1. The power supply design project pays attention to the details of each parameter in the early stage.
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Take a look at this TEA1832 drawing from NXP. Analyze the design and optimization of circuit parameters and achieve certification to mass production. Try to select the components in the company warehouse and the large components in all the components to facilitate the subsequent cost reduction.
The chip resistor is 5% of 0603, 5% of 0805, 1%. The larger the capacitance of the chip capacitor, the higher the price, and the design should be considered.
1. Input, FUSE selection needs to take into account the I^2T parameter. The classification of fuses, fast break, slow break, current, voltage value, fuse certification is complete. The safety distance before the fuse is 2.5mm or more. Design as much as possible to 3mm or more. Need to consider whether there is a margin for the fuse I2T when lightning strikes, will not hang up.
2, this figure can add a varistor, generally use 14D471, also use 561, the larger the diameter, the greater the surge current resistance, there are also enhanced version of 10S471, 14S471, etc., generally 14D471 hit 1KV, 2KV lightning strike enough Increase the lightning strike voltage and replace it with MOV+GDT. If necessary, a heat shrinkable sleeve is placed on the outside of the varistor.
3, NTC, this figure can add an NTC, some customers limited cooling start surge current does not exceed 60A, 30A, another purpose of NTC can also smash part of the voltage during lightning strikes, reduce the pressure of the MOSFET. Pay attention to NTC voltage, current, temperature and other parameters when selecting.
4, common mode inductance, conduction and radiation is a very important filter component, common mode inductance has a ring of high-conductivity materials 5K, 7K, 0K, 12K, 15K, commonly used winding has a slot winding, and winding, butterfly winding method Etc., there is a UU type, divided into 4 slots of the ET type. If this can share the best of the old model, the cost considerations can only be finalized after the conducted radiation test is completed.
5, X capacitor selection, this need to be combined with common mode inductance test conduction and radiation to constant volume value, in general, the greater the power, the larger the X capacitance.
6. If there is a discharge time requirement for input L and N when performing the authentication, it is necessary to discharge 2 and 2 strings of resistors to discharge the capacitor under the X capacitor.
7. The choice of bridge stack generally needs to consider the surge current, voltage and heat dissipation of the bridge stack to prevent it from hanging when lightning strikes.
8, VCC starting resistance, pay attention to the power consumption of the starting resistor, mainly the withstand voltage value, the general withstand voltage of 1206 is 120V, 0805 is generally withstand voltage of 150V, can leave more room for better.
9. Input filter electrolytic capacitor, generally consider the cost consideration, the output retention time is 10mS, according to the minimum case of electrolytic capacitor capacity value 80% capacity design, different manufacturers and different design experience is a little bit different, one should pay attention to ordinary electrolysis Capacitor and 电解 lightning electrolytic capacitor, electrolytic capacitor ripple current is related to capacitor life, this look at the brand and the specific series.
10. There is a small ceramic capacitor connected in parallel to the input electrolytic capacitor. This usually does not show its usefulness. It has an effect when conducting the conducted immunity.
11. RCD absorption part, the value of R corresponds to the peak voltage value on the MOSFET. If the chip resistor is used, pay attention to the voltage derating and power consumption. C generally takes 102/103 1KV high-voltage ceramic tiles, and it may be changed to film capacitors when rectifying radiation. D generally uses FR107, FR207, when the radiation is changed, it is also changed to 1N4007 or other slow tubes, or magnetic beads (K5A, K5C, etc.) are placed on D. For low power, RC can be replaced by TVS tube, such as P6KE160.
12, MOSFET selection, start and short circuit conditions need to pay attention to SOA. Current derating at high temperatures and voltage derating at low temperatures. Generally 600V 2-12A is enough to use the flyback within 100W, and weigh the selection according to the cost. When many methods have no effect when rectifying radiation, it is often the case that a MOSFET is over.
13, MOSFET drive resistance is generally 10R + 20R, the resistance value corresponds to the switching speed, efficiency, temperature rise. This parameter needs to be adjusted when the radiation is rectified.
14. The GATE to SOURCE terminal of the MOSFET needs to add a 10K-100K resistor discharge.
15. There is an Isense resistor between SOURCE and GND of the MOSFET. The power should be as large as possible. Try to use a wound non-inductive resistor. The power is small, or there is a phenomenon of a bomber when there is a short circuit in the sense resistor.
16. Isense resistance to IC's Isense increases by 1 RC, the value is 1K, 331, may have a role in debugging. If this TEA1832 circuit is used as a reference, add a C parallel to GND.
17, different IC peripheral pin reference design manual, according to their own experience, put the filter capacitor at the IC pin.
18, before the change: the design of the transformer, the discussion of the flyback transformer design forum, not much to say. Still consider the cost, try not to add a shield inside the transformer, at most, add a cross shield outside the transformer. The transformer must check the delta B value, delta B = L * Ipk / (N * Ae), L (uH), Ipk (A), N is the primary number of turns (T), Ae (mm2) is interested in verifying this formula can At the lowest voltage input, the output load is continuously increased. When the transformer saturation waveform is seen, the calculation result should be about 500mT when saturated. The VCC auxiliary winding of the transformer should be wound with more than 2 wires as much as possible. In the previous large batch, there were several auxiliary windings with insufficient light load voltage or VCC overvoltage at heavy load. 2 and above VCC auxiliary winding The line can be coupled as much as possible to solve the problem of large voltage differences.
18, after the change: the design of the transformer, the discussion of the flyback transformer design forum, not much to say. Still consider the cost, try not to add a shield inside the transformer, at most, add a cross shield outside the transformer. The transformer must check the delta B value to prevent the core from saturating at high temperatures. Delta B = L * Ipk / (N * Ae), L (uH), Ipk (A), N is the primary number of turns (T), Ae (mm2). (Refer to TDG's core characteristics (100 °C) saturation magnetic flux density 390mT, remanence magnetic 55mT, so the ΔB value is generally within 330mT, the abnormal situation is not saturated, generally less than 300mT. I used to do the flyback transformer Values ​​are less than 0.3), learn zhangyiping experience (so the general flux density chooses 1500 Gauss, the transformer can be smaller, the transformer should be smaller, the frequency is lower. The lower one can be bigger.)
The VCC auxiliary winding of the transformer should be wound with more than 2 wires as much as possible. In the previous large batch, there were several auxiliary windings with insufficient light load voltage or VCC overvoltage at heavy load. 2 and above VCC auxiliary winding The line can be coupled as much as possible to solve the problem of large voltage differences.
Note: If you are interested in verifying this formula, you can input at the lowest voltage and increase the output load. When you see the saturation waveform of the transformer, the calculation result should be about 500mT when saturated (the saturation magnetic flux density is 510mT at 25°C).
Learn from the basic characteristics of the magnetic core of TDG.
19. When the output diode efficiency is high, an ultra-low dropout Schottky diode can be used. When the cost is high, an ultrafast recovery diode can be used.
20. The RC in parallel with the output diode is used to suppress voltage spikes and also suppress radiation.
21, optocoupler and 431, the optocoupler diode can increase a resistance of about 1K-3K, the resistance of Vout series to the optocoupler is generally between 100 ohm-1K. C and RC on 431 are used to adjust loop stability, dynamic response, and so on.
22, Vout detection resistance needs to have a current of about 1mA, the current is too small, the output error is large, the current is too large, affecting standby power consumption.
23. The output capacitor is selected. The ripple current of the output capacitor is approximately equal to the output current. When the capacitor is selected, the ripple current is amplified by 1.2 times or more. 24, a small inductance can be added between the two output capacitors to help suppress the radiation interference. With a small inductance, the ripple current of the first output capacitor will be much larger than the ripple current of the second output capacitor. Therefore, in many circuits, the first capacitor has a large capacity, and the second capacitor has a small capacity.
25. The output Vout terminal can add a common mode inductor in parallel with the 104 capacitor to help conduct and radiate, and also reduce the peak-to-peak ripple.
26, the need to do constant current can be used professional chip, AP4310 or TSM103 and other similar chips to do, with 431 + 358 are OK, pay attention to VCC voltage range, loop adjustment is similar.
27. If there are multiple output load conditions, the main feedback circuit of the power supply must have a fixed output or a dummy load. Otherwise, the output voltage of other channels may be unstable due to coupling, burst mode and other problems. 28. There is a Y capacitor between the primary secondary ground. The general capacity is less than or equal to 222, and the leakage current is less than 0.25mA. Different product certifications have requirements for leakage current.
Counting down so much, the electronic components can basically be finalized, and the entire initial BOM can be reviewed and referenced. Components in the BOM can be placed in several brands to facilitate the cost of the core. If the customer has special requirements, you can add functional circuit implementation in the circuit. If it is not possible to find a new IC to complete, IC power changes have little effect on peripheral devices at equal power and frequency. If the customer's temperature range is relatively high, the corresponding component options need to refer to the component temperature and derating.
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