同步整流变换器电流检测方法的研究

The Study of A Synchronous Rectifier Converter Current Detection MethodLiu ShulinSchool of Electric and Control Engineering Xi’an University of Science and TechnologyXi’an, Chinalsigma@Lu Xiaoyue,YangboSchool of Electric and Control Engineering Xi’an University of Science and TechnologyXi’an, Chinagogobenxiao@Abstract—For the traditional current detection method of a synchronous rectifier converter is easy to make the additional power loss and low detection accuracy such problems, it proposes a new current detection method, which detects the current using the conduction voltage drop of the synchronous rectifier, by introducing a negative feedback loop to force the voltage equal, which across the operational transconductance amplifier, adjusting the resistance value connected to both ends of the operational transconductance amplifier to increase the accuracy of current detection. This method is no additional power relative to the entire system, which overcomes the conventional problems in the current detection. Simulation result shows that the lossless current sensing method is feasible, and can be achieved.Keywords- synchronous rectifier; detection accuracy; power loss; conduction voltage drop; lossless current sensingI.I NTRODUCTIONAs the technology advances, DC-DC switching power supply towards high reliability, high stability, low noise, interference and modular direction to develop[1]. A variety of portable products keep high current, low power and low voltage. They are powerful, energy-saving, simple and efficient circuit which is easy to use, so such power management chip is getting more and more popular[2]. Current detection module is the most important module of realizing switching power function. It can open real-time detection of the load, short-circuit and over current condition, which is an important part of the current loop. Largely determine the chip's power consumption and efficiency[3]. Current detection in the current mode control must accurately detect different load plus slope compensation signal as a feedback control signal to control power management work. Therefore it is necessary to study synchronous rectifier converter current detection methods[4].The most commonly used current detection technique is to use a current proportional to the characteristics of parallel MOSFET. Width to length ratio of SENSEFET and MOSFET is 1/n. Current flew through SENSEFET reduced to 1/n proportion to the power tube. A resistor was connected to the end of the SENSEFET, then design a basic current detection circuit to detect the current flowing through SENSEFET which can detect the voltage across the resistor, multiplied by n is the current flowing through MOSFET. But this method has the sense resistor, more power loss, and it is difficult to control[6,9]. Another method of detecting switch conduction voltage drop is to make switch as a resistor, which is a non-destructive testing. But the resistance will change with temperature, and detection accuracy is not high enough[8,10]. Thus the study of new current detection method and technology has become an important switching power supply research direction present.It has been proved that the current detection needing current detecting elements and easy to generate additional power in this paper. Detecting conduction voltage drop of continued flow tube which is the circuit device itself will be no additional loss. Use a transistor and an operational transconductance amplifier to make a negative feedback circuit, then combine current mirror, leading to the detection current. And by adjusting the resistance connected to both ends of the operational transconductance amplifier, achieving the current non-destructive testing, to provide a solution on static power reduction of switching power supply.II.W IDELY U SED C URRENT D ETECTION M ENTHODSN OWADAYSRecently this conventional current is widely used to test current，the circuit is shown in Figure 1.Figure 1. Widely used current detection methodM1 is the power MOSFET, M3 is the SENSEFET, SENSEFET is in parallel with MOSFET. Transistor’s source is connected together, operation amp is used to ensure that two of the drain voltage is equal, avoiding the current mirror is not accurate caused by FET channel length modulation effect, the current Flowing SENSEFET and MOSFET will follow the two tubes of the distribution width to length ratio 1/n. To ensure the extra power loss of SENSEFET is small enough, we should tryto make the MOSFET's effective width to length ratio is 100 times more than SENSEFET.A resistor is connected to the end of SENSEFET, design a basic current detection circuit which can detect the voltage across the resistor, then it can detect the current flowing through SENSEFET, multiplied by n is the current flowing through the MOSFET. In this detection circuit detects a high precision, but a small resistance will bring additional power loss, affecting the size of switching power supply power. Between the power tubes and sampling tubes even a small coupling can also cause a big error, sampled signal will be added into a large peak signal, this technique can be used only when the power management is integrated into the chip or in particular match with the MOSFET.III. C IRCUIT A NALYSIS OF C URRENT D ETECTION Lossless current sensing circuit is a new circuit in the improvements on detecting the voltage drop of power MOSFET in this paper. Use original devices in the power circuit, that is detecting current through detect the voltage drop of continued flow tube to achieve lossless testing. Because VT will change with the temperature, RDS will also change. Adjust the resistance connected to both ends of the operational transconductance amplifier according to changes can reduce the error and improve the accuracy.A.Lossless current sensing circuit work principlesDetect current using voltage drop of the original device in the power loop circuit, it will have no additional power relative to the entire system [5]. The basic principle circuit figure is shown in Figure 2, by introducing a negative feedback loop forces the voltage across the operational transconductanceamplifier equivalent.Figure 2. Current detection basic schematicWhen the voltage of left resistor R1 rises, that is thenegative voltage of operational transconductance amplifier increases. The output of operational transconductance amplifier is low. Through a transistor, output is still low. Therefore, it is a negative feedback. According to the principle that the voltage drop of a circuit loop is zero, the sum voltage drop on R1, R2 and Rs is zero. It can be sampled out current IX and the relationship between IL. The relationship between IX and IL is decided by the ratio of R1 and R2. Current IX flowing through transistor can be reduced to Rs /R1 of IL. Then the current sampling can be obtained using a current mirror to make it output. IL will be detected by IOUT in this way. Detection accuracy use series resistors at both ends of the sense amplifier to optimize the adjustment. VT will be larger while thetemperature grows, RDS will become larger too. Make R1 and R2 larger appropriate can improve the detection accuracy. B.Circuit design of lossless current senseing amplifier According to the work principle of lossless current sensing circuit, it can be seen that the voltage drop of loop circuit made up of operational transconductance amplifier, R 1, R 2 and Rs from Figure 2, this results. 11122X L s I R I R I R I R ×+×−×=× (1)Because 1I =2I , and make 1R =2R , Then we can get1R R I I SL X ×=(2) Detected current is proportional to the size of the inductor current through equation (2), by adjusting the size of R 1 can improve the detection accuracy I L , according to the continued flow tube open and operate in the linear zone. Assuming the drain voltage and source voltage are small, continued flow tube can be used as a switch, which can be seen as a resistor R S , The principles is the same as the switch tube. The resistor R S is as follows()S OX GS T LR W C V V μ=− (3)Among, μ is the mobility of channel charge, Cox is the Unit Gate oxide capacitance, VT is the threshold voltage. W is the conductive channel effective width of the power tube, L is conductive channel effective length of the power tube. Gate voltage can be measured, so V GS can be known. The value can be calculated into the value of R S . But with the temperature increases, V T gradually increases, R S has gradually become larger. It can be appropriate to transfer a little large number of R 1 to achieve the target of reduced errors and improved accuracy. R 1 can be made into trimming resistor within the integrated circuit. It is easy adjustment according to the results of measurements. Then make changes and adjust the size of R1. This measurement of I L will be more accurate. We know that the inductor current is equal to the ratio of U DS and R S . It is as followsS DSL R U I =(4)R S can be obtained according to equation (3), because the source of continued flow tube is grounded, that the voltage of point B is zero. Now U DS can be obtained by measuring the voltage of point A, which is the drain of continued flow tube. Thus I L can be obtained according to equation (4).Figure 3 shows the internal circuit of Figure 2. The left part is operational transconductance amplifier, the outputs through RC and fed back to ISP2_R, then pass the transistor, forcing the input voltage of operational transconductance amplifier equal. The circuit is as follows.Figure 3. Circuit detection circuitIV. L OSSLESS C URRENT S ENSING C IRCUIT S IMULATION Use a current source instead of the synchronous rectifier in Figure 2 to simulate the output current when in simulation. The transient simulation waveforms are shown in Figure 4. The above figure is the simulated load current I out , while the below chart is the current which has been detected. It can be seen that the changes of current was successfully detected from Figure 3 in the case of 1MHz, which illustrate the detection methodfeasible.Figure 4. Figure 4. Simulation waveform of transient current detectionIt can be seen that in our design circuit I out and I sense have the consistent changes of transient current detection from the simulation waveforms, I out detected out the changes of I sense . Simulation results validate the accuracy and feasibility of the circuit design. However, if R S is too small, when R S equals 0.5Ω, circuit may not track the current change, as it is shown in figure 5. The above figure is the simulated load current, themiddle figure is the current detected out. You can see the current change has not come out of the test. To obtain detection accuracy without changing R S according to equation (3) to adjust the sense amplifier connected to both ends of the resistors R 1 and R 2, eventually get an accurate current sensing.Figure 5. Changes failure detection currentV. C ONCLUSIONThis paper presents a new lossless current sensing technology, which used the voltage drop of original device in power circuit to detect current, that is to get the test current through test voltage drop of the continued flow tube, and take out the current using current mirror. Resistances connected to both ends of the operational transconductance amplifier were adjustment to improve the detection accuracy, and to achieve lossless testing, which overcome the traditional current detection needing circuit detection elements and easy to make additional power. This method is feasible through simulation, and can eliminate the disadvantage that lack of precision caused by temperature changes. The circuit is clear, the structure is simple, which has highly practical and economic value.R EFERENCES[1] Liushu Lin, Liujian. Analysis and design of switching converter[M]Machinery Industry Press,2010[2] Zhang Huaien. 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