The electrothermal macromodel of voltage mode PWM controllers for SPICE
Introduction
Today, switching power electronic converters (SPEC) are a very important class of power electronic circuits. While considering typical operating conditions of semiconductor devices—especially power switching devices and other components (subcircuits) used in SPEC, selfheating should be taken into account. It influences considerably the important parameter values and the reliability of SPEC. As a result of selfheating, the device junction (inner) temperature Tj differs, sometimes considerably, from the ambient temperature Ta [1], [2].
In the computer-aided design and analysis of SPEC the proper and accurate models of the whole circuits or their components including, among others, selfheating are needed [3]. Such models, called here electrothermal ones, are formulated first of all for power devices [4], [5], [6], [7], [8], [9], [10].
However, there are no trends to formulate the electrothermal models of low-power or signal devices and circuits operating in SPEC. The authors proved that the temperature affects strongly the performance of a class of the integrated PWM controllers, especially its parameter values and characteristics—called here nonisothermal ones.
In literature one can find information on the PWM controller models for SPICE worked out by other authors [11], [12], [13]. Unfortunately, in these models selfheating is not included and moreover, the model from [12] describes the idealized PWM controller deprived of any nonlinearities and protection circuits.
In the paper the electrothermal macromodel of PWM SG3525A controller dedicated for SPICE, appropriated to the electrothermal transient analysis, is proposed. This macromodel is an improved version of the other ones, proposed also by the authors [14], [15], [16], [17]. The results of measurements and calculations illustrating the correctness and usefulness of the proposed macromodel working both in the catalogue test circuits and in the boost converter are presented as well.
Section snippets
The structure and the library isothermal macromodel
The considered here PWM controllers are appropriated for controlling dc–dc converters. The block diagram of SG3525A controller is shown in Fig. 1. The specified blocks in Fig. 1, such as: voltage reference source (VRS), undervoltage lock out (UVLO), Flip–Flop (FF), oscillator (OSC), PWM comparator (PWM), error amplifier (EA), output stage (OS) and shutdown circuit (SC) are described in the catalogue data [18], [19] in detail.
The operating frequency range from 100 Hz to 400 kHz is determined by
The electrothermal macromodel form
According to the principles of electrothermal models formulation, e.g. see [1], the worked out by the authors electrothermal macromodel of the controller SG3525A, presented in Fig. 2, consists of three essential parts, representing the electrical model along with the parameters dependent on temperature, the thermal model and the model of the power dissipated in the considered device.
The structure of the electrical model has been created on the basis of the analysis of the PWM controller
Calculations and measurements
The library and electrothermal macromodels of the controller SG3525A have been implemented to SPICE with the use of ABM option. To confirm the correctness of these macromodels some transient analysis and measurements of the both isothermal and electrothermal properties of the controller SG3525A, operating in the selected circuits have been carried out.
At first, the characteristics of the investigated controller (DUT) operating in the catalogue test circuit [18] (Fig. 5) are considered.
The
Conclusions
Due to some disadvantages of the available library macromodel of SG3525A controller, the new, more comfortable and accurate macromodel including selfheating is proposed in the paper. Worked out by the authors the electrothermal macromodel (ETM) of SG3525A includes the phenomena omitted in the library macromodel: selfheating, the influence of the supply voltage VCC and the temperature on time tw, as well as the dependence of the supply current and voltage shapes on the temperature and supply
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