Question

A photovoltaic module is used to supply constant 12 V that needed by a Tablet. Depending on insolation from the Sun, the photovoltaic module’s output voltage is changing between 18 V to 36 V (dc). Thus, a buck converter is used between the netbook and the photovoltaic module.

1) What is the duty ratio range of the dc/dc converter?

2) If the inductance of the buck converter inductor is 40 uH, what is the minimum switching frequency to limit the current ripple in the inductor to less than 1 A? 3) At constant 12 V, what is the minimum power consumption of the Tablet to keep the inductor current always continuous?

4) For the operation condition stated in 3), draw (not simulate and plot) the waveforms of i) switching status, ii)voltage cross the diode , and iii) inductor current; mark the values for each waveform

Answer 1

1.

Design parameters of the buck converter is as below

Out put voltage of buck converter

Input voltage is in the range below

Minimum duty cycle is when input voltage is maximum

Maximum duty cycle is when input voltage is minimum

Duty cycle varies from 0.33 to 0.667

2.

equation for the inductor ripple current of a buck converter is as below

on re arranging

Maximum switching frequency occur at maximum input voltage

3.if the inductor current is just continious average inductor current

In a buck converter average inductor current and average load current are same

Power consumption =

4.For the Boundary of continuous condition in question 3 wave forms are shown as below