Power Supply Design Seminar Current Sensing Solutions for Power Supply Designers Topic Categories: Basic Switching Technology Design Reviews – Functional Circuit Blocks Reproduced from 1997 Unitrode Power Supply Design Seminar SEM1200, Topic 1 TI Literature Number: SLUP114 © 1997 Unitrode Corporation © 2011 Texas Instruments Incorporated Power Seminar topics and online power- training modules are available at: power.ti.com/seminars Current Sensing Solutions for Power Supply Designers By Bob Mammano ABSTRACT sensing applications, there are many issues facing the designer. Among these include: While often considered a minor overhead func­ tion, measuring and controlling the currents in a • The need for AC or DC current information power supply can easily become a major contributor • Average, peak, RMS, or total waveform of the to the success or failure of a design. To aid in current achieving optimum solutions for this task, this topic • Isolation requirements will review the many strategies for current sensing, • Acceptable power losses in the measurement describe the options for appropriate sensing devices, process and illustrate their application with practical design • Accuracy, stability, and robustness techniques. The distinction between current control • Bandwidth and transient response and fault protection will be explored and applied to • Mechanical considerations various power supply topologies with emphasis on • Implementation cost performance-defining characteristics. CURRENT SENSING IN POWER Since there has been no published report thus far SUPPLIES of any single current monitoring approach which is clearly superior in all the aspects given above, an im­ While it would be easy to classify the design of portant task in any new project is to prioritize the re­ the typical power supply as a voltage regulation quirements as each options will have its strengths and problem, experienced designers recognize that very weaknesses in satisfying this list of issues. With high up on any priority list of tasks are issues related these characteristics in mind, let us first look at some to the measurement and control of current. And, as it of the possibilities in measuring current. turns out, these are usually where more of the diffi­ culties lie. At the risk of being too philosophical, let SENSING AND MEASURING me suggest that the measurement of voltage is a CURRENT "passive" activity in that it can readily be done at al­ RESISTIVE SENSING most any place in the system without affecting per­ formance. Current measurements, however, are Discrete Resistors for Current Sensing "intrusive" in that we have to "insert" some type of In almost all cases, current sensing means devel­ sensor and, in so doing, run a greater risk of affect­ oping a voltage signal which is representative of the ing the system we are trying to monitor or control. current flowing at the particular place of interest in Which is, of course, justification for this topic in the circuit. Thus, a current sensing resistor should be which we will attempt to present a compendium of considered as a current-ta-voltage converter which, in current sensing devices, techniques and applications order to perform its function, must be selected on the as an aid to designers of a broad range of power sys­ basis of a set of attributes such as the following: tems. • Low value in order to minimize power losses There are many reasons for monitoring current in (Rcs<1 n for this discussion) power supplies, and as many or more uses to make of • Low inductance because of high dildt the information once obtained. At least three of these • Tight tolerance on initial value and low tem­ which will be discussed herein, pertain to the use of perature coefficient for accuracy current sensing for fault protection, for current-mode • High peak power rating to handle short duration high current pulses controlled voltage regulation, and for commanding • High temperature rating for reliability current as a goal in itself. In all of these current Texas Instruments 1 SLUP114 Low value resistors in today's market consist of ~ - The first criteria in determining a sense re­ wirewound, thick film, thin film, metal element, and sistor's value is often the voltage threshold of the perhaps even other technologies. They come in a following circuitry which is going to operate on the wide variety of form factors including axial and ver­ sensed current information. The simpler Ie control­ tical through-hole mounted, power and chip surface lers, for example, will have a defined voltage range mounted devices, two terminal and four terminal for current sensing, or a specific threshold for initi­ (Kelvin), on metallic or alumina substrates, and some ating over-current protection, and the value of the with integral heat sinks. Representative samples of sensing resistor is then uniquely defined by the value this variety are pictured in Figure 1. The following of the current to be measured. In more sophisticated discussion is offered as an aid to understanding some applications where some signal amplification is of the considerations in selecting and applying these available, the choice would be made on the basis of devices to perform the function of current sensing. minimizing the voltage drop across the sense resistor. Caddock Electronics' Type MV lsotek's PMA, PMD Surface Mount Caddock Electronics ' Type SR10 Cur­ Power Film Resistor. Design uses four terminal package, 10mm x 6mm rent Sense Resistor. Kelvin connection interdigitated non-inductive termina­ x 1.5mm. Resistance down to 3mI2, vertical mount 1 W non-inductive de­ tions. Power ratings from 1.5W to with <30ppmi'C TCR sign. O.008W to 1.0W, 1% standard lOW tolerance. Isotek's RTO Heat Sink Base Mount lRC's OARS series Surface Mount two and four terminal configurations. sense resistor. As low as 5mD stan­ Uses TO-238 package and screw ter­ dard, rated at 1 W with <30ppmi'C minal connections. Rated to 50W with TCR. Mounting configuration mini­ <50ppm TCR and <2nH inductance mizes TCR of copper leads Figure I A sampling of commercially available current sensing resistor products. Texas Instruments 2 SLUP114 This process may be driven by the voltage itself - as connect to it. Note that these derating curves are in low headroom regulators, for example - or by unique to the style of resistor, its value, its material, power loss considerations stemming from either and its mounting configuration. thermal requirements or efficiency goals. In either case, minimizing the voltage drop requires a trade-off 150 study as there is almost always an accuracy or cost Cl <C penalty incurred. For example, amplifier or com­ 0 ...J parator input offset voltage variation represents a Cl w 100 fixed range of ambiguity which becomes an ever I- <C more significant variable as the sense voltage is re­ a:: 17S·C - """ SOLDER I- 120·C duced. Other errors which might not be noticed with z 50 ~ w CONTINUOUS 1 ~F\W- higher voltage drops include losses in contacting the 0 RATING OF ; a:: FR · 4CB _, sensing resistor and, potentially, even the effects of w a. MA TERIALS I ! thermal EMF generation at the connections. o ~ o 50 100 150 200 Another important consideration relative to re­ AMBIENT TEMPERATURE (·C) sistor value is the variability of resistor materials chosen by the various manufacturers of these devices. Figure 2 - A typical power derating curve for a current These low values of resistance demand specialized sense resistor materials and the manufacturers have responded with a wide range of proprietary formulations, all with Peak or pulsed power capability is a function of variations in their characteristics. Even within a par­ energy (watt-seconds) because it is energy which cre­ ticular part type from a particular manufacturer, dif­ ates heat, not just power alone. Peak power can ferent resistor values may be made with different therefore be hundreds or thousands of times higher materials with resultant differences in performance. than steady state power as long as the pulse duration This is an important point and will be illustrated fur­ is limited. An example of this is shown in the curves ther in the discussion which follows. of Figure 3 which describe a one-watt surface mount device from IRe which can have a pulse power rating Power Ratin~ - This parameter is often the driving of up to 40 watt-seconds. (Note that the discontinuity indicator for selection of the proper technology for a in the curve is due to a material change at 0.0150.) sense resistor. An important part of the power con­ siderations is an understanding of the nature of the current waveform to be experienced. The device may 50 be intended to sense a DC current but may also need (.) 40 to survive - or perhaps even measure - transient peak w CI) ...... currents caused from PWM switching, output ca­ ..:. 30 \ I- ...... pacitor charging, inrush current surges, load tran­ « \ s: \ ..... sients, short circuit peaks, and random externally 20 '" '--.... caused surge currents. "'- ~ 10 Data on steady state power dissipation capability 0.01 0.02 0.03 0.04 0.05 is most often supplied as a curve giving allowable power rating versus ambient temperature, where the RESISTANCE (0) curve is designed to manage the temperature rise in Figure 3 - Pulse-power, or energy rating, for a one-watt the resistor. Figure 2 gives an example of a typical IRe resistor series. derating curve for a sense resistor making note of the temperature limitations of the PC board under the resistor and the melting point of the solder used to Texas Instruments 3 SLUP114 Some technologies and package options may vice has a finite and non-linear characteristic, the limit the surge current rather than the power, due to industry practice is to specify TCR as a linear rela­ limiting constraints on leads, wire bonds, or in the tionship between specified temperatures, such as case of film devices, the rating of the film itself.