Chapter 3 special_purpose_diodes Phy.ppt
- 1. Chapter 3 Special Purpose Diodes • Diode A diode has two leads connected to the external circuit • Since a diode behaves differently depending upon forward or reverse bias, it is critical to be able to distinguish the leads. 1 • The anode connects to the p-type material, the cathode to the n-type material of the diode.
- 2. Ideal Diode 2 • In an ideal diode, current flow freely through the device when forward biased, having no resistance. • In an ideal diode, there would be no voltage drop across it when forward biased. All of the source voltage would be dropped across circuit resistors. • In an ideal diode, when reverse biased, it would have infinite resistance, causing zero current flow.
- 3. Practical Diodes • A practical diode does offer some resistance to current flow when forward biased. • Since there is some resistance, there will be some power dissipated when current flows through a forward biased diode. Therefore, there is a practical limit to the amount of current a diode can conduct without damage. • A reverse biased diode has very high resistance. • Excessive reverse bias can cause the diode to conduct.
- 4. Special Diodes • Zener Diode • LEDs • Photo Diode • Varactor Diode • Tunnel Diode • Schottky Diode • Schokley Diode
- 5. 3.1 The Zener Diode • A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property • It is heavily doped in comparison with normal diode to reduce the breakdown voltage. • Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts.
- 6. Zener Diodes – Operating Range A zener diode is much like a normal diode, the exception being is that it is placed in the circuit in reverse bias and operates in reverse breakdown. This typical characteristic curve illustrates the operating range for a zener. Note that its forward characteristics are Operating range
- 7. Zener Breakdown Characteristics The zener diode’s breakdown characteristics are determined by the doping process. Low voltage zeners (>5V), operate in the zener breakdown range. Those designed to operate <5 V operate mostly in avalanche breakdown range. Zeners are available with voltage breakdowns of 1.8 V to This curve illustrates the minimum and maximum ranges of current operation that the zener can effectively maintain its voltage. Zener zone Diode zone Avalanch e zone 5V.
- 8. Zener Diodes – Equivalent Circuit • Ideal Zener exhibits a constant voltage, regardless of current draw. • Ideal Zener exhibits no resistance characteristics.
- 10. Zener Diodes – Equivalent Circuit Figure 3–4 shows the ideal model (first approximation) of a zener diode in reverse breakdown and its ideal characteristic curve. It has a constant voltage drop equal to the nominal zener voltage. This constant voltage drop across the zener diode produced by reverse breakdown is represented by a dc voltage symbol even though the zener diode does not produce a voltage.
- 16. 3.2 Zener Diode - Applications Regulation with variable input Voltage In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages. Zener current will increase or decrease directly with voltage input changes. The zener current, Iz, will vary to maintain a constant Vz. Note: The zener has a finite range of current operation. VZener remains constant
- 17. Zener Diode - Regulations Acrobat Document Calculate VZRegulate: (pg.118) VinMIN = VR + VZ = 55mV + 10V = 10.055V.VR = IZR = (100mA)(220) = 22V. Vin(max) = 22V + 10V = 32V 1N4740 PDMAX = 1W. VZ = 10V. IZK = 0.25mA to IZM = 100mA VRmin = IZKR= .25mA x 220 = 55mV VRmax = IZM = 100mA x 220 = 22V. **
- 19. Zener Regulation with Variable Load Acrobat Document In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage. Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation. VZener remains constant See Ex. 3-5
- 24. Zener Limiting In addition to voltage regulation applications, zener diodes can be used in ac applications to limit voltage swings to desired levels. Figure 3–18 shows three basic ways the limiting action of a zener diode can be used. Part (a) shows a zener used to limit the positive peak of a signal voltage to the selected zener voltage. During the negative alternation, the zener acts as a forward-biased diode and limits the negative voltage to -0.7 V. When the zener is turned around, as in part (b), the negative peak is limited by zener action and the positive voltage is limited to +0.7 V. Two back-to-back zeners limit both peaks to the zener voltage ±0.7 V,as shown in part During the positive alternation, D2 is functioning as the zener limiter and D1 is functioning as a forward-biased diode. During the negative alternation, the roles are
- 26. 3.3 The Varactor Diode
- 49. 3.5 the Other Diodes
- 50. Schottky Diode The Schottky diode’s (hot-carrier diodes) significant characteristic is its fast switching speed. This is useful for high frequencies and digital applications. It is not a typical diode in that it does not have a p-n junction. Instead, it consists of a lightly-doped n-material and heavily-doped (conduction-band electrons) metal bounded together. Response is very quick…high speed digital communications. Lightly doped Heavily doped (conduction-band electrons)
- 52. Tunnel Diode The tunnel diode exhibits negative resistance. It will actually conduct well with low forward bias. With further increases in bias it reaches the negative resistance range where current will actually go down. This is achieved by heavily-doped p and n materials that create a very thin depletion region which permits electrons to “tunnel” thru the barrier region. Germanium or Gallium Tank circuits oscillate but “die out” due to the internal resistance. A tunnel diode will provide “negative resistance” that overcomes the loses and maintains the oscillations.
- 53. Tunnel Diodes Tank circuits oscillate but “die out” due to the internal resistance. A tunnel diode will provide “negative resistance” that overcomes the loses and maintains the oscillations.
- 58. Troubleshooting Although precise power supplies typically use IC type regulators, zener diodes can be used alone as a voltage regulator. As with all troubleshooting techniques we must know what is normal. A properly functioning zener will work to maintain the output voltage within certain limits despite changes in load.
- 61. Assignment # 4 Chapter 3 Due on Monday July 31, 2023 Serial 1-8
- 62. Assignment # 4 Chapter 3 Due on Monday July 31, 2023 Serial 9-18
- 63. Serial 19-26
- 64. Serial 27-33