Decompose the power supply circuit layer by layer, accurate to each component, so that the analysis is twice the result with half the effort!

1. This time the explanation of the power supply takes a 13.2W power supply as an example. Input: AC90~264V Output: 3.3V/4A Schematic diagram: Transformer is the important core of the entire power supply, so the calculation and verification of the transformer is very important. Determine the material and size of the transformer: According to the transformer calculation formula 2. Determine the decision of the primary side filter capacitor filter capacitor, you can determine the Vin (min) on the capacitor, the larger the filter capacitor, the higher the Vin (win), and the larger the watt can be The number of Power, but the relative price is also higher. 03 Determine the transformer wire diameter and number of wires. After the transformer is determined, the Bobbin of the transformer can be determined. According to the slot width of Bobbin, the wire diameter and number of the transformer can be determined, and the current density of the wire diameter can also be calculated. The current density is generally 6A /mm2 is a reference, the current density can only be used as a reference value for the design of the transformer, and the temperature rise record shall prevail. 04Determine the duty cycle (duty cycle) The duty cycle can be determined by the following formula. The design of the duty cycle is generally based on 50%. If the duty cycle exceeds 50%, it is easy to cause oscillation. 5. Determine the Ip value based on the transformer's turn ratio relationship, which can determine the number of turns and voltage of the auxiliary power supply. Determine the stress of the MOSFET and the secondary diode: According to the turns ratio of the transformer, it can be preliminarily calculated whether the stress of the transformer meets the specifications of the selected parts. The input voltage is 264V (380V on the capacitor). Benchmark. 7. Other if the output voltage is below 5V and TL431 instead of TL432 must be used, one more set of windings must be considered for use with Photo coupler and TL431. Substitute the obtained data into 8. The transformer calculates the output wattage 13.2W (3.3V/4A), Core u003d EI-28, the winding area (slot width) u003d 10mm, Margin Tape u003d 2.8mm (each side), the remaining winding area u003d4.4mm.09 Transformer material and size. From the above assumptions, it can be seen that the material is PC-40, sizeu003dEI-28, Aeu003d0.86cm2, and the winding area (groove width)u003d10mm. Because Margin Tape uses 2.8mm, the remaining can be The winding area is 4.4mm. Assuming that the filter capacitor uses 47uF/400V, Vin(min) is tentatively set at 90V. 010 determines the wire diameter and number of the transformer 011 determines the Duty cycle 12. Determines the Ip value 13. Determines the number of turns of the auxiliary power 14. Determines the Stress (stress) of the MOSFET and the secondary diode 15. Others are because the output is 3.3V, and TL431 The Vref value is 2.5V, and if the voltage drop on the photo coupler is about 1.2V, the output voltage will not be able to drive the photo coupler and TL431, so an additional set of coils must be added to provide the voltage required for the feedback path. 16. Wiring diagram of transformer 17. Selection of parts ● FS1: The Iin value is calculated by the transformer. From this Iin value (0.42A), the company shared material 2A/250V is used, and the design must also consider whether the Iin at Pin(max) Will exceed the rating of the fuse. ● TR1 (Thermistor): At the moment of power activation, due to the short circuit of C1 (primary side filter capacitor), the Iin current is very large. Although the time is short, it may also cause damage to Power, so it must be installed before the filter capacitor A thermistor to limit the Iin within the Spec (115V/30A, 230V/60A) at the moment of power-on, but because the thermistor will also consume power, it should not be too large (otherwise it will affect the efficiency), generally When using SCK053 (3A/5Ω), if the C1 capacitor uses a larger value, you must consider increasing the resistance of the thermistor (usually used on power with large wattage). ● VDR1 (surge absorber): When lightning occurs, parts may be damaged, which will affect the normal operation of Power. Therefore, a surge absorber must be added to the AC input terminal (after Fuse) to protect Power (generally 07D471K) is commonly used, but if there are price considerations, you can ignore it first. ● CY1, CY2 (Y-Cap): Y-Cap can generally be divided into Y1 and Y2 capacitors, if AC Input has FG (3 Pin), generally use Y2-Cap, if AC Input is 2Pin (only L, N), generally use The difference between Y1-Cap, Y1 and Y2, in addition to the price (Y1 is more expensive), the insulation level and withstand voltage are also different (Y1 is called double insulation, the insulation withstand voltage is about twice that of Y2, and it will be on the body of the capacitor. There is a 'back' symbol or indicate Y1). Because this circuit has FG, Y2-Cap is used. Y-Cap will affect EMI characteristics. Generally speaking, the larger the better, but leakage and price issues must be considered, leakage current (Leakage Current) Must meet safety requirements (3Pin company standard is 750uA max). ● CX1 (X-Cap) and RX1: X-Cap are EMI prevention parts. EMI can be divided into two parts: Conduction and Radiation. Generally, the Conduction specification can be divided into: FCC Part 15J Class B and CISPR 22 (EN55022) Class B. The FCC test frequency is 450K~30MHz, the CISPR 22 test frequency is 150K~30MHz. Conduction can be verified by a spectrum analyzer in the factory, and Radiation must be verified in the laboratory. X-Cap generally has low frequency bands (150K ~ several M EMI prevention is effective. Generally speaking, the larger the X-Cap, the better the EMI prevention effect (but the higher the price). If the X-Cap is above 0.22uf (including 0.22uf), the safety regulations must There is a bleeder resistor (RX1, generally 1.2MΩ 1/4W). ● LF1 (Common Choke): EMI prevention parts, which mainly affect the medium and low frequency bands of Conduction. EMI characteristics and temperature rise must be considered at the same time when designing. For the Common Choke of the same size, the greater the number of coils (relative wire diameter) The thinner), the better the EMI prevention effect, but the temperature rise may be higher. ● BD1 (rectifier diode): AC power is converted to DC by full-wave rectification. The Iin value calculated by the transformer shows that as long as the 1A/600V rectifier diode is used, the withstand voltage is only 600V because of the full-wave rectification. .
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