Chapter 2 ZENER DIODE

Zener diodes The breakdown characteristics of diodes can be tailored by controlling the doping concentration – Heavily doped p + and n - regions result in low breakdown voltage (Zener effect) Used as reference voltage in voltage regulators I V Region of operation Region of operation Region of operation

ZENER DIODE

Basic Zener regulator V i and R L Fixed and If V≥V Z and Iz >Izmin that means the Zener diode is on or If V<V Z or Iz <Izmin that means the Zener diode is off And:

Basic zener regulator V i and R L Fixed 2. Substitute the appropriate equivalent circuit and solve for the desired unknowns.

Example For the network of Figure below determine V L, V R, I Z and P Z. Repeat with R L =3K .

Example (solution)

As R L parallel connected with Zener diode: V Z =V L Example (solution)

Fixed V i, Variable R L OR: If Izmin is given: Using VDR 2- Finding R Lmax : 1- Finding R Lmin :

Example For the network determine the range of R L and I L that will result in V RL being maintained at 10 V. Determine the maximum wattage rating of diode.

Solution

Finding …contiued 40 mA- 32 mA= 8mA

Example

Fixed R L, Variable V i The minimum value of Vi must be sufficiently large to turn the Zener diode on. The maximum value of Vi limited by maximum Zener current I ZM Using VDR

Example For the network of Fig. determine the range of Vi that will maintain the Zener diode in on state..

Solution =23.67 V =16.67 mA =76.67 mA =36.87 V  Finding minimum value of V i  Finding maximum value of V i

Applications Simple square-wave generator

AC Limiter Applications Protection circuit

Problem 42 I 20 V I (a)Determine V L, I L, I Z,and I R for the network if R L = 180  (b)Repeat part (a) if RL = 470 . (c)Determine the value of R L that will establish maximum power conditions for the Zener diode. (d)Determine the minimum value of R L to ensure that the Zener diode is in the “on” state.

Problem 43 (a)Design the network of Fig to maintain VL at 12 V for a load variation (IL) from 0 to 200 mA. That is,determine Rs and VZ. (b)Determine PZ for the Zener diode of part (a).

Problem 44 Determine the range of V i that will maintain V L at 8V and not exceed the maximum power rating of the Zener diode. =11.3V =36mA =86mA =15.85V

Half Wave Voltage Doubler High Voltage transformers are expensive and impractical at voltages above 1000V The peak inverse voltage PIV across each diode is 2Vp

Half Wave Voltage Doubler First Half Cycle – D 1 conducting, C 1 charged. C 2 remains uncharged because of the reverse biased D2 Second Half Cycle – D 2 conducting, C 2 charged by (source + C1) – about twice the source voltage Next Half Cycle – Repeat of first stage with the addition that C 2 can now discharge through R L. Notice that C 2 is charged once/cycle, so output has the same frequency as input.

Full Wave Voltage Doubler When the secondary voltage is positive, D 1 is forward-biased and C 1 charges to approximately Vp During the negative half-cycle, D 2 is forward-biased and C 2 charges to approximately Vp. The output voltage 2Vp- is taken across the two capacitors in series. The peak inverse voltage PIV across each diode is 2Vp

Voltage Tripler The addition of another diode-capacitor section to the half-wave voltage doubler creates a voltage tripler. The operation is as follows: On the positive half-cycle of the secondary voltage. C 1 charges to Vp, through D1. During the negative half-cycle, C 2 charges to 2Vp through D2 as described for the doubler. During the next positive half-cycle, C 3 charges to 2Vp through D3 the tripler output is taken across C 1 and C 3.

Voltage Quadrupler The addition of still another diode-capacitor section produces an output four times the peak secondary voltage. C 4 charges to 2Vp through D 4 on a negative half-cycle. The 4V p output is taken across C 2 and C 4, as shown. In both the tripler and quadrupler circuits. the PlV of each diode is 2Vp

Voltage Quadrupler Can extend this circuit to produce extremely high voltages (~750kV). Voltage Quadrupler

Special Purpose Diodes

Varactor Diodes A varactor diode is best explained as a variable capacitor. Think of the depletion region a variable dielectric. The diode is placed in reverse bias. The dielectric is “adjusted” by bias changes.

Varactor Diodes The varactor diode can be useful in filter circuits as the adjustable component.

Other Diode Types The Schottky diode’s significant characteristic is it’s fast switching speed. This is useful for high frequencies and digital applications. It is not a typical diode in the fact that it does not have a p-n junction, instead it consists of a heavily doped n-material and metal bound together.

Other Diode Types The PIN diode is also used in mostly microwave frequency applications. It’s variable forward series resistance characteristic is used for attenuation, modulation, and switching. In reverse bias exhibits a nearly constant capacitance.

Other Diode Types The tunnel diode is also used for fast switching applications. This is achieved by reduced doping at the junction.

Other Diode Types The laser diode (light amplification by stimulated emission of radiation) produces a monochromatic (single color) light. Laser diodes in conjunction with photodiodes are used to retrieve data from compact discs.