AN45

AN45
High voltage current monitoring using the ZXCT
series in power supplies
by Peter Abiodun Bode, Snr. Applications Engineer
Introduction
Power supply monitoring requirements
All power supplies and charging units have some current measurement requirement. The current
levels measured will vary dependent upon the application. Operating input and required output
voltage levels will differ in accordance with the system. For example, battery charger modules for
PDA’s can operate below 20V whilst measuring 1A to 2A, however a power supply for a bus
converter will have very different requirements. A 700W power supply module will typically have
current measuring requirements of tens of amps. By carefully setting the sense voltage to be used
and determining the corresponding sense resistor, RS, the ZXCT series in their basic form can
cope with all of these.
Sometimes, it is necessary to monitor high side current circuits with operating voltages in excess
of what the ZXCT series were designed for. The circuits outlined below demonstrate how a 20V
current monitor can be used in applications with supply rails up to 250V and above.
High side high voltage current monitoring
One of the key benefits of the Zetex range of current-output current monitors (COCM's) is the very
fact that their output is a current which, unlike their voltage-output (VOCM) counterpart, does not
require an absolute ground reference to function. This means that the COCM device can be
floated at a higher voltage and still ensure that its output current is available at a lower potential
for translation to a ground-referenced output voltage.
RS
VSUPPLY
ILOAD
VSENSE
2
S-
S+
ZXCT1008 / 9
IOUT
GND
OUT
2
3
VOUT
IOUT
VOUT
RG
Figure 1
5
S-
ZXCT1010 & 12
OUT
1
ILOAD
VSENSE
4
3
S+
RS
VSUPPLY
RG
Simplest 3-terminal COCM
Figure 2
Simplest 4-terminal COCM
Figure 1 and Figure 2 show the basic configuration of 3- and 4-terminal current-output current
monitors. In this form, the maximum operating voltage will be limited to that of the COCM itself
(typically 20V, 40V or 60V). However, with the addition of only one or three components, the range
of operation can be extended to much higher voltages. The methods for achieving this are
illustrated in Figure 3, Figure 4 and Figure 5
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ILOAD
3
S-
R1
ZXCT1008 / 9
S+
RS
VSUPPLY
S-
Z1
ZXCT10xx
GND
OUT
1
ILOAD
VSENSE
VSENSE
2
S+
RS
VSUPPLY
OUT
S+
SZXCT10xx
GND
OUT
IOUT
IOUT
Z1
VOUT
Figure 3 Simplest supply
range extension
IOUT
FMMT597
FMMT597
TR1
TR1
VOUT
R2
RG
ILOAD
VSENSE
15V - 20V
RS
VSUPPLY
VOUT
R2
RG
Figure 4 Improved supply
range extension
RG
Figure 5
Best supply range
extension
Suitable devices: All COCM's
Circuit explanation
The three circuits are discussed in detail below.
High voltage Option 1
Figure 3 is the simplest to use if the supply voltage is essentially fixed and does not vary much
and satisfies the following criteria,
∆VSUPPLY ≤ VMAX − VDO
Equation 1
VSUPPLY (min) ≥ VDO + VOUT (max) + VZ
Equation 2
VSUPPLY (max) ≤ VWM + VZ
Equation 3
where,
VZ = Zener operating voltage
VMAX = Maximum operating voltage (20V in most cases)
VSUPPLY(min) = Minimum supply operating voltage,
VSUPPLY(max) = Maximum supply operating voltage,
VDO = Drop-out voltage (absolute minimum voltage across device, S+ & OUT pins)
It can only be used with a 3-terminal COCM. If the supply varies too much and/or it is required to
use a 4-terminal COCM, either Figure 4 or Figure 5 will have to be used.
Besides the limitations in Equation 1 and Equation 2, the only other limitation on this method is
the power dissipation in the zener diode.
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Design Example 1
It is required to measure the load current of a 100V power supply which delivers 5A into the load.
The supply's tolerance is stated at ±5%. The output voltage needs to be scaled to 5V at full load.
Solution
The minimum to maximum supply range is 95V to 105 representing a change of 10V. This change
is well within the operating range of the ZXCT1009 of 2.5V to 20V and meets the requirements of
Equation 1. Therefore use the option in Figure 3.
Transposing Equation 1 above,
VZ ≤ VSUPPLY (min) − (VDO + VOUT (max) )
Equation 4
VZ ≤ 87.5V
Transposing Equation 2 above,
VZ ≥ VSUPPLY (max) − VWM
Equation 5
VZ ≥ 85V
Hence, the zener diode's voltage rating needs to be between 85V and 87.5V. If the range of zener
voltage does not cover common standard values, as in this case, the required voltage could be
made up with two zener diodes in series. For example it is possible to use two 43V zeners in series
to form an 86V zener diode.
The next parameter to check is to make sure that the zener diode(s) dissipation is taken into
consideration. For this it is necessary to know what IOUT is. This can be approached in one of two
ways, either from the input to the output or, from the output to the input depending on which
parameter there is greater control of. If optimum accuracy is paramount and it is possible to have
full control of the choice of zener dissipation, work from the input. If zener dissipation is a given,
then work from the output. In either case the set of equations required are the same except that
they need to be worked iteratively to make sure they are not breaking any of the design
parameters.
So, assume that a zener diode rated at 300mW. As a general rule, it is necessary to apply a
derating factor to this, for example 50%. Hence IOUT is given by:
I OUT
P
0.3 ⋅ 0.5
= 1.74mA
= Z ⋅ 0.5 =
VZ
86
Equation 6
Since VOUT is known, RG can now be determined, but it is wise to determine RS first in case of the
need to adjust IOUT to take into consideration the range limitations of VSENSE, power dissipation
in RS and the limited choice of RS values. There is far more freedom in choosing RG than in RS
which is typically less than 1 Ohm.
Check for sensible values of RS and VSENSE to obtain an output current of around 1.74mA.
VSENSE =
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I OUT 1.74
=
= 174mV
GT
0.01
3
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Which will require an RS value of
RS =
V SENSE 0.174
=
= 34.8mΩ
I LOAD
5
It is unlikely to find a 34.8m⍀ resistor so it is necessary to choose the nearest standard value.
33m⍀, a value within the E12 value series, is more likely and represents only -5% deviation from
the calculated value (remember the zener power is derated by 50%, so there is plenty of margin).
Using this value, the true values of VSENSE and IOUT will be
VSENSE = RS ⋅ 5 = 0.033 ⋅ 5 = 165mV
I OUT = GT ⋅VSENSE = 0.01 ⋅ 0.165 = 1.65mA
which is even less than the original estimate, so it is known to be within acceptable limits.
Finally RG can be determined by,
RG =
VOUT
5
=
= 3.03kΩ
I OUT 1.65
So, use a 3k⍀ resistor for a cumulative error of 1%, or determine if 3.03k⍀ can be found in higher
electrical (E) series, or make up this value with a series or parallel resistor combination. For
example 3k in series with 30⍀ or 3k3 in parallel with 36k or 39k.
The solution of the problem is shown below in Figure 6.
Figure 6
Solution to Design Example 1
High voltage Option 2
The previous example in Figure 3 has a very limited supply variation range. Figure 4 is a little
more flexible as it dynamically varies the voltage drop across both R1 and R2 to compensate for
varying supply voltage.
TR1 is used in the common base configuration and is used to drop most of the supply voltage
between collector and emitter. When the current gain is reasonably high (>100), IC≈IE and IOUT
still flows through RG and hence VOUT can still be calculated in the normal way.
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Ideally, R1 must be chosen to keep within the ZXCT's normal supply range, large enough in value
to provide the minimum operating voltage to the device at the lowest supply voltage but not too
large that the maximum device operating voltage is exceeded at the highest input voltage.
Procedure 1 - Design steps for Figure 4
1. Determine or estimate IOUT (it doesn't need to be precise at this stage)
2. Determine the required minimum supply voltage, VSUPPLY(min).
3. Determine device's maximum working voltage, VMAX.
I
4. Calculate transistor bias current IB from I B = OUT
hFE (min)
5. Calculate bias resistor RB from
RB =
(VSUPPLY (min) − VDO − Veb )
IB
=
⎛
VSUPPLY (max)
6. Calculate R1 from R1 = ⎜
⎜V
⎝ SUPPLY (max) − VMAX
(VSUPPLY (min) − VDO − Veb ) ⋅ hFE (min)
I OUT
=
R1⋅ R 2
R1 + R 2
⎞
⎟ ⋅ RB
⎟
⎠
⎛V
⎞
7. Calculate R2 from R 2 = ⎜⎜ SUPPLY (max) ⎟⎟ ⋅ RB
⎝ VMAX
⎠
High voltage Option 3
In a situation where a higher supply voltage is required or where the supply voltage varies over
a wide range, the scheme in Figure 5 could be used where resistor R1 in Figure 4 is replaced with
a zener diode rated within the maximum working voltage of the COCM. The design steps are
similar to those in Procedure 1 but slightly simpler.
Procedure 2- Design steps for Figure 5
1. Determine or estimate IOUT (it doesn't need to be precise at this stage)
2. Determine device's maximum working voltage, VMAX.
3. Chose the value of Z1 to be within VMAX e.g. VZ=15V for a 20VMAX device.
In general, make sure (VDO + Vbe ) < VZ ≤ VMAX
4. Determine the required minimum supply voltage, VSUPPLY(min).
5. Compute transistor bias current IB from I B =
6. Compute resistor R2 from
R2 =
(VSUPPLY (min) − VZ )
IB
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=
I OUT
hFE (min)
(VSUPPLY (min) − VZ ) ⋅ hFE (min)
I OUT
5
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High voltage Option 4
Both Options 2 and 3 (Figure 4 and Figure 5) provide wider range operation than is possible with
Figure 3. However neither would be suitable for devices such as ZXCT1050 whose common mode
range include ground. What is required is a scheme that extends the supply voltage (or common
mode) range but does not at the same time raise it from ground. Figure 7 below shows how this
can be done.
A resistor, R3, is connected from the S- pin to ground so as to form a potential divider with the
transconductance resistor, RGT. The S+ pin is similarly connected to another potential divider
formed by R1, R2. It must be ensured that the ratios (not the absolute values) of the two potential
dividers are exact. In other words, R1/R2 must be equal to RGT/R3. Failure to observe this rule will
result in massive common mode error that would render the scheme practically useless. In
addition, the resistors themselves need to be very closely matched to much better than 1%.
RS
RS
VSUPPLY
VSUPPLY
R1
R1
RGT
S+
SVCC ZXCT1050
GND
OUT
VCC
VCC
IOUT
VOUT
RGT
S+
SVCC ZXCT1050
GND
OUT
R2V
IOUT
R2F
RG
VOUT
R2
R2
RG
R3
Figure 7 Extending the CM range of the
ZXCT1050 (using precision resistors)
R3
Figure 8 Using non-precision resistors to
extend CM range (using standard resistors)
Hence, one resistor could be replaced by a trimmable resistor to balance both legs. This way, less
than precise values could be used to start with as shown in Figure 8. Here, R2 has been replaced
by the combination of a fixed and a variable resistor1. Now, the resistors do not have to be low
tolerance ones and standard 1% or even 2% resistors can be used. What is more important is
stability. So, in any case, always make sure that high stability resistors are used. Metal film
resistors are generally very good for this.
Procedure 3- Design steps for extending CM range and Figure 7 and Figure 8
1. Determine the maximum required supply voltage, VS(max).
2. Calculate R3 from R3 =
RGT
⎛ VS (max) ⎞
⎜⎜
⎟⎟ − 1
⎝ VCC − 2 ⎠
3. Make R3 the nearest lower preferred value. E.g. if the result of 2 above were 69.35k, then
choose 68k as the nearest lower preferred value.
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R
4. Next, determine R1 and R2 from R1 = GT . The easiest thing to do is to simply make
R2
R3
R1=RGT and R2=R3. It’s possible to make R1=nRGT and R2=nR3 where n is any arbitrary
number, preferably not less than 1. The advantage of making n greater than 1 is that the
current down the potential divider network formed by R1, R2 can be kept to a minimum. Be
careful however not to make n too high as it then begins to introduce offset errors into the
circuit. A value of n between 1 and 10 is quite reasonable.
1Note
that it is not recommended to make all of R2 variable as this would result in very low resolution, increased
potential for long term drift and make the circuit more susceptible to thermal and mechanical shock effects.
This is all that is required as far as using high precision resistors is concerned (Figure 7). In order
to use standard resistors however (Figure 8) the following steps are required as well.
5. Determine the tolerance, Tol, of resistors being used, e.g. 1%.
8 ⋅ Tol
6. Calculate R2V from R 2V ≥
⋅ R 2 and select the nearest higher preferred value.
100
4 ⋅ Tol ⎞
7. Calculate R2F from R 2 F ≤ ⎛⎜1 −
⎟ ⋅ R 2 and select the nearest lower preferred value.
100 ⎠
⎝
Make sure that R2V is a good quality variable resistor (e.g. cermet type). If the circuit is going to
be subjected to a wide temperature range, it would also be advisable to make sure that the
temperature coefficient of R2V is comparable to that of the fixed resistors.
Conclusion
Current output current monitors have a limited voltage range. However, use of a few extra
components allows their voltage capability to be extended to hundreds of volts. Several
techniques have been discussed which shows the flexibility and usefulness of current output
current monitors.
Recommended further reading
1. AN39 - Current Measurement Applications Handbook
2. DN77 - Transient and noise protection for current monitors
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The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
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or
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