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Introduction.

 If the DC supply shown in Fig. 1 can be switched rapidly on and off, the load waveforms will be those shown. A continuously variable average voltage from (nearly) zero to (nearly) the full supply voltage is available by changing the duty cycle of the switching. The “nearly” has to be added, because switching devices in the real world have minimum on times that produce a small output and minimum off times that prevent them from reaching full output voltage in a controlled manner. They can be switched completely on or completely off, but small prohibited areas of control always exist near those extremes.

a)

b)

Figure 1 Basic pulse width modulation.

    The limitations imposed by the need for AC line voltage or forced commutation in SCRs limited their applications in pulse width modulated systems, and they spurred the development of SCR derivatives that could be turned off by a control signal. The SCR morphed in one direction into the gate turnoff thyristor (GTO). This device can be turned off by a gate signal with a very high peak power but a small average power. The GTO is available in high-current and high-voltage ratings and has found niche applications where its characteristics are favorable. But it remains very expensive, and it cannot be classed as a general-purpose industrial control device at this time.
   Another SCR derivative is the integrated gate controlled thyristor (IGCT). The IGCT can interrupt anode current by having a current larger than the anode current injected into the gate. It requires a very low inductance in the gate circuit to create a current equal to the anode current with a low-voltage supply, and that mandates incorporating the gate driver into the physical packaging of the IGCT. The required energy storage for the gate signal is aided by the use of “supercaps,” low-voltage electrolytic capacitors with a very high energy density. Again, the device is not really suitable for general-purpose use, although it has been used successfully in high-power applications. Both the GTO and the IGCT are somewhat handicapped by the regenerative, latching characteristic of the parent SCR. They do not cease conduction gracefully but must be driven out from the gate. Developments on power transistors continued through the years and resulted in a breakthrough when a field effect transistor was wedded to a conventional bipolar transistor. The result, the integrated gate bipolar transistor (IGBT), is rapidly becoming the device of choice for many applications. It has engendered a host of new high-power equipment designs featuring switchmode operation. The device has the ability to operate at frequencies several orders of magnitude higher than the SCR, although the higher-power IGBTs are more limited in their maximum frequency.
    The IGBT has a PNP power transistor controlled by a field effect transistor such that it has the best properties of each. It can handle very high powers and yet be controlled by a gate that, being insulated and field controlled, draws no current. Figure 13.2 shows the collector-emitter voltage as a function of the collector current for several different gate-emitter voltages. As the gate voltage is increased, the IGBT begins to draw current, and this current increases with collector voltage until a region is reached at which the current is constant for a given gate voltage. At high levels of gate voltage, however, a saturated region is reached in which the collector-emitter voltage varies only slightly with collector current. Although the IGBT can be operated as a linear amplifier in the same manner as a transistor, it is almost always operated as a switch between off and on with enough gate drive so it remains at the lowest possible voltage drop for the required current. The saturated voltage drop is typically in the 3.0-V range. IGBTs are currently available in voltages to 6000 V and currents to 2000 A. Most large IGBTs are packaged in a molded plastic assembly on an integral mounting plate that serves as a contact medium to a heat sink. The active device itself consists of a large number of small IGBTs connected in parallel with bond wires to the external electrodes. Many have an integral inverse parallel diode.

Figure 2 - IGBT schematic and characteristics.

    External circuitry for the IGBT must recognize the fast switching characteristics of the device. Turn-off time can be as little as 500 ns and, with collector currents as high as several thousand amperes, the circuit di/dt can reach fearsome levels. It is imperative that stray inductances in snubber circuits be kept to an absolute minimum to protect the device from overvoltage. It is also necessary to prevent excessive load current from pulling the IGBT out of saturation. The dissipation will rise to high levels that can destroy the device in a relatively few microseconds. Yet, immediate cessation of gating may create an excessive di/dt and an overvoltage on turn-off from faults. The designer must balance the need to limit dissipation with the need for voltage protection by controlling the rate of gate signal turn-off.
   Lest one conclude from the above that the IGBT is a fragile device, nothing could be further from the truth. IGBTs have proven themselves as the power conversion elements in AC motor drives over 20,000 hp and in choppers for DC electric arc furnaces at power levels above 100 MW. They are rugged, reliable devices.

The “H” Bridge

    Four IGBTs can be combined to form a bipolar switching circuit that is the basis for many motor drives and other applications. Figure 3shows the basic H-bridge. The IGBTs are each equipped with an antiparallel diode that provides a free-wheeling path for load current and clamps any switching transients to the DC buses. If IGBTs 1 and 2 are turned on, load current flows in the direction of arrow F. If 3 and 4 are on, the current flows in direction R. Both polarities can be pulse width modulated to form an AC output waveform.

Figure 3 - The “H” bridge.

If a sine wave is used as a command waveform, the switching series is shown in Fig. 4. The output waveform will be somewhat “ragged,” because the switching frequency is quite low to illustrate the process more clearly. The output waveform is sometimes cleaned up with an output filter to lessen harmonics at the load.

Figure 4 - PWM sine wave switching.

    The proper switching protocol is essential to operation of the “H” bridge. If, say, IGBTs 1 and 2 are switched on at the same time that 3  and 4 are switched off, there will be a short circuit because of the turn off times of 3 and 4. An “underlap” circuit is necessary to ensure that 3 and 4 are turned off and recovered some time before 1 and 2 are turned on.
   One more section can be added to this circuit to form the threephase motor drive shown schematically in Fig. 5.

Figure 5 - IGBT motor drive.

    Many variations on the three-phase circuit have been devised to meet the objectives of lower harmonic distortion at the load, higher voltage output, and a host of other specialized needs. One objection to the basic H-bridge is that the load is switched from peak positive voltage to peak negative, and alternative circuits have been developed to provide lower voltage switching steps to reduce the dv/dt at the motor. Among these circuits is the neutral-point-clamped inverter, an arrangement that offers five switching voltage levels instead of only two. In addition to the size of the voltage step, the dv/dt tends to rise with the switching frequency, because the higher-frequency semiconductors switch faster. Motor windings are subject to the front-of-wave effects, and the application of solid-state drives to older motors led to problems with insulation failures. Motor manufacturers developed enhanced insulation techniques, and motors are now available that are rated for solid-state drives. Various types of transistors can be used in the basic H-bridge to operate directly from the AC line and power a high-frequency transformer. Small, lightweight, high-frequency transformers and filters have revolutionized the design of DC power supplies for all manner of equipment operated from a 120-Vac line.

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