Zener diodes
If we connect a diode and resistor in
series with a DC voltage source so that the diode is forward-biased, the
voltage drop across the diode will remain fairly constant over a wide
range of power supply voltages:
According to the "diode equation," the
current through a forward-biased PN junction is proportional to e
raised to the power of the forward voltage drop. Because this is an
exponential function, current rises quite rapidly for modest increases
in voltage drop. Another way of considering this is to say that voltage
dropped across a forward-biased diode changes little for large
variations in diode current. In the circuit shown above, diode current
is limited by the voltage of the power supply, the series resistor, and
the diode's voltage drop, which as we know doesn't vary much from 0.7
volts. If the power supply voltage were to be increased, the resistor's
voltage drop would increase almost the same amount, and the diode's
voltage drop just a little. Conversely, a decrease in power supply
voltage would result in an almost equal decrease in resistor voltage
drop, with just a little decrease in diode voltage drop. In a word, we
could summarize this behavior by saying that the diode is regulating
the voltage drop at approximately 0.7 volts.
Voltage regulation is a useful diode
property to exploit. Suppose we were building some kind of circuit which
could not tolerate variations in power supply voltage, but needed to be
powered by a chemical battery, whose voltage changes over its lifetime.
We could form a circuit as shown and connect the circuit requiring
steady voltage across the diode, where it would receive an unchanging
0.7 volts.
This would certainly work, but most
practical circuits of any kind require a power supply voltage in excess
of 0.7 volts to properly function. One way we could increase our voltage
regulation point would be to connect multiple diodes in series, so that
their individual forward voltage drops of 0.7 volts each would add to
create a larger total. For instance, if we had ten diodes in series, the
regulated voltage would be ten times 0.7, or 7 volts:
So long as the battery voltage never
sagged below 7 volts, there would always be about 7 volts dropped across
the ten-diode "stack."
If larger regulated voltages are
required, we could either use more diodes in series (an inelegant
option, in my opinion), or try a fundamentally different approach. We
know that diode forward voltage is a fairly constant figure under a wide
range of conditions, but so is reverse breakdown voltage, and
breakdown voltage is typically much, much greater than forward voltage.
If we reversed the polarity of the diode in our single-diode regulator
circuit and increased the power supply voltage to the point where the
diode "broke down" (could no longer withstand the reverse-bias voltage
impressed across it), the diode would similarly regulate the voltage at
that breakdown point, not allowing it to increase further:
Unfortunately, when normal rectifying
diodes "break down," they usually do so destructively. However, it is
possible to build a special type of diode that can handle breakdown
without failing completely. This type of diode is called a zener
diode, and its symbol looks like this:
When forward-biased, zener diodes behave
much the same as standard rectifying diodes: they have a forward voltage
drop which follows the "diode equation" and is about 0.7 volts. In
reverse-bias mode, they do not conduct until the applied voltage reaches
or exceeds the so-called zener voltage, at which point the diode
is able to conduct substantial current, and in doing so will try to
limit the voltage dropped across it to that zener voltage point. So long
as the power dissipated by this reverse current does not exceed the
diode's thermal limits, the diode will not be harmed.
Zener diodes are manufactured with zener
voltages ranging anywhere from a few volts to hundreds of volts. This
zener voltage changes slightly with temperature, and like common
carbon-composition resistor values, may be anywhere from 5 percent to 10
percent in error from the manufacturer's specifications. However, this
stability and accuracy is generally good enough for the zener diode to
be used as a voltage regulator device in common power supply circuit:
Please take note of the zener diode's
orientation in the above circuit: the diode is reverse-biased,
and intentionally so. If we had oriented the diode in the "normal" way,
so as to be forward-biased, it would only drop 0.7 volts, just like a
regular rectifying diode. If we want to exploit this diode's reverse
breakdown properties, we must operate it in its reverse-bias mode. So
long as the power supply voltage remains above the zener voltage (12.6
volts, in this example), the voltage dropped across the zener diode will
remain at approximately 12.6 volts.
Like any semiconductor device, the zener
diode is sensitive to temperature. Excessive temperature will destroy a
zener diode, and because it both drops voltage and conducts current, it
produces its own heat in accordance with Joule's Law (P=IE). Therefore,
one must be careful to design the regulator circuit in such a way that
the diode's power dissipation rating is not exceeded. Interestingly
enough, when zener diodes fail due to excessive power dissipation, they
usually fail shorted rather than open. A diode failed in this
manner is easy to detect: it drops almost zero voltage when biased
either way, like a piece of wire.
Let's examine a zener diode regulating
circuit mathematically, determining all voltages, currents, and power
dissipations. Taking the same form of circuit shown earlier, we'll
perform calculations assuming a zener voltage of 12.6 volts, a power
supply voltage of 45 volts, and a series resistor value of 1000 Ω (we'll
regard the zener voltage to be exactly 12.6 volts so as to avoid
having to qualify all figures as "approximate"):
If the zener diode's voltage is 12.6
volts and the power supply's voltage is 45 volts, there will be 32.4
volts dropped across the resistor (45 volts - 12.6 volts = 32.4 volts).
32.4 volts dropped across 1000 Ω gives 32.4 mA of current in the
circuit:
Power is calculated by multiplying
current by voltage (P=IE), so we can calculate power dissipations for
both the resistor and the zener diode quite easily:
A zener diode with a power rating of 0.5
watt would be adequate, as would a resistor rated for 1.5 or 2 watts of
dissipation.
If excessive power dissipation is
detrimental, then why not design the circuit for the least amount of
dissipation possible? Why not just size the resistor for a very high
value of resistance, thus severely limiting current and keeping power
dissipation figures very low? Take this circuit, for example, with a 100
kΩ resistor instead of a 1 kΩ resistor. Note that both the power supply
voltage and the diode's zener voltage are identical to the last example:
With only 1/100 of the current we had
before (324 µA instead of 32.4 mA), both power dissipation figures
should be 100 times smaller:
Seems ideal, doesn't it? Less power
dissipation means lower operating temperatures for both the diode and
the resistor, and also less wasted energy in the system, right? A higher
resistance value does reduce power dissipation levels in the
circuit, but it unfortunately introduces another problem. Remember that
the purpose of a regulator circuit is to provide a stable voltage for
another circuit. In other words, we're eventually going to power
something with 12.6 volts, and this something will have a current draw
of its own. Consider our first regulator circuit, this time with a 500 Ω
load connected in parallel with the zener diode:
If 12.6 volts is maintained across a 500
Ω load, the load will draw 25.2 mA of current. In order for the 1 kΩ
series "dropping" resistor to drop 32.4 volts (reducing the power
supply's voltage of 45 volts down to 12.6 across the zener), it still
must conduct 32.4 mA of current. This leaves 7.2 mA of current through
the zener diode.
Now consider our "power-saving" regulator
circuit with the 100 kΩ dropping resistor, delivering power to the same
500 Ω load. What it is supposed to do is maintain 12.6 volts across the
load, just like the last circuit. However, as we will see, it cannot
accomplish this task:
With the larger value of dropping
resistor in place, there will only be about 224 mV of voltage across the
500 Ω load, far less than the expected value of 12.6 volts! Why is this?
If we actually had 12.6 volts across the load, it would draw 25.2 mA of
current, as before. This load current would have to go through the
series dropping resistor as it did before, but with a new (much larger!)
dropping resistor in place, the voltage dropped across that resistor
with 25.2 mA of current going through it would be 2,520 volts! Since we
obviously don't have that much voltage supplied by the battery, this
cannot happen.
The situation is easier to comprehend if
we temporarily remove the zener diode from the circuit and analyze the
behavior of the two resistors alone:
Both the 100 kΩ dropping resistor and the
500 Ω load resistance are in series with each other, giving a total
circuit resistance of 100.5 kΩ. With a total voltage of 45 volts and a
total resistance of 100.5 kΩ, Ohm's Law (I=E/R) tells us that the
current will be 447.76 µA. Figuring voltage drops across both resistors
(E=IR), we arrive at 44.776 volts and 224 mV, respectively. If we were
to re-install the zener diode at this point, it would "see" 224 mV
across it as well, being in parallel with the load resistance. This is
far below the zener breakdown voltage of the diode and so it will not
"break down" and conduct current. For that matter, at this low voltage
the diode wouldn't conduct even if it were forward-biased! Thus, the
diode ceases to regulate voltage, for it can do so only when there is at
least 12.6 volts dropped across to "activate" it.
The analytical technique of removing a
zener diode from a circuit and seeing whether or not there is enough
voltage present to make it conduct is a sound one. Just because a zener
diode happens to be connected in a circuit doesn't guarantee that the
full zener voltage will always be dropped across it! Remember that zener
diodes work by limiting voltage to some maximum level; they
cannot make up for a lack of voltage.
In summary, any zener diode regulating
circuit will function so long as the load's resistance is equal to or
greater than some minimum value. If the load resistance is too low, it
will draw too much current, dropping too much voltage across the series
dropping resistor, leaving insufficient voltage across the zener diode
to make it conduct. When the zener diode stops conducting current, it
can no longer regulate voltage, and the load voltage will fall below the
regulation point.
Our regulator circuit with the 100 kΩ
dropping resistor must be good for some value of load resistance,
though. To find this acceptable load resistance value, we can use a
table to calculate resistance in the two-resistor series circuit (no
diode), inserting the known values of total voltage and dropping
resistor resistance, and calculating for an expected load voltage of
12.6 volts:
With 45 volts of total voltage and 12.6
volts across the load, we should have 32.4 volts across Rdropping:
With 32.4 volts across the dropping
resistor, and 100 kΩ worth of resistance in it, the current through it
will be 324 µA:
Being a series circuit, the current is
equal through all components at any given time:
Calculating load resistance is now a
simple matter of Ohm's Law (R = E/I), giving us 38.889 kΩ:
Thus, if the load resistance is exactly
38.889 kΩ, there will be 12.6 volts across it, diode or no diode. Any
load resistance smaller than 38.889 kΩ will result in a load voltage
less than 12.6 volts, diode or no diode. With the diode in place, the
load voltage will be regulated to a maximum of 12.6 volts for any load
resistance greater than 38.889 kΩ.
With the original value of 1 kΩ for the
dropping resistor, our regulator circuit was able to adequately regulate
voltage even for a load resistance as low as 500 Ω. What we see is a
tradeoff between power dissipation and acceptable load resistance. The
higher-value dropping resistor gave us less power dissipation, at the
expense of raising the acceptable minimum load resistance value. If we
wish to regulate voltage for low-value load resistances, the circuit
must be prepared to handle higher power dissipation.
Zener diodes regulate voltage by acting
as complementary loads, drawing more or less current as necessary to
ensure a constant voltage drop across the load. This is analogous to
regulating the speed of an automobile by braking rather than by varying
the throttle position: not only is it wasteful, but the brakes must be
built to handle all the engine's power when the driving conditions don't
demand it. Despite this fundamental inefficiency of design, zener diode
regulator circuits are widely employed due to their sheer simplicity. In
high-power applications where the inefficiencies would be unacceptable,
other voltage-regulating techniques are applied. But even then, small
zener-based circuits are often used to provide a "reference" voltage to
drive a more efficient amplifier-type of circuit controlling the main
power.
- REVIEW:
- Zener diodes are designed to be
operated in reverse-bias mode, providing a relatively low, stable
breakdown, or zener voltage at which they being to conduct
substantial reverse current.
- A zener diode may function as a
voltage regulator by acting as an accessory load, drawing more current
from the source if the voltage is too high, and less if it is too low.
|
