Simulating Switching Power Supplies with SPICE and PSPICE

Due to the nonlinearity of DC/DC PWM power converters and the possible multiple operating modes (CCM mode or DOM mode), the analysis is very difficult.

Simulation plays an important role in designing or analyzing switching power supplies. Digital simulation can be used to verify whether the design meets performance requirements. Digital simulation can reduce the experimental work of the circuit. Compared with circuit experiments, simulation takes much less time and can be carried out more comprehensively and completely to help improve the quality of the design. In addition, simulation can also provide certain information. Therefore, simulation can accelerate the analysis and design evaluation of switching power supplies. For large signal analysis, it is generally difficult to solve it with analytical methods, and digital simulation is more necessary. Therefore, simulation is an important step between the theoretical design of switching power supplies and hardware circuit board experiments. Sometimes the application of simulation methods can understand the problems and solutions in the theoretical design more thoroughly than hardware experiments. After the theoretical design is completed, a simple circuit simulation model can be used to verify it; there are many non-ideal characteristics in the actual circuit, such as noise, parasitic capacitance, leakage inductance and line inductance, switching time, diode recovery process, etc. Non-ideal components can be considered in the SPI (E model, such as each simulation, only one or two of them are considered to study their impact on the performance of the switching power supply, thereby avoiding many confusing or complex phenomena caused by nonlinearity.

Some theoretical problems are too complex or underdeveloped (such as resonant converters, the effect of leakage inductance on cross regulation, circuit losses, etc.). When applying these theories to design, you can first use SPICE simulation to test (Trial & Error) analysis. SPICE simulation can also be used to analyze some potential problems, such as volt-ampere imbalance causing transformer saturation and uncertain RC clamping voltage levels. In actual circuits, these problems may damage power transistors or rectifiers; therefore, it is necessary to do simulation research and analysis in advance. Since PSPICE is derived from SPICE, this chapter mainly introduces its application in conjunction with SPICE. In principle, these discussions also apply to PSPICE.

After verifying the design with simulation, the SPICE program can give the small signal open-loop frequency characteristics (Bode curve) to verify the transient response and startup characteristics of the switching power supply. Once the simulation test is passed, the hardware circuit experiment is the last step to verify the design. The hardware experiment should only make some small modifications to the design. Only when such a result is obtained can it be considered a satisfactory design process.

Many people abroad have done a lot of modeling development and research work on using SPICE to analyze switching power supplies, such as Bello, MON-teith, Griffin, Y, S. Lee, etc.

In the large signal mathematical model of the switching power supply, there are two (dynamic) variables multiplied by terms, such as dU or ax (J is the conduction ratio or duty cycle, U is the input voltage, and X is the state variable). The SPICE software package can handle this type of transient nonlinear quadratic terms. Therefore, not only can the SPI (E program be used to perform DC analysis and AC small signal analysis on the switching power supply, but the network can also analyze the large signal transient process of the open-loop or closed-loop system, such as the startup process or the load current small signal analysis, and can also analyze the large signal transient process of the open-loop or closed-loop system, such as the startup process or the load current large changes. In recent years, people have used SPICE software to analyze switching power supplies with feedforward and current control, and some people have used it to simulate resonant converters.

The main component of the switching converter main circuit is the switching transistor controlled by the on-time pulse train d. When using the SPICE simulation program, a dedicated simulation model of the power semiconductor switching device and the control circuit must be established first, which includes three contents:

(1) The model of the power semiconductor switch tube is represented by an ideal transformer controlled by a conduction ratio d. The transformer ratio is related to J. Therefore, in addition to the four terminals of the ideal transformer primary and secondary sides, the simulation model also has input terminals for realizing the conduction ratio ' control. The control circuit should also be represented by certain symbols and the input and output terminals should be marked.

(2) Equivalent subcircuit: The equivalent subcircuit of the above two models is composed of components such as current source, voltage source, resistor, capacitor, etc.

(3) Subcircuit simulation program: The subcircuit topology and component parameters are input into the computer using a description language in the specified format.

By combining the above unused simulation models with the SPICE general circuit program, and using its subcircuit simulation program as a subprogram of SPICE, various switching converters or switching regulated power supplies can be simulated and analyzed.

Using SPICE simulation program, its accuracy depends on the step size and integration order.