Dc Converters And The Power Crisis Engineering Essay

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In real world, power crisis was climbing up-and-up provided that uses of portable electronic device with other devices were being increased. Majority portion of these portable devices operates in DC current. Operating voltage of these DC electronics is starting from 3.3V. So after getting the voltage lower than 3.3 or depending on the applications, the operation would stop and the rest of energy would have lost.

DC/DC converters could be a relief in this case. DC/DC converter is a device that converts one level of DC voltage to another level. This conversion could be of different types, depending on different structure of circuit and their applications. Some generic DC to DC converters were step-up, step-down, and invert. Here concern would be centred to step-up converters and 'Maxim MAX756' [1] would be up to its extent.

Fig 1: MAX756 [Embedded in an application device]

The vision was making the automatic measurement systems independent of power source in near future. So the objectives of this assignment for the device MAX756 was set to followings.

Investigating alternative electric energy source to power a typical real life application

Evaluating a DC/DC converter [experimentally] how this device can aid the technical solution of low power energy problem.

Problem analysis

2.1 Overview

Fig 2: Applications (5v) can't run with Low energy (below threshold)

From figure-2 above, it can be observed that the applications were unable to run with low power input. Below a threshold voltage of the application, the application would turn off. For an example: If the threshold voltage of the application is of 3.3 V and the low power supply would have either of the following situation, the applications would not run.

Below 3.3 V

Normally has 5v but occasionally goes lower than 3.3 V

So one of the long term and viable solution would be using the DC/DC converter like the following approach,. Here the low energy would be boosted up to the required energy of the operating voltage of the application. But this DC/DC has also some limitations. One of them would be the amount of low input range.

Fig 3: Applications (5v) run with energy below even threshold with DC/DC converter

2.2 MAX756

Fig 4: DC/DC converter circuit

The key part of the given circuit is MAX756. And the key part of MAX756 would be switching regulator. Here in this circuit MAX756 acts as boost regulator. In fact the output voltage is directly controlled by the ON time of the switch of switching regulator. How this controls is being explained here.

2.2.1 Switch ON

Switch is ON when pulse from the regulator goes high. At this time most of the current would go through the inductor and storing the energy to the inductor. By this time the charge stored in the capacitor would release and the constant voltage at output remains.

Fig 5: Operations at switching (On) mode [2]

2.2.2 Switch OFF

Switch is OFF when pulse from the regulator is LOW. At this time the energy stored in the inductor would release and would induce a reverse voltage across the inductor. This is known as voltage kick. Declining current through the inductor makes diode as for forward bias. This helps Capacitor charge to a voltage that is higher than the input voltage.

Fig 6: Operations at switching (Off) mode [2]

2.2.3 Constant Output

If the output voltage would try to increase there would have implied a less feedback voltage. This would impact to have a less comparator output, consequently a less duty cycle of the frequency generator and less inductive kick. Less inductive kick would produce less output voltage.

Again when the output voltage would try to decrease there would have implied a higher feedback voltage. This would impact to have a high comparator output, consequently a high duty cycle of the frequency generator and high inductive kick. High inductive kick would produce high output voltage.

As well as MAX756 is operating at 500 KHz, DC output looks very much stable in reality.

2.2.4 Calculations

Min. Duty Cycle (D_min) = 1 - (Vimax/Vomin) [3]

Max. duty cycle (D_max) = 1 - (Vimin/Vomax) [3]

Min. Inductor size (L) > D * Vin * (1-D) / (freq * 2 * Iout ) [3]

Minimum capacitor (Cap) > Iout / (Vripple * freq) [3]

2.3 Applications

2.3.1 Humidity Control

Watering the crop fields, rooms need humidity measuring. Measuring the humidity needs humidity sensors to be used. T. Calculating true relative humidity needs

Supply voltage,

Sensor temperature, and

Output voltage

Fig 7: Humidity measuring sensor

The following formula to calculate the relative humidity (RH) could be [4].

Sensor RH = (VOUT / VSUPPLY) - 0.16 / 0.0062, [For 25°C]

True RH = sensor RH / (1.0546 - 0.00216 T), where T is in °C

True RH = sensor RH / (1.093 - 0.0012 T), where T is in °F

The device normally operates at 5 VDC. But it could operate over a 4-5.8 VDC range. At room temperature, the output voltage ranges from 0.8 to 3.9 VDC as the humidity varies from 0% to 100%.

2.3.2 USB IPOD/mobile charger

IPOD or mobile are two most used portable devices today. These devices can be charged with USB cable though it was known that USB is mostly used for transferring data, like scanning, printing etc. But this UBS in this case would be used as to charge device directly from power source. In this picture among four pins, only two pins needs to be used to get power from the source. This pins are Vcc and GND. The other two D+ and D- for data pins and would be unused.

Fig 8: Practical USB connection (Only power here)

2.3. 3 Child Toy

Child toys are likely to use 1,2,3,4 cell battery sources. A four cell battery powered toy needs 4*1.5=6 V power to get operated. After using a toy, children are likely to keep these batteries connected to this application and the batteries go discharged. So toys need a huge number of batteries for operation. DC to DC converter would be one of the solutions for that type of application.

2.3.4 Motion Detection and Lightening

At night to detect motion near home, people normally use a device with a set of light that flashes after having some movement near. Motion can be detected by various technologies today.

One of the ways could be using pressure sensor. Among other possible ways another one could be using microwave radio energy. Receiver waits any reflected energy to bounce back. When a person moves through the microwave energy it changes the amount of reflected energy and trigger the action of having sensed the movement. Ultrasonic sound waves can also be used to determine the motion by reflected wave [10].

2.3.5 Automatic Door

Shops, library, canteen doors open automatically. This automatic doors needs to detect the presence of pedestrian. After sensing the presence of pedestrian, the door needs to get opened and it could have opened by magnetic induction and rolling simple steeper motor for several rounds.

Fig 9: Pedestrian Detection for automatic door

2.4 Criteria of Choosing Application

Some criterion was considered for choosing the right application. Main concern was decided as low power consumption, another concern was the usability in daily basis. Other one was the high frequency of usability in a particular day so that, though it takes small energy its total energy consumption goes very high for high frequency using. Automation was chosen as a selection criterion because of having fascination on these types of works. And finally whether the application is able to commercially approvable was taken for so.

Table 1:

Comparing and analyzing each criterion for every application it was found 'Automatic door' the best suitable application among discussed.

2.5 Selected Application

Automatic door was then selected as the application. Here to detect the presence of pedestrian it needs to use a sensor like LDR or other types of sensors. In this case it was considered to use LDR for low costs. Light source could be placed at the transmitting end Tx as in the fig. LDR can be used to detect whether the light source has been cut or not. If light source would cut, it would realize that there is someone is passing / coming towards door. Fig 10: LDR sensor system

2.5.1 Assumptions for Application

Here some assumptions could be assimilated,

As a controller, it could be used PIC18f2520 [5].

Power requirement considering for the controller and for LDR (sensor), and light source only.

Power would be provided only for the MCLR PIN [5].

LDR would take the power from pins RC0.

Light would consume power from RC1.

2.5.2 Operational Description

PIC18f2520 as a microcontroller has got 3 PORTS [A, B and C]. Each of the PORT has 8 pins. So there are 24 operational pins in total. Some of them could be used as input as well as output depending on how pins were configured. Here 0 for 'OUTPUT' and 1 for 'INPUT'. Pins can take digital as well as the analogue input also.

Fig 11: PIC18f2520 uses C port for I/O.

If 3.3V or greater could be applied to any operational pins it would be in high state. Comparing the high or low state by checking the particular pin, it could be determined whether in the passage there was any pedestrian or not.

2.5.3 Input Output Selection

As long as it was used PORT C in this case, the configuration bit for selecting the pins would be 0b00000000(BIN) or 0x00(HEX) which means all the pins of port C is being used as output. It could be 0b11111111 (BIN) or 0xFF (HEX) which would mean all the pins are input.

So as pins C0 and C1 are being used as output for feeding LDR and the light, and output from LDR as input goes to the third pin of C2, the configuration should like 0b00000100(BIN) / 0x04(HEX) or 0b11111100 (BIN) / 0xF4 (HEX). Here pins RC-4 to RC7 were considered don't care in this case.

Table 2:

So by declaring PORT C=0xF4 or PORT C=0x04 it could be configured the input and output. Here RC3 is being as output and its function is to give a high pulse of 1 or low pulse by which the door would be opened or not. 1 in pin CR3 will open door otherwise not.

2.5.4 Voltage Requirement

As controller is the vital part of this system the electrical characteristics of it was taken into account.

Voltage range of MCLR (X) = 4.2 V< X >=5.5 V

Operating voltage range (X) = 2 V< X >=5.5 V

Fig 12: [5].

2. 5.5 Current Requirement

Current requirement for PIC18f2520 was excavated from datasheet, where current range was presented for numerous range of frequency. These ranges were compiled in the form of table structure for different frequency range. The Vdd in the following table is input voltage.

Table 3: Compiled from Datasheet for 4 MHz [5].

Table 4: Compiled from Datasheet for 1 MHz [5].

Table 5: Compiled from Datasheet for 31 kHz [5].

2. 5.6 Power Requirement

So if the system was designed to be operated at 4 MHz, the power calculation could be like the following table.

Table 6: Compiled from required voltage of operation and Current at 4 MHz

So at temperature of 25 degrees, if the input voltage was set to 3V in a particular moment, the maximum power would then be required as 3*2.7=8.1 mW. If the current is greater than 80mA, and the voltage at MCLR goes below the Vss, it could cause latch up, that's why it was recommended to use a resistor of amount 50-100 Ohm in series of MCLR and source.

Problem solutions

3.1 Analyze source of power

3.1.1 Solar Cell

Solar cells are photovoltaic cells made from germanium, silicon. They can produce electricity directly from solar light. Upon heating the light in form of photon, silicon/ germanium atoms release the free electrons cause generating electricity. A solar cell provides direct current (DC) [6].

So these types of cell can be used for generating electricity while it is

Abundant in nature

Environmentally friendly

Low/no maintenance cost

But this source has some limitations too for practical considerations. But these are not that major considering its overall impactions.

No availability of power at night

Dependent on the weather conditions

Initial cost

As it needs large space for installations depends on the applications.

Location of solar panel could affect the efficiency.

Not a good choice for portable device where it needs huge power (Still now).

3.1.1.1 Maximum Cell Power

If a solar cell of radius is 5 cm and if it is made of silicon then the maximum power from a solar cell would be:

= Ns*Nt *PI*r2

= 0.25*0.85*1000*3.142*25*10^-4

=1.67 W

If each cells are of about 0.45 V, then current would be of 3.74 ampere.

3.1.1.2 Higher Current

Connecting cells in parallel would give the higher. Possible connection of this type of solar cell would be in fig--.

Fig 13: Parallel Connection of solar Cell

3.1.1.3 Higher Voltage

Connecting cells in series would produce the higher voltage. Possible connection of this type of solar cell would be in fig.

Fig 14: Series Connection of Cells

3.1.2 Bicycle Dynamo

According to the faradays law when a magnet is in motion and if it is constantly cutting the magnetic flux, their current will produce [7].

Fig 15: Bicycle Dynamo

A dynamo can either be A.C or D.C according to the brushes and number of commutators used. The faster the rider rides the faster the change in magnetic flux and hence the more power. There are three main kinds of bicycle generators. Normally a bottle dynamo can produce about 3W of power, 6Volt and 0.5 amps. This power was enough to light the path while bicycle riding.

Hub Generators,

Sidewall Generators, and

Drum Generators

3.1.3 Knee Brace Generator

While walking human muscles break/slow down knees and energy is absorbed. The new knee-brace generator helps the muscles do this and then "harvests" Regenerative brakes collect the kinetic energy. This knee brace harvests the energy lost when a human breaks the knee after swinging the leg forward to take a step [8].

Using a series of gears, the knee brace assists the hamstring in slowing the body just before the foot hits the ground, whilst simultaneously generating electricity. Sensors on the device switch the generator off for the remainder of each step. In this way, the device puts less strain on the wearer than if it was constantly producing energy [9]. Fig 16: Biometric energy Harvester [9]

Tests of the 1.6kg device produced an average of 5 watts of electricity from a slow walk and explored ways of generating more electricity and found that we can get as much as 13 watts from walking.

3.1.4 Piezo Electric Generator

One of the most promising techniques of mechanical power harvesting is 'Piezo Electric components where deformation is converted to electrical charge via direct piezo electric effect [10]. Some materials generate an electric charge when placed under mechanical stress. Voltage created by an applied stress is called piezoelectricity. Piezoelectric materials produce a voltage in response to an applied force, usually a uni-axial compressive force.

These materials are usually ceramics with a perovskite structure. The perovskite structure exists in two crystallographic forms

Tetragonal structure and

Cubic structure.

In the tetragonal state, each unit cell has an electric dipole, i.e. there is a small charge differential between each end of the unit cell. A mechanical deformation can decrease the

separation between the cations and anions which produces an internal field or voltage. The relationships between a force applied to a piezoelectric ceramic element and the electric field and charge produced by the element are:  E = - (g33T), Q = - (d33F)

Where

E: electric field,

g33: piezoelectric voltage constant

T: stress on ceramic element

Q: generated charge

d33: piezoelectric charge constant Fig 17: Bending Generator[10]

F: applied force

One of the studies [11] has shown that with a resonance frequency of 291 Hz a piezo cantilever can produce 96 microwatt power. Notable phenomenon is that the higher the frequency the higher the power. Most curial part here is vibrator generation. This can be solved by using spring downside of the crystal plate and a considerable mass upon the crystal.

Table 7: Data [11]

Fig 18: Voltage generation Vs Frequency [11]

3.1.5 Energy from Vibration

Magnetic field converts the mechanical energy to electrical energy. When pressure is applied coil oscillates through the magnetic field created by the permanent magnet. Passing through the flux a small voltage is induced in the coil. Then this voltage can be used for the low power applications. Increasing the number of turns of the coil and the powerful magnet, power can be increased.

Fig 19: Vibration Generator [12]

From Faraday's law of magnetism it was found that the voltage of generated electricity is

V= dΦ/dt; [13]

B= magnetic flux density in teslas (T)

Φ= magnetic flux in Webbers (Wb)

A= Area in square meters (m2)

3.2 Criteria of Choosing Source

The criteria of choosing the source was taken into account for the following factors.

Whether the source in environmentally independent or not .Like, solar energy is not possible to harvest at night time, at cloudy sky. Bicycle dynamo is not too much affected by the external environment other than passing through a strong magnetic field.

Whether the source has time limitation. Application can't be operated whole day if it was used as the solar power.

Whether the source has space limitation for its implementation.

Whether the source produces electricity automatically?

Whether the source is more or less fits with the practical use of the application chosen.

Whether the application produces the considerable amount of energy for the application

The summery of the criteria could be represented by the comparison table 3. Here it was chosen by the highest number of source owner. Piezo electric Generator and Bicycle dynamo had the same weight of 5. However Piezo electric Generator was considered for sensor powering of an automatic door as because the person's pressure would create the energy itself so the true automation comes.

Table 8: Criteria analysis of choosing sources

Problem Implementations

4.1 Prescribed Power Solution

Fig 20: Prescribed simple power solution

4.1.1 Power Required for Application from Converter

Here application was considered to be operated at 4 MHz and operating temperature was considered from -40 degree Celsius to 85 degree Celsius. So application would have required having followings energy ratings.

Current: 2.5 mA - 5 mA

Voltage: 4.2 V to 5 V

Power : 10.5mW to 25 mW

So this specification would have to be generated from DC/DC converter.

4.1.2 Power Required for Converter

Among all the experiment data, it was found when DC/DC input voltage is 1.25 V and the current is 29 mA and as a load of application would use 1K ohm, then the application would viable to be in running mode. Other two close specifications were found that is showing in the following table. In this case the efficiency of DC/DC converter was found to be happen 53.65%.

Table 9:

4.1.3 Power Produced by Source

So power source would need to provide 1.25 V and 29 mA of current. But the specified source was found to be given highest of 287µA of current with various voltage range. This current could be produced only when the resonance frequency would be of 569 Hz and the size of the cantilever was of 11*25.4 mm2. Here problem arises how to generate this frequency of almost 0.57 kHz of frequency. Human step can't easily produce this type of frequency. That's why the specific type of spring would need to be used to generate the frequency for the cantilever in a suitable mechanical way.

Fig 21: Cantilever producing 1.25 V with 550-569 sHz

With this configuration it would need to use [29 * 1000 µA / 287 µA= 101.04] 101 of these power points at least in the pathway to enter the gate. If it takes a distance of 1.5 feet between two steps of a human the path way would need to be at least of 1.5*101 feet= 151.5 feet=50.5 yards long.

4.2 Use of Super capacitor

In this case as because the energy from the footsteps would sum up to get the sensor activated, a storage system would have needed to store the electricity. As a storage device super capacitor was considered because it would not need to change time to time like as batteries.

4.3 Power Management

It was not straight forward to achieve the required current in the application, as because the required current is far more than that was found from a single power generator point. So it

Fig 22: Block diagram for power arrangement

was considered parallel connection of each single power generator points showing in the power arrangement diagram. This arrangement boosts the current by taking current from each of the power points and stores in a super capacitor. This super capacitor is connected to a relay system that after storing a particular amount of energy, it would get contacted and the discharging of the super capacitor would have started.

Fig 23: Block Diagram for Sensor System (Power)

When pedestrian would be near the sensor by pressing the power points, the required amount of power should be generated by the system and would be stored in super capacitor. And then relay would be connected after getting particular power, letting the super capacitor discharge. Then Sensor would be activated. Light would get the power to fall over the sensor. If the light gets distracted by the pedestrian while his crossing the sensor point and the signal

4.4 Prototype

4.4.1 Test Method

The only way to evaluate whether the converter could produce the expected outcome was using the DC power supply as power input. It was used various loads including system requirement of 1k ohm. Input current was measured by series connection with the millimetre and the circuit. The other load and input was varied to check the stability of the required voltage output from the converter. At result section it has been presented.

Results

As application needs 2.5 mA to 5.3 mA and 4.2 V to 5 V for its operation as was stated in datasheet of the main controller, it was tested with various ranges of loads and the inputs which can arise at real situation to produce the output for the application. Some relevant output combinations [table -9].

Iout=4.41 mA, Vout 4.41

Iout=5.36 mA, Vout 4.29

5.1 Iout=4.41 mA, Vout 4.41

This situation is desirable for the application noted earlier. It was found with the input voltage of 1.25V and input current of 28.9, almost 29 mA. Here load was used with1 kOhm. Hence output current would be 4.41 mA. It's a theoretical operation mode of application with temperature of -43 degree to 83 degree of Celsius.

Fig 24: Iout=4.41 mA, Vout 4.41

5.2 Iout=5.36 mA, Vout 4.59

This was also a desirable condition for the application having slight current range higher from Iout of 5.3. Here it was used with 800 Ohm [table -9]. Unfortunately this evidence was not captured in a moment that could be presented here.

5.3 Probable Practical Situations

In practical sense, frequency would be variable generating from spring below the cantilever; hence the voltage would be varied from source so input voltage for the converter could be changed. One of the same situation was found from the data while it was experimenting was with 1.5 input volt direct from the DC to DC input power.

Table 10: Situation having 1.5 Vin

It shows that, with a high frequency if the voltage from the source goes higher than 1.25 V say 1.5, the output from source really exceeds the expected range of our application generated by the controller in this its 5.84.

Other two satiations were experimented. With the source output of 1.43V and with some varying current, though current variation not likely to be happened. It shows that thought the current level is bit close to the expected 5.82 and 6.47 [2.5 mA to 5.3 mA] but the voltage exceeds the higher range showing in the following table.

Table 11: Situation having 1.43 Vin with various load

Discussion of Results

Among various range it was examined [Appendix], the practicable solutions were compiled in an excel table below. It was found only single experiment that shows in tick mark that could solve the total package for real world. As any other disruption was intolerable for the application and the source, because of having limitations in producing power from source and the limitation of producing power for the application by the converter also.

Table 12:

So in real sense, this one is very limited, that doesn't full fill the practical consequences, because source would produce variable range of input power that doesn't match for the application. It needs to be constant that is not practical for the source.

This implies that, to make the system viable for real word, it would need to have some modification to make this converter applicable for the prescribed application with the source mentioned. Table 13: Expected Values

To stabilize the application the new design would be like the following table.

Table 14: Converter Property Values (evaluated by excel)

Conclusion

The successful experiments on MAX657 were made to evaluate the implacability of the device for a sensor based application to make the sensor based application independent of power source. Experiment reveals the limited applicability in such application with the configuration of the circuit.

But by reconfiguring the equipments [inductor, capacitor and Schottky diode- not calculated here though] would make the circuit applicable to that type of application. That is how world can save a huge amount of power needs for sensors in near future.

Future Work

In this experiment the data was experimented randomly, having no particular dimension or view. While it was known the view, why and which data should be experimented, it was late. But by now to make this application viable just needs few other experiments to check whether the application is really suits to the desired level.

If the experiment results fail to prove this device viable for wide range, it would need to remake the design to try to help the world to save huge power.

References

Technische Universität Graz Institut für Elektronik. (25.01.2010). MaXIM. Available: http://www.ife.tugraz.at/datashts/Maxim/MAX756.pdf. Last accessed 25 March 2010.

National Semiconductor Corporation. (14.01.2010).Switching Regulators. Available: www.national.com/appinfo/power/files/f5.pdf. Last accessed 26 March 2010.

Limor. ( 8 April 2010). DC/DC Boost calc. Available: http://www.ladyada.net/library/diyboostcalc.html. Last accessed 15 April 2010.

Questex Media Group LLC. (2010). A 1-Wire Humidity Sensor. Available: http://www.sensorsmag.com/sensors/humidity-moisture/a-1-wire-humidity-sensor-1080. Last accessed 15 April 2010.

Microchip. (10/05/07). AN1151. Available: http://ww1.microchip.com/downloads/en/AppNotes/01151a.pdf. Last accessed 25 March 2010.

ICRA. (2005). What are Solar Cells?. Available: http://www.makeitsolar.com/solar-energy-information/07-solar-cells.htm. Last accessed 29 March 2010

Nordic Group. (2008). Dynamo Powered Lights. Available: http://www.nordicgroup.us/s78/dynamo.html. Last accessed 15 April 2010.

Roger Highfield. (07 Feb 2008). Knee-brace gizmo to generate electricity. Available: http://www.telegraph.co.uk/science/science-news/3324635/Knee-brace-gizmo-to-generate-electricity.html. Last accessed 15 April 2010

Jonathan Fildes . (7 February 2008). Knee dynamo taps people power. Available: 1. http://news.bbc.co.uk/1/hi/7226968.stm. Last accessed 15 April 2010

PIEZO SYSTEMS, INC.. (2005). INTRODUCTION TO PIEZO TRANSDUCERS. Available: http://www.piezo.com/tech2intropiezotrans.html. Last accessed 29 March 2010.

Mikko Leinonen. (2002). Piezo Electric Energy Harvesting For powering Low Power Electronics. Available: http://nortech.oulu.fi/EnePro/Proceedings/Leinonen_pp105-109.pdf. Last accessed 29 March 2010.

Gorgia Tech. (2005). Harvesting Ambiant energy. Available: http://users.ece.gatech.edu/~rincon/publicat/trade_jrnls/ee_times_0805_harvest.pdf. Last accessed 25 March 2010.

ThinkQuest . (1996). Induced Electromotive Force. Available: http://library.thinkquest.org/16600/advanced/inducedemf.shtml. Last accessed 25 March 2010

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