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AN-862 Application Note INTERFACING DIGITAL CIRCUITS to THYRISTOR CONTROLLED AC LOADS

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AN-862 Application Note INTERFACING DIGITAL CIRCUITS to THYRISTOR CONTROLLED AC LOADS AN-862 Application Note INTERFACING DIGITAL CIRCUITS TO THYRISTOR CONTROLLED AC LOADS This article describes the interfacing of triacs with integrated circuit drivers (e.g. logic gates) for the control of AC power loads. Included are the more popular logic families, such as TTL (including LSTTL) and CMOS with both positive and negative supply voltages. Examples of circuit isolation with opto- coupling are provided and the use of microprocessors as driving elements is discussed. Utilization of interfacing NPN and PNP transistors for thyristors requiring higher drive current is covered. THYRISTORS Thyristors are available as unidirectional devices Quadrant IV, where the gate is positive and MT2 is (SCR's) or bidirectional devices (triacs) The SCR negative, is the least sensitive one; therefore, other (Silicon Controlled Rectifier) conducts current in one quadrants of operation should be considered when direction only (from anode to cathode) in response adequate gate trigger current might be a problem. Use to a positive-going gate signal The gate signal causes of a negative gate will assure triggering in quadrants a current flow from gate to cathode and will turn II and II!. on the SCR only with the anode is positive with Quadrants II and III operation is particularly desirable respect to the cathode. When the anode is when the triac is driven by 1C logic - not on Iy negative, the SCR is in its blocking state. (See Figure 1a.) because less gate trigger current is required in Once the SCR has been triggered on, it will remain in the on state as long as the anode current remains above a specified minimum holding level (I H)' even if THYRISTOR CHARACTERISTICS the triggering gate signal is removed. If the anode SCR TRIAC current drops below this "holding" current (IH), the SCR unlatches (turns off) With an AC load, this IT = On-State happens at the end of the positive half cycle of anode Current voltage, resulting in a pulsating DC anode current in VT = On-State Voltage response to an AC anode voltage. For AC load requirements, triacs are normally employed The triac can be thought of as two SCR's connected in an inverse-parallel arrangement with one common gate lead. (See Figure 1 b) Without a gate trigger signal the device will be in its off state, resulting in a high impedance between terminals MT1 and MT2. When the triac is triggered on, a low MT1-MT2 Vr ' Breakover I Forward impedance results. Typically, in the "on" condition, Voltage I Breakover a triac (or SCR) will drop one to two volts across Revl'rse Voltage its main terminals at rated current. Breakover Once turned on, the triac, like the SCR, will stay Voltage on whether or not the gate signal is present, until the current flowing through it drops below the holding Equivalent current (IH). This happens at the end of every half cycle of applied 60Hz. If a continuous "on" (low Input Circuit impedance) state is desired, the triac must be triggered Equivalent (a) Cathode Input at the beginning of every half cycle, or a continuous Circuit gate signal must be present. The triac can be triggered on by either a positive or negative gate signal, on either the positive or negative half-cycle of applied MT2 voltage. This provides Figure 1. Static characteristics of SCRs (a) and Triacs (b). In the convenient triggering regardless of the gate trigger absence of a gate signal, very little current flows through either signal polarity, or MT2 voltage polarity Triggering device if the anode or MT2 voltage (SeR and Triac, respectively) sensitivity, however, is determined by the polarity remains below the breakover points (normal operating condition). relationship between the trigger signal and the MT2 When an appropriate gate trigger current is applied (see text), the voltage. In Figure 2, this relationship is divided into devices go into conduction, resulting in a very low resistance across four quadrants depicting the possible phase relations them. Turn-off can be achieved only by reducing the anode or MT2 between the trigger signal and the MT2 voltage. current below the holding current, IH. these more sensitive quadrants, but also because IC This voltage is needed to overcome the input threshold power dissipation is reduced since an "active low" output voltage of the device. To prevent thyristor triggering, from the IC is used for triggering. gate voltage should be kept to approximately O.4V There are other advantages to operating in Quadrants or less. II and III. Since the rate of rise of on-state current of Like ICT' V GTincreases with decreasing temperature. a triac (di/dt) is a function of how hard the triac's gate TABLE 1. is turned on, a given IC output in Quadrants II and III Triacs with Various Current Ratings will produce a greater di/dt capability than in the less Sensitive Gate Triacs sensitive Quadrant IV. Moreover, harder gate turn-on could reduce di/dt failure caused by localized junction burn-out that occurs when the thyristor tries to conduct MAC92, 92A, 93, 93A 0.6A too much current just after triggering, before the 2N6068A, B-75A, B 4A entire device has a chance to turn on. One additional MAC228, A 8A advantage of quadrant II and III operation is that devices specified in all four quadrants are generally Non-sensitive Gate Triacs more expensive than devices specified in quadrants I, Triac II and III due to the additional testing involved and MAC91, A 0.6 the resulting lower yields 2N6068-75 4.0 2N6342-49 8.0 Using Triacs 2N6342A-49A 12 Once the triac load requirements are defined, an MAC15, A Series 15 MAC223, A Series appropriate device selection can be made by referring 25 2N6157-65 30 to the triac current ratings of Table 1. 2N5441-46 40 Two important thyristor parameters are gate trigger current (ICT) and gate trigger voltage (V GT)' IGT (Gate Trigger Current) is the amount of gate A trigger current required to turn the device "on". IGT has a negative temperature coefficient- that is, the trigger current required to turn the device on increases with decreasing temperature. If the triac I60 Hz must operate over a wide temperature range, its I GT Line requirement could double at the low temperature extreme from that of its 25°C rating. IGT_vs_tempera- ture information is usually on the data sheet in the form of a graph, or in a specifications table. Voltage ~ It is good practice, if possible, to trigger the thyristor Applied to Terminals with three to ten times the IGT rating for the device. This increases its di/dt capability and ensures adequate A and B gate trigger current at low temperatures. \ V GT (Gate Trigger Voltage) is the voltage the thyristor gate needs to ensure triggering the device on. tl IGT 1. _ ~ , 1~~:~~~ubber Change in Triac Voltage during \I Network ----Turn-offL (dvl MT2(-) Undesired Triggering Figure 2. The Four Quadrants of Triac Operation. A Triac may be due to Feedback triggered into conduction by either a positive or negative-going gate signal, with either a positive or negative voltage applied between MT2 and MTl. Inductive Load Switching Switching of inductive loads, using triacs, may require special consideration in order to avoid false triggering. ------..., Load This false-trigger mechanism is illustrated in Figure 3 Source connection Current which shows an inductive circuit together with the for I Sink accompanying waveforms. R1 current As shown, the triac is triggered "on", at t1, by the I Current 1.4 k I sink condition positive gate current (lCT)' At that point, triac current I flows and the voltage across the triac is quite low ,/ "" since the triac resistance,during conduction, is very low. V .•..~.ut Load , From point t1 to tl the applied ICT keeps the triac in a conductive condition, resulting in a continuous I connection for sinusoidal current flow that leads the applied voltage I current 1.0 k I by 90°, due to the inductive nature of the circuit. source At t2, I is turned off, but triac current continues I CT ______ .J condition to flow until it reach,esa value that is less than the sustaining current (IH), at point A. At that point, triac current is cut off and triac voltage is at a maximum. Some of that voltage is fed back to the gate via the internal capacitance (from MT2 to gate) of the triac. m·TO·THYRISTOR INTERfACE The subject of interfacing requires a knowledge of the output characteristics of the driving stages as Source well as the input requirements of the load. This Current section describes the driving capabilities of some of Sink the more popular TTL circuits and matches.these to Current the input demands of thyristors under various practical operating conditions. m Orcuits with Totem·Pole Outputs (e.g. MC5400 series) The configuration of a typical totem-pole connected TTl output stage is illustrated in Figure Sa. This stage is capable of "sourcing" current to a load, when the load is connected from Vout to ground, and of "sinking" current from the load when the latter is connected from Vout to VcC' If the load happens to be the input circuit of a triac (gate to MT,), the triac will be operating in quadrants I and IV (gate goes positive) when connected from Vout to ground, and of "sinking" II and III (gate goes negative) when connected from Vout to V cc. Quadrant I-IVOperation Consideringfirst the gate-positive condition, Figure Sb, Cs 60 Hz Line Figure 6(a). Totem-pole output circuit of MC5400-type TIL logic.
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