Green Energy Powered Multiphase Buck Converter for Microprocessors

Swapnajit Pattnaik1, B. S. Umre2 , Diksha Khare3

Electrical Engineering Department, Visvesvaraya National Institute of Technology, Nagpur Nagpur, Maharashtra, India

1swapnajit.pattnaik@gmail.com, 2bsumre@rediffmail.com, 3dipavali_786@yahoo.co.in

Abstract—Power supply requirements of new generation microprocessors are one of the problems of latest generation. New generation microprocessors require 40-100A and voltage in the range of 1 V or 2 V. Lower voltages can be obtained from DC-DC converter namely the buck converter. The three phase DC-DC converter of 100W, 12V/1V with efficiency 97% is modeled. With the help of soft switching techniques, this multiphase technology gives excellent transient response and increased efficiency. The input to the DC-DC converter is provided from a hybrid combination of wind and solar system. Owing to the present scenario of the fuel- crisis, renewable energy sources are explored to a greater extent. Renewable energy sources being environment friendly are called as Green Energy Sources.

Key words--- Green energy, DC-DC, Multiphase

I. INTRODUCTION Due to rapid industrialization and with advent of new

technologies, demand for more power supply is increasing day by day. There is a need to explore renewable energy sources to generate electricity as they are cheap, efficient and environment friendly. As these sources of energy are pollution free and environment friendly, they are called as Green Energy Sources. Worldwide renewable energy capacity has grown at the rate of 60% annually [1].

Today, a single digital IC can require 20 to 50 Amps [2-3] and as much as 200 Amps in the future [4]. Even though voltage is dropping, the overall power required (Volts x Amps) is increasing from today's 30 to 50 W to 200 W within the next several years. To reduce power dissipation, it is common to start and stop operation during periods of inactivity [5]. The advantage of the multiphase buck topology is that it is relatively simple and provides excellent transient response, high efficiency, small size, and low cost. The effective switching frequency is multiplied by the number of phases while the load is divided by the number of phases [6]. The multiphase buck converter is commonly powered from a 5V or 12V bus derived from an AC-DC power supply. The trend is towards using 12V to lower the bus current and therefore reduce resistive losses in the Printed Circuit Board (PCB) and connectors.

In this paper, Green Energy Sources are used to provide supply to the DC-DC converter connected to the low voltage powered microprocessor. In the next section, a brief summary about renewable sources are presented. Section III describes about multiphase buck converter. The proposed green energy

system is elaborated in section IV. Current control technique used in proposed system is discussed in section V. The design process is followed in section VI. Results and discussions part is presented in section VII. The last section provides the conclusions.

II. GREEN ENERGY SOURCES FOR HYBRID POWER SYSTEM Various Power Sources i.e. Thermal, Hydro and Nuclear called as Conventional sources accounts for most of the contribution in generation of power. The contribution of various power sources to generation of Electricity is as shown in table 1.

TABLE.1 Contribution of various power plants. Power Plant Electricity Generated

Thermal 64.75 %

Hydroelectric 21.73 %

Nuclear 2.78 %

Renewable Energy Sources 10.73 %

Since the natural fossil energy resources are limited on this planet, we have to put our focus on green power generation like solar and wind power. Hybrid Power Systems (HPS) are the power systems combining different power systems. HPS being combination of more than one energy source, the system needs not have to be dependent on any one system; making the system more efficient and reliable.

A. Solar Power System The output of sun is 2.8×1023 kW. The Solar energy intercepted by earth is about 1.8x1011 MW. The energy reaching the earth is 1.5 ×1018 kWhr/year. While travelling from outer space to earth, solar energy is lost because of scattering, absorption, cloud cover, reflection and climate. Solar energy is a free and readily available source of energy. This energy can be converted into electrical energy by means of solar or PV Cells. Electric current start to flow in the external circuit, when solar radiations are incident on the PV cells, At present time, the popularity of the photovoltaic and wind energy has been increasing since the year 2000 because of people wanting clean energy. As of 2008, world PV installations were at 5.95 GW which is a 110% increase from previous year, 2.826 GW. The leading countries are Germany, Spain, United States and Japan.

B. Wind Power System The wind is identified as a key natural which contributes in reducing undesirable fossil fuel power plant operation. Worldwipower capacity has reached 120GW at the e36% increase in comparison to the previous y Wind turbines are used to convert the electric power. Electric generator inside thethe mechanical power into the electric powsystems are available ranging from 50W to 2 Mechanical output of turbine of widependent on the speed of turbine. For smspeed is about 3.5m/s. The large wind powwind speed of 6m/s but wind speeds higavailable in many locations.

C. Hybrid Power System (HPS) Hybrid electric systems combine wphotovoltaic technologies offering several each other.

III. MULTIPHASE BUCK CONVER

The block diagram of a multiphase buccurrent controlled closed loop is shown proposed topology with soft switching is shMultiphase PWM DC-DC converter is dauxiliary circuit with closed loop control tolosses across the switches.We have useswitching devices. As the power requgeneration microprocessors is 40-100A currand better current transient response, MConverter is the efficient converter to Multiphase topology also benefits in componas power density increases allowing powersize and increase efficiency and performanchigh current buck converters are mostlgraphics and telecom applications. Themultiphase buck topology are such as 1. It ptransient response and 2. It is having highsize and low cost.

Increasing the number of phases reducesin the input and output filters. The effrequency is multiplied by the number of load is divided by number of phases.Witnumber of phases, efficiency can be improve

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IV. PROPOSED

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Soft switching losses are reduced by using soft switching techniques, but some conduction losses remains in switches and other elements. Converter comprises an auxiliary circuit and an control circuit having PID Controllers, SR Flipflops and logical operators.

V. CURRENT MODE CONTROL The multiphase DC-DC converter with current control

circuit consists of an SR flip-flop with an XOR gate connected to feedback path by the PID controller and a relational operator. SR flip-flop with XOR gate acts as driver for the MOSFETS in each phase. Equal current sharing for each phase is obtained by current mode control. In this scheme, PID controller block provides faster transient response and reduced steady state error. Being gate driver, XOR gate, SR flip flop and pulse generator generates PWM signal.

A. Mode of Operation Operations of first phase are described in details as: Prior to t = t0, the auxiliary switch s4 will be turned on while

the main switch s1 is off. At t = t0, the auxiliary switch s7 will be turned on which realizes zero current turned on as it is in series with the resonant inductor Lr. During this stage ilr rises and current iD4 through body diode of switch s1 falls simultaneously at the same rate. Resonance occurs between Lr and Cs1 (charged previously). This mode ends at t = t1 when iLr reaches I0/3 and iD4 becomes zero. The body diode D4 is turned off with ZCS. The diode Ds1 starts conducting at the instant when body diode DS4 is turned off.

At t = t1, is7 = iLr= I0/3. In this interval, capacitor cs1 is forced to conduct through body diode of main switch s1. This mode ends with turned off main switch s1 under ZVS and the main switch s1 will be turned off as per required duty cycle. This operation is repeated for second and third phase.

VI. DESIGN CRITERIA The following design criteria are considered for optimal

solution.

A. Number of Phases : Increase in number of phases reduces the ripple current in

the input and output filters and improves the transient performance but it also increases circuit complexity and cost. The choice is made on how many power MOSFETs are required in order to handle the current in each phase, one should consider increasing number of phases. This requires an additional output inductor but the increased cost and PCB area of the components tends to be offset by reduction in the cost and size of the input filter and output capacitor.

B. Current per Phase: The performance of the MOSFETs determine the optimal

current per phase which today ranges from 10 A-25 A. Designs operating at lower switching frequencies using state of the art MOSFETs, and having low thermal impedance (heat sinks) tend to be in the upper end of this range. Designs targeting the

compact size, maximum efficiency and fast transient mature lower cost MOSFETs tend to be in the lower end of this range.

C. Transient Response: The value of the output inductor and number of phases place a theoretical limit on the ability of the multiphase converter to slew its output current. This is due to the fact that the rate of change of current in an inductor is equal to the voltage placed across it divided by the value of its inductance.

The minimum response time to a load to decrease can be calculated as follows

min ( ) /r MAX MIN OUTT L I I V= − (1)

Minimum response time to a load to increase can be calculated by

min ( ) / ( )r MAX MIN IN OUTT L I I V V= − − (2)

The theoretical minimum response is determined by the slew rate of the each inductor divided by each number of phases. This time is further increased by the response time of the PWM controller and MOSFETs.

The change in output voltage and due to a load step can be calculated as follows

( / / )o o o oV I ESL t ESR T CΔ = Δ Δ + + (3)

where ΔI0 is the change in load current, ESL is equivalent series inductance, Δt is the load current slew rate, ESR is equivalent series resistance. To is the regulator response time and is equal to the equations (1) and (2) plus PWM response time, and Co is the total value of the output capacitors.

D. Equal Current Sharing: If the current sharing is not equal, there will be large ripple

content in the output voltage. We have used control mechanism called current mode control technique to verify equal sharing of current by each phase. Current measurement for each phase is connected to feedback for a constant, same for every phase. The final inductor current will be compared with that constant and if the output inductor current is greater than the constant, then the relational operator will give output as high. When both inputs are one for XOR gate, result of those inputs which corresponds to R of the SR flip-flop will be zero and the input for S will be one. As a result MOSFET will be turned off. The same process is repeated for each and every feedback path. Hence we can limit the output inductor current up to a certain level.

VII. RESULTS AND DISCUSSIONS The multiphase buck converter with closed loop topology is

simulated using PSIM 9.0 Software. The converter delivers a load of 100 W and an output voltage of 1V at constant frequency of 1 MHz under steady state.

The voltages across the main switches S1, S2 and S3 and currents through the same main switches are shown in Fig. 3 (a), (b) and (c) respectively. The output inductor current waveform is shown in Fig. 4. The output load voltage waveform is shown in Fig. 5.

It is observed that the currents through every output inductor are same and having same value. Thus equal current sharing is achieved by current mode control. Reduction of switching losses and improvement of efficiency by the use of ZVS can be clearly seen from the Fig. 6, where efficiencies with ZVS and without ZVS are shown.

(a)

(b)

(c)

Fig. 3 (a) Switching wave forms across the main switch S1 (b) Switching wave forms across the main switch S2 (c) Switching wave forms across the main switch S3

Fig. 4 Output inductor currents of the multiphase topology

Fig. 5 Steady state output voltage across the load

Fig. 6 Graph between efficiency and load current

VIII. CONCLUSIONS The topology suitable for new generation microprocessor

is proposed. The topology is powered by a hybrid combination of PV and wind power system. Use of multiphase converter with control circuit to eliminate switching losses is the added feature. The converter has high efficiency, high power density, and least size due to multiple phases. It is cheaper and is having faster switching frequency of 500 kHz.

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