2 2555 - elfit.ssru.ac.th · 2. DC-to-AC conversion (Inverter) DC/AC converters (inverters) are used primarily in UPS or emergency light. During normal electricity condition, the
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Steady DC from a battery or regulated power supply, this is ideal for electronic circuits
Smooth DC from a smoothed power supply,this is suitable for some electronics. * ระดับแรงดันที่เพิ่มขึ้นและลดลงเล็กน้อยเช่นน้ีเรียกว่ำค่ำริปเปิ้ล (Ripple)
Varying DC from a power supply without smoothing, this is not suitable for electronics
AC/DC converters (rectifiers) are used every time an electronic device is connected to the mains (computer, television and etc) 2. DC-to-AC conversion (Inverter)
DC/AC converters (inverters) are used primarily in UPS or emergency light. During normal electricity condition, the electricity will charge the DC battery. During blackout time, the DC battery will be used to produce AC electricity at its output to power up the appliances.
3. DC-to DC conversion (Chopper)
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DC/DC converters are used in most mobile devices (mobile phone, pda and etc) to maintain the voltage at a fixed value whatever the charge level of the battery is. These converters are also used for electronic isolation and power factor correction.
4. AC-to-AC conversion. (Cycloconverter) AC/AC converters are used to change either the voltage level or the frequency
(international power adapters, light dimmer). In power distribution networks AC/ AC converters may be used to exchange power between utility frequency 50 Hz and 60 Hz power grids
รูปที่ 1.4 Power diode (a) Symbol (b) v-i Characteristic TYPES OF POWER DIODE
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2) GTO – Gate Turn Off Thyristor GTO (Gate Turn Off Thyristor)
• 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) • Gate drive design is very difficult due to very large reverse gate current at turn off.
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• Ratings: Highest power ratings switch - Voltage: Vak < 5kV - Current: Ia < 5kA - Frequency: f < 5KHz
รูปที่ 1.5 GTO (a) Symbol and (b) v-i characteristic 125
3) TRIAC • Semiconductor device that electrically equivalent to two SCRs, connected anti parallel, although
internal structure are not exactly the same as that two SCRs • 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
• Combination of BJT and MOSFET characteristics. - Gate behaviour similar to MOSFET ,easy to turn on and off. - Low losses like BJT due to low on-state Collector- Emitter voltage (2-3V).
• Switching frequency up to 100KHz. Typical applications: 20-50KHz.
รูปที่ 1.7 IGBT (a) Symbol (b) v-i characteristic
5) DIAC • The construction of a diac is similar to an open base NPN transistor. • The diac is similar to having two parallel Shockley diodes turned in opposite directions
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• The bidirectional transistor-like structure exhibits a high-impedance blocking state up to a voltage breakover point (VBO) above which the device enters a negative- resistance region.
• These basic diac characteristics produce a bidirectional pulsing oscillator in a resistor- capacitor AC circuit.
• Since the diac is a bidirectional device, it makes a good economical trigger for firing triacs in phase control circuits such as light dimmers and motor speed controls.
(a)
(b)
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รูปที่ 1.8 DIAC (a) Symbol (b) v-i characteristic of bilateral trigger DIAC
6) PUT – PROGRAMMABLE UNIJUNCTION TRANSISTOR • The PUT is actually a type of thyristor • It can replace the UJT in some applications. • It is more similar to an SCR (four-layer device) except that its anode-to-gate voltage can be used
to both turn on and turn off the device. • Notice that the gate is connected to the n region adjacent to the anode. • The gate is always biased positive with respect to the cathode. • When VA - VG > 0.7 V, the PUT turns on. • The characteristic plot of VAK versus IA is similar to the VE versus IE plot of the UJT.
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รูปที่ 1.9 PUT (a) Basic construction (b) Symbol and biasing
7) SCR • If the forward break over voltage (Vbo) is exceeded, the SCR “self-triggers” into the conducting
state. • The presence of gate current will reduce Vbo. • “Normal” conditions for thyristors to turn on:
- 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. Vak collapses to normal forward volt- drop, typically 1.5-3V.
• In reverse -biased mode, the SCR behaves like a diode. • Thyristor cannot be turned off by applying negative gate current.
- It can only be turned off if Ia goes negative (reverse) - This happens when negative portion of the of sine-wave occurs (natural commutation)
• Another method of turning off is known as “forced commutation” - The anode current is “diverted” to another circuitry.
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รูปที่ 1.10 SCR (Thyristor) (a) Symbol (b) v-i characteristic
Air conditioning Cooking Lightning Electronics (personal computers, other entertainment equipment)
(b) Commercial
Heating, ventilating, and air conditioning Central refrigeration Lighting Computers and office equipment Uninterruptible power supplies (UPSs) Elevators
(d) Transportation Traction control of electric vehicles Battery chargers for electric vehicles Electric locomotives Street cars, trolley buses Subways Automotive electronics including engine controls
(e) Utility systems High-voltage dc transmission (HVDC) Static var compensation (SVC)
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Supplemental energy sources (wind, photovoltaic), fuel cells Energy storage systems Induced-draft fans and boiler feed water pumps
(f) Aerospace Space shuttle power supply systems satellite power systems Aircraft power systems
(g)Telecommunications Battery chargers Power supplies (dc and UPS)
1. A computer system needs various power supplies and the total cost of a computer is at present 1/3 power supplies. Figure right shows a power system employing one DC supply created from the mains and distributed individual power supplies for each sub-system.
2. The cable that is plugged into the aircraft that is waiting at the gate. In general terms the power conversion provides the following.
1 W < P < 10 MW: 107 W range The more Electric Aircraft initiative intelligently applies new, innovative electrically driven smart power sub-system technologies for aircraft secondary power which traditionally use hydraulic, pneumatic, mechanical and electrical components.
เอกสำรอ้ำงอิง [1] http://www.slideshare.net/mkazree/chapter-6-power-electronicdevices [2] Daniel W.Hart, “Introduction To Power Electronics,” Prentice-Hell,Inc.,USA.,1997 [3] http://www.kpsec.freeuk.com/acdc.htm#props [4] http://protorit.blogspot.com/2011/06/applications-of-power-electronics.html