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b c Primary ic c + vcn vbn + b Secondary D1 n v + an ib Bridge rectifier ia a D4 D6 R D2 D3 iL D5 + vL
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Figure 1121 Waveforms and conduction times of three-phase bridge recti er
Figure 1120 Three-phase diode bridge recti er
11
Power Electronics
EXAMPLE 115 Three-phase Bridge Recti er
Problem
Simulate the three-phase bridge recti er of Figure 1120 and verify numerically that the average and rms output voltages are given by equations 119 and 1110, respectively
Solution
Known Quantities: Source voltage; load resistance
Find: Vripple
Schematics, Diagrams, Circuits, and Given Data: The load resistance is R = 12
the source is 208 V, 3 phase
Focus on Computer-Aided Solutions: The analysis of this design has been conducted in simulation, using Electronics WorkbenchTM The simulation of this circuit may be found in the accompanying CD-ROM Comments: What is the effect of an inductive load on the load current and voltage
waveforms Try adding a 001-H series inductance to the load circuit
Thyristors and Controlled Recti ers In a number of applications, it is useful to be able to externally control the amount of current owing from an AC source to the load A family of power semiconductor devices called controlled recti ers allows for control of the recti er state by means of a third input, called the gate Figure 1122 depicts the appearance of a thyristor, or silicon controlled recti er (SCR), illustrating how the physical structure of this device consists of four layers, alternating p-type and n-type material Note that the circuit symbol for the thyristor suggests that this device acts as a diode, with provision for an additional external control signal The operation of the thyristor can be explained in an intuitive fashion as follows When the voltage vAK is negative (ie, providing reverse bias), the thyristor acts just like a conventional pn junction in the off state When vAK is forwardbiased and a small amount of current is injected into the gate, the thyristor conducts forward current The thyristor then continues to conduct (even in the absence of gate current), provided that vAK remains positive Figure 1123 depicts the i-v curve for the thyristor Note that the thyristor has two stable states, determined by the bias vAK and by the gate current In summary, the thyristor acts as a diode with a control gate that determines the time when conduction begins A somewhat more accurate description of thyristor operation may be provided if we realize that the four-layer pnpn device can be modeled as a pnp transistor connected to an npn transistor Figure 1124 clearly shows that, physically, this is a realistic representation Note that the anode current, iA , is equal to the emitter current of the pnp transistor (labeled Qp ) and the base current of Qp is equal to the collector current of the npn transistor, Qn Likewise, the base current of Qn is the sum of the gate current and the collector current of Qp The
A (anode) A P G (gate) N P N G K iA
+ vAK
K (cathode)
Figure 1122 Thyristor structure and circuit symbol
Part II
Electronics
iA Holding current Latching current Holding current Reverse breakdown voltage iL iH Forward leakage current vAK Gate triggered G P N P N
Reverse leakage current
Figure 1123 Thyristor i-v characteristic
behavior of this transistor model is explained as follows Suppose, initially, iG and iBn are both zero Then it follows that Qn is in cutoff, and therefore iCn = 0 But if iCn = 0, then the base current going into Qp is also zero and Qp is also in cutoff, and iCp = 0, consistent with our initial assumption Thus, this is a stable state, in the sense that unless an external condition perturbs the thyristor, it will remain off Now, suppose a small pulse of current is injected at the gate Then iBn > 0, and Qn starts to conduct, provided, of course, that vAK > 0 At this point, iCn , and therefore iBp , must be greater than zero, so that Qp conducts It is important to note that once the gate current has turned Qn on, Qp also conducts, so that iCP > 0 Thus, even though iG may cease, once this on state is reached, iCp = iBn continues to drive Qn so that the on state is also self-sustaining The only condition that will cause the thyristor to revert to the off state is the condition in which vAK becomes negative; in this case, both transistors return to the cutoff state In a typical controlled recti er application, the device is used as a half-wave recti er that conducts only after a trigger pulse is applied to the gate Without concerning ourselves with how the trigger pulse is generated, we can analyze the general waveforms for the circuit of Figure 1125 as follows Let the voltage vtrigger be applied to the gate of the thyristor at t = The voltage vtrigger can be a short pulse, provided by a suitable trigger-timing circuit ( 13 will discuss timing and switching circuits) At t = , the thyristor begins to conduct, and it continues to do so until the AC source enters its negative cycle Figure 1126 depicts the relevant waveforms Note how the DC load voltage is controlled by the ring time , according to the following expression: vL = VL = 1 T
T /2
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