design procedure for buck regulator

8/8/2019 Design Procedure for Buck Regulator

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Introduction to Buck Converter

Buck converter is one of the Switched mode power supply (SMPS) which outputs a lower

voltage than a given input voltage. Due to this special characteristic it is also known as step-

down converter, current step-up converter, chopper, direct converter.

Solution to Automobiles by using Buck Converters

Li-Polymer rechargeable batteries are commonly used as power supplies in mobile phone

chargers. But these devices have a minor drawback when it comes to mobility, where the user

will not always have main power to recharge the batteries which are powering the system. Our

project is about a power supply for a Li-Polymer battery charger for an automobile which is

using a 12 input in car cigarette lighter socket.

This solution is suitable for a Li-Polymer battery having a capacity of 700 mAh to 900 mAh,

discharged voltage of 3.6 V, and a maximum charged voltage of 4.9 V.

Design Architecture for 12v-4.9v Buck converter

Li – Polymer

Battery



Design Specifications

Topology: Buck converter

Input Voltage: 12VDC

Output Voltage: 4.9VDC

Output Voltage ripple: 50mV

Total current 600mA

Switching frequency 200kHz

In this project we are going to come up with a solution for the main drawback of the Li-Polymer

rechargeable batteries by using Buck converter as the power supply unit.

This application will be very useful when traveling in remote areas where charging a battery will

be difficult or impossible. By using a Buck converter we can reduce the power dissipated as

heat and produce the best environment for the battery charging.



Design Procedure for Buck regulator

First calculate Duty ratio to obtain required output voltage.Average ouput voltage is given by;

=

=

where; T= total period = 1 , = , =

In our SMPS design, =4.9

12= 0.4083

Next, we have to select a particular switching frequency, preferably > 20 kHz for negligible

acoustic noise. Higher fs results in smaller L, but higher device losses. In our case we select

= 200. Possible devices for provide a signal at this frequency range : MOSFET, IGBT

and BJT. Low power MOSFET can reach MHz range.

Average, Maximum and Minimum inductor current

Average inductor current = Average current in RL



From figure ,capacitor C is charging during the period that > , thus increase in charge on

∆ . When < , C is discharging ,thus;

∆ =1

2∗ ∆

2∗ 2=

∗∆8

, thus ∆ = ∆ =

∗ 28 (1− )

Or finally,

=∆ ∗

2

8 ∗ 1− For our design calculated value of Capacitor value

=4.9

50∗10−3

∗

1(200∗103)2

8∗120.80∗10−6

∗1

−0.4083 = 1.5

However , in most practical circuits, the output ripple voltage is more likely to be caused by the

ripple current through the capacitor ESR value, this value is given by,

=∆∆ =

50

120= 0.4167Ω

Capacitor ratings:

Must withstand peak ouput voltage

Must carry required RMS current.RMS

value for is given by;

For our case calculated rms value is 0.60099

Considering these factors we selected our simulation value as 10 times calculated value.,

= 15

Other factors

In here either P-Channel or a N-Channel MOSFET can be used. For our case we selected

IRF034 as our PWM driver.

It’s obviously that we have to select schotky diode as diode D, because of the low losses.

We select 120NQ045Schotky diode. From datasheet we have found this diode can

tolerate 12v perfectly.



Circuit Diagram

Simulation results

The simulation results have been confirming our calculations,

Following simulation graphs depict the state between 690us to 700us

Pulse



Load Ripple Current

Load Voltage Ripple



Comparison

Quantity Calculated Simulated

Output voltage 4.9 v 4.8864 V

Voltage Ripple 50 mV 48.8 mV

Output Current 600 mA 598.355 mA

Current Ripple 120 mA 122.003 mA

design procedure for buck regulator

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