Introduction. Vishay Telefunken

Transcription

1 Introduction Optocouplers incorporating optotriacs at the output side are best suited for the transmission of control signals at the interface between low voltage and power electronics. Optical signal coupling and appropriate designing cater for very good potential galvanically separation, this being an aspect of most importance at such interfaces. These are the only optocouplers whose outputs can be connected up directly to an AC voltage of several 100 V. This facilitates direct control of small AC voltage appliances or the switching and control of large AC power outputs via power triacs. The VISHAY Telefunken optocouplers with triac outputs as well as their characteristic features will be found summarised in Table 1. All triac optocouplers are built inside a 6 PIN plastic DIP housing and tested in accordance with VDE Owing to these components being chiefly employed for mains voltage applications, VDE Safety Regulations definitely need to be observed. Those types marked K3010P(G) series are suitable for use on 115 V mains for Class I IV applications, whereas types K3020P(G) series can be suitably employed on 220 V mains for Class I III applications. The final digit in the type designations corresponds to grouping according to the input switch on current, the trigger current of the optocoupler. The various parameters listed in this table will be explained and discussed shortly as based on the application of triac optocouplers. Fig. 1 (see next page) shows the two forms of housing used, the 6 PIN standard type and the 6 PIN G type featuring extended pin spacing. The minimum insulation spacing distance, i.e. the insulation thickness between the input and the output side, is 0.75 mm in respect of both housings. The 6 PIN G type is suitable for applications on which an external creepage path of at least 8 mm is necessary. Table 1: V IOTM V DRM T V TM I H dv/dt I TRMS Optocouplers min. min. typ. max. typ. max. typ. typ. max. V V ma ma V V µa V/µs ma K3010P(G) 8 < 15 K3011P(G) < K3012P(G) 2 < 5 K3020P(G) 15 < 30 K3021P(G) 8 < K3022P(G) 5 < 10 K3023P(G) 2 < Note: The applied trigger current is the maximum current at which the coupler must switch from the off state to the on state. For the designer it is recommend to chose the resistor for setting the current for a minimum current of 15 ma (e.g. for K3010P). We further recommend to add 50% of this current ( compensation aging plus temperature behavior ) to assure a long life operation

2 K3010P / K3020P K3010PG / K3020PG weight: ca.0.50 g creepage distance: 6 mm air path: 6 mm after mounting on PC board weight: ca.0.50 g creepage distance: 8 mm air path: 8 mm after mounting on PC board ~ ~ A (+) C ( ) nc Note: Pin 5 must not be connected Figure 1. Illustration of optocoupler; Standard type and G type Infrared Transmitter (emitter side) As on all optocouplers, those parts incorporating a triac output also make use of a GaAs diode to serve as an infrared emitter. Those parameters of the IR emitter diodes of importance for dimensioning a control circuit the forward voltage V F and its temperature dependence will be found illustrated in Fig. 3. The data sheet specifies a typical V F value of 1.25 V and a maximum value of 1.6 V for a current of 50 ma. Refer to the characteristics in Fig. 2 for values needed for other currents. The lowest current flows at the maximum forward voltage which explains why this value has to be included in calculations. As temperature increases, the forward voltage decreases by approx. 1.5 mv/c, Fig. 3. Here too, the worst case is therefore encountered at low temperature. At 25 C, the forward voltage is found to have increased by approx. 6.5 %. As specified in the data sheet, the permissible reverse voltage of the IR diode is 5 V. Although, in most cases, actual reverse voltage is much higher, the IR diode should not be subjected to higher reverse voltages as this may impair the service life of the component. In normal forward operation, a slight decrease in the radiant intensity will occur in the over of time. This minimum ageing increases along with the forward current and generally also with the temperature. To avoid trigger problems at low temperatures, an appropriate allowance should be added to the forward current of the diode

3 =10mA Forward Current ( ma ) T amb =25 C Scattering Limit V Frel Forward Voltage ( V ) V F Forward Voltage ( V ) T amb Ambient Temperature ( C ) Figure 2. Forward Current vs. Forward Voltage Phototriac (receiver side) As regards functioning a normal triac corresponds approximately to two counter connected thyristors. Fig. 4 illustrates the voltage current characteristic of a triac in diagram form along with the underlying principle. ON State V Reverse Breakover Voltage / Current +I +I T I Forward Breakover Voltage / Current V DRM +I DRM +V DRM ( I G > I GT ) Quadrant III V T +I H I DRM I H I T +V T ( I G > I GT ) Quadrant I Figure 4. Voltage current characteristic of a triac The triac is switched on via the gate current I G. This current exercises the function of a trigger with a threshold I GT which transfers the triac from the OFF state to the ON state. As long as I G is < I GT, the very low off state current I DRM passes through the triac. The triac is thus blocked and the applied voltage is able +V Figure 3. Forward Voltage vs. Ambient Temperature to build up without the triac cutting through. In the event of I G exceeding the trigger value I GT, this will switch on the triac; its voltage current characteristic now passes through the branch on the voltage V TM, the forward voltage of the triac. Once switched on, the triac will remain in the ON state as long as the current I flowing through it does not fall below the IH value (hold current) irrespective of the polarity of the applied voltage and value of the gate current I G. On the optotriac a gate is not led out, its function being assumed by I R radiation from the transmitter diode built into the optocoupler and optically coupled to the triac. In this case, the current passing through the transmitter diode replaces the gate current of the triac. Here, the corresponding trigger value is designated as T. Otherwise an optotriac behaves in exactly the same way as a normal triac. Fig. 5 contains a diagram illustrating the characteristic curve of an optotriac. Although the negative characteristic curve portion between V TM and V DRM (portion of I H characteristic indicated by dotted line in Fig. 4) does not appear here, the hold current I H nevertheless also exists for the optotriac. The optotriac also remains switched on (ON state) as long as the I H is not fallen below. If the main current I is smaller than I H, the optotriac will switch itself off regardless of whether the transmitter current of the optocoupler exceeds T or not

4 +I < T 100 < T > T ON state I H IDRM ON state IF> IFT V + V I V TM V TM Figure 5. Characteristic curve of an optotriac Those optotriacs used on the K3010P(G) Series or K3020P(G) Series optocoupler have a hold current I H of approx. 100 µa. Triac peak reverse voltage V DRM and reverse current I DRM As will be gathered from the characteristic curves in Fig. 4 and Fig. 5, the triac switches itself on (ON state) as soon as the applied voltage V T exceeds the value V DRM by a certain small degree. This process of switching on the triac without a control signal being received from the transmitter side of the optocoupler is referred to as overhead firing. It may also be caused by glitches of short duration and is undesirable on optotriac applications (malfunctioning). Accordingly, when using optocouplers, it should be ensured that the V DRM is on no account whatsever exceeded, not even for a short duration. For the VISHAY optocoupler K3010P(G) Series (suitable for 115 V networks) minimum tested V DRM voltage is 250 V, whereas for types K3020P(G) Series (designed for 220 V systems) the minimum tested V DRM voltage is 400 V. As long as the voltage applied on the optotriac lies between V TM and V DRM and no control signal from the optocoupler input switches on the triac, the optotriac will remain blocked, in which case only the off state current I DRM will flow. Its value at 25 C is far below 10 na and can be disregarded in most cases. However, it should be borne in mind that its value increases by approximately factor 10 per 30 C temperature rise. Fig. 6 shows a typical off state current curve via temperature. I DRM Off State Current ( na ) V DR =100V = T amb Ambient Temperature ( C ) Figure 6. Off state current vs. Ambient Temperature Triac forward voltage V TM Triac forward voltage V TM After firing, the forward voltage V T remains across the triac, or in AC mode the period peak forward voltage V TM. This must not be confused with the forward voltage V F of the I R diode. It corresponds approximately to the saturation voltage V CEsat of the phototransistor. The forward voltage V TM applies to both directions and, in the case of triac couplers, is approx. 2 V at 100 ma and a maximum of 3 V (refer to characteristic in Fig. 7). This value appears to be high in comparison with the saturation voltage of a phototransistor on the standard optocouplers but it needs to be borne in mind that a triac coupler is capable of switching much higher voltages. At the same time, the remaining voltage is less than 1 % of the switch voltage. The forward voltage V TM of the triac increases along with the temperature (Fig. 8) but at approx. 0.1/K the rise is so slight that it can generally be ignored. I TM On State Current ( ma ) =30mA 100 T amb =25 C V TM On State Voltage ( V ) Figure 7. On State Current vs. On State Voltage

5 V TMrel Relative On State Voltage T I T =100mA T amb Ambient Temperature ( C ) Figure 8. Relative On State Voltage vs. Ambient Temperature Critical rate of voltage rise dv/dt Critical rate of voltage rise dv/dt A common feature of all thyristors and triacs is that the rate of the voltage rise at its main terminals (anode/cathode) needs to be limited. If the voltage rises faster than the so called rate of rise, the thyristor or triac will fire even if no control signal is applied. However, individual misfiring due to interfering spikes from the power mains can be tolerated in many cases since the component switches off again the next time the AC current passes through zero. Two very different values are generally given in the data sheets of triac couplers as applied to the critical rate of voltage rise. 1. The critical static rate of rise or critical rate of rise in the OFF state (dv / dt) cr refers to cases when the emitting diode of the triac coupler is currentless ( = 0). Typical values of 10 V/µs are given here but particularly resistant types with 50 V/µs or more are also encountered. 2. The critical dynamic rate of rise or the critical commutation rate of rise (dv / dt) crq refers to the moment when a control signal previously applied to the IR diode of the triac coupler is deactivated. Table 2: Examples of the dv/dt values of sinusoidal AC voltages. f Hz V eff V At high frequencies, there is a risk that the charge carriers remaining in the depletion layer from the fired state will result in renewed firing during the subsequent half wave of opposite polarity. Typical values of 0.2 V/µs (min. 0.1 V/µs) are specified in triac coupler data sheets. These values are therefore lower by a factor of 50 than the static values. The critical rate of rise as applied to triacs can be measured with a Iinear rising sawtooth voltage or with an exponential voltage increase (DIN ). A method of measurement using a sinusoidal AC voltage of variable amplitude and frequency has proved successful as applied to triac couplers. From these parameters the dv/dt can be calculated in the range when the voltage passes through zero, i.e. when firing generally occurs. Rate of rise of a sinusoidal AC voltage: The momentary value of a sinusoidal AC voltage (Fig. 11) can be represented by the formula: v = V M sin (ϖt) where, V M = peak voltage ϖ = 2 π f = radian frequency f = frequency. The momentary value of the voltage rise is obtained by differentiation: dv/dt = V M cos (t ) The voltage rise is at its steepest when the voltage passes through zero and, in this case, the following applies: t = 0 and cos (t ) = 1 and thus dv/dt: V M Where V M = 2 V rms and = 2πf becomes dv/dt = 2 V rms 2πf = 8.89 V rms f dv / dt V / s

6 Information regarding dimensioning and circuitry engineering of optocouplers with a phototriac at the output. In the same way as Schmitt phototriggers, triac couplers function as threshold switches, for which reason a current transfer ratio (CTR) cannot be specified as in the case of couplers with phototransistors. Here, the trigger current T serves as a means for measuring switching sensitivity. At 25 C the typical trigger current T corresponds to data sheet 2 ma or 15 ma depending on the type. However, a circuit intended for volume production must be designed for the maximum value which amounts to 5 ma or 30 ma at 25 C. As shown in Fig. 9, the trigger current decreases in conjunction with an increase in temperature and the triac coupler becomes more sensitive. Therefore, the trigger current assumed for 25 C will suffice for reliable triggering even at higher temperatures. If the circuit is also to function at low temperatures, the planned forward current must be correspondingly higher. For instance, the additional value should be approximately 40 % for 25 C. As specified in the data sheet, the maximum permissible forward current is 80 ma. However, to ensure long service life, the IR diode should not be operated continuously at this current. Trel Relative Threshold Forward Current V S =3V R L = T amb Ambient Temperature ( C ) Figure 9. Relative Threshold Forward Current vs. Ambient Temperature Those values given for the trigger current in the data sheet are static values. In other words, a direct current flows through the emitter diode in the course of measurement. If, when employing the optocouplers, triggering does not take place using direct current, but with relatively short pulses, dependence of the trigger current on the applied pulse width tp should definitely be observed. Fig. 10 illustrates the curve pattern T = f (tp). Here, the relative value of T based on the static value is applied over the pulse width tp. As shown in Fig. 10, proceeding from a pulse width of approx. 4 µsec, three times the value of the static T is required to switch on the phototriac. This dynamic trigger current increases dramatically for shorter pulses. However, in the case of pulse widths exceeding 20 µsec, the trigger current T hardly differs from its static value. I Relative Threshold Forward Current FTrel tp ( µsec ) V T = 100V I T = 100mA Figure 10. Relative Threshold Forward Current vs. Pulse duration In frequent cases when dimensioning and developing applications with triac optocouplers, it is important to know how quickly the triac switches at the output of the optocoupler compared with the triggering action. This information is obtained from the switch on time ton of the optocoupler. Fig.11 below explains in diagram form how ton is defined in this case

7 10% von max V T max t on 10% von (V T V TM ) V TM Figure 11. Defination t on time Figure 12. Sine wave a.c The switch on time ton of the triac optocoupler is dependent on the degree of the trigger current employed. Fig. 13 shows the measured response curve of the switch on time via, in which case has been assumed as a multiple of the trigger current T. This curve applies to typical specimens on both the K3010P(G) and K3020P(G) optocoupler. t on Turn On Time ( s) Factor of T V T =10V R L =100 Figure 13. Turn on Time vs. Trigger current As shown in Fig. 13, t on is approx. 35 µsec for = T but decreases to approx. 12 µsec as soon as is doubled ( = 2 T ). If switch on times shorter than 30 to 35 µsec are important for an application, should be selected correspondingly higher than 2 T. Triggering the triac optocoupler The IR diode of a triac optocoupler is triggered in exactly the same way as all other optocouplers. But there is another determination by means of the T max. The applied trigger current is the maximum current at which the coupler must switch from the off state to the on state. For the designer it is recommend to chose the resistor for setting the current for a minimum current of 15 ma (e.g. for K3010P). We further recommend to add 50% of this current ( compensation aging plus temperature behavior ) to assure a long life operation. Fig. 14 shows the basic circuit for triggering the IR diode. V in corresponds to a DC voltage or the level of a pulse. The value of R V should be determined as an example for a K3020P(G) optocoupler at an ambient temperature of 25 C. From the data sheet for the K3020P(G) coupler a maximum trigger current follows of max = 30 ma. For this current a forward voltage of the IR diode results from the diagram V F = f( ) from Fig. 2: V F =1.5 V. If, for example, the input voltage is assumed as being V in =5 V, this results in the following: R V V in V F approx. 120 Tmax If the optocoupler is to be used up to a temperature of 25 C, the max. trigger current must be increased by approx. 40 % in accordance with characteristic curve T = f (T) in Fig. 9. This results in V F = 1.65 V and R V leading to R V 5V 1.65V approx mA However, proceeding from this resistance, a higher current will flow through the IR diode at +25 C which will also lead to higher dissipated power. For a typical K3020P(G) specimen proceeding from V F typ. = 1.25 V at 25 C the current will become: V in GND V F R V R L V T Figure 14. Basic driver circuit for IR diode

8 5V 1.25V 75 approx. 50mA and the dissipated power of the IR diode P V = 62.5 mw. Where higher temperatures prevail and greater currents, the max. permissible dissipated power of the I R diode should be observed. This amount to 100 mw at 25 C for the IR diode of the coupler. A disadvantage of the simple circuit in Fig. 14 is that the current is dependent on the level of the input voltage V in and thus too, on the forward voltage V F of the IR diode itself. In actual practice, triggering the IR diode with an impressed current works considerably better. This possibility is illustrated in Fig. 15. Here, the IR diode is located inside the collector of a transistor in such a way that V F has no influence on. GND V in R 1 I q R 2 I b V 2 V b +V S R V V T A value of 0.7 V with close approximation can be used for V b. A precise value will be gathered from the input characteristic curve of the transistor employed. The resistor R V in the emitter circuit of the transistor results in negative feedback being generated, in which case the set current Tmax remains stable to changes in temperature and component tolerances. The resistor R 2 can also be replaced by a zener diode or two Si. diodes so that a voltage V 2 results between 1.2 V and 2 V. The result in this case is as follows: R V V 2 V b Tmax Tmax is now in close approximation independent of V in as well as being constant. Triac couplers can be controlled by conventional integrated standard TTL or CMOS driver circuits (Table 3 see next page). Thus they form the ideal interface between microcomputer control systems and mains operated loads. The required forward current is adjusted using a series connected resistor. In the circuit shown in Fig. 16, the output has low level in active state and functions as a current sink. +V R V V T Figure 15. Transistor controlled IR diode driver circuit Here, the current is determined by R v ; V 2 and V b : V in = (I b + I q ) R 1 + Iq R 2, V 2 = I q R 2 = V b + I V R V, I V = I b + based on the assumption I b << I q and I b <<, the following results for the required resistor R V so that a certain current Tmax is set: V 2 I q xr 2 V in xr 2 (R 1 R 2 ) I V R V = V 2 V b and with I V = Tmax R V V in xr 2 (R 1 R 2 )xtmax V b Tmax V in GND Figure 16. IR diode driver circuit with IC (active low). Here, the bias resistance is: R V (V S V Fmax V OL ) Tmax For the TTL drivers 7407/17 and TTL inverters 7406/7416 with open collector outputs, a maximum output voltage V QL = 0.7 V is specified for the maximum output current I QL = 40 ma, thus:

9 R V = ( 5V 1.5V 0.7V ) / 30mA = 100 (rounded). +V S V in In the circuit shown in Fig. 17, the output reveals high level in the active state (active high), and functions as a current source. Here, the following applies: R V V QH V Fmax Tmax V T GND Figure 17. IR diode driver circuit with IC (active high). R V Table 3: Integrated driver circuits for triac couplers Logic Function Type max. / QL max. V QL hex driver 7407/17 40mA 0.7 hex inverter 7406/16 40mA mA 0.4 TTL quad NAND driver mA F38 48mA 0.5 quad NOR driver mA 0.4 hex bus driver mA 0.5 Interface hex inverter mA 0.8 hex driver 4050B 20mA 1.5 CMOS hex inverter 4049B 20mA

10 Switching resistive loads The main function in applying triac couplers is to control power triacs which in turn serve to switch loads having a high power consumption. A simple case involves the switching of purely resistive loads such as incandescent bulbs and heating elements (Fig. 18). The power triac must be selected in such a way that it will withstand the power on spikes of these loads which can often be considerable. At a maximum effective output current of 100 ma, a triac coupler is also capable of controlling high power triacs. R S R g I RL V ac/ass main Triac R L Figure 18. Resistive load switching V MT In Fig. 18 a resistor Rg is illustrated at the gate of the power triac. This Rg is necessary in conjunction with triacs which become very highly resistive at the gate. Such triacs need relatively low trigger currents for switching purposes. On the other hand they are also sensitive to glitches of short duration as well as to other forms of disturbance at the gate. The value of Rg is normally between 100 Ω and a few kω depending on the type of triac. The first circuit focuses on a power triac without gate resistor Rg. Here, the optocoupler acts as a driver to the power triac, its output being located directly on the mains voltage of 220 V. A maximum permissible current is taken from the data sheet for the phototriac of 1.5 A proceeding from a duration of <= 10 µsec. To make sure that this value is not exceeded, a protective resistor R s is required. The value of R s is computed from the peak value of the mains voltage plus a safety margin of approx. 10%. pp = V / 2 = 340 V with I TMS = 1.5 A thus R s = pp / I TMS = 240 (rounded). If the power triac used requires a firing current of I GT = 50 ma and a firing voltage of V GT = 3 V, the voltage needed for triggering is TR = I GT R S + V GT + V TM = 50 ma V V = 16.5 V The momentary value of the voltage must have risen to this value before the power triac fires. If a gate resistor R g is used, the transverse current V GT / R g through R g is added to the I GT of the power triac. Thus TR ( I GT V GT R g ) R S + V GT + V TM However, as power triacs with a highly resistive gate also need a correspondingly lower gate current for switching purposes, the rise in current by the transverse current is not of much significance as regards loading the phototriac. However, the resistor R g has an effect on the switching behaviour of the power triac. Here, R g in conjunction with the gate capacity or parasitic capacities may cause a time lag to occur when switching on, to which special attention must be paid in the case of phase controlled power triacs. Figure 19. Signal curves for resistive loads I RL V MT

11 Switching inductive loads A more complex case of triac coupler operation involves the control of inductive loads such as motors, relays and magnets via power triac (Fig.20 ). The problem here is that the load current through the triac lags behind the mains voltage by a specific phase angle. After deactivation of the triac coupler control signals, the power triac remains conductive until the load current has dropped below its holding current value. The voltage (of opposite polarity) applied to the load up to this moment now also reaches the triac coupler via the power triac. At the same time, the voltage rise is limited only by the capacitance of the triac and stray capacitance. If the voltage rate of rise dv/dt now exceeds the critical static voltage slope when the triac is reverse biased (dv/dt) cr (amounting only to a typical value of 10 V/µs for the triac coupler, but amounting to 50 to 500 V/µs in the case of the power triac and depending on the type) firing again takes place immediately i. e. the load cannot be switched off. To keep voltages with a high rate of rise away from the sensitive triac coupler, an R C network is wired between the power triac and the triac coupler. The additional series resistance R S limits discharge current peaks of the capacitor. Its value can be determined in the same way as described in the chapter entitled Switching resistive loads. On the basis of dv/dt = V S / (R C) the time constant of the R C network is R C = V S / (dv / dt ) = 340 V / (10 V / µs) = 34 µs. The series circuit with the two resistances determines the trigger current of the power triac. Their values must therefore be dimensioned in accordance with the trigger data of the power triac. If the power triac is to be triggered at a voltage of 50 V, for instance, and if the trigger current is 50 ma, the total of the resistances is R S + R = 50 V / 50 ma = 1 kω and R = 1 kω 240 Ω = 750 Ω. Thus, C = 34 µs / 750 Ω = 50 nf (rounded). For applications subject to increased ambient temperature, it should be borne in mind that the critical voltage slope decreases along with increases in temperature (by approx. 2% per degree). R S R 1 C R g I L L V MT Figure 20. Inductive load switching Figure 21. Signal curves with inductive loads I L V MT

12 Examples of application Motor brake Electric motors in machines which tend to continue running after deactivation pose a risk to safety. Motor brakes are therefore specified in the relevant regulations. Fig. 22 shows a simple circuit in which a triac coupler controls a power thyristor which in turn short circuits the winding of a DC motor. (This diagram does not show the power supply for operating the motor.) At a supply voltage V S = 12 V and for operation from 25 C the bias resistance R V for the triac coupler s IR diode is R V = ( V QH V Fmax ) / Tmax R V = (10.5 V 1.5 V ) / 30 ma = 300 Ω V S = +12V R S 22KΩ 10KΩ M 1MΩ Timer 555 V T 30mA 240 R L R g 22µF GND C 47K Start Reset R V 330 Figure 22. Motor brake circuit Dark room exposure control Let us take a look at a circuit for a dark room exposure control as an example based on the applications of a triac coupler. The circuit shown in Fig. 23 only contains those components that are basically needed, and its details can still be refined. A type 555 timer IC wired as a monostable multivibrator ensures time delay. When idle, its output pin 3 has low potential. When the START pushbutton is pressed, the output switches to H level and a current flows through the IR diode of the coupler and the bias resistor to ground. If correctly dimensioned, the triac of the coupler (and thus too, the powertriac) is fired and the lamp is lit for the following time t = 1.1 R C provided it is not prematurely deactivated by pressing the RESET pushbutton. 22 µf has been selected as the value for the time defining capacitance. R = 22 kω is thus needed for a maximum lamp lighting time of 0.5 s, while R = 1 MΩ is needed for a maximum lighting time of 25 s. Figure 23. Dark room exposure control Series connection If the peak reverse voltage of a triac coupler turns out to be inadequate for an application due to VDE specifications (VDE 0160) prescribing higher values, for instance, two triac couplers can be series connected on the emitter and receiver sides (Fig. 24). A high resistance voltage divider ensures balancing and prevents one of the two phototriacs from firing too soon. 1MΩ 1MΩ R S Figure 24. Series connection