12V power adapter control loop part

　　1. Kickback voltage and maximum duty cycle

　　Considering the 600V withstand voltage of the switch tube, the recoil voltage on the primary side of the transformer is generally about 120V, and the corresponding maximum duty cycle is about 0.37. The corresponding transformer primary and secondary windings have a turns ratio of 5:1, which actually corresponds to The recoil voltage is 125V.

　　2. Determination of switching frequency

　　The switching frequency needs to be determined according to EMC requirements. If the EMC of the power adapter can be done well, the higher the switching frequency, the better, so that the volume of passive components can be reduced.

　　But in fact, the FEC characteristics of many power adapters, the EM generated at the switching frequency is relatively large, if the switching frequency is within the EMC detection range, it is difficult to pass EMC, so the AC/DC converter generally chooses the switching frequency at least EMC The minimum detection frequency is 1/2 or even 1/3, that is, 63-65kHz or 41-43kHz. In the case of strict EMC requirements, the switching frequency is generally 41-43kHz, and if the EMC requirements are slightly looser, you can choose 63-65kHz, and EMC is loose These can choose 132kHz.

　　This design chooses 63-65kHz. The timing capacitor is 22nF, and the timing resistance is about 750-8500. Obviously this is a parameter setting with a large dead time, which can limit the maximum duty cycle to no more than 0.5, avoiding switching tubes and The transformer is working in an awkward state greater than 0.5

　　3. Output voltage detection and error amplification

　　The output voltage detection adopts TL431 and its auxiliary circuit.

　　In order to make the output voltage accurate, an adjustable resistor is set in the output voltage detection resistor network. From the perspective of reliability, this adjustable resistor should be installed close to the negative terminal of the output voltage. In this way, the feedback resistor will not output even if it is open. The phenomenon of overvoltage, and the adjustable resistor is set at the positive end of the output voltage, the open circuit or increase of the resistance of the adjustable resistor will inevitably lead to the output overvoltage.

　　The output voltage detection network adds an RC differential circuit to improve the response speed and stability.

　　If you consider that only the open loop gain of T1A31 is not enough to obtain the required power adapter voltage regulation rate and load current regulation rate, you can also use the error amplifier of UC3842 to form a cascade of two-stage amplifiers.

　　Selection of power adapter components

　　AC input section

　　The fuse is 250V3.15A; the anti-surge current negative temperature coefficient thermistor model is 5D-13; the varistor model is 14D471K.

　　Common mode power filter: differential mode (X2) capacitor selection 275V/0.47pF (CX1) and 275V0.luF (Cx2); common mode inductor model: WD03007; common mode (Y2) capacitor (CYA､CY2) selection 400V/ 2.2nF.

　　Rectification and filtering part

　　1. Selection of rectifier bridge

　　According to the relevant content in section 5.2 of this book, the rectifier bridge requires at least 5A rated current and at least 600V rated voltage. Actually, the 8A/600V GBU806 is selected.

　　2. Filter capacitor

　　For a flyback power adapter powered by a 220V voltage level, the input rectifier filter capacitor actually needs to withstand a ripple current of about 0.8A. Due to the height limitation of the shell, a low-height electrolytic capacitor is required. You can choose the 400V120μF low-height produced in recent years ( 25mm) electrolytic capacitor. The withstand voltage meets the requirements and can withstand 0.75A (general specifications) -1.31A (high ripple current). This choice can basically meet the requirements.

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