Discrete diodes-those packaged in individual cases with externally accessible anode and cathode connections-arc commercially available in a wide range of types designed for different kinds of service and for a variety of applications. We find. for example, switching diodes designed specifically for use in logic circuit applications, like those discussed in the last section. These diodes typically have low power dissipation ratings, are small in size, and are designed to respond rapidly to pulse-type inputs, that is, to switch between their “ON” and “OFF” states with minimum delay. Rectifier, or power, diodes are designed to carry larger currents and to dissipate more power than switching diodes. They are used in power supply applications, where heavier currents and higher voltages are encountered. Small signal diodes arc general-purpose diodes used in applications like those discussed in Section 3-5.
Figure 3-36 illustrates the variety of sizes and shapes that commercially available diodes may have. Each of those shown has a designation that identifies the standard case size it has (00-4, 00-7, etc.). Materials used for case construction include glass, plastic, and metal. Metal cases are used for large, rectifier-type diodes to enhance the conduction of heat and improve their rower dissipation capabilities.
There are two particularly important diode ratings that a designer using commercial, discrete diodes should know when selecting a diode for any application: the maximum reverse ooltage (VRM) and the maximum forward current. The maximum reverse voltage, also called the peak inverse voltage (PI V), is the maximum reverse biasing voltage that the diode can withstand without breakdown. If the PIV is exceeded, the diode “breaks down” only in the sense that it readily conducts current in the reverse direction. As discussed in Chapter 2, breakdown may result in permanent failure if the power dissipation rating of the device is es+ceded. The maximum forward current is the maximum current that the diode can istain when it is forward biased. Exceeding this rating will cause excessive heat to tIC generated in the diode and will lead to permanent failure. Manufacturers’ ratings for the maximum forward current will specify whether the rating is for continuous, peak, average, or rms current, and they may provide different values for each. The symbols I; and /1′ are used to represent forward current.
Example 3-9
In the circuit of Figure 3-37, a rectifier diode is used to supply positive current pulses to the 100-n resistor load. The diode is available in the combinations of ratings listed in the table portion of the figure. Which is (he least expensive diode that can be used for the application?
Solution.
The applied voltage is 120 V rms, Therefore. when the diode is Il’\ ersc biased by the peak negative value of the sine wave, it will he subjected to a maximum reverse-biasing voltage of (1.414)(120) = 169.7 V. The Viii/ rating must be greater than 169.7 V.
The average value of the current is one-half the average value of a single sinusoidal pulse: IAII(; = (V2)(O.6371/.) A, where 1/. is the peak value of the pulse. (Note that the factor I/~must be used because the pulse is present for only onehalf of each full cycle.) The peak forward current in the example (neglecting the drop across the diode) is 1/. = (169.7 V)/(lOO 0) = 1.697 A. Therefore, the average forward current through the diode is lsvo = (V;!)(O.637)( 1.697) = 0.540 A.
The least expensive diode having ratings adequate for the peak inverse voltage and average forward current values we calculated is the one costing $2.00
Figure 3-3R shows a typical manufacturer’s specification shed for a line of silicon small-signal diodes. Like many other manufactured electronic components. diodes arc often identified by a standard type number in accordance with JEDEC (Joint Electron Devices Engineering Council) specifications. Diode type numbers have the prefix ‘IN, like those shown in the leftmost column of Figure 3-30. (Not all manufacturers provide JEDEC numbers; many use their own commercial part numbers.) The second column in the specification sheet shows the maximum reverse voltage, VR,\{, for each of the diode types. Note that VRM ranges from 20 V to 200 V for the diodes listed. The third column shows the rated average forward current I” of each diode in mA, and these range from 0.1 mA to 200 mA. The next two pairs of columns list values of reverse current IR for different values of reverse voltage Vii and ambient temperature T,,,••b» The next column gives capacitance values in pF, an important specification in high-frequency and switching applications. The column headed t; lists the reverse recovery time of each diode, in nanoseconds; This specification relates to the time required for a aiode to switcil from its ON to its OFF state and is another important parameter in switching circuit design. Finally. the maximum rated power dissipation is given in mW. The product of diode voltage and diode current should never exceed this rating in any application (unless there is some auxiliary means for removing heat, such as a cooling fan).
Figure 3-39 shows a .ypical specification sheet for a line of silicon rectifier diodes. Note that the forward current ratings for these diodes are generally larger than those of the small-signal diodes. The current ratings arc given as InAI,) (average), and lIS,\{, each in units or amperes. l rs» is the maximum non-repetitive forward current that the diode can sustain. that is. the maximum value of momentary or surge current it can conduct. Note thai the ft.·SM values are much larger than the 1/(,11′) values. The voltage ratings arc specified by ~//IIW. the maximum repetitive reverse voltage that each diode can sustain. Also note the large physical sizes and the metal cases of the stud-mounted rectifiers that arc capable of conducting currents from 12 to 40 A.