There are three common methods of current sensing for switch-mode power supplies: using a sense resistor, using a MOSFET RDS(ON), and using an Inductor‘s DC resistance (DCR). Each method has advantages and disadvantages that should be considered when selecting a detection method.
sense resistor current
The sense resistor as a current sense element produces the lowest sense error (typically between 1% and 5%) and a very low temperature coefficient of about 100 ppm/°C (0.01%). In terms of performance, it provides the most accurate power supply, helps achieve extremely accurate supply current limiting, and also facilitates precise current sharing when multiple supplies are connected in parallel.
Figure 1. RSENSE Current Sensing
On the other hand, resistors also generate additional power dissipation due to the addition of current-sense resistors to the power supply design. As a result, sense resistor current monitoring techniques may have higher power dissipation compared to other sensing techniques, resulting in lower overall solution efficiency. Dedicated current sense resistors may also add to the solution cost, although a sense resistor typically costs between $0.05 and $0.20.
Another parameter that should not be ignored when choosing a sense resistor is its parasitic inductance (also known as effective series inductance or ESL). The sense resistor can be properly modeled with a resistor in series with a finite inductor.
Figure 2. RSENSE ESL Model
This inductance depends on the specific sense resistor chosen. Certain types of current sense resistors, such as sheet metal resistors, have lower ESL and should be used in preference. In contrast, wire wound sense resistors have high ESL due to their package construction and should be avoided. In general, the ESL effect becomes more pronounced as the current increases, the detection signal amplitude decreases, and the layout is unreasonable. The total inductance of a circuit also includes parasitic inductances caused by component leads and other circuit components. The overall inductance of a circuit is also affected by layout, so component placement must be properly considered. Improper layout can affect stability and exacerbate existing circuit design problems.
The effect of the sense resistor ESL can be mild or severe. ESL can cause significant oscillations in the switching gate driver, which adversely affects switch turn-on. It also increases the ripple of the current sense signal, causing voltage steps in the waveform instead of the expected sawtooth waveform shown in Figure 3. This reduces the current detection accuracy.
Figure 3. RSENSE ESL can adversely affect current sensing.
To minimize resistance ESL, sense resistors with long loops (like wirewound resistors) or long leads (like thick resistors) should be avoided. Low profile surface mount devices are preferred, examples include board structure SMD sizes 0805, 1206, 2010 and 2512, and better choices include inverted geometry SMD sizes 0612 and 1225.
Current Sensing Based on Power MOSFET
Current sensing using MOSFET RDS(ON) enables simple and cost-effective current sensing. The LTC3878 is a device that uses this approach. It uses a constant on-time valley mode current sense architecture. The top switch is turned on for a fixed time, after which the bottom switch is turned on, and its RDS drop is used to detect the current valley or current lower limit.
Figure 4. MOSFET RDS(ON) Current Sensing
While inexpensive, this method has some drawbacks. First, its accuracy is not high, and the RDS(ON) value may vary widely (about 33% or more). Its temperature coefficient can also be very large, even exceeding 80% above 100°C. Also, if an external MOSFET is used, the MOSFET parasitic package inductance must be considered. This type of detection is not recommended for very high currents, especially for polyphase circuits, which require good phase current sharing.
Inductor DCR Current Sensing
Inductor DC resistance current sensing uses the parasitic resistance of the inductor winding to measure current, eliminating the need for a sense resistor. This reduces component cost and improves power efficiency. Compared to the MOSFET RDS(ON), the device-to-device variation of the inductive DCR of the copper wire winding is generally smaller, although it still varies with temperature. It is favored in low output voltage applications because any voltage drop across the sense resistor represents a significant portion of the output voltage. An RC network is connected in parallel with the series combination of inductance and parasitic resistance, and the sense voltage is measured on capacitor C1 (Figure 5).
Figure 5. Inductor DCR Current Sensing
With proper component selection (R1 × C1 = L/DCR), the voltage across capacitor C1 will be proportional to the inductor current. To minimize measurement error and noise, it is best to choose a lower value of R1.
The circuit does not measure the inductor current directly, so it cannot detect inductor saturation. Soft saturation inductors such as powder core inductors are recommended. These inductors typically have higher core losses than equivalent iron-core inductors. Compared with the RSENSE method, inductive DCR detection does not have the power loss of the sense resistor, but may increase the core loss of the inductor.
When using the RSENSE and DCR detection methods, Kelvin detection is required due to the small detection signal. It is important to keep the Kelvin sense traces (SENSE+ and SENSE- in Figure 5) away from noisy copper pours and other signal traces to minimize noise extraction. Some devices, such as the LTC3855, feature temperature compensated DCR detection to improve accuracy over temperature.
Table 1 summarizes the different types of current sensing methods and their advantages and disadvantages.
Table 1. Advantages and Disadvantages of Current Sensing Methods
Each method mentioned in Table 1 provides additional protection for switch-mode power supplies. Depending on design requirements, trade-offs in accuracy, efficiency, thermal stress, protection, and transient performance may affect the selection process. Power supply designers need to choose the current sensing method and power inductor carefully, and design the current sensing network correctly. Computer software programs, such as Analog Devices’ LTpowerCAD design tool and LTspice® circuit simulation tool, can go a long way to simplifying design work and achieving optimal results.
Other current detection methods
There are other current sensing methods available. For example, current sense transformers are often used with isolated power supplies to provide protection for current signal information across the isolation barrier. This method is generally more expensive than the three techniques above. In addition, new power MOSFETs with integrated gate driver (DrMOS) and current sensing have appeared in recent years, but so far, there is not enough data to infer how well DrMOS performs in terms of the accuracy and quality of the sensed signal.
LTspice software is a powerful, fast, free simulation tool, schematic capture and waveform viewer with enhancements and models to improve simulation of switching regulators. Click here to download LTspice.
LTpowerCAD Design Tool is a complete power supply design tool program that significantly simplifies the task of power supply design. It guides users to find solutions, select power stage components, provide detailed efficiency information, Display fast loop Bode plot stability and load transient analysis, and export the final design to LTspice for simulation.
About the Author
Henry Zhang is Director of Applications Engineering for Power Products at Analog Devices. He joined Linear Technology (now part of Analog Devices) in 2001 as a power applications engineer and began his career. He became Head of Applications Division in 2004 and Applications Engineering Manager in 2008. His team supports a wide range of products and applications, from small form factor integrated power modules to large kW class high power, high voltage converters. In addition to supporting power applications and new product development, his team has developed the LTpowerCAD power design tool program. Henry has a broad interest in power management solutions and analog circuits. He has published more than 20 technical articles, published many seminars and videos, and has more than 10 power supply patents granted or pending.
Henry is a graduate of Virginia Polytechnic Institute and Blacksburg State University, Virginia, with an MS and PhD in electrical engineering.
Kevin Scott is Product Marketing Manager in the Power Products Group at Analog Devices, where he manages boost, buck-boost and isolation converters, LED drivers and linear regulators. He has served as a senior strategic marketing engineer, responsible for developing technical training content, training sales engineers, and authoring numerous website articles on the technical advantages of the company’s many products. He has 26 years of experience in the semiconductor industry in applications, business management and marketing roles. Kevin graduated from Stanford University in 1987 with a bachelor’s degree in electrical engineering.