The Design and Simulation of Controlled Converter
Circuit for Smart Phone Battery Charger at Rated
1.5A and 3.8V load
Ban Siong Lee1, Kang Yung Yee1, Yoong Xiang Wong1
1

Faculty of Electrical and Electronics Engineering, Universiti Teknologi Petronas, 31750, Perak, Malaysia leebansiong92@gmail.com, ky.christopher@gmail.com, tikuz92@gmail.com

Abstract— There have been increasing demand on low power application over the years, particularly in charging devices for device such as smartphones. A buck converter with controlled
PWM is proposed in this paper to achieve 1.5A and 3.8V load.
This method can ensure that the output current and voltage stay within the satisfactory range. A buck converter with rectified DC voltage of 240V DC as input voltage, and output voltage of 3.8 with 1.5A output current and duty cycle of 0.016 is designed. P pice is used in designing simulation and the results of the circuit have been obtained to verify the operation and performance of the concept. The theoretical calculations are compared with the simulation results. The ripple output voltage and current obtained is relatively low as calculated of around 1%. Losses can be further controlled by adjusting the dead time and duty ratio of the circuit.
Keywords— Full Wave Bridge Rectifier; Buck Converter; DCDC Converter; Smartphone Battery Charger

I.

INTRODUCTION

Portable electronic industry has grown over the years, many different demands has increased for instance, small and cheap systems, photographing and taking video, brighter and fullcolour display and a demand for increased talk-time in cellular phones. As a result, smartphone battery is drained very rapidly with all these demands and consumer always need to charge back their device quickly. Hence, to keep up with these demands from the consumer, a more efficient system must introduce to meet these demands and reduce the losses during switching. DC-DC converter is one of the electronic systems used to step down a pre-set voltage such as 9V, 5V DC to 3V,
2V or less for different types of applications on a smartphone.
The advantage of DC-DC converter is to be able to change from one DC level to another while minimizes power dissipation during switching, which in other words, to perform a conversion at the highest possible power efficiency.
One of the circuit topologies is the buck converter topology.
It has been chosen to convert the input voltage, 9V DC to
3.8V DC with 1.5A current. A buck converter has similar idea as step down transformer where it converter a voltage level to a lower value.
This paper presents a system where it includes full-bridge rectifier, buck converter and PWM as a control element in the system as a solution to obtain desired output at 1.5A and 3.8V
DC. Its process is evaluated and the design areas are

deliberated. The performance of this system is validated and proved with the simulation results.
II.

LITERATURE REVIEW

A DC-DC converter is an electronic circuit used to convert a source from one voltage level to another. It is a type of power converter. These converters are ubiquitous and are important in portable electronic devices such as hand-phones and laptops, which primarily receive power from batteries.
These devices often contain several sub-circuits, each with its own voltage level requirement which is different from that supplied by the battery or an external power supply
(sometimes higher or lower than the supply voltage).
Additionally, the battery voltage declines as its stored power is drained. DC to DC converters offer a way to solve this problem. The power from the supply can be converted into any specific value that is required by the device.

Figure 1: Schematic of a buck converter [1]

Figure 2: Schematic of a boost converter [2]
One of the most common converters is the buck converter.
The buck converter functions as a step-down converter. It converts the voltage from the power supply into a lower level of voltage as its output [1]. The schematic of the circuit can be seen from Figure 1.
Another common type of converter is the boost converter.
This converter is the opposite of the buck converter because instead of converting the voltage into a lower level, it amplifies the voltage of the power supply [2].
There are several topologies to perform DC-DC conversion.
One of the topologies is the usage of an H-bridge inverter
(DC/AC) in tandem with a full-bridge diode rectifier
(AC/DC). It can be improved by adding voltage regulators, reducing the heat sinks, etc. [3].
To obtain high performance control of a dc-dc converter, a good model of the converter is needed. The load usually affects the dynamics and one way to take this aspect into consideration is to consider the load as a part of the converter.
The load is often the most variable part of this system. If the load current and the output voltage are measured, then there are good possibilities to obtain a good model of the load online. Adaptive control can then be applied to improve the control [4].
The averaging approach is commonly used for fast and economic analysis of switched DC-DC converters. The switched DC-DC converter is usually regarded as an analogue linear time-varying system, where the externally controlled switches are modelled by using time-varying resistances. To model such systems, the so-called generalized transfer functions (GTFs) are applied. GTFs are functions of two wellknown s and z operators, simultaneously describing both the continuous-time and the discrete-time behaviour of switched converter. In comparison with the classical methods based on averaged modelling, the advantages of GTFs consist in the simulation of more believable converter behaviours. However, the disadvantage is this method results in more complicated mathematical models and difficult implementation in current simulation programs. [5]
DC-DC converters can also be divided into two categories; namely, non-isolated and isolated. Non-isolated DC-DC converters employ an inductor and there is no dc voltage isolation between the input and output. The vast majority of applications do not require dc isolation between the input and

output voltages. The non-isolated dc-dc converter has a dc path between its input and output.
On the other hand, an isolated converter employs a transformer to provide dc isolation between the input and output voltages which eliminates the dc path between the two.
These converters employ the usage of a switching transformer whose secondary is either diode-or synchronous-rectified to produce a dc output voltage using an inductor and capacitor output filter. This configuration has the advantage of being able to produce multiple output voltages by adding secondary transformer windings [6].
Besides DC-DC converters, there are also AC-AC converters. These converters convert AC waveforms to other
AC waveforms and the output voltage and frequency can be modified. [7]

Figure 3: Ideal switching waveforms for AC-AC converters

Figure 4: A schematic of an AC-AC Converter [7]

Back on the topic of topologies for DC-DC converters, a recently popular topology for high-power and high-density switching converter designs is a phase-shifted, full-bridge
DC/DC converter. It is favoured mainly because of its capability for zero-voltage switching (ZVS) operation, which minimizes switching losses.
However, a large circulating current in this topology causes significant conduction loss at heavy loads, while at light loads the circulating current becomes too little for switches to achieve ZVS. Both characteristics impact the ability to achieve maximum efficiency. Reducing circulating circuit and extending soft switching over a wider load range are two key areas to improve a phase-shifted, full-bridge converter’s performance. One way is to implement an open-loop, half-bridge bus converter operating with the switching duty cycle set at near
50% which results in the utilization of magnetic components and activation of ZVS and high efficiency over a wide load range. Such a converter does not and cannot regulate its output voltage because the pulse width modulation (PWM) is fixed.
However, if two such converters operate together in phaseshift mode and superimpose their inverter stage outputs on a common output filter, it is possible for the circuit to maintain the bus converter’s merits while regulating its output voltage
[8].
Normally a boost converter is employed for power factor correction (PFC) applications to improve performance or a fly-back converter is used to reduce the cost. However, the single-stage method using the simplest fly-back converter is not able to tightly regulate the output voltage. Paralleling of the converter power modules is a well-known technique that is often used in high-power applications to achieve the desired output power with smaller size power transformers and inductors [9].
Conventionally, DC-DC converters have been controlled by linear voltage mode and current mode control methods.
Unfortunately, under large parameter and load variation, their performance degrades. To solve this problem, sliding mode
(SM) control techniques can be implemented as they are well suited to DC-DC converters. This is due to the fact that they are inherently variable structure systems. These controllers are robust concerning converter parameter variations, load and line disturbances [10].
III.

bridge rectifier will give only positive upper half waves. In order to remove the ripples and obtain a constant 240V DC power supply, a smoothing capacitor is added into the circuit.
This smoothing capacitor will charge the output of the bridge rectifier to the peak and maintains a constant value.
For the system, switching frequency, fs = 50Hz, and output ripple voltage, ∆Vo/Vo = 1% is assigned, the smoothing capacitor value just after the full wave bridge rectifier can be obtained From the input voltage and output voltage values, the duty cycle, D is found out to be

Next, the value of the inductor is measured using the maximum inductor current formula with ILmax = Io = 1.5A

The critical value of the inductor, Lcrit can then be determined using the following formula

Since L is greater than Lcrit, the buck converter operates in continuous conduction mode (CCM).
With ∆Vo/Vo = 1% = 0.01 assigned, the value of output capacitor value, Co can be obtained using the formula of output voltage ripple

METHODOLOGY

In this section, theoretical analysis is carried out to complete the design of the controlled converter circuit for smart phone battery charger at rated 1.5A and 3.8V. The inductor value, L and the capacitor value, Co are determined by using the conventional buck converter formulas. With input voltage, Vin = 240V, output voltage, Vo = 3.8V and output current, Io = 1.5A, the output resistor, Ro is found to be

From 240V AC, the system is converted into 240V DC using a full wave bridge rectifier. The output of the full wave

IV.

SIMULATION RESULTS

Figure 5 shows the design of the controlled converter circuit for smart phone battery charger with combined full wave bridge rectifier and buck converter circuit to achieve a rated
1.5A and 3.8V output. The parameter values calculated in previous section are used to simulate the circuit.

+

Figure 5: Controlled converter circuit with combined full
+
wave bridge rectifier and buck converter circuit
Figure 6 and 7, the system is converts 240V AC into 240V
_
DC using a full wave bridge rectifier. The output of the full wave bridge rectifier gives only positive upper half waves. A smoothing capacitor is added into the circuit to remove the ripples and obtain a constant 240V DC. The capacitor charges the output of the bridge rectifier to the peak and maintains a constant value of 240V DC.

Figure 9 and Figure 10 shows the resultant steady-state waveforms of the buck converter with combined full wave bridge rectifier circuit from simulation. Figure 6 depicts the output voltage, Vo waveform while Figure 7 depicts the output current, Io waveform for 2 seconds. Based on the graphs, it is clearly seen that the system successfully reaches a steady state of 3.8V and 1.5A of output after being simulation for 2 seconds. Both voltage and current initially undergo a transient rise, which exceeds the required voltage and current due to reverse recovery voltage and current. Q1 transistor is used as a switch in the circuit to control the ON-OFF state across the collector and emitter.

Figure 9: Output voltage, Vo at the point of output resistance, Ro

Figure 6: Input voltage before full wave bridge rectifier
Figure 10: Output current, Io at the point of output resistance, Ro
V.

RESULTS AND DISCUSSIONS

Figure 7: Input voltage after full wave bridge rectifier
A. Full wave Bridge Rectifier Analysis
Figure 8 shows the duty ratio obtained from the circuit.
From the calculation above, the duty ratio calculated was
0.016, which means that the ON ratio of the switch is 0.016.
However, due to the design of this circuit, the ON time and
OFF time are reversed, because ON time should be higher compared to OFF time to obtain the results. This is also due to the characteristics of the device and circuit. From the figure, the ON time obtained was 9 s while the OFF time was 1 s. In other words, the ON time is 99% and the OFF time is 1%

Figure 8: Duty ratio of the switch 1:9

Figure 11: Full wave bridge rectifier
Full wave bridge rectifier utilizes 4 diodes to perform full wave rectification from AC to DC. Figure 11 shows 4 diodes labelled D1 to D4 are arranged in pairs with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D4 are forward biased while diodes D3 and D4 are reverse biased. During the negative half cycle of the supply, diodes D2 and D3 are forward biased while diodes D1 and D4 are now reverse biased. After rectification from both the cycles, a smoothing capacitor is required to ensure the rectification is almost

identical to the direct current. In other words, the smoothing capacitor converts the full-wave rippled output of the rectifier into a smooth DC output voltage.
Figure 6 shows the input voltage before full wave bridge rectifier where it is an AC supply. After the AC voltage gone through the full wave bridge rectifier, the voltage became a smooth straight line DC voltage as shown in figure 7.
B. Buck Converter Steady State Analysis

After the switch is turn off as shown in figure 14, the charged inductor now discharge with the diode. This diode is called free-wheeling diode which enable inductor to discharge as a closed circuit.
For the design, converter must operate in the continuous conduction mode (CCM). In order for the converter to operate in CCM, the value of inductor must not be zero. A minimum value of inductor is called critical inductor value, Lcrit, where it is the lowest value to ensure CCM, value below Lcrit will result discountinous conduction mode.
The parallel capcitor between inductor and resistor is remove voltage ripple in practical. It is typically large to limit the ac ripple across Vo.

Figure 12: Buck converter
Figure 12 shows the transistor-diode implementation of buck converter. This topology is used in design with PWM control at the transistor.
Buck converter has two modes of operation: switch
(transistor) ON and OFF. To simplify explanation of the operation, several assumptions can be made: I) All the components are lossless and ideal switching devices, the average input power, Pin and the average output power Po is equal; II) Assume steady state operation, the inductor current and the capacitor voltage are periodic over one switching iL (t0) = iL (t0 +T)
VC (t0) = VC (t0 +T)
III) Assume ideal capacitors and inductors, thus, the average inductor voltage and the average capacitor current are zero.
Mode On:

Duty cycle, D is the period where the switch is turn on for a period of DT. For turn off, the formula is (1-D)T. PWM is the pulse that has the duty cycle. Duty cycle is crucial in design as it ensure the circuit turn on and turn off at a precise timing to obtain desired output.

VI.

CONCLUSION

In this paper, a combined full wave bridge rectifier and buck converter circuit has been presented for a controlled converter circuit for smart phone battery charger application.
The circuit presented here gives the required output voltage and current to charge a smart phone which is 3.8V and 1.5A.
The ripple output voltage and current obtained is relatively low as calculated of around 1%. Losses can be further controlled by adjusting the dead time and duty ratio of the circuit. Full wave bridge rectifier and buck converter are indeed two very important circuits that are used to convert AC to DC and step down voltages. Both are widely used in the industry as they can easily derive to the operational principals and design procedures that electric gadgets often require.

ACKNOWLEDGMENT
This paper was produced to fulfill the requirements of the
Power Electronics course. All thanks for the information gathered go to the authors whose works are referenced below.
Figure 13: Mode on
As shown in the figure 13, when the switch is on, the input voltage Vin, put the diode into the reverse biased. Thus, the input voltage charges the inductor.
Mode Off:

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Figure 14: Mode off

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