Buck converter design report 2 PDF

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Power Electronic converter design in electrical engineering....

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Buck converter design report 1. Introduction It was required to design a converter with a range of input voltage between 10 and 20 volts, and output voltage 10volts. Therefore, the Buck converter topology was chosen for this purpose as it suits the application since it reduces the voltage level (from input to output). It is easy to design and if implementation has to be made, it is cheap and not cumbersome to build. 2. Design specification Input voltage (Vin) Output voltage (Vo) Output current (Io) Switching frequency (fsw) Current ripple ( ∆ il ) Voltage ripple ( ∆ Vo )

10 – 20V 10 V 1A 50 KHz < 0.5 A < 0.1 V

3. Selection of components The selection of the components was based on the design specifications, and some calculations had to perform to ensure the right choice of the components. The calculations were performed as shown: 3.1 Choice based on design specifications Mohan [1] suggest the following criteria for the selection of components characteristics: D=

Vo Vin

Criterion Transistor control voltage

Requirements Vin

Values / Choice 12 volts

Transistor control current Iavg Transistor

Io D* Io

1A 0.833 A

Diode I inductor R Load resistor Transistor type

(1-D) Io Io any Switching

0.167 1a 10 ohms Mosfet

frequency With Vo chosen to be 12 volts because 12 volts output to transformers are very common in practice. The choice of the values to use in the design was to be greater than the computed values for the components parameters to avoid using them at a risk of burning them, if the chosen values were equal to the computed values, and to cater for any kind of fluctuations the circuit might be subjected to. Calculation of the minimum values These are the values of L and C required for the converter to operate in continuous conduction mode at D =

Vo Vin

For the inductor The choice of the inductance value was based on the ripple current since the inductor behaves as short circuit for the steady state part of the signal ∆ il=

Vin ( 1−D ) D L fsw

∆ il < 0.5 A

Using the equation above the minimum inductance was calculated and found to be Lmin 66.667 uH. It is the minimum value because the maximum ripple current of 0.5A was used and since these two values are inversely proportional. For the capacitor

The capacitance value was calculated using the maximum ripple voltage and current, and the minimum inductance value as follows: ∆ Vo=

Vin ( 1−D ) D 8∗L∗C∗fsw

∆ Vo < 0.1 V The minimum capacitance value was found to be Cmin 12.5 uF. Mohan also suggests that a converter should be designed to function in both Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM). This is achieved by implementation of the worst-case design whereby the ranges in which the input voltage and the output load voltage vary. The converter to be designed has to operate in CCM even under light load therefore, the inductance value chosen should not be larger than 3 times the critical inductance(Lmax< 3Lcrit), where the critical inductance Lcrit is the value of the inductor that will make the converter operate at the border of CCM and DCM at full load. Calculation of critical values The critical value of the boundary current Ilb was kept to be equal to Io = 1A Ilb=

Vin D 8∗Lcrit∗fsw

Lcrit = 30 uH Lmax < 3*30 uH The capacitance value follows the same procedure of choice as the inductor, knowing that a higher capacitance value will lead to less ripple in the output voltage which is a desired output. The critical capacitance value was obtained by substituting the Lcrit in the equation of ∆ Vo=

Vin (1−D ) D 8∗Lcrit∗Ccrit∗fsw

Ccrit = 27.778 uF Cmax < 3*27.778 uF

The values had to be greater than the minimum and less than 3 times the critical values The value of L had to be 66.667 uH < L chosen < 90 uH The value of C had to be 12.5 uF...