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 ADI 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.
Where is the best place to put the detection resistor?
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 3), it senses the peak inductor current when the top MOSFET is on, allowing it to be used for peak current mode controlled power supplies. 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 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 4, 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 4. 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 5, 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 5. 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 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.
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. 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 7. 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 8 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 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 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 9 solves this problem by placing a sense resistor in series with the inductor, which allows 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 9. LT8390 buck-boost converter with RSENSE in series with the 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, 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.
Current detection method instruction manual
There are three common current sensing methods for switch mode power supplies: using a sense resistor, using the MOSFET RDS(ON), and using the DC resistance (DCR) of the inductor. Each method has advantages and disadvantages that should be considered when selecting a sensing method.
Sense Resistor Current Sensing
The sense resistor as the current sensing element produces the lowest sensing error (typically between 1% and 5%) and has a very low temperature coefficient of about 100 ppm/°C (0.01%). In terms of performance, it provides the highest accuracy power supply, facilitates very accurate power supply current limiting, and facilitates precise 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 generate additional power dissipation. Therefore, compared with other detection technologies, the detection resistor current monitoring technology may have higher power consumption, resulting in a decrease in the overall efficiency of the solution. Dedicated current sensing resistors may also increase the solution cost, although the cost of a detection resistor is usually 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 correctly modeled as 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 structure and should be avoided. In general, ESL effects become more pronounced with increasing current, decreasing sense signal amplitude, and improper layout. The total inductance of the circuit also includes parasitic inductance caused by component leads and other circuit elements. The total inductance of the circuit is also affected by the layout, so the layout of the components must be properly considered. Improper layout may affect stability and exacerbate existing circuit design problems.
The effects of sense resistor ESL can be mild or severe. ESL can cause significant ringing in the switch gate driver, which can adversely affect switch turn-on. It can also increase the ripple of the current sense signal, resulting in voltage steps in the waveform instead of the expected sawtooth waveform as shown in Figure 13. This can degrade current sensing accuracy.
Figure 13. RSENSE ESL can 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 structure SMD sizes 0805, 1206, 2010, and 2512, and better choices include reverse geometry SMD sizes 0612 and 1225.
Current sensing based on power MOSFET
Using MOSFET RDS(ON) for current sensing allows for simple and cost-effective current sensing. The LTC3878 is a device that uses 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 floor.
Figure 14. MOSFET RDS(ON) current sensing
Although inexpensive, this method has some disadvantages. First, it is not very accurate and the RDS(ON) value can vary over a wide range (approximately 33% or more). Its temperature coefficient can also be very large, exceeding 80% above 100°C. In addition, if an external MOSFET is used, the MOSFET parasitic package inductance must be considered. This type of sensing is not recommended for very high currents, especially for multiphase 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 the current, eliminating the need for a sense resistor. This reduces component cost and improves power supply efficiency. The inductor DCR of the copper winding generally has less device-to-device variation than the MOSFET RDS(ON), 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 the inductor and parasitic resistance, and the sense voltage is measured across capacitor C1 (Figure 15).
Figure 15. Inductor DCR current sensing
By choosing the proper 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 low R1 value.
The circuit does not measure the inductor current directly, so it cannot detect inductor saturation. It is recommended to use an inductor with soft saturation, such as a powder core inductor. Such inductors generally have higher core losses than equivalent iron core inductors. Compared to the R SENSE method, inductor DCR sensing does not have the power loss of the sense resistor, but may increase the inductor core loss.
When using both R SENSE and DCR sensing methods, Kelvin sensing is required due to the small sense signal. It is important to keep the Kelvin sense traces (SENSE and SENSE- in Figure 5) away from noisy copper areas and other signal traces to minimize noise pickup. Some devices, such as the LTC3855, have temperature compensated DCR sensing to improve accuracy over the entire temperature range.
Table 1. Advantages and disadvantages of current sensing methods
Each of the methods mentioned in Table 1 provides additional protection for switch-mode power supplies. Depending on the design requirements, trade-offs in accuracy, efficiency, thermal stress, protection, and transient performance may affect the selection process. The power supply designer needs to carefully select the current sensing method and power inductor and properly design the current sensing network. Computer software programs such as ADI's LTpowerCAD design tool and LTspice circuit simulation tool can go a long way in simplifying the design work and achieving the best results.
Other Current Sensing Methods
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 technologies 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 the detection signal.
Source: New Energy BMS, author Hu Yaoshan
Disclaimer: This article is authorized to be reproduced from the "New Energy BMS" public account. Reprinting is for learning reference only and does not mean that this account agrees with its views. This account does not assume any infringement responsibility for its content, text, and pictures.
提升卡
变色卡
千斤顶
京公网安备 11010802033920号
