Switched-Mode Power Supplies
Introduction:
Despite being introduced in the early 1960's, active development in Switched Mode Power Supply
(SMPS) technology continues to be an area of advanced research to this day. Most of these research
efforts have been geared towards the development of battery-operated devices, which have been a major
driving force in the development of SMPS for the past three decades [1]. Advances in this technology
have also been associated with the search for renewable alternatives to energy sources based on fossil
fuels [2]. This paper reviews the current state of SMPS technology, including commercial applications
and areas of ongoing research.
Commercial Applications of SMPS
SMPS, in the form of DC to DC converters, are a common presence in the field of portable electronics.
These converters are responsible for regulating the output battery voltage to levels suitable for the
application at hand. Traditionally, Linear Mode Power Supplies (LMPS) achieved conversion to the
desired voltage by dissipating excess energy as heat. Nevertheless, since battery life is crucial for handheld devices, methods capable of managing excess energy via smart switching must be used.
The high efficiency of switch-mode technology has also made it an attractive option for harnessing the
power of renewable energy sources. In particular, SMPSs have been used to maximize the energy harvest
from solar photovoltaic cells. Based on the voltage input from the solar cells, special DC to DC
converters present an optimal electrical load to the solar panels and produce a voltage suitable for that
load. These high efficiency DC to DC converters are called Maximum Power Point Trackers (MPPTs).
SMPSs are available in a wide range of sizes and forms. Austria Microsystems produces integrated circuit
implementations of DC to DC converters such as the AS1326B at a unit price of $1.79 [3]. At the
consumer end, SMPSs are targeted towards specific applications. For example, Nady Systems
manufactures a 48V microphone SMPS (SMPS-1X) for $19.99 [4]. Similarly, Samsung electronics'
BN94-02071 is a television set SMPS with a unit price of $70.16 [5]. Commercial SMPSs are classified
by their voltage range and conversion efficiency. For example, the AS1326B (up to 96% efficiency) has
an input ranging from 0.7V to 5.0V while its output varies from 2.5V to 5.0V.
Underlying Technology
SMPSs can be classified into distinct types according to whether the input and output waveforms are AC
or DC [6]. Special focus is placed on DC to DC converters due of their ability to serve as MPPTs.
Electronic switch-mode DC to DC converters convert one DC voltage level to another, by temporarily
storing the input energy and then releasing it to the output at a different voltage. The energy is stored in
magnetic or electric storage elements (such as inductors and capacitors). These components are controlled
by a transistor switch driven by an external signal. The signal is determined by an algorithm programmed
into an embedded control unit. The frequency and duty cycle of this signal regulates the circuit's
impedance and output voltage. Taken together, the input signal, the switching algorithms, the storage
elements, and the circuit configuration are the four parameters that characterize a given SMPS.
The main operating principle of DC to DC SMPSs is the tendency of an inductor to resist the changes in
current induced by the transistor switch. In order to resist, the inductor will either absorb energy (charge)
or produce energy (discharge). The discharge voltage is proportional to the rate of current change, and not
to the original charging voltage. This mechanism allows an output voltage to be different from the input
one.
SMPS Implementation and Innovation
The building blocks of SMPSs consist of inductors, capacitors, and a transistor switch. These elements
have become available in Integrated Circuit (IC) form which can easily be connected to the control unit
[6]. Both IC and discrete implementations must include a safety diode connected across the load to
provide a safe path for the current to decline in case the connection to the load is suddenly broken.
A micro-controller unit (MCU) is used to produce the control signal for the switching transistor. Voltage
and current readings allow the MCU to constantly calculate and optimize the power output. In this way,
optimal operation is guaranteed regardless of changes in external conditions, such as objects blocking the
sunlight in the case of solar panels.
SMPS power conversion efficiency (75% to 98%) exceeds that of existing LMPS technology [7].
This high efficiency is due to recent innovations which have led to the use of power field effect transistors
(FETs) to implement the SMPS switch and the safety diode [7]. Power FETs are able to switch more
efficiently than bipolar transistors at high frequencies. This benefit allows the SMPS to reduce switch
losses while using simpler driving circuits. The only potential disadvantage of SMPSs involves the
generation of unwanted radio emissions from high frequency switching. Nevertheless, current research
efforts have begun to propose solutions to reduce electromagnetic interference (EMI). As a result, EMI
considerations now form an important step in the early stages of SMPS design [8].
References:
[1]
A. Pressman, K. Billings, T. Morey, Switching Power Supply Design , 3rd ed., New Delhi:
McGraw-Hill Professional, 2009, p. 22.
[2]
Electric Power Research Institute, “DC Power Production, Delivery and Utilization : An EPRI
“White Paper” June 2006. [Online]. Available: http://www.epri.org [Accessed: February 3, 2011]
[3]
Austria Microsystems, “AS1326,” austriamicrosystems.com, January, 2011. [Online]. Available:
http://www.austriamicrosystems.com/eng/Products/Power-Management/DC-DC-Step-upConverters/AS1326/ [Accessed February 3, 2011].
[4]
Amazon.com, Inc. “Nady SMPS-1X Phanom Power Supply” amazon.com, February 2011
[Online] http://www.amazon.com/Nady-SMPS-1X-Phantom-Power-Supply/dp/B000XUUXB8
[Accessed February 3, 2011].
[5]
Part Store “Samsung BN9402071A” PartStore.com , October 2010 [Online]
http://www.partstore.com/Part/Samsung/Samsung/BN9402071A/New.aspx
[Accessed: February 2, 2011]
[6]
S. Salivahanan, N. Kumar and A. Vallavaraj, Electronic Devices and Circuits, 2nd ed., New
Delhi: McGraw-Hill, 2008, p. 14.
[7]
V. A. Lalithambika et al “Switch mode power supply controllers” U.S. Patent 7,551,460 B2, June
23, 2009.
[8]
K. Mainali, and R. Oruganti, “Conducted EMI Mitigation Techniques for Switch-Mode Power
Converters: A Survey.” IEEE Transactions on Power Electronics, Vol 25, No 9, September 2010.