notes%208 - Class notes #8 Ideal diode Diodes are devices...

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Class notes #8 Ideal diode Diodes are devices that conduct better in one direction than in the other direction. The ideal diode is a theoretical device that carries this idea to the extreme: it is a perfect conductor in one direction and a perfect insulator in the other direction. If such a device could be built it would have an IV (current versus voltage) characteristic as depicted in Figure 1. The horizontal part of the IV corresponds to zero current indicating that the ideal diode is a perfect insulator in one direction (the diode is blocking current). The vertical part of the IV indicates where the diode is acting as a perfect conductor: the voltage is zero and the current is arbitrarily large. Real diodes Ideal diodes do not exist. Instead we make due with real semiconductor diodes whose IV characteristics are indicated in Figure 2. There are many differences between real and ideal diodes. The principle ones are the forward voltage, breakdown voltage, and leakage current. The difference that is obvious when comparing Figure 1 and Figure 2 is that the voltage is not zero when the real diode is conducting. The real current depends exponentially on the applied voltage which makes for a very sharp increase in current at a certain voltage. In fact a look at the curve suggests that the voltage is Figure 1: Ideal diode IV characteristic Figure 2: Real diode IV characteristic
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nearly unchanged over a wide range of currents. We call this voltage the forward voltage , and it depends on the materials used to make the diode and on the diode temperature. The common silicon diodes have a forward voltage of around 0.6 to 0.7 volts. The 1N5819 Schottky diodes we use in the lab are slightly better, with a forward voltage closer to 0.4 volts. Since the forward voltage is not zero, the real diode absorbs power and this wasted power is equal to the product of the current and the forward voltage. We are using the slightly more expensive Schottky diodes because their forward voltage is lower (i.e. their efficiency is higher) and this allowed us to use a much cheaper transformer. The fact that the forward voltage is not zero means that our real diode is not an ideal
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This note was uploaded on 10/21/2010 for the course ESE 123 taught by Professor Westerfield during the Fall '07 term at SUNY Stony Brook.

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notes%208 - Class notes #8 Ideal diode Diodes are devices...

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