Abstract--This paper is to investigate the possibility of

providing a smooth DC power supply transition for electronic

devices. The transition is meant from power outlet as the main

supply to battery as a backup supply and vice versa. The

important requirements during the power-source transition

process are to provide supply that is constant, reliable and free

from fluctuations. The power supply smooth transition is

important especially in electronic devices that utilizing

microcontroller and microprocessor. Unstable power transition

might create power transients and hence leads to unsynchronized

between the processor and other components. This paper

explains a circuit design with low-cost and effective method for

monitoring and maintaining power supply transition. The

purpose of this circuit is to check continuously the level of main

supply to a load with a backup supply that will turn on whenever

the main supply drops below specified minimum voltage level.

When the main supply has reached above the specific minimum

level, the control circuit will shut the backup supply and resumes

the main supply to the load. The design of this circuit shows that

the transition process runs smoothly and eliminates the

transients. Although the transition process shows a slight drop in

term of voltage supply, the effect does not give significant

problem to the target system. It is concluded that the prototype

design of this circuit is acceptable and meets the objective of the

design.

Index Terms-- monitoring power supply transition, stable

supply, backup power supply, power transient.

I. INTRODUCTION

LECTRONIC system which is related to ac power line

disturbances can increase the rate of system maintenances.

Therefore, the end user is needed a back up power supply that

can deliver power continuously, regardless of disturbances

that may occur such as outages, brownouts, sags, swells,

blackouts and other electrical glitches [1]. With the increasing

need to provide complete power loss protection, the market for

external power backup has expanded rapidly, particularly for

low power workstations and file servers [2]. The common

battery backup is the uninterruptible power supplies (UPS),

which helps the electronic device to ride through those periods

when the power goes off for a few minutes during

undetermined circumstances. Although the UPS has been

Rahmat Sanudin, Mohd Zainizan Sahdan and Siti Hawa Ruslan are with

Faculty of Electrical and Electronic Engineering, Universiti Tun Hussein Onn

Malaysia (e-mail: rahmats@uthm.edu.my).

CT Salwanee Bahayakhi is a graduate student in Faculty of Technical

Education, Universiti Tun Hussein Onn Malaysia.

widely known and used, the UPS system have some

disadvantages such as less backup time, regular battery

replacement and high tag price. The objective of this paper is

to design a circuit that is capable of providing DC backup

supply in order to reduce the effect of disturbances from

interrupted power supply. This paper will discuss the theory,

design work and construction of backup power supplies. The

first part gives an overview of theory in backup power supply

followed by the construction of low-cost backup power supply

and finally the analysis and result obtained.

II. DC POWER SUPPLY

A. Power Supply Conversion

The power supply converts the alternating current (AC) line

to the direct current (DC) needed by an electronic devices. It is

designed to convert 110V or 230V AC power from the main

supply to lower DC voltage for the electronic components of

the device [3]. As shown in Fig. 1, the first circuitry inside a

power supply consists of voltage reduction module that

converts the high 110 or 240 VAC input to a lower voltage. It

protects the rest of the circuitry from voltage surges on the

input and cleanly shut off the power supply outputs if voltage

drops too low.

Fig. 1: Block diagram of an electronic device power supply

The voltage regulation circuit monitors the DC output

levels and adjusts the amount of current flowing to the output

of transformer to keep the DC output level remains constant.

There should be one voltage regulator for each output voltage.

The voltage conversion and output circuit converts 50 Hz or

60 Hz AC to 30 kHz AC through an oscillating dual transistor

Low-cost Circuit for Implementing Smooth

Power Supply Transition

Rahmat Sanudin, CT Salwanee Bahayakhi, Mohd Zainizan Sahdan and Siti Hawa Ruslan

E

The 5th

Student Conference on Research and Development –SCOReD 2007 11-12 December 2007, Malaysia

1-4244-1470-9/07/$25.00 ©2007 IEEE.

that acts as a “switcher” and transformer coils. It then converts

the AC to DC. Finally, it filters the entire AC ripple out of the

DC voltage to make it perfectly smooth, flat DC. The DC

output circuit should monitor the output and shut off the

voltage if the outputs get accidentally shorted.

B. DC-DC Converter Circuit

A range of dc – dc switch mode converters are used to

convert an unregulated dc input to a regulated dc output at a

required voltage level. They achieve the voltage regulation by

varying the on – off or time duty ratio of the switching

element. There are two main applications of dc – dc converter:

one is to provide a dc power supply with adjustable output

voltage that requires an isolating transformer and the other is

to transfer power from a fixed dc supply [4]. Basic types of dc

– dc converter are:

(a) Step down an unregulated dc input voltage to produce a

regulated dc output voltage using a circuit known as buck

converter or step-down switch mode power supply (SMPS).

(b) Step up an unregulated dc input voltage to produce a

regulated dc output voltage using a circuit known as boost

converter or step-up SMPS.

(c) Step up or step down an unregulated dc input voltage to

produce a regulated dc output voltage.

(d) Produce multiple dc outputs using a circuit such as the

fly-back converter.

(e) To provide an SMPS with backup power features the

circuit integrates with a fly-back converter and a buck

converter. It can accept a high voltage of main power input

and a low voltage of backup battery input [5].

C. Disturbances in Power Supply

One of common electrical power disturbances is transient.

A transient is a voltage surge of short duration that exceeds the

nominal voltage by more than 10%. It is commonly referred to

as a “spike” or “glitch”. Transient could be harmful either to

electronic devices such as computer hardware or to sensitive

stored data. The effect of transients is substantial and should

be avoided at all times on the DC output supply [6]. Transients

on the DC output are usually caused by surges in the AC

power supply. A defective power supply can cause transients

too, but it is rare compared to AC transients. An AC transient

occurs when the power distribution switches the power off and

on, or by a motorized appliance (air conditioner, refrigerator,

air compressor) near the computer switching that kept

switching on or off.

D. Computer Glitches

As mentioned previously, power disturbances could

damage sensitive electronic equipments especially the

computer. One of serious damage in computer system is

known as computer glitch. A computer glitch is the failure of

a system, usually containing a computing device, to complete

its functions or to perform them properly. It frequently refers

to an error which is not detected at the time it occurs but

shows up later in data errors or incorrect human decisions.

While the fault is usually attributed to the computer hardware,

this is often not the case since hardware failures are rarely

undetected.

III. METHODOLOGY

A. Circuit Description

The main objective of the circuit design is to check

continuously the level of main supply to a digital system

circuit that ranges from 0 – 15V. As shown in Fig. 2, the

switching circuit comprises a comparator IC, MAX 931,

which monitors the main supply voltage. When the main

voltage drops below the minimum specified level, it triggers

DC backup supply to turn on and supply the load. The control

circuit waits until the main supply increases above the

maximum specified level in order to shut the backup supply

and resumes the main supply to the load. A 9V battery is used

as the backup supply when the main supply drops below the

minimum specified voltage level. Components D1, C1 and R6

are used to introduce a delay in the gate drive which

eliminates a supply rail glitch that would otherwise occur

when switching from the battery to the main supply. Such

glitches can cause an unacceptable reset in the system’s

microcontroller.

Fig. 2: Circuit layout for backup supply circuit

Fig. 3 shows the flow chart on the circuit operation in

switching between main supply and the backup supply. It

starts with monitoring the voltage level of main supply and

compare with the minimum voltage level. If the power from

main supply is below the minimum specified level, then the

main supply is shut and the backup supply will supply the

load. Otherwise, the main supply will continue the load. When

the backup supply, the circuit will monitor whether the main

supply has surpassed the minimum level and if it so, then the

main supply will take over from backup supply to supply the

load.

The voltage comparison is done using hysteresis function

of IC MAX 931. Hysteresis increases the comparators noise

margin by increasing the upper threshold and decreasing the

lower threshold. To add hysteresis to the MAX931 connect

resistor R1 between REF and HYST and connect resistor R2

between HYST and V-. If no hysteresis required connect

HYST to REF. When hysteresis added, the upper threshold

increases by the same amount that the lower threshold

decreases. The hysteresis band (the difference between upper

and lower threshold VHB) is approximately equal to twice the

voltage between REF and HYST. The HYST input can be

adjusted to a maximum voltage of REF and minimum voltage

(REF -50mV). The maximum difference between REF and

HYST (50mV) will therefore produce a 100 mV max

hysteresis band.

Fig.3: Flow chart of circuit operation

B. IC MAX 931

The main component inside IC MAX931 is comparator.

The comparator accepts input of linear voltage and provides a

digital output. The output is a digital signal that stays at a high

voltage level when the noninverting (+) input is greater than

the voltage at the inverting (-) input and switches to a lower

voltage level when the noninverting input voltage goes below

the inverting input voltage [12].

For the typical connection with one input connected to a

reference voltage, the other connected to the input signal

voltage. When the input (pin 4) above the reference voltage

(pin 3), the output remains at low voltage level. But, when the

input drops below the reference voltage, the output will

quickly switch to a high voltage level.

The output stage of comparator eliminates crowbar

glitches during output transitions. This makes them immune to

parasitic feedback which causes instability and provides

excellent performance [13].

C. IC IRF 7805

This device is an n – channel MOSFET transistor which is

a voltage controlled device. The main current flow is

controlled by an electrostatic field generated by the voltage

applied between the gate or source terminals. The main

current is turned on and off by the level of voltage on the gate.

During normal operation, the comparator output is low, the n-

channel FET is off, and battery negative terminal floats. Power

flows from the main supply to the load. When the comparator

output goes high, the n– channel FET turns on and grounding

the negative terminal of the battery. Power then flows from

battery to load. The IRF 7805 SO8 can reduce conduction and

switching losses that make them ideal for high efficiency DC

– DC converter that power the latest generation of mobile

microprocessors. Through this converter circuit, it converts a

source of DC from one voltage to another [14].

IV. RESULT AND ANALYSIS

This part is to discuss the results and the analysis obtained

from the circuit base on waveform measured at input voltage,

output voltage (Rload) and voltage at nFET gate. The voltages

are measured during low voltage and high voltage to see the

ability of the circuit switch from AC to DC or vice versa. The

voltage transition needs to be determined first with analysis on

the design.

A. Voltage Transition

It is important to validate the voltage transition of the

circuit for determine the circuit condition either it is high or

low supply. Based on Fig. 4 and Fig. 5, the circuit is in battery

supply mode support when main supply is below than 9.6V. In

this condition, nFET voltage gate is at high level. Meanwhile,

when voltage exceed over 10.6V main supply take over the

circuit with nFET gate voltage in low level.

Fig. 4: Capture of main supply voltage level

Fig 5: Capture of gate level during operation

B. Switching Condition

Switching conditions are shown in figure below prove the

ability of the circuit switch in changing the main supply and

battery backup supply mode. From Fig. 6, measurement is

taken from high voltage level (15V) to low voltage level (0V)

and back to high level voltage (15V).

Fig. 6: Capture of supply switching during operation

Fig. 7 shows when the circuit was at main supply, the

voltage at Rload is 15V, which was similar to input voltage. But

when the circuit was supplied from battery, voltage at Rload

was constant at 8.73V even the supply had dropped until 0V.

It means the lowest voltage on the circuit is 8.73V and the

circuit was able to operate even main supply was dropped.

Fig. 7: Capture of Rload voltage level

Fig. 8 illustrates the digital system circuit received at least

8.73V from this circuit to maintain digital system output 5V

(high) when main supply was dropped. It explains each

voltage regulator needed input voltage at least 2V higher than

output voltage.

Fig. 8: Output voltage supplied to the load

In Fig. 9, the input voltage was changed repeatedly to show

the circuit ability in changing supply mode. However, setting

at the waveform edge on input voltage and output voltage is

not descending and ascending linearly because of DC power

supply for input voltage was controlled manually. Fig. 10

shows voltage at Rload and voltage at nFET gate change

parallel with input voltage. Meanwhile, Fig. 11 shows that

even main supply had dropped; it still produced 5V output

voltage as long as it is supported by battery backup circuit.

Fig. 9: Capture of changing input voltage

Fig. 10: Capture of Rload and nFET voltage

Fig. 11: Output voltage at load

Table I summarises the voltage level of the comparator IC,

nFET, Rload and output during the circuit operation. It shows

that the output produced maintains at 5V irrespective of main

supply voltage level. Thus, it is proven that the circuit is able

to maintain the DC output voltage even though the main

supply level has dropped below the minimum specified level.

TABLE I

VOLTAGE LEVEL AT IC COMPARATOR, NFET, RLOAD AND OUTPUT

VOLTAGE

Main

Supply

(V)

Comparator

IC MAX931

(V)

N channel

switching

(V)

Battery

Output

Voltage

(V) at

Rload

Output

Voltage at

Voltage

Regulator

(V)

5 5.02 4.48 On 8.74 5

9 5.02 4.48 On 8.75 5

10 0 0 Off 9.60 5

15 0 0 Off 14.63 5

10 0 0 Off 9.60 5

9 5.02 4.48 On 8.75 5

5 5.02 4.48 On 7.54 5

V. CONCLUSION

Based on the result obtained, it is concluded that the

objective to design a DC backup power supply has been

achieved. All major components in the circuit are properly

functioning. It is shown that the circuit is capable to make

supply switching in the range of 9.6V to 10.6V. However, the

capability of this circuit to eliminate glitch in electronic device

is difficult to notice. It is believed that the introduction of

delay in gate drive could possible to overcome the problem.

VI. REFERENCES

[1] W. E. Kazibwe, R. J. Ringlee, G. W. Woodzell and H. M. Sendaula, "

Power Quality: A review," IEEE Computer Application in Power, vol. 3,

pp. 39-42, Apr. 1990.

[2] R.A. Priegnitz, “Comparing Battery Back Up Units.” Applied Power

Electronics Conference and Exposition Proc 1991.,pp. 480 – 483.

[3] National Semiconductor Application Note. “Introduction to Power

Supplies”, 2002.

[4] W. Shepherd and L. Zhang. “Power Converter Circuits” New York,

USA: Marcel Dekker Inc. pp. 445 – 447, 2004.

[5] D. K. W. Cheng and F.H. Leung, “Design Of a Switching Mode Power

Supply with UPS features” 1995 IEE Region 10 International

Conference on Microelectronics and VLSI,. pp. .444 – 447.

[6] A. C. Bierbaum, (1988). “Monitoring and Investigating Power

Disturbances Problems” 32nd Annual Conference of Rural Electric

Power Conference 1988..B1-1 –B1-6

[7] K. Saito, T. Shodai, K. Yamashita, and H. Wakaki, (). “High

Perfomance Backup Supply System” INTELEC”03, Oct,19. 23. pp. 261

– 267, 2003

[8] Power Quality for Business’. [Online]. Available:

http://www.aps.com/aps_services.

[9] D..P. Tryling,.“Uninterruptible power supplies broaden their industrial

appeal” Electrical Apparatus Magazine, July 2003

[10] M. S Racine,. J. D. Parham and Rashid M.H. “An Overview of

Uninterruptible Power Supplies.” 2005 Power Symposium Proceedings

of the 37th Annual North American. pp. 159 – 163

[11] M.E. Baran, J. Maclaga, A.W. Kelley, and K. Craven, (1997). “Effects

of Power Disturbances on Computer Systems” IEEE Transaction on

Power Delivery. Vol 13. pp. 1309 – 1315. 1998.

[12] R. l. Boylestad, and L. Nashelky, Electronic Devices and Theory

Circuit. 9th. ed. Upper Saddle River, N.J: Pearson Prentice Hall. 2006,

pp. 702 – 705

[13] MAX 931 – MAX 934 Data Sheet. Maxim Integrated Product Inc. 1997.

[14] IRF 7805 Data Sheet. International Rectifier. 2003.

[15] K. C., Wu, “Switch Mode Power Converter – Design and Analysis”

London, U.K: Elsevier Academic Press. 2006, pp. 249 – 348

[16] I. M. Gottlieb, “Power Supplies, Switching Regulators, Inverters and

Converter.” 1st. ed. Blue Ridge Summit, P.A: Tab Book Inc. 299 – 329.

1984.

VII. BIOGRAPHIES

Rahmat Sanudin is a staff in Department of

Electronic Engineering, Faculty of Electrical and

Electronic Engineering, Universiti Tun Hussein Onn

Malaysia. He graduated from Unversiti Tenaga

Nasional in 2001 and later received M.Eng

(Electrical) from Universiti Teknologi Malaysia in

2005. Upon joining UTHM, he was an R & D

engineer in JVC Electronics (M) Sdn. Bhd. for two

years.

CT Salwanee Bahayakhi is a graduate student in Faculty of Technical

Education, Universiti Tun Hussein Onn Malaysia. She received B. Eng.

(Electrical) from Universiti Tun Hussein Onn Malaysia in 2007. Her major

interest in the field of medical electronic related to diagnosis system.

Mohd Zainizan Sahdan is a staff in Department of

Electronic Engineering, Faculty of Electrical and

Electronic Engineering, Universiti Tun Hussein Onn

Malaysia. He received B. Eng Electrical (Computer

Eng) and M. Eng. Electrical (Microelectronics) from

Kolej Universiti Teknologi Tun Hussein Onn. His

research interest is in IC fabrication technology. He

has joined UTHM upon graduated in 2004.

Siti Hawa Ruslan is an Associate Professor and

Head of Department of Electronic Engineering,

Faculty of Electrical and Electronic Engineering,

Universiti Tun Hussein Onn Malaysia. She received

B. Sc. Electrical Eng from Univ. of Miami, Florida,

USA and M. Eng. (Electrical) from Universiti

Teknologi Malaysia. Her research interest is in IC

design. Upon joining UTHM, she was a staff of

Faculty of Electrical Engineering in Universiti

Teknologi Malaysia.