Solar Compatible Multiple Output DC To DC Converter For Personal Computer System.

Prajakta Shendage, Pranita Sontakke

Department Of Electronics And Telecommunication, Vishwakarma Institute Of Information Technology,

Pune.

Abstract— In a conventional SMPS, five times ac to dc & dc to ac conversions are required, out of these two conversions are from UPS system connected to the SMPS and three in the SMPS circuit. These conversions produce loss up to 5% in each stage. This reduces overall efficiency of system . This project aims to minimize disadvantages of conventional SMPS by use of triple Buck-converters. To minimize multiple conversions and limit to a single DC to DC conversion, we are going to design a single DC to DC convertor that is Solar Compatible Multiple Output Dc To Dc Converter For Personal Computer System. . Here we use solar panel which is DC voltage source providing 48V DC voltage and feeding this to computer motherboard which is as DC load which requires +5V, -5V, +12V, -12V and +3.3V DC. We are using buck topology, to convert fixed dc input voltage which is from solar panel DC source into constant dc output voltage (+5v, +12v, and 3.3v). We are going to use double buck converter , as we get DC input from solar pannel that is 48v, and we step it down it up to +5v, and 3.3v, so for that we are going to use IC LTC 3890-01, which is double buck converter IC, by linear tecnology, and we are going to use single buck converter to get 12v from IC3524D PWM generator output.

I. Introduction Conventional desk top UPS[2] systems store electricity in form of chemical energy in batteries and when ever mains fail the batteries provide DC power which is then converter to ac by inverter and then fed to the computer in order to save critical data. Thus power from mains or from UPS is fed to switched mode power supply (SMPS) unit which converts it to dc to supply the computer motherboard and accessories. Leaving the case of the computer powered by mains, during power failure the dc power is unnecessarily converted to ac and then to dc again in SMPS hence high switching losses and heat is produced there by making the entire system very lower efficient. In this work it is proposed to directly feed the dc power from the battery to the computer SMPS system by building a UPS arrangement within the computer system. This not only saves space, it is highly efficient compared to the conventional ups system, easy to

carry, low cost and reliable since there is no change over operation is required.

Fig 1. Block diagram of a mains operated AC/DC SMPS. .

II. Working of conventional SMPS [1]

Input rectifier

The AC input from mains is first rectified in the SMPS using a rectifier to convert it into DC. The rectifier consisting of a full wave diode bridge or module that produces an unregulated DC voltage to the Smoothing capacitor. The input AC passing into the rectifier has AC voltage pulses that may reduce the power factor. So control techniques are used to force the average input current to follow the sine wave.

Inverter

This stage converts the rectified DC into AC using a power oscillator. The power oscillator has a small output transformer with a few windings at the frequency 20-100 kHz. Switching is controlled by a MOSFET amplifier. The output AC voltage is usually isolated optically from the input AC by using an Optocoupler IC for safety reasons.

application of the desired mean voltage along with ripple voltage of a very high frequency to the output side, consisting of a low pass filter circuit followed by the load. The high frequency ripple in voltage is effectively filtered using small values of filter capacitors and inductors.

Voltage converter

This stage has a high frequency transformer and the inverted AC drives its primary windings. This creates the up and down voltage at the output. If DC is required, the output AC is converted to DC using a rectifier circuit using Silicon diodes or Schottky diodes( fast recovery and minimum loss of current and low forward voltage drop) . The rectified output DC is then filtered using the filter section consisting of inductors and capacitors. Some non isolated SMPS contains an inductor instead of the transformer and the circuit act as boost converter or buck converter. In high voltage SMPS, Capacitor-Diode multiplier is used instead of inductors or transformer.

Output regulator

The output stage always monitors the output voltage by comparing with a reference voltage using a feedback system. For safety reasons, the output stage is isolated by an optoisolator as seen in the SMPS of computers. In some SMPS, Open loop regulation is used without feedback circuit and constant voltage is fed to the transformer input.

The feedback circuit needs power to run before it can generate power, so an additional non-switching power-supply for stand-by is added.

Advantages of SMPS:

1. Light weight since the transformer is too small and it it operates at high frequency of 50Hz-1MHz.

2. Output voltage is well regulated and controlled by duty cycle and there is little resistive loss since the transistor fully on or off during switching.

3. Greater efficiency since the switching transistor dissipates very little heat. The SMPS can fail and can cause very high output voltage that destroys the equipment.

Disadvantages of SMPS:

Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof. Very low cost SMPS may couple electrical switching noise back onto the mains power line, causing interference with A/V

equipment connected to the same phase. Non-power-factor-corrected SMPSs also cause harmonic distortion.

Losses in SMPS: .

A. Power factor:

Simple off-line switched mode power supplies incorporate a simple full-wave rectifier connected to a large energy storing capacitor. Such SMPSs draw current from the AC line in short pulses when the mains instantaneous voltage exceeds the voltage across this capacitor. During the remaining portion of the AC cycle the capacitor provides energy to the power supply. As a result, the input current of such basic switched mode power supplies has high harmonic content and relatively low power factor. This creates extra load on utility lines, increases heating of building wiring, the utility transformers, and standard AC electric motors, and may cause stability problems in some applications such as in emergency generator systems or aircraft generators.

B. Transformer Losses:

Any switched-mode power supply that gets its power from an AC power line (called an "off-line" converter) requires a transformer for galvanic isolation. Some DC-to-DC converters may also include a transformer, although isolation may not be critical in these cases. SMPS transformers run at high frequency. Most of the cost savings (and space savings) in off-line power supplies result from the smaller size of the high frequency transformer compared to the 50/60 Hz transformers formerly used. There are additional design tradeoffs.

The terminal voltage of a transformer is proportional to the product of the core area, magnetic flux, and frequency. By using a much higher frequency, the core area (and so the mass of the core) can be greatly reduced. However, core losses increase at higher frequencies. Cores generally use ferrite material which has a low loss at the high frequencies and high flux densities used.

C. Copper losses:

Switching power supplies must pay more attention to the skin effect because it is a source of power loss. At 500 kHz, the skin depth in copper is about 0.003 inches (0.076 mm) – a dimension smaller than the typical wires used in a power supply. The effective resistance of conductors increases, because current concentrates near the surface of the conductor and the inner portion carries less current than at low frequencies.

The skin effect is exacerbated by the harmonics present in the high speed PWM switching waveforms. The appropriate skin depth is not just the depth at the fundamental, but also the skin depths at the harmonics.

Solution to the Problem: To minimize theses losses of conventional SMPS ,we are going to design Solar Compatible Multiple Output Dc To Dc Converter For Personal Computer System using triple buck converter topology.

III. Design

Project Specifications :-

a. Input Voltage: 48v DC nominal b. Input Variation: 40v – 60v c. Output: +5v , 10A,

-5v, 1A +12v, 8A -12v, 1A +3.3v, 10A d. Switching frequency: 100KHz / 200KHz

IV. Components Of the Schematic

fig 2. Block Diagram

Basically we are going to use LTC3890-01 ic by linear technology which gives us dual output as +5v and +3.3v when we apply 48v input voltage from solar panel. for the negative voltage we use multi winding inductors . Same as for single buck converter we are going to use LM 3524D PWM regulator to get +12v output and multi winding inductor to get -12v output. simply, by using one dual buck converter ic and one single buck converter ic

we are trying to implement triple buck converter to overcome disadvantages of SMPS . Buck converter operation:[4]

The demand for high efficiency DC-DC converters is increasing dramatically, especially for use in battery operated devices such as cellular phones and laptop computers. In these devices, it is intrinsic to extend battery life. By employing DC-DC converter power-saving techniques, power efficiency can be significantly increased, thereby extending battery life . Switching power converters inherently generate ripple, and typically require output filtration to meet ripple and EMI specifications. The buck type switched dc to dc converter is well known in power electronics. Due to the fact that the converter contains two energy storing elements, a coil and a capacitor, smooth dc output voltages and currents with very small current ripple can be generated . Passive LC low-pass filters have been employed to achieve the necessary degree of ripple attenuation. The passive filter components often account for a large portion of converter size, weight.

Fig 3. buck Converter Block Diagram

If the buck converter operates in Continuous Conduction Mode (CCM), the relationship between the input voltage (Vi) and the output voltage ( Vo) is:

D=Vo/Vi (1) where, D is the conducting ratio or duty ratio or duty cycle and , is the switching period and is conducting time of the switch. D= Ton/Ts (2) where Ts is switching Period and Ton is conducting Period Hence Vo/Vi= Ton /Ts=D (3) Since, VL= Ldi/dt during on period the change in inductor current is given as : ∆IL= Vi-Vo/L* Ton (4) During off period , the inductor current is given as: -∆IL= -Vo/L* Toff (5) the switching frequency is expressed as : f=1/T= 1/ (Ton+ Toff) = Vo(Vi-Vo)/ ViL∆IL (6) The peak to peak ripple current can be found as ∆IL = V o(Vi-Vo) / LVif (7) The peak to peak ripple voltage of the capacitor is : ∆Vc= ∆IL/ 8fC (8)

V. Experimentation and Results:

fig 4. Schematic For single Buck Converter (+12v,-12v)[5]

fig 5. Results For single Buck Converter (+12v,-12v):

Input Voltage: 48V Output Voltage: 12.09V Frequency: 100KHz

fig 7. Schematic For Dual Buck Converter (+5v, +3.3v)

VI.

Application :

SYSTEM PROVIDING POWER SUPPLY TO PC MOTHERBOARD

VII. FUTURE SCOPE:

Simply using triple buck converters we can

overcome disadvantages of SMPS, replacing conventional SMPS for personal computer system by this circuit configuration. using triple buck converter topology we can minimize the conversion losses with great efficiency more than 95% for frequency range above 1MHz.

VIII. CONCLUSION: Initial testing results represents that we have

minimized the conversion losses in SMPS to great extend. The project aims to overcome disadvantages of conventional SMPS by use of triple Buck-converters is satisfied.

This project is part of funded project by ISRO UoP

Space Technology Department Cell.

IX. ACKNOWLEDGMENT We would like to take this opportunity to thank everyone

who has been involved with this project. Prof. V.M. Aranake and Prof. R.S Pol (V.I.I.T., Pune)

who have been our guiding lights, who gave us the opportunity to work for ISRO and to encourage & inspire us.

We also take this opportunity to thank Prof. Dr. P. D. Khandekar (H.O.D. E&TC) to provide us with well equipped Lab’s and permission for late working hours.

X. REFERENCES

[1] SWITCHED-MODE POWER SUPPLY[1] FROM WIKIPEDIA.

[2] Computer SMPS hoe stuff works by Electronics Hobby[2]

[3]Ganesh S, Chiranjit Ghosh T, Kokilasree R,

Nandhakumar : “Dc UPS For Critical Loads.: [3]

[4]Power Electronics & Applications- Rashid [4]

[5] LM3524D PWM Generator Datasheet[5]

[6] www.linear.com [6]