Switch Mode Power Supply Current Sensing - Part 2: Where to Place the Sense Resistor

Where to Place the Current Sense Resistor

The placement of the current sense resistor in conjunction with the switching regulator architecture determines what current is being sensed. Currents that are sensed include the peak inductor current, the valley inductor current (the minimum value of the inductor current when in continuous conduction mode) and the average output current. The location of the sense resistor affects power loss, noise calculations and the common mode voltage seen by the sense resistor monitoring circuitry.

Buck Regulator High Side Placement

For a step-down (buck) regulator, the current sense resistor can be placed in several locations. When placed on the high side of the top MOSFET (as shown in Figure 1), it detects the peak inductor current when the top MOSFET is on and thus can be used for peak current mode controlled supplies. However, it does not measure inductor current when the top MOSFET is off and the bottom MOSFET is on.

Current Sense Blog RSENSE Placement Buck HS

Figure 1. Buck Converter with High Side RSENSE

In this configuration, current sensing can be noisy, because the turn-on edge of the top MOSFET has strong switching voltage ringing. To minimize this affect, a long current comparator blanking time (the time during which the comparator ignores the input) is needed. This limits the minimum switch ON time and can limit the minimum duty cycle (duty cycle= VOUT / VIN) and maximum converter step-down ratio. Note in the high side configuration, the current signal can be riding on top of a very large common mode voltage (VIN).

Buck Regulator Low Side Placement

In Figure 2, the sense resistor is placed below the bottom MOSFET. In this configuration it detects the valley mode current. To further reduce power loss and save component cost, the bottom FET RDS(ON) can be used to sense current without using an external current sensing resistor RSENSE.

Current Sense Blog RSENSE Placement Buck Low Side

Figure 2. Buck Converter with Low Side RSENSE

This configuration is usually used for a valley mode controlled power supply. It can also be sensitive to noise, but in this case it is when the duty cycle is large. A valley mode controlled buck converter allows high step-down ratios; however, its maximum duty-cycle is limited due to its fixed/controlled switch ON time.

Buck Regulator Placement in Series with the Inductor

In Figure 3, the current sensing resistor RSENSE is placed in series with the inductor, so it can detect the continuous inductor current, which can be used for average current monitoring, and peak or valley current monitoring. Accordingly, this configuration allows peak, valley or average current mode controls.

Current Sense Blog RSENSE Placement Buck Post Inductor

Figure 3. RSENSE in Series with the Inductor

This sensing method provides the best signal-to-noise ratio performance. An external RSENSE usually can provide a very accurate current sensing signal for accurate current limit and sharing. However, the RSENSE also causes additional power loss and component cost. To reduce the power loss and cost, the inductor winding DC resistance (DCR) can be used to sense current without an external RSENSE.

Boost and Inverting Regulators High Side Placement

For a step-up (Boost) regulator, the sense resistor can be placed in series with the inductor providing high side sensing (Figure 4).

Current Sense  Blog RSENSE Placement Boost High Side

Figure 4. Boost Converter with High Side RSENSE

Since the boost has continuous input current, a triangular waveform results and current is continuously monitored.

Boost and Inverting Regulators Low Side Placement

The sense resistor can also be placed on the low side of the bottom MOSFET as shown in Figure 5. Here, the peak switch current (which is also the peak inductor current) is monitored, resulting in a current waveform every half cycle. Due to the MOSFET switching, the current signal has strong switching noises.

Current Sense Blog RSENSE Placement Boost Low Side

Figure 5. Boost Converter with Low Side RSENSE

Buck-Boost Low Side SENSE Resistor Placment or in Series with the Inductor

A 4-switch buck-boost converter is shown below in Figure 6 with the sense resistor on the low side. The converter operates in buck mode when the input voltage is much higher than the output voltage, and in boost mode when the input voltage is much lower than the output voltage. In this circuit, the sense resistor is located at the bottom of the 4-switch H-bridge configuration. The mode of the device (buck mode or boost mode) determines what current is being monitored.

Current Sense Blog RSENSE Placement Buck-Boost Low Side

Figure 6. Buck-Boost with RSENSE on the Low Side

In buck mode (switch D always on, switch C always off), the sense resistor monitors the bottom side switch B current and the supply operates as a valley current mode buck converter.

In boost mode (switch A always on, switch B always off) the sense resistor is in series with the bottom MOSFET (C) and measures peak current as the inductor current rises. In this mode, since the valley inductor current is not monitored, it is difficult to detect the negative inductor current when the supply is in light load condition. Negative inductor current means energy is simply being transferred from the output back to the input, but due to losses associated with the transfer, efficiency suffers. So for applications such as battery-powered systems for which light load efficiency is important, this current sensing method is undesirable.

The circuit of Figure 7 resolves this issue by placing the sense resistor in series with the inductor so that the inductor current signal is continually measured in both buck and boost modes. Since current sensing RSENSE is connected to the SW1 node which has high switching noises, the controller IC needs to be carefully designed to allow sufficient blanking time for the internal current comparator.

8390 Current Sensing RSENSE on H-Bridge

Figure 7. LT8390 Buck-Boost with RSENSE in series with the Inductor

An additional sense resistor can also be added at the input for input current limiting or at the output (as shown below) for constant output current applications such as battery charging or driving LEDs. In this case, since the average input or output current signal is needed, a strong R/C filter can be added to the current sensing path to reduce current sensing noise.

In most of the above examples, the current sensing element is assumed to be a sense resistor. However, this does not have to be and often is not the case. Other sensing techniques include using the voltage drop across a MOSFET or the DC resistance (DCR) of the inductor. These current sensing methods are addressed in Part 3 "Current Sensing Methods".

Authors

Kevin Scott

Kevin Scott

Kevin Scott works as a Product Marketing Manager for the Power Products Group at Analog Devices, where he manages Boost, Buck-Boost and Isolated Converters, LED Drivers and Linear Regulators. He previously worked as a Senior Strategic Marketing Engineer, creating technical training content, training sales engineers and writing numerous website articles about the technical advantages of the company’s broad product offering. He has been in the semiconductor industry for 26 years in applications, business management and marketing roles.

Kevin graduated from Stanford University in 1987 with a BS in Electrical Engineering and started his engineering career after a brief stint in the NFL.

henry-jindong-zhang

Henry (Jindong) Zhang

Henry Zhang is an applications engineering manager for power products at Linear Technology. He began his Linear career as an applications engineer in 2001. He became an applications section leader in 2004 and applications engineering manager for power products in 2008. His group supports wide range of products and applications, from small monolithic regulators and power modules, to large kW-level high power, high voltage converters. In addition to supporting power applications and new product developments, his group also develops the LTpowerCAD supply design tool. Henry has broad interests in power management solutions and analog circuits. He has over twenty technical articles, seminars and videos published and 8 power supply patents granted or pending.

Henry graduated from Virginia Polytechnic Institute and State University in Blacksburg, Virginia with his masters and Ph.D. degrees in electrical engineering.