ТППГ

Transformer model

Fig, 20,45 Transformer-isolated dc-dc cciiveitercontaining з resonant switch network, Hroblem 20.3.

20.5 И is desired lo design a half-wave zero-current-swilching quasi-resonant forward converter to operate with the followingspecificadons: 1 = 320 V. V= 42V. 5 W < f < 100 W. Design the converter lo operate with a maximum switching frequency of I MHz and a switch conversion ratio oft = 0.45. Attempt to minimize the peak transistor current, while maintaining zero current switching at all operating point.s. You may neglect the transformer magnelizinj; current, and ignore the transformer reset scheme.

(a) Specify your choices fur the lurns ratio , and the lank elements and C referred to the transformer secondary side.

(b) For your design of part (a), what is the minimum switching frequency?

(c) What is the worst-case peak transistor current ?

20.6 Analysis of the ZVS qiiasi-resonanl swiich of Fig. 20.24.

(a) For each suhinlerval, sketch the resonant switch cell circuit, and derive expressions for the lank inductor currenl and capacitor vollage waveforms.

(b) For subinterval 2, in which Qj/fJ( are off and conducts, write the кюр equation which relates the tank capacitor voltage, tank inductor voltage, and any other network voltages as appropriate. Hence, for subinterval 2 relate the integral of the tank tapachor vollage to the change in lank inductor current.

(c) Determine the switch-network terminal-waveform average values, and hence derive an expression for the switch conversion rado Verify that your result coincides with Eq, (20.61).

20.7 Analysis of the full-bridge zero-vohage transition converter of Section 20.4.1. The converter of Fig. 20.36 operates with the waveforms illnstrated in Fig. 20.38. According to Eq. (20.68), the conversion ratio of this converter Is given approximately by Л/(ф) = пф.

Derive an exact expression for iVf, based on the waveforms given in Fig. 20.3S, Your result should be a function of the length of subinterval 4, the load current, the switching frequency, and the values of the inductance and capacitances. Note: there Is a reasonably simple answer lo this ijueslion.

Appendices

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