Project Report Mobile Phone Charger
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Abstract The purpose of this document is to provide the design solution for ELEC3 204 Project for a Mobile Phone Charger. It contains an overview on how the circuitry system works in conjunction with the feedback control system. Descriptions, schematic circuit diagrams are provided for the purpose of explaining how the controlled PWM technique works with the DC/DC buck converter in order to give out the desired output voltage for charging the phone. Furthermore the hardware and software modules used in the design are discussed. The System Performance section contains captured waveforms on various parts of the circuit and efficiency data that validates the functionality of the controlled converter system as well as problems encountered during the designing and testing phase. Lastly this paper concludes the design solution as well as providing references used for the project.
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Content
System Overview...............................................3 System Requirements and Specificati ons............6 Operational Scenario..........................................7 System Perf ormance..........................................8 Perf ormance Analysis ......................................13 Software Code..................................................14 Conclusion........................................................15
Reference.........................................................17
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System Overview The battery charger works by using PICAXE programming software to monitor the output voltage value sensed by feedback 1 and load current value sensed by feedback and pin
2
2
read by pin 1
of the PICAXE microcontroller during the charging state. This is shown on Figure 2.
The schematic system block diagram is shown in Figure 3. This reference design can charge or discharge mobile battery of the same type. The PICAXE
08
(M) microcontroller contains the
embedded charging algorithm and controls the external charging components and the buck converter. The voltage for battery charging is generated through a buck converter, which is controlled by the PICAXE
08s
Hardware PWM signal. The PICAXE
08
controls battery charging
and discharging through PWM signal. Battery Voltage is feedback into the microcontroller through Feedback 1, it is necessary for over voltage protection. The Current is sensed using feedback
2,
it shows the current being delivered to the battery during charge for status
information and output voltage regulation.
Figure 2. Sensing circuit of mobile phone charger
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Figure 3. Hardware overview of the whole buck converter circuitry system
From Figure 3 the PICAXE
08M
is used to generate a sequence of 5V PWM (Pulse-Width-
Modulation) signal out of pin 2, sending the required duty cycle into the gate-source terminals of the MOSFET
2N7000
switch. Q1
2N7000
boost the PWM signal to a 12V pulse which the
signal is further boosted by the IR2117 driver to with its R1, R2 and D2 components in order to drive the switch Q 2 which controls the output voltage connected to the load R 01 of the buck converter . Only one converter is used for the mobile phone charger application.
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System Requirements and Specificati ons The specifications of the circuit for battery charger parameters are as follows: Input voltage = 12V ± 20% Output voltage = 5.0 V ± 20% (varied depending different mobile phones) Switching frequency = 75 kHz Output current ripple = 400mA Efficiency no less than 50% , , , , y y y y
y y
Converter inductor value can be calculated from the following Equations:
As the inductor Equation shows, the higher the PWM switching frequency (as T is smaller), the lower value of inductor is required. Note that the capacitor Cf and resistor Rf in Figure 2 is simply a ripple current reducer. In this case, larger is better, as ripple is inversely proportional to the value of this capacitor and to ensure that the PICAXE is sensing the correct load current.
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Operational Scenario The charging of the mobile phone is achieved by connecting the mobile phone with the output voltage Vo of the buck converter shown in a generalised form in Figure 4.The charging state starts with the start-up delay when PICAXE gradually output PWM pluses which increase from zero duty cycle to near about 41% reaching the current limit of 350 mA approximately . This is achieved through load current sensed by PICAXE from feedback 2. PICAXE program regulates the charging current by sending PWM pluses with appropriate duty cycle to output a constant current value between 300 mA 400 mA. During the charging state the output voltage value is kept constant with the value of 2.64 V. At any time when the output voltage exceeds 120% of its rated voltage OPV (overvoltage protection) is activated. OPV is achieved by feedback 1 which is used to sense the output voltage. When PICAXE sensed this exceeding voltage value the program immediately sends continuous PWM pluses with decreasing duty cycle until the voltage value read from feedback 1 falls within the limited range again.
Figure 4. Simplified buck converter circuit. To see more specific feedback connections at Vo refer to Figure 2.
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System Perf ormance PICAXE PWM Output Voltage across D3 Vo (output voltage) IL (output current) V (voltage ripple) I (current ripple) Dummy load is Resistor R = 8.2 ohm Captured Wavef orms:
V 12 V 2.64 V (across dummy load) 337 mA (across dummy load) 100 mV 60 mA 5
Figure 5. PICAXE PWM Output = 5v
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Figure 6. Voltage across D3 = 12 V
Figure 7. Vo (output voltage) = 2.64 V
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Figure 8. V = 100 mV
Figure 9. I = 60 mA
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Overvoltage Protection Wavef orms
Fig1. Vo = 0 V (initially from start-up delay)
Fig3. Vo = 4v (reaching OVP output value) after
Fig2. Vo increasing
Fig4. Vo decreasing to the limit voltage range after OVP.
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Perf ormance Analysis The waveforms illustrate the dcdc converter output response for a current step from zero current to Io. The response of the buck converter changes depending on the current at the output. The response time for the current to increase and reach the stable v alue is about 0.5 seconds. All waveforms are taken across the dummy load (Resistor of 8.2 ohm). We are controlling the voltage using the current so the microcontroller is programmed to control the current. The waveforms are little bit noisy. To reduce the current ripple to about 40mA, Rf and Cf are used to be about 1k and 1 00nF.
We got efficiency ranging between 52 and 58 %.
Efficiency:
=
2.64
* 0 .334 / (12 * 0.134)
=
54.8 %
Efficiency Factors: Conduction losses that depend on load: y y y y
Resistance when the transistor or MOSFET switch is conducting. Diode forward voltage drop (usually 0.7 V ) Inductor winding resistance Capacitor equivalent series resistance
Switching losses: y y
Voltage-Ampere overlap loss 2 Frequencyswitch*CV loss
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Conclusion
This project has given a basic insight in to the working of a buck converter. It is seen that if ripple increases, then the efficiency decreases. An optimum switching frequency and inductor current ripple pair is important as it maximizes the efficiency of a buck converter. We have generated the voltage and current depending on the specifications given. If high efficiency is the most important parameter to achieve, then a low switching frequency is the best choice. As different mobile takes in different voltage input, so current sensing is used to limit the duty cycle.
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Software Code init: high 2 b0=0 b1=0 b2=53
'Initialize the program 'variable to read the voltage 'variable to read the current 'make duty cycle 0 at start
main: pwmout 2,12,b2 readadc 4,b0
'varying PWM according to the current value 'reading the voltage
debug b0 if b0>125 then ' for overvoltage protection b2= b2 + 1 MAX 53 MIN 21 ' if v>6 volt then duty cycle decreases endif readadc 1,b1
' reading value of current
debug b1
b3=2048/100 if b1>b3 then b2=b2+1 MAX 53 MIN 21 endif if b1<16 then b2=b2-1 MAX 53 MIN 21
' to make b3= 400 mA
'decrease duty cycle when current is > .4 amp
' increase duty cycle when current is < .3 amp
debug b2 endif goto main
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References
Papers
[1] Jinwen Xiao, Angel Peterchev, Jianhui, Seth Sanders, An Ultra-Low-Power DigitallyControlled Buck Converter IC for Cellular Ph one Applications, Applied Power Electronics Conference and Exposition, 2004. Nineteenth Annual IEEE, Volume 1, Issue, 2004 Page(s): 383 - 391 Vol.1 [2] Michael D. Mulligan, Bill Broach, and Thomas H. Lee, A Constant- Frequency Method for Improving Light-Load Efficiency in Synchronous Buck Converters, Power Electronics Letters, IEEE Volume 3, Issue 1, March 2005
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