電力電子技術(shù)外文翻譯-資料下載頁(yè)
2025-01-06 17:19本頁(yè)面
【正文】 drop below the forwardvoltage drop of the SCR so that the current drops below a minimum value, called the holding current , or the polarity from anode to cathode must actually reverse. The fact that the SCR cannot be turned off easily limits its use in dc applications to those cases where some method of reducing the forward current to below the holding values can be provided. In ac circuits, the SCR will automatically turn off in every half cycle when the ac voltage applied to the SCR reverses polarity. Characteristics and specifications of SCRs are as follows. 1. Maximum forward current. There is a maximum current that the SCR can carry in the forward direction without damage. This value varies from a few hundred milliamps to more than a thousand amps, for large industrial types. 2. Peak reverse voltage. Like a diode, there is a perk reversebias voltage that can be applied to the SCR without damage. The value varies from a few volts to several thousand volts. 3. Trigger voltage. The minimum gate voltage to drive the SCR into conduction varies between types and sizes, from a few volts to 40 V. 4. Trigger current. There is a minimum current that the source of trigger voltage must be able to provide before the SCR can be fired. This varies from a few milliamps to several hundred milliamps. 5. Holding current . This refers to the minimum anodetocathode current necessary to keep the SCR conducting in the forwardconducting state. The value varies from 20 to 100 mA. AC Operation The operation of an SCR varies in the rms dc voltage in halfwave operation. The trigger voltage is developed by some circuit that produces a pulse at a certain selected phase of the applied ac signal. Thus, the SCR turns on in a repetitive fashion as shown. The SCR is turned back off, of course, in each half cycle when the ac polarity reverses. By changing the part of the positive half cycle when the trigger is applied, the effective (rms) value of dc voltage applied to the load can be increased. Of course, with this circuit the maximum possible rms dc voltage is that which would be developed by a halfwave rectifier. If more power is required, the SCR can be used with a fullwave bridge circuit. The trigger voltage must now be generated in each half cycle and applied to the SCR trigger (gate) terminal. In a processcontrol application, the controller output signal would be used to drive a circuit that changed the time at which the pulses were applied to the gates, and thus changed the power applied to the load. The voltage applied to the load is pulsating dc. This configuration could not be used with a load that required ac voltage for operation. Trigger Control To use the SCR in processcontrol applications, special circuitry to convert control signalsinto suitable trigger signals to the SCRs is required. These circuits are usually posed ofelectronic systems that use the control voltage to determine the phase of the ac load voltage atwhich the SCR should be turned on. The controlsignal voltage is used to provide base drive to a transistor via an LED that ensures isolation of the control circuit from the power circuit. At lowbase drive the capacitor is charged slowly, and will not reach the SCR trigger voltage until late in the cycle (hence low load power). A large control signal will provide high base drive, and the capacitor will charge much more quickly. Then the SCR will turn on much earlier in the cycle, and more power will be delivered to the load. 教師評(píng)語(yǔ) 教師簽名： 20 年 月 日
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