28-01-2016 | | By Stephen Oxley
Stephen Oxley, Senior Engineer at TT Electronics, advises on the correct way to use current sense resistors.
Designers of power supplies, battery management systems and motor drives are commonly faced with the need to measure current accurately. Whether for fault protection or functional control, the simplest method is often to use a current sense resistor. With this type of resistor circuit performance is dependent on good layout design as much as on component selection. The designer must also minimize the voltage dropped across the sense resistor and take care of errors that can arise from sensing a low voltage. The heat dissipated by the sense resistor and its ability to withstand surge currents are other factors to take account of. The ten tips presented here will help to address all of these issues.
Current Sense Methods
The first task for a designer with a current to measure is to select which of the four common current sensing methods to use. These are listed below, together with the advantages and disadvantages of each. Current sense resistors, with the advantages of low cost and accurate wideband measurement are frequently the best answer.
Choosing the best ohmic value Choosing the best ohmic value is a question of balance. If it is too high then power will be wasted, excess heat generated, and voltage regulation lost. If it is too low then the sense voltage will be correspondingly low, so issues of noise and resolution will limit accurate measurement.
You can calculate the ideal ohmic value from the lowest value of sense voltage consistent with an acceptable accuracy divided by the lowest value of current in the range to be measured. Then choose the closest available value above this ideal value. Current sense resistors are often available at integer milliohm values up to 10 milliohms and at multiples of 5 milliohms above this, in addition to standard E24 values.
Picking the right power rating Once the ohmic value is fixed you can calculate the power dissipation under operating conditions from P = Irms2.R. Allowing for overload or fault conditions and high ambient temperature if applicable, select the required power rating. For many current sense products, only the maximum temperature of the solder joints limits the power rating. Power rating is thus sometimes a function of the PCB layout design as well as of component selection. It may be further be influenced by the use of a thermal substrate such as DBC or IMS. For example, LRMAP3920 is rated at up to 5W on FR4 but up to 10W on a thermal substrate.
Selecting the resistor technology Many current sense resistors are in the category of bulk metal technology. This means that the element is a self-supporting piece of resistance metal alloy. This gives the lowest ohmic values available, with SMD parts extending down to fractions of 1 milliohm. Bulk metal also offers the benefits of good surge tolerance and low temperature coefficient of resistance (TCR). At values around 100 milliohms and above, a supporting substrate is needed and the resulting technologies of wirewound and metal foil offer similar benefits.
Thick film chip resistors are available for current sensing at values down to a few milliohms and offer a wide selection of ohmic values and very low internal thermal impedance. However, the TCR at lower values and the surge performance are generally not as good as for bulk metal. Thin film chip resistors offer an improvement in TCR but should never be used in circuits where current surges might occur.
Optimising the PCB design PCB track design around current sense resistors is generally more critical to performance than for normal resistors. The main difference is that four rather than two tracks must be provided to form a Kelvin connection, even where the component itself has only two terminals. The aim is to minimize the conductive path shared between the current path and the sensing loop (figure 2), which would increase both the tolerance and the TCR of the mounted part. You can do this by connecting the voltage sense tracks to the inner edges of the solder pads (figure 3.) You can also take this a step further and split the voltage sense pads from the current path pads, so that the solder joints are also removed from the shared path (figure 4.) By this method it is possible to approach the accuracy obtained from a true four terminal resistor.
Managing heat Whether your design requires the resistor to dissipate power at a significant proportion of its rating, or to have minimal temperature rise in order to minimize TCR errors, it’s important to understand how heat will be removed and where it will go. There are two options; either the heat can go mostly into the PCB or it can go mostly into the surrounding air. A flat chip format will deliver heat mainly through its solder joints into the PCB tracks, and this means that the effective power rating may be enhanced by providing additional area of copper around both terminations, and by placing the resistor away from other heat generating components if possible. Where it is preferable to deliver heat to the air, parts with higher internal thermal impedance should be used. One example is OARS-XP series (figure 5) in which the element is lifted away from the PCB. The corresponding thermal image (figure 6) shows that the solder joints remain at around 110°C even when the element temperature exceeds 200°C. This format will minimize heating of the PCB and can better take advantage of forced air cooling if provided.
Allowing for current surges Often a design must accommodate high surges in current above the maximum current which needs to be accurately measured. These surges can arise momentarily as a result of inrush conditions or the delay between a motor stalling and a protective current trip occurring. If this is a possibility, then the current sense resistor must have an appropriate level of surge tolerance.
The best technology for surge tolerance is bulk metal. This is because energy capacity is dependent on the heat capacity of the resistive element, and both the mass and the permissible peak temperature are relatively high for bulk metal. The energy capacity is normally stated on the datasheet and is generally dependent on ohmic value.
Thick film technology can give moderately good surge performance, and where this is specified, as in the case of LRF3W, it is stated on the datasheet as a graph of maximum peak power against surge duration. The surge performance in this case does not depend on ohmic value. Thin film types are generally unsuitable where surges may occur.
Derating for high temperature As with any resistor, if the ambient temperature is higher than the rated temperature, power de-rating must be applied, and this is generally indicated in graphical form on the datasheet. Ambient temperature is defined as the temperature which would be adopted by the body of the resistor under application conditions except with negligible power dissipation in the resistor. For air cooled types this is the surrounding air temperature, and for chip packages it is the local PCB temperature.
In some cases, such as high power chip parts like LRMAP3920, the de-rating graph is plotted against terminal temperature rather than ambient temperature. This is ambient temperature plus the temperature rise at the terminals. It is essential to note which temperature is on the X-axis when comparing between datasheets.
Understanding thermal EMF When using a metallic element shunt with high heat dissipation and low sense voltage, consideration may need to be given to thermoelectric voltages. The junction between a metallic resistance element and metal terminations acts as a thermocouple, generating a voltage proportional to the temperature difference across it. A metal element sense resistor is therefore like two thermocouples back to back. This means that, if the temperature differences across both junctions are equal, the error voltage is cancelled out. You can achieve this by making the design thermally symmetrical, that is, by presenting both terminals with similar levels of heatsinking and by keeping any other heat sources thermally distant.
A further benefit may be obtained by choosing an alloy with an inherently low thermal EMF against the termination material. For example, a junction between copper and copper-manganese alloys can develop just 3µV/°C which is over an order of magnitude lower than for a copper-nickel alloy. An example of a product with low thermal EMF is LRMAM.
Reducing inductive errors The combination of a high current path and a low signal voltage makes current sense resistor circuits particularly vulnerable to inductive errors. These may be due either to self-inductance or to mutual inductive coupling to other circuits. Self-inductance is important for the measurement of high frequency or rapidly changing currents and is generally lowest for SMD bulk metal parts at around 1 to 5nH. But even wirewound parts, known for high inductance, tend to have figures below 50nH for values below 1O. For a quick check on the importance of this factor, evaluate for the highest frequency, f, the magnitude of inductive impedance |Z|=2pfL and check whether this is negligible compared to the ohmic value.
The second source of inductive error is due to the voltage sensing loop linking changing magnetic fields. In order to reduce this, the loop area contained by the sense resistor, the voltage sense tracks and the sense circuit input should be minimised. This means keeping the sense circuitry as close as possible to the sense resistor and running the voltage sense tracks close to each other. A good way to keep these tracks really close is to superimpose them in different PCB layers.
Combining multiple resistors Designers are sometimes forced to use more than one current sense resistor connected in parallel, either to meet a high power or surge rating, or to achieve an ohmic value lower than the minimum available. This is problematic but possible. Resistors may be connected in parallel with voltage sense connections made to just one of the resistors, provided the track layout ensures equal distribution of current between resistors. For example (figure 7), the position in the current trace in which the resistors are placed should be well clear of bends or constrictions which could affect the distribution of current density. The goal is to ensure that the total track resistance in series with each resistor should be the same (figure 8), so that the sensed resistor carries the required fraction of the total current.
If it is not possible to design for equal current distribution, or if true four terminal resistors are to be used, you can connect the sense terminals of multiple resistors together into two common tracks, one for each polarity. To prevent high currents flowing in these sense connections, a ballast resistor of at least 1000 times the value of the current sense resistor should be used in series with each connection.