An RC compensation method suitable for synchronous rectification technology

Abstract

The application is a compensation method of RC compensation circuit suitable for synchronous rectification technology, which adjusts the time constant of the compensation capacitor (Cc) and the compensation resistor (Rc) to be equal to the ratio of the parasitic inductance Ls of the printed circuit board and the synchronous rectification tube and the conduction resistance R DS of the synchronous rectification tube, so that the application adopts an RC compensation network to sample V DS , and effectively solves the early turn-off problem of the high-frequency synchronous rectification circuit by parameter matching and compensation of the parasitic inductance effect. The improved RC compensation with low loss, small size and simple circuit can effectively eliminate the early turn-off problem of the synchronous rectification tube and provide the possibility of improving the working frequency of the switching power supply to a greater extent.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of switching power supply design, and in particular to an RC compensation method suitable for synchronous rectification technology. BACKGROUND

[0002] Switching power supply is widely used in computer, communication, aerospace and other fields, which stimulates the development and research of high frequency, high efficiency and high power density power supply. In low voltage and large current application occasions, synchronous rectification technology (SR) is usually used to reduce the loss caused by high output current. Synchronous rectification technology uses a special MOSFET with low on-resistance to replace the Schottky barrier diode, which can effectively reduce the rectification loss and further improve the efficiency of the power supply system.

[0003] One of the obstacles to improve switching frequency is the premature turn-off of the synchronous rectification circuit. The synchronous rectification controller mainly realizes control by sampling the drain-source voltage V DS When V DS is less than the turn-on threshold, the synchronous rectification tube is turned on, and when V DS is greater than the turn-on threshold, the synchronous rectification tube is turned off. However, the actual measured V DS is actually the sum of the on-resistance voltage drop and the parasitic inductance voltage drop, so that the turn-off time of the synchronous rectification tube based on V DS is earlier than expected, causing additional conduction loss. The above process is expressed as:

[0004]

[0005] Where i s is the transformer secondary current, R DS is the on-resistance of the synchronous rectification tube, and Ls is the sum of the synchronous rectification tube package and the parasitic inductance of the printed circuit board. It can be seen that when the current i s drops, the parasitic inductance will generate a voltage drop opposite to the on-resistance, making V DS greater than the expected value, so that the actual V DS reaches the turn-on threshold in advance, causing the synchronous rectification tube to turn off, resulting in a large efficiency loss.

[0006] In order to achieve the exact turn-off of the synchronous rectification tube, there are currently three main solutions: (1) Add parameter matching RC compensation method. This method has high requirements for parameter accuracy, which is difficult to meet strictly in the whole range, and is prone to over-compensation and under-compensation problems. (2) Detect V DSThe peak judgment body diode of the peak judgment body diode is turned on, and the self-adapting adjustment is carried out. The method has the best effect, but the control is complex, and the original and auxiliary signals need to be transmitted, and the volume is large. SUMMARY

[0007] TECHNICAL PROBLEM: The purpose of the present application is to solve the problems mentioned in the above background art. Based on the consideration of high power density, an RC compensation method suitable for synchronous rectification technology is proposed. The improved RC compensation has low loss, small volume and simple circuit, can effectively eliminate the premature turn-off problem of the synchronous rectifier, and provides the possibility for higher working frequency of the switching power supply.

[0008] TECHNICAL SCHEME: The RC compensation circuit suitable for synchronous rectification technology comprises a synchronous rectifier controller circuit, a synchronous rectifier and an RC compensation circuit. The RC compensation circuit comprises a compensation capacitor, a compensation resistor, a clamping diode and a discharge diode.

[0009] The anode of the clamping diode and the discharge diode is connected, the cathode of the clamping diode is connected to the source end of the synchronous rectifier, and the cathode of the discharge diode is connected to the drain end of the synchronous rectifier. One end of the compensation capacitor is connected to the source end of the synchronous rectifier, the other end is connected to the compensation resistor, and the other end of the compensation resistor is connected to the drain end of the synchronous rectifier. The anode of the clamping diode and the discharge diode is connected to the connection point of the compensation capacitor and the compensation resistor. The positive and negative terminals of the clamping diode are the output terminals of the RC compensation circuit, and are connected to the input terminals of the synchronous rectifier controller circuit.

[0010] The time constant of the compensation capacitor and the compensation resistor is adjusted to be equal to the ratio of the parasitic inductance Ls of the printed circuit board and the synchronous rectifier and the conduction resistance R DS of the synchronous rectifier, that is, to make

[0011] The voltage across the compensation capacitor is the voltage across the conduction resistance R DS of the synchronous rectifier, and the voltage across the compensation capacitor is the voltage across the conduction resistance R DS of the synchronous rectifier. By collecting the voltage across the compensation capacitor, the voltage difference between the source and the flow of the rectifier can be obtained, so as to compensate the voltage drop caused by the parasitic inductance and eliminate the premature turn-off problem.

[0012] The clamping diode provides a fast discharge circuit for the compensation capacitor, and solves the delay turn-on problem caused by RC.

[0013] The discharge diode clamps the voltage of the compensation capacitor to the source end potential of the synchronous rectifier, and solves the risk of damage caused by excessive current of the discharge diode.

[0014] The synchronous rectifier controller circuit uses an operational amplifier U to form a basic single limit comparator, wherein the reference ground of the operational amplifier U is the source end of the synchronous rectifier, the positive input end is connected to the source end of the synchronous rectifier, and the negative input end is connected to the anode of the two diodes, that is, the voltage Vc across the compensation capacitor (Cc) is compared, when Vc is less than the threshold voltage, the operational amplifier outputs a high level, and the synchronous rectifier is turned on; otherwise, when Vc is greater than the threshold voltage, the synchronous rectifier is turned off, and the turn-off of the synchronous rectifier is controlled.

[0015] Advantages: the application has the following advantages by adopting the above technical scheme:

[0016] (1) When the synchronous rectification technology is used for a higher switching frequency, the early turn-off problem becomes more serious, and a large efficiency loss is caused. In view of the early turn-off problem, the application adopts an RC compensation network to sample V DS , through parameter matching, the influence of the parasitic inductance is compensated, and the early turn-off problem of the high-frequency synchronous rectification circuit is effectively solved.

[0017] (2) Because of the existence of the clamping diode, the discharge diode will not bear a large current at the moment when the synchronous rectifier is turned on. Therefore, the application has low requirements for the discharge diode, and the circuit cost is reduced.

[0018] (3) The circuit of the application is simple, and can be applied in various scenes; no additional floating ground driven switch tube is needed, and it is easy to implement in engineering; and the occupied volume is small, meeting the miniaturization requirement in the field of switching power supply. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The traditional synchronous rectification schematic diagram mentioned in the background art is shown.

[0020] Figure 2 The circuit diagram of the sampling circuit and the synchronous rectification controller of the application is shown.

[0021] Figure 3 The output simulation diagram of the traditional synchronous rectification technology, the existing RC compensation scheme and the application is shown, wherein, Figure 3 a represents the output simulation diagram of the traditional synchronous rectification, Figure 3 b represents the output simulation diagram of the existing RC compensation scheme, Figure 3 c represents the output simulation diagram of the application.

[0022] In the figure: compensation capacitor Cc, compensation resistor Rc, clamping diode D1, discharge diode D2, source end Source of the synchronous rectifier, and drain end Drain of the synchronous rectifier. DETAILED DESCRIPTION

[0023] An RC compensation circuit for synchronous rectification technology according to the present invention includes a synchronous rectifier controller circuit, a synchronous rectifier, and an RC compensation circuit. The RC compensation circuit includes a compensation capacitor Cc, a compensation resistor Rc, a clamping diode D1, and a discharge diode D2. The positive terminals of the clamping diode D1 and the discharge diode D2 are connected together. The negative terminal of the clamping diode D1 is connected to the source terminal of the synchronous rectifier, and the negative terminal of the discharge diode D2 is connected to the drain terminal of the synchronous rectifier. One end of the compensation capacitor Cc is connected to the source terminal of the synchronous rectifier, and the other end is connected to the compensation resistor Rc. The other end of the compensation resistor Rc is connected to the drain terminal of the synchronous rectifier. The positive terminals of the clamping diode D1 and the discharge diode D2 are connected to the connection point of the compensation capacitor Cc and the compensation resistor Rc. The positive and negative terminals of the clamping diode D1 are the output terminals of the RC compensation circuit and are connected to the input terminals of the synchronous rectifier controller circuit.

[0024] By adjusting the time constants of the compensation capacitor Cc and the compensation resistor Rc to make them equal to the parasitic inductance L of the PCB and the synchronous rectifier tube. s and synchronous rectifier diode on-resistance R DS Even if the ratio is obtained The voltage across the compensation capacitor Cc is equal to the on-resistance R of the synchronous rectifier diode. DS By measuring the voltage across the terminals, the voltage drop caused by parasitic inductance can be compensated, eliminating the problem of premature turn-off. In addition, the discharge diode D2 provides a fast discharge circuit for the compensation capacitor Cc, solving the problem of delayed turn-on caused by RC; the clamping diode D1 clamps the voltage of the compensation capacitor Cc, forcibly clamping it to the source potential of the synchronous rectifier, thus eliminating the risk of the discharge diode D2 being damaged by excessive current.

[0025] In addition to the sampling circuit, a synchronous rectification controller is also needed to control the turn-off of the synchronous rectifier. This invention uses only one operational amplifier U to construct a basic single-threshold comparator. The reference ground of op-amp U is the source terminal of the synchronous rectifier, the non-inverting input terminal is connected to the source terminal of the synchronous rectifier, and the negative input terminal is connected to the anode of two diodes, i.e., the voltage V across the compensation capacitor Cc. C Compare. When V C When the voltage is below the threshold voltage, the operational amplifier outputs a high level, turning on the synchronous rectifier diode; otherwise, V... C The synchronous rectifier is turned off when the voltage exceeds the threshold voltage, thus achieving switching control.

[0026] The invention will now be further explained with reference to the accompanying drawings.

[0027] like Figure 2The application is applicable to the RC compensation method of the synchronous rectification technology, and mainly solves the problem of the early turn-off in the traditional synchronous rectification control, and comprises a synchronous rectification controller, a synchronous rectification tube and a sampling circuit.

[0028] Firstly, the embodiment of the sampling circuit is introduced. When the synchronous rectification tube is turned off, almost no current flows, and at this time, the V DS obtained by sampling is a larger positive value. When the synchronous rectification tube is turned on, through the relationship between the compensation circuit and the equivalent circuit of the synchronous rectification power tube, the following complex frequency domain expression can be listed:

[0029]

[0030] Among them, i s is the transformer secondary side current, R DS is the on-resistance of the synchronous rectification tube, L s is the sum of the parasitic inductance of the synchronous rectification tube package and the printed circuit board, Cc is the compensation capacitor, Rc is the compensation resistor, and V c is the voltage across the compensation capacitor. According to the above formula, when , the above formula can be changed to:

[0031] i s R DS = V C

[0032] It can be known that i s R DS = V DS , so it can be concluded that the voltage across the compensation capacitor Cc is equal to the voltage of the on-resistance of the synchronous rectification power tube. Therefore, when sampling, the voltage across the compensation capacitor Cc is obtained, which can eliminate the influence of the parasitic inductance L s of the printed circuit board and the synchronous rectification tube package, measure the more accurate V DS , reduce the efficiency loss, and improve the switching power supply working frequency.

[0033] In order to facilitate subsequent control of the control circuit, two floating ground drive switch tubes are usually added to the RC drive circuit. One switch tube controls the discharge of the compensation capacitor Cc, and the other switch tube controls the connection of the compensation capacitor Cc and the compensation resistor Rc. In this way, the voltage across the compensation capacitor Cc sampled when the synchronous rectification tube is turned off is 0V, and the voltage across the compensation capacitor sampled when the synchronous rectification tube is turned on is equal to V DS , which is convenient for subsequent formation of a control signal. However, the control of the two floating ground drive switch tubes is difficult, and it is difficult to realize in engineering.

[0034] A current RC compensation circuit scheme is to connect a discharging diode in parallel with the compensation resistor. When the synchronous rectifier is turned on, a fast discharging loop is formed to solve the delay turn-on problem caused by RC. In this way, the problem of using floating ground to drive the switch tube is avoided. However, the discharging diode will bear a large current when the synchronous rectifier is turned on, so the requirement for the diode is high, which increases the manufacturing cost.

[0035] Based on the above problems, the diode is connected in parallel with the compensation capacitor, and the anode is connected to the source of the synchronous rectifier. In addition, the clamping diode D1 in the compensation circuit clamps the voltage of the compensation capacitor Cc to the source potential of the synchronous rectifier, which solves the problem of large current of the discharging diode and reduces the requirement for the diode.

[0036] Then the control circuit of the synchronous rectifier is introduced. A basic single-limit comparator composed of an operational amplifier can meet the demand. The reference ground of the operational amplifier U is the source of the synchronous rectifier, the positive input terminal is connected to the source of the synchronous rectifier, and the negative input terminal is connected to the anode of the two diodes, i.e. the voltage V C of the compensation capacitor Cc is compared with the threshold voltage. C When V C is less than the threshold voltage, the operational amplifier outputs a high level to turn on the synchronous rectifier; otherwise, the synchronous rectifier is turned off, thereby realizing switch control.

[0037] Figure 3 The output simulation diagrams of the traditional synchronous rectification technology, the current RC compensation scheme and the present application are shown in FIG. 1, wherein, Figure 3 a represents the output simulation diagram of the traditional synchronous rectification, and it can be seen that when there is no RC compensation, the synchronous rectifier will be turned off in advance, which makes the body diode of the synchronous rectifier also conduct, resulting in a large efficiency loss; Figure 3 b represents the output simulation diagram of the current RC compensation scheme, and after adding the RC compensation, the problem of turning off in advance is effectively solved, but the discharging diode D2 will bear a large discharging current when the synchronous rectifier diode is turned on, which may damage the diode and affect the normal operation of the entire circuit; Figure 3 c represents the output simulation diagram of the present application. It can be seen that when the RC compensation circuit of the present application is applied to the switching power supply, the problem of turning off in advance is greatly eliminated, a good synchronous rectification effect is achieved, and the discharging current of the discharging diode D2 is significantly reduced, which can ensure the normal operation. The present application can be used in the synchronous rectification control of current high-frequency power converters, and can also be used in the synchronous rectification control of future higher frequency.

[0038] The preferred embodiment of the application has been described in detail. It should be appreciated that those skilled in the art can make many modifications and variations without departing from the spirit of the application. The scope of the application should therefore be determined not with reference to the above description but instead with reference to the appended claims along with their full scope of equivalents.

Claims

1. A compensation method for an RC compensation circuit suitable for synchronous rectification technology, characterized by, A synchronous rectifier controller circuit, a synchronous rectifier and an RC compensation circuit are included. A synchronous rectifier controller circuit, a synchronous rectifier and an RC compensation circuit are included. Wherein RC compensation circuit includes: compensation capacitor C c , compensation resistance R c , clamping diode D1 and discharge diode D2, wherein; the positive of the clamping diode D1 and discharge diode D2 is connected, the negative of clamping diode D1 is connected with the source end Source of synchronous rectifier tube, the negative of discharge diode D2 is connected with the drain end Drain of synchronous rectifier tube;The one end of the compensation capacitor C c is connected with the source end Source of synchronous rectifier tube, the other end is connected with compensation resistance R c , the other end of compensation resistance R c is connected with the drain end Drain of synchronous rectifier tube;The positive of clamping diode D1 and discharge diode D2 is connected with the connection point of compensation capacitor C c compensation resistance R c ; The positive and negative of clamping diode D1 is the output end of RC compensation circuit, which is connected with the input end of synchronous rectifier controller circuit. adjusting the time constant of the compensation capacitor C c and the compensation resistor R c to be equal to the ratio of the parasitic inductance L s of the printed circuit board and the synchronous rectifier tube and the on-resistance R DS of the synchronous rectifier tube, i.e. so that R C C C = L S / R DS , the synchronous rectifier controller circuit is formed by using an operational amplifier U as a basic single limit comparator, wherein the reference ground of the operational amplifier U is the source terminal of the synchronous rectifier tube, the positive input terminal is connected to the source terminal of the synchronous rectifier tube, and the negative input terminal is connected to the anode of two diodes, i.e. the voltage Vc across the compensation capacitor C c is compared, when Vc is less than the threshold voltage, the operational amplifier outputs high level, and the synchronous rectifier tube is turned on; otherwise, when Vc is greater than the threshold voltage, the synchronous rectifier tube is turned off, and the turn-off of the synchronous rectifier tube is controlled.

2. The compensation method for RC compensation circuit suitable for synchronous rectification technology according to claim 1, characterized in that, The compensation capacitor C c The voltage across the two ends of the synchronous rectifier tube is the conduction resistance R DS The voltage across the two ends, collecting the voltage across the two ends can be equivalent to the voltage difference between the source and the flow of the rectifier tube, thereby compensating for the voltage drop caused by the parasitic inductance and eliminating the premature shutdown problem.

3. The compensation method for RC compensation circuit suitable for synchronous rectification technology according to claim 1, characterized in that, The clamping diode D1 is a compensation capacitor C c A fast discharge circuit is provided to solve the problem of delay opening caused by RC.

4. The compensation method for RC compensation circuit suitable for synchronous rectification technology according to claim 1, characterized in that, The discharge diode D2 compensates for the capacitor C c The voltage is clamped to the source potential of the synchronous rectifier, which solves the risk of damage to the discharge diode D2 due to excessive current.

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