Introduction and Fundamentals of Power Electronics
Power electronics is a subdivision of science and engineering which is a combination of power engineering, semiconductor devices, analog electronics and control systems. Here the fundamentals of each subject are applicable and mingled in a way so as to get a regulated form of electrical energy. Itself, Electrical energy is not useful until it is converted into a tangible form of energy such as motion, light, sound, heat, etc. In too many ways power electronics affect our daily lives in today’s scenario, from power circuits that extend battery life to voltage regulator circuits that help to manage and distribute energy efficiently, from the grid to the consumer. Power electronics basically deals with the process of controlling the voltage and current flow and converting it to a form that is appropriate for user loads. Most desired electronic power system is one whose reliability and efficiency is 100%.
In practical applications power, electronic devices alone are not that useful and hence required to designed with a circuit along with other supporting components. This supporting circuitry includes a decision-making block that controls the power electronics, switches in order to achieve the preferred output, and the firing circuit and sometimes feedback circuit.
The above figure shows the basic block diagram of the electronic power supply. Control Unit takes the output feedback from sensors and compares it with the reference value and accordingly gives input to the firing circuit. Firing circuit is a pulse generating circuit which gives pulse output in a manner so as to control the power electronic switches in the main circuit block. Here the net result is that the load receives the desired electrical power and hence delivers the desired result. A typical example of the above system would be the speed control of motors.
The figure above shows the basic architecture of the electronic power system and the interconnection between them. A power electronic system converts electrical energy from one form to another and to ensures the following is achieved.
• Maximum efficiency
• Maximum reliability
• Maximum availability
• Minimum cost
• Least weight
• Small size
Applications of power electronics include industrial uses, commercial uses, residential purposes, electric vehicles, aerospace and space technologies.
Electronic Power supply Classification
|Input||Output is DC||Output AC|
|Different Types of Power supply Based on input and output||Wall wartBattery ChargerBench Power supply||Isolation TransformersVariable AC supplyFrequency Charger|
|When Input DC||DC-DC converters||InvertersGeneratorsUPS|
Different Types of Power supply Based on input and output
Input-Output is DC Output AC
• When Input AC
• Wall wart
• Battery Charger
• Bench Power supply • Isolation Transformers
• Variable AC supply
• Frequency Charger
• When Input DC
• DC-DC converters
• Variable AC power supply
Variable AC power supply is designed with transformers or adjustable autotransformers. Variable AC power supply is used to convert AC input-to-AC output step down voltage levels. A transformer with several windings may be used to design such power supply else adjustable autotransformer can be used. These supplies convert input AC voltage and current levels while the frequency of the source power remains unchanged.
• Frequency changers
Frequency changes are devices used to alter the frequency of AC input power. These power supplies can be designed using electromechanical devices like a motor-generator set or designed with the help of a rectifier-inverter set. First, the rectifier converts AC to DC, after that then the inverter converts DC back to AC of different frequencies.
• Isolation transformers
The isolation transformer is used to step up or step down voltage levels while keeping the mains and output circuits isolated through CE Certified Reinforced Insulation. Transformers are used for AC-to-AC supply, here impedance matching is essential between the power source and the load circuit. These are useful for connecting balanced and unbalanced circuits. Isolation transformers do not convert voltage levels or frequency of the source power circuit.
UNREGULATED POWER SUPPLY (UPS)
UPS Typically designed to produce a specific voltage at a specific maximum output load current. These are usually the block wall chargers that chance AC into a small trickle of DC and often used to power, for example, household electronics devices. They are the most commonly used power adapters and also called as“wall wart”.The DC output voltage depends on an internal voltage reduction transformer and should be matched as closely as possible to the load current requirements. The output voltage will decrease as the output to the load current increases. With an unregulated DC power supply, the voltage output varies with the size of the load. It consists of a transformer, rectifier section, and capacitor smoothing filter, but no regulation provided to steady the voltage. It can have safety circuits and would be best for the applications where precision not required
• Power input filtering:
Some it is necessary to ensure that spikes or noise signals from the power line do not enter the power supply. To achieve this and to remove noise and limit the effects of incoming spikes, the filter is placed at the input to the power supply. In most cases, any filtering at this point is quite minimal, although for specific power supplies these complicated circuits may be used.
• Input transformer
If a power supply use mains supplies of 110 or 240 volts AC. Then a transformer is used at the input side to transform the incoming line voltage to the desired level power of supply design.
• Rectifier section
To change the incoming AC waveform to a DC waveform. This is achieved using an AC rectifier circuit. Two types of rectifier circuits may be used – half-wave rectifiers and full-wave rectifiers. These effectively block the part of the waveform on one side and allow through the part of the waveform on the other side. The rectifying action of a diode is shown
•Fig.Unregulated power supply
REGULATED POWER SUPPLY
A regulated DC power supply is essentially an unregulated power supply with the addition of a voltage regulator. This allows the voltage to stay constant irrespective of the amount of current consumed by the load, provided the predefined limits are not exceeded. The regulated power supply is of two types.
Linear vs. Switch-Mode Power Supplies
There are two basic types of power supply arrangements used with dc power management subject: linear and switch-mode power supply. Linear power supplies constantly conduct current. Switch-mode supplies transform dc to a switched signal that is rectified to produce a dc output.
Differences between these two arrangements include size and weight, power-handling capability, EMI, and regulation. In linear regulator’s main components are pass transistor, error amplifier, and voltage reference. The linear regulator maintains a fix output voltage by using the error amplifier that compares a portion of the output voltage with a stable reference voltage. If the output voltage increase, the feedback signal generated that causes the pass transistor to lower the output voltage and vice versa.
Fig 1.AC-DC Linear Power Supply
Typical isolated switch-mode supply. Here, the inputac voltage is rectified and filtered to obtain a dc voltage for the other power-supply components. One usually used approach is, to use the on and off pulse-width modulation (PWM) to control the power-switch output voltage. The ratio of on-time to the switching period time is known as the duty cycle. Higher the duty cycle, the higher the output power from the power semiconductor switch.
Fig 2.Isolated AC-DC Switch-Mode Power Supply(SMPS)
Error amplifier in SMPS compares a portion of the output voltage that is feedback, with a stable voltage reference value to provide the drive for the PWM circuit. The resulting drive for the PWM circuit controls the duty cycle of the pulsed signal fed to the power switch, which in turn controls the power-supply output dc voltage. If the dc output voltage tends to rise or fall, the PWM changes the duty cycle accordingly so that the dc output voltage remains constant.
An isolation circuit is required to sustain isolation between the output ground and the power supplied to the power supply’s components. Usually, an optocoupler used to provides isolation while permitting the feedback voltage to control the output of the power supply.
The inductor-capacitor low-pass output filter changes the switched voltage from the switching transformer to a dc voltage. The filter is not perfect, so there is always some residual output noise called “ripple.” The amount of this ripple depends upon the effectiveness of the low-pass filter at the switching frequency. Power-supply switching frequencies range between 100kHz to over 1MHz. higher switching frequencies allow the utilization of smaller-size, lower-value inductors and capacitors within the output low-pass filter. However, higher frequencies also can increase power semiconductor losses, which reduce power-supply efficiency.
The power switch is a key component of the power supply in terms of power dissipation. The switch is typically a power MOSFET that operates in only two modes—on and off. In the off condition, the power switch draws a very small current and dissipates a very small amount of power. Where in the on the state, the power switch draws the maximum current, with low on-resistance, so in most cases its power dissipation is minimal. In the transition from the on s to the off state and off to on state, the power switch pass through its linear region so it can consume moderate power. The total loss for the power switch is, therefore, the sum of the on and off state plus the transition through its linear regions.