Design of Buck-Boost Converter Using Multisim Software

Design of Buck-Boost Converter Using Multisim Software

Mousumi Mishra, Asst. Professor, KIIT University

Apurba Abhijeeta, MTech

Abstract

The demand for switching power supply devices is rapidly increasing in this scientific era. The design engineers never get the particular supply voltage which they wish to use in their work. So, to regulate the desired voltage supply we need a converter or inverter. In case we consider a DC-DC Converter, we can step up the voltage level using a boost converter or step down the voltage level using a buck converter. Here, in this paper we have discussed about the design of buck-boost converter using multisim software, where after necessary simulations we convert a supply voltage of 21V to 12V and this output voltage can be used in the applications which require 12V supply voltage.

1. Introduction

   To work in today's technical environment we have to deal with a rapidly changing market of electronic products and components. As new technology develops, integrated circuits function works faster and are smaller in size. Still today many integrated circuits still require a voltage of 12 Volts or 24 Volts in order to function. The DC-DC converters are widely used in regulated switch-mode dc power supplies and in dc motor drives applications. Often the input of these converters is an unregulated dc voltage, which is obtained by rectifying the line voltage, and therefore it will fluctuate due to changes in the line voltage magnitude. Switch-mode DC- DC converters are used to convert the unregulated dc input into a controlled dc output at a desired voltage level.

   Here, our aim is to get a 12 to 13 V output from a 21 V input voltage supply. So, for this voltage regulation we have designed a Buck-boost converter. It will buck (step down) the supply voltage first and then it will boost (step up) the output of buck converter so as to get the desired 12 to 13 V.

   To calculate the power stage of a converter, following four parameters are always considered. They are:

   1. Input voltage range: VIN(min) and VIN(max) 2.Nominal output voltage, VOUT

      1. Maximum output current, IOUT(max)

      2. An integrated circuit used to build the buck converter

      So, to get into the basic configuration, firstly we need to determine the duty cycle, D, for the minimum input voltage. The minimum input voltage is used as it leads to the maximum switch current.

      VIN(min) = minimum input voltage VOUT = output voltage

      = efficiency of the converter(say 90%)

      The efficiency is added to the duty cycle calculation, since the converter also has to deliver the energy dissipated. Thus, this calculation gives a more realistic duty cycle than just the formula without the efficiency factor.

      The next step in determining the maximum switch current is to find the inductor ripple current.

      In this paper we have the introduction in Section 1, the basic configuration of buck converter and the basic configuration of boost converter in Section 2, the basic calculations on which the design is done in Section 3,the simulation of buck converter, boost converter and the buck- boost converter in Section 4 and conclusion at the end.

   2. Basic Configurations

      Basic Configuration of Buck Converter

      Following figure shows the basic configuration of a buck converter where switch is integrated in the selected integrated circuit (IC). Certain converters have diode replaced by a second switch that is integrated into the converter .

      VIN(max) = maximum input voltage VOUT= desired output voltage

      D = duty cycle calculated

      fs = minimum switching frequency of the converter L = selected inductor value

      Now, it has to be determined if the selected IC can deliver the maximum output current.

      ILIM(min) = the minimum current limit value of the integrated switch

      IL = inductor ripple current calculated

      If the calculated value for the maximum output current of the selected IC, IMAXOUT, is below the system's maximum output current, the switching frequency has to be increased to reduce the ripple current. Again, a higher inductance

      reduces the ripple current and thus increases the maximum output current of the selected IC. If the calculated value is above the maximum output current of the application, the maximum switch current in the system is calculated:

      IL = inductor ripple current calculated

      IOUT(max) = maximum output current necessary in the application.

      IOUT(max) is the peak current that the inductor, the integrated switch and the external diode have to withstand. Again, the higher the inductor value, the higher is the maximum output current since the ripple current is reduced. The inductor must always have a higher current rating than the maximum output current as current increases with the decreasing inductance.

      For parts where no inductor range is given, the following equation is a good estimation for the choice of inductor:

      VIN = input voltage

      VOUT = desired output voltage

      fs = minimum switching frequency of the converter. IL = estimated inductor ripple current

      The inductor ripple current cannot be calculated from the previous equation as the inductor value is unknown. So, a good estimation for the inductor ripple current is 20% to 40% of the output current, i.e,

      IL = estimated inductor ripple current

      IOUT(max) = maximum output current necessary in the application

      Now coming to diode selection, we know to reduce losses, we use Schottky diodes. The forward current rating needed is equal to the maximum output current:

      IF = average forward current of the rectifier diode IOUT(max) = maximum output current necessary in the application.

      Schottky diodes have a much higher peak current rating than average rating. Therefore the higher peak current is not a problem. The other parameter that has to be checked, is the power dissipation of the diode. It has to handle:

      IF = average forward current of the rectifier diode VF = forward voltage of the rectifier diode

      D = duty cycle calculated

      Again, for selecting the capacitor, the minimum value for the input capacitor is taken because it is necessary to stabilize the input voltage due to the peak current requirement of a switching power supply. The best practice is to use low-equivalent series resistance (ESR) ceramic capacitors. Otherwise, the capacitor loses much of its capacitance due to dc bias or temperature. The value can be increased if the input voltage is noisy.

      With external compensation, the following equation can be used to adjust the output capacitor value for a desired output voltage ripple:

      COUT(min) = minimum output capacitance IL = estimated inductor ripple current

      fs = minimum switching frequency of the converter VOUT= desired output voltage ripple

      The ESR of the output capacitor also adds some more ripple and the output voltage is given by:

      VOUT(ESR) = additional output voltage ripple due to capacitors ESR

      ESR = equivalent series resistance of the output capacitor IL = inductor ripple current

      The selection of the output capacitor is not driven by the steady-state ripple, but by the output transient response. The output voltage deviation is caused by the time it takes the inductor to catch up with the increased or reduced output current needs.

      Basic Configuration of Boost Converter

      The following figure shows the basic configuration of a boost converter where the switch is integrated in the integration circuit(IC).In case of lower power conveters the diode is replaced by a second switch integrated into the converter.

      The first step is to determine the duty cycle, D, for the minimum input voltage. The minimum input voltage is considered as this leads to the maximum switch current. So, duty cycle is :

      VIN(min) = minimum input voltage VOUT = desired output voltage

      = efficiency of the converter

      Here, efficiency is added to the duty cycle calculation, as the converter has to deliver the energy dissipated.

      The next step to get the maximum switch current is to determine the inductor ripple current.

      VIN(min) = minimum input voltage

      D = duty cycle calculated in previous equation

      fs = minimum switching frequency of the converter L = selected inductor value

      Now it has to be determined if the selected IC can deliver the maximum output current, i.e ,

      ILIM(min) = minimum current limit value of the integrated switch.

      IL = inductor ripple current.

      D = duty cycle calculated in the previous equation.

      A higher inductance reduces the ripple current and thus increases the maximum output current. If the calculated value is above the maximum output current of the application, then maximum switch current in the system is calculated:

      IL = inductor ripple current

      IOUT(max) = maximum output current necessary in the application

      A good estimation for the choice of inductor value is given as:

      VIN = input voltage

      VOUT = desired output voltage

      fs = minimum switching frequency of the converter IL = estimated inductor ripple current

      The inductor ripple current cannot be calculated with the previous equation the inductor value is unknown. A good estimation for the inductor ripple current is 20% to 40% of the output current, given by:

      IL = estimated inductor ripple current

      IOUT(max) =maximum output current necessary in the application

      Here also, the best practice is to use low ESR capacitors to minimize the ripple on the output voltage. With external compensation, to adjust the output capacitor values for a desired output voltage ripple:

      COUT(min) = minimum output capacitance

      IOUT(max) = maximum output current of the application D = duty cycle

      fs = minimum switching frequency of the converter VOUT = desired output voltage ripple

      The ESR of the output capacitor also adds some ripple, given by:

      VOUT(ESR) = additional output voltage ripple due to capacitors ESR

      ESR = equivalent series resistance of the used output capacitor IOUT(max) = maximum output current of the application

      D = duty cycle

      IL = inductor ripple current

      Thus, the basic configuration of buck and boost converter cited above is used in the designing of buck-boost converter.

   3. Basic Calculations

Based on the above formulae, the following calculations were done.

BUCK CONVERTER :

Assuming input voltage, VIN = 21V, maximum output voltage, VOUT = 12V and maximum current, Imax= 900mA, the following values are calculated using the above equations:

1. Duty Cycle, D = 0.63

2. Inductor ripple current, IL =270 mA

3. Inductor value, L = 1.26 uF

4. Average forward current of the rectifier diode, IF = 360mA

5. Maximum output capacitance, COUT = 0.02uF

BOOST CONVERTER :

Assuming the input voltage, VIN =8V, output voltage, VOUT = 12V, maximum output current, IMAX = 900mA, the following values are calculated using the above equation:

1. Duty cycle, D = 0.59

2. Inductor ripple current, IL =562.5 mA

3. Inductor value, L = 6.58 uH

4. Average forward current of rectifier diode, IF = 90 mA

5. Output capacitance, COUT = 2.5uF

   1. Simulation Output with its Scope

      BUCK CONVERTER

      Figure 1: Design of Buck Converter

      Figure 2: Output Scope of Buck Converter

      BOOST CONVERTER

      Figure 3: Design of Boost Converter

      Figure 4: Output Scope of Boost Converter

      BUCK-BOOST CONVERTER

      Figure 5: Design of Buck-Boost Converter

      Figure 6: Output Scope of Buck-Boost Converter

   2. Conclusion

      Thus, the designing of buck-boost converter using multisim software is done and is verified. The input voltage supplied to the buck converter is 21V,then the obtained output of the buck converter is 3.21V(approx) is given as the input voltage to the boost converter and then the obtained output of the boost converter is 13V. So, combining the buck converter and boost converter we got the buck-boost converter output to be 13V,when 21V supply was given.

   3. References

[1]-Understanding Boost Power Stages in Switch mode Power Supplies (SLVA061)

[2]-Understanding Buck Power Stages in Switch mode Power Supplies (SLVA057)

[3]-Robert W. Erickson: Fundamentals of Power Electronics, Kluwer Academic Publishers, 1997

1. -Mohan/Underland/Robbins: Power Electronics, John Wiley & Sons Inc., Second Edition, 1995

2. -Improve Your Designs with Large Capacitance Value Multi-Layer Ceramic Chip (MLCC) Capacitors by George

M. Harayda, Akira Omi, and Axel Yamamoto, Panasonic