Various definitions on What is Power electronics?

  • Power Electronics is the study of switching electronic circuits in order to control the flow of electrical energy. It is the technology behind switching power supplies, power converters, power inverters, motor drives, and motor soft starters.
  • Power electronics is the application of solid-state electronics for the control and conversion of electric power. It applies to both the systems and products involved in converting and controlling the flow of electrical energy, allowing the electricity needed for everyday products to be delivered with maximum efficiency in the smallest and lightest package.
  • Power Electronics is basically an interface between unstable and stable Electrical Energy. Electrical Energy (AC or DC, voltage or current) obtained from any sources can be converted into well-regulated AC or DC, Voltage or Current of any desired level.
  • The task of power electronics is to process and control the flow of electric energy by supplying voltages and currents in a form that is optimally suited for user loads.

 

Different Applications of Power Electronics:

  • Residential: Refrigeration and freezers, Space heating, Air conditioning, Cooking, Lighting Electronics (personal computers, other entertainment equipment).
  • Commercial: Heating, ventilating and air conditioning, Central refrigeration, Lighting, Computers and office equipment, Uninterruptible power supplies (UPSs), Elevators.
  • Industrial: Pumps, Compressors, Blowers and fans, Machine tools (robots), Arc furnaces, induction furnaces, Lighting, Industrial lasers, Induction heating, Welding.
  • Transportation: Traction control of electric vehicles, Battery chargers for electric vehicles, Electric locomotives, Street cars, trolley buses, Subways, Automotive electronics including engine controls.
  • Utility systems: High-voltage dc transmission (HVDC), Static var compensation (SVC), Supplemental energy sources (wind, photovoltaic), fuel cells, Energy storage systems, Induced-draft fans and boiler, feedwater pumps.
  • Aerospace: Space shuttle power supply systems, Satellite power systems, Aircraft power systems.
  • Telecommunications: Battery chargers, Power supplies (dc and UPS).

 

Power Electronic Converters:

  • The power converter usually consists of more than one power conversion stage where the operation of these stages is decoupled on an instantaneous basis by means of energy storage elements such as capacitors and inductors. Therefore, the instantaneous power input does not have to equal the instantaneous power output. Thus, a converter is a basic module (building block) of power electronic systems. It utilizes power semiconductor devices controlled by signal electronics (integrated circuits) and possibly energy storage elements such as inductors and capacitors.
  • Based on the form (frequency) on the two sides, converters can be divided into the following broad categories:
  1. AC TO DC (Rectifiers).
  2. DC TO AC (Inverters).
  3. DC TO DC (Choppers).
  4. AC TO AC (Cycloinverters) (controlled voltage and frequency).
  5. AC TO AC (AC voltage controller) (controlled voltage)
  • Further insight can be gained by classifying converters according to how the devices within the converter are switched.
  1. Line frequency (naturally commutated) converters where the utility line voltages present at one side of the converter facilitate the turn-off of the power semiconductor devices. Similarly, the devices are turned on, phase-locked to the line voltage waveform. Therefore, the devices switch on and off at the line frequency of 50 or 60 Hz.
  2. Switching (forced-commutated) converters, where the controllable switches in the converter are turned on and off at frequencies that are high compared to the line frequency. Despite the high switching frequency internal to the converter, the converter output may be either dc or at a frequency comparable to the line frequency. As a side note in a switching converter, if the input appears as a voltage source, then the output must appear as a current source or vice versa.
  3. Resonant and quasi-resonant converters, where the controllable switches tum on and/or tum off at zero voltage and/or zero current.

 

Linear and Switched Mode Converters:

  • Block Diagram and Circuit diagram of Linear regulated Power Supply can be seen in attached image1 and 2.
  • Block Diagram and circuit diagram of Switched Mode Power Supply can be seen in attached image 3 and 4.

 

Some pros and cons of using Power Electronic Converters:

 

Pros

Cons

High Energy Efficiency

Fast Dynamic Response

Higher reliability and Cost-effective

Environmentally clean and safe

Quiet operation

Generates harmonics

Poor power factor operation

 

Power Electronic Switching Devices:

  • Power switches play a main role in power electronic converters. They operate in 2 states, Conducting state (that means the switch is closed) and Blocking state (that means the switch is off).
  • Based on their control input requirements, they are categorized into 3 types,

 

Current driven devices

Bipolar Junction Transistors (BJT)

MD switch

Gate Turn Off (GTO)

Voltage Driven devices

Metal Oxide Semiconductor Field-effect transistor (MOSFET)

Insulated gate bipolar transistor (IGBT)

MOS controlled thyristor (MCT)

Pulse driven devices

Silicon controlled rectifier (SCR)

Three terminal AC switch (TRIAC)

 

  • Furthermore, they are also categorized into 3 types based on turn-on and turn-off control,

 

Uncontrolled

Diode

Diode for alternating current (DIAC)

Semi-controlled

Silicon controlled rectifier (SCR)

Three terminal AC switch (TRIAC)

Fully controlled

Bipolar Junction Transistors (BJT)

Gate Turn Off (GTO)

Metal Oxide Semiconductor Field-effect transistor (MOSFET)

Insulated gate bipolar transistor (IGBT)

Integrated gate-commutated thyristor (IGCT)

 

  • The following characteristics are desired in a power semiconductor switch,
  1. Small leakage current in the off state.
  2. Small on-state voltage Von to minimize on-state power losses.
  3. Short tum-on and tum-off times. This will permit the device to be used at high switching frequencies.
  4. Large forward- and reverse-voltage-blocking capability.
  5. High on-state current rating. In high-current applications, this would minimize the need to connect several devices in parallel, thereby avoiding the problem of current sharing.
  6. Positive temperature coefficient of on-state resistance. This ensures that paralleled devices will share the total current equally.
  7. Small control power required to switch the device. This will simplify the control circuit design.
  8. Capability to withstand the rated voltage and rated current simultaneously while switching. This will eliminate the need for external protection (snubber) circuits across the device.
  9. Large dv/dt and di/dt ratings. This will minimize the need for external circuits otherwise needed to limit dv/dt and di/dt in the device so that it is not damaged.

 

  • Comparison of different switches,

 

Switch

Power Capability

Switching speed

Application

BJT

Medium

Medium

SMPS

Bridge inverters

DC-DC converters

Power factor correction circuits

MOSFET

Low

Fast

SMPS

DC-DC Converters

GTO

High

Slow

HVDC transmission

IGBT

Medium

Medium

Inverters

Choppers

UPS

Harmonic compensators

MCT

Medium

Medium

Rectification

Inverters

Choppers

 

Switched Mode Power Supply (SMPS):

  • The main components of an SMPS consist of a rectification block (which is used to convert AC line voltage to controlled DC voltage) and a DC-DC converter block (which is used to convert varying DC voltage to fixed DC voltage based on the load demand).
  • The different types of the rectifier and their classification block diagram can be seen in the attached image 5.
  • Typically for an SMPS application, we use Single-phase uncontrolled full-wave bridge rectifier.
  • The classification of DC-DC converters can be seen in attached image 6.
  • The typical power ratings and the applications of some of the DC-DC converters are as mentioned below,

 

Converter

Maximum Power (Watt)

Typical Efficiency

Magnetics Required

(for energy storage during operation at each half cycle)

Buck

500

85

Inductor

Boost

150

70

Inductor

Buck-Boost

150

70

Inductor

Sepic

150

75

Dual or coupled inductor

Cuk

150

75

Dual or coupled inductor

Fly-back

150

75

Transformer

Forward

150

75

Transformer Inductor

Push-pull

500

80

Transformer Inductor

Half-bridge

1000

85

Transformer Inductor

Full bridge

2000

85

Transformer Inductor

 

  • For a typical USB application, we use an isolated Fly-back converter.
  • The basic circuit diagram of Fly-back is attached in image7

The basic operation of Fly-back converter:

  • High Input voltage Vin is rectified using a diode bridge rectifier.
  • The rectified voltage is filtered using necessarily designed passive electronic components.
  • Then obtained DC voltage is switched at high frequencies using switch S1. The main objective of switching is to convert the constant DC voltage to pulsating DC voltage and pass it through high-frequency transformer, where the DC voltage is stepped down to required levels.
  • The stepped down pulsating DC voltage is rectified and passed through a filter and is fed to the load.
  • More about the working of closed-loop DC-DC converters and their in-depth role in USB application with detailed circuit explanations will be spoken in my upcoming blog.

(credits: Ned Mohan. Conveters, Applications and Designs)

 

Topics I will be discussing in part 2,

  • USB power specifications.
  • USB Power Supply.
  • Primary and Secondary control of Fly-back converters.
  • Circuit level understanding of 2-switch and 4-switch Buck-Boost converter.

 

 

Regards

Abhilash P