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SCR’s AND TRIACS IN AUTOMOTIVE APPLICATIONS REV. 2 INTRODUCTION In power electronics the choice of the switch to be used is always very important for the designer: cost, reliability, ruggedness and easiness to be driven must be permanently kept in mind. This is specially im- portant in automotive where the designer has to optimize the numerous switching devices implemented for a wide field of applications: ignition circuits, vari ous electric motor controls, actuators in security sys- tems, etc... In the large family of electronic switches, the thyristor must be considered as a low cost and powerful de- vice for many functions in a car. This paper reminds the reader of the main characteristics of the thyristors and their advantages in certain automotive applications. Some examples are presented to illu strate how SCR’s and triacs can bring innovative solutions in new electronic designs. THE THYRISTOR In fact the term "thyristor" defines a silicon bipola r semi-conductor family in cluding any switch whose bistable action depends on PN-PN regenerative feed back. The main members of this family are the SCR and the Triac (see appended table). On the other hand, this structure is also widely used in the new product family integrating several "discrete" components to build various power functions. This new integrated circuits range is called ASD™ (Application Specific Discrete). MAIN CHARACTERISTICS AND BENEFITS Switching on One of the major advantages of the thyristor is its ability to switch from off-state to on-state in response to a low gate current. According to the type of device used, a gate current of a few tens of microampere or a few milliampere is sufficient to turn the switch on. The triggering signal can be a short duration pulse in the range of 100 microseconds or less. As soon as the latching current through the device is reached, the switch remains ON and the gate current can be removed. This "memory" effect can be used in a broad area of applications, reducing the consumption of drivers and saving auxiliary components. An important point worth noting here is the four quadrants triggering capability of triacs. Whatever the po- larity of the current through the device, it can be fired either with positive or negative gate current. This characteristic allows a wide flexibility in the circuitry (see Figure 1).
Figure 4. Ignition circuit with SCR: storage capacitor connected across input voltage source The triac can be used as a very flexible unidirectional or bidirectional switch in such a circuit. circuit for the gate control (see Figure 5).
Figure 5. Ignition circuit with Triac: In case B the Triac is triggered by a negative gate current. In trigger mechanism in action: Air bag, belt pretensionner, etc... provides a high reliability (see Figure 6).
Figure 6. Example of application of SCR in safety equipment. The thyristor is turned on when the function is activated and inactivated by reversing the current. bridge: four power MOS transistors associated with the heavy gate drive circuit inherent to this topology. A good example is the central door lock system shown below.
Figure 7. Typical circuit using triac as a bidirectional switch to control each door separately In this circuit, the terminal A1 of the triac is sometimes connected to the ground or to the battery voltage. Therefore, a small interface circuit is needed to drive the gate from a standard micro controller output. requirement concerning gate triggering current (see Figure 8).
Figure 8. Example of gate drive interface circuits Note: (A): the triac is always fired by a negative gate current pulse (Quadrants II and III).
Figure 11. Sensitive SCR can be used as upper arm of H-bridge. The driver, is this case, is turns off and then the SCR opens. liable electronic switch in today’s car applications. The particular characteristics of thyristor offer a powerful and cost effective solution in many applications. drives can give new ideas to the designer. SCR’s and triacs are today fully compatible with automotive requirements in terms of quality and reliability. integration of several "discrete" functions on the same silicon die (ASD™ Application Specific Discrete). quality inherent in the planar technology.
REVISION HISTORY
Table 1. Revision History 3-June-2004 2 Stylesheet update. No content change.
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