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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
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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.
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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
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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.
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Circuit Diagram
Simulation results
The simulation results have been confirming our calculations,
Following simulation graphs depict the state between 690us to 700us
Pulse
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Load Ripple Current
Load Voltage Ripple
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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