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Boost Converter with PFC & Thermal Model

Overview

This demonstration shows a 300 W switched-mode power supply. The AC input voltage may vary between 85 Vrms and 265 Vrms. The controlled output voltage is 390 V DC.

The simulation combines the electrical power circuit, the control with a standard IC and the thermal behavior of the semiconductors.

Power circuit

The power supply is based on a diode rectifier and a power factor correction circuit in boost topology. After having passed the input EMI filter in CLC configuration, the single-phase line voltage is rectified by a standard diode bridge. To improve the efficiency, the boost converter is realized with a CoolMOSTM power switch and a Silicon Carbide (SiC) Schottky diode.

Control

The controller IC (ICE1PCS01 from Infineon) is modeled with functional blocks in a separate subsystem. It operates in continuous conduction mode (CCM) with average current control.

The control is cascaded and consists of the inner current loop and the outer voltage loop. The inner current loop controls the sinusoidal profile for the average input current. It uses the dependency of the PWM duty cycle on the line input voltage to determine the corresponding input current. This means the average input current follows the input voltage as long as the device operates in CCM. Under light load condition, depending on the choke inductance, the system may enter into discontinuous conduction mode (DCM). In DCM, the average current waveform will be slightly distorted.

The outer voltage loop controls the output bus voltage. The output voltage contains a ripple with double the frequency of the input voltage (e.g. 120 Hz in North America). A compensation network connected to the IC suppresses this ripple which must not be amplified by the control loop.

The mask of the controller block contains the parameters of passive components that in reality are directly connected to the IC. The switching frequency can be specified directly, although in reality it is programmed via an external resistor.

The model of the control IC supports soft start for low output voltage. However, other protection features such as overvoltage and peak current protection are not implemented.

Thermal model

A special component model is used to simulate the thermal behavior of the MOSFET. Look under the mask of this component to see the thermal equivalent circuit of the device:

Within the MOSFET model, the heat sink (i.e. the blue frame) represents the thermal capacitance of the chip. It collects all switching loss energy and conduction loss power from the dies of the MOSFET and the reverse diode. It also propagates its temperature back to the chip in order to receive the correct loss readings. The losses are propagated via a thermal RC chain which models the case of the component. The Ambient Temperature component at the end connects the component with an outside heat sink modeling the dissipator that the component is mounted on.

In the SiC boost diode model, the thermal impedance of the device has been entered directly in the thermal description:

In the main circuit it can be seen that the dissipator is split into two heat sink frames in order to comprise both the rectifier diodes and the semiconductors of the PFC stage. A thermal resistance connects the dissipator with the temperature of the ambient air.

The thermal descriptions are stored in a private thermal library in the directory /demos/SMPS_CCM_plecs/Infineon/.

Simulation

The simulation shows a start-up of the power supply under constant load. The Scope with the electrical quantities shows the sinusoidal mains current and the ramp-up of the output voltage. A 120 Hz ripple in the output voltage can be observed.

The second Scope shows the junction temperature of the MOSFET (yellow) and the boost diode (purple). The temperature of the external heat sink is rising very slowly in the timeframe of the simulation.

Steady-state operation

It is not practical to determine the stationary temperature levels of the dissipator and the semiconductors by running a simulation until the thermal transients have settled. Due to the large thermal time constants such a simulation would take several hours.

Instead, you can perform a Steady-State Analysis on the circuit. From the Simulation menu choose Analysis tools.... This opens a dialog, in which the analysis has already been pre-configured. To start the analysis, click on the Start analysis button. You can display the progress of the analysis by clicking on the Show log button. When the analysis has finished, a simulation of five steady-state cycles is displayed.

Reference

For further information regarding the control IC, the circuit and the power semiconductors please refer to: http://www.infineon.com/pfc.