Application design of PWM flyback switching power supply based on FAN6754A

　　This paper introduces the working characteristics and principles of the new peak current type PWM control chip FAN6754A, and analyzes the design principle and working process of the flyback switching power supply . According to the secondary circuit structure, a new flyback switching voltage regulator is designed. The transformer design process of the flyback switching power supply is emphasized, including the calculation of the inductance value, the selection of the magnetic core, the determination of the number of winding turns, and the air gap. The three-terminal voltage regulator TL431 is used in conjunction with FAN6754A to achieve the control of the power supply voltage and the regulated output, and the optocoupler device is used to achieve the isolation and feedback of the input/output. And thermistors and overvoltage and overcurrent protection measures are added to the power supply circuit. The experimental test results show that the designed power supply has the advantages of close to 89% efficiency, excellent voltage regulation performance, small ripple, high voltage regulation rate and load regulation rate.

　　The flyback topology has been proven to be an effective solution in terms of both cost and technology, and is implemented using PWM power conversion in laptop AC-DC adapters and chargers. Here, this article designs a new flyback switching power supply using the FAN6754A control chip for a 65W/19V laptop power adapter.

　　FAN6754A Overview

　　FAN6754A is a highly integrated green PWM controller for general switching power supplies and flyback including power adapters from Fairchild Semiconductor. It can meet the current stringent international energy-saving regulations. FAN6754A can provide high startup voltage, which can improve the energy efficiency under light load by 25%. The built-in 8ms soft-start circuit can greatly reduce the current spike and output voltage overshoot when the MOSFET starts. FAN6754A can reduce EMI by up to 5-10dB of frequency jitter function. In addition, FAN6754A has added several design functions to reduce overall power consumption, such as the proprietary green mode function, which provides off-time modulation to continuously reduce the switching frequency under light load conditions.

　　FAN6754A has built-in multiple robust and accurate protection functions to protect the power supply from failures, without adding external components or circuits, such as undervoltage protection, undervoltage lockout (UVLO), overvoltage protection (OVP), overload protection (OLP) and overtemperature protection (OTP), overcurrent protection (OCP) and overcurrent limit (OCL). The VDD overvoltage protection (OVP) function prevents damage caused by abnormal conditions such as open feedback loops. When VDD exceeds 24V due to abnormal conditions, the PWM output will be turned off. The undervoltage lockout (UVLO) circuit has two thresholds, namely the turn-on and turn-off thresholds, which are fixed at 17V and 10V respectively. The UVLO here has a two-stage turn-off threshold. When the controller's protection is activated, the VDD voltage drops below the UVLO turn-off threshold of 10V, and the PWM output will be stopped. However, VDD will not rise again immediately at this time. It will continue to drop to the completely shut-off voltage point of 6.5V before VDD rises to the start-up voltage point again. The PWM controller will then output pulses again. This mechanism allows the average input power of the power supply to be greatly reduced under abnormal conditions such as output short circuit or open loop, and the power supply will not overheat. Unlike previous PWM controllers, the HV4 pin of FAN6754A can also perform AC undervoltage protection. A fast diode and a start-up resistor are used to sample the AC line voltage (sampling once every 180μS, pulse width 20μS). The peak value of each sampling cycle is updated and stored in the register. This peak value can be used for undervoltage and current level limit adjustment. When the voltage on the HV pin is lower than the undervoltage voltage, the PWM output is turned off. In addition, the HV pin can adjust the current limit value to reduce the overcurrent protection tolerance over the entire AC voltage range.

Design of 　　Flyback Switching Power Supply

　　The main circuit of the power supply adopts a single-ended flyback topology. After the power is turned on, the 220V mains electricity is converted into about 310V DC after passing through the EMI filter, the rectifier bridge BD and the filter capacitor; the 220V mains electricity triggers the constant current source inside the chip through the start-up resistor R7 to charge the VDD capacitor. When VDD reaches the conduction threshold voltage, FAN6754A outputs a pulse and the power supply starts working. After that, the chip is powered by the auxiliary winding, and the voltage is maintained at about 17V. After the main switch tube is turned on, the secondary Q3 is in the off state, the current of the primary N1 winding increases linearly, and the inductor energy storage increases; after the switch tube is turned off, the current of the N1 winding is cut off, and the magnetic field energy in the transformer is released to the output end through the secondary winding and Q3. The PWM pulse output generated by the 8-pin of FAN6754A drives the conduction and cutoff of the switch tube, and outputs the voltage to the secondary winding through the high-frequency transformer. The high-frequency square wave pulse voltage is then rectified and filtered to become a DC voltage output. The output voltage obtained by the feedback winding is input into the error amplifier after voltage division and sampling, and compared with the reference voltage to generate the control voltage, that is, the output pulse width, to achieve voltage stabilization. The design requirements of this article are: input AC 90V~264V, one way is the power output 19V/3.42A, and one way is the power supply 17V/1.5mA for the PWM controller; the output has no post-stage linear regulator, the frequency is 65kHz, and the PWM output can be automatically adjusted with the change of input voltage to ensure the stability of the output voltage, the total output power is 65W, and the efficiency is 80%. The design is shown in the figure below.

　　According to the design requirements, when the HV starts, the input DC voltage is typically 104V (equivalent to AC 80.6V), and the power supply starts. The driving HV current is a minimum of 2.0mA, and the typical current is 3.5mA. According to the input voltage range and the HV internal resistance of 1.62K, a 1N4007 and a 200K high-voltage resistor are selected considering the margin. In addition, in order to take into account the startup time and VDD power supply capacity, two electrolytic capacitors are used, and the power supply line is separated by a diode in the middle. When the power is turned on, the mains only charges the first capacitor close to the IC through the HV pin. After the IC starts quickly, the diode is turned on, and the two capacitors supply power to the IC together. The switching frequency of the power supply in normal mode is 65KHz. In the specific design, Rt=5.6K and Ct=1nF are selected.

　　The flyback converter requires an input AC voltage of 90V~264V to be used normally. The rectified DC voltage applied to the converter at the maximum input voltage is:

　　The maximum drain-source voltage that the MOSFET tube can withstand here is Vds_max=Uinmax+nVo. Assuming n is 4.5 and Vo is 19V, then Vds_max=373+4.5*19=458.5(V). Because there is also a spike voltage generated by the leakage inductance, a certain margin should be left, and a 650V withstand voltage MOSFET is selected. The selection standard of the MOSFET tube is: under the premise of meeting the switching stress of the device, the driving circuit makes the output driving waveform have steep rising and falling edges. The MOSFET model selected in the design is: SPA07N65C3 (drain-source voltage 650V, drain current 7A, on-resistance 0.6Ω).

　　Due to the leakage inductance caused by the winding process of the transformer and the excessive switching voltage stress caused by the inductance of the load, the switch tube may be damaged. Here, a spike voltage absorption network consisting of a transient voltage suppressor and a diode in series is used to effectively prevent the reverse voltage from being borne during the shutdown process of the power MOSFET tube. D8 is a TVS (Transient Voltage Suppressor) model P6 KE150A, with a clamping voltage of 150V/dissipated power of 600W, and D9 is a BYV95C (1 A/1KV) diode with soft recovery characteristics. The maximum clamping voltage of the switch tube Vds: Vds(clamp)=Vds_max+VD5=458.5+150=608.5(V), which is less than the MOSFET rated value of 650V. The difference between this absorption network and the traditional RCD absorption network is that under low voltage input or light load conditions, the Vds voltage of the switch tube is not enough to make it operate, which reduces power loss and improves the efficiency of the entire machine under low voltage input and light load conditions.

　　FAN6754A has an open-loop protection (OLP) function, which ensures the safety of the system when an open-loop or output short-circuit fault occurs. The external resistance value input to the SENSE6 pin can be converted into a voltage value, which forms a current inner loop control with the chip. Taking the feedback voltage VFB of the voltage feedback loop as the reference value, once VFB is lower than the threshold voltage, the switching frequency will continue to decrease. At present, most switching power supplies adopt an offline structure, which is generally sampled from the secondary winding loop through a resistor divider. However, since the feedback cannot be directly sampled from the output voltage, there is no isolation, so the anti-interference ability is poor, which is not suitable for occasions with high accuracy requirements or wide load variation range. Here, an adjustable precision shunt regulator TL431 is used in conjunction with an optocoupler to form a feedback loop. The output voltage of the parallel regulator TL431CLP is about 2.5V, IF50mA, CTR>5 0%. TL431CLP and the optocoupler FOD817A form a precise feedback loop, make fine adjustments to Vo19V, and form a precise switching power supply, so that the voltage regulation rate and negative regulation rate can reach less than 0.2%. The PWM duty cycle is determined by the FB voltage and current sampling. Take R21=0.15Ω, when the current flows through the MOSFET short-circuit ground, the internal current amplifier of FAN6754A narrows the conduction width, and the output voltage drops until FAN6754A stops working, no trigger pulse output, and the MOS tube is cut off, achieving the purpose of protecting the power tube. When Vsense is less than about 0.46V, the SENSE6 pin short-circuit protection is entered. If the feedback voltage (FB) is greater than 4.6V for a certain period of time, the PWM pulse is disabled.

　　By using an external negative temperature coefficient (NTC) thermistor to sense the temperature of the external system, the over-temperature protection (OTP) function can be realized. The impedance of the NTC thermistor TR1 decreases as the temperature increases, and the voltage VRT on the RT5 pin decreases accordingly. If VRT is less than 1.035V, PWM is turned off after 12mS. If VRT is less than 0.7V, PWM is turned off after 100uS.

　　The design of the output diode RC absorption network can follow the following steps: (1) Test the diode reverse voltage resonance period Tr when no RC network is added; (2) Select a ceramic capacitor Cdsn in parallel with the diode so that its reverse voltage resonance period is 2*Tr; (3) Calculate the absorption resistor Rdsn according to the following formula: Rdsn=3*Tr/(2π*Cdsn). According to the above actual values ​​of R19 and C21, they are 47Ω and 1nF respectively.

　　The high-frequency transformer is responsible for energy storage, voltage transformation, energy transfer, etc. Its design is as follows.

　　(1)Power selection.

　　The secondary winding is the W2 working power supply and W3 output of FAN6754A. The working voltage of W2 is 19V and the peak current is about 3.42A, so the estimated output power is about 65W.

　　(2) Selection of magnetic core.

　　The switching frequency used in this design is 65kHz. From the power-core size diagram, we can see that the RM10 ferrite core is selected. Its effective area A e is 98mm2 and its saturation flux density is 390mT at 100℃.

　　(3) Determination of the number of winding turns.

　　The maximum on time of the primary winding switch tube corresponds to the lowest input voltage and maximum load. In this design, the maximum duty cycle is: D=Ton/T=0.5, and the corresponding cycle length is: T=l/f=1 5us, then Ton=7.5us.

　　It can be obtained that the number of turns of the primary side of the transformer is:

　　△B is 0.2Tesla, and Np=50 turns. According to the relationship between the primary and secondary voltages of the flyback circuit: Vo= [(Ns/Np)*D/(1-D)]*Vp, calculate the number of turns of the W2 winding (+19V). Assuming the rectifier diode voltage drop is 0.7V and the winding voltage drop is 0.6V, the winding output voltage value is:

　　Similarly, VDD17V/1.5mA can also be obtained when powering FAN6754A.

　　(4) Calculation and verification of transformer primary inductance

　　When the switch is turned on, the average input current of the primary side is: I1 =Po/ηVs, where the efficiency of the transformer is η=85%, then:

　　I1=65/(0.85*90√2)=0.60A

　　The peak current is Ipeak=2*I1/Dmax=2*0.60/0.50=2.4A.

　　Design the change in transformer primary current: △I=(2/3)*I peak=2.40*2/3=1.6A.

　　From the relationship between voltage and current change V=L(△I/△T), we can get:

　　Meets the job requirements here.

　　The lowest voltage input and maximum peak current are the worst working conditions for the transformer. Based on this condition, we verify whether the transformer is saturated. By the formula: Bmax=L*Ipeak/(Ae*Np), substituting relevant parameters into Bmax=0.3T, which is less than the saturation flux density of 0.39T, the design is passed.

　　According to the calculated inductance, an appropriate air gap of 0.36mm is added to ensure that the primary side of the transformer can store a certain amount of energy when it is turned on, and transfer this energy to the secondary side during the off period, so that the transformer can work reliably.

　　Experimental results and analysis

　　Based on the above analysis, a flyback switching power supply was developed and the performance of the system was tested. The following table shows the efficiency data measured at the 20AWG output line end:

　　Table 1 Power performance test results

　　As can be seen from Table 1, under the input condition of 115Vac/50Hz, the average efficiency of the FAN6754A converter is 88.66%, and under the input condition of 230Vac/50Hz, it is 88.88%. This is mainly due to the fact that the operating frequency of the FAN6754A can change with the load. When the output is unloaded, the prototype works in intermittent working mode. When the input is 264Vac, the input power is 88mW. At the same time, the voltage regulation rate and load regulation rate of the prototype are within ±2%, and the output ripple is no more than 25 mV.

　　Figure 2 Output at 90V

　　Figure 3 Output at 264V

Figures 2 and 3 are the output waveforms of the switching power supply　　at 90V and 264V AC input respectively . CH1: Vds, CH2: Vgs, CH3: Ids. Figure 2 is the full-load working waveform of the MOSFET, showing that the power supply works in the current continuous mode at low voltage 90Vac and in the current discontinuous mode at high voltage 264Vac, and the respective voltages and currents meet the expected design. The switching power supply designed here has 2 outputs at the secondary, 1 +19V and 1 +17V. Here, the entire switching power supply system is shown to have a stable and smooth DC output within a wide voltage input range after initial design and continuous debugging. The experimental results fully meet the design requirements, with excellent voltage regulation performance, small ripple, good load regulation, voltage regulation and output voltage accuracy, and the power supply driving capability meets the requirements.

　　in conclusion

　　This paper develops a single-ended flyback switching power supply with stable performance based on FAN6754A, introduces the design of peripheral circuits, buffer circuits and transformers, and realizes feedback regulation of output voltage and various protections. The experimental results show that the designed flyback switching power supply using FAN6754A can well meet the requirements of the system, with few peripheral components and meets the expected requirements. It can be used in situations where the input voltage range is wide and the load changes are large.