Switched-Mode Boost Converter

In power generation plants, e.g. in wind turbines, electronic converters and inverters are used. Faults in these components can degrade the performance of power distribution systems and lead to failures. In [19, 22], it is reported that semiconductors and electrolytic capacitors in power power electronic converter systems have a higher failure rate than other components. 

The conversion from one voltage level to another is achieved by storing the input energy temporarily in the inductor when the controlled switch is ON and the diode is OFF, and then releasing that energy to the output at a different voltage value when the controlled switch is turned OFF and the diode is ON. In some applications, the diode is replaced by a pFET in order to avoid the diode s forward voltage drop. 

Unknowns in (8.1)-(8.3) can be eliminated by means of the following constitutive element equations 

Rsw and Rd denote the ON-resistances of the power MOSFET switch and the diode respectively. 

The structural parameter sensitivity of the ARRs is reflected by the FSM in Tables.1. All possible faults are detectable but none can be isolated with the two detectors. 

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The previous chapters address various aspects of quantitative bond graph-based FDI and system mode identification for systems represented by a hybrid model. This chapter illustrates applications of the presented methods by means of a number of small case studies. The examples chosen are widely used switched power electronic systems. Various kinds of electronic power converters, e.g. buck- or boost converters, or DC to AC converters are used in a variety of applications such as DC power supplies for electronic equipment, battery chargers, motor drives, or high voltage direct current transmission line systems [1]. 

This chapter considers a simple boost converter often used in power electronic systems. Figure 8.1 depicts its circuit schematic. In this circuit, the MOSFET transistor and the diode may be considered non-ideal switches. The transistor is a controlled power switch. Boost converters are designed that they operate either in so-called continuous conduction mode or in discontinuous conduction mode. In continuous conduction mode the inductor current never falls to zero. Accordingly, the converter assumes two states per switching cycle. When the transistor is on, the diode is off and vice versa. The diode commutates autonomously and oppositely to the transistor. Hence, there are two system modes in a healthy boost converter. 

In a healthy system operating in continuous conduction mode, the switch and the diode open and close oppositely (mi + m2 = 1). Let / sw = Rd = Ron- Then the ARRs simplify and the dynamic behaviour of a correctly operating boost converter is given by the state equations... 

The boost converter circuit has got two switches. In a healthy system, they commutate oppositely so that there are only two system modes. As two sensors have been attached to the circuit, each healthy mode is identified by two ARRs. 

The previous section considers a simple DC to DC boost converter with two switches controlled by two complementary signals. The healthy boost converter thus may be in one of two feasible modes. Reference [18] studies switch faults in a simple single phase half-bridge inverter. In [17], bond graph-based FDI is applied to a single phase H-bridge inverter. Both works represent switches by means of controlled junctions, i.e. use hybrid bond graphs. 

ZVS QRCs (Kit Sum, 1988 Liu and Lee, 1986) are similar to ZCS QRCs. The auxiliary LC elements are used to shape the switching device s voltage waveform at off time in order to create a zero-voltage condition for the device to turn on. Figure 10.87(d) shows an example of ZVS QR boost converter implemented using ZV resonant switch. The circuit can operate in the half-wave mode (Fig. 10.87(e)) or in the fuU-wave mode... 

As one can notice, the boost-mode converter has the same parts as the forward-mode converter, but they have been rearranged. This new arrangement causes the converter to operate in a completely different fashion than the forward-mode converter. This time, when the power switch is turned on, a current loop is created that only includes the inductor, the power switch, and the input voltage source. The diode is reverse-biased during this period. The inductor s current waveform (Figure 3-4) is also a positive linear ramp and is described by... 

As seen in Section 4.1, the major types of losses are the conduction and switching losses. Conduction losses are addressed by selecting a better power switch or rectifier with a lower conduction voltage. The synchronous rectifier can be used to reduce the conduction loss of a rectifier, but it can only be used for forward-mode topologies, and excludes the discontinuous boost-mode converters. The synchronous rectifier will improve the efficiency of a power supply about one to six percent depending upon the average operating duty cycle of the supply. For further improvements, other techniques must be pursued. 

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