Boost-mode converters

The second family of converters are the boost-mode converters. The most elementary boost-mode (or boost-derived) converter can be seen in Figure 3-3. It is called a boost converter. 

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... 

Figure 3-28 Output stages for forward and boost-mode converters (a) balf-wave forwardmode (b) center-tapped forward-mode (c) full-wave bridge forward-mode (d) boost-mode.

The output filter converts the rectified rectangular ac waveform into the dc output. Forward-mode converters have a two-pole L-C filter which produces the dc average of the rectified rectangular waveform. Boost-mode converters have a single-pole, capacitive input filter which produces a dc voltage which is the peak voltage of the rectified waveform. Both are reactive impedance filters and exliibit very little loss. 

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. 

Continuous-mode converters, both forward and boost, suffer from one common problem. The output rectifiers have forward current flowing through them just... 

The operation of a discontinuous-mode, flyback converter is quite different from that of a forward-mode converter, and likewise their control-to-output characteristics are very different. The topologies that fall into this category of control-to-output characteristics are the boost, buck/boost, and the flyback. The forward and flyback-mode converters operating under current-mode control also fall into this category. Only their dc value is determined differently. Their representative circuit diagram is given in Figure B-12. 

We can also use the fact that the output voltage of a discontinuous mode converter at a given duty cycle depends on its inductance. So we can tune the slave buck-boost to have the required output level (at its expected maximum load current) by a careful choice of inductance. Within a valid range, this technique provides completely adjustable auxiliary output voltages, something we cannot normally expect from composite topologies based only on continuous conduction modes. 

Figure 3-4 Waveforms for a discontinuous-mode boost converter.

Figure 3-5 Waveforms for a continuous-mode boost converter.

Another form current-mode control is called hysteretic current-mode control. Here both the peak and the valley currents are controlled. This is obviously better for continuous-mode forward for boost converters. It is somewhat complicated to set-up, but it does offer very fast response times. It is not a very common method of control and its frequency varies. 

So how did we manage to achieve automatic line regulation As the input voltage increases, the feedback loop of the regulated buck converter commands its duty cycle to decrease to maintain output regulation. It just so happens that this decrease in duty cycle is exactly what was required by the discontinuous mode buck-boost to regulate its own output almost perfectly. 

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... 

Equation (8.13) holds under the assumption that the boost converter operates in continuous conduction mode and reduces for Rl =0 into the well known formula for the voltage conversion... 

The expression for the voltage conversion in (8.14) is to be replaced by a more complicated one in case the boost converter is operated in discontinuous conduction mode. 

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. 

ARR-based system mode identification has been illustrated for the boost converter with only two modes and for the three-phase diode bridge inverter with six active... 

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... 

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