Download Section B9: Zener Diodes

Section B9: Zener Diodes
When we first talked about practical diodes, it was mentioned that a
parameter associated with the diode in the reverse bias region was the
breakdown voltage, VBR, also known as the peak-inverse voltage (PIV). This
was a bad thing before – the whole avalanche breakdown, large current,
overheating device and total destruction thing...
Well, guess what? Under specific
fabrication conditions, a diode may
be created that will not be destroyed
if
the
breakdown
voltage
is
exceeded, as long as the current
does not exceed a defined maximum
(to prevent overheating). These
devices are known as zener diodes
and they are designed to have an
avalanche characteristic that is very
steep. The IV characteristic curve
and diode symbols for the regular
and zener diodes are given to the
right (Figure 3.38 of your text).
In the forward bias region, the zener behaves like a regular diode within
specified current and/or power limits. The magic of these devices comes in
when we get into the reverse bias region. As previously mentioned, the
zener is designed to have an almost vertical avalanche characteristic at the
breakdown voltage – hereinafter also called the zener voltage, and it is ideal
for use in voltage regulation. The limiting (maximum) power for a zener
diode is given by Pz=VzIzmax and is a function of the design and construction
of the diode. The knee of the curve (the current for which |vD|=VZ) is
generally approximated as 10% of Izmax, or Izmin=0.1Izmax.
There are two distinctly different mechanisms that may cause breakdown in
a zener diode:
1. Above approximately eight (8) volts, the predominant mechanism is
avalanche breakdown, also referred to as impact ionization or
avalanche multiplication. This process begins with thermally generated
minority carriers that acquire enough kinetic energy to break covalent
bonds and create an EHP through collisions with crystal atoms. The free
carriers created through this collision contribute to the reverse current
and may also possess enough energy to participate in collisions, creating
further EHPs and the avalanche effect.
2. The high field emission or zener breakdown mechanism is the second
method of disrupting the covalent bonds of the crystal and increasing the
reverse bias diode current. The reverse voltage where this occurs is
determined by the diode doping and occurs when the depletion layer field
is large enough to break covalent bonds and cause the number of free
carriers due to EHP generation to multiply.
Either of these effects, or a combination of the two, significantly increases
the current in the reverse bias region while having a negligible effect in the
voltage drop across the junction. Although “breakdown” and “disruption” and
words of that order have been liberally used in the previous discussion,
please realize that the zener process in not inherently destructive unless the
maximum power dissipation specified for the device is exceeded.
Zener Regulator
As mentioned earlier, the characteristics
of the zener diode make it ideal for
application as a voltage regulator.
Placing the zener diode in parallel with
the load as shown in Figure 3.39
(reproduced to the right) ensures an
essentially constant output voltage even though the load current and the
source voltage may vary. The key to the design of this voltage regulator is
to choose the resistor Ri to keep the zener in the breakdown region, while
ensuring that the diode current never exceeds Izmax.
Your text derives an expression for this circuit parameter by developing the
nodal expression for the zener current and defining the two extremes for iZ
in terms of the input/output conditions:
1. Izmin occurs when the load current is maximum and the source voltage is
minimum.
2. Izmax occurs when the load current is minimum and the source voltage is
maximum.
Circuit analysis techniques yield an expression for the resistor of interest as
v − VZ
in Equation 3.56, where Ri = s
and iR=iZ+iL. Equating the
iR
characteristics for the extremes of iZ (Izmin and Izmax) in the expression for Ri
yields:
Ri =
Vs min − Vz
V
− Vz
= s max
IL max + I z min IL min + I z max
(Equation 3.58)
using the approximation Izmin=0.1Izmax, stirring liberally and rearranging...
I z max =
IL min(Vz − Vs min ) + IL max (Vs max − Vz )
Vs min − 0.9Vz − 0.1Vs max
(Equation 3.61)
The variation in load current (or, equivalently, load resistance) and source
voltage may be given or measured. Be aware that these derivations are
based on the rule of thumb relationship between the maximum and
minimum zener current. If another relationship is defined, you’ll have to
back up to Equation 3.58 and rework Equation 3.61.
Once Izmax is known, the second expression in Equation 3.58 may be used to
solve for Ri.
Equivalently, Equation 3.58 (or Equation 3.61) may be solved for Izmin by
rearranging our rule of thumb to be Izmax=10Izmin:
I z min =
IL min(Vz − Vs min ) + IL max (Vs max − Vz )
10Vs min − 9Vz − Vs max
(Equation 3.61: Modified)
In this case, the first expression in Equation for 3.58 may be used to solve
for Ri.
One final note before we leave this topic. Be aware that it is not written in
stone that every combination of source voltage and/or load current will
result in a viable circuit. As illustrated in Example 3.6(b), sometimes there is
no resistor Ri that will make the regulator operate correctly. As your author
states, something in the design conditions will have to change – an increase
in source voltage or a reduction in load current for the desired output
voltage (This is obviously assuming that you are totally committed to the
zener voltage!).
Full-Wave Zener Regulator
The filtering discussion in Section B7 introduced the reduction of output
ripple by using a parallel RC combination. As shown in Figure 3.41 of your
text (reproduced below), this process may be enhanced by introducing a
zener regulator after the RC filter. Note that in the circuit above, the resistor
in the RC combination is no longer the load, but is what’s known as a
bleeder resistor and is denoted RF. The purpose of RF is to provide a
discharge path for the capacitor if the load is the removed. It is desired that
this resistor absorb as little power as possible when the circuit is in
operation, so it usually has a very high resistance (higher resistance, lower
current for same voltage).
Anyway, we can still use Equation 3.52 to solve for the capacitor (now called
CF to match the RF notation), with, of course, a couple of modifications:
¾ In Equation 3.52, we were only concerned with the maximum swing of
the source with respect to the zero axis. However, since the voltage
across Ri will not go to zero as long as the zener is operating, the Vmax
term is replaced by Vsmax-Vz.
¾ The equivalent resistance across the capacitor CF is evaluated under
forward bias conditions and is the parallel combination of RF and Ri.
However, since RF is generally very much larger than Ri, this parallel
combination is approximately equal to Ri.
¾ The remainder of the terms are as defined in Equation 3.52:
o ∆V is the peak-to-peak ripple allowable at the output, and
o fp is the frequency of the rectified waveform (twice the original
frequency for full-wave rectification).
Putting this all together, an expression for the capacitance in a full-wave
zener rectifier is given by
CF =
Vs max − VZ
∆VfpRi
(Equation 3.62)
Practical Zener Diodes and Percent Regulation
We’ve been toodling along in this discussion acting like the zener diode is
going to behave absolutely perfectly at all times – the characteristic in the
breakdown region is perfectly vertical (infinite slope implies zero resistance),
it’s going to give us an absolutely perfectly constant voltage as long as we
don’t exceed some simple power rating, right? Well, not exactly (big
surprise)...
Actually, the zener is pretty well behaved and is extremely useful if used
correctly. However... to account for the fact that the curve is not perfectly
vertical, we must include a nonzero zener resistance in series with our ideal
diode as shown in Figure 3.42 and
reproduced to the right. The effect
of this resistor (RZ) is to cause the
output voltage to deviate from the
constant value of the ideal zener
voltage (VZ’ in the figure) What
we’ve got now at the output is the
practical zener voltage,
VZ = VZ’+iZRZ.
Your text illustrates a numeric example of this effect in Equation 3.63. By
using the two extremes of zener current (IZmax and IZmin), it can be seen that
the output of a practical zener diode varies between Voutmax and Voutmin. Since
we have a nonideal output, the concept of percent regulation is
introduced.
Percent regulation is defined as the total variation over the nominal
(desired) value. For voltage regulation of a zener diode, this translates to
%Regulation =
V out max−Vout min
* 100 (Equation 3.64: Modified)
Voutno min al
The lower the percent regulation, the better the regulator – an ideal zener
diode would have a percent regulation of zero (Voutmax =Voutmin).