llc变压器的设计2.pdf

CHAPTER 5The Parallel Resonant Converterhe objective of this chapter is to describe the operation of the parallel resonant converter in detail. The concepts developed in chapter 3 are used to derive closed-form solutions for the output characteristics and steady-state control characteristics, to determine operating mode boundaries, and to find peak component stresses. General results are presented using frequency control for both the continuous and the discontinuous conduction modes. This chapter also explains the origin of the discontinuous conduction mode, which is in many ways the dual of the series resonant discontinuous conduction mode. The characteristics of the parallel resonant converter are quite different from those of the series resonant converter, and from those of conventional PWM converters. The parallel topology can both step up and step down the dc voltage. Although the output characteristics are again elliptical, near resonance they exhibit a current-source characteristic. The discontinuous conduction mode occurs under heavy loading (or short-circuit conditions, in the limit). The transistor current stresses and conduction loss depend on the output voltage, and are nearly independent of load current. Although these features may make the parallel resonant converter ill-suited to some conventional power supply applications, they can be used to advantage in others. An example is given in section 5.4, in which the parallel resonant converter is used to construct a 24V:10kV high voltage power supply with current source characteristics. Design considerations are outlined, and the near-ideal operation of an experimental circuit is described. A second application example is also explored, in which the parallel resonant converter is used as an off-line low harmonic rectifier. The converter input characteristics are found, and the advantages and disadvantages of the PRC in this application are discussed.TPrinciples of Resonant Power Conversion25.1.Ideal Steady-State Characteristics in the Continuous Conduction ModeA full bridge isolated version of the parallel resonant converter is given in Fig. 5.1. For this discussion, a 1:1 turns ratio is assumed. The converter differs from the series resonant converter because it is the tank capacitor voltage, rather than the tank inductor current, which is rectified and filtered to produce the dc load voltage. A two-pole L-C low pass filter (LF and CF)performs this filter function. Hence, we haveV = (5-1)by use of the flux-linkage balance principle (Chapter 3) on inductor LF. The magnitude of the quasi-sinusoidal voltage vC(t) is controllable by variation of the switching frequency — vCbecomes large in amplitude near resonance. Hence, the dc output voltage V is controllable by variation of the normalized switching frequency F = fS / f0.LC VgQ1Q2D1D2Q3D3Q4D41 : nD5D6D8D7 V+–LFCFIIF+ vC –vT+– iLRFig. 5.1.Full bridge realization of the parallel resonant converter.The LF and CF filter elements are “large”, i.e., their switching ripple components are small compared to their respective dc components in a well-designed converter. Hence, IF and V are essentially dc. Also, by charge balance on CF, we have in steady-stateIF = I(5-2)Typical waveforms are drawn in Fig. 5.2 for above-resonance operation with zero voltage switching. The input bridge produces a square wave output voltage vT(t), which is applied acrossthe LC tank circuit. In response, the tank current and tank capacitor voltage ring with quasi- sinusoidal voltages. The bridge rectifier now switches when the tank voltage passes through zero. The peak values of the tank waveforms vC(t) and iL(t) do not, in general, occur at the transistor orChapter 5, The Parallel Resonant Converter3diode switching times. In the continuous conduction mode, four subintervals occur during each switching period. The circuit topologies during these subintervals depend on the conducting states of the input and output bridges, which in turn depend on the input bridge drive signal and the polarity of the tank capacitor voltage. The tank circuits during each of the four subintervals are drawn in Figs. 5.3 – 5.6. In each case, the tank circuit topology is identical to that of Fig. 3.15, repeated in Fig. 5.7. The applied tank voltage VT is ±Vg, dependingon the conducting states of the input bridge switches, and the applied tank current IT is ±IF (= ±I),depending on the polarity of the tank capacitor voltage. These voltages are summarized in Table 5.1.Table 5.1. Applied tank voltages and currents for the parallel resonant converter operating in continuous conduction modeIntervalVTMTITJT1+Vg+1–I–J2+Vg+1+I+J3–Vg–1+I+J4–Vg–1–I–Jω0tvT +Vg-VgiLIL1IL0αβ vC-IL0-VC0VC0|vC|-IL1V = Fig. 5.2.Typical waveforms for the parallel resonant converter operating in continuous conduction mode.Principles of Resonant Power Conversion4State plane portraitThe normalized state plane trajectory for the circuit of Figs. 5.7 and 3。