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Line Regulation: R V V - V V Ever Change? V

The document discusses line regulation in shunt voltage regulators. It explains that since Zener diodes have a small but non-zero dynamic resistance, changes in the source voltage will cause small changes in the load voltage. It performs a small-signal analysis to show that the change in load voltage is directly proportional to the change in source voltage, with the ratio defined as the line regulation. For typical shunt regulators with very small dynamic resistance, the line regulation is also small, meaning the load voltage remains well regulated despite small source voltage variations.

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Mayank Sinha
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62 views4 pages

Line Regulation: R V V - V V Ever Change? V

The document discusses line regulation in shunt voltage regulators. It explains that since Zener diodes have a small but non-zero dynamic resistance, changes in the source voltage will cause small changes in the load voltage. It performs a small-signal analysis to show that the change in load voltage is directly proportional to the change in source voltage, with the ratio defined as the line regulation. For typical shunt regulators with very small dynamic resistance, the line regulation is also small, meaning the load voltage remains well regulated despite small source voltage variations.

Uploaded by

Mayank Sinha
Copyright
© Attribution Non-Commercial (BY-NC)
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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9/8/2005

Line Regulation.doc

1/4

Line Regulation
Since the Zener diode in a shunt regulator has some small (but non-zero) dynamic resistance rZ, we find that the load voltage VO will have a small dependence on source voltage VS. In other words, if the source voltage VS increases (decreases), the load voltage VO will likewise increase (decrease) by some very small amount. Q: Why would the source voltage VS ever change? A: There are many reasons why VS will not be a perfect constant with time. Among them are: 1. Thermal noise 2. Temperature drift 3. Coupled 60 Hz signals (or digital clock signals) As a result, it is more appropriate to represent the total source voltage as a time-varying signal (vS (t ) ), consisting of both a DC component (VS) and a small-signal component ( v s (t ) ): vS (t ) =VS + vs (t ) vS

VS t
Jim Stiles The Univ. of Kansas Dept. of EECS

9/8/2005

Line Regulation.doc

2/4

As a result of the small-signal source voltage, the total load voltage is likewise time-varying, with both a DC (VO) and smallsignal ( vo ) component:

vO (t ) =VO + vo (t )
So, we know that the DC source VS produces the DC load voltage VO, whereas the small-signal source voltage v s results in the small-signal load voltage vo .

R
+ + -

v s
VS

VO + vo
-

RL

say, 500 mV, what will value of vo be?

Q: Just how are v s and vo related? I mean, if v s equals,

A: Determining this answer is easy! We simply need to perform a small-signal analysis. In other words, we first replace the Zener diode with its Zener PWL model.

Jim Stiles

The Univ. of Kansas

Dept. of EECS

9/8/2005

Line Regulation.doc

3/4

R
+ + + +

v s
VS

VZ0
_

VO + vo

RL

rz

We then turn off all the DC sources (including VZO) and analyze the remaining small-signal circuit!

R
+ -

v s
rz

vo
-

RL

From voltage division, we find:

r R vo = v s Z L R + rZ RL

However, recall that the value of a Zener dynamic resistance rZ is very small. Thus, we can assume that rZ >> RL, and therefore rZ RL rZ , leading to:

Jim Stiles

The Univ. of Kansas

Dept. of EECS

9/8/2005

Line Regulation.doc

4/4

r R vo = v s Z L R + rZ RL r v s Z rZ + R

Rearranging, we find:
vo rZ = v s rZ + R

line regulation

This equation describes an important performance parameter for shunt regulators. We call this parameter the line regulation. * Line regulation allows us to determine the amount that the load voltage changes ( vo ) when the source voltage changes

( v s ).

* For example, if line regulation is 0.002, we find that the load voltage will increase 1 mV when the source voltage increases 500mV (i.e., vo = 0.002 v s = 0.002(0.5) = 0.001 V ). * Ideally, line regulation is zero. Since dynamic resistance rZ is typically very small (i.e., rZ R ), we find that the line regulation of most shunt regulators is likewise small (this is a good thing!).
Jim Stiles The Univ. of Kansas Dept. of EECS

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