SMPS Transformer Manufacturers in Mumbai, India.

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

See also: 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.


Switched-mode power supplies can be classified according to the circuit topology. The most important distinction is between isolated converters and non-isolated ones.

Non-isolated topologies

Non-isolated converters are simplest, with the three basic types using a single inductor for energy storage. In the voltage relation column, D is the duty cycle of the converter, and can vary from 0 to 1. The input voltage (V1) is assumed to be greater than zero; if it is negative, for consistency, negate the output voltage (V2).

Type Power [W] Relative cost Energy storage Voltage relation Features
Buck 0–1,000 1.0 inductor 0 ≤ Out ≤ In, Current is continuous at output.
Boost 0–150 1.0 Single inductor Out ≥ In, Current is continuous at input.
Buck-boost 0–150 1.0 Single inductor Out ≤ 0, Current is dis-continuous at both input and output.
Split-pi (or, boost-buck) 0–4,500 >2.0 Two inductors and three capacitors Up or down Bidirectional power control; in or out
Ćuk Capacitor and two inductors Any inverted, Current is continuous at input and output
SEPIC Capacitor and two inductors Any, Current is continuous at input
Zeta Capacitor and two inductors Any, Current is continuous at output
Charge pump Capacitors only Low performance. Like a CW multiplier, the disadvantages of charge pumps for power conversion can be somewhat mitigated through proper component sizing and drive frequency, since output energy is proportional to capacitance and frequency.

When equipment is human-accessible, voltage and power limits of <=42.4 V peak/60 V dc and 250 VA apply for Safety Certification (UL, CSA, VDE approval).
The buck, boost, and buck-boost topologies are all strongly related. Input, output and ground come together at one point. One of the three passes through an inductor on the way, while the other two pass through switches. One of the two switches must be active (e.g., a transistor), while the other can be a diode. Sometimes, the topology can be changed simply by re-labeling the connections. A 12 V input, 5 V output buck converter can be converted to a 7 V input, −5 V output buck-boost by grounding the output and taking the output from the ground pin.
Likewise, SEPIC and Zeta converters are both minor rearrangements of the Ćuk converter.
Switchers become less efficient as duty cycles become extremely short. For large voltage changes, a transformer (isolated) topology may be better.

Isolated topologies

All isolated topologies include a transformer, and thus can produce an output of higher or lower voltage than the input by adjusting the turns ratio. For some topologies, multiple windings can be placed on the transformer to produce multiple output voltages. Some converters use the transformer for energy storage, while others use a separate inductor.

Type Power [W] Relative cost Input range [V] Energy storage Features
Flyback 0–250 1.0 5–600 Transformer Isolated form of the buck-boost converter.
Ringing choke converter (RCC) 0–150 1.0 5–600 Transformer Low-cost self-oscillating flyback variant.
Half-forward 0–250 1.2 5–500 Inductor
Forward2 100-200 60–200 Inductor Isolated form of buck-boost converter
Resonant forward 0–60 1.0 60–400 Inductor and capacitor Single rail input, unregulated output, high efficiency, low EMI.
Push-pull 100–1,000 1.75 50–1,000 Inductor
Half-bridge 0–2,000 1.9 50–1,000 Inductor
Full-bridge 400–5,000 >2.0 50–1,000 Inductor Very efficient use of transformer, used for highest powers.
Resonant, zero voltage switched >1,000 >2.0 Inductor and capacitor
Isolated Ćuk Two capacitors and two inductors
  • ^1 Flyback converter logarithmic control loop behaviour might be harder to control than other types.
  • ^2 The forward converter has several variants, varying in how the transformer is "reset" to zero magnetic flux every cycle.

Quasi-resonant zero-current/zero-voltage switch

Quasi-resonant switching switches when the voltage is at a minimum and a valley is detected In a quasi-resonant zero-current/zero-voltage switch (ZCS/ZVS) "each switch cycle delivers a quantized 'packet' of energy to the converter output, and switch turn-on and turn-off occurs at zero current and voltage, resulting in an essentially lossless switch." Quasi-resonant switching, also known as valley switching, reduces EMI in the power supply by two methods:

  • By switching the bipolar switch when the voltage is at a minimum (in the valley) to minimize the hard switching effect that causes EMI.
  • By switching when a valley is detected, rather than at a fixed frequency, introduces a natural frequency jitter that spreads the RF emissions spectrum and reduces overall EMI.

Zero voltage switched power supplies require only small heatsinks as little energy is lost as heat. This allows them to be small. This ZVS can deliver more than 1 kilowatt. Transformer is not shown.
Quasi-resonant switching switches when the voltage is at a minimum and a valley is detected

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

For failure in switching components, circuit board and so on read the failure modes of electronics article.
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.


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.



Main article: Switched-mode power supply 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.[citation needed]
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.


The term switchmode was widely used until Motorola claimed ownership of the trademark SWITCHMODE, for products aimed at the switching-mode power supply market, and started to enforce their trademark. Switching-mode power supply, switching power supply, and switching regulator refer to this type of power supply.

Switched mode mobile phone charger
A 450 Watt SMPS for use in personal computers with the power input, fan, and output cords visible
A switched-mode power supply (also switching-mode power supply, SMPS, or simply switcher) is an electronic power supply unit that incorporates a switching regulator in order to provide the required output voltage. The switch mode transformer is used in conjunction with the switching regulator to form a switch mode power supply.

Switch mode power transformers are used extensively in electronic applications, usually within a switch mode power supply:

Switching Mode Power Transformers, Basic Application Circuits

The design of a switch mode power transformer will differ depending upon the type of circuit used. There are many variations of switching mode power supplies, but they can be narrowed down to three basic circuit configurations (each also has a mirrored configuration); Buck, Boost, and flyback. Be aware that the name for the Buck circuit varies from industry to industry and from person to person. It may also be referred to as an inverter, D.C. converter, forward converter, feed forward, and others. There are also unipolar and bipolar (push-pull) versions.