This paper was presented by Stephen Oxley, TT Electronics at the 3rd PCNS 7-10th September 2021, Milano, Italy as paper No.3.1. and voted by attendees as:
OUTSTANDING PAPER AWARD
The most cost effective and simplest way of converting a measured current to a voltage signal is to use a low ohmic value current sense resistor. The increase in products containing batteries, motors or actuators which call for current monitoring or control has led to huge growth in the market for current sense chip resistors with values below one ohm over the last two decades. But more recently, driven by power efficiency demands and enabled by low noise sense voltage amplifiers, the value range has been extended downwards from milliohms to hundreds of micro-ohms.
Such low ohmic values present challenges to the user at many stages in their design and manufacturing processes. This paper considers the nature of these challenges and suggests strategies to overcome them. The stages considered are component selection, PCB layout design, verification of the ohmic value of unmounted components, critical assembly processes, and expected ohmic values during product life. At each stage there are potential pitfalls but also opportunities to quantify and minimise error and variation.
Although sub-milliohm chip resistors are still just chip resistors, it is advisable to treat them as being a separate class of component, and to discover the particular considerations and techniques that enable their successful use.
PCB LAYOUT DESIGN
Near-Kelvin Connection
The PCB layout design around a very low value resistor is critical to its performance and the most important aspect of this is the fact 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 voltage sensing loop (Figure 4a), which would increase both the effective ohmic value and the TCR of the mounted part. This may be achieved by connecting the voltage sense tracks to the inner edges of the solder pads (Figure 4b.) One can also take this a step further and split the voltage sense pads from the current path pads, so that the solder joints themselves are also removed from the shared path (Figure 4c.) By this method it is possible to approach the accuracy obtained from a true four terminal resistor.
Furthermore, a study [1] by Analogue Devices based on TT Electronics’ ULR3 0.5mΩ mounting pad options has shown that a mounted value tolerance close to 1% may be achieved on a 1% tolerance component, indicating low additional error due to mounting effects, by using a centralised, isolated sense pad design similar to that of Figure 4c.
Minimisation of Sense Loop Area
A source of error where high currents which are AC or changing DC are involved is due to the voltage sensing loop linking with changing magnetic fields. This can induce a noise signal superimposed on the desired voltage sense signal. In order to reduce this, the loop area contained within the sense resistor, the two 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. Where long track runs are unavoidable, it is also possible to use periodic vias to cross over the tracks into alternate layers. This replicates on a PCB the effect of a twisted pair cable, which, by means of cancellation of induced voltages, allows the circuit to withstand the effect of any changing magnetic fields which have small variation across the spatial periodicity of the twisting.
Connecting Multiple Resistors in Parallel
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 all of the 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. Moreover, this ensures that the proportion of the total current carried by the sensed resistor does not vary with temperature, as would otherwise happen with unequal series track resistances as a result of the high TCR of the copper PCB tracks.
Design for Heatsinking
A flat chip resistor dissipates over 80% of its heat by conduction into PCB tracks, and so providing sufficient copper area to act as a heatsink is important. Copper area is for this purpose defined as the total area directly surrounding the solder pads including the first two squares of connected tracks. The general relationship between effective power rating and PCB copper area is as indicated in Figure 9. This can be divided into three distinct regions.
In region (A) there is relatively low thermal conduction through copper connected to the pads and conduction by substrate and convection to air predominate. In region (B) the copper connected to the pads acts as a heatsink to raise the effective power rating. In region (C) further increase in copper area gives diminishing returns as the internal thermal impedance of the chip restricts the rating.
This limiting factor of the internal thermal impedance may be lowered significantly by changing the orientation of the resistor. If the terminations are formed on the longer edges of the chip rather than the shorter edges, the solder joint width is approximately doubled and the maximum distance from film centre to termination is approximately halved. The principle is illustrated in Figure 10a. TT Electronics’ ULR2N and ULR3N, shown in Figure 10b are examples of products which make use of this enhanced cooling method.
The resistor datasheet should contain information on the mounting conditions used to obtain the rated power and this indicates the minimum copper area which a designer should provide.
