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Note for Power Electronics - PE by Premananda Pany

  • Power Electronics - PE
  • Note
  • Gandhi Institute of Engineering and Technology University - GIET
  • Electrical Engineering
  • B.Tech
  • 4 Topics
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POWER ELECTRONICS MODULE-I (As per BPUT Syllabus) B.Tech (5th Semester) Prepared by Dr. Premananda Pany Power semiconductor devices 1. Switching and V-I characteristic of devices: power diode Diode is a two terminal P-N junction semiconductor device, with terminals anode (A) and cathode (C). It is a uncontrolled device. The symbol of the Power diode is shown below which is same as signal level diode. Fig.1.Power diode symbol. When anode (terminal A) is made at a higher potential thancathode (terminal K), the device is said to be forward biased and a forward current will flow from anode to cathode.A small voltage drop arises across the device typically <1V called as forward voltage drop(Vf), which under ideal conditions is usually ignored.By contrast, when a diode is reverse biased, it does not conduct and the diode then experiences a small current flowing in the reverse direction called the leakage current. It is shown below in the VI characteristics of the diode. Fig.2 v-i characteristic of power diode. The structure of power diode is different from the low power signal diode. 1

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Power Diode Characteristics: The reverse recovery characteristics of the Power diode is shown in the figure 2. From the figure, we can understand the turn off characteristic of the diode. The Reverse recovery time trr is the time interval between the application of reverse voltage and the reverse current dropped to 0.25 of Irr.Parameter ta is the interval between the zero crossing of the diode current to it reaches Irr. Parameter tb is the time interval from the maximum reverse recovery current to 0:25 of IRR. The lower trr means fast diode switching. The ratio of the two parameters ta and tb is known as the softness factor SF. For power diodes, there are two voltage ratings. One is the repetitive peak inverse voltage (VRRM) and the other is the non-repetitive peak inverse voltage.The non-repetitive voltage (VRM) is the diode’s capability to block a reverse voltage that may occur occasionally due to over voltage surge.The data sheet of a diode normally specifies three different current ratings. They are: (1) Average current; (2) RMS current; and (3) Peak current. A design engineer must ensure that each of these values are never exceeded. Diode Protection: Snubber circuits are essential for diodes used in switching circuits. It can save a diode from overvoltage spikes, which may arise during the reverse recovery process. A very common snubber circuit for a power diode consists of a capacitor and a resistor connected in parallel with the diode as shown. Diode is widely used in rectifiers, Voltage Clamping circuit, As freewheeling Diode in a Voltage Multiplier. Types of Power Diode: Schottky diodes: These diodes are used where a low forward voltage drop (usually 0.3V) is needed in low output voltage circuits. These diodes are limited in their blocking voltage capabilities to 50 – 100V. Fast Recovery diodes: These are used in high frequency circuits in combination with controllable switches where a small reverse recovery time is needed. At power levels of several hundred volts and several hundred amperes, these diodes have trr ratings of less than a few microseconds. Line - frequency diodes: The on-state voltage of these diodes is designed to be as low as possible and as a consequence have larger trr, which are acceptable for line frequency applications. These diodes are available with blocking voltage ratings of several kilovolts and current ratings of several kilo amperes. Moreover, they can be connected in series and parallel to satisfy any voltage and current requirement. 2

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Thyristor family: SCR Fig.3. Symbol of thyristor. A thyristor is generallya four layer three-terminal device. Four layers are formed by alternating p-type and n-type semiconductor materials. Subsequently, there are three pn junctions formed in the device. It is a bistable device. The three terminals of this device are called anode (A), cathode (K) and gate (G) respectively. The gate terminal is control terminal of the device. That means, the current flowing through the device is controlled by electrical signal applied to the gate terminal. The anode and cathode are the power terminals of the device handle the large applied voltage and conduct the main current through the thyristor. Fig.4. P-N junctions of thyristor. A thyristors acts as an on-off switch which is used to control output power of an electrical circuit by switching on and off the load circuit periodically in a pre-set interval. The main difference of thyristors with other electronics switches is that, a thyristor can handle large current and can withstand large voltage, whereas electronic switches handle only small current and voltage. When positive potential applied to the anode with respect to the cathode, ideally no current will flow through the device and this condition is called forward – blocking state but when appropriate gate signal is applied, a large forward anode current starts flowing, with a small anode-cathode potential drop and the device becomes in forward-conduction state. Although after removing the gate signal, the device will remain in its forwardconduction mode until the polarity of the load reverses.Therefore, thyristor is also called a latching device. Some thyristors are also controllable in switching from forward-conduction back to a forward-blocking state. 3

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(a) (b) Fig.5. (a) circuit for obtaining v-I characteristic of thyristor (b) static v-I characteristic of a thyristor TRIAC The Triac is a member of the thyristor family. But thyristor conducts only in one direction whereas triac can conduct in both directions. A triac hast hree terminals similar to two anti-parallel connected thyristors. The triac conducts by injecting a current pulse into the gate terminal. The gate loses control over conduction once the triac is turned on. The triac turns off only when the current through it become zero. Therefore, a triac is known as a minority carrier, a bidirectional semi-controlled device. They are widely used in residential lamp regulators, heater control and for speed control of small single-phase series and induction motors.Fig. 6 (a) and (b) show the triac symbol and equivalent circuit of a triac respectively. The three terminals are marked as MT1 (Main Terminal 1), MT2 (Main Terminal 2) and the gate by G. TRAC symbolEquivalent Circuit Fig. 6(a) Fig. 6(b) Fig. 6. TRIAC symbol and Equivalent circuit 4

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