Technical Article: What is Switch Mode Power Supply Current Sensing?

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Technical Article: What is Switch Mode Power Supply Current Sensing? [Copy link]

Current mode control is widely used in switch mode power supplies due to its high reliability, simple loop compensation design, and simple and reliable load sharing function. The current sense signal is an important part of the current mode switch mode power supply design. It is used to regulate the output and provide overcurrent protection. Figure 1 shows the current sense circuit of the LTC3855 synchronous switch mode buck power supply. The LTC3855 is a current mode control device with cycle-by-cycle current limiting function. The sense resistor RS monitors the current.

Figure 1. Switch-mode power supply current sense resistor (RS)

Figure 2 shows an oscilloscope image of the inductor current for two cases: the first case with a load that the inductor current can drive (red line), and in the second case with the output shorted (purple line).

Figure 2. LTC3855 current limiting and foldback example, measured on a 1.5 V/15 A rail.

Initially, the peak inductor current is set by the selected inductor value, the power switch on-time, the input and output voltages of the circuit, and the load current (indicated by “1” in the figure). When the circuit is short-circuited, the inductor current rises rapidly until it reaches the current limit point, where RS × IINDUCTOR (IL) equals the maximum current sense voltage, to protect the device and downstream circuitry (indicated by “2” in the figure). Then, the built-in current foldback limit (number “3” in the figure) further reduces the inductor current to minimize thermal stress.

Current sensing has other uses as well. In multiphase power designs, it allows for accurate current sharing. For light-load power designs, it improves efficiency by preventing reverse current flow (reverse current is current flowing through an inductor in the opposite direction, from output to input, which can be undesirable or even destructive in some applications). Also, when the loads in multiphase applications are light, current sensing can be used to reduce the number of phases required, thus improving circuit efficiency. For loads that require a current source, current sensing can convert the power supply into a constant current source for applications such as driving LEDs, charging batteries, and driving lasers.

In the second part of this series, “Where to Place the Current Sense Resistor,” we explain in which branch of the circuit to place the current sense resistor and how it affects operation.

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