced beyond 100 A/ms to reliably withstand the spurious discharge of the motor capacitor. Noise immunity is reinforced up to 750 V/µs at Tj = 125°C and is doubled in comparison with any equivalent triac, allowing the size of its noise suppressor to be reduced or even eliminated. The ACST8-8CFP is housed in a 1500 V isolated fully-moulded package, the TO-220 FPAB, which offers the possibility to mount all switches on a single heatsink and to operate in a very high moisture environment.
Overvoltage robustness
Built with the ASD technology, the ACST8-8C is designed like a snubberless triac and integrates an overvoltage protection. This unique construction allows the ACST devices to resist the mains voltage surges, such as those described in the IEC61000-4-5 standard. The ACST switch safely turns on in 'crowbar' mode when the applied voltage exceeds its avalanche voltage. Therefore, the surge energy is sent back to the AC line, and, since robustness is built in, the external overvoltage protection is no longer required in parallel with the switch.
To illustrate this robustness, a new symbol has been created for ACST devices. A 'crowbar' is added to the traditional triac symbol voltage-current characteristic.
Key features of the ACST8-8C are:
* Blocking voltage: VOFF > 800 V.
* Avalanche voltage: VCL = 1200 V.
* Surge line voltage: Vpp > 2000 V.
* Junction temperature range: -40°C < T < +125°C.
* Gate triggering current: IGT < 30 mA.
* High off-state static dV/dt: >750 V/µs.
* On-state current: IRMS = 8 A @ TCASE = 90°C.
* Turn off commutation: (dl/dt)C > 4,5 A/ms.
* 1500 V isolated with TO-220 FPAB package.
Developed for motor driving, the ACST8-8CFP offers a robust and high-quality switch for the design of electronic control for appliances.
Example applications are: AC solid-state relay; compressor induction motor drive; power tool trigger.
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