Switching Power Supplies Selection Guide     Power Supplies Selection Guide     Switching Power Supplies Selection Guide

Image Credit: RS Components, Ltd.| Digi-Key Corporation | eUrasia Power

 

Power supplies are electrical devices that deliver electric power to one or several loads. They generate the output power by converting an input signal into an output signal. Switching power supplies make use of a switching element or regulator (normally a power transistor) in order to generate the desired voltage. Switching power supplies are also called switch-mode products or switching mode power supplies (SMPSs). Compared to linear power supplies, switching power supplies offer greater efficiencies and are noticeably smaller.

 

SMPS Operation

Switching power supplies incorporate electronic components that continuously switch ON and OFF at a very high frequency. This switching action connects and disconnects energy-storing devices (inductors or capacitors) to and from the input source voltage or the output load. SMPS design results in a smaller power supply since the size of the power transformers, inductors, and capacitors is inversely proportional to the switching frequency. Switch-mode operation also reduces the power consumption because when a switch is OFF, its current is almost zero. When a switch is ON, its voltage is very small. A switching power supply is more efficient than a linear power supply because in a linear power supply the excess power is wasted (in form of heat); in a SMPS, all the power is used to convert input power into output power. The switching elements (normally inductors, capacitors or transistors operating in cut-off or saturation) have no dissipating resistance, so no waste of power occurs.

 

Types of Switching Power Supplies

Switching power supplies are typically classified based on circuit structure or input/output configuration.

 

Types Based on Circuit Topology

Switched-mode power supplies can be classified according to the circuit topology of their voltage converter. The following questions should be asked when selecting the appropriate topology:

  1. Is the output voltage higher or lower than the range of the input voltage?
  2. How many outputs are required?
  3. Is input to output dielectric isolation required?
  4. Is the input/output voltage very high?
  5. Is the input/output current very high?
  6. What is the maximum voltage applied across the transformer primary and what is the maximum duty cycle?

SMPS topologies (circuit structures) are classified into two families: non-isolating and isolating.

 

Non-Isolating

Non-isolating topologies are simpler designs that transfer energy to the load during the conduction cycle of a switching transistor. The basic types use a single inductor for energy storage.

  • Step-down (buck) - This type of converters is used to down-convert a DC voltage to a lower DC voltage of the same polarity, i.e. the input voltage must be greater than the output voltage. The step-down converter uses a transistor as a switch that alternately connects and disconnects.
  • Step-up (boost) - The step-up regulator takes a DC input voltage and produces a DC output voltage that is higher in value than the input but of the same polarity. In other words, the input voltage must be smaller than the output voltage.
  • Inverting (buck-boost) - The output voltage is inverted respect to the input voltage. The inverting regulator takes a DC input voltage and produces a DC output voltage that is opposite in polarity to the input. The negative output voltage can be either larger or smaller in magnitude.
  • CUK - CUK converters are used for either step-down or step-up conversion, but they can be voltage polarity reversers or inverters. The CUK converter is named after its originator, Slobodan Cuk, of Cal Tech University in California.
  • Charge pump - The charge pump converter is used for either voltage step-up or for voltage inversion, but generally in low power applications. A charge pump operates by storing energy as electric charge in a capacitor, instead of in an inductor like most other converters.

Isolating

Isolating topologies include a transformer to produce an output of higher or lower voltage than the input by adjusting the turnings ratio. Some converters in this category use the transformer for energy storage, while others use a separate inductor.

  • Flyback - Flyback converters are isolating converters. Flyback converters can be designed as an extension of the buck-boost type with the difference that flyback converters are isolating converters. Instead of using an inductor to store energy it uses a transformer.
  • Forward - Forward converters are isolating converters. They are similar to flyback converters. Instead of using an inductor to store energy they use a transformer. The difference between a flyback and a forward converter is that forward converters use the transformer to step down the primary voltage and provide isolation between the input and output voltage.
    • Two-switch forward - These forward converter variants use an additional switch to limit the maximum voltage stress of the switch to a value equal to the input voltage.
  • Push-pull - Push-pull converters are transformer-isolated converters based on the basic forward topology. The transformer used in a push-pull converter consists of a center-tapped primary and a center-tapped secondary.
  • Half-bridge - Half-bridge converters are transformer-isolated converters based on the basic forward topology. The transformer is used to step down the pulsating primary voltage.
  • Full-bridge - Full-bridge converters are transformer-isolated buck converters. Since the shape of the converter looks like an H, a full-bridge converter is also known as an H-bridge converter. A full-bridge converter configuration retains the voltage properties of the half-bridge topology, and the current properties of push-pull topology. They are used for high power applications.
    • ZVT full bridge - Zero voltage transition (ZVT) full bridge topology is a full bridge converter that uses the ZVT full bridge technique. In a phase-shift ZVT converter, the gate drive of both of the diagonal switches is phase shifted. In addition, both halves of the bridge switch network are driven through the complementary gate pulse with a fixed 50% duty cycle.

The appropriate isolating SMPS topology can usually be selected based on input voltage and output power, as shown in the table below. Selection may vary based on other specific requirements such as cost, size, and personal experience of the designer.

 

Input Voltage

Output Power

Preferred Topology

Universal input (90-264) VAC

Po < 150 W, Load current < 10A

Flyback, Forward

Universal input (90-264)VAC

Po < 150 W, Load current > 10A

Forward

Universal input (90-264) VAC

150 < Po > 350 W

Two-Switch Forward, Half-Bridge, Push-Pull

Universal input (90-264) VAC

Po < 500 W

Half-Bridge, Push-Pull

Vin > 350 VDC

Po < 750 W

Half-Bridge

Vin < 200 VDC

Po < 500 W

Push-Pull

Vin > 350 VDC

500 < Po > 1000 W

Full-Bridge

Vin > 350 VDC

Po > 1000 W

ZVT Full-Bridge

Vin > 350 VDC

Po > 2000 W

More than one ZVT full-bridge in parallel, interleaved with more than one ZVT full-bridge

 Table Credit: Microchip Technology Inc.

 

Types Based on Input/Output

Based on the type of input/output voltages, the power supplies can be classified into the following general categories:

  • DC Power Supplies (AC-DC devices) - Provides a DC output voltage when an AC voltage is applied to the input. These are normally known as simply "power supplies".
  • DC-DC Converters - These are power sources that produce a DC output voltage from a DC input voltage. Strictly speaking, these devices are converters that convert a DC voltage into another DCvoltage with different magnitude.
  • DC-AC Converters (Inverters) - These devices produce AC voltage from a DC input voltage. These are better known as "inverters".
  • AC Power Sources (AC-AC devices) - These devices convert an AC voltage into another AC voltage with (in general) different amplitude. They are also known as AC power sources.

Specifications

 

Performance

There are many parameters needed to fully characterize a power supply; however for most power supply types there are a set of parameters that are common. These include input and output voltage (specified in volts [V]), the output current (in amps [A]), the rated output power (in watts [W]), the input signal frequency (in Hertz [Hz], kilohertz [kHz], or megahertz [MHz]), and the regulation.

  • Input voltage is the magnitude and type of the voltage applied to the power supply.
  • Input frequency is the frequency of the input signal.
  • Output voltage is the magnitude of the voltage at the output of the device.
  • Output current is the current associated with the output voltage.
  • Output power is the power (in watts) delivered to the load.
  • Regulation indicates the stability of the output voltage.
    • Line regulation is the maximum steady-state amount that the output voltage changes as a result of a specified change in input line voltage.
    • Load regulation is the maximum steady-state amount that the output voltage changes as a result of a specified change in load.

Mounting

Mounting specifications are of less importance but should be considered as needed for proper fitting to the application or system. Mounting options include:

  • Board mount
  • Circuit mount
  • Wall mount
  • DIN rail mount
  • Rack mount
  • Desktop

Features

Features for power supplies add functionality such as circuit protection and cooling which may be important for certain applications.

 

Protection

Several factors can affect the performance and/or the physical integrity of power supplies. Circuits to protect the power supplies are normally included in the design and construction of the device. Some of these are:

  • Short circuit protection
  • Overload protection
  • Over current protection
  • Over voltage protection
  • Under voltage protection
  • Over temperature protection

Cooling

Several cooling methods are used to protect power supplies:

  • Fan cooling
  • Heat sink cooling
  • Water cooling

Other Features

Power supplies can also incorporate a number of other features:

  • Battery backup
  • Hot swappable
  • Power factor correction
  • Temperature compensation
  • Weatherproof

Application

In terms of usage, the most important feature of a switching power supply is its efficiency. As explained above, with a SMPS there is a very small amount of power loss from the conversion of input to output power. Therefore, SMPSs are used in applications that require high efficiency. Some of these applications include:

  • Computers
  • Mobile devices chargers
  • Automobile chargers
  • Medical devices

For a more in-depth overview of power supply selection, visit the Power Supplies Selection Guide on GlobalSpec.

 

References

 

SMPS Switching Power Supply Design Basics

 

Integrated Publishing - Solid-State Power Supplies

 

Microchip - Switch Mode Power Supply Topologies (pdf)