Current sensing – Different techniques

Part I

• Historically current was measured directly by moving coil meters where the current flowing through the coil moves a needle against the force of a spring. The deflection of the needle is directly proportional to the current. The sensitivity of the meter has to be high. Typically this current used to be 50 micro Amp.

• To measure voltage a series high value resistor is connected from the source to the meter.

• To measure high current a shunt resistor of low value is connected in parallel to the meter.

• Subsequently when digital systems became popular, Analog to Digital Converters were used to measure a voltage. The current required is very low less than a nA.

• Interface to most of the ADCs require a voltage to be generated.

• To sense current a very low resistor called shunt is used in the current path and the voltage across this shunt is connected to the ADC.

• It may be noted that a shunt is in fact connected in series with the path where we need to measure the current. It is connected in parallel to the moving coil meter and hence the name “shunt”

• Shunt is one of the methods to convert current into voltage.

• There are other popular methods for measuring current and they are– transformer (for ac measurement)– Hall effect sensor for both ac and dc.– Flux gate sensing– Rogowski coil

• This presentation deals with shunt and Current Transformer. Other types are covered in the remaining parts.

• This presentation also contains 2 viewgraphs on lossless inductor current sensing method.

Comparison of different methods of measuring current

LEM- different current sensing techniques

Measurement using shunts

• To ensure minimum loss we should use as low a resistor as possible.

• The shunt resistor should have very low temperature coefficient so that the value of resistor and hence the sensed voltage does not vary due to the self heating in the resistor.

• An amplifier will amplify the sensed signal to match the output to the ADC.

• ADCs typically have a full scale range of 1 to 5 V, while the sensed voltage across the shunt will be typically 60 to 100 mV.

Typical applications of a shunt

• To monitor current in a power supply, making sure it is not damaged by the load.

• To set the current in an LED lighting system.• To sense how much current is being applied to a battery

pack during charging to prevent overcharging or overheating in the batteries.

• To measure the amount of current being applied to a circuit from the battery pack, enabling estimation of the battery life before charging is needed.

• To estimate the rotational speed of a motor, since the speed of a motor is directly proportional to the amount of current applied to it.

• In ac applications a transformer may thought of as the first option.

Important Parameters for Current sense resistors.

• Tight Tolerance• Typically ±1% or tighter.• Low Temperature Coefficient

Typically 50 to 500 ppm. • Low Thermal EMF• Resistance Value

– For maximizing energy conversion efficiency, this should be kept as low as possible.

tti current sense resistor

Four Terminal Measurement

When current measured is large, the points of measurement is very critical and Four Terminal measurement technique is usually adopted.

Figure above illustrates the layout to be used to get an accurate and repetitive measurement.

Placement of the shunt

• Where the shunt is inserted is a very important decision. • To insert it in the ground path is easy from the amplifier

point of view.• But in many applications it may not be feasible to break

the different ground paths. Positive bus or some intermediate point may be the place where the shunt has to be inserted. Such a sensing is called High Side Sensing. To make the matters worse the voltage at the intermediate point may have large excursions of voltage.

• Since the voltage at the High Side can be anywhere from 5V to 100 V (it may even be 400V), normal op amps can not be directly connected to the shunt.

• Most semiconductor manufacturers have ICs for high side sensing. I have taken examples from TI.

• INA 138, 168 and 193 series are the popular ones. The links of these are given below:

• http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=426&uiTemplateId=NODE_STRY_PGE_T

INA 138 block diagram

INA 138

Q1

INA 138

• Load current IS is drawn from supply VS through shunt resistor RS.

• The voltage drop in shunt resistor (VS) is forced across the input terminals through resistors causing current to flow into the collector of Q1.

• External resistor RL converts this current to a voltage, VOUT, at the OUT pin.

• The output voltage is given by the equation

VOUT = (IS) (RS) (200μA/V) (RL)

Gain-Bandwidth of INA 138

Band width is a function of the gain. Higher the gain lower the bandwidth.

For a gain of 10, the bandwidth is 100 KHz.

The bandwidth also comes down with capacitor across the output resistor.

Common mode noise

Since the current sense signal is at the positive high side, there is a high common mode voltage.

Any ac at this point can affect the performance further.

The dc common rejection is 120 db. Thus a 60 V common mode voltage gives an error of just 60 µV.

Things are different at high frequencies. At 100 KHz, for a differential gain of 10, the CMR is 60 dB. A 60 V swing will give 60 mV error which is quite high. The signal measured is also in the similar order.

Extending the range of common mode voltage range.

Breakdown voltage of MOSFET Q1 decides the maximum common mode voltage. INA sees only a max voltage of only 39 V

Extending the range of high side sensing

Current Sense amplifiers from other manufacturers

• Analog Devices– http://www.analog.com/en/subCat/0,2879,759%255F7

77%255F0%255F%255F0%255F,00.html

• Linear Technology– http://www.linear.com/pc/categoryProducts.jsp

• Maxim– http://para.maxim-ic.com/results.mvp?q=cs_amp&r=0

&an_1=Family&av_1=Current%20Sense%20Amplifiers&mh_1=1&wt_1=0&tree=amps&p=1

Current Transformer sensing

• To measure ac, current transformer is the best answer.

• But current transformer at 50 Hz is quite bulky.

• Design of a current transformer is very similar to a voltage transformer.

• Next few slides give the details.

Current Transformer Design Procedure

• Spec :– Imax = 50 A– Frequency = 50 Hz– Turns ratio =100– Full scale sense voltage = 1 V.

• Design:– Output current = Imax / Turns ratio = 0.5 A– Output Power = 0.5 VA.– There are 2 software which will help us in doing the

further design. One is from ORCAD 15.1. Magnetics Part Editor which may be invoked under the “PSpice Accessories” is the tool. The other is from “Magnetics”. This software is downloadable free from the following url. http://www.mag-inc.com/software/ctd1_3.zipPackage

• This software can design 3 types of current transformer– a) Traditional (like 50 HZ one)– b) Hall effect– c) SMPS (High frequency).

• For the 50 Hz one we need to use a material that has a large Bmax.

• Magnetics offers 3 types of material suitable for this application – a) Magnesil Bmax = 1.8 Tesla– b) 48 Alloy Bmax = 1.4 Tesla– c) Supermalloy Bmax = 0.8 Tesla

• Burden refers to the resistor that needs to be connected across the secondary to limit the voltage at the output for max current.

• For our example the secondary current is 50/100 = 0.5 A. The burden resistance for limiting the output to 1 V will be 1/ 0.5 A. This gives a resistance value of 2 ohms. Bmax has been chosen as 500 Gauss. This determines the accuracy of measurement. To improve accuracy we need to reduce Bmax which eventually increases the size of the CT.

Few design possibilities Bmax Burden Material Accuracy Core

OD

5000 2 Ω Magnesil 5.25% 1.5 “

5000 2 Ω 48 Alloy 0.27% 1.5 “

5000 2 Ω Supermalloy

1.13% 1.5 “

3000 2 Ω Magnesil 8.32 % 3.5”

3000 1 Ω Magnesil 2.2% 1.15”

3000 0.5 Ω Magnesil 0.79% 1.0”

• Accuracy and size are dependent upon burden, Bmax and material.

Lossless current sensing for inductor

Conventional approach

Lossless approach

So, if we make Lo/Rdc = R1*C1, thenVC =Io * Rdc

Inductor current sensing