Switching power supply current detection method, this will make it clear

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 sensing circuit for the ADI LTC3855 synchronous switch-mode buck power supply. The LTC3855 is a current mode control device with cycle-by-cycle current limiting. The sense resistor RS monitors the current.

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

Figure 2 shows an oscilloscope image of the inductor current in two cases: the first case uses a load that the inductor current is capable of driving (red line), while in the second case the output is shorted (purple line).

Figure 2. LTC3855 current limit and foldback example, measured on 1.5 V/15 A supply 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 a "1" in the diagram). When the circuit is short-circuited, the inductor current rises rapidly until it reaches the current limit point, that is, RS × IINDUCTOR (IL) is equal to the maximum current detection voltage to protect the device and downstream circuits (indicated by "2" in the figure). Then, a built-in current foldback limit (number "3" in the figure) further reduces the inductor current to minimize thermal stress.

Current sensing also has other uses. In multi-phase power supply designs, it can be used to achieve accurate current sharing. For light load power supply designs, it improves efficiency by preventing reverse current flow (reverse current refers to the current flowing in the opposite direction through the inductor, that is, from the output to the input), which may be undesirable or even harmful in some applications. destructive). Additionally, when multiphase applications have smaller loads, current sensing can be used to reduce the number of phases required, thereby increasing circuit efficiency. For loads that require a current source, current sensing can convert the power source into a constant current source for applications such as LED driving, battery charging, and driving lasers.

For buck regulators, there are multiple locations for the current sense resistor. When placed on the high side of the top MOSFET (as shown in Figure 3), it senses the peak inductor current when the top MOSFET is on, allowing it to be used in 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 3. Buck converter with high-side RSENSE

In this configuration, the current sensing can be very noisy due to the strong switching voltage oscillation of the top MOSFET's conduction edge. To minimize this effect, a longer current comparator blanking time (the time the comparator ignores the input) is required. This limits the minimum switch on-time and may limit the minimum duty cycle (Duty Cycle = VOUT/VIN) and the maximum converter step-down ratio. Note that in high-end configurations, the current signal may be above a very large common-mode voltage (VIN).

In Figure 4, the sense resistor is located below the bottom MOSFET. In this configuration, it detects valley mode current. To further reduce power losses and save component costs, the bottom FET RDS(ON) can be used to sense current without using an external current-sense resistor RSENSE.

Figure 4. Buck converter with low-side RSENSE

This configuration is typically used for valley mode controlled power supplies. It may also be sensitive to noise, but in this case it is sensitive at large duty cycles. Valley mode controlled buck converters support high buck ratios but have a limited maximum duty cycle due to their fixed/controlled switch on-time.

In Figure 5, the current sensing resistor RSENSE is connected in series with the inductor, so the continuous inductor current can be detected. This current 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 5. RSENSE in series with inductor

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

For a boost regulator, a sense resistor can be placed in series with the inductor to provide high-side sensing (Figure 6).

Figure 6. 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.

The sense resistor can also be placed at the low side of the bottom MOSFET, as shown in Figure 7. Here the peak switch current (also the peak inductor current) is monitored, producing a current waveform every half cycle. MOSFET switching causes the current signal to have strong switching noise.

Figure 7. Boost converter with low-side RSENSE

Figure 8 shows a 4-switch buck-boost converter with the sense resistor on the low side. When the input voltage is much higher than the output voltage, the converter operates in buck mode; when the input voltage is much lower than the output voltage, the converter operates in boost mode. In this circuit, the sense resistor is located at the bottom of the 4-switch H-bridge configuration. The mode of the device (buck or boost) determines the current monitored.

Figure 8. Boost converter with low-side RSENSE

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 acts as a valley current mode buck converter.

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

The circuit in Figure 9 solves this problem by placing a sense resistor in series with the inductor, allowing continuous measurement of the inductor current signal in both buck and boost modes. Since the current sense RSENSE is connected to the SW1 node which has high switching noise, the controller IC needs to be carefully designed to allow a long enough blanking time for the internal current comparator.

Figure 9. LT8390 buck-boost converter, RSENSE in series with inductor

Additional sense resistors can also be added to the input to achieve input current limiting, or added to the output 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 sensing path to reduce current sensing noise.

As a current sensing element, the sense resistor produces the lowest sense error (typically between 1% and 5%) and has a very low temperature coefficient of approximately 100 ppm/°C (0.01%). In terms of performance, it provides the highest precision power supply, enabling extremely precise power supply current limiting and precision current sharing when multiple power supplies are connected in parallel.

Figure 10. RSENSE current sensing

On the other hand, because the current sensing resistor is added to the power supply design, the resistor will also cause additional power consumption. Therefore, sense resistor current monitoring technology may have higher power consumption compared to other sensing technologies, resulting in a decrease in 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 selecting a sense resistor is its parasitic inductance (also called effective series inductance or ESL). The sense resistor can be modeled correctly with a resistor in series with a finite inductor.

Figure 12. RSENSE ESL model

This inductance depends on the specific sense resistor selected. Certain types of current sense resistors, such as metal plate resistors, have lower ESL and should be used in preference. In contrast, wirewound sense resistors have higher ESL due to their package construction and should be avoided. Generally speaking, the ESL effect will become more obvious as the current increases, the detection signal amplitude decreases, and the layout is unreasonable. The total inductance of a circuit also includes parasitic inductance caused by component leads and other circuit elements. The total inductance of a circuit is also affected by layout, so component placement must be properly considered. Improper placement can affect stability and exacerbate existing circuit design problems.

The impact of sense resistor ESL can be mild or severe. ESL can cause significant oscillations in the switch gate driver, adversely affecting switch conduction. It also increases ripple in the current sense signal, causing a voltage step in the waveform instead of the expected sawtooth waveform as shown in Figure 13. This reduces current sensing accuracy.

Figure 13. RSENSE ESL may adversely affect current sensing

To minimize resistor ESL, avoid using sense resistors with long loops (such as wirewound resistors) or long leads (such as thick resistors). Low profile surface mount devices are preferred, examples include board construction SMD sizes 0805, 1206, 2010 and 2512, better options include inverted geometry SMD sizes 0612 and 1225.

Using MOSFET RDS(ON) for current sensing enables simple and cost-effective current sensing. The LTC3878 is a device that takes this approach. It uses a constant on-time valley mode current sensing architecture. The top switch is turned on for a fixed time, after which the bottom switch is turned on, and its RDS voltage drop is used to detect the current valley or current lower limit.

Figure 14. MOSFET RDS(ON) current sensing

Although inexpensive, this method has some disadvantages. First, its accuracy is not high and the RDS(ON) value may vary over a wide range (approximately 33% or more). Its temperature coefficient can also be very large, even exceeding 80% above 100°C. Additionally, if an external MOSFET is used, the MOSFET parasitic package inductance must be considered. This type of sensing is not recommended for very high current situations and is especially not suitable for multiphase circuits, which require good phase current sharing.

Inductor DC resistance current sensing uses the parasitic resistance of the inductor winding to measure the current, eliminating the need for a sense resistor. This reduces component costs and improves power efficiency. The device-to-device variation in the inductance DCR of copper windings is generally smaller compared to MOSFET RDS(ON), but it will still vary 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 across capacitor C1 (Figure 15).

Figure 15. Inductor DCR current detection

By selecting appropriate components (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 R1 value.

The circuit does not directly measure the inductor current and therefore cannot detect inductor saturation. It is recommended to use soft saturation inductors, such as powder core inductors. The core losses of this type of inductor are usually higher compared to equivalent iron core inductors. Compared with the RSENSE method, inductor DCR detection does not have the power loss of the detection resistor, but may increase the core loss of the inductor.

When using the two detection methods RSENSE and DCR, since the detection signal is small, Kelvin detection is required. It is important to keep the Kelvin detection traces (SENSE+ and SENSE- in Figure 5) away from high-noise copper areas and other signal traces to minimize noise extraction. Some devices, such as the LTC3855, feature temperature-compensated DCR sensing to improve accuracy over the entire temperature range.

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 the design requirements, tradeoffs in accuracy, efficiency, thermal stress, protection, and transient performance may all affect the selection process. Power supply designers need to carefully select the current sensing method and power inductor and correctly design the current sensing network. Computer software programs such as Analog Devices' LTpowerCAD design tool and LTspice® circuit simulation tool can go a long way in simplifying design work and obtaining optimal results.

There are other current sensing methods available. For example, current sensing transformers are often used with isolated power supplies to protect current signal information across the isolation barrier. This method is usually more expensive than the three techniques mentioned above. In addition, new power MOSFETs with integrated gate drivers (DrMOS) and current sensing have also appeared in recent years, but so far there is not enough data to infer how DrMOS performs in terms of accuracy and quality of detection signals.