How is supply current detected in switch mode?These 12 circuits & 10 knowledge points are explained

Current detection technology is widely used in today’s life and work. In many systems, it is necessary to detect the inflow and outflow current. Detecting the current can avoid device errors. So our protagonist today is “current detection technology of switch mode power supply”.

Current detection technology is widely used in today’s life and work. In many systems, it is necessary to detect the inflow and outflow current. Detecting the current can avoid device errors. So our protagonist today is “current detection technology of switch mode power supply”.

basic knowledge

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 design of a current-mode switch-mode power supply, which regulates the output and provides overcurrent protection. Figure 1 shows the current sense circuit for the ADI LTC3855 synchronous switch-mode step-down 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. Switch-Mode Supply Current Sense Resistor (RS)

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

Figure 2. LTC3855 Current Limit 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 (represented by a “1” in the diagram). When the circuit is shorted, 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 circuits (represented by a “2” in the diagram). Then, the built-in current foldback limit (number “3” in the figure) further reduces the inductor current to minimize thermal stress.

Current sensing also serves other purposes. Use it to achieve accurate current sharing in multiphase power supply designs. For light-load power supply designs, it prevents reverse current flow, thereby improving efficiency (reverse current refers to the current flowing in the reverse direction through the inductor, i.e. from the output to the input, which may be undesirable in some applications, or even destructive). Additionally, current sensing can be used to reduce the number of phases required when the load for multiphase applications is small, thereby increasing circuit efficiency. For loads that require a current source, current sensing converts the power source into a constant current source for applications such as LED driving, battery charging, and driving lasers.

Where is the best place to put a sense resistor?

The location of the current sense resistor along with the switching regulator architecture determines the current to be sensed. The sensed currents include peak inductor current, valley inductor current (the minimum value of inductor current in continuous conduction mode), and 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 circuit.

placed on the high side of the buck regulator

For a buck regulator, there are several places for the current sense resistor to be placed. 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, which can 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 3. Buck Converter with High-Side RSENSE

In this configuration, the current sensing can be very noisy due to strong switching voltage oscillations at the turn-on edge of the top MOSFET. 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 possibly 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 may be above a very large common-mode voltage (VIN).

placed on the low side of the buck regulator

In Figure 4, the sense resistor is below the bottom MOSFET. In this configuration, it senses 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 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 to a large duty cycle. Valley-mode controlled buck converters support high step-down ratios, but have a limited maximum duty cycle due to their fixed/controlled switch on-times.

Buck regulator in series with inductor

In Figure 5, the current-sense resistor RSENSE is in series with the inductor, so it can sense the continuous inductor current, 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 5. RSENSE in series with inductor

This detection method provides the best signal-to-noise performance. An external RSENSE typically provides a very accurate current sense signal for accurate current limiting and sharing. However, RSENSE also incurs additional power losses and component costs. To reduce power loss and cost, the inductor coil DC resistance (DCR) can be used to sense current instead of an external RSENSE.

placed on the high side of the 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 6).

Figure 6. Boost Converter with High-Side RSENSE

The boost converter has a continuous input current, so it generates a triangular waveform and continuously monitors the current.

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 7. The peak switch current (and also the peak inductor current) is monitored here, 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

SENSE resistor placed at the low side of the buck-boost converter or in series with the inductor

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 works in buck mode; when the input voltage is much lower than the output voltage, the converter works in boost mode. In this circuit, the sense resistor is 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), a sense resistor is placed in series with the bottom MOSFET (C) and the peak current is measured 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 lightly loaded. Negative inductor current means that power is transferred from the output back to the input, but since there are losses in this transfer, efficiency suffers. For applications such as battery-powered systems, where light-load efficiency is important, this method of current sensing is undesirable.

The circuit of Figure 9 solves this problem by placing a sense resistor in series with the inductor to continuously measure the inductor current signal 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 so that the internal current comparator has a long enough blanking time.

Figure 9. LT8390 Buck-Boost Converter with RSENSE in Series with Inductor

Additional sense resistors can also be added at the input for input current limiting, or at the output for constant output current applications such as battery charging or driving LEDs. In this case, the input or output current signal needs to be averaged, so a strong RC filter can be added in the current sense path to reduce current sense noise.

Instruction Manual for Current Detection Method

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 Sensing

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 10. RSENSE Current Sense

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 12. 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 13. This reduces the current detection accuracy.

Figure 13. RSENSE ESL may 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 14. 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 inductor and parasitic resistance, and the sense voltage is measured on capacitor C1 (Figure 15).

Figure 15. 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. 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 ADI’s LTpowerCAD design tool and LTspice® circuit simulation tool, go a long way in 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.

The Links:   2MBI100U4H-170-50 DG12864-75