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POWER ELECTRONICS ACS Control with opto-Triac Laurent GONTHIER Application Manager for Appliance & Industrial segments, STMicroelectronics Jean - Michel SIMONNET Application Engineer for Appliance & Industrial segments, STMicroelectronics A C switches are silicon devices which control AC loads directly connected to the AC mains. This means that the driving reference terminal of the AC switch can be connected to the Line potential. This paper discusses the need for an insulation layer for the control unit and the way to implement it for the specific case of an AC switch device. Is insulation required between MCU and AC switch? The drive reference terminal of an AC switch can be connected to the Line potential. If an MCU is used to directly control this device, it will then also be linked to the Line potential. It was thought in the past that connecting an MCU to the Line has to be avoided as it will lead to a poor appliance immunity. But it has been demonstrated for many years that such topology has good immunity. Connecting an MCU supply to a stable non-floating reference is even better for immunity. In terms of safety for people, animals and goods, appliances sold in Europe have to fulfill the 73/23/CEE directive, called the "low voltage directive". Manufacturers must then follow some European harmonized standards (such as the EN60335-2-X for home appliances, EN60730-2-X for automatic electrical controls for household or EN60669-2-for static or mechanical switches for home applications). These standards specify how to implement operational or safety insulation. Operational insulation is required between two conductors, biased by two different potentials, to allow proper operation of the appliance. Such an insulation level doesn’t deal primarily with safety but the requirements also help to minimize exposure to ignition or fire due a too high leakage current between two printed circuit board tracks. Safety insulation has to be implemented between Antoine PASSAL Application engineer for Appliance & Industrial segments, STMicroelectronics accessible parts and high voltage circuits to protect end users against electric shocks. Several levels of insulation are defined according to the equipment class and the way it is implemented. It’s not required to ensure safety insulation by insulating low voltage control circuits (like MCU) from highvoltage parts (like AC switches). Indeed the insulation could be implemented elsewhere, for example on the keyboard to which the end user has access, and leave the MCU connected to the Line. This could be cost effective as a non-insulated power supply and noninsulated-drivers would then be sufficient. Operational insulation is required when the control circuit reference is not the same as the AC switch reference. This is the case with new appliances using an inverter for 3-phases motor control, where the MCU is connected on the DC rail, and the ACS switch is referenced to Line. Then a level-shifter has to be used to allow communication between the MCU and the power switch. A usual way to implement this is to use an optoTriac but such a device will not work properly for whole AC switches. Could an opto-Triac be used to drive an ACS? Among AC switches available today, different technologies and designs are proposed. The main known families are the standard Triacs, the snubberless Fig. 1: ACS symbol (a) and simplified silicon structure (b) POWER ELECTRONICS Triacs and also the ACS devices released in the early 90’s. To switch-on a Triac or an ACS, a gate current must be applied between Gate (G) and terminal A1 for Triac, or between Gate and terminal COM for ACS. For Triac, the gate current could be positive and negative thanks to the two back-to-back diodes, implemented between G and A1. The silicon structure of an ACS is different from a Triac (see Figure 1). Here the gate is the emitter of an NPN bipolar transistor. So there is only one PN junction (implemented by P1 and N1, see Figure 1). The gate current can then only be sunk from the gate, and not sourced to it. The usual solution to trigger a Triac with an insulated control is to use an opto-Triac connected in series with the Triac A2 and G terminals. A resistor is also connected in series to reduce the current applied to the gate. The device is then triggered by a positive gate current when the voltage across the Triac, just before turn-on, is positive, and by a negative current in the reverse situation. Such a solution works for all Triacs. The Triac is then triggered in what is called Q1 and Q3 quadrants. For ACS devices, as they can only be triggered by a negative current, an opto-Triac will drive the ACS only when the Line voltage is negative. This will lead to a load half-cycle conduction and this is not convenient for most applications. However there are new applications where such an operation is requested. This is, for example, the case for some pumps used in coffee machines which feature an internal diode, and also for some electromagnets used for door-lock function in washing machines. Then an ACS driven by an opto-Triac could be used for such applications. The only point to deal with is that a voltage higher than 10 V has not be applied to G Fig. 2: Solution for half-cycle ACS control with series diode Fig. 3: Solution for half-cycle ACS control with parallel diode referred to COM, not to damage the G-COM junction. To avoid the positive Line voltage to be applied when the opto-Triac is ON, there are two different solutions: - adding a high-voltage diode in series with the optoTriac (see Figure 2), to block the positive voltage. - adding a low-voltage diode in parallel with the COM-G junction (see Figure 3) to bypass the high positive bias. It should be noted that in the 1st case, the opto-Triac and the diode could be replaced by a reverse blocking opto-thyristor, with its Anode terminal connected to ACS gate. How to implement a full-wave ACS control with an opto-Triac? For home appliances, most loads have to be controlled in full-cycle mode. The previous schematics are then not adapted to ensure ACS triggering for each cycle. The solution is to add a low-voltage capacitor to store some charges to apply a gate current at the beginning of the positive conduction. The schematic of this Fig. 4: Schematic for full-cycle ACS control with an opto-Triac solution is presented on Figure 4, which shows two lowvoltage diodes. 1: Opto-Triac is turned on and charge capacitor C up to VGT parameter (~ 0.7 V), then COM-G junction is forward biased and the ACS is triggered by a negative gate current. 2: ACS is ON up to the next zero current crossing point. G-COM voltage is kept down to -0.7 V due to ACS conduction and C is kept charged. 3: As ACS current increases, VG-COM increases and so a negative current is applied by C which triggers the ACS for the next cycle. In this solution, the ACS is off at each cycle beginning for the time required to recharge capacitor C. The ACS then turns on when the voltage across its terminals equals approximately 10 V. This behavior doesn’t result in high conducted noise as the Line current is still approximately sinusoidal thanks to the charge current that flows through capacitor C at Zero Voltage crossing point.