1. EEE 205 / ECE 202 Electronic Devices and Circuits Fall 2010 Lecture 5: Zener Diodes

2. 2 Diode Applications – Rectifier Circuits Block diagram of a DC power supply. One of the most important applications of diodes is in the design of rectifier circuits. Rectifier circuits can be classified in two groups: 1. Half-wave (HW) rectifier 2. Full-wave (FW) rectifier

3. 3 Zener Diodes FIGURE 3-1 Zener diode symbol. IZIZ VZVZ + -

4. 4 Zener Diodes Zener Diode Characteristics  Unlike ordinary diodes, zener diodes are used in the reversed biased mode, in the breakdown region.  Zener diodes can maintain a constant voltage across its terminal despite large variations in the zener current.  Therefore, they are used to get reasonably regulated dc voltage when the input voltage and load resistance vary.

5. 5 Zener Diodes General diode I-V characteristic

6. 6 Zener Diodes  The application of too negative a voltage across the zener diode results in a sharp change in the I-V characteristics.  The current increases at a very rapid rate in a direction opposite to that of the positive voltage region.  The reverse-bias potential that results in this dramatic change in characteristics is called the Zener potential or the Zener voltage and is given the symbol V Z.  As the voltage across the diode increases in the reverse-bias region, the velocity of the minority carriers responsible for the reverse saturation current I s also increases. Zener Diode Characteristics

7. 7 Zener Diodes 1.Avalanche Breakdown 2.Zener Breakdown 1. Avalanche Breakdown  As the voltage across the diode increases in the reverse-bias region, the velocity of the minority carriers responsible for the reverse saturation current Is also increases.  Eventually, their velocity and associated kinetic energy will be sufficient to release additional carriers through collisions with otherwise stable atomic structures.  That is, an ionization process will result whereby valence electrons absorb sufficient energy to leave the parent atom.  These additional carriers can then aid further ionization to the point where a high avalanche current is established and the avalanche breakdown region determined. Diode Breakdown Mechanism

8. 8 Zener Diodes 2. Zener Breakdown  The avalanche region (V Z ) can be brought closer to the vertical axis by increasing the doping levels in the p- and n-type materials.  However, as V Z decreases to very low levels, such as 5 V, another mechanism, called Zener breakdown, will contribute to the sharp change in the characteristic.  It occurs because there is a strong electric field in the region of the junction that can disrupt the bonding forces within the atom and “generate” carriers.  Although the Zener breakdown mechanism is a significant contributor only at lower levels of V Z, this sharp change in the characteristic at any level is called the Zener region and diodes employing this unique portion of the characteristic of a p- n junction are called Zener diodes. Diode Breakdown Mechanism

9. 9 Zener Diodes Ideal Zener Diode model VzVz

10. 10 Zener Diodes IV Characteristics: Zener Diode Voltages V ZK, I ZK – Zener knee voltage and current, zener just turns on V Z – Nominal zener voltage at zener test current I ZT, the diode should be operated around this point V Z0 – Intersect of the slope at the operating point Q r z – Resistance of the zener diode at point Q  V =  I.r z – Change in the diode voltage due to change in the current

11. 11 Zener Diodes Practical Zener Diode Model V Z0

12. 12 Zener Diodes Practical Zener Diode Model

13. 13 Zener Diodes Q IV Characteristics: Zener Diode Currents

14. 14 Zener Diodes Analysis of Zener Diode Circuits: Line Regulation Analysis of Zener Diode Circuits: Line Regulation Line regulation,  V 0 /  V IN (mV/V), is a measure of fluctuation in the output voltage due to variation in the supply voltage.

15. 15 Zener Diodes Analysis of Zener Diode Circuits: Load Regulation Analysis of Zener Diode Circuits: Load Regulation Load regulation,  V 0 /  I L (mV/mA), is a measure of fluctuation in the output voltage due to variation in the load current.

16. 16 Exercise Zener Diodes V Z =6.8V, I Z =5 mA, r z = 20 ohm, I ZK =0.2 mA. The nominal supply voltage V + = 10 V, and can vary by ±1 V. a. Find V 0 with no load and V + at its nominal value. b. Find the change in V 0 resulting from the ±1V change in V +. (Line regulation) c. Find the change in V 0 resulting from connecting a load resistance R L that draws a current I L =1mA. Find load regulation. d. Find V 0 when R L =2 K ohm. Find change in V 0. e. Find V 0 when R L =0.5 Kohm. f. What is the minimum value of R L for which the diode still operates in the breakdown region?