电气工程及其自动化专业英语 Chapter 3 Power Electronic Technology

Chapter 3 Power Electronic Technology,Section 1 Semiconductor Switches,Text,New Words and Expressions,Exercises,End,Transition of part of speech,Section 1 Semiconductor Switches,Semiconductor switches are very important and crucial components in power electronic systems. These switches are meant to be the substitutions of the mechanical switches, but they are severely limited by the properties of the semiconductor materials and the process of manufacturing.,Section 1 Semiconductor Switches,Switching lossesPower losses in the power electronic converters are comprised of the switching losses and the parasitic losses. The parasitic losses account for the losses due to the winding resistances of the inductors and transformers, the dielectric losses of capacitors, the eddy and the hysteresis losses. The switching losses are significant and can be managed. They can be further divided into three components: (a) the on-state losses, (b) the off-state losses and the losses in the transition states.,Section 1 Semiconductor Switches,On-State LossesThe electrical switches conduct heavy current and have nonzero voltage across the switch in the on-state. The on-state power losses are given by,(3-1),The uson and if are respectively the switch voltage in the on-state and the forward current through the switch. For example, the typical power diodes and the power transistors have nearly 0.5 to l volt across them in the on-state. The forward currents can be hundreds to thousands of amperes. The on-state,Section 1 Semiconductor Switches,power losses are very significant. Off-State LossesThe electrical switches withstand high voltages and have nonzero leakage current through the switch in the off-state. The off-state power losses are given by,(3-2),Section 1 Semiconductor Switches,The usoff and ir are respectively the reverse bias voltage in the off-state and the reverse current through the switch. For example, the typical power diodes and the power transistors have high reverse voltages in hundreds to thousands of volts and microamps to milliamps through them in the off state. Transition-State LossesThe practical switching devices have limited capabilities of rate of voltage transition and the rate of current steering. These nonabrupt transition rates,Section 1 Semiconductor Switches,give rise to power losses in the switching devices. We will examine these switching losses in two cases separately: the inductive and capacitive loads. Switching with Inductive LoadThe inductor is assumed to be large so that the current through it in steady state is nearly constant Io. Assume that initially the switch is off. The inductor current is +Io and freewheels through diode V1. When the switch is turned on, the current through the switch begins to build up linearly (an assumption) to +Io while the diode V1 is still on.,Section 1 Semiconductor Switches,The on diode has zero voltage across it (an ideal diode), hence, the voltage on the switch is held constant at +Us. When the current buildup is over, the diode Vl ceases to conduct and the voltage on the switch ramps linearly (again an assumption) down to zero.When the switch is turned off, the voltage begins to build up linearly to +Us while the diode V1 is off. While the diode is off the current through the switch equals the inductor current, which is constant Io. After the switch voltage reaches zero,Section 1 Semiconductor Switches,the current through the switch begins to decrease below Io, as the remaining current is now steered through the diode V1, which has now turned on. The current through the switch ramps down to zero ultimately. Switching waveforms with inductive load are shown in Fig.3-1.,Fig.3-1 Switching waveforms with inductive load,Section 1 Semiconductor Switches,The switching losses are given by,(3-3),The switching power losses increase linearly with the switching frequency like in the resistive case but about six times more. The upper bound on the switching frequency is also about half.,(3-4),Section 1 Semiconductor Switches,Switching with capacitive load The capacitor is assumed to be large so that the voltage through it in steady state is nearly constant Uo. Assume that initially the switch is on, hence, the current through the switch is IS. The capacitor voltage is Uo, the voltage across the switch is zero and the diode V1 is reverse biased. When the switch is turned off, the switch voltage begins to ramp up to + Uo while the diode V1 is still off. During this buildup, the current through the switch is held constant at IS . When the voltage buildup is,Section 1 Semiconductor Switches,over, the diode Vl begins to conduct and the voltage on the switch is clamped at Uo, and the current through the switch ramps linearly (again an assumption) down to zero.When the switch is closed, the current begins to build up linearly to IS while the diode V1 is still on. The voltage on the switch remains clamped at UO. After the switch current reaches IS, the diode turns off and the voltage on the switch begins to ramp down to zero.,