1. INTRODUCTION:
Batteries are devices that are made up of two or more electrochemical cells that help convert chemical energy to electrical energy. Batteries play an important role in storing and generating electrical energy. The global battery market is worth $ 50 billion in US alone. Batteries find their application in a wide range of products like electronic equipment such as cellular phones, portable radio etc. An electrochemical cell typically contains two electrodes (one positively charged; the other negatively charged) and an electrolyte. A battery supplies direct current. There are two main categories of batteries: primary and secondary. Primary batteries cannot be recharged and once they discharge, they have to be discarded. Secondary batteries, unlike primary batteries, can be recharged and reused.
Although there have been advancements in battery technology in recent years, smart phones usually have to be recharged daily. Hence there have been numerous innovations in the charging techniques of batteries. Wireless charging technology helps a power source to transfer electrical energy to a load through an air gap without any connecting wires. Nowadays, there is lots of research going on in wireless charging technology for commercial products like smartphones and other smart portable devices. According to IMS research, by 2016 wireless charging would be a 4.5 billion market. According to pike research, by 2020 wireless powered devices market would be worth 15 billion. In this review article, we have done a comprehensive study on two major directions in which this field is advancing (1) non radiative (coupling based) wireless charging[1,2] (2) radiative (RF based) wireless charging[3].DC-DC converters are the most important part on the smart phone platform. The power losses which take place in these converters are the major cause of decrease in battery runtime. There have been advancements made in recent years to decrease the power loss in the converters.In this article, we have reviewed two optimization methods which help reduce the power loss in the converter[4].Next we have discussed Zero Current Switching (ZCS) Buck converter. ZCS leads to zero current during switching transition decreasing the switching losses and increasing the reliability of the battery chargers [5].
2. CHARGING TECHNOLOGY
2.1 Wireless Charging
2.1.1 Wireless Charging using inductive coupling
Most common way of transferring energy wirelessly is via inductive coupling.Inductive coupling has the ability to transfer energy wirelessly and safely. There is no radiation of Radio Frequency (RF) or Infrared (IR) signals. Because of the many benefits offered by this technology it is currently the most preferred choice for wireless power transfer. It has been shown to be able to transfer adequate amount of power as required for smartphone wireless charging applications. At the Massachusetts Institute of Technology transfer of 60W of electricity over the distance of 2 m has been successfully demonstrated [6].
An air core transformer is used to transfer power wirelessly with inductive coupling. Such power transfer systems are known to be feasible as they are used for powering passive Radio Frequency Identification (RFID) systems, which require much less power is smaller as compared to mobile phone charging [7, 8]. The basic circuit of an inductively coupled system used for wireless power transfer is given in Fig. 1.It consists of two coils- primary and secondary. The power is transferred through the primary side, which is shown in Fig.1 as a power supply (Us, Rs) and a coil (L1). The secondary side is formed by a parallel resonant circuit (consisting of L2 and C2) followed by a rectifier (D and Cload). Resistive load (Rload)represents power consumption of the mobile phone battery charger.
Fig.1Wirelesspowertransfersystem[1]
The power transfer happens via mutual inductance (M) between the inductively coupled coils L1 and L2.
The maximum achievable voltage on the secondary side (V2) depends upon the value of mutual inductance M. The secondary side voltageV2 attains a maxima for a particular value of M. This is the optimal value of mutual inductance.Quality factor of an inductor is the ratio of its inductive reactance to its series resistance at a given frequency. Hence Q is a measure of an inductor’s efficiency.
Here, R1and R2 are the series resistances of the coils L1 and L2 respectively.
Analysis of V2 for various values of R1and R2, done at the optimal value of mutual inductance shows that with a higher resistive part of the coupled coils, i.e. with lower Q factor, a maximal achievable voltage at the secondary side lowers.
Fig. 2 V2 as a function of R1and R2 [1]
From Fig.2 we can see that V2 is remains more or less constant with change in R1 (at constant R2). However, V2 varies significantly with R2 (at constant R1) i.e. the Q factor of the secondary side has significant impact on power transfer performance, hence the coil design with highest possible Q is the main focus of many design requirements [1]. The size of the coil has to be about the same as the smartphone because the transmitting and the receiving coil should be ideally of the same size. The frequency of operation should be as high as possible without transmitting radio waves (RF); which is possible if the wavelength is small enough with respect to the dimensions of the coil itself. The high frequency region (HF) which spans from about 3 to 30 MHz satisfies this requirement. The best spectrum region of operation is the one with the widest spectrum of unused frequencies. A downside of operating in this region is the significant skin effect, which affects the final charger performance. In Skin effect the current is forced to the outer edges of a conductor, which decreases the equivalent cross-sectional area of conductor and hence increases the resistance thus contributing losses. This effect is more evident at higher frequency. This is mitigated by making the coils and conductors on a printed circuit board (PCB). Present wireless charger devices have the power transfer capability of around 0.5 W at a maximum distance of 2.5 cm, which is sufficient to charge a regular mobile .
According to [11] power consumption, power transfer efficiency, wave forms and spectrum analysis are the most important measurements to be made.Voltage received at secondary side is much higher than required; but this voltage drops under load depending on transmitting coil inductance and quality factor of the secondary side. Major voltage drop occurs due to load and the rest is removed by a zener diode; the final charger is tuned so there is not a lot of excess voltage. If series resonance is used, on the receiving coil voltage would drop and the current would rise. Hence parallel LC receiving setup is used so that exact resonance does not have to be achieved and tuning the circuit becomes easier. Another significant effect on the performance of the charger due to the detuning over time by aging components and by accident is mitigated by this setup.
Fig. 3 Waveform of oscillator, showing the harmonics constituting noise [1]
Smartphones have antennas built in them, and the conductors inside them can pick up noise (unfiltered harmonics) from the wireless charger. Hence filters are present to remove higher harmonics and prevent them from being amplified or transmitted. The charger design is simple enough and it does not interfere with the mobile phone functionality while the device is charging. Phone is able to establish a connection with the base station and send or receive messages. With presented design no problems were located and every tested function worked without problems .
2.1.2WirelessChargingusingNFC:
Near field communication (NFC) is based on inductive coupling between two loop antennas (transmitter and receiver) with tuning networks so that the antenna circuits operate close to their resonant frequency. This lends this technology the capability of supplying power to passive radio frequency identification (RFID) tags during their readout [13]; a feature which is exploited in supplying power to sensors and other small devices while taking their readout, discussed in [9].NFC is different from the Wireless Power Consortium (WPC) Qi specification- which gives the guidelines for wireless power transfer- in its higher operating frequency and in its lower Radio Frequency (RF) power levels. The operating frequency in the Qi specification is 110–205 kHz and in the NFC specification it is 13.56 MHz [10, 11]. These make NFC-enabled charging more suitable for applications with lower charging power levels (like mobile handsets).
NFC already facilitates power transfer as well as bidirectional communication in the same implementation. The limited power transfer capacity due to certain usage issues and technical features of NFC such as typical NFC interactions are of short duration, power transfer for charging requires much longer interactions, which affects the way in which NFC devices operate; Wireless power transfer with practical efficiency requires shorter distances than those in typical NFC interactions, which effects the design and placement of antennas.Input power to the antenna circuit of an active NFC function of a smartphone is typically 100–750 mW, which is ten times lower than in the WPC’s Qi Low Power specification [12]; short duty cyclesform basis of NFC operation, entailing intermittent RF generation and power transfer, but for reasonable charging efficiency continuous RF generation is required.The current NFC specification does not have a charging management feature which requires a continuous exchange between the devices and embedded data processing that need to be implemented without degrading the power transfer performance.
The implementation of the charging add-on is based on a shared 13.56 MHz antenna and tuning network with conventional NFC operating modes to keep the overall implementation as compact as possible [13]. Typically, NFC interactions are “touch-based” but wireless charging scenarios require continuous placement of the charger device and the smartphone in each other’s vicinity to keep the distance between the antennas considerably smaller than the antenna dimensions for an optimal coupling coefficient [2].Due to the relatively high operating frequency of NFC, power transfer capability is somewhat low with a fixed output voltage. But, smaller and lighter antennas can be made due to smaller antenna inductances at high operating frequency with fixed output power and voltage. From equation (4) the antenna quality factor on the receiver side is more at higher frequency and thus power transfer efficiency of tuned inductive links is more, according to [17]. Antenna circuit tuning is an important requirement of both low-power and high-power NFC charging as it facilitates reliable NFC interactions along with efficient and controllable power transfer. It involves proper tuning and load matching both with a low coupling coefficient (for NFC interactions) and with a high coupling coefficient (wireless charging operations), to support the various RF energy levels in both these interactions and to eliminate detuning (due to impedance mismatch) and pole splitting caused by the high coupling coefficient [14].
Fig. 4 Maximum link efficiency vs. coupling coefficient [2]
For convenient and energy-efficient charging continuous generation of RF and appropriate charging management is required which needs continuous data exchange between the devices for purposes such as mutual idling of the charger and the smartphone, continuous control of the charging power level (including security measures), agreement on the maximum power, initiation and termination of the charging through battery monitoring or user involvement, and exchange of status information for the device’s energy management.
Fig. 5 Exchange of power and data in during an NFC charging interaction [2]
Identification is an integral part of the NFC Technology Detection Activity polling cycle, performed by the initiator of the NFC link [16] (which will be the charging device in this case). The continuous exchange of data requires standardization for interoperability: what protocols are followed; what information/signals are exchanged to initialize, maintain, and stop the charging transaction; how the device differentiates between charging management data exchange and generic NFC data transfer; how high-power NFC charging will be authenticated without risking damage to other conventional NFC devices that may be present in the vicinity of an NFC charger; etc. The role of the NFC initiator should implicitly be taken by the charger so that it is required only for the initiator to generate the 13.56 MHz carrier that is also needed during power transmission. This is suitable as smartphones need not generate the HF carrier; this enables the charging of even the devices running on low battery; they can use load modulation to communicate back to the charging device, which requires lower power than carrier generation.
2.1.3 RF energy harvesting
Radio waves can be used for carrying information by varying the combination of frequency, phase and amplitude of the wave within a frequency band. It is a part of the electromagnetic spectrum. When the EM radiation comes in contact with the conductor (antenna) it induces an electrical current on its surface. This is known as skin effect. This principle can be used in wireless power transfer.
Fig. 6 Block diagram of the system [3]
Above figure shows the block diagram of the system. It consists of multiple RF sources which are captured by antenna. DC power is generated using matching circuits and rectifier circuits which is used to recharge the battery. Hence circuit converts RF signal into a DC signal[15]. One of the way to build such a circuit is using Schottky diode. It offers low forward voltage and high switching speed. Matching circuit is as shown in figure. Only capacitor is used. It is used to tune the antenna at its resonant frequency. Resonant frequency is given by.
Since RF signal is an AC signal, a rectifier circuit is used to convert it to DC signal. A voltage doubler circuit is used to rectify the input voltage. The output voltage is twice the input voltage minus twice the diode threshold voltage. For n stage voltage doubler circuit output voltage is n times the input voltage. Diodes in the circuit are required to operate at high frequency. Also they must have low turn on voltage. Schottky diode is used for this purpose. A modified villiard voltage doubler can be used to improve the efficiency[16]. It is a three stage voltage doubler circuit. Another voltage doubler used is CMOS transistor based. In a circuit which involves CMOS operations Schottky diode cannot be used. This is because such processes involve signals less than the threshold voltage of the diode. Hence a transistor based circuit is required for RF energy harvesting.
Fig. 7 Traditional CMOS circuitFig.8 Modified CMOS circuit [3]
A traditional CMOS circuit is shown. In ideal case, high output voltage is obtained at resonant frequency. An improvement was made in this circuit to increase the efficiency. nMOS was replaced by pMOS in order to eliminate power losses since no current would flow from pMOS body to the bulk substrate although it would be connected to the output. This is due to the fact that pMOS FET are placed in n typed doped inside a p type substrate[3].
2.1.4Comparison:
|
Wireless Charging technique |
Advantage |
Disadvantage |
Effective Charging Distance |
|
Inductive Coupling |
Safe for humans, simple implementation |
Short charging distance, heating effect, not suitable for mobile applications, needs tight alignment between charger and charging devices |
From few millimeters to a few centimeters |
|
Magnetic Resonance Coupling |
Loose alignment between chargers and charging devices, charging multiple devices simultaneously on different power, high efficiency of charging, non-line-of-sight charging |
Not suitable for mobile applications, limited charging distance, complex implementation |
From a few centimeters to a few meters |
|
RF Radiation |
Long effective charging distance, suitable for mobile applications |
Not safe when RF density exposure is high, low charging efficiency, line-of-sight charging |
Typically within several tens of meters, up to several kilometers |
2.2 Optimization of power delivery network (PDN) in smartphones
Due to advancements in smart phone functionality there have been inclusion of many high performance modules like sophisticated sensors, fast wireless interface, high resolution display on smart phone platform. There has been relatively slow advancements in electrical storage density of modern batteries. This has led to low service time between successive charging of smart phones. One of the critical factor that has been often overlooked is the power conversion efficiency of the (PDN). DC-DC converter is the heart of PDN. It provides the battery power to different modules.
Fig.8ConceptualdiagramofthePDNinasmartphoneplatform.[4]
To minimize the power loss due to dc-dc converter there are two optimization methods:
1) Static Switch Sizing (S3) 2)Dynamic switch modulation (DSM)
There are three types of dc-dc converters in smart phone platform namely inductive dc-dc converters, low dropout linear regulator (LDO) and capacitive dc-dc converter[4]. Major power losses arise due to resistance of passive elements and power switches in the design. Efficiency of dc-dcconverter depends on gate widths of switches. Gate width is inversely proportional to switching losses but directly proportional to conduction losses. Hence optimizing the gate width can decrease the power loss of converter.
2.2.1 Static switch sizing method
In static switch sizing method, for a fixed output
current Iout, optimum gate widths are derived. Iout in a
smart phone is different for different applications[17-19]. Therefore the total
current is a continuous random variable and its probability density function is
denoted by f(Iout). Accordingly width of pMOS and nMOS is
calculated. Expression for optimum gate width for pMOS is obtained by solving
[20,21]. Similarly expression for gate
width for nMOS is obtained.
2.2.2 Dynamic switch modulation
Fig. 9 Circuit diagram for dynamic switch modulation. [4]
Static switch sizing method is not efficient when the variance of load current (Iout) distribution is high. Hence optimum efficiency in charging load current can be obtained by turning ON and OFF some parallel connected switches. This is done using dynamic switch modulation method. N pairs of switches are connected in parallel. First switch has the minimum width and last switch has the maximum width. Depending on Ioutwe get different combination of ON and OFF switches which is used to achieve maximum dc-dc conversion efficiency. Based on the number of ON switches effective width Weff,type,i is calculated where subscripts type implies PMOS or nMOS, i implies smallest effective width in switch configuration. There are three effective widths for two pMOS switches Weff,p,1, Weff,p,2, Weff,p,3. Hence Ioutis divided into three operation ranges. For each output range optimum switch combination is found.
Fig. 10 Concept of DSM operation with two parallel connected pMOS switches. [4]
Thus S3 was based on the principle of configuration of switches in dc-dc converter in such a way that optimal efficiency conditions of converter match with the load conditions. Experimental results show that S3 increases the efficiency of PDN by 6% which results into 19% power loss reduction. Also it is easy to implement. For dynamically varying load conditions DSM is used to overcome the lack of capability of S3. Similar enhancement is obtained in DSM operation. However if some new applications are added to smart phone platform it will change the output current. In such a case S3 will fail but DSM will continue to provide power efficiency enhancement.
2.3 Zero current switching (ZCS) buck converter
Today, it is an era of smart phones. Everyday new smart phones are launched in the market. Consequently, there has been advancements in pulse width-modulated (PWM) dc-dc converters for battery chargers consisting of semiconductor switches[22-25]. These switches are used to turn ON/OFF the whole load current due to which they are subjected to high switching stress and high switching losses.
Fig. 11 Traditional ZCS buck converter for battery charging [5]
Zero current switching (ZCS) buck converter significantly decreases the switching losses by eliminating the current and voltage overlap by forcing the switch current zero before the switch voltage . Most of the ZCS converters operate with constant on time control. There have been many designs of battery charging circuits with ZCS. [26-31]. Auxiliary switch present in the quasi-resonant circuit is the one of primary design features of ZCS PWM converter. This is because auxiliary switch generates the resonance and temporarily stops a period that can be regulated. Due to this the disadvantages of fixed conduction or cutoff time in a traditional quasi resonant converter is overcome. To shrink the volume of passive components and for higher power density converters are operated at high frequencies. Metal oxide field effect transistor (MOSFET) with soft switching is used for this purpose. An additional LC resonant tank is connected to shape the switching device’s current waveform at on time. Thus a zero current condition is generated for the device to turn off. In order to limit di/dt of the power switch an inductor Lr is connected in series.[32]
Fig.12 Novel ZCS buck converter for battery charging [5]
Cr works as an auxiliary energy transfer element. Dm is called freewheeling diode. The capacitor Cfa and inductor Lfform a low pass filter. It filters high frequency ripple signals thus providing a stable source for charging. Diode Df prevents backward transfer of energy i.e. from battery to circuit. In traditional ZCS buck converter, after the power switch is turned on inductor and capacitor oscillates. The power switch is turned off with ZCS after resonance brings the inductor current to zero. On time of power switch is determined by resonant period which is constant. Hence it operates at fixed on time control. Consequently, there is generation of unpredictable frequencies as output is obtained by varying the off time of switches. Hence in the proposed ZCS converter, the auxiliary power switch is kept off when main power switch is on. Resonance occurs only when auxiliary switch Sa is on. The main power switch is turned off with ZCS after the current is made zero by the resonance. Hence, Sa holds off the resonance for some time. Thus novel ZCS battery charger operates at constant switching frequency. For a lead acid battery, it took 350 min for terminal voltage to rise from 10.5 to 16V.The average efficiency during the charging period of ZCS battery charger circuit was 85.2% [5].
Fig.13 Equivalent circuit of novel ZCS buck converter [5]
3. CONCLUSION:
Over last few years, advancements in charging systems of smartphone batteries are happening at an exponential rate. Wireless power transfer is a promising technique that can address the challenge of improving the battery life. With the integration of inductively couplingpower transfer systems and implementation of smart charging methods, wireless charging can become an essential and useful tool for charging smartphones.In comparison to dedicated wireless charging solutions, integration of NFC and wireless charging into the same implementation results in a more compact and cost-efficient charging interface for portable devices such as smartphones.The NFC chip and antenna have already been integrated into the design of smartphones. Majority of new NFC enabled handsets are smartphones that require frequent and portable charging solutions. Implementation of NFC enabled charging systems has the potential for improving charging infrastructure in future with reduced costs.In a RF energy harvesting circuit Schottky diodes are most suitable due to its low series resistance and high forward bias current for a given voltage. However, in the circuits involving CMOS operations where operating signals are way below the threshold diode voltage a CMOS based villiard voltage multiplier is used. This technique proves helpful in charging a mobile handset with a fully charged mobile handset kept in its vicinity. There is significant power loss in the PDN of the smart phone platform during power conversion. The losses can be minimized by optimizing the gate widths of the switches used. S3 and DSM are the two methods used for the purpose. S3 is simple to implement but does not work in dynamically varying load conditions. On the other hand DSM takes more area but is able to achieve high conversion efficiency under all load conditions. A novel ZCS buck converter increases power transfer efficiency by decreasing the switching losses of dc-dc converters used in smart phone.Switching takes place at zero current condition. Also its simple circuit makes it easy to implement it.
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Received on 19.02.2017 Accepted on 29.05.2017 © EnggResearch.net All Right Reserved Int. J. Tech. 2017; 7(1): 69-78. DOI:10.5958/2231-3915.2017.00012.8 |
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