How to Use Optocoupler Normalized Curves - Vishay

1 Vishay SEMICONDUCTORSO ptocouplers and Solid-State RelaysApplication Note 45 How to Use Optocoupler Normalized CurvesAPPLICATION NOTE Rev. , 18-Oct-111 Document Number: 83706 For technical questions, contact: DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENTARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT Optocoupler provides insulation safety, electrical noiseisolation, and signal transfer between its input and insulation and noise rejection characteristics of theoptocoupler are provided by the mechanical packagedesign and insulating phototransistor Optocoupler provides signal transferbetween an isolated input and output via an infrared LEDand a silicon NPN current is forced through the LED diode, infrared lightis generated that irradiates the photosensitive base collectorjunction of the phototransistor.

2 The base collector junctionconverts the optical energy into a photocurrent which isamplified by the current gain (HFE) of the gain of the Optocoupler is expressed as a CurrentTransfer Ratio (CTR), which is the ratio of the phototransistorcollector current to the LED forward current. The currentgain (HFE) of the transistor is dependent upon the voltagebetween its collector and emitter. Two separate CTRs areoften needed to complete the interface design. The firstCTR, the non-saturated or linear operation of the transistor,is the most common specification of a phototransistoroptocoupler and has a VCE of 10 V. The second is thesaturated or switching CTR of the coupler with a VCE of Figures 1 and 2 illustrate the Normalized CTRCE for thelinear and switching operation of the phototransistor.

3 Figure1 shows the Normalized non-saturated CTRCE operation ofthe coupler as a function of LED current and ambienttemperature when the transistor is operated in the linearmode. Normalized CTRCE(SAT) is illustrated in figure 2. Thesaturated gain is lower with LED drive greater than 10 mA. Fig. 1 - Normalized CTR vs. IF and TA Fig. 2 - Normalized Saturated CTRThe following design example illustrates how normalizedcurves can be used to calculate the appropriate 1 Using an IL1 Optocoupler in a common emitter amplifier(figure 3) determine the worst case load resistor under thefollowing operation conditions: Fig. 3 - IL1 to 74HC04 InterfaceTA = 70 C, IF = 2 mA, VOL = V, logic load = 74 HC04IL1 Characteristics: CTRCE(NON SAT) = 20 % min.

4 At TA = 25 C, IF = 10 mA,VCE = 10 CT R17485TA = 25 CTA = 50 CTA = 70 CTA = 100 CNormalized to:IF = 10 mA, VCE = 10 VTA = 25 CIF - LED Current (mA) = 25 CIF - LED Current (mA) Normalized CTRVCE(SAT) = V17486TA = 50 CTA = 70 CTA = 100 CNormalized to:IF = 10 mA, VCE = 10 VTA = 25 CHC04 LROLVFICCV17487 How to Use Optocoupler Normalized CurvesAPPLICATION NOTEA pplication Note semiconductors Rev. , 18-Oct-112 Document Number: 83706 For technical questions, contact: DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENTARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT CTRCE(SAT) using the normalization factor(NFCE(SAT)) found in figure 2. Fig. 4 - Normalized Saturated CTRCTRCE(SAT) = CTRCE(NON SAT) NFCE(SAT) CTRCE(SAT) = 20 % x (1)CTRCE(SAT) = %Step the minimum load resistor using the followingequation:RL(MIN) = k , select 51 k 5 %The switching speed of the Optocoupler can be greatlyimproved through the use of a resistor between the baseand emitter of the output transistor.

5 This is shown in figure5. This resistor assists in discharging the charge stored inthe base to emitter and collector to base junctioncapacitances. When such a speed-up technique is used theselection of the collector load resistor and the base emitterresistor requires the determination of the photocurrent andthe hFE of the photocurrent generated by the LED is described by theCTRCB of the coupler. This relationship is shown inequations 3 and 4. Equation 5 shows that CTRCE is theproduct of the CTRCB and the hFE. The hFE of the transistoris easily determined by evaluating equation 4, once theCTRCE(SAT) and CTRCB are known. The Normalized CTRCB isshown in figure 6. Equations 5, 6, and 7 describe thesolution for determining the RBE that will permit reliableoperation.

6 Fig. 5 - Optocoupler /Logic Interface with RBE Resistor Fig. 6 - Normalized CTRCB vs. LED CTRCB17488 Normalized to:IF = 10 mA, VCB = VTA = 25 CSCTRCB - 25 SCTRCB - 50 SCTRCB - 70 SCTRCB - 100IF - LED Current (mA)( ) 2 mA100 %RL(min.) =- 50 A5 V- VCTRCE(SAT) IF100 %RL(min.) =- ILVCC-VOL(2) CTRCBIF - LED Current (mA) Normalized to:IF = 10 mA, VCB = VTA = 25 CSCTRCB - 25 SCTRCB - 50 SCTRCB - 70 SCTRCB - 100 ICBIFCTCB =100 %(3)CTRCB100 %ICB = IF(4)CTRCE(SAT) = CTRCB HFE(SAT)(5)CTRCE(SAT)hFE(SAT) =CTRCB(6)VBERBE =ICB - IBE(7)VBE HFE(SAT) RLRBE =ICB HFE(SAT) RL - [VCC- VCE(SAT)](8)VBECTRCE NFCE(SAT)CTRCB NFCBIF CTRCE NFCE(SAT) RL100 %RLRBE =- [VCC- VCE(SAT)](9)How to Use Optocoupler Normalized CurvesAPPLICATION NOTEA pplication Note semiconductors Rev. , 18-Oct-113 Document Number: 83706 For technical questions, contact: DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE.

7 THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENTARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT 2 Using an IL2 Optocoupler in the circuit shown in figure 6,determine the value of the collector load and base emitterresistor, given the following operational conditions:TA = 70 C, IF = 5 mA, VOL = V,Logic load = 74HC04IL2 Characteristics:CTRCE = 100 % at Tamb = 25 C, VCE = 10 V, IF = 10 mACTRCB = % at Tamb = 25 C, VCB = V, IF = 10 mASOLUTIONStep CTRCE(SAT), and figure 2 the CTRCE(SAT) = 55 %, [NFCE(SAT) = ]From figure 6 the CTRCB = %, [NFCB = ]Step equation 2 RL = k Select RL = k Step RBE, using equation = 199 k , select 220 k Using a k collector and a 220 k base emitter resistorgreatly minimizes the turn-off propagation delay time andpulse distortion.

8 The following table illustrates the effect theRBE has on the circuit only does this circuit offer less pulse distortion, but it alsoimproves high-temperature switching and common modetransient rejection while lowering static DC V(100 %)( )( %)( )(5 mA)(100 %)( )( k )100 %( k )RBE =- [5 V- V](10)TABLE 1IF = 5 mA, VCC = 5 VRL = k RBE = RL = k RBE = 220 k tdelay1 s2 strise4 s5 ststorage17 s10 stfall5 s12 s7 stplh22 s12 sPulse distortion 50 s pulse37 %10 %