A switched-mode power supply (switching-mode power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a source, like mains power, to a load, such as a personal computer, while converting voltage and current
characteristics. An SMPS is usually employed to efficiently provide a
regulated output voltage, typically at a level different from the input
voltage.
Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation,
full-on and full-off states, and spends very little time in the high
dissipation transitions (which minimizes wasted energy). Ideally, a
switched-mode power supply dissipates no power. Voltage regulation
is achieved by varying the ratio of on-to-off time. In contrast, a
linear power supply regulates the output voltage by continually
dissipating power in the pass transistor.
This higher power conversion efficiency is an important advantage of a
switched-mode power supply. Switched-mode power supplies may also be
substantially smaller and lighter than a linear supply due to the
smaller transformer size and weight.
Switching regulators are used as replacements for the linear
regulators when higher efficiency, smaller size or lighter weight are
required. They are, however, more complicated; their switching currents
can cause electrical noise problems if not carefully suppressed, and
simple designs may have a poor power factor.
History
1926: "Electrical condensors" by Coursey
mentions high frequency welding
and furnaces.
1959: Transistor oscillation and rectifying converter power supply system
U.S. Patent 3,040,271 is filed.
1970: High-Efficiency Power Supply produced from about 1970 to 1995.
1972: HP-35, Hewlett-Packard's first pocket calculator, is introduced with transistor switching power supply for light-emitting diodes, clocks, timing, ROM, and registers.
1976: "Switched mode power supply"
U.S. Patent 4,097,773 is filed
1977: Apple II is designed with a switching mode power supply. "
For its time (1977) it was a breakthrough, since until then switching mode power supplies weren’t used. Designed by Rod Holt,".
"
Rod
Holt was brought in as product engineer and there were several flaws in
Apple II that were never publicized. One thing Holt has to his credit
is that he created the switching power supply that allowed us to do a
very lightweight computer".
1980: The HP8662A
10 kHz – 1.28 GHz synthesized signal generator went with a switched power supply.
Explanation
A linear regulator provides the desired output voltage by dissipating excess power in ohmic losses
(e.g., in a resistor or in the collector–emitter region of a pass
transistor in its active mode). A linear regulator regulates either
output voltage or current by dissipating the excess electric power in
the form of heat,
and hence its maximum power efficiency is voltage-out/voltage-in since
the volt difference is wasted. In contrast, a switched-mode power supply
regulates either output voltage or current by switching ideal storage
elements, like inductors and capacitors,
into and out of different electrical configurations. Ideal switching
elements (e.g., transistors operated outside of their active mode) have
no resistance when "closed" and carry no current when "open", and so the
converters can theoretically operate with 100% efficiency (i.e., all
input power is delivered to the load; no power is wasted as dissipated
heat).
The basic schematic of a boost converter.
For example, if a DC source, an inductor, a switch, and the corresponding electrical ground are placed in series and the switch is driven by a square wave,
the peak-to-peak voltage of the waveform measured across the switch can
exceed the input voltage from the DC source. This is because the
inductor responds to changes in current by inducing its own voltage to
counter the change in current, and this voltage adds to the source
voltage while the switch is open. If a diode-and-capacitor combination
is placed in parallel to the switch, the peak voltage can be stored in
the capacitor, and the capacitor can be used as a DC source with an
output voltage greater than the DC voltage driving the circuit. This boost converter acts like a step-up transformer for DC signals. A buck–boost converter
works in a similar manner, but yields an output voltage which is
opposite in polarity to the input voltage. Other buck circuits exist to
boost the average output current with a reduction of voltage.
In an SMPS, the output current flow depends on the input power
signal, the storage elements and circuit topologies used, and also on
the pattern used (e.g., pulse-width modulation with an adjustable duty cycle) to drive the switching elements. Typically, the spectral density of these switching waveforms has energy concentrated at relatively high frequencies. As such, switching transients, like ripple, introduced onto the output waveforms can be filtered with small LC filters.
Advantages and disadvantages
The main advantage of this method is greater efficiency because the
switching transistor dissipates little power when it is outside of its
active region (i.e., when the transistor acts like a switch and either
has a negligible voltage drop across it or a negligible current through
it). Other advantages include smaller size and lighter weight (from the
elimination of low frequency transformers which have a high weight) and
lower heat generation due to higher efficiency. Disadvantages include
greater complexity, the generation of high-amplitude, high-frequency
energy that the low-pass filter must block to avoid electromagnetic interference (EMI), a ripple voltage at the switching frequency and the harmonic frequencies thereof.
Very low cost SMPSs 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.
Theory of operation
Input rectifier stage
If the SMPS has an AC input, then the first stage is to convert the input to DC. This is called rectification.
The rectifier circuit can be configured as a voltage doubler by the
addition of a switch operated either manually or automatically. This is a
feature of larger supplies to permit operation from nominally
120 V or
240 V
supplies. The rectifier produces an unregulated DC voltage which is
then sent to a large filter capacitor. The current drawn from the mains
supply by this rectifier circuit occurs in short pulses around the AC
voltage peaks. These pulses have significant high frequency energy which
reduces the power factor. Special control techniques can be employed by
the SMPS to force the average input current to follow the sinusoidal
shape of the AC input voltage, correcting the power factor. An SMPS with
a DC input does not require this stage. An SMPS designed for AC input
can often be run from a DC supply (for
230 V AC this would be
330 V DC),
as the DC passes through the rectifier stage unchanged. It's however
advisable to consult the manual before trying this, though most supplies
are quite capable of such operation even though nothing is mentioned in
the documentation. However, this type of use may be harmful to the
rectifier stage as it will only use half of diodes in the rectifier for
the full load. This may result in overheating of these components, and
cause them to fail prematurely.
If an input range switch is used, the rectifier stage is usually configured to operate as a voltage doubler when operating on the low voltage (
~120 V AC) range and as a straight rectifier when operating on the high voltage (
~240 V AC)
range. If an input range switch is not used, then a full-wave rectifier
is usually used and the downstream inverter stage is simply designed to
be flexible enough to accept the wide range of DC voltages that will be
produced by the rectifier stage. In higher-power SMPSs, some form of
automatic range switching may be used.
Inverter stage
The inverter stage converts DC, whether directly from the input or from
the rectifier stage described above, to AC by running it through a power
oscillator, whose output transformer is very small with few windings at
a frequency of tens or hundreds of kilohertz.
The frequency is usually chosen to be above 20 kHz, to make it
inaudible to humans. The switching is implemented as a multistage (to
achieve high gain) MOSFET amplifier. MOSFETs are a type of transistor with a low on-resistance and a high current-handling capacity.
Voltage converter and output rectifier
If the output is required to be isolated from the input, as is
usually the case in mains power supplies, the inverted AC is used to
drive the primary winding of a high-frequency transformer.
This converts the voltage up or down to the required output level on
its secondary winding. The output transformer in the block diagram
serves this purpose.
If a DC output is required, the AC output from the transformer is
rectified. For output voltages above ten volts or so, ordinary silicon
diodes are commonly used. For lower voltages, Schottky diodes
are commonly used as the rectifier elements; they have the advantages
of faster recovery times than silicon diodes (allowing low-loss
operation at higher frequencies) and a lower voltage drop when
conducting. For even lower output voltages, MOSFETs may be used as synchronous rectifiers; compared to Schottky diodes, these have even lower conducting state voltage drops.
The rectified output is then smoothed by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.
Simpler, non-isolated power supplies contain an inductor instead of a transformer. This type includes boost converters, buck converters, and the buck-boost
converters.
These belong to the simplest class of single input, single output
converters which use one inductor and one active switch. The buck
converter reduces the input voltage in direct proportion to the ratio of
conductive time to the total switching period, called the duty cycle.
For example an ideal buck converter with a 10 V input operating at a 50%
duty cycle will produce an average output voltage of 5 V. A feedback
control loop is employed to regulate the output voltage by varying the
duty cycle to compensate for variations in input voltage. The output
voltage of a boost converter
is always greater than the input voltage and the buck-boost output
voltage is inverted but can be greater than, equal to, or less than the
magnitude of its input voltage. There are many variations and extensions
to this class of converters but these three form the basis of almost
all isolated and non-isolated DC to DC converters. By adding a second
inductor the Ćuk and SEPIC converters can be implemented, or, by adding additional active switches, various bridge converters can be realised.
Other types of SMPSs use a capacitor-diode voltage multiplier instead of inductors and transformers. These are mostly used for generating high voltages at low currents . The low voltage variant is called charge pump.
Regulation
A feedback
circuit monitors the output voltage and compares it with a reference
voltage, which shown in the block diagram serves this purpose. Depending
on design/safety requirements, the controller may contain an isolation
mechanism (such as opto-couplers)
to isolate it from the DC output. Switching supplies in computers, TVs
and VCRs have these opto-couplers to tightly control the output voltage.
Open-loop regulators do not have a feedback circuit. Instead,
they rely on feeding a constant voltage to the input of the transformer
or inductor, and assume that the output will be correct. Regulated
designs compensate for the impedance of the transformer or coil. Monopolar designs also compensate for the magnetic hysteresis of the core.
The feedback circuit needs power to run before it can generate power,
so an additional non-switching power-supply for stand-by is added.
Transformer design
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 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, higher frequency also means more energy lost during
transitions of the switching semiconductor. Furthermore, more attention
to the physical layout of the circuit board is required, and the amount of electromagnetic interference will be more pronounced.
Core losses increase at higher frequencies. Cores use ferrite
material which has a low loss at the high frequencies and high flux
densities used. The laminated iron cores of lower-frequency (<400 Hz)
transformers would be unacceptably lossy at switching frequencies of a
few kilohertz.
Copper loss
Main article: Copper loss
At low frequencies (such as the line frequency of 50 or 60 Hz), designers can usually ignore the skin effect.
For these frequencies, the skin effect is only significant when the
conductors are large, more than 0.3 inches (7.6 mm) in diameter.
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.
In addition to the skin effect, there is also a proximity effect, which is another source of power loss.
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. Harmonics can be removed by filtering, but the filters are
expensive. Unlike displacement power factor created by linear inductive
or capacitive loads, this distortion cannot be corrected by addition of a
single linear component. Additional circuits are required to counteract
the effect of the brief current pulses. Putting a current regulated
boost chopper stage after the off-line rectifier (to charge the storage
capacitor) can correct the power factor, but increases the complexity
and cost.
In 2001, the European Union put into effect the standard
IEC/EN61000-3-2 to set limits on the harmonics of the AC input current
up to the 40th harmonic for equipment above 75 W. The standard defines
four classes of equipment depending on its type and current waveform.
The most rigorous limits (class D) are established for personal
computers, computer monitors, and TV receivers. To comply with these
requirements, modern switched-mode power supplies normally include an
additional power factor correction (PFC) stage.
Efficiency and EMI
Higher input voltage and synchronous rectification mode makes the
conversion process more efficient. The power consumption of the
controller also has to be taken into account. Higher switching frequency
allows component sizes to be shrunk, but can produce more RFI. A resonant forward converter produces the lowest EMI of any SMPS approach because it uses a soft-switching resonant waveform compared with conventional hard switching.
Failure modes
Power supplies which use capacitors suffering from the capacitor plague
may experience premature failure when the capacitance drops to 4% of
the original value. This usually causes the switching semiconductor to
fail in a conductive way. That may expose connected loads to the full
input volt and current, and precipitate wild oscillations in output.
Failure of the switching transistor is common. Due to the large switching voltages this transistor must handle (around
325 V for a
230 VAC mains supply), these transistors often short out, in turn immediately blowing the main internal power fuse.
Precautions
The main filter capacitor will often store up to
325 V
long after the power cord has been removed from the wall. Not all power
supplies contain a small "bleeder" resistor to slowly discharge this
capacitor. Any contact with this capacitor may result in a severe
electrical shock.
The primary and secondary side may be connected with a capacitor to reduce EMI
and compensate for various capacitive couplings in the converter
ciruit, where the transformer is one. This may result in electric shock
in some cases. The current flowing from line or neutral through a
2000 Ω resistor to any accessible
part must according to
IEC 60950 be less than
250 μA for IT equipment.
Applications
Switched-mode power supply units (PSUs) in domestic products such as personal computers often have universal inputs, meaning that they can accept power from mains supplies
throughout the world, although a manual voltage range switch may be
required. Switch-mode power supplies can tolerate a wide range of power frequencies and voltages.
In 2006, at an Intel
Developers Forum, Google engineers proposed the use of a single 12 V
supply inside PCs, due to the high efficiency of switch mode supplies
directly on the PCB.
Due to their high volumes mobile phone chargers have always been particularly cost sensitive. The first chargers were linear power supplies
but they quickly moved to the cost effective ringing choke converter
(RCC) SMPS topology, when new levels of efficiency were required.
Recently, the demand for even lower no load power requirements in the
application has meant that flyback topology is being used more widely;
primary side sensing flyback controllers are also helping to cut the bill of materials (BOM) by removing secondary-side sensing components such as optocouplers.
Switched-mode power supplies are used for DC to DC conversion as well. In automobiles where heavy vehicles use a nominal
24 VDC
cranking supply, 12 volts for accessories may be furnished through a
DC/DC switch-mode supply. This has the advantage over tapping the
battery at the 12 volt position that all the 12 Volt load is evenly
divided over all cells of the 24 volt battery. In industrial settings
such as telecommunications racks, bulk power may be distributed at a low
DC voltage (from a battery back up system, for example) and individual
equipment items will have DC/DC switched-mode converters to supply
whatever voltages are needed.
