Teach you how to design a power supply step by step
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The whole process of power supply design
This time, a 13.2W power supply is taken as an example Input: AC90~264V Output: 3.3V/4A Schematic diagram: The 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: Based on the transformer calculation formula Determine the primary side filter capacitor: The filter capacitor can determine the Vin(min) on the capacitor. The larger the filter capacitor, the higher the Vin(win), which can produce a larger wattage of power, but the price is also higher. Determine the transformer wire diameter and number of wires: After the transformer is determined, the transformer Bobbin can be determined. Based on the Bobbin slot width, the transformer wire diameter and number of wires can be determined, and the current density of the wire diameter can also be calculated. The current density is generally based on 6A/mm2. The current density can only be used as a reference value for the design of the transformer, and the temperature rise record should be used as the final reference. Determine the Duty Cycle (Working Cycle): The Duty Cycle can be determined by the following formula. The Duty Cycle is generally designed based on 50%. If the Duty Cycle exceeds 50%, it is easy to cause oscillation. Determine the Ip value: Determine the number of turns of the auxiliary power supply: Based on the turn ratio of the transformer, the number of turns and voltage of the auxiliary power supply can be determined. Determine the stress of MOSFET and secondary diode: Based on the transformer's turn ratio, we can preliminarily calculate whether the transformer's stress meets the specifications of the selected parts. The calculation is based on an input voltage of 264V (380V on the capacitor). Others: If the output voltage is below 5V and TL431 must be used instead of TL432, an extra set of windings must be considered for the photo coupler and TL431. Substitute the obtained data into the formula, and we can get B(max). If the B(max) value is too high or too low, the parameters must be readjusted. Transformer calculation: Output wattage 13.2W (3.3V/4A), Core = EI-28, windable area (slot width) = 10mm, Margin Tape = 2.8mm (each side), remaining windable area = 4.4mm. [attach]407535 [/attach] Calculation formula: Transformer material and size: From the above assumptions, we can know that the material is PC-40, size = EI-28, Ae = 0.86cm2, windable area (slot width) = 10mm. Because the Margin Tape uses 2.8mm, the remaining windable area is 4.4mm. Assume that the filter capacitor uses 47uF/400V, and Vin (min) is temporarily set at 90V. ●Determine the wire diameter and number of wires of the transformer: 407536[/attach] ●Determine the Duty cycle: 407538[/attach] ●Determine the Ip value: 407539[/attach] ●Determine the number of turns of the auxiliary power supply: 407540[/attach] ●Determine the Stress of the MOSFET and the secondary-side diode: 407542[/attach] ●Others: Because the output is 3.3V and the Vref value of TL431 is 2.5V, if the voltage drop of about 1.2V on the photo coupler is added, the output voltage will not be able to drive the Photo coupler and TL431, so another set of coils must be added to provide the voltage required for the feedback path. ● Transformer wiring diagram: Parts selection: ● FS1: The Iin value is calculated from the transformer. Based on this Iin value (0.42A), it can be known that the company's shared material 2A/250V is used. When designing, it is also necessary to consider whether the Iin at Pin (max) will exceed the rated value of the fuse. ● TR1 (thermistor): At the moment of power activation, due to the short circuit of C1 (primary filter capacitor), the Iin current is very large. Although the time is very short, it may also cause damage to the power supply. Therefore, a thermistor must be installed before the filter capacitor to limit the Iin within the specification (115V/30A, 230V/60A) at the moment of power on. However, since the thermistor also consumes power, it cannot be too large (otherwise it will affect the efficiency). Generally, SCK053 (3A/5Ω) is used. If the C1 capacitor uses a larger value, it is necessary to consider increasing the resistance of the thermistor (generally used on high-wattage power supplies). ●VDR1 (surge absorber): When lightning occurs, it may damage the parts and affect the normal operation of the power supply. Therefore, a surge absorber must be added to the AC input end (after the fuse) to protect the power supply (generally 07D471K is commonly used). However, if there are price considerations, it can be ignored for now. ●CY1, CY2 (Y-Cap): Y-Cap can be generally divided into Y1 and Y2 capacitors. If the AC Input has FG (3 Pin), Y2-Cap is generally used. If the AC Input is 2Pin (only L, N), Y1-Cap is generally used. The difference between Y1 and Y2 lies in 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 there will be a "return" symbol or Y1 on the capacitor body). This circuit uses Y2-Cap because of FG. Y-Cap will affect EMI characteristics. Generally speaking, the larger the better, but leakage and price issues must be considered. The leakage current must meet safety requirements (the 3Pin company standard is 750uA max). ●CX1(X-Cap), RX1:X-Cap is an EMI prevention component. EMI can be divided into two parts: Conduction and Radiation. Conduction specifications can generally be divided into two types: FCC Part 15J Class B and CISPR 22(EN55022) Class B. The FCC test frequency is 450K~30MHz, and the CISPR 22 test frequency is 150K~30MHz. Conduction can be verified in the factory with a spectrum analyzer, while Radiation must be verified in the laboratory. X-Cap is generally effective for EMI prevention in the low frequency band (between 150K and several M). 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 require a bleeder resistor (RX1, generally 1.2MΩ 1/4W). ●LF1 (Common Choke): EMI prevention components mainly affect the low and medium frequency bands of conduction. EMI characteristics and temperature rise must be considered at the same time during design. For the same size of Common Choke, the more coils there are (the thinner the wire diameter is), the better the EMI prevention effect is, but the temperature rise may be higher. ●BD1 (Rectifier Diode): AC power is converted to DC by full-wave rectification. From the Iin value calculated by the transformer, it can be seen that as long as a 1A/600V rectifier diode is used, because it is full-wave rectification, the withstand voltage is only 600V. ●C1 (filter capacitor): The size of C1 (capacitance value) can determine the Vin(min) value in the transformer calculation. The larger the capacitance, the higher the Vin(min) but the higher the price. This part can be used to actually verify whether Vin(min) is correct in the circuit. If the AC Input range is 90V~132V (Vc1 voltage is up to about 190V), a 200V withstand voltage capacitor can be used; if the AC Input range is 90V~264V (or 180V~264V), because the Vc1 voltage is up to about 380V, a 400V withstand voltage capacitor must be used. 407546●D2 (auxiliary power diode): Rectifier diode, commonly used FR105 (1A/600V) or BYT42M (1A/1000V), the main difference between the two: 1. Different withstand voltage (the difference does not matter here) 2. Different VF (FR105=1.2V, BYT42M=1.4V)●R10 (auxiliary power resistor): Mainly used to adjust the VCC voltage of PWM IC. For the currently used 3843, VCC must be greater than 8.4V (Min. Load) during design. However, in order to consider the output short circuit, the VCC voltage cannot be designed too high to avoid no protection when the output is short-circuited (or the input wattage is too large). ●C7 (filter capacitor): Auxiliary power supply filter capacitor, providing PWM IC with a more stable DC voltage, generally using 100uf/25V capacitor. ●Z1 (Zener diode): When the feedback fails, the output voltage surges, and the auxiliary power supply voltage increases relatively. If there is no protection circuit at this time, it may cause damage to the parts. If a Zener Diode is added between 3843 VCC and 3843 Pin3, when the feedback fails, the Zener Diode will collapse, causing Pin3 to reach 1V in advance, thereby limiting the output voltage and achieving the purpose of protecting the parts. The value of Z1 depends on the level of the auxiliary power supply. The decision of Z1 must also consider whether it exceeds the VGS withstand voltage of Q1. In principle, the company's existing materials can be used (generally 1/2W is sufficient). ●R2 (activation resistor): Provides the path for the first activation of 3843. When it is activated for the first time, C7 is charged through R2 to provide the voltage required by 3843 VCC. When the resistance of R2 is large, the turn on time is longer, but the Pin wattage is small when short-circuited. When the resistance of R2 is small, the turn on time is longer. The on time is shorter, and the Pin wattage is larger when short-circuited. Generally, 220KΩ/2W MO is used. . R4 (Line Compensation): Used for high and low voltage compensation, so that the 3843 Pin3 foot is close to the same at 90V/47Hz and 264V/63Hz (generally use 750KΩ~1.5MΩ 1/4W). ●R3, C6, D1 (Snubber): These three parts make up the Snubber. The purpose of adjusting the Snubber is: 1. When Q1 is turned off, a spike will be generated. Adjusting the Snubber can ensure that the spike will not exceed the withstand voltage of Q1. 2. Adjusting the Snubber can improve EMI. Generally speaking, D1 uses 1N4007 (1A/1000V) for better EMI characteristics. R3 uses a 2W MO resistor, and the withstand voltage of C6 is based on the actual voltage difference between the two ends (usually a ceramic capacitor with a withstand voltage of 500V is used).
●Q1 (N-MOS): Currently, 3A/600V and 6A/600V are commonly used. The RDS(ON) of 6A/600V is smaller than that of 3A/600V, so the temperature rise will be lower. If the IDS current does not exceed 3A, 3A/600V should be considered first and verified by the temperature rise record, because the price of 6A/600V is much higher than 3A/600V. The use of Q1 also needs to consider whether VDS exceeds the rated value. ●R8: The role of R8 is to protect Q1 and prevent Q1 from floating. ●R7 (Rs resistor): The maximum voltage of 3843 Pin3 is 1V. The size of R7 must be matched with R4 to achieve the purpose of high and low voltage balance. Generally, 2W MO resistor is used. When designing, R7 is determined first and then R4 is added for compensation. Generally, the voltage of 3843 Pin3 is designed to be between 0.85V and 0.95V (depending on the wattage. If the wattage is small, it cannot be too close to 1V to avoid reaching 1V due to component error). ●R5, C3 (RC filter): Filter out the noise of 3843 Pin3. R5 generally uses 1KΩ 1/8W, and C3 generally uses 102P/50V ceramic capacitor. If C3 uses a smaller capacitance value, it may not start up under heavy load (because 3843 Pin3 instantly reaches 1V); if a larger capacitance value is used, there may be problems such as not starting up under light load and the short-circuit Pin is too large. ●R9 (Q1 Gate resistor): The size of R9 resistor will affect EMI and temperature rise characteristics. Generally speaking, the larger the resistance value, the slower the speed of Q1 turn on / turn off, and the better the EMI characteristics, but the higher the temperature rise of Q1 and the lower the efficiency (mainly because the turn off speed is slow); if the resistance value is small, Q1 turn on / turn off faster, Q1 temperature rise is lower, the efficiency is higher, but the EMI is poor. Generally, 51Ω-150Ω 1/8W is used. ●R6, C4 (control oscillation frequency): Determine the working frequency of 3843. The working frequency composed of R and C can be obtained from the Data Sheet. C4 is generally a 10nf capacitor (with an error of 5%), and R6 uses a precision resistor. Taking DA-14B33 as an example, C4 uses a 103P/50V PE capacitor, and R6 is a 3.74KΩ 1/8W precision resistor. The oscillation frequency is about 45 KHz. ●C5: Similar to RC filter, its main function is to make high voltage and light load less prone to oscillation. Generally, 101P/50V ceramic capacitor is used. ●U1(PWM IC): 3843 is a kind of PWM IC. The Duty Cycle is controlled by the feedback signal of Photo Coupler (U2). Pin3 has the function of current limiting (maximum voltage 1V). Among the 3843 currently used, there are KA3843 (SAMSUNG) and UC3843BN (ST). The pins of the two are the same, but the oscillation frequency generated is slightly different. UC3843BN is about 2KHz faster than KA3843. The increase of fT will lead to some problems (such as EMI problem and short circuit problem). Because KA3843 is more difficult to buy, UC3843BN is used as much as possible when designing new models. ●R1, R11, R12, C2 (primary side loop gain control): 3843 has an Error AMP inside. R1, R11, R12, C2 and Error AMP form a negative feedback circuit to adjust the stability of the loop gain. Improper adjustment of the loop gain may cause oscillation or incorrect output voltage. Generally, C2 uses a vertical multilayer capacitor (with better temperature stability).
●U2 (Photo coupler) The photocoupler (Photo coupler) mainly converts the signal on the secondary side to the primary side (in the form of current). When the TL431 on the secondary side is turned on, U2 will convert the current on the secondary side to the primary side in proportion. At this time, 3843 outputs an off signal (Low) from Pin6 (output) to turn off Q1. The reason for using a Photo coupler is to meet safety requirements (the distance from primacy to secondary must be at least 5.6mm). ●R13 (secondary side loop gain control): controls the current flowing through the Photo coupler. When the resistance value of R13 is small, the current flowing through the Photo coupler is large, U2 conversion current is large, and the loop gain is fast (it is necessary to confirm whether it will cause oscillation). When the resistance value of R13 is large, the current flowing through the Photo coupler is small, U2 conversion current is small, and the loop gain is slow. Although it is less likely to cause oscillation, it is necessary to pay attention to whether the output voltage is normal. ●U3 (TL431), R15, R16, R18 adjust the output voltage, the output voltage should not exceed 38V (because the maximum VKA of TL431 is 36V, if the VF value of Photo coupler is added, Vo should be safer below 38V), the Vref of TL431 is 2.5V, the purpose of R15 and R16 in parallel is to make the output voltage fine-tuned, and the value of R15 and R16 after parallel connection should not be too large (try to be below 2KΩ) to avoid inaccurate output. ●R14, C9 (secondary loop gain control): To control the loop gain of the secondary side, generally speaking, increasing the capacitance will slow down the gain; decreasing the capacitance will speed up the gain. The characteristics of the resistor are just the opposite of the capacitor. When the resistor is increased, the gain will speed up; when the resistor is decreased, the gain will slow down. As for the best value of gain adjustment, it can be measured by dynamic load to obtain an optimal value. ●D4 (rectifier diode): Because the output voltage is 3.3V, and the output voltage regulator (Output Voltage Regulator) uses TL431 (Vref=2.5V) instead of TL432 (Vref=1.25V), an additional set of windings must be added to provide the power required by the photo coupler and TL431. Because the current required by U2 and U3 is not large (about 10mA), the diode withstand voltage value of 100V is sufficient, so only 1N4148 (0.15A/100V) is needed. ●C8 (filter capacitor): Because the current required by U2 and U3 is not large, only 1u/50V is needed. ●D5 (rectifier diode): Output rectifier diode, the use of D5 needs to consider: a. Current value b.Taking this as an example, the output current is 4A, and a 10A diode (Schottky) should be used, but after the temperature rise test, it was found that the temperature of D5 was too high, so it must be replaced with a 15A diode, because the VF of 10A is larger than that of 15A. The withstand voltage of 40V was verified to be in compliance, so the 15A/40V Schottky was finally used. ●C10, R17 (secondary side snubber): D5 will generate a spike at the moment of cutoff. If the spike exceeds the withstand voltage of the diode (D5), the diode will be in danger of breakdown. Adjusting the snubber can appropriately reduce the voltage value of the spike. In addition to protecting the diode, it can also improve EMI. R17 generally uses a 1/2W resistor and C10 generally uses a 500V ceramic capacitor. During the snubber adjustment process (264V/63Hz), pay attention to whether R17 and C10 will overheat, and this should be avoided. ●C11, C13 (filter capacitor): The first-stage filter capacitor on the secondary side should use a capacitor with a smaller internal resistance (LXZ, YXA...). The appropriateness of the capacitor selection can be determined based on the following three points: a. The output ripple voltage meets the specifications b. Whether the capacitor temperature exceeds the rated value c. Whether the voltage across the capacitor exceeds the rated value ●R19 (dummy load): Proper use of a dummy load can make the circuit more stable, but the resistance of the dummy load should not be too small, otherwise it will affect the efficiency. When using it, you must also pay attention to whether it exceeds the rated value of the resistor (generally, the design only uses half of the rated wattage). ●L3, C12 (LC filter circuit): LC filter circuit is the second-stage filter. In the case of not affecting the stability of the line, L3 is generally enlarged (larger inductance), so C12 can use a smaller capacitance value. Switching power supply design verification 407527 Design verification: (can be divided into three parts) a. Design stage verification b. Sample production verification c. QE verification Design stage verification During the design experiment stage, you should develop the habit of recording. Recording can verify whether the experimental results are consistent with the electrical specifications. The following is an explanation of the verification of the power supply design stage (the verification items depend on the specifications). ●Electrical specification verification: 3843 PIN3 pin voltage (full load 4A): ●Duty Cycle, fT: ●Vin(min) = 100V (90V / 47Hz full load) ●Stress (264V / 63Hz full load) : Q1 MOSFET: D5:
D4:407554●Auxiliary power supply (start-up, full load), short circuit Pin max.:407555●Static (full load)407556[/attach]407557[/attach]●Full Range load (0.3A-4A) (verify whether there is oscillation)407558[/attach]●Feedback failure (output light load)407559[/attach] ●OCP(Over Current Protection) ●Pin(max.) ●Dynamic test ●HI-POT test:
This power supply belongs to input 3 PIN HI-POT test is 1500Vac/1 minute. ●Grounding test: For 3-pin input (with FG), the grounding resistance (Grounding test) is generally required. Safety regulations stipulate that the grounding resistance from FG to the output wire (output end) cannot exceed 100mΩ (25A/3 Second). ●Temperature rise record After the design experiment is finalized (tentative), the overall temperature rise and EMI need to be evaluated. If the temperature rise or EMI cannot meet the specifications, the experiment needs to be repeated. Please refer to the attachment for the temperature rise record. D5 originally used BYV118 (10A/40V Schottky), but it was changed to PBYR1540CTX (15A/40V) due to the higher temperature rise. ●EMI test: 407565 ●Mechanical dimensions: The design phase should verify the mechanical dimensions. The verification items include: PCB dimensions, part height limit, part forbidden area, screw hole position and diameter, shell hole size, etc. If the verification cannot be done during the design phase, it must be done during the sample phase. Sample verification phase: After the sample is made, in addition to the temperature rise record and EMI test (whether it needs to be re-verified depends on the situation), each sample should be verified (including electrical and mechanical dimensions). The electrical verification at this stage can be completed by ATE (Chroma) testing. The ATE test must be consistent with the electrical specifications. QE verification: QE verifies the samples provided by the engineering department. The engineering department should provide the following delivery items and samples for QE verification. Source: Network compilation. If copyright is involved, please contact us to delete.
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