Introduction
The insulated gate bipolar transistor (IGBT) combines the positive attributes of BJTs and MOSFETs.  BJTs have lower conduction losses in the on-state, especially in devices with larger blocking voltages, but have longer switching times, especially at turn-off while MOSFETs can be turned on and off much faster, but their on-state conduction losses are larger, especially in devices rated for higher blocking voltages.  Hence, IGBTs have lower on-state voltage drop with high blocking voltage capabilities in addition to fast switching speeds. The symbol for an IGBT is shown on the right.

The IGBT is used in medium- to high-power applications such as switched-mode power supply, traction motor control and induction heating. Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities in the order of hundreds of amps with blocking voltages of 6,000 V.

Introduction
A rectifier is an electrical device that converts alternating current AC to direct current DC, a process known as rectification. The primary application of rectifiers is to derive DC power from an AC supply. Virtually all electronics except simple motor circuits such as fans require a DC supply but mains power is AC so rectifiers find uses inside the power supplies of virtually all electronic equipment. The symbol for a rectifier is shown on the right.

Current only conducts in the direction of the arrow (from anode to cathode) when the voltage between them exceeds the ON voltage.

Introduction
A low dropout or LDO regulator is a DC linear voltage regulator which can operate with a very small input-output differential voltage. The main components are a power FET and a differential amplifier (error amplifier). One input of the differential amplifier monitors a percentage of the output, as determined by the resistor ratio of R1 and R2. The second input to the differential amplifier is from a stable voltage reference. If the output voltage rises too high relative to the reference voltage, the drive to the power FET changes so as to maintain a constant output voltage. The circuit diagram is shown on the right.

Operation
A regulator's dropout voltage determines the lowest usable supply voltage. If, for example, the LDO has a dropout voltage around 700mV (0.7V), a 3.3V output would require the input to be at least 4V. Thus the LDO may be specified to provide a fixed 3.3V output with a 4V to 5.5V input. The dropout voltage is related to the output current via the pass device on resistance. LDO differ from standard regulators in the type of pass device employed. A standard regulator may use a NMOS transistor output which is very easy to control, whereas a LDO will use a PMOS output which is harder to control due to its large output impedance. However the PMOS transistor will not require its gate voltage to be driven high (threshold voltage). Thus the dropout voltage is only limited by the PMOS transistor on resistance. Alternative strategies include gate voltage pumping, which is often dismissed due to noise, power consumption and startup time constraints.

Applications

• Consumer applications for conversions from 3.3V or 5V rails

• Cellular headsets, PDAs and wireless network

• Powering low-noise amplifiers, VCOs and RF receivers

• Medical instrumentation, automated test equipment and measurement devices

Basics
A buck converter is a step-down DC to DC converter. The simplest way to reduce a DC voltage is to use a voltage divider circuit; however a lot of the power is wasted (half if the voltage divider is converting 10V to 5V for example). A buck converter works on the concept of transferring packets of energy from the input to output via a transistor used as a switch. This greatly increases the efficiency. The diagram below shows the buck converter in its simplest form.

Function: Step-down  
When to use: Typically when Vin is 3x to 5x Vout and Iout is 0.5A to 5A
Characteristics: Easy to design and good efficiency above conditions mentioned above

Operation
The operation of the buck converter is fairly simple, with an inductor and two switches (usually a transistor and a diode) that control the inductor. It alternates between connecting the inductor to source voltage to store energy in the inductor and discharging the inductor into the load. The relationship between the input voltage and output voltage is: (find out whether this is for CCM or DCM or both)

Vout = VinD where D is the duty cycle ratio of the waveform applied to the switch.

The buck converter can operate in one of two modes, continuous and discontinuous. A Buck converter operates in continuous mode if the current through the inductor (IL) never falls to zero during the commutation cycle. In some cases, the amount of energy required by the load is small enough to be transferred in a time lower than the whole commutation period. In this case, the current through the inductor falls to zero during part of the period, this is known as discontinuous mode.

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Synchronous rectification
A synchronous buck converter is a modified version of the basic buck converter circuit topology in which the diode is replaced by a second switch. This modification is a trade off between increased cost and improved efficiency. In a standard buck converter, the freewheeling diode turns on, on its own, shortly after the switch turns off, as a result of the rising voltage across the diode. This voltage drop across the diode results in a power loss. The modified circuit is shown below.

Function: Step-down  
When to use: When high efficiency is required with high output current (>5A) or low duty cycles (Vin > 5x Vout and/or Iout <0.5A)
Characteristics: A second switch replaces the diode in the basic buck topology, reducing losses in the conditions mentioned above

Applications

• Powering FPGAs, DSPs and microprocessors

• Networking equipment

• Optical networks

• Industrial power supplies

Basics
A boost converter is a power converter with an output DC voltage greater than its input DC voltage. It is a class of switching-mode power supply (SMPS) containing at least two semiconductor switches (a diode and a transistor) and at least one energy storage element. Filters made of capacitors (sometimes in combination with inductors) are normally added to the output of the converter to reduce output voltage ripple. The diagram on the right shows the boost converter in its simplest form.

Function: Step-up 
When to use: Typically used when transformerless, regulated output voltages larger than input voltages at output currents beyond 100mA-200mA are required
Characteristics: Best for low-power conversion (up to 10W or 20W) and output voltages less than or equal to 7x Vin

Operation
The key principle that drives the boost converter is the tendency of an inductor to resist changes in current. When being charged it acts as a load and absorbs energy (somewhat like a resistor), when being discharged, it acts as an energy source (somewhat like a battery). The voltage it produces during the discharge phase is related to the rate of change of current, and not to the original charging voltage, thus allowing different input and output voltages. The relationship between the input and output voltage is:
Vout = Vin/(1-D)    where D is the duty cycle ratio of the waveform applied to the switch

As with the buck converter the boost converter can operate in continuous mode or discontinuous mode.

Applications

• TFT-LCD Bias

• USB bus power

• White LED flash/Torch applications

Basics
The schematic of a flyback converter can be seen on the right. It is equivalent to that of a buck-boost converter, with the inductor split to form a transformer. Therefore the operating principle of both converters is very close. When the switch is on the primary of the transformer is directly connected to the input voltage source. This results in an increase of magnetic flux in the transformer. The voltage across the secondary winding is negative, so the diode is reverse-biased (i.e blocked). The output capacitor supplies energy to the output load. When the switch is off, the energy stored in the transformer is transferred to the output of the converter.
 

Function: Step-up, step-down, invert or buck-boost
When to use: Typically when multi-output or isolation is required, when step-up beyond 8x Vin is required or when the max voltage or current of the switch needs to be extended in order  to take advantage of the turns ratio conversion from the transformer
Characteristics: Best for medium power conversion (5W to 100W)

Operation
The flyback converter is an isolated power converter; therefore the isolation of the control circuit is also needed. The two prevailing control schemes are voltage mode control and current mode control. Both require a signal related to the output voltage. There are two common ways to generate this voltage. The first is to use an optocoupler on the secondary circuitry to send a signal to the controller. The second is to wind a separate winding on the coil and rely on the cross regulation of the design.
Applications
• Low-power switch-mode power supplies (cell phone charger, standby power supply in PCs)
• Low cost multiple-output power supplies (e.g. main PC supplies < 250 W)
• High voltage supply for the CRT in TVs and monitors (the flyback converter is often combined with the horizontal deflection drive).
• High voltage generation, e.g. for Xenon flash lamps, lasers, copiers etc.
• The ignition system in Spark-Ignition engines is also a flyback converter, the ignition coil being the transformer and the contact breaker forming the switch element.
• Isolated gate driver.

Basics
A SEPIC (single ended primary inductor converter) is a DC-DC converter which allows the output voltage to be greater than, less than, or equal to the input voltage. The output voltage of the SEPIC is controlled by the duty cycle of the control transistor. The largest advantage of a SEPIC over the buck-boost converter is a non-inverted output (positive voltage). SEPICs are useful in applications where the battery voltage can be above and below the regulator output voltage. For example, a single lithium ion battery typically has an output voltage ranging from 4.2 volts to 3 volts. If the load requires 3.3 Volts, then the SEPIC would be effective since the battery voltage can be both above and below the regulator output voltage. Other advantages of SEPICs are input/output isolation provided by C1 and true shutdown mode: when the switch is turned off output drops to 0V. The circuit diagram for a SEPIC is shown on the right. 

Function: Buck-boost (VinMAX > Vout > VinMIN)
When to use: Excellent option when buck-boost operation is needed and no transformer is desired
Characteristics: Lower input ripple than flyback, no snubber circuit required

Operation
The basic schematic for a SEPIC is shown on the right. As with other Switched mode power supplies, the SEPIC exchanges energy between the capacitors and inductors in order to change energy from one voltage to another. The amount of energy exchanged is controlled by S1, which is typically a MOSFET. MOSFETs are used instead of BJTs due to the extremely high input impedance and the low voltage drop across the MOSFET when turned on.

Basics
The Cuk converter is a type of DC-DC converter that has an output voltage magnitude that is either greater than or less than the input voltage magnitude, with an opposite polarity. It uses a capacitor as its main energy-storage component, unlike most other types of converters which use an inductor. The diagram is shown on the right.

 
Function: Inverting (Vout is negative; Vin is positive)
When to use: When a regulated, negative, low-ripple voltage is needed from a positive supply
Characteristics: Continuous current at input and output translates into a very low-ripple/very low-noise output

Operation
A Cuk converter comprises two inductors, two capacitors, a switch (usually a transistor), and a diode. Its schematic can be seen in figure 1. It is an inverting converter, so the output voltage is negative with respect to the input voltage. The capacitor C is used to transfer energy and is connected alternately to the input and to the output of the converter via the commutation of the transistor and the diode (see figures 2 and 3). The two inductors L1 and L2 are used to convert respectively the input voltage source (Vi) and the output voltage source (Co) into current sources. Indeed, at a short time scale an inductor can be considered as a current source as it maintains a constant current. This conversion is necessary because if the capacitor were connected directly to the voltage source, the current would be limited only by (parasitic) resistance, resulting in high energy loss. Charging a capacitor with a current source (the inductor) prevents resistive current limiting and its associated energy loss. As with other converters (Buck converter, Boost converter, Buck-boost converter) the Cuk converter can either operate in continuous or discontinuous current mode. However, unlike these converters, it can also operate in discontinuous voltage mode (i.e the voltage across the capacitor drops to zero during the commutation cycle). The relationship between the input and output voltage is:

Vout = -VinD/(1-D)    where D is the duty cycle ratio of the waveform applied to the switch