AC220V-DC12V flyback switching power supply design

Abstract

The invention provides an AC220V-DC12V flyback switching power supply design. The AC220V-DC12V flyback switching power supply design comprises an input filter circuit, a power level circuit, a feedback circuit and a PWM control circuit, the input filter circuit converts a commercial power AC 220V AC power supply into a DC power supply. The power level circuit reduces the DC voltage to 12V for output and realizes cold ground and hot ground isolation; the feedback circuit realizes hot ground and cold ground isolation through an optocoupler; the PWM control circuit controls the duty ratio of the output PWM waveform according to the feedback voltage so as to achieve the voltage stabilization function of the output voltage, voltage stabilization of the output voltage is achieved through the PWM control circuit, it can be guaranteed that the output voltage of the power supply is kept stable when the load changes, and the power supply is suitable for equipment sensitive to voltage; by using the full-wave rectifier bridge and the filter capacitor, 220V alternating current can be effectively converted into smooth direct current, the ripple voltage is reduced, and the use efficiency of the power supply is improved.

Description

Technical Field

[0001] The present invention relates to the field of power supply design, and in particular to a design of an AC220V-DC12V flyback switching power supply. Background Art

[0002] LLC power topology is an efficient power conversion solution, usually used in high-power applications. Its working principle is to use the resonance characteristics to reduce switching losses and improve efficiency. It usually includes two switches (such as MOSFET) and a resonant cavity (composed of an inductor and a capacitor); its advantages are high efficiency, good load adaptability, and small output voltage fluctuation; its disadvantages are complex design, large size, high thermal management requirements, and high material and component costs.

[0003] The forward converter uses the voltage step-up and step-down principle of the transformer to convert AC power into DC power. Its working mode is to adjust the output voltage by controlling the switching frequency and duty cycle of the switching element. Its advantages are relatively simple structure and easy implementation; its disadvantages are relatively large size, relatively obvious electromagnetic interference (EMI), especially under high-frequency operation, and high cost.

[0004] As electronic devices move towards miniaturization, lightness and high performance, power supply design is constantly improving. New materials (such as wide bandgap semiconductors) and new technologies (such as digital control and intelligent power management) are being widely used to improve the energy efficiency of power supplies and reduce size and cost. Summary of the invention

[0005] The purpose of the present invention is to solve the shortcomings of the prior art and to propose an AC220V-DC12V flyback switching power supply design.

[0006] To achieve the above-mentioned purpose, the present invention adopts the following technical scheme: an AC220V-DC12V flyback switching power supply design, including an input filter circuit, a power stage circuit, a feedback circuit and a PWM control circuit; the input filter circuit converts the AC220V AC power supply into DC power; the power stage circuit steps down the DC voltage to 12V output and realizes the isolation of the cold ground and the hot ground; the feedback circuit realizes the isolation of the hot ground and the cold ground through an optocoupler; the PWM control circuit controls the duty cycle of the output PWM waveform according to the feedback voltage to realize the voltage stabilization function of the output voltage.

[0007] Preferably, the input filter circuit includes an AC power supply model, a rectifier bridge model and a filter capacitor; the AC power supply model simulates a 220V 50Hz mains power input; the rectifier bridge model is used to rectify AC power into DC power; the filter capacitor is configured to smooth the rectified DC voltage to ensure a stable output voltage.

[0008] Preferably, the rectifier bridge model is a full-wave rectifier bridge, model BYV32-400, having the characteristics of 400V reverse withstand voltage and 3A RMS current.

[0009] Preferably, the power stage circuit includes a transformer, an auxiliary power winding and a MOSFET switch; the transformer has a primary winding and a secondary winding, which are responsible for energy conversion and electrical isolation; The auxiliary power winding supplies power to the PWM controller; the MOSFET switch is configured to control the charging and discharging of the transformer.

[0010] Preferably, the PWM control circuit includes a current detection circuit and a voltage regulator diode; the current detection circuit is configured to monitor and feedback current to achieve current protection; the voltage regulator diode is configured to limit the voltage at the VCC terminal of the PWM controller to prevent overvoltage damage.

[0011] Preferably, the power supply design has the function of effectively suppressing EMI radiation and high-frequency noise.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention realizes voltage stabilization of the output voltage through the PWM control circuit, can ensure that the output voltage of the power supply remains stable when the load changes, and is suitable for voltage-sensitive equipment; the use of a full-wave rectifier bridge and a filter capacitor can effectively convert 220V AC power into smooth DC power, reduce the ripple voltage, and improve the use efficiency of the power supply; through the X capacitor and Y capacitor in the design, as well as the common-mode inductor, the electromagnetic interference of the power supply to the outside can be effectively suppressed, and the impact of the equipment on the power grid and other equipment during operation can be reduced; through the design of an optical coupler and a transformer, the isolation of the hot ground and the cold ground in the circuit is realized, the safety of the circuit is improved, and the possibility of interference and failure is reduced; the use of a current detection resistor allows the current to be monitored and limited, preventing overcurrent from damaging the power supply and the load, and improving the reliability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The power supply design schematic diagram of the present invention; Figure 2 This is a schematic diagram of the splitting of the first part of the power supply design principle diagram of the present invention; Figure 3 This is a schematic diagram of the splitting of the second part of the power supply design principle diagram of the present invention; Figure 4 It is a schematic diagram of the splitting of the third part of the power supply design principle diagram of the present invention; Figure 5 This is a schematic diagram of the splitting of the fourth part of the power supply design principle diagram of the present invention; Figure 6 FIG. 2 is an application diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0014] In order to provide a further understanding of the purpose, structure, features, and functions of the present invention, the following detailed description is given in conjunction with the embodiments.

[0015] Please refer to Figures 1 to 6 The present invention provides an AC220V-DC12V flyback switching power supply design, and the power supply design includes four parts: an input filter circuit, a power stage circuit, a feedback circuit, and a PWM control circuit. The function of the input filter circuit is to filter the AC220V AC power supply into a DC power supply. The power stage circuit is to step down the DC voltage after the AC power supply is rectified into a 12V output, and at the same time realize the function of isolating the cold ground and the hot ground. The feedback circuit is to feed back the output circuit to the PWM control current, and also realize the function of isolating the hot ground and the cold ground through an optocoupler. The PWM control circuit controls the duty cycle of the output PWM waveform according to the feedback voltage to realize the function of stabilizing the output voltage.

[0016] The AC2020V circuit enters from output ①, passes through the rectifier circuit and is output from ② to the power circuit. The power circuit outputs from ⑦ and enters the feedback circuit. The feedback circuit output ⑤ is given to the PWM control circuit. The PWM control circuit output 4 controls the power circuit, so that the power circuit output ⑦ can stably output a 12V voltage, forming a control closed loop.

[0017] Combined with reference Figure 2 ① Port VAC is the AC voltage source model, simulating 220V 50Hz AC power input; ② port is the rectifier filter voltage output terminal; C17 is the X capacitor, which is used to suppress the differential mode conduction and radiation interference of the power supply to the outside, and the high frequency interference transmitted from the AC power line to the power supply; L1 K2 L2 is the common mode inductor model, C15 C16 is the Y capacitor, and the circuit combination can effectively suppress the common mode interference from the outside to the power supply; D5 D6 D7 D8 is the full-wave rectifier bridge model, model BYV32-400, 400V reverse withstand voltage, 3A RMS current; C9, C18 are filter capacitors, and the input rectifier diode constitutes a rectifier filter circuit to rectify the AC power into DC, and the output voltage of port ② is calculated as follows: Rectified maximum DC voltage: ; Rectified minimum DC voltage: ; Therefore, C9 and C18 are filter capacitors, and they form a rectifier filter circuit with the input rectifier diode to rectify the AC mains into DC. The withstand voltage is 400V and the 180uF parameters are selected to ensure that the rated withstand voltage of the capacitor is greater than the maximum voltage during actual operation, thereby ensuring the stability of the input rectifier circuit.

[0018] like Figure 3As shown, port ② is the high-voltage power input, connected to the rectifier filter circuit output 198V; port ③ is the auxiliary power output, connected to the driver chip power input; port ④ drives the PWM input of the MOSFET, connected to the PWM control chip drive output; port ⑥ is the current detection circuit output, connected to the current detection end of the PWM control chip; ⑦ is the 12V voltage output; R12 is the control IC power startup resistor; D3 is the auxiliary power voltage regulator diode; D1 is the auxiliary power rectifier diode; R5 is the auxiliary power current limiting resistor; LT1 is the transformer primary auxiliary winding; R20 is the capacitor C12 charge release resistor; C12 is the transformer primary reflected voltage absorption capacitor; D10 is the reflected voltage absorption circuit protection diode; transformer LT1, LT2, LT3, L4, K1 constitute a flyback transformer model, LT1 is the primary winding, LT3 is the secondary winding, LT2 is the auxiliary power winding, providing power for the PWM controller, L4 The primary parasitic leakage inductance is 47uH, and K1 is the magnetic core; the transformer mainly has the function of energy conversion and isolating the primary and secondary; R20, C12, and D10 constitute the primary RDC absorption circuit; the ③ end is connected to the VCC pin of the PWM controller, and R12 provides the startup power supply for the PWM controller; D1 is a rectifier diode, R5 is a current limiting resistor, and at the same time forms an RC filter circuit with the C5 capacitor. After startup, LT2 obtains the induced voltage from the primary and converts it into 12V DC through D1 rectification and R5 filtering. D3 is a 15V voltage regulator tube as a voltage clamp to prevent the PWM controller VCC terminal voltage from being too high and damaging the PWM controller; X7 is a switching MOSFET. When X7 is turned on, the power flows through the transformer primary coil LT1 to charge the transformer. When X7 is turned off, LT1 is open and LT2 releases the energy in the magnet; R7 is a current detection resistor, and current feedback is performed through path ③; the primary current can be limited by setting the R7 pin, thereby playing a role in current protection; Q3 is the X7 Ciss capacitor that quickly releases the triode. When the output of ③ is high, the Ciss of X7 is charged through R10 to support X7 conduction, and Q3 is cut off at this time; when the output of ③ is low, R10Q3 is turned on, and the Ciss charge of X7 is quickly released; LT3 is the secondary winding of the transformer. When the primary X7 MOS tube is turned off, the induced voltage is released; X6 is a rectifier diode. When the X7 MOS tube is turned on, the secondary coil generates a negative induced voltage that is cut off by the diode; C19 is a 100nF ceramic capacitor that filters high frequency and suppresses EMI radiation; C11 is an electrolytic capacitor that filters the switch ripple current and voltage; R27 is a load circuit that simulates the load state; C12 is an AC coupling capacitor that absorbs the leakage inductance of L4 at X7; the MOSFET generates an instantaneous pulse voltage when it is turned off; R2 is the C12 capacitor that releases the charge to zero the voltage across C12; D10 is an anti-reverse diode to avoid the LT1 primary forward AC short circuit.

[0019] At the maximum voltage of AC 230V, the rectified DC voltage is Vdc=325V. Considering the secondary reflected voltage of the transformer and the reverse electromotive force of the primary leakage inductance, the MOSFET uses the parameter of VDSS=600V withstand voltage, and a 10% safety margin is reserved. The reverse electromotive force of the leakage inductance is 30% of the primary voltage. Therefore: The maximum operating voltage of MOSFET and the primary reflected voltage are calculated as follows: MOS tube maximum voltage: ; Leakage inductance and complex reflected voltage: ; Calculation of the turns ratio between the primary and secondary sides of the transformer: Secondary diode forward voltage drop: ; Primary to secondary transformer winding turns ratio: ; Output power calculation: ; Input power calculation: Assuming the flyback power supply efficiency is 75%: Assuming power supply efficiency: ; Input Power: ; Input current calculation: ; Primary winding current calculation: ; Maximum duty cycle calculation: ; Secondary winding ramp center current: ; Primary winding slope center: ; The primary winding peak current is calculated. When the current ripple factor is 0.5, the transformer core efficiency is the highest, so the current ripple factor r is set to 0.5.

[0020] Current wave coefficient: ; Primary winding peak current: ; Switching frequency setting, the switching frequency is set to 110KHz, and the actual simulation frequency is 90KHz: ; Primary MOSFET turn-on time: ; Volt-second product calculation: ; Calculation of primary winding inductance: ; Secondary winding inductance calculation: ; Calculation of the resistance value of the current limiting resistor R7: The minimum current limiting threshold of UC1842 is 0.9V; UC1842 minimum current limiting voltage threshold: ; Calculation of the resistance of current limiting resistor R7: ; Based on the above analysis, a transformer with a primary inductance of 4.36mH, a turns ratio of 9, a primary maximum current of 1A, and a peak current of 0.618A is selected. The primary current limiting resistor R7 is set to 1.456ohm.

[0021] The output filter capacitor is 2200uF, the equivalent series resistance is 50mohm, and the 16V aluminum electrolytic capacitor is selected.

[0022] Switching frequency: ; Secondary peak current: ; Maximum Duty Cycle: ; Output capacitor value: ; Output capacitor equivalent series resistance: ; Output ripple peak-to-peak value: ; The primary auxiliary winding supplies power to the PWM controller UC1842. The current consumption of UC1842 is at the milliampere level. Therefore, the auxiliary winding current is not considered in the above current calculation process. The turns ratio of the primary winding and the primary auxiliary winding of the transformer can be directly calculated based on the voltage required by the PWM controller UC1842. The operating voltage range of UC1842 is 12V~28V, the starting voltage is 16V, and the chip operating voltage is set to 18V, so D7 is selected as BZX84J-B18.

[0023] Considering the working efficiency and diode voltage drop, the output voltage is set to 22V, and the primary auxiliary winding is calculated as follows: The primary reflected voltage and primary auxiliary power supply voltage are calculated as follows: Leakage inductance and secondary side reflected voltage: ; Primary auxiliary winding supply voltage: ; Calculation of turns ratio between primary winding and auxiliary winding: ; Forward voltage of the primary auxiliary winding rectifier diode: ; Primary auxiliary winding inductance calculation: ; Based on the above calculation, it can be known that the turns ratio of the primary winding to the primary auxiliary winding is set to 5, and the corresponding inductance value of the primary auxiliary winding is 168uH.

[0024] like Figure 4 As shown in the figure, the feedback circuit connection method is: port ⑦ is connected to the 12V power output; X2: optocoupler; X5: TL431 voltage regulator circuit; R3: circuit voltage conversion resistor; R21: optocoupler current limiting resistor; R16: TL431 current limiting resistor; R13, C10: loop RC compensation circuit; R15, R14: feedback voltage divider resistor.

[0025] Working principle: 1) Port ① is connected to the reference voltage 5V output by the controller, ② is connected to the FB feedback pin of the controller, and ③ is connected to the voltage output Vout; R15 and R14 are feedback resistors; R13 and C10 are zero-point compensation RC networks; X2 is an isolation optocoupler, which is used to isolate the primary and secondary.

[0026] 2) When the output voltage 12V increases, the R15 and R14 voltage divider resistances increase↑àX5 TL431 feedback pin voltage increases↑àWhen it is higher than the TL431 internal comparator reference voltage (VREF_TL431) by 1.25VàX5 output voltage decreases↓àX2 optocoupler light emitting diode voltage difference increases↑àphotodiode current increases↑àX2 optocoupler emitter current increases↑àoptocoupler photodiode brightness increases↑, forming a negative feedback closed loop with the PWM control circuit and power conversion circuit.

[0027] 3) When the output voltage 12V decreases, the voltage divider resistance of R15 and R14 decreases↓àX5 TL431 feedback pin voltage decreases↓àTL431 output increases↑àX5 output voltage increases↑àX2 optocoupler light emitting diode voltage difference becomes smaller↓àphotodiode current decreases↓àX2 optocoupler emitter current decreases↓àoptocoupler photodiode brightness decreases↓, forming a negative feedback closed loop with the PWM control circuit and power conversion circuit.

[0028] Based on the above analysis, it can be concluded that the voltage relationship between the power supply output port ③ and the feedback circuit output port ② changes.

[0029] Table 1: Feedback logic table Power output terminal ③ Optocoupler diode brightness Voltage increase↑ Brightness increase↑ Voltage reduction↓ Brightness decrease↓ For programmable reference source, select Texas Instruments TL431, and the output voltage formula is as follows.

[0030] Output voltage calculation formula: ; Output voltage calculation process: Typical value of Iref current of TL431: ; TL431 reference voltage: ; Feedback resistor: ; Feedback resistor: ; Output voltage: ; Set R15 to 9.53Kohm, R14 to 2.49Kohm, and the output voltage to 12.097V, which is within the design requirement of 12V±0.5V.

[0031] like Figure 5 As shown, the PWM controller circuit connection: ②Port is connected to the output of the input rectifier filter circuit, and the voltage is 200V.

[0032] ③Port is connected to the primary auxiliary power output terminal.

[0033] ④Port connects to the primary drive MOSFET.

[0034] ⑤Port connects to current detection resistor.

[0035] Circuit principle: Calculation of power supply startup time: The startup voltage of UC1842 is VCC=16V, so the startup time is the time it takes for 200V to charge C5 from R12 to 16V. Select a suitable RC to confirm the charging time.

[0036] 1) When the AC220V power supply is connected to the circuit, port ② is 200V DC voltage, which charges capacitor C5 through R12. When the voltage rises to 12V, it is clamped by the voltage regulator diode D2, and the X9 UC1842 circuit starts to work.

[0037] 2) One end of capacitor C8 is connected to the COMP pin of the control chip through Q1. The initial voltage of capacitor C2 is zero. In the initial state, the COMP voltage is clamped at 0V+Vbe=0.3V. VCC 12V charges C8 through R9. The voltage of the COMP pin gradually increases. Since the COMP pin is connected to the PWM modulation comparator, the PWM pulse width also gradually increases, and the output voltage gradually rises from 0V to 12V. This process is the power soft start.

[0038] 3) When the output power supply 12V increases↑àthe brightness of the light-emitting diode in the feedback circuit increases↑àthe current flowing through the emitter resistor R3 of the optocoupler X2 increases↑àthe voltage at the feedback pin VFB of the controller increasesàthe duty cycle of the PWM output decreases↓àthe output voltage decreases↓, forming a negative feedback closed loop.

[0039] 4) When the output power supply 12V decreases↓àthe brightness of the light-emitting diode in the feedback circuit decreases↓àthe current flowing through the emitter resistor R3 of the optocoupler X2 decreases↓àthe voltage at the feedback pin VFB of the controller decreases↓àthe duty cycle of the PWM output increases↑àthe output voltage increases↑, forming a negative feedback closed loop.

[0040] 5) The calculation formula of UC1842 switching frequency is as follows: ; By setting the RT and CT parameters, R2=15.4KΩ, C2=1000pF, the switching frequency is set to 110KHz.

[0041] When the input voltage is powered on quickly, the voltage of the output capacitor is 0V, which is equivalent to a short circuit. The control circuit will set the control PWM to the maximum duty cycle according to the feedback voltage of the feedback circuit. Since the output transient condition is equivalent to a short circuit state, the starting current at this time will be very large, which will damage the power devices in the circuit. The existence of the soft start circuit avoids high current startup. When the input power is powered on, the reference voltage of the controller is in a low voltage state due to the existence of the CSS capacitor. Compared with the input feedback voltage, the generated duty cycle is relatively small, and the starting current will be controlled within a smaller range to avoid damage to circuit components. Figure 6 As shown, after power-on, it is best to gradually widen the PWM pulse width from zero duty cycle. The UC1842 device can be easily implemented externally with three components. An R / C network is used to provide a time constant to control the output of the error amplifier. In addition, a transistor is used to isolate the components from the normal operation of any node. The effect of the load on the RT / CT time constant is also minimized by amplifying the transistor gain. By adjusting the RSS and CSS values, the soft start time is set, the design points are verified by simulation to ensure that the soft start process does not trigger the primary current detection protection. Here Rss=R9 is set to 10Kohm and Css=C8 is set to 2.2uF.

[0042] like Figure 6 As shown, the AC220V to 12V power supply circuit has a stable output voltage of 12.115V, a slow start time of 3.19 milliseconds, a stable working state, and meets the design requirements.

[0043] The present invention has been described by the above-mentioned relevant embodiments, however, the above-mentioned embodiments are only examples for implementing the present invention. It must be pointed out that the disclosed embodiments do not limit the scope of the present invention. On the contrary, changes and modifications made without departing from the spirit and scope of the present invention are all within the scope of patent protection of the present invention.

Claims

1. A design of AC220V-DC12V flyback switching power supply, characterized by: It includes an input filter circuit, a power stage circuit, a feedback circuit and a PWM control circuit; the input filter circuit converts the AC220V AC power supply into DC power; the power stage circuit steps down the DC voltage to 12V output and realizes the isolation of the cold ground and the hot ground; the feedback circuit realizes the isolation of the hot ground and the cold ground through an optocoupler; the PWM control circuit controls the duty cycle of the output PWM waveform according to the feedback voltage to realize the voltage stabilization function of the output voltage.

2. The AC220V-DC12V flyback switching power supply design as claimed in claim 1, characterized in that: The input filter circuit includes an AC power supply model, a rectifier bridge model and a filter capacitor; the AC power supply model simulates a 220V 50Hz mains power input; the rectifier bridge model is used to rectify AC power into DC power; the filter capacitor is configured to smooth the rectified DC voltage to ensure a stable output voltage.

3. The AC220V-DC12V flyback switching power supply design as claimed in claim 2, characterized in that: The rectifier bridge model is a full-wave rectifier bridge, model BYV32-400, with the characteristics of 400V reverse withstand voltage and 3A RMS current.

4. The AC220V-DC12V flyback switching power supply design as claimed in claim 1, characterized in that: The power stage circuit includes a transformer, an auxiliary power winding and a MOSFET switch; the transformer has a primary winding and a secondary winding, which are responsible for energy conversion and electrical isolation; The auxiliary power winding supplies power to the PWM controller; the MOSFET switch is configured to control the charging and discharging of the transformer.

5. The AC220V-DC12V flyback switching power supply design as claimed in claim 1, characterized in that: The PWM control circuit includes a current detection circuit and a voltage regulator diode; the current detection circuit is configured to monitor and feedback current to achieve current protection; the voltage regulator diode is configured to limit the voltage at the VCC terminal of the PWM controller to prevent overvoltage damage.

6. An AC220V-DC12V flyback switching power supply design as described in any one of claims 1-5, characterized in that: The power supply design described herein has the function of effectively suppressing EMI radiation and high-frequency noise.

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