AN59

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AN59
Designing with Shunt Regulators - Series regulation
Peter Abiodun A. Bode, Snr. Applications Engineer, Diodes Incorporated
Introduction
Series regulators are versatile means of supplying power to many types of electronic circuits.
Although both fixed and adjustable regulators are readily available off the shelf at competitive
prices, there are instances when it may be desirable or necessary to design a discrete solution.
Such reasons might be due to requirement for a more accurate source than is commercially
available - i.e. a precision regulator. These might be for instrumentation, calibration or
environmental reasons. Low noise or level of required power might be other possible reasons for
seeking a discrete solution.
The Zetex precision references can make implementation of these discrete solutions relatively
simple. This document details the design considerations of the series regulator.
Basic Series Regulator
ZXTN25020CFH
Vin
I R3
Q1
R3
IKA
REF1
IB
IL
Vout
IR
R1 ⎞
⎛
VO UT = VREF ⎜ 1 +
⎟
R
2⎠
⎝
R1
VREF
C1
R3 =
(I
R2
VIN (min) − (VOUT + VBE (ma x) )
B (ma x)
IR 3
+ I KA (min) ) ≤ IR 3 ≤ I KA (ma x)
GND
Figure 1 Basic series regulator using a reference
A series regulator has the advantage that the series element (Q1) has a different voltage across it
from that across the load. This voltage can be arranged to be very low in comparison leading to a
much lower power dissipation in the series element, Q1, than is being delivered to the load. In
addition, There is little or no power dissipation in Q1 if there is no load (i.e. when IL = 0) other
than the very small current due to IR. These are the reasons why it is a better method for medium/
high power applications than the shunt regulator alone.
It is an irony and a practical benefit that the most important basic building block of a series regulator
is a shunt regulator which can be readily identified in Figure 1. The series regulator is, at most,
only as good as the quality of this shunt regulator which, in its crudest form, could be a zener diode.
The function of the shunt regulator is not to power the load but to drive the transistor which powers
the load. As long as Q1 is suitably sized, this simple circuit can be configured to supply very high
power and current running into several 10's of amperes and hundreds of Watts.
Q1 could be a single device as shown or it could be a Darlington pair or even several transistors in
parallel as the need may be.
Accuracy
The accuracy of the circuit is affected only by three components: the tolerance of the reference
and that of resistors R1 and R2. See Appendix in AN57 for further information on this.
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AN59
Calculated Example 1
Requirement
Supply Voltage:
Output voltage:
Load current:
12V to 15V
10V ±1%
100mA
Assume the use of TLV431.
Discussion
The ZXTN25020CFH transistor is used in this example because it offers a high forward gain
(180 - 500) and a fairly high power handling capability (1.25W) for a device in a SOT23 package.
The equations for determining R1, R2 and accuracy are the same as in all reference applications.
The objective here is to determine R1, R2 and R3 and check that the transistor Q1 can handle any
resultant power dissipation.
Solution
R1 and R2 are chosen to be high enough in value that significant power is not wasted. On the other hand they need to be low enough that regulation accuracy and stability are achieved. Either
R1 or R2 can be arbitrarily fixed and the other calculated from
R1 ⎞
⎛
VOUT = VREF ⎜1 +
Equation 1
⎟
R
2⎠
⎝
AN58 showed the benefit of choosing a value of R1 in the region of 100kohm. Assume R1 is 100k
and rearrange Equation 1 to obtain
R1
R2 =
⎛ VOUT
⎞
⎜⎜
− 1⎟⎟
⎝ VREF
⎠
R2 =
100k
⎛ 10
⎞
− 1⎟
⎜
⎝ 1.24
⎠
= 14.15k
Or
R2 = 14.2k
to the nearest E192 value
and within 0.35%.
Determine maximum required base current, IB(max)
I
I B(max) = OUT (max)
h FE (min) +1
=
Hence,
100mA
181
IB(max) = 552.5µA
This is the maximum base current required by the transistor. R3 needs to be able to supply this
plus, at least, the minimum cathode current for the TLV431 which is 100µA.
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AN59
Therefore,
IR3(min) = 652.5µA
R3 =
R3 =
Or
VIN (min) − (VOUT + VBE (max) )
I R 3(min)
12 − (10 + 0.9)
652.5 μA
= 1.68 k⍀
1.6 k⍀
to the nearest lower E24 value.
Off-load and at maximum input, all of IR3 will flow into the TLV431 and it is necessary to check
that this current will not be excessive.
Hence,
VIN (max) − (VOUT + VBE (min) )
I R 3(max) =
R3
15 − 10.6
1600
= 2.75mA
less than 15mA as required.
The last thing to check is that the transistor is suitably power-rated for this application.
Hence,
⎛ h
⎞
FE (max)
⎟IOUT
PQ1(max) = (VIN (max) − VOUT ) ⋅ ⎜
⎜h
⎟
⎝ FE (max) + 1 ⎠
= (15 − 10) ⋅ 0.998 ⋅ 0.1
= 0.5W
This is comfortably within
the capability of the
ZXTN25020CFH when
suitably mounted.
Accuracy
It is necessary to determine the required component tolerances for meeting the specified accuracy. Since the design calls for an accuracy of ±1%, it follows that the TLV431 used must be the 0.5%
tolerance part. The resistors' tolerance can then be calculated as follows (see AN57 Shunt Regulators for more information):
This is the error caused
⎛ R1 ⎞
First determine α RD
α RD = ⎜
⎟(α R1 − α R 2 )
by using preferred
⎝ R1 + R 2 ⎠
resistors as opposed to
calculated values.
100 k
⎛
⎞
⎟(0 − 0.35)
⎝ 100 k + 14.15k ⎠
α RD = ⎜
α RD = −0.31%
R1, R2 tolerance,
⎡⎛ α VOUT − (α TLV 431 + α RD ) ⎞⎛ R1 + R 2 ⎞⎤
⎟⎜
⎟⎥
2
⎠⎝ R1 ⎠⎦
⎣⎝
α R = ± ⎢⎜
⎡⎛ 1 − (0.5 − 0.31) ⎞⎛ 114.2 ⎞⎤
⎟⎜
⎟⎥
2
⎠⎝ 100 ⎠⎦
⎣⎝
α R = ± ⎢⎜
α R = ± 0.463%
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Summary
Using a TLV431B, R1 = 100k, R2 = 14.2k (both 0.463% or better) and R3 = 1.6k will satisfy the
requirement.
Series Regulator with Current Limit
I LIM Rs
ZXTN25020CFH
Vin
I R3
IB
R3
Q1
I OUT
VRE F
REF2
I CONT
VREF
REF1
R1
Vout
R1 ⎞
⎛
VOUT = VREF ⎜1 +
⎟
R
2⎠
⎝
R3 =
(I
C1
R2
VIN − (VOUT + VREF + VBE )
IR 3
B (max)
RS =
+ I KA(min) ) ≤ I R 3 ≤ I KA(max)
VREF
I LIM
GND
Figure 2 Series regulator with current limit
The circuit in Figure 1 has no current limit. If a short circuit were to be applied to the output, the
resultant current, ISC, that would flow can potentially be
⎛ VIN (max) − VBE ) ⎞
⎟⎟
ISC = (hFE (max) + 1)⎜⎜
R3
⎝
⎠
⎛ 15 − 0.9 ⎞
= (501)⎜
⎟
⎝ 1600 ⎠
ISC = 4.4A
The ZXTN25020CFH, with its IC(cont) rating of 4.5A could handle this current. A much more
serious issue is that all of the supply voltage now appears across Q1 at the same time. At 15V this
amounts to a power dissipation of 66W. According to the Pulse Power Dissipation chart in the
ZXTN25020CFH datasheet, the transistor will fail in less than 0.3ms if subjected to this level of
power.
Figure 2 adds current limit to the series regulator in Figure 1 using a second reference (REF2), also
a TLV431. For currents below the limit, the circuit works normally supplying the required load
current at the design voltage. However, should attempts be made to exceed the design current
set by REF2, the device begins to shunt current away from the base of Q1. This begins to reduce
the output voltage and thus ensuring that the output current is clamped at the design value.
Subject only to Q1's ability to withstand the resulting power dissipation, the circuit can withstand
either a brief or indefinite short circuit.
Calculated Example 2
Requirement
Add a 105mA ±5% current limit to Calculated Example 1 (Figure 1 Basic series regulator).
Solution
RS =
=
or
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VREF
I LIM
1.24
0.105
= 11.81
RS = 11.8 ⍀
To nearest E48 value.
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Using a 1% device (TLV431A), an RS value of 11.8⍀ 1% makes the worst case cumulative error 2%,
within ±5% as required.
Determination of actual short circuit current
When the current limit circuit is operative, the output current consists of two components as
shown below. ILIM remains constant, ICONT varies continuously with the level of overload until
reaching a maximum at full short circuit i.e. when VOUT = 0.
Hence
IOUT (max) = I LIM + ICONT
⎛ VIN (max) − (VREF + VBE )
⎞
I
= I LIM + ⎜⎜
− LIM ⎟⎟
R3
hFE + 1⎠
⎝
⎛ h
⎞ VIN (max) − (VREF + VBE )
= I LIM ⎜⎜ FE ⎟⎟ +
R3
⎝ hFE + 1⎠
≈ I LIM +
Therefore,
VIN (max) − (VREF + VBE )
IOUT (max) =
since hFE >> 1
R3
1.24 15 − (1.24 + 0.6)
+
11.8
1600
IOUT(max) = 113.3mA
This represents the maximum short circuit current, it is not the current seen by transistor Q1
which is given by
⎛ h
IC = I LIM ⎜⎜ FE
⎝ 1 + hFE
⎞
⎟⎟
⎠
if hFE >> 1
≈ I LIM
Therefore
IC = 105mA
This means that a direct short circuit would not immediately result in a failure. It is still however
necessary to estimate how long the circuit could withstand such an overload condition for as
follows.
Overload duration
With a short circuit, the current will be limited to a maximum of 107mA (105 +2%). Worst case
voltage across Q1 will be 13.76V (i.e. 15V - 1.24V). Therefore, Q1's dissipation will be 1.47W.
Referring to the Pulse Power Dissipation chart in the ZXTN25020CFH datasheet, it can be seen
that Q1 will withstand this condition for between 10s and 100s. This is if there is a direct short
circuit of the output voltage. If there is only a partial overload, the situation will be far less severe.
There are a number of steps that can be taken if an indefinite short circuit handling capability is
required. The simplest action would be to use a slightly bigger transistor for Q1. For example,
the ZXTN2005G is a transistor in a SOT223 package and will dissipate up to 3W continuously
when suitably mounted.
Another method that could be used is to apply a re-entrant or "fold-back" current limiting rather
than the simple current limiting above. This is a method which adjusts the over-load current limit
according to the value of the output voltage such that, by the time the output voltage drops to
zero - i.e. a short circuit, the current limit has dropped to a very small value, typically 5% or less
of the full load current. This is a more complex solution and is outside the scope of this document.
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Conclusion
Precision series regulators can be implemented using references. These allow the user to have
more control of both the qualitative (e.g. accuracy) and quantitative (e.g. output voltage, current
limit or power delivery) to suit the application.
Recommended further reading
AN58 - Designing with Shunt Regulators - Shunt Regulation
AN60 - Designing with Shunt Regulators - Fixed Regulators and Opto-Isolation
AN61 - Designing with Shunt Regulators - Extending the operating voltage range
AN62 - Designing with Shunt Regulators - Other Applications
AN63 - Designing with Shunt Regulators - ZXRE060 Low Voltage Regulator
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