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Introduction to Power Electronics

EMT 369 2013/2014. Introduction to Power Electronics. Dr Ruslinda A.Rahim. What is Power Electronic. ' Power Electronics is Power Processing Electronics'. ‘ The applications of solid-state electronics for the control and conversion of the electric power M.H Rashid, Prentice Hall.

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Introduction to Power Electronics

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  1. EMT 369 2013/2014 Introduction to Power Electronics Dr Ruslinda A.Rahim

  2. What is Power Electronic 'Power Electronics is Power Processing Electronics' ‘The applications of solid-state electronics for the control and conversion of the electric power M.H Rashid, Prentice Hall ‘ the technology associated with the efficient conversion, control and conditioning of electric power by static means from its available input form into the desired electrical output form’ Wikipedia

  3. Modern Power Electronics is the use of power semiconductor devices and electronic circuits to process electrical power for a multitude of uses ranging from the spaces programme to home appliances and theme park roller coaster rides.

  4. Power Electronic rapid growth due to: – Advances in power (semiconductor) switches – Advances in microelectronics (DSP, VLSI, microprocessor/microcontroller, ASIC) – New ideas in control algorithms – Demand for new applications

  5. Power Electronic is an INTERDISCIPLINARY field:

  6. Power Electronic Converter source load Power Electronic is about controlling electrical energy flow from a source to a load with the help of a power electronic converter. • constant voltage • constant frequency • constant current • constant phase angle • variable voltage • variable frequency • variable current • variable phase angle Power Electronic Converter

  7. DSP ASP Analogue Signal Processing Digital Signal Processing SIGNAL PROCESSING PROCESSORS

  8. DC POWER PROCESSED OUTPUT INFORMATION INPUT INFORMATION analogue signal processing D-A conv. A-D conv. digital signal processing SIGNAL PROCESSING SIGNAL PROCESSOR

  9. control information PROCESSED OUTPUT POWER INPUT POWER ac-dc conv. dc-ac conv. ac-ac conversion POWER PROCESSING POWER PROCESSOR dc-dc conversion

  10. POWER PROCESSING • The Power Electronic Converter : basic types of power converter and names in common usage. • Diode Rectifier • 2. AC to DC Converter (Controlled Rectifier) • 3. DC to DC Converter (DC Chopper) • 4. AC to AC Converter (AC voltage regulator) • 5. DC to AC Converter (Inverter)

  11. Diode Rectifiers. A diode rectifier circuit converts AC voltage into a fixed DC voltage. The input voltage to rectifier could be eithersingle phase or three phase. AC to DC Converters. An AC to DC converter circuit can convert AC voltage into a DC voltage. The DC output voltage can be controlled by varying the firing angle of the thyristors. The AC input voltage could be a single phase or three phase. AC to AC Converters. This converters can convert from a fixed ac input voltage into variable AC output voltage. The output voltage is controlled by varying firing angle of TRIAC. These type converters are known as AC voltage regulator. DC to DC Converters. These converters can converter a fixed DC input voltage into variable DC voltage or vice versa. The DC output voltage is controlled by varying of duty cycle.

  12. POWER CONVERTER TOPOLOGY A system approach reduces the power converter to a number of specialist technologies as shown by the generalized power converter block diagram.

  13. MULTI DISCIPLINARY

  14. Power Electronics Application

  15. Conversion concept: example • Supply from TNB:50Hz, 240V RMS(340V peak). Customer need DC voltage for welding purpose, say. • TNB sine-wave supply gives zero DC component! • We can use simple half-wave rectifier. A fixed DC voltage is now obtained. This is a simple PE system.

  16. Conversion concept (cont..) How if customer wants variable DC voltage? More complex circuit using SCR is required. By controlling the firing angle, α,the output DC voltage (after conversion) can be varied.. Obviously this needs a complicated electronic system to set the firing current pulses for the SCR.

  17. Current issues related to power electronics: Energy Scenario • Need to reduce dependence on fossil fuel (coal,natural gas, oil) and nuclear power resource (uranium). Depletion of these sources is expected • Effort to tap renewable energy resources such as solar, wind, fuel-cell etc. need to be increased. • Energy saving: 15-20% of electricity can be saved by PE applications. • E.g. variable speed drives (air conditioned, fans,pumps). Variable speed compressor airconditioning system saves up to 30% of energy • compared to conventional thermostat-controlled system. • Electrical lighting using electronics ballast can boost the efficiency of fluorescent lamp by 20%.

  18. POWER SEMICONDUCTOR DEVICES

  19. POWER SEMICONDUCTOR DEVICES(Power Switches) Power switches are the work-horses of Power E systems. • Power Electronic switches works in two states only: – Fully on (conducting); – Fully off (blocking) • Can be categorised into three group – PowerDiode : on and off states controlled by power circuit only – Thyristor (SCR) : Latched on by low-power control signal but must be turned off by power circuit. Cannot be turned off by control signal. – Controllableswitches: Can be turned on and off by low-power control signals (e.g. BJT, MOSFET, IGBT, GTO)

  20. Power Diode On and off states controlled by the power circuit

  21. Power Diode • When diode is forward biased, it conducts current with a small forward voltage (Vf) across it (0.2-3V) • • When reversed (or blocking state), a • negligibly small leakage current (uA to • mA) flows until the reverse breakdown • occurs. Diode should not be operated at • reverse voltage greater than Vr

  22. Power Diode Fast-recovery diodes have a small reverse-recovery time Diode Turn-Off

  23. Power Diode • When a diode is switched quickly from forward to reverse bias, it continues to conduct due to the minority carriers which remains in the p-n junction. • The minority carriers require finite time, i.e, trr (reverse recovery time) to recombine with opposite charge and neutralize. • Effects of reverse recovery are increase in switching losses, increase in voltage rating, over-voltage (spikes) in inductive loads

  24. Types of diode • Line frequency (general purpose): – on state voltage very low (below 1V) – large trr (about 25us) – very high current (up to 5kA) and voltage (5kV) ratings – Used in line-frequency (50/60Hz) applications such as rectifiers • Fast recovery diode – very low trr (<1us). – Power levels at several hundred volts and several hundredamps – Normally used in high frequency circuits • Schottky diode – very low forward voltage drop (typical 0.3V) – limited blocking voltage (50-100V) – Used in low voltage, high current application such as switched mode power supplies.

  25. Thyristors • Thyristors can only be turned on with two conditions: – the device is in forward blocking state (i.e Vak is positive) – a positive gate current (Ig) is applied at the gate • Once conducting, the anode current is LATCHED (continuously flowing).

  26. Turning on/off mechanism • In reverse -biased mode, the SCR behaves like a diode. It conducts a small leakage current which is almost dependent of the voltage, but increases with temperature. • When the peak reverse voltage is exceeded, avalanche breakdown occurs,and the large current will flow. • In the forward biased mode, with no gate current present (i.e. in the untriggered state, the device exhibits a leakage current. • If the forward breakover voltage (Vbo) is exceeded, the SCR “self-triggers” into the conducting state and and the voltage collapses to the normal forward volt-drop,typically 1.5-3V. The presence of any gate current will reduce the forward breakover voltage.

  27. Thyristor in a Simple Circuit • For successful turn-off, reverse voltage required for an interval greater than the turn-off interval

  28. Bipolar Junction Transistors (BJT) • Ratings: Voltage: VCE<1000, Current:IC<400A. Switching frequency up to 5kHz.Low on-state voltage: VCE(sat) : 2-3V • Low current gain (β). Need high base current to obtain reasonable IC . Expensive and complex base drive circuit. • Used commonly in the past • Now used in specific applications • Replaced by MOSFETs and IGBTs

  29. BJT Characteristics • To turn on/off the device, a base drive circuit is connected to the base and emitter terminal. • To turn on, current is injected into the base terminal. When turned on, conventional current passes from collector to emitter. • To turn-off, the base current is removed. • The current gain of a BJT ends to be low when operated in the saturated ON condition. β<10 is common. It deteriorates as voltage ratings increases. • It is normal to use Darlington connection for higher current gain.

  30. MOSFETs • Ratings: Voltage VDS<500V, current IDS<300A. • Very fast device: >100KHz. For some low power devices (few hundred watts) may go up to MHz range. • Easy to control by the gate • Optimal for low-voltage operation at high switching frequencies • On-state resistance a concern at higher voltage ratings

  31. MOSFET Characteristics • Turning on and off is very simple. Only need to provide VGS =+15V to turn on and 0V to turn off. Gate drive circuit is simple. • Basically low voltage device. High voltage device are available up to 600V but with limited current. Can be paralleled quite easily for higher current capability. • Internal (dynamic) resistance between drain and source during on state, RDS(ON), ,limits the power handling capability of MOSFET. High losses especially for high voltage device due to RDS(ON) . • Dominant in high frequency application (>100kHz). Biggest application is in switched-mode power supplies.

  32. IGBT • Combination of BJT and MOSFET characteristics. Compromises include: – Gate behaviour similar to MOSFET - easy to turn on and off. – Low losses like BJT due to low on-stateCollector-Emitter voltage (2-3V).

  33. IGBTCharacteristics • Ratings: Voltage: VCE<3.3kV, Current,:IC<1.2kA currently available. Work in under progress for 4.5kV/1.2kA device.Constant improvement in voltage and current ratings • Good switching capability (up to 100KHz) for newer devices. Typical application, IGBT is used at 20-50KHz. • For very high power devices and applications, frequency is limited to several KHz. • Very popular in new products; practically replacing BJT in most new applications.

  34. Gate-Turn-Off Thyristors (GTO) • Behave like normal thyristor, but can be turned off using gate signal • However turning off is difficult. Need very large reverse gate current (normally 1/5 of anode current) • Slow switching speeds • Used at very high power levels • Require elaborate gate control circuitry

  35. GTO Characteristics • Ratings: Voltage: Vak<5kV; Current:Ia<5kA. Highest power ratings switch. Frequency<5KHz. • Gate drive design is very difficult. Need very large reverse gate current to turn off.Often custom-tailored to specific application. • Currently getting very stiff competition from high power IGBT. The latter has much simpler and cheaper drivers. • GTO normally requires snubbers. High power snubbers are expensive. • In very high power region (>5kV, >5kA),development in gate-controlled thyristor (GCT) may effectively end the future of GTO

  36. Switch comparison (2000)

  37. Power Switch Losses • It is important to consider losses of power switches: – to ensure that the system operates reliably under prescribed ambient conditions – so that heat removal mechanism (e.g. heat sink,radiators, coolant) can be specified. Heat sinks and other heat removal systems are costly and bulky. – losses in switches affects the system efficiency • Main losses occurs in power switches are – forward conduction losses, – blocking state losses – switching losses

  38. Forward conduction losses Ideal switch has zero voltage drop across it during turn-on (Von). Although the forward current ( Ion ) may be large, the losses on the switch is zero. • But for real switches, e.g. BJT, IGBT, GTO, SCR, GCT have forward conduction voltage (on state) between 1- 3V. MOSFET has on state voltage which is characterised by the RDS(ON).

  39. Forward conduction andblocking state losses Forward conduction and blocking state losses • Losses is measured by product of volt-drop across the device Von with the current, Ion,averaged over the period. • Forward conduction losses is the major source of loss at low frequency and DC operation. • During turn-off, the switch blocks large voltage. Ideally no current should flow through the switch. But for real switch a small amount of leakage current may flow. This creates turn-off or blocking state Losses • The leakage current during turn-off is normally very small, Hence the turn-off losses are usually neglected.

  40. Switching losses • During turn-on and turn off, ideal switch requires zero transition time. Voltage and current are switched instantaneously. • In real switch,due to the non-idealities of power switches, the switching profile is as shown in above. • The switching losses occurs as a result of both the voltage and current changing simultaneously during the switching period.

  41. Switching losses • The product of device voltage and current gives instantaneous power dissipated in the device. • The heat energy that developed over the switching period is the integration (summation) of instantaneous power over time as shown by the shaded area under the power curve. • The average power loss is the sum of the turn-on and turn off energies multiplied by the switching frequency. • When frequency increase, switching losses increases. This limits the usable range of power switches unless proper heat removal mechanism is employed.

  42. Snubbers

  43. Snubbers • From previous equation, the voltage across the switch is bigger than the supply (for a short moment). • The spike may exceed the switch rated blocking voltage and causes damage due to overvoltage. • To prevent such occurrence, a snubber is put across the switch. An example of a snubber is an RCD circuit shown below. • Snubber circuit “smoothened” the transition and make the switch voltage rise more “slowly”. In effect it dampens the high voltage spike to a safe value. • Switches and diodes requires snubbers.However, new generation of IGBT,MOSFET and GCT do not require it.

  44. RCD Snubbers In general, snubbers are used for: – turn-on: to minimise large overcurrents through the device at turn-on – turn-off: to minimise large overvoltages across the device during turn-off. – Stress reduction: to shape the device switching waveform such that the voltage and current associated with the device are not high simultaneously.

  45. The Ideal Switch - The Power Electronics Engineer's Dream

  46. The Ideal Switch - The Power Electronics Engineer's Dream • An impossible dream but advances in device and packaging technology have produced • conductivity modulated bipolar devices for high power applications at high temperatures • mosfet fabrication for high frequency switching

  47. Characteristics of Practical Devices • Switching Power Loss is proportional to: • switching frequency • turn-on and turn-off times

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