PSpice Modeling and Simulation of Darlington Transistor

0 Introduction

SPICE is a powerful general-purpose analog and mixed-mode circuit simulator, which is mainly used to verify circuit design to predict circuit function. This is especially important for integrated circuits. For this reason, SPICE was born in the Electronic Research Laboratory of the University of California, Berkeley, as its name means: Simulation Programl for Integrated Circuits Emphasis.

PSpice is the PC version of SPICE (from OrCAD Corp. of Cadence Design Systems, Inc.). Although originally used for IC design, more and more circuit and system designers have realized the advantages of analog circuit simulation due to the promotion of low-cost computing and stable design. As a result, the device model requirements for active devices have been reconsidered, so many semiconductor companies now provide appropriate SPICE models as part of their technical support functions.

PSpice has embedded algorithms or models that describe the functionality of many devices. A set of model parameters are used to describe the functionality of the embedded model. We can define a device based on an embedded model by applying the PSpice model syntax to set all or any of the model parameters.

1 PSpice modeling of Darlington transistor

The PSpice Model Editor is a model extractor that is used to generate model definitions for PSpice A/D during simulation. The most common way to describe model characteristics is to enter data sheet information for each device characteristic. Once we are satisfied with each characteristic, we can use the Model Editor to estimate (or extract) the corresponding model parameters and generate characteristic curve charts. This is called the prototyping process. Usually, we always repeat this process until we get satisfactory results and save them. The Model Editor generates a model library, which contains appropriate model and subcircuit definitions.

A Darlington transistor (also often called a Darlington pair) is a semiconductor device that contains two bipolar transistors, so the current amplifier is further amplified by the second amplification after the first amplification. The total current gain is equivalent to the product of the two individual gains together:

Darlington pair current gain, hFE = hFEl × hFE2

(hFF1 and hFE2 are the gains of the individual transistors, respectively, where hFE = I(CdllectorCurrent)/IBaseCurrent)

This way the Darlington transistor has a very high gain, say 10,000, so only a very small base current is needed to turn on the Darlington transistor.

A Darlington transistor behaves like a single transistor with a very high current gain. It has three pins (B, C and E) which are equivalent to the three pins of a standard single transistor. The maximum collector current Ic(max) is equivalent to the Ic(max) of T2, the second transistor in Figure 1 below. The resistors in the figure are used to reduce the switching delay when turning off a Darlington transistor.

To model a Darlington transistor, we take the data sheet of the chip TIPl20 as a reference. As we mentioned above, the information we input from this device data sheet can be converted into a parameter model group set using PSpice model syntax, or a subcircuit netlist generated using PSpice SUBCKT syntax, through the model editor, and then save these definitions to the model library so that PSpice can search for them when the simulation model is needed.

After the model definition is completed, we can create the components for a Darlington pair, as shown in Figure 2:

2 Typical characteristics of PSpice simulation of Darlington transistor TIPl20

To analyze the typical characteristics of TIP120, we use the equivalent circuit of Figure 1, where R1 = 8KΩ and R2 = 0.12KΩ according to the data sheet of TIP120.

Since Darlington transistors are often used to amplify weak signals so that the weak signals can be clearly detected by another circuit or computer/microprocessor, they are used to evaluate the current gain (hFE) characteristics. The PSpice simulation results are shown in Figure 3.

At the same time, the collector current electrical performance vs. input current. Under different collector-emitter saturation voltage states, the simulation results of the relationship between collector-emitter saturation voltage and collector current are shown in Figures 4 and 5. The simulation results show that they match the data sheet of TIP120 very well.

3 Conclusion

The PSpice model extracted from TIPl20 and the above. PSpice simulation results show that we can conclude that the PSpice program is really a very useful research tool for electrical engineering professionals. It allows us to simulate individual components and electronic circuits and perform a large number of different circuit verification and circuit performance prediction analyses. It is so flexible and generally so stable that many engineers use it as a "software oscilloscope".

However, whether the results of SPICE simulation are satisfactory depends largely on the component models and device parameters in the simulation. The technology in the electronics industry is developing rapidly, and the device characteristics vary so much that using only the default parameters cannot effectively reflect the device characteristics. If the wrong device parameters or models are applied in a SPICE simulation, then all the power of the computer is wasted, as the old saying goes: "Garbage in, garbage out."