EXPERIMENT 7

EE 3401 Electronics I Laboratory

  Zener Diodes

Objectives:

1. To understand the operating characteristics and parameters of the Zener diode.

2. To be able to determine the optimum circuit element values to insure that the diode remains in the constant-voltage region.

3. To observe the effects of heat (due to excess power dissipation) on the nominal Zener voltage.

4. To calculate the percent regulation for specific circuit parameters.

Parts and Equipment:

Zener diodes: 2-1Nxxxx (6.2 V, 500mW)
Resistors:

1-100 W-1 Watt
1-150 W-1 Watt
1-220 W-1/2 Watt
1-390 W-1/2 Watt
1-470 W
1-1.8 kW
2-1.0 kW

Textbook Reference:

Serda and Smith, " Microelectronic Circuits" 3rd, Oxford University Press, 1991, pp. 116-190, or Savant, Roden, and Carpenter, "Electronic Design," The Benjamin/Cummings Publishing Company, Inc., Redwood City, CA, 1991, pp.44-50

Material Review:

As you recall from the Material Review of Experiment 2, under sufficiently strong reverse bias, diodes will exhibit a dramatic increase (in the negative sense) in current due to the combination of high electric field - induced ionization of electrons from atoms (Zener Breakdown) and/or a high kinetic energy - activated chain reaction of collision-induced ionizations known as Avalanche Breakdown.

Figure 1a and 1b exhibit the I-V curves with Fig. 1a exhibiting the breakdown region in the Third Quadrant consistent with the normal diode polarity convention, while Fig. 1b exhibits the Third Quadrant rotated about the origin into the First Quadrant, the manner in which it is normally presented for a commercial Zener Diode which is deliberately doped/fabricated to operate in the breakdown region.

Fig. 2 exhibits the schematic symbol and appropriate polarity for a Zener diode. Zener breakdown is one example of "tunneling" phenomena that occurs in solid state devices as electrons and holes actually go "through" rather than "over" a potential energy barrier due to quantum mechanical effects associated with the wave characteristics of electrons and the Heisenberg Uncertainty Principle. In general, heavy doping of the p-n junction (depletion) region favors Zener breakdown while with lighter doping, avalanche breakdown dominates. In both cases, the heavier the doping the smaller in magnitude will be the breakdown voltage; thus, this voltage can be "tailored".

A rigorous theoretical description of the I-V curves during breakdown is very complicated so, in many cases, an approximate "curve-fitting" method is more appropriate. Referring to the rotated I-V characteristic of Fig. 1b, one might use a general Taylor Series expansion:

I_z = A + BV_z + CV_z² + DV_z³ + ..., (1)

where I_z and V_z are both positive and measured with respect to the origin : (I_z = 0, V_z = 0). However, since I_z I_s (the reverse saturation current) before breakdown, a better expansion might be

I_z I_s + B(V_z - V_zk) + C(V_z - V_zk)² + ..., (2)

where V_zk is the onset or "knee" voltage of the breakdown. Of course, the greater the number of terms, the better the fit between a finite-term, truncated series and the actual I-V data. If even more accuracy is needed, one should replace I_s with the actual I_zk corresponding to V_zk; however, this is always close to I_s.

In general, retaining the terms out to an exponent of four and, perhaps, three should provide a good fit over a limited rated operating range; a purely quadratic equation may yield only a rough fit. One will often find that the coefficient of one of the higher order terms (for example, the third order or fourth order term) may be much larger than the others so that another useful approximation is

I_z I_s + (I_zk - I_s)(V_z/V_zk)ⁿ, (3)

where k is a constant and n the corresponding positive exponent. An easier but less accurate approximate is

In general, to find all of the constants in any of these approximate equations will require a number of actual data points equal to the number of unknown constants in the particular approximation. For example, if one chose

I_z A + BV_z + CV_z² + DV₃³, (5)

then four (I_z, V_z) data points from the actual I-V curves would be required to find A, B, C, and D. These might be (0,0), (I_zk, V_zk), (I_zn, V_zn), and (Izm, Vzm), where sub-k represents the "knee" values, sub-n the normal/nominal or midrange operating values, and sub-m the maximum values dictated by thermal limits.

A better but more complicated method of finding the constants would not involve exact fitting of the approximation to a number (finite) of points with possible major deviations between the points but would involve a general nonlinear "least squares"/"regression" method that insures a good fit at all points but not necessarily an exact fit at any point.

Generally, one would like to operate near the nominal value to guarantee no problems with either thermal burnout due to power dissipation or the operating point "sliding" off of the breakdown region into the nearly horizontal position of the I-V curves. Practically, it often suffices to use the graphical characteristics of the I-V curves provided by the manufacturer (recalling that they themselves are only nominal and rated to within a certain percentage error). The extremely steep slope of the breakdown region means that huge variations in current can be accommodated with negligible variations in voltage; this allows a Zener diode to be used as a voltage regulator. Consider the circuit shown in Fig. 3. The corresponding equation is

For a given theoretical approximation to i_z(t) = f(v_L(t)), the equation could be solved to yield v_L at each instant of time (this might be best achieved by numerical computer techniques).

However, what one would find is that

Thus, Fig. 4 exhibits an approximate v_L(t), showing the saturation or limiting of V_L at V_Z(approximately V_Zn) when the diode is reverse biased and at -0.7 V when the diode is forward biased.

As a good approximation,

By superimposing the equation (or load line)

on the graphical I_z-V_z curve (Fig. 5),

one can determine the actual operating point (I_zo, V_zo) from the intersection of the curves, independently of any functional approximation for i_z = f(v_z). One notes that the slope of the load line is

and that as R_L becomes smaller for a fixed R_i, the operating point eventually slides off of the breakdown region and regulation is lost. As R_i approaches 0 for a fixed R_L, the load line becomes a vertical line at v_Z = v_S and I_z becomes f(v_S). Note that in a DC case, since the output voltage is clamped to V_Z(approximately V_Zn), I_L and, hence, P_L = V_L I_L are also clamped to guarantee both proper load operation and no thermal problems.

By placing two Zener diodes "back to back", one can clip the bottom and the top portions of an input sinusoid to create a quasi-square wave (Fig. 6). This method has applications in signal processing. A back to back pair of Zener diodes can also be used to clip off high voltage spikes from the incoming power line voltage as long as the total energy of the spike is below the maximum energy level of the diode. Zener diode voltages generally range out to roughly 20 V in fairly small steps and then in larger steps up to above 100 V. A fair alternative to a Zener diode can be produced by placing several standard forward biased diodes in series; each will take on a voltage of approximately 0.7 V (Si) or 0.3 V (Ge) and the voltage across the series combination of n diodes will be 0.7n or 0.3n with a large dI/dV.

This experiment will provide you with practical experience with Zener diodes and provide impetus to use these useful devices in electronic design.

Procedure 1: Zener Characteristics.

1. Build the circuit below using two 6.2 V Zener diodes and two 1 kOhm resistors. Then display the voltage current curve similar to the way you did in the first diode experiment.  Use the signal generator set for a triangular waveform at about 1 kHz and 5 or 6 Vrms. Note the forward characteristic is the same as before.  Change the scope settings to show the reverse characteristic as well by setting the vertical zero to the center of the screen and the horizontal zero at 1 division from the right edge of the screen. Use 1V/div on both the axes.  If the reverse breakdown is not observed increase the source voltage until the reverse current just reaches the bottom of the screen. (About 4 mA with the above settings.)  Check how the Zener voltage changes with temperature.  Does V_Z increase or decrease with temperature? How does this compare with the forward bias voltage change with temperature observed in your first diode experiment?  Does the forward bias voltage of the zener behave the same way as the diode in the last experiment?

Procedure 2: Zener Diode Regulators .

1.  Construct the circuit below, with the following values:
 

Zener diode - 1Nxxxx (6.2 V, 1/2W)
R_i = 390 W
DC power supply

2. Repeat the following steps:  First with V_S = 15 V_DC, then repeat with with V_S = 8 V_DC.

First with R_i = 390 W,  connect V_S to the circuit and measure V_L after it has stabilized.  As the diode heats up due to the current flow through it, the voltage will change until it reaches an equilibrium state.  This heating effect will be greatest with the highest value of source voltage, lowest value of  R_i and the largest value of  R_L.  Wait until the 3rd digit on the meter does not change for about 5 seconds and then record the voltage reading.

a) With R_L = 1.8 kW
b) Repeat with R_L = 470 W

Then repeat with each of the following values of  R_i 220 W, 150 W, and 100 W.  Note: with  R_i = 390 W, record    V_L immediately after V_S is first connected and again after it has reached equilibrium.

3. For each value of V_S ,  calculate I_L(mA) and I_Z(mA) for each of the eight combinations of resistors (R_i and R_L).

4. For each of the values of R_i; calculate the line regulation for  R_L = 470 Ohms and then the line regulation for R_L = 1.8k Ohms.   The line regulation should always be positive.  If you obtain any negative values, comment on the possible causes.

5.  For each of the values of R_i; calculate the load regulation for V_S = 15 V_DC and then the load regulation for V_S = 8 V_DC. 

Remember you will have the maximum load current flowing through the minimum resistance and the minimum current through the maximum resistance.  The load regulation should always be negative.  If you obtain any positive values, comment on possible causes.

6.  Check the results for cases where the zener diode drops out of regulation.  That is I_Z goes to zero or close to zero so the diode is no longer on the steep part of the reverse breakdown curve.  In this case the output voltage depends on the value of V_S, R_i, and R_L and not on V_Z.  Comment on the results. 

7.  Calculate P_Z(mW) and P_Ri(mW) for each of the 16 sets of data.  Compare the results to the maximum power rating of the components.  In which cases would you expect each of the components to be the hottest?

Questions:

1. Describe the difference between a Zener diode and a standard diode.

2. What voltage do you expect would be across the Zener diode if it was accidentally reversed in the voltage regulator circuit?

3. Explain how Zener diodes are often used as the "second line" (after a fuse or circuit breaker) of defense in protecting a commercial electronic appliance from a brief high voltage surge in the input power line. Describe the limitations of this method.

4. Which value of R_i gives the best overall regulation operation?  Explain why the the regulation is worse at the other values of R_i.
 
 
Conclusions:

What can you say about Zener Diodes and Zener diode regulators based on your experimental observations?

This page last updated 10/16/2007