Feed-forward circuit with adjustable gain

Abstract

The utility model discloses a feed-forward circuit with adjustable gain, which comprises a sampling unit, an operational amplifier unit, a sampling filter unit and a sampling proportion adjusting unit, the sampling proportion adjusting unit is used for changing the operational amplifier proportion by increasing capacitance and resistance through switching of an analog switch, the input end of the sampling unit receives load current, and the output end of the sampling unit is connected with the operational amplifier unit. The operational amplifier unit is further connected with the sampling filtering unit and the sampling proportion adjusting unit. The utility model has the advantage that the stability and the precision of the system under different load conditions are ensured.

Description

Technical Field

[0001] The utility model relates to the field of load current feedforward control, in particular to a gain-adjustable feedforward circuit. Background Art

[0002] In the prior art, the voltage-current dual-loop control, which is the most widely used in AC power supply control, is equivalent to a non-ideal voltage source with internal impedance. The output voltage fluctuates with the change of load current, and the output external characteristics of the inverter are poor. To suppress the fluctuation of the inverter output voltage, the most direct method is to increase the loop gain of the inverter control system, but the control performance and system stability are mutually constrained, resulting in unsatisfactory control effect. In order to avoid the system instability problem that may be caused by increasing the control gain, using a current sensor to collect load current information for feedforward compensation becomes a simple and easy method. In actual engineering, proportional feedforward compensation is often used. This method weakens the influence of low-frequency current on the output voltage, but the suppression effect on medium and high frequency harmonic current is not obvious. The proportional differential feedforward method can theoretically suppress the influence of load current in the entire frequency domain, but the additional impedance characteristics introduced by the feedforward link must completely fit the original inverter output impedance characteristics to eliminate the influence of load current on the output voltage.

[0003] Therefore, the difficulty of the feedforward compensation method of the prior art is that the high-frequency harmonic current suppression effect is not obvious. When the load current changes, the ripple change introduced into the control loop interferes with the control system, making it difficult to ensure the stability and accuracy of the system. Utility Model Content

[0004] The technical problem to be solved by the utility model is that the feedforward compensation method of the prior art is difficult to ensure the stability and accuracy of the system.

[0005] The utility model solves the above technical problems through the following technical means: a gain-adjustable feedforward circuit, comprising a sampling unit, an operational amplifier unit, a sampling filter unit, and a sampling ratio adjustment unit that changes the operational amplifier ratio by increasing capacitance and resistance through the engagement of an analog switch, wherein the sampling unit input receives a load current, the sampling unit output is connected to the operational amplifier unit, and the operational amplifier unit is also respectively connected to the sampling filter unit and the sampling ratio adjustment unit.

[0006] Further, the sampling unit is a resistor RSENSE, one end of the resistor RSENSE receives the load current and is connected to the operational amplifier unit, and the other end of the resistor RSENSE is grounded and connected to the operational amplifier unit.

[0007] Furthermore, the operational amplifier unit is an operational amplifier U1A, the non-inverting input terminal of the operational amplifier U1A is connected to the other end of the resistor RSENSE, the inverting input terminal of the operational amplifier U1A is connected to one end of the resistor RSENSE, and a sampling filtering unit and a sampling ratio adjustment unit are connected between the inverting input terminal and the output terminal of the operational amplifier U1A and between the non-inverting input terminal and the ground terminal.

[0008] Furthermore, the sampling and filtering unit includes capacitors C1 to C4, and the sampling ratio adjustment unit includes resistors R1 to R4, analog switches U2A, U2B, U2C and U2D. One end of the resistor R2 and one end of the capacitor C2 are both connected to the inverting input terminal of the operational amplifier U1A, the resistor R2 and the capacitor C2 are respectively connected between the inverting input terminal and the output terminal of the operational amplifier U1A, the resistor R4 and the analog switch U2A are connected in series between the inverting input terminal and the output terminal of the operational amplifier U1A, the capacitor C4 and the analog switch U2B are connected in series between the inverting input terminal and the output terminal of the operational amplifier U1A, the capacitor C1 and the resistor R1 are respectively connected between the non-inverting input terminal and the ground terminal of the operational amplifier U1A, the capacitor C3 and the analog switch U2C are connected in series between the non-inverting input terminal and the ground terminal of the operational amplifier U1A, and the resistor R3 and the analog switch U2D are connected in series between the non-inverting input terminal and the ground terminal of the operational amplifier U1A.

[0009] Furthermore, the models of the analog switch U2A, the analog switch U2B, the analog switch U2C and the analog switch U2D are all ADG1412.

[0010] The advantages of the utility model are:

[0011] The sampling ratio adjustment unit of the utility model changes the operational amplifier ratio by increasing capacitance and resistance through the engagement of analog switches, so that the proportional coefficient can be adjusted by switching the analog switches under different load conditions, so that the output results are roughly the same, thereby achieving the stability and accuracy of the system under different load conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is an overall control block diagram of the AC power supply disclosed in the embodiment of the utility model;

[0013] Figure 2 The schematic diagram of the unipolar frequency-doubling inverter circuit with load current feedforward disclosed in the embodiment of the utility model;

[0014] Figure 3 The waveform of the 10Ω load current Io disclosed in the embodiment of the utility model;

[0015] Figure 4The output voltage and bridge arm midpoint waveform disclosed in the embodiment of the utility model;

[0016] Figure 5 The waveform of the 100Ω load current Io disclosed in the embodiment of the utility model;

[0017] Figure 6 This is a schematic diagram of FFT analysis of the 10Ω load output voltage disclosed in the embodiment of the utility model;

[0018] Figure 7 This is a schematic diagram of FFT analysis of the 100Ω load output voltage disclosed in the embodiment of the utility model;

[0019] Figure 8 This is a schematic diagram of a gain-adjustable feedforward circuit disclosed in an embodiment of the utility model. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the embodiments of the utility model clearer, the technical solution in the embodiments of the utility model will be clearly and completely described below in combination with the embodiments of the utility model. Obviously, the described embodiments are part of the embodiments of the utility model, not all of the embodiments. Based on the embodiments of the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

[0021] This embodiment provides a gain-adjustable feedforward circuit, which is mainly used to perform adjustable filtering on the load current collected by the current sensor, and is controlled by the filtering and feedback coefficient gear selection circuit. The feedback amount of the feedforward in the control loop is changed by different current feedback ratios. In order to clearly introduce the overall circuit principle and process, the relevant contents of AC power supply control are briefly described below. The overall circuit uses instantaneous voltage as the outer loop, introduces load current feedforward after PI adjustment, and then generates the drive control signal of the inverter circuit through the inductor current inner loop. The overall circuit control block diagram is shown as follows: Figure 1 As shown, it expresses the transfer function relationship of the entire circuit. Figure 2It is a schematic diagram of a unipolar frequency-multiplied inverter circuit with load current feedforward. The inverter modulation method adopts unipolar frequency-multiplied SPWM technology. The power circuit adopts a bridge inverter structure, and a Hall sensor is used to collect the inductor current and output current. After the inverter output voltage Vo is adjusted by error amplification control through the sampling circuit and the set reference Vref, the load current Io is superimposed on the feedforward, and the output result is then put into the loop adjustment with the inductor current inner loop. The output modulation wave is compared with the fixed carrier (Vc1, Vc) to obtain the drive waveform T1 to T4, and the inverter circuit MOS tube is controlled. The generated output is passed through the filter circuit to obtain the final AC output. Among them, the gain-adjustable feedforward circuit of the utility model is used in the process of load current Io feedforward. The amplification ratio of the operational amplifier is adjusted through this circuit, so that a waveform with the same voltage amplitude and stable and reliable can be output under different load conditions.

[0022] The output voltage is 220Vrms AC voltage. When a 10Ω load is added, the output load current is as follows: Figure 3 As shown, a current sensor is used to sample the load current and the result is brought into the feedback loop. The output voltage at this time is similar to the waveform at the midpoint of the bridge arm as shown in Figure 4 As shown in Figure 2, FFT analysis of the output voltage shows that the fundamental THD is 0.44%. Figure 5 When using a 100Ω load, the output load current is as follows. Figure 6 As shown in Figure 2, FFT analysis shows that the THD of the fundamental wave is 0.52%. Figure 7 As shown in the figure, under two different load conditions, the amount of feedforward current introduced is also different. By adjusting the filter coefficient and ratio, the output voltage waveform that can be obtained remains almost unchanged.

[0023] The specific structure and principle of the gain-adjustable feedforward circuit provided by the utility model are introduced as follows: Figure 8 As shown, the gain-adjustable feedforward circuit includes a sampling unit, an operational amplifier unit, a sampling filter unit, and a sampling ratio adjustment unit that changes the operational amplifier ratio by increasing capacitance and resistance by switching on an analog switch. The input end of the sampling unit receives a load current, the output end of the sampling unit is connected to the operational amplifier unit, and the operational amplifier unit is also connected to the sampling filter unit and the sampling ratio adjustment unit, respectively.

[0024] The sampling unit is a resistor RSENSE, one end of which receives the load current Io, and the other end of which is grounded. The op amp unit is an operational amplifier U1A, the in-phase input of which is connected to the other end of the resistor RSENSE, the inverting input of which is connected to one end of the resistor RSENSE, and the sampling filter unit and the sampling ratio adjustment unit are connected between the inverting input of the operational amplifier U1A and its output, and between the in-phase input and the ground. The sampling filter unit includes capacitors C1 to C4, the sampling ratio adjustment unit includes resistors R1 to R4, an analog switch U2A, an analog switch U2B, an analog switch U2C and an analog switch U2D, the models of the analog switches U2A, U2B, U2C and U2D are all ADG1412, and the analog switches U2A, U2B, U2C and U2D can be four different switch switching channels of the same switch of model ADG1412. One end of the resistor R2 and one end of the capacitor C2 are both connected to the inverting input terminal of the operational amplifier U1A, the resistor R2 and the capacitor C2 are respectively connected between the inverting input terminal and the output terminal of the operational amplifier U1A, the resistor R4 and the analog switch U2A are connected in series between the inverting input terminal and the output terminal of the operational amplifier U1A, the capacitor C4 and the analog switch U2B are connected in series between the inverting input terminal and the output terminal of the operational amplifier U1A, the capacitor C1 and the resistor R1 are respectively connected between the non-inverting input terminal and the ground terminal of the operational amplifier U1A, the capacitor C3 and the analog switch U2C are connected in series between the non-inverting input terminal and the ground terminal of the operational amplifier U1A, and the resistor R3 and the analog switch U2D are connected in series between the non-inverting input terminal and the ground terminal of the operational amplifier U1A. Among them, resistors R1 to R4 are sampling ratio resistors, capacitors C1 to C4 are sampling filter capacitors, and each analog switch controls the ratio of the peripheral capacitors and resistors of the sampling circuit. After the resistor RSENSE samples the feedforward current, it is input to the operational amplifier U1A. The amplification ratio of the operational amplifier U1A is mainly achieved by the analog switch U2A, analog switch U2B, analog switch U2C and analog switch U2D, so that the ratio of the connected resistors and capacitors is different, thereby different amplification ratios. After the amplification ratio is adjusted, the feedforward current Io is adjusted and output at the corresponding amplification ratio, and the output result is added as the feedforward control. Figure 2 The loop shown.

[0025] Through the above technical scheme, the utility model has few input signals and a simple structure. It only increases capacitance and resistance by adding analog switches and controls the ratio of the peripheral capacitance and resistance of the gain-adjustable feedforward circuit to change the amplifier ratio, thereby achieving the stability and accuracy of the system under different load conditions.

[0026] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A gain-adjustable feedforward circuit, characterized in that: The invention comprises a sampling unit, an operational amplifier unit, a sampling filter unit and a sampling ratio adjustment unit which changes the operational amplifier ratio by increasing capacitance and resistance by switching on an analog switch. The input end of the sampling unit receives a load current, the output end of the sampling unit is connected to the operational amplifier unit, and the operational amplifier unit is also connected to the sampling filter unit and the sampling ratio adjustment unit respectively.

2. A gain-adjustable feedforward circuit according to claim 1, characterized in that: The sampling unit is a resistor RSENSE, one end of the resistor RSENSE receives the load current and is connected to the operational amplifier unit, and the other end of the resistor RSENSE is grounded and connected to the operational amplifier unit.

3. A gain-adjustable feedforward circuit according to claim 2, characterized in that: The operational amplifier unit is an operational amplifier U1A, the non-inverting input terminal of the operational amplifier U1A is connected to the other end of the resistor RSENSE, the inverting input terminal of the operational amplifier U1A is connected to one end of the resistor RSENSE, and a sampling filtering unit and a sampling ratio adjustment unit are connected between the inverting input terminal and the output terminal of the operational amplifier U1A and between the non-inverting input terminal and the ground terminal.

4. A gain-adjustable feedforward circuit according to claim 3, characterized in that: The sampling and filtering unit includes capacitors C1 to C4, and the sampling ratio adjustment unit includes resistors R1 to R4, analog switches U2A, U2B, U2C and U2D. One end of the resistor R2 and one end of the capacitor C2 are both connected to the inverting input terminal of the operational amplifier U1A, the resistor R2 and the capacitor C2 are respectively connected between the inverting input terminal and the output terminal of the operational amplifier U1A, the resistor R4 and the analog switch U2A are connected in series between the inverting input terminal and the output terminal of the operational amplifier U1A, the capacitor C4 and the analog switch U2B are connected in series between the inverting input terminal and the output terminal of the operational amplifier U1A, the capacitor C1 and the resistor R1 are respectively connected between the non-inverting input terminal and the ground terminal of the operational amplifier U1A, the capacitor C3 and the analog switch U2C are connected in series between the non-inverting input terminal and the ground terminal of the operational amplifier U1A, and the resistor R3 and the analog switch U2D are connected in series between the non-inverting input terminal and the ground terminal of the operational amplifier U1A.

5. A gain-adjustable feedforward circuit according to claim 4, characterized in that: The models of the analog switch U2A, the analog switch U2B, the analog switch U2C and the analog switch U2D are all ADG1412.

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