Devices part 2
- 4. Power Semiconductor Devices: • Firing circuits and commutation circuits. • Cooling and heat sink. • Snubber circuits • Fuse selection. • Series and parallel operation of SCR • Switching Losses in Semiconductor Devices This Module will cover the following topics
- 5. SCR Firing Circuits (Manual)
- 6. Firing Circuits As the capacitor charges up, the potential difference across its plates slowly increases with the actual time taken for the charge on the capacitor to reach 63%of its maximum possible voltage, in our curve 0.63Vs being known as one Time Constant, ( T ).
- 7. SCR Firing Circuits (UJT)
- 9. Brushless motors rely on semiconductor switches to turn stator windings on and off at the appropriate time. The process is called electronic commutation, borrowing on terminology used for the mechanism in dc motors, called a commutator, that switches current into the rotarcoiles. Commutation circuits As we know that once a thyristor starts conducting then it continue to conduct till the current flowing through it reduced below the holding current. In commutation we mainly reduce thyristor's current from holding current. Also Read: Triggering Methods Of Thyristor or SCR
- 10. There are two methods of commutation of thyristors : Natural Commutation Forced Commutation. 1. Natural Commutation: The process of turning off a thyristor without using any external circuits is known as Natural commutation. This type of commutation is only possible in AC Applications. • When using A.C. Supply, the current passing through the device is alternating. • This alternating current go to peak positive value , passes through its zero and then go through peak negative value. When the alternating quantity at zero then the current passing through the device also zero and at same time a reverse polarity develops across the thyristor. • This will quickly Turn OFF the thyristor. This process of commutation is known as Natural Commutation.
- 11. 2. Forced Commutation : Classification of Forced Commutation Methods The forced commutation can be classified into different methods as follows: Class A: Self commutated by a resonating load Class B: Self commutated by an LC circuit Class C: Cor L-C switched by another load carrying SCR Class D: C or L-C switched by an auxiliary SCR Class E: An external pulse source for commutation Class F: AC line commutation Forced commutation can be observed while using DC supply; hence it is also called as DC commutation. The external circuit used for forced commutation process is called as commutation circuit and the elements used in this circuit are called as commutating elements
- 14. Class C, C or L-C switched by another load–carrying SCR This configuration has two SCRs. One of them may be the main SCR and the other auxiliary. Both may be load current carrying main SCRs. The configuration may have four SCR with the load across the capacitor, with the integral converter supplied from a current source. Assume SCR2 is conducting. C then charges up in the polarity shown. When SCR1 is triggered, C is switched across SCR 2 via SCR 1 and the discharge current of C opposes the flow of load current in SCR2.
- 15. Natural VS. Forced Commutation of thyristors
- 16. Heat Sink Losses in power electronic converters produce heat that must be transferred away and dissipated to the surroundings The major losses occur in semiconductor power switches, while their small size limits their thermal capacity High temperatures of the semiconductor structures cause degradation of electrical characteristics of switches, such as the maximum blocked voltage or the turn-off time Serious overheating can lead to destruction of a semiconductor device in a short time
- 17. • Natural convection cooling • Forced air cooling • Liquid cooling Heat Sink
- 18. Heat Sink
- 20. 20 Rugged, reliable, efficient, long lived, but not very fast A switch
- 21. 21 Snubbers are energy-absorbing circuits used to suppress the voltage spikes caused by the circuit's inductance when a switch, electrical or mechanical, opens up. The most common snubber circuit is a capacitor and resistor connected in series across the switch (transistor) Snubbers Circuits Voltage Spikes
- 22. Switch is making/On V = 0, At the moment the switch is opening (contact breaking) the voltage will increase and overshoot (spike)will occur then settled to source value At this moment the inductor will behave like a source and will provide the energy to CAP to remain energized is reflected by the –ve dip in the current. The moment Switch is making/On the voltages will reduce across the SW and instantaneous current will rush find the reassessment then settled down.
- 23. • Reduce or eliminate voltage or current spikes • Limit dI/dt or dV/dt • Shape the load line to keep it within the safe operating area (SOA) • Transfer power dissipation from the switch to a resistor or a useful load • Reduce total losses due to switching • Reduce EMI by damping voltage and current ringing Snubbers are circuits which are placed across semiconductor devices for protection and to improve performance. Snubbers can do many things: Two most common snubbers are: The resistor capacitor (RC) damping network The resistor-capacitor-diode (RCD) turn-off snubber. RC Snubber Circuits
- 27. Circuit Protection Circuit protection requires : Over Current protection Over voltages Protections Fuses are use d in between the source and the load. The fuse is a reliable over current protective device, primarily used as a circuit protection device for over currents, overloads and short circuits Fuse possesses an element that is directly heated by the passage of current and is destroyed when the current exceeds a predetermined value Overloads Short-Circuits
- 28. Normal load vs. Overload Fuse reaction time. Fuse Reaction time
- 30. Fuse Reaction time Normal load vs. Overload Fuse reaction time. Pre-arc time tpa This refers to the time range between the onset of a current that is large enough to melt the fusible element. Arcing time ta This refers to the time lapse between the generation of the arc and its (final) extinction. Operating time top This is the sum of the pre-arcing time and the arcing time. tT
- 31. In distribution systems on bases of speed ratio Fast blow : K Type Slow blow: T Type Fast Below and Slow Blow Fuses
- 32. There are a number of standards to classify fuses according to the rated voltages, rated currents, time/current characteristics, manufacturing features and other considerations Circuit Protection ANSI/UL 198‐1982 standards: Low voltage fuses are 600 V or less. Medium and high voltage fuses are in range 2.3‐138 kV fuse cutout and a backup current-limiting fuse A Fault operated can be easily identified from the ground since it drops open. Both the fuse tube and the backup limiter can be quickly removed from the mounting with a telescoping hot stick. American National Standards Institute
- 33. Working
- 34. Overload protection requires a current transformer which simply measures the current in a circuit. Overload CT for operation on a 110 kV grid
- 35. TOC protection operates based on a current vs time curve. Based on this curve if the measured current exceeds a given level for the preset amount of time, There are two types of overload protection: Instantaneous overcurrent Time over current (TOC). Instantaneous overcurrent requires that the current exceeds a predetermined level for the circuit breaker to operate.
- 36. MOV Technology • Contains a ceramic mass of zinc oxide grains combined with small quantities of bismuth, cobalt and manganese sandwiched between two metal plates • The boundary between each grain and its neighbor forms a diode junction, allowing current to only flow in one direction • Equivalent to a mass of back-to- back diode pairs, each in parallel Schematic Symbols
- 37. Surge Protective Device (SPD) • A surge protective device, or SPD, reduces the magnitude of a voltage transient surge thus protecting equipment from damaging effects. SPD’s were commonly known in the past as TVSS (Transient Voltage Surge Suppressor) • A SPD tries to: – Send surge away (to ground) – Acts as a momentary ‘short circuit’ • ‘short circuit’ ≈ voltage equalization ≈ no overvoltage ≈ protected load
- 38. How a SPD Works • The SPD acts as a pressure relief valve • The pressure relief valve (SPD) does nothing until an over- pressure pulse (voltage surge) occurs in the water (power) supply SPD Transient Voltage SPD Shunt Path Surge Protective Device
- 39. Metal Oxide Varistors (MOV) Contains a ceramic mass of zinc oxide grains, combined with other metal oxides sandwiched between two metal plates forming a network of back-to-back diode pairs Silicon Junction Diode The diode is installed reverse-biased under normal conditions. When the voltage rises above normal conditions the diode becomes forward-biased Spark Gap If a voltage surge is experienced a spark ignites gases creating an arc across the gap Gas Tube Arrestor Commonly used for telephone lines as they enter a building Sophisticated spark gap that safely shunts the surge to ground Types of SPD Technologies Surge Protective Device
- 40. MOV Failure Modes • There are two types of MOV failure modes: 1. High energy over-voltages 2. Lower energy repetitive pulses
- 41. MOV Failure due to High Energy Over Voltages • Event: Large single energy event spike or transient beyond the rated capacity of the device – Failure: Device will rupture or explode • Event: Sustained over-voltage condition building up energy – Failure: Device will go into thermal overheating and rupture (thermal overload) • Event: Repeated lower level spikes or transient over-voltage conditions – Failure: Device will slowly degrade until failure Due to the destructive nature of this failure surge rated fuses are required for all MOV installations. (Except TPMOV®) MOV Failure Every time an MOV switches, it’s life is slightly degraded. The greater the transient hit, the greater the degradation of the MOV
- 42. Series and parallel operation of SCR SCRs are available of ratings up to 10 KV and 3 KA. Sometimes system face demand, more than these ratings. That requires combination of more than one SCRs . Connecting SCRs in Series meets high voltage demand. Connecting SCRs in parallel meets high current demand.
- 43. Series and Parallel Connection of SCR or Thyristor Series Operation of SCR When the operating voltage is more than the rating of one SCR the multiple SCRs of same ratings are used in series. Parallel Operation of SCR When the operating current is more than the individual current ratings of SCRs then we use more than one SCRs in parallel.
- 44. Switching Losses in Semiconductor Devices Power dissipate is common in all semiconductor devices. Device fail and destroy themselves if it dissipate too much power, that may also damage the other system components. What happened when switch is turned ON and OFF Switch is ON Entire current Io flows through the switch and the diode is reverse biased Switch is turned OFF I0 flows through the diode and a voltage equal to the input voltage Vd appears across the switch. (assuming a zero voltage drop across the ideal diode)
- 45. Switching Losses in Semiconductor Devices When the switch is turned ON by applying positive control signal to the switch, as is shown During the turn-on transition of this generic switch The current buildup consists of a short delay time td(on) followed by the current rise time tri. The current Io flows entirely through the switch can the diode become reverse biased and the switch voltage fall to a small on-state value of Von, voltage fall time of tfv . The waveforms in Fig indicate that large values of switch voltage and current are present simultaneously during the turn-on crossover interval tc(on) Voltages across switch
- 46. Switching Losses in Semiconductor Devices The energy dissipated in the device during this turn-on transition can be approximated from Fig switching power loss Ps in the switch is due to these transitions can be approximated from Eqs The energy dissipation Won in the switch during this on-state interval can be approximated as
- 47. Switching Losses in Semiconductor Devices The other major contribution to the power loss in the switch is the average power dissipated during the on- state Pon , which varies in proportion to the on-state voltage. From Eq. which shows that the on-state voltage across the switch should be as small as possible Therefore, the total average power dissipation PT in a switch equals the sum of Ps and Pon . PT = Ps + Pon