In dcdc converters used in battery-powered portable equipment, it is sometimes required that the…

In dcdc converters used in battery-powered portable equipment, it is sometimes required that the converter continue to regulate its load voltage with high efficiency while the load is in a low-power sleep mode. The power required by the transistor gate drive circuitry, as well as much of the switching loss, is dependent on the switching frequency but not on the load current. So to obtain high efficiency at very low load powers, a variable-frequency control scheme is used, in which the switching frequency is reduced in proportion to the load current. Consider the boost converter system of

In dcdc converters used in battery-powered portable equipment, it is sometimes required that the converter continue to regulate its load voltage with high efficiency while the load is in a low-power sleep mode. The power required by the transistor gate drive circuitry, as well as much of the switching loss, is dependent on the switching frequency but not on the load current. So to obtain high efficiency at very low load powers, a variable-frequency control scheme is used, in which the switching frequency is reduced in proportion to the load current. Consider the boost converter system of Fig. 5.29a. The battery pack consists of two nickelcadmium cells, which produce a voltage of Vg = 2.4V } 0.4 V. The converter boosts this voltage to a regulated 5V. As illustrated in Fig. 5.29b, the converter operates in the discontinuous conduction mode, with constant transistor on-time ton. The transistor offtime to f f is varied by the controller to regulate the output voltage. (a) Write the equations for the CCM-DCM boundary and conversion ratio M = V/Vg, in terms of ton, to f f , L, and the effective load resistance R. For parts (b) and (c), the load current can vary between 100 A and 1 A. The transistor on-time is fixed: ton = 10 s. (b) Select values for L and C such that: The output voltage peak ripple is no greater than 50mV Fig. 5.29 Boost converter employed in portable battery-powered equipment with sleep mode, Problem 5.18: (a) converter circuit, (b) inductor current Waveform The converter always operates in DCM, and The peak inductor current is as small as possible. (c) For your design of part (b), what are the maximum and minimum values of the switching frequency?

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