2nd episode of "Theory of operation of inverter from scartch"

Characteristics

The voltage-current characteristics of a diode are shown in Fig. 1.3. In the forward region, the diode starts conducting as the anode voltage is increased with respect to the cathode. The voltage where the current starts to increase rapidly is called the knee voltage of the diode. For a silicon diode, the knee voltage is approximately 0.7 V. Above the knee voltage, small increases in the diode voltage produce large increases in the diode current. If the diode current is too large, excessive heat will be generated, which can destroy the diode. When the diode is reverse-biased, diode current is very small for all values of reverse voltage less than the diode breakdown voltage. At breakdown, the diode current increases rapidly for small increases in diode voltage.

Simple application :-

Rectifier Circuits

Rectifier circuits produce a DC voltage or current from an AC source. The diode is an essential component of these circuits. Figure 1.4 shows a half-wave rectifier circuit using a diode. During the positive half cycle of the source voltage, the diode is forward-biased and conducts for vs(t) > Ef. The value of Ef for germanium is 0.2 V and for silicon it is 0.7 V. During the negative half cycle of vs(t) , the diode is reverse biased and does not conduct. The voltage vL(t) across the load RL is shown in Fig. 1.5.

The half-wave rectifier circuit produces a pulsating direct current that uses only the positive half cycle of the source voltage. The full-wave rectifier shown in Fig. 1.6 uses both half cycles of source voltage. During the positive half cycle of vs(t), diodes D1 and D2 are forward-biased and conduct. Diodes D3 and D4 are reverse-biased and do not conduct. During the negative half cycle of vs(t), diodes D1 and D2 are reverse-biased and do not conduct, whereas diodes D3 and D4 are forward-biased and conduct. The voltage vL(t) across the load RL is shown in Fig. 1.7.

Thyristors

Thyristors are four-layer pnpn power semiconductor devices. These devices switch between conducting and nonconducting states in response to a control signal. Thyristors are used in timing circuits, AC motor speed control, light dimmers, and switching circuits. Small thyristors are also used as pulse sources for large thyristors. The thyristor family includes the silicon-controlled rectifier (SCR), the DIAC, the Triac,the silicon-controlled switch (SCS), and the gate turn-off thyristor (GTO) .

The SCR is the most commonly used electrical power controller. An SCR is sometimes called a pnpn diode because it conducts electrical current in only one direction. Figure 1.8a shows the SCR symbol. It has three terminals: the anode (A), the cathode (K), and the gate (G). The anode and the cathode are the power terminals and the gate is the control terminal. The structure of an SCR is shown in Fig. 1.8b.

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When the SCR is forward-biased, that is, when the anode of an SCR is made more positive with respect to the cathode, the two outermost pn-junctions are forward-biased. The middle pn-junction is reversebiased and the current cannot flow. If a small gate current is now applied, it forward-biases the middle pnjunction and allows a much larger current to flow through the device. The SCR stays ON even if the gate current is removed. SCR shutoff occurs only when the anode current becomes less than a level called the holding current (IH).

Characteristics

The volt-ampere characteristic of an SCR is shown in Fig. 1.9. If the forward bias is increased to the forward breakover voltage, VFBO, the SCR turns ON. The value of forward breakover voltage is controlled by the gate current IG. If the gate-cathode pn-junction is forward-biased, the SCR is turned ON at a lower breakover voltage than with the gate open. As shown in Fig. 1.9, the breakover voltage decreases with an increase in the gate current. At a low gate current, the SCR turns ON at a lower forward anode voltage. At a higher gate current, the SCR turns ON at a still lower value of forward anode voltage. When the SCR is reverse-biased, there is a small reverse leakage current (IR). If the reverse bias is increased until the voltage reaches the reverse breakdown voltage (V(BR)R), the reverse current will increase sharply. If the current is not limited to a safe value, the SCR may be destroyed.

Simple application :-

SCR Turn-Off Circuits

If an SCR is forward-biased and a gate signal is applied, the device turns ON. Once the anode current is above IH, the gate loses control. The only way to turn OFF the SCR is to make the anode terminal negative with respect to the cathode or to decrease the anode current below IH. The process of SCR turnoff is called commutation. Figure 1.10shows an SCR commutation circuit. This type of commutation method is called AC line commutation. The load current IL flows during the positive half cycle of the source voltage. The SCR is reverse-biased during the negative half cycle of the source voltage. With a zero gate current, the SCR will turn OFF if the turn-off time of the SCR is less than the duration of the half cycle.

Power Bipolar Junction Transistors

Power bipolar junction transistors (BJTs) play a vital role in power circuits. Like most other power devices, power transistors are generally constructed using silicon. The use of silicon allows operation of a BJT at higher currents and junction temperatures, which leads to the use of power transistors in AC applications where ranges of up to several hundred kilowatts are essential.

The power transistor is part of a family of three-layer devices. The three layers or terminals of a transistor are the base, the collector, and the emitter. Effectively, the transistor is equivalent to having two pn-diode junctions stacked in opposite directions to each other. The two types of a transistor are termed npn and pnp. The npn-type transistor has a higher current-to-voltage rating than the pnp and is preferred for most power conversion applications. The easiest way to distinguish an npn-type transistor from a pnp-type is by virtue of the schematic or circuit symbol. The pnp type has an arrowhead on the emitter that points toward the base. Figure 1.11 shows the structure and the symbol of a pnp-type transistor. The npn-type transistor has an arrowhead pointing away from the base. Figure 1.12 shows the structure and the symbol of an npn-type transistor.

When used as a switch, the transistor controls the power from the source to the load by supplying sufficient base current. This small current from the driving circuit through the base–emitter, which must be maintained, turns on the collector—emitter path. Removing the current from the base–emitter path and making the base voltage slightly negative turns off the switch. Even though the base–emitter path may only utilize a small amount of current, the collector–emitter path is capable of carrying a much higher current.

The volt-ampere characteristics of a BJT are shown in Fig. 1.13 Power transistors have exceptional characteristics as an ideal switch and they are primarily used as switches. In this type of application, they make use of the common emitter connection shown in Fig. 1.14 The three regions of operation for a transistor that must be taken into consideration are the cutoff, saturation, and the active region. When the base current (I_B) is zero, the collector current (I_C) is insignificant and the transistor is driven into the cutoff region. The transistor is now in the OFF state. The collector–base and base–emitter junctions are reversebiased in the cutoff region or OFF state, and the transistor behaves as an open switch. The base current(I_B) determines the saturation current. This occurs when the base current is sufficient to drive the transistor into saturation. During saturation, both junctions are forward-biased and the transistor acts like a closed switch. The saturation voltage increases with an increase in current and is normally between0.5 to 2.5 V. The active region of the transistor is mainly used for amplifier applications and should be avoided for switching operation. In the active region, the collector–base junction is reversed-biased and the base–emitter junction is forward-biased.

When a transistor is used as a switch, the control circuit provides the necessary base current. The current of the base determines the ON or OFF state of the transistor switch. The collector and the emitter of the transistor form the power terminals of the switch.

The DC load line represents all of the possible operating points of a transistor and is shown in Fig. 1.14.The operating point is where the load line and the base current intersect and is determined by the values of V_CC and R_C.

In the ON state, the ideal operating point occurs when the collector current I_C is equal to V_CC /R_C and V_CE is zero. The actual operating point occurs when the load line intersects the base current at the saturation point. This occurs when the base current equals the saturation current or I_B = I_B(sat). At this point, the collector current is maximum and the transistor has a small voltage drop across the collector–emitter terminals called the saturation voltage V_CE(sat).

In the OFF state, or cutoff point, the ideal operating point occurs when the collector current I_C is zero and the collector–emitter voltage V_CE is equal to the supply voltage V_CC. The actual operating point, in the OFF state, occurs when the load line intersects the base current (IB = 0). At the cutoff point, the collector current is the leakage current. By applying Kirchoff ’s voltage law around the output loop, the collector–emitter voltage (V_CE) can be found.

The operating points between the saturation and cutoff constitute the active region. When operating in the active region, high power dissipation occurs due to the relatively high values of collector current I_C and collector–emitter voltage V_CE. For satisfactory operation, a slightly higher than minimum base current will ensure a saturated ON state and will result in reduced turn-on time and power dissipation.

References :-

1-power electronics HandBook , Sohail Anwar

2-power electronics devices, circuits and application , Muhammad H. Rasheed

3-power electronics devices, circuits and passive elements ,Barry williams

4-power electronics devices , Mohan

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Thanks.