Design method of medium-frequency induction heating power supply control system

Abstract

The invention discloses a design method of a medium-frequency induction heating power supply control system, relates to a design method of a power supply control system, and aims to solve the problem of poor output waveform stability of an existing induction heating power supply. Based on the induction heating principle, the technical characteristics of the medium-frequency induction heating power supply are determined; designing a control system main circuit according to the technical characteristics of the medium-frequency induction heating power supply, and calculating parameters of the control system main circuit; a virtual power supply is constructed by using the calculated parameters of the main circuit of the control system, 600Hz pure sine fundamental waves are generated by using a Wien bridge oscillating circuit, steamed bread waves are obtained through a closed-loop voltage stabilizing circuit and a precise rectifying circuit and serve as input signals of a control chip, and 20Hz pulse signals are obtained; and comparing and fitting the 20Hz pulse signal with the 600Hz square wave to obtain a 600Hz pulse signal, and taking the 600Hz pulse signal as an MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) driving signal to finish the design. The beneficial effect is that the output waveform stability of the induction heating power supply is good.

Description

Technical Field

[0001] The invention relates to a power supply control system design method. Background Art

[0002] According to the frequency, there is no specific method and standard for the power supply used in the industrial field at present; generally, the power supply with a frequency range of 150Hz to 20kHz is called a medium frequency power supply; medium frequency power supplies are generally divided into two types: the first is a 400Hz medium frequency power supply, which is mainly used for aviation power and communication power supply; the second is the power supply used in induction heating, referred to as induction heating power supply; induction heating is divided into melting, welding, quenching, etc.; different application directions also have different frequency requirements; in order to operate the power supply more efficiently, the load resonant frequency must be effectively utilized; for induction heating power supplies, China began to study in the early 1990s, especially induction heating power supplies for high-frequency switching devices such as IGBT and MOSFET; among them, IGBT is an insulated gate bipolar transistor, and MOSFET is a metal oxide semiconductor field effect transistor. In the mid-1990s, IGBT has been widely used in the medium frequency field; researchers have developed a high-frequency induction heating power supply with MOSFET as a switch; the induction heating power supply is used for surface heat treatment of metal tools, and is also used to check the temperature change stress of aircraft engine blades; the application of IGBT and MOSFET is the development trend of induction heating power supplies in recent years; their appearance has further developed induction heating technology, and the research on induction heating power supplies has continued to deepen;

[0003] At present, after a long period of development, the induction heating power supply has formed a complete supporting industrial system and opened up a very broad market; however, the control of the induction heating power supply is still mainly controlled by analog integrated circuits. The output waveform of the induction heating power supply controlled by analog integrated circuits is poorly stable and cannot meet the needs of induction heating. Summary of the invention

[0004] The purpose of the present invention is to solve the problem of poor stability of the output waveform of the existing induction heating power supply, and proposes a design method for a medium frequency induction heating power supply control system.

[0005] The invention provides a method for designing a medium frequency induction heating power supply control system, the method comprising the following steps:

[0006] Step 1: Based on the principle of induction heating, determine the technical characteristics of the medium frequency induction heating power supply;

[0007] Step 2: design the main circuit of the control system according to the technical characteristics of the medium frequency induction heating power supply determined in step 1, and calculate the parameters of the main circuit of the control system;

[0008] Step 3. Use the parameters of the control system main circuit calculated in step 2 to build a virtual power supply, and use the Wien bridge oscillation circuit to generate a 600Hz pure sinusoidal fundamental wave. After a closed-loop voltage stabilization circuit and a precision rectifier circuit, a steamed bun wave is obtained as the input signal of the control chip, and a 20Hz pulse signal is obtained. The 20Hz pulse signal is compared and fitted with the 600Hz square wave to obtain a 600Hz pulse signal as the MOSFET drive signal to complete the design.

[0009] Furthermore, the technical features of the medium frequency induction heating power supply in step 1 include electrical efficiency, thermal efficiency, input overvoltage protection, input undervoltage protection, overcurrent protection, overheating protection and leakage protection;

[0010] The electrical efficiency refers to the degree to which the input power can be transferred to the output; that is:

[0011]

[0012] Among them, P L1 is the electrical efficiency; P i is the input power of the heating power supply; P0 is the output power of the heating power supply; η1 is the efficiency of the heating power supply;

[0013] The thermal efficiency refers to the output power of the heating power supply; that is:

[0014]

[0015] Among them, η2 is the thermal efficiency; P RP is the actual effective power in induction heating;

[0016] During the heating process, the difference between the effective power obtained by the heating power output power transmitted to the heated workpiece, that is, the thermal power loss P L2 for:

[0017]

[0018] The total efficiency η is the product of the electrical efficiency η1 and the thermal efficiency η2 of the heating power supply, that is:

[0019]

[0020] According to formula (4), the efficiency of the entire electromagnetic induction heating system is the effective power P obtained by heating the workpiece. RP With input power P i The percentage of the ratio;

[0021] The input overvoltage protection means that when the input power voltage is higher than the upper limit of the allowable voltage fluctuation, the heating power supply should enter an automatic protection state, which is shutdown or alarm; when the input power voltage is lower than the upper limit voltage, the heating power supply automatically resumes operation;

[0022] The input undervoltage protection means that when the input power supply voltage is lower than the lower limit voltage allowed to fluctuate, the heating power supply should enter the automatic protection state; when the input power supply voltage is higher than the lower limit voltage, the heating power supply automatically resumes operation;

[0023] The overcurrent protection means that when the input current or load current of the heating power supply exceeds the specified rated current, the heating power supply should automatically enter the current limiting state; when the current is short-circuited or short-circuited, the driving pulse of the power switch should be turned off;

[0024] The overheat protection refers to the overheat protection object of the heating power supply, which is mainly the power switch tube, heating winding, and resonant compensation capacitor of the inverter, protecting thermistors and key components from overheating, and entering the self-protection state when the temperature reaches the set temperature;

[0025] The leakage protection means that when the heating power supply equipment leaks and exceeds the specified leakage current, it should enter the automatic protection state.

[0026] Furthermore, the calculation formula for the input and output characteristics of the main circuit in step 2 is:

[0027] Inverter circuit input voltage U dcim for:

[0028]

[0029] Among them, U acim Input voltage for the main circuit;

[0030] The effective value of the fundamental wave of the output voltage of the inverter circuit is:

[0031]

[0032] Among them, U d The DC bus input voltage of the inverter circuit; U AB is the effective value of the output voltage waveform of the inverter circuit;

[0033] The effective value of the inverter circuit output current I H for:

[0034]

[0035] Wherein, R is the equivalent resistance of the series resonant resistor in the inverter circuit; is the phase difference between output voltage and current;

[0036] Inverter circuit output power P H for:

[0037]

[0038] In actual work, there must be a dead time t d , that is, the on-duty ratio during the switching period is D, and the effective value of the output voltage waveform of the inverter circuit is:

[0039]

[0040] At this time, the output power of the inverter circuit is P H for:

[0041]

[0042] Adjust the DC bus voltage U d , adjust the duty cycle D and adjust the output voltage and current phase difference All are output power values ​​for induction heating power supply;

[0043] When the inverter circuit frequency f s When it is equal to the natural frequency f0 of the series resonant circuit, other relevant parameters are as follows:

[0044] The equivalent impedance Z0 is:

[0045] Z0=R (11)

[0046] The current I in the resonant tank H0 for:

[0047]

[0048] Voltage U across inductor L L for:

[0049] U L =jQU AB (13)

[0050] Where Q is the reactive power of the resonant circuit; j is the imaginary unit;

[0051] The voltage U on the capacitor C C for:

[0052] U C =jQU AB (14)

[0053] The reactive power of the inductor is:

[0054]

[0055] Active power P of equivalent load RR for:

[0056]

[0057] Furthermore, the specific method for calculating the parameters of the main circuit of the control system in step 2 is:

[0058] According to the power output frequency of 600Hz, the resonant inductance and capacitance values ​​are selected as 2mH and 350μF respectively, R is 20Ω, and the duty cycle D is 0.8; the resonant frequency is:

[0059]

[0060] The maximum input voltage of the inverter circuit is:

[0061]

[0062] The maximum output voltage of the inverter circuit is:

[0063]

[0064] The maximum output current of the inverter circuit is:

[0065]

[0066] The output voltage of the inverter circuit is:

[0067]

[0068]

[0069] The effective current output by the inverter circuit is:

[0070]

[0071] The useful power of the equivalent load R is:

[0072] P H =U H I H =280×14=4kW (24).

[0073] Furthermore, the specific structure of generating the MOSFET drive signal using the constructed virtual power supply in step 3 includes: a 600Hz fundamental oscillator, a closed-loop voltage regulator circuit, a precision rectifier circuit, a synchronous square wave circuit, a 20HzSPWM generator circuit, a timing circuit and an SPWM drive circuit;

[0074] The current output by the virtual power supply passes through a 600Hz fundamental wave oscillator, a closed-loop voltage stabilization circuit and a precision rectifier circuit in sequence to generate a 20Hz pulse signal; at the same time, the current output by the virtual power supply passes through the 600Hz fundamental wave oscillator and then passes through a synchronous square wave circuit to generate a 600Hz square wave signal; the 20Hz pulse signal and the 600Hz square wave signal are passed through a timing circuit for comparison and fitting to obtain a 600Hz pulse signal; the obtained 600Hz pulse signal is passed through an SPWM drive circuit to generate a drive signal.

[0075] The beneficial effects of the present invention are as follows: the present invention analyzes in detail the control principle and the control system principle and process, uses a sine wave as the fundamental wave to generate the required steamed bun wave, combines the functional characteristics and its control characteristics to generate a pulse trigger control signal, and uses series resonance as a load to design an induction heating power supply; through current tests and temperature tests, the overcurrent and overheating protection circuit designed in this application has timely feedback and can successfully complete the protection work of the hardware circuit; at the same time, multiple groups of experimental verifications were carried out during simulation, which fully explained the conversion process of the medium frequency induction heating power supply; the output results of the actual circuit and the simulation circuit are consistent, and the expected driving effect can be successfully obtained, and the output waveform of the induction heating power supply has good stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 A flow chart of a design method of a medium frequency induction heating power supply control system described in the first specific implementation mode;

[0077] Figure 2 is a circuit diagram of a voltage source series resonant inverter circuit in a specific implementation mode 1;

[0078] Figure 3 It is a block diagram of the specific structure principle of the virtual power supply generating a MOSFET driving signal in the first specific implementation mode;

[0079] Figure 4 is a circuit diagram of a virtual power supply in a specific implementation mode 1;

[0080] Figure 5 is a circuit diagram of an RC bridge oscillator circuit in a specific implementation mode 1;

[0081] Figure 6 is a circuit diagram of a 600 Hz fundamental wave oscillator in a specific implementation mode 1;

[0082] Figure 7 is a circuit diagram of a closed-loop voltage stabilizing circuit in a specific implementation mode 1;

[0083] Figure 8 It is a circuit diagram of the precision rectifier circuit in the first specific implementation mode. DETAILED DESCRIPTION

[0084] Specific implementation method 1. Combination Figures 1 to 8 This embodiment describes a design method of a medium frequency induction heating power supply control system, which includes the following steps:

[0085] Step 1: Based on the principle of induction heating, determine the technical characteristics of the medium frequency induction heating power supply;

[0086] Step 2: design the main circuit of the control system according to the technical characteristics of the medium frequency induction heating power supply determined in step 1, and calculate the parameters of the main circuit of the control system;

[0087] Step 3. Use the parameters of the control system main circuit calculated in step 2 to build a virtual power supply, and use the Wien bridge oscillation circuit to generate a 600Hz pure sinusoidal fundamental wave. After a closed-loop voltage stabilization circuit and a precision rectifier circuit, a steamed bun wave is obtained as the input signal of the control chip, and a 20Hz pulse signal is obtained. The 20Hz pulse signal is compared and fitted with the 600Hz square wave to obtain a 600Hz pulse signal as the MOSFET drive signal to complete the design.

[0088] In this embodiment, the induction heating power supply is the core of the induction heating system and the source of heat energy; it ensures the normal operation of the induction heating; therefore, some basic performance requirements of the heating power supply should be proposed, including performance requirements and electrical performance technical parameters; the performance requirements of the induction heating power supply mainly include the following items:

[0089] (1) Reliability requirements

[0090] The induction heating power supply should have high reliability technical indicators, which can be measured by the mean time between failures (MTBF). The induction heating power supply is a special power supply in the power supply category; therefore, the reliability indicators are entered into the cutter body through the provisions of the general power supply standard, that is, the mean time between failures must not be low.

[0091] (2) Safety requirements

[0092] The designed and manufactured induction heating power supply should meet the safety indicators required by relevant standards or product specifications, such as insulation requirements, electrical strength requirements, protection against electric shock, etc., to prevent power failure or endanger personal and equipment safety under extreme conditions and harsh environmental conditions.

[0093] (3) Maintainability requirements

[0094] Maintainability requirements can be measured by the mean recovery time MTTR, and the maintainability of the induction heating power supply should have the following characteristics. The power supply should be designed with a fault self-diagnosis function. When the power supply fails, the phenomenon and location of the fault should be diagnosed in time according to the alarm display. The power supply should be designed with a fault self-diagnosis function. When the power supply fails, the phenomenon and location of the fault should be diagnosed in time according to the alarm display.

[0095] (4) High power density requirements

[0096] The induction heating power supply should have high requirements for power capacity per unit volume (W / cm2) and power capacity per unit mass (W / g) to reduce the volume and mass of the power supply equipment, facilitate installation and use, and occupy a small area.

[0097] (5) Environmental suitability performance requirements

[0098] High-quality induction heating power supply should have strong environmental adaptability and relatively loose requirements on environmental conditions. Environmental suitability requirements mainly refer to the requirements of ambient temperature, humidity, altitude and surrounding environment purification, as well as the requirements for the quality of power grid power.

[0099] In a preferred embodiment, the technical features of the medium frequency induction heating power supply in step 1 include electrical efficiency, thermal efficiency, input overvoltage protection, input undervoltage protection, overcurrent protection, overheating protection and leakage protection;

[0100] The electrical efficiency refers to the degree to which the input power can be transferred to the output; that is:

[0101]

[0102] Among them, P L1 is the electrical efficiency; P i is the input power of the heating power supply; P0 is the output power of the heating power supply; η1 is the efficiency of the heating power supply;

[0103] The thermal efficiency refers to the output power of the heating power supply; that is:

[0104]

[0105] Among them, η2 is the thermal efficiency; P RP is the actual effective power in induction heating;

[0106] During the heating process, the difference between the effective power obtained by the heating power output power transmitted to the heated workpiece, that is, the thermal power loss P L2 for:

[0107]

[0108] The total efficiency η is the product of the electrical efficiency η1 and the thermal efficiency η2 of the heating power supply, that is:

[0109]

[0110] According to formula (4), the efficiency of the entire electromagnetic induction heating system is the effective power P obtained by heating the workpiece. RP With input power P i The percentage of the ratio;

[0111] The input overvoltage protection means that when the input power voltage is higher than the upper limit of the allowable voltage fluctuation, the heating power supply should enter an automatic protection state, which is shutdown or alarm; when the input power voltage is lower than the upper limit voltage, the heating power supply automatically resumes operation;

[0112] The input undervoltage protection means that when the input power supply voltage is lower than the lower limit voltage allowed to fluctuate, the heating power supply should enter the automatic protection state; when the input power supply voltage is higher than the lower limit voltage, the heating power supply automatically resumes operation;

[0113] The overcurrent protection means that when the input current or load current of the heating power supply exceeds the specified rated current, the heating power supply should automatically enter the current limiting state; when the current is short-circuited or short-circuited, the driving pulse of the power switch should be turned off;

[0114] The overheat protection refers to the overheat protection object of the heating power supply, which is mainly the power switch tube, heating winding, and resonant compensation capacitor of the inverter, protecting thermistors and key components from overheating, and entering the self-protection state when the temperature reaches the set temperature;

[0115] The leakage protection means that when the heating power supply equipment leaks and exceeds the specified leakage current, it should enter the automatic protection state.

[0116] In this embodiment, the thermal efficiency of the induction heating device refers to the output power of the heating power supply, that is, the degree to which the power required to heat the workpiece can be changed to the effective power of the workpiece heating. Or the thermal efficiency is the percentage of the effective power obtained by heating the workpiece to the power required to heat the workpiece. From the heating winding of the inverter to the heated workpiece, there are air gaps, heat insulation layers, heat-resistant layers and table tops, etc., which will cause heat convection, radiation, conduction and other heat power losses, so the workpiece is heated. The effective power is always less than the power required to heat the workpiece. Assume that the actual effective power of induction heating is P RP , the power lost when the heating power is transmitted to the workpiece is P2, then the thermal efficiency of the thermal induction heating device refers to the output power of the heating power, that is, the degree to which the power required to heat the workpiece can be changed to the effective power of the workpiece heating.

[10] . Or thermal efficiency is the percentage of the effective power obtained by heating the workpiece to the power required to heat the workpiece. From the heating winding of the inverter to the heated workpiece, there are air gaps, insulation layers, heat-resistant layers and countertops, etc., which will cause heat convection, radiation, conduction and other heat power losses, so the workpiece is heated. The effective power is always less than the power required to heat the workpiece. Assume that the actual effective power of induction heating is P RP , the power lost when the heating power output is transmitted to the workpiece is P2.

[0117] In a preferred embodiment, the calculation formula of the input and output characteristics of the main circuit in step 2 is:

[0118] Inverter circuit input voltage U dcim for:

[0119]

[0120] Among them, U acim Input voltage for the main circuit;

[0121] The effective value of the fundamental wave of the output voltage of the inverter circuit is:

[0122]

[0123] Among them, U d The DC bus input voltage of the inverter circuit; U AB is the effective value of the output voltage waveform of the inverter circuit;

[0124] The effective value of the inverter circuit output current I H for:

[0125]

[0126] Wherein, R is the equivalent resistance of the series resonant resistor in the inverter circuit; is the phase difference between output voltage and current;

[0127] Inverter circuit output power P H for:

[0128]

[0129] In actual work, there must be a dead time t d , that is, the on-duty ratio during the switching period is D, and the effective value of the output voltage waveform of the inverter circuit is:

[0130]

[0131] At this time, the output power of the inverter circuit is P H for:

[0132]

[0133] Adjust the DC bus voltage U d , adjust the duty cycle D and adjust the output voltage and current phase difference All are output power values ​​for induction heating power supply;

[0134] When the inverter circuit frequency f s When it is equal to the natural frequency f0 of the series resonant circuit, other relevant parameters are as follows:

[0135] The equivalent impedance Z0 is:

[0136] Z0=R (11)

[0137] The current I in the resonant tank H0 for:

[0138]

[0139] Voltage U across inductor L L for:

[0140] U L =jQU AB (13)

[0141] Where Q is the reactive power of the resonant circuit; j is the imaginary unit;

[0142] The voltage U on the capacitor C C for:

[0143] U C =jQU AB (14)

[0144] The reactive power of the inductor is:

[0145]

[0146] Active power P of equivalent load R R for:

[0147]

[0148] In a preferred embodiment, the specific method for calculating the parameters of the main circuit of the control system in step 2 is:

[0149] According to the power output frequency of 600Hz, the resonant inductance and capacitance values ​​are selected as 2mH and 350μF respectively, R is 20Ω, and the duty cycle D is 0.8; the resonant frequency is:

[0150]

[0151] The maximum input voltage of the inverter circuit is:

[0152]

[0153] The maximum output voltage of the inverter circuit is:

[0154]

[0155] The maximum output current of the inverter circuit is:

[0156]

[0157] The output voltage of the inverter circuit is:

[0158]

[0159]

[0160] The effective current output by the inverter circuit is:

[0161]

[0162] The useful power of the equivalent load R is:

[0163] P H =U H I H =280×14=4kW (24).

[0164] In this embodiment, the IRFP460A semiconductor power device is selected as the switch tube. DSS =500V, I D =30A, load selection voltage type series resonant inverter circuit, such as Figure 1 As shown, it is powered by a voltage source, and the inverter circuit bus DC power supply is composed of a large-capacity energy storage filter capacitor and can be regarded as a constant voltage source. Therefore, the series resonant inverter circuit is also called a voltage source series inverter circuit.

[0165] exist Figure 2 In the figure, the load is a series resonant circuit. The resonant compensation capacitor C, the resonant inductor L and the equivalent resistor R are connected in series and then connected between the two bridge arms A and B of the full-bridge circuit. Under the control of the driving pulse, the power switching devices Q1, Q4 and Q3, Q2 are turned on and off alternately, and during the commutation process of the switching devices, the upper and lower switching tubes in the same bridge arm must have dead time.

[0166] In a preferred embodiment, the specific structure of generating a MOSFET drive signal using the constructed virtual power supply in step 3 includes: a 600Hz fundamental oscillator, a closed-loop voltage regulator circuit, a precision rectifier circuit, a synchronous square wave circuit, a 20HzSPWM generator circuit, a timing circuit and an SPWM drive circuit;

[0167] The current output by the virtual power supply passes through a 600Hz fundamental wave oscillator, a closed-loop voltage stabilization circuit and a precision rectifier circuit in sequence to generate a 20Hz pulse signal; at the same time, the current output by the virtual power supply passes through the 600Hz fundamental wave oscillator and then passes through a synchronous square wave circuit to generate a 600Hz square wave signal; the 20Hz pulse signal and the 600Hz square wave signal are passed through a timing circuit for comparison and fitting to obtain a 600Hz pulse signal; the obtained 600Hz pulse signal is passed through an SPWM drive circuit to generate a drive signal.

[0168] In this embodiment, Figure 3 The specific structural principle block diagram of the virtual power supply generating the MOSFET drive signal. The virtual power supply generates a 6V DC voltage as a virtual "ground". The Wien bridge fundamental oscillator generates a 1.5VPP (peak-to-peak voltage) and 600Hz sine fundamental wave in the virtual "ground" environment. After voltage stabilization and rectification, a 2.3VPP, 5.4Vmin steamed bun wave is obtained, which is converted to 2.3VPP, 0.6Vmin in the actual ground environment. The 2.3VPP, 5.4Vmin steamed bun wave in the virtual "ground" environment is input into the non-inverting input of the comparator in the SG3525 in the actual ground environment. The synchronous square wave in the virtual "ground" environment generates a 600Hz square wave, which is fitted with the 20kHz pulse output by the SG3525 to obtain a 600Hz pulse signal.

[0169] Virtual power is a virtual "ground" with direct current, which is actually -6V. The three parts of the control circuit, namely the "fundamental oscillator circuit", "precision rectifier circuit" and "synchronous square wave circuit", work in the virtual "ground" or virtual power mode, while the "SG3525 generation circuit", "timing circuit" and "dead zone adjustment circuit" work in the real environment. In the SPWM generation circuit working in the real environment, the SG3525 requires the input voltage to rise to 0.9V, and the required 2.3VPP front-end voltage is sufficient. If the amplitude of the input head wave is large, distortion will occur, so 6V grounding is selected as the reference, which can reduce the average value of the circuit in the actual environment. Figure 4 The positive input terminal of the operational amplifier TL084CN is divided by two resistors to obtain a voltage of 6V. Because the reverse input terminal is connected to the output terminal and connected in a follower mode, the output terminal is also 6V. This 6V terminal is a virtual "ground", and the real ground wire becomes -6V at this time, and the +12V terminal is regarded as +6V.

[0170] The sine wave oscillation circuit uses a bridge oscillation circuit, also known as a Wien bridge oscillation circuit, such as Figure 5 As shown, this circuit consists of two parts, namely the frequency selection network and the amplifier circuit A V . F V It is composed of Z1 and Z2, and also serves as a positive feedback network.V It is a voltage series negative feedback amplifier circuit based on an integrated operational amplifier, with high input impedance and low output impedance. Figure 5 It can be seen that Z1, Z2 and R1, R2 form a four-arm bridge, and the two input ends of the amplifier circuit are respectively connected to the diagonal corners of the bridge. Figure 5 The structure in the dotted box is an RC series-parallel frequency selection network, which has the function of frequency selection, but its frequency response is uneven.

[0171] Figure 5 The structure in the dotted box is an RC series-parallel frequency selection network. The two Rs have the same resistance value, which has the function of frequency selection, but its frequency response is uneven. Figure 5 It can be seen that:

[0172]

[0173]

[0174] Therefore, the feedback coefficient of the feedback network is:

[0175]

[0176] Substituting s=jω into equation (27), we get:

[0177]

[0178] Will Substituting into formula (28), we get:

[0179]

[0180] Therefore, the amplitude-frequency response and phase-frequency response of the RC series-parallel frequency selection network can be obtained:

[0181]

[0182]

[0183] According to formula (30) and (31), it can be seen that when

[0184]

[0185] When , the amplitude of the frequency response is the largest, that is, The phase angle of the phase-frequency response is zero, that is,

[0186] when When , the oscillator output voltage amplitude is maximum, the output voltage is 1 / 3 of the input voltage, and the output voltage and input voltage are in the same phase.

[0187] The RC bridge oscillator circuit is a circuit that can convert direct current into an alternating energy output with a certain amplitude, frequency and waveform. The Wien bridge oscillator circuit is widely used because of its simple circuit, good waveform and easy oscillation, but its amplitude stability is not good, so field effect tubes were initially selected to control the gain of the oscillator using voltage-controlled resistors to achieve the purpose of stabilizing the amplitude. However, field effect tubes have thermal stability problems, and the output amplitude will increase significantly. After the output reaches 4VPP, the amplitude drops from 4VPP to about 2.6VPP after more than ten seconds. The reason is that the two amplitude stabilizing diodes change their conduction internal resistance after the temperature rises, causing the circuit gain to decrease, and ultimately causing the output amplitude to change. Therefore, in order to stabilize the output amplitude, the gain of the circuit must first be stabilized.

[0188] Therefore, modifications were made to the Wien bridge oscillator, such as Figure 6 As shown, a negative thermistor R7 is connected in parallel with the resistor R6. When the temperature changes, the resistance of the thermistor R7 changes at the same time to achieve the purpose of stabilizing the gain of the entire circuit. Figure 6 shown.

[0189] Thermistor R7 is used to compensate for the rate of change of thermistor. Experiments have shown that when the temperature rises, if the circuit output still decreases, it means that the compensation is insufficient and the value of resistor R6 can be increased. If the output increases, it means that the compensation is excessive and the value of resistor R6 can be reduced. The output amplitude of the improved Wien bridge oscillator becomes very stable and can be controlled at around 0.1VPP. Experiments have found that the output waveform of the fundamental oscillation circuit changes with the output voltage. The larger the output value, the greater the distortion. The output voltage can be adjusted by adjusting the sliding rheostat RT1. After debugging, it was found that only when the output is controlled below 4VPP, the distortion is minimal, at around 0.4%. The smaller the output, the smaller the distortion. Therefore, the output value is selected as 4VPP. Adjusting the sliding rheostat RT1 can make the circuit output a sine waveform of 4VPP. According to It can be seen that the fundamental frequency is 600Hz.

[0190] The error amplifier control method uses AC feedback to stabilize AC. However, under the condition of strong voltage output, interference is difficult to avoid, and the circuit is too complicated, which does not meet the requirements of simplicity and integration of this project. Therefore, the error amplifier control method has been improved. The circuit diagram is as follows Figure 7 As shown, the SPWM driving principle of the microcontroller is used to use DC feedback for AC voltage regulation, so that the gain can be controlled by current. Later, junction field effect transistors were used as voltage-controlled resistors to adjust the gain, which has the following advantages:

[0191] (1) There is a large dynamic flip, the output can be adjusted from 0V to 10VPP.

[0192] (2)Good linearity.

[0193] (3) Rapid response and short delay.

[0194] (4) Additional distortion of about 0.2%.

[0195] The improved error amplifier control method can obtain a stable 4.6VPP sine wave.

[0196] In order to obtain the steamed bun wave required by SG3525, the previously obtained sine wave needs to be converted into a steamed bun wave of a certain size through a rectifier circuit. The circuit diagram is as follows Figure 7 As shown, in the precision rectifier circuit, two uncontrolled full-wave half-bridge rectifier circuits are selected. In the amplifier IC102c, diodes D3 and D4 are mutually conductive, with a phase difference of 180. In the positive half cycle of the sine wave, diode D3 is conductive and diode D4 is cut off. Because there is a diode conduction voltage drop, the output voltage is -0.7V. In the negative half cycle, diode D3 is cut off and diode D4 is conductive, because R 401 =R 402 , so the amplifier is an inverting input negative feedback amplifier with an amplification factor of 1, and the output voltage is a sine wave with an average value reduced by 0.7V. Similarly, in the amplifier IC102D, the output of IC102C is used as the inverting input of IC102D. During the positive half cycle of the sine wave, D5 is turned on and D6 is turned off. During the negative half cycle, D5 is turned off and D6 is turned on, and the average value is reduced by 0.7V. Therefore, the output of the precision rectifier circuit is a steamed bun wave with an average value reduced by 1.4V.

[0197] The control principle and control system principle and process are analyzed in detail. The sine wave is used as the fundamental wave to generate the required steamed wave. The pulse trigger control signal is generated by combining the functional characteristics and control characteristics. The induction heating power supply is designed with series resonance as the load. Through current tests and temperature tests, the overcurrent and overheating protection circuit designed in this paper has timely feedback and can successfully complete the protection work of the hardware circuit. At the same time, multiple groups of experimental verifications were carried out during the simulation to fully illustrate the conversion process of the medium-frequency induction heating power supply.

[0198] Through debugging, the control principle and results of the induction heating power supply were analyzed in detail. Combined with simulation experiments, some simulation and actual data and waveforms were obtained, and the conclusions were drawn as follows:

[0199] (1) The control driving circuit designed in the present invention has been proved through experiments that the output results of the actual circuit and the simulation circuit are consistent, and the expected driving effect can be obtained smoothly. In addition, the output is stable and the waveform is good, which can achieve the expected effect.

[0200] (2) The auxiliary power supply works normally, can obtain the voltage designed in the design scheme, and successfully provides the voltage required for normal operation of each chip in the control drive circuit. And the AC-DC-AC frequency conversion technology is adopted, which is more mature and reliable to use.

[0201] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A design method for a medium frequency induction heating power supply control system, characterized in that: The design method includes the following steps: Step 1: Based on the principle of induction heating, determine the technical characteristics of the medium frequency induction heating power supply; Step 2: design the main circuit of the control system according to the technical characteristics of the medium frequency induction heating power supply determined in step 1, and calculate the parameters of the main circuit of the control system; Step 3. Use the parameters of the control system main circuit calculated in step 2 to build a virtual power supply, and use the Wien bridge oscillation circuit to generate a 600Hz pure sinusoidal fundamental wave. After a closed-loop voltage stabilization circuit and a precision rectifier circuit, a steamed bun wave is obtained as the input signal of the control chip, and a 20Hz pulse signal is obtained. The 20Hz pulse signal is compared and fitted with the 600Hz square wave to obtain a 600Hz pulse signal as the MOSFET drive signal to complete the design.

2. The design method of a medium frequency induction heating power supply control system according to claim 1 is characterized in that: The technical features of the medium frequency induction heating power supply in step 1 include electrical efficiency, thermal efficiency, input overvoltage protection, input undervoltage protection, overcurrent protection, overheating protection and leakage protection; The electrical efficiency refers to the degree to which the input power can be transferred to the output; that is: Among them, P L1 is the electrical efficiency; P i is the input power of the heating power supply; P0 is the output power of the heating power supply; η1 is the efficiency of the heating power supply; The thermal efficiency refers to the output power of the heating power supply; that is: Among them, η2 is the thermal efficiency; P RP is the actual effective power in induction heating; During the heating process, the difference between the effective power obtained by the heating power output power transmitted to the heated workpiece, that is, the thermal power loss P L2 for: The total efficiency η is the product of the electrical efficiency η1 and the thermal efficiency η2 of the heating power supply, that is: According to formula (4), the efficiency of the entire electromagnetic induction heating system is the effective power P obtained by heating the workpiece. RP With input power P i The percentage of the ratio; The input overvoltage protection means that when the input power voltage is higher than the upper limit of the allowable voltage fluctuation, the heating power supply should enter an automatic protection state, which is shutdown or alarm; when the input power voltage is lower than the upper limit voltage, the heating power supply automatically resumes operation; The input undervoltage protection means that when the input power supply voltage is lower than the lower limit voltage allowed to fluctuate, the heating power supply should enter the automatic protection state; when the input power supply voltage is higher than the lower limit voltage, the heating power supply automatically resumes operation; The overcurrent protection means that when the input current or load current of the heating power supply exceeds the specified rated current, the heating power supply should automatically enter the current limiting state; when the current is short-circuited or short-circuited, the driving pulse of the power switch should be turned off; The overheat protection refers to the overheat protection object of the heating power supply, which is mainly the power switch tube, heating winding, and resonant compensation capacitor of the inverter, protecting thermistors and key components from overheating, and entering the self-protection state when the temperature reaches the set temperature; The leakage protection means that when the heating power supply equipment leaks and exceeds the specified leakage current, it should enter the automatic protection state.

3. The design method of a medium frequency induction heating power supply control system according to claim 1 is characterized in that: The calculation formula for the input and output characteristics of the main circuit in step 2 is: Inverter circuit input voltage U dcim for: Among them, U acim Input voltage to the main circuit; The effective value of the fundamental wave of the output voltage of the inverter circuit is: Among them, U d The DC bus input voltage of the inverter circuit; U AB is the effective value of the output voltage waveform of the inverter circuit; The effective value of the inverter circuit output current I H for: Wherein, R is the equivalent resistance of the series resonant resistor in the inverter circuit; is the phase difference between output voltage and current; Inverter circuit output power P H for: In actual work, there must be a dead time t d , that is, the on-duty ratio during the switching period is D, and the effective value of the output voltage waveform of the inverter circuit is: At this time, the output power of the inverter circuit is P H for: Adjust the DC bus voltage U d , adjust the duty cycle D and adjust the output voltage and current phase difference All are output power values ​​for induction heating power supply; When the inverter circuit frequency f s When it is equal to the natural frequency f0 of the series resonant circuit, other relevant parameters are as follows: The equivalent impedance Z0 is: Z0=R (11) The current I in the resonant tank H0 for: Voltage U across inductor L L for: U L =jQU AB (13) Where Q is the reactive power of the resonant circuit; j is the imaginary unit; The voltage U on the capacitor C C for: U C =jQU AB (14) The reactive power of the inductor is: Active power P of equivalent load R R for:

4. The design method of a medium frequency induction heating power supply control system according to claim 1 is characterized in that: The specific method for calculating the parameters of the main circuit of the control system in step 2 is: According to the power output frequency of 600Hz, the resonant inductance and capacitance values ​​are selected as 2mH and 350μF respectively, R is 20Ω, and the duty cycle D is 0.8; the resonant frequency is: The maximum input voltage of the inverter circuit is: The maximum output voltage of the inverter circuit is: The maximum output current of the inverter circuit is: The output voltage of the inverter circuit is: The effective current output by the inverter circuit is: The useful power of the equivalent load R is: P H =U H I H =280×14=4kW (24)。 5. The design method of a medium frequency induction heating power supply control system according to claim 1 is characterized in that: The specific structure of generating the MOSFET drive signal using the constructed virtual power supply in step 3 includes: a 600Hz fundamental oscillator, a closed-loop voltage regulator circuit, a precision rectifier circuit, a synchronous square wave circuit, a 20Hz SPWM generator circuit, a timing circuit and an SPWM drive circuit; The current output by the virtual power supply passes through a 600Hz fundamental wave oscillator, a closed-loop voltage stabilization circuit and a precision rectifier circuit in sequence to generate a 20Hz pulse signal; at the same time, the current output by the virtual power supply passes through the 600Hz fundamental wave oscillator and then passes through a synchronous square wave circuit to generate a 600Hz square wave signal; the 20Hz pulse signal and the 600Hz square wave signal are passed through a timing circuit for comparison and fitting to obtain a 600Hz pulse signal; the obtained 600Hz pulse signal is passed through an SPWM drive circuit to generate a drive signal.

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