Switch-Mode Power Supply Current Sensing: How to Place the Sense Resistor

okhxyyo

Switch-Mode Power Supply Current Sensing: How to Place the Sense Resistor [Copy link]

The location of the current sense resistor, along with the switching regulator architecture, determines the current to be sensed. The sensed currents include the peak inductor current, the valley inductor current (the minimum value of the inductor current in continuous conduction mode), and the average output current. The location of the sense resistor affects the power loss, noise calculations, and the common-mode voltage seen by the sense resistor monitoring circuit.

Placed in the buck regulator high side

For a buck regulator, there are several locations where the current sense resistor can be placed. When placed on the high side of the top MOSFET (as shown in Figure 1), it senses the peak inductor current when the top MOSFET is on, allowing it to be used for peak current mode control of the power supply. However, it does not measure the inductor current when the top MOSFET is off and the bottom MOSFET is on.

Figure 1. Buck converter with high-side RSENSE

In this configuration, the current sense can be very noisy due to the strong switching voltage ringing on the turn-on edge of the top MOSFET. To minimize this effect, a long current comparator blanking time (the time the comparator ignores the input) is required. This limits the minimum switch on-time and can limit the minimum duty cycle (duty cycle = VOUT/VIN) and the maximum converter step-down ratio. Note that in the high-side configuration, the current signal can be on top of a very large common-mode voltage (VIN).

Placed on the low side of the buck regulator

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

Figure 2. Buck converter with low-side RSENSE

This configuration is often used for valley mode controlled power supplies. It can also be sensitive to noise, but in this case it is sensitive when the duty cycle is large. Valley mode controlled buck converters support high step-down ratios, but since their switch on-time is fixed/controlled, the maximum duty cycle is limited.

Buck regulator in series with inductor

In Figure 3, the current sense resistor RSENSE is connected in series with the inductor so that the continuous inductor current can be sensed, which can be used to monitor the average current as well as the peak or valley current. Therefore, this configuration supports peak, valley, or average current mode control.

Figure 3. RSENSE in series with an inductor

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

Placed in the high-side of boost and inverting regulators

For a boost regulator, a sense resistor can be placed in series with the inductor to provide high-side sensing (Figure 4).

Figure 4. Boost converter with high-side RSENSE

The boost converter has a continuous input current, so a triangular waveform is generated and the current is continuously monitored.

Placed on the low side of the boost and inverting regulators

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 (also the peak inductor current) is monitored, generating a current waveform every half cycle. The current signal has strong switching noise due to the MOSFET switching.

Figure 5. Boost converter with low-side RSENSE

The SENSE resistor is placed at the low end of the buck-boost converter or in series with the inductor.

Figure 6 shows a 4-switch buck-boost converter with the sense resistor at 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 at the bottom of the 4-switch H-bridge configuration. The mode of the device (buck mode or boost mode) determines the current monitored.

Figure 6. Buck-boost converter with RSENSE on the low side

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

In boost mode (switch A is always on and switch B is always off), a sense resistor is placed in series with the bottom MOSFET (C) and measures the peak current as the inductor current ramps up. In this mode, since the valley inductor current is not monitored, it is difficult to sense negative inductor current when the power supply is lightly loaded. Negative inductor current means that energy is transferred from the output back to the input, but efficiency suffers because of the losses in this transfer. For applications such as battery-powered systems, where light-load efficiency is important, this current sensing method is undesirable.

The circuit in Figure 7 solves this problem by placing a sense resistor in series with the inductor, allowing the inductor current signal to be continuously measured in both buck and boost modes. Since the current sense RSENSE is connected to the SW1 node with high switching noise, the controller IC needs to be carefully designed to allow the internal current comparator to have a sufficiently long blanking time.

Figure 7. LT8390 buck-boost converter with RSENSE in series with the inductor

Additional sense resistors can also be added at the input to implement 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, an average input or output current signal is required, so a strong RC filter can be added to the current sense path to reduce current sense noise.

Most of the examples above assume that the current sensing element is a sense resistor. This is not a requirement and is often not the case. Other sensing techniques include using the voltage drop across a MOSFET or the DC resistance (DCR) of an inductor.

software

LTspice

LTspice software is a powerful, fast and free simulation tool, schematic capture and waveform viewer with enhanced features and models to improve the simulation of switching regulators.

LTpowerCAD

LTpowerCAD design tool is a complete power design tool program that greatly simplifies the task of power supply design. It guides the user to find a solution, select power stage components, provide detailed efficiency information, display fast loop Bode plot stability and load transient analysis, and can export the final design to LTspice for simulation.

Jacktang

LTspice software seems to be very powerful in simulating this circuit

Thanks for sharing

annysky2012

This article is good. It is not only for switching power supplies, but also for high-power devices that need to detect current.

btty038

Does anyone have any recommendations for current sensing chips?

For example, TI ADI?

That kind of mini type device,,,,

okhxyyo

btty038 posted on 2021-2-26 22:26 Can anyone recommend a small current detection chip? For example, TI ADI? The kind of mini type device,,,, ...

I don't know about this. I'm not familiar with it. You can check the official website yourself.

btty038

okhxyyo posted on 2021-2-28 20:29 I don't know about this. I'm not familiar with it, you can check the official website yourself

Oak

lehuijie

btty038 posted on 2021-3-1 12:15 Ouke

Chip with Hall detection, normal chip size