Modeling phototransistor optocouplers using PSPICE simulation software

Category: RF/IF
Manufacture: California Eastern Laboratories
Datasheet: Download this application note

Ap p l i c A t i o n no t e AN 3005
Modeling phototransistor optocouplers using PSPICE simulation software
Staff Applications Engineer, CEL Opto Semiconductors
Introduction PSPICE is a circuit simulation program that's used to provide a reasonably detailed analysis of circuits containing active components such as bipolar transistors, field effect transistors, diodes, and op-amps. PSPICE can also help characterize lumped components like resistors, capacitors and inductors. PSPICE programs are comfortable with measurement parameters like Voltage and Current. However, when it comes to modeling optoelectronic components the PSPICE program does not possess the capability to evaluate or simulate components with outputs measured in radiometric or photometric units like Watts (w) or Lumens (lm), or other variables like optical intensity, radiant power, irradiance with unit measurements in mW/sr, mW/m2, lumens. This application note provides a guideline to model phototransistor optocouplers with first order approximation using PSPICE models. An Optocoupler Model Typically, an optocoupler is an optically-coupled isolator that uses a GaAs LED as a light source and a bipolar NPN phototransistor as a receiver. In this note, the optocoupler will be modeled by a current-controlled current source. The forward current If through the LED emitter will act as current control and current source acts an output of the phototransistor. The output of the phototransistor will be a product of If and Current Transfer Ratio or CTR. The internal capacitance Cint of the optocoupler output will also be added across the output terminals of the current-controlled current source for transient analysis, the internal capacitance, Cint, is calculated based on the formula Tr = 2.2 x Cint x RL where tr and RL are the rise time and load resistor provided in the data sheet, respectively. Please note that the CTR, tr or the internal capacitance of the optocoupler will vary depending on the forward current If through the LED, power supply VCC, and load resistance RL on the detector side. As a result, any changes to the If, VCC or RL will lead to a change in CTR, tr and capacitance Cint. To the right are some graphs from the PS2501 data sheet for reference.
Forward Current I f (mA) Switching Time vs Load Resistance
I f = 5 mA VCC = 5.0 V
Switching Time t (s)
tr 1 100 td 500 1k 5k 10k 50k 100k
Load Resistance RL ( ) Switching Time vs Load Resistance
I f = 2 mA VCC = 10.0 V
Switching Time t (s)
Load Resistance RL ( )
PS2501-1 Electrical Characteristics ( TA = 25C )
PARAMETER DIODE Forward Voltage Reverse Current Terminal Capacitance TRANSISTOR COUPLED Collector to Emitter Dark Current Current Transfer Ratio (IC/IF) Note 1 Collector Saturation Voltage Isolation Resistance Isolation Capacitance Rise Time See Test Circuit Fall Time See Test Circuit NOTE 1. PS2501-1 CTR Rank: K L M D 300 to 600 (%) 200 to 400 (%) 80 to 240 (%) 100 to 300 (%) H W Q 80 to 160 (%) 130 to 260 (%) 100 to 200 (%) SYMBOL VF IR Ct ICEO CTR VCE(sat) RI-O CI-O tr tf CONDITION IF = 10 mA VR = 5 V V = 0 V, f = 1.0 MHz VCE = 80 V, IF = 0 mA IF = 5 mA, VCE = 5 V IF = 10 mA, IC = 2 mA VI-O = 1.0 kVDC V = 0 V, f = 1.0 MHz VCC = 10 V, IC = 2 mA, RL = 100 1011 0.5 3 5 80 300 50 100 600 0.3 MIN TYP 1.17 MAX 1.4 5 UNIT V A pF nA % V pF s
From this data, let's set the CTR = 300% at If = 1mA, and Cint = 3 s /2.2 x 100W = 14nF, VCC = 10V, and model the PS2501-1 using the emitter follower configuration with RL = 100W.
Modeling the PS2501-1 optocoupler for DC and transient analysis The following is an example of a modeling of the PS2501 optocoupler with load resistance RL, in emitter follower configuration. From the Electrical Characteristics in the table above, the CTR can vary from 80% to 600% at If = 5 mA and VCC = 5.0 V.
DC Analysis The PSPICE model and Netlist for If = 1 mA, IC = 3mA and RL = 100W are shown in Figure 2 below and on the next page. Rise time tr is 3s, based on VCC =10V, IC = 3mA, and RL =100W. The test circuit is shown in Figure 1. + F1 10 V VCC
0R0 0 IDC I1 1 mA 1 RL 100 FPOLY
PW = 100s Duty Cycle = 1/10
Figure 2. DC Analysis -- Schematics Netlist
Figure 1. Test circuit for determining Switching Time
$N_0002 $N_0001 1000 0 $N_0002 DC 1mA $N_0003 $N_0004 POLY(1) VF_F1 3 $N_0001 0 DC 0V 0 $N_0004 100 $N_0003 0 10V

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