64 details that must be paid attention to in power supply design[Copy link]
1. The transformer flying lead numbers on the transformer drawing, PCB, and schematic diagram must be consistent.
Reason: Safety certification requirements
This is a mistake that many engineers make when submitting information for safety certification.
2. Two sets of discharge resistors are required for the X capacitor.
Reason: UL62368 and CCC certification require disconnecting a set of resistors before testing the residual voltage of the X capacitor
This is a mistake that many novices make. The only way to correct it is to re-change the PCB Layout, wasting their own time and that of the purchasing staff.
3. The PCB hole diameter of the transformer flying lead must take into account the maximum flying lead diameter. If necessary, two sets of PCB holes, one large and one small, should be reserved.
Reason: To avoid assembly difficulties or empty welding problems
Because safety certification applications usually have a series, for example, 24W applies for a series, which includes a 4.2V-36V voltage segment, and the flying wire diameters for outputting low-voltage 4.2V high current and high-voltage 36V low current are different.
The calculation of multiple flying wire diameters is shown in the following table:
5. Circuit debugging: the resistance values of multiple parallel OCP current limiting resistors should be designed to be the same.
Reason: The resistor with a larger resistance value can withstand greater power.
6. Circuit design: the holes for the heat sink pins are made into rectangular ovals (experience value: 2*1mm).
Reason: Avoid assembly difficulties
The oval hole allows the radiator to have space to move, which is very beneficial for assembly and furnace transfer.
7. During circuit debugging and abnormal testing, the output voltage or OVP design should be less than 60Vac (Vpk)/42.4Vdc (Vrms).
Reason: Safety regulations
This is something that newbies tend to overlook, so products applying for certification must undergo an OVP test to capture the output instantaneous waveform.
8. Circuit design: the explosion-proof hole distance of the electrolytic capacitor is greater than 2mm, and the horizontal bent legs leave 1.5mm.
Reason: Quality improvement
Generally, all regular companies have this requirement. Japan pays more attention to the issue of explosion-proof holes, except in special circumstances.
9. EMI rectification case of a 36W adapter, output 12V/3A, multiple pictures for comparison, the rectification took 3 weeks.
As shown in Figure 1, the situation at 130-200M is not optimistic;
The main reason for 130-200M lies in the PCB layout problem and the Schottky circuit on the secondary side. Changing other places has little effect. The Schottky sleeve magnetic bead can be completely pressed down. I forgot to save the picture.
In order to save costs, the company did not allow me to do this because the installation of magnetic beads affected the cost. This PCB layout was immediately discarded and the key PCB layout routing was adopted as shown in Figure 1a.
The transformer winding method remains unchanged: Np1→VCC→Ns1→Ns2→copper shield 0.9Ts→Np2
PCB key layout: Y capacitor ground → transformer ground → large capacitor ground
Note: The primary and secondary wires inside the transformer are crossed.
As can be seen from Figure 1a, after changing the PCB layout, 130M-200M has been completely attenuated, but 30-130M is not as good as Figure 1, maybe the transformer output line is not crossed. Careful observation shows that this IC has a frequency jitter function, and some peaks are cut off in the conduction frequency band;
The primary side absorption capacitor is increased from 471P to 102P, and the 65M position is pressed down a little, but it is still a little high at the back, as shown in Figure 1b-1;
The transformer shield is still replaced with copper foil shield (the number of turns is changed from 0.9Ts to 1.3Ts), and the effect is good, as shown in Figure 1b-4.
Second: The Y capacitor loop should be as short as possible, passing through the transformer ground and then returning to the large capacitor ground, without crossing other signal lines;
10. A 48W (36V/1.33A) EMI rectification case, which reduced 30-40M by simply adjusting the Schottky absorption.
When I first started, I used the L command to trace one by one, sweating profusely.
After using it many times, the solution is to use the X command to turn it into a single line
b. Method of converting CAD wireframe to PADS to make PCB frame drawing:
step1．在CAD里面刪掉没有的线，只剩下板框，其它线也可以不删。
step2. Press PE on the keyboard, press Enter, click one side with the mouse, press Y, press Enter, press J, press Enter, drag the mouse to select the entire frame, press Enter, and press Esc to exit this mode.
step3. Adjust the proportion, SC press the space bar, select the entire frame, press the space bar, click the mouse anywhere, proportion: 39.37, press the space bar.
13. When drawing the PCB to define the transformer pins, it is necessary to consider whether the input and output lines of the transformer will cross, because the windings between the windings cross at 45-90 degrees at the boundaries, and a sleeve needs to be added to the pin at the crossing point.
14. The hot spot area of PCB must be far away from the input and output terminals to prevent the noise source from being connected to the line and causing EMI to deteriorate. When necessary, add a ground wire or other shielding methods for isolation. As shown in the figure below, a ground wire is added for effective isolation.
Pay attention to the safe distance of this ground wire.
15. The driving resistor should be as close to the MOS as possible, and the current sampling resistor should be as close to the chip as possible to avoid other invisible consequences.
Iron Laws of PCB Layout
16. Let me share a radiation rectification case. A long heat sink has two legs. Both legs are grounded. The radiation cannot be rectified. Later, one of the legs is left hanging, and the radiation frequency band becomes better. Later, it is analyzed that the reason is that the grounding of the two legs will generate a magnetic field loop.
This renovation cost a lot of money.
17. For power supplies equipped with fans, the PCB layout must take the air path into consideration.
Let the wind go
18. Remember, remember, remember, do not run weak signal lines between the two legs of the rod-type inductor, otherwise you will not be able to find the cause of any accidents that may occur.
Remember, I suffered a lot on this before.
19. Summary of transformer core shape selection:
a..EE, EI, EF, EEL, often used to make small and medium power transformers, low cost, simple process
b..EFD, EPC, often used to make products with height restrictions, suitable for small and medium power
c..EER, ERL, ETD types are often used to make medium and large power transformers, especially suitable for making multi-channel output high-power main transformers, and the transformer leakage inductance is small, which is easier to meet safety regulations.
d..PQ, EQ, LP types. The middle column of this core is larger than that of the general core. The leakage inductance of the product is small. It is suitable for small volume and high power transformers. The number of output groups cannot be too many.
e..RM, POT type, often used to make communication or small and medium power high frequency transformers, the magnetic shielding itself is very good, easy to meet the EMC characteristics
f..EDR type, generally used for LED driving, product thickness is required to be thin, transformer manufacturing process is complex
20. There may be a high potential difference between some components or wires, so the distance between them should be increased to avoid discharge causing accidental short circuits.
For example, the distance between D and S of the high-voltage MOS on the primary side of the flyback is 0.85mm for 500V according to the formula, and 0.9mm for DS voltage below 700V. Considering pollution and moisture, 1.2mm is generally used.
21. If a magnetic bead is connected to the D pin of the TO220 packaged MOS, it is necessary to consider increasing the safety distance of the T pin.
I encountered the problem of the machine crashing before, but it was solved by increasing the safety distance, because the magnetic beads are easily stained with residue.
22. Here is a simple method to verify VCC: put the product in a low temperature environment (refrigerator) for a few minutes, and test whether the VCC waveform voltage triggers the chip undervoltage protection point.
Small companies don't have that many equipment. If you're interested, you can make a comparison to see how big the VCC difference is.
There are many factors to consider when designing the number of VCC turns.
23. Adding ventilation holes on the PCB at the bottom of the transformer is conducive to heat dissipation. The same applies to small boards, and the air path must be considered.
In safety certification, when the transformer temperature exceeds 2 degrees, this method can be used
24. When there are high-voltage components next to the jumper, a safe distance should be maintained, especially components that are easy to move or tilt.
Ensure product stability during mass production
25. When a jumper has to be used at the bottom of the large output electrolytic capacitor, the jumper should be a low voltage or ground wire. In order to prevent the capacitor from being burned by wave soldering, a sleeve is usually added.
When designing, try to avoid running jumpers at the bottom of the capacitor, as this will increase costs and potential risks.
26. When the high-frequency switching tube is placed flat on the PCB, do not place chips or other sensitive devices on the other side of the PCB.
Reason: When the switch is working, it is easy to interfere with the chip on the back, causing system instability. The same applies to other high-frequency devices.
27. When designing the PCB, the output DC line should be designed to be consistent in length and the spacing between pad holes should be small.
Reason: The tail of SR is the same length. When the two pad holes are too far apart, it will cause inconvenience in production and welding.
28. Place MOS tubes and transformers away from the AC end to improve EMI conduction.
Reason: High-frequency signals will be coupled out through the AC terminal, so the noise source will be detected by the EMI equipment and cause EMI problems.
29. The driving resistor should be close to the MOS tube.
Reason: Increase anti-interference ability and improve system stability
30. A constant voltage and constant current PCB design routing method with rotating lights and a failure case.
Please see the figure for PCB design routing method:
As shown by the green lines in (1)(2)(3), the ground of R11 and R14 are connected to the ground of the chip, and then to the ground of the EC4 electrolytic capacitor. Note that it cannot be connected to the ground of the transformer, because the transformer secondary A->D3->EC4->secondary B forms a power loop. If the ground of the ME4312 chip is connected between the secondary B line and the EC4 capacitor, it will be subject to strong di/dt interference, which will cause system instability and other factors.
Through the above processing, the light flickering problem has been solved, and the test results are as follows:
CV15V 1.043A
CV14V 1.043A
CV13V 1.043A
CV12V 1.043A
CV11V 1.043A
CV10V 1.043A
CV9V1.043A
CV8.5V 1.043A
CV8V VCC undervoltage protection
0-94mA turns green, 96mA and above turns red
The ratio of turning lights is 94/1043=9%, and the ratio of turning lights can be controlled at 3-12%.
31. A little trick to deal with the recent price increase of chip capacitors is to reserve a plug-in position for chip capacitors, or change 104 to 224P, which is relatively cheaper.
32. Circuit debugging: The circuit with LC filter output needs to be aged to confirm the ripple. If the ripple is abnormal, please adjust the loop.
Reason: To verify product stability
This is very important. I have often encountered this problem before. After the production line ages, the test ripple will become higher, and the phenomenon is loop oscillation.
33. When debugging the circuit, when diodes are connected in parallel, the abnormality caused by the failure of one diode to open the circuit should be tested (including the two diodes in TO-220).
Reason: Quality improvement
Small companies generally don't do this. A good product must be able to withstand any test.
34. Circuit design: If there is sufficient PCB space, please design it to meet all safety standards.
Reason: Reduce the number of PCB modifications.
If one of your products complies with the UL60335 standard, and one day a customer wants it to meet UL1310, you will have to change the PCB Layout and submit it for safety compliance reporting. If the board you draw meets various standards, the subsequent work will be much easier.
35. For circuit design, please design ESD to the standard of contact ±8KV/air ±15KV.
Reason: To reduce the number of subsequent rectifications.
Customers like Philips have very strict ESD requirements. I heard that Foxconn also requires it to reach ±20KV. If a customer has such requirements one day, you will be busy for a while.
36. Circuit design, when designing the transformer, the VCC voltage under light load should be greater than the undervoltage shutdown voltage value of the IC.
Determine that the no-load VCC voltage must be greater than the chip shutdown voltage of about 5V, and at the same time confirm that it cannot be greater than the chip overvoltage protection value when fully loaded
37. Circuit design, design of shared transformer needs to consider the VCC voltage when using the maximum output voltage. At low temperatures, VCC has a slight NOSIE which will trigger the OVP action.
If your product 9V-15V shares a transformer, please confirm the VCC voltage and the withstand voltage of the power tube
38. Circuit debugging, Rcs and Ccs values cannot be too large, otherwise VDS will exceed the maximum withstand pressure and explode the machine.
If the LEB leading edge blanking time is set too short, shorter than the peak pulse time, it will have no effect and will still cause misjudgment; if it is set too long, it will not play a protective role when a real overcurrent occurs.
The RC value of Rcs and Ccs cannot exceed 1NS Delay, otherwise when the output is short-circuited, Vds will be higher than when fully loaded, exceeding the maximum withstand voltage of MOSFET and may cause the machine to explode.
The empirical value of 1nS Delay is approximately equal to 1K to 100PF, which is also equal to 100R to 102PF.
39. When drawing the small board, add a circular drill hole at the 90-degree corner of the small board pin.
This will make the small board fit tightly with the large PCB board without any floating phenomenon.
40. Circuit design, the heat sink of Schottky can be connected to the output positive line, so that the iron-sealed Schottky does not need insulating pads and insulating particles.
41. For circuit debugging, do not use 1N4007 for RCD absorption with power above 15W, because 1N4007 is 300uS slower and has a voltage drop of 1.3V. The temperature is very high during the aging process, and it is easy to fail and cause the machine to explode.
42. Circuit debugging, the withstand voltage of the output filter capacitor must meet at least 1.2 times the margin to avoid damage during mass production.
I made this very low-level mistake before. I used 16V withstand voltage capacitors for 14.5V output, and 1% of the capacitors in mass production failed.
43. In circuit design, when large capacitors or other capacitors are made horizontally, if there is a jumper at the bottom, it must be placed at the negative potential so that the jumper does not need to be put through a sleeve.
This can save costs.
44. For rectifier bridge stack, diode or Schottky, the wafer size should be described in the component approval or in the BOM, such as 67mil.
Reason: To control the consistency of supplier delivery and avoid suppliers cutting corners and affecting product efficiency
What is annoying is that the supplier tampered with the product, causing an entire batch of trial-produced products to fail to pass the sixth-level energy efficiency test. The reason is that the internal Schottky chip is too small.
45. Circuit design, Snubber capacitors, because of the noise problem, Mylar capacitors are preferred.
One way to deal with abnormal sound
46. The noise generated by the varnished TDK RF inductor and the unvarnished drum differential mode inductor is 12dB smaller than that of the varnished core .
Method 2 for dealing with abnormal sound
47. The transformer is vacuum-painted during production to enable it to operate at a lower magnetic flux density, and epoxy resin black glue is used to fill the gaps on the three center columns.
Method 3 for dealing with abnormal sound
48. In circuit design, if the starting resistor is used before rectification, a resistor of several hundred K should be added in series.
Reason: When the resistor is short-circuited, the IC and MOSFET will not be damaged.
49. Circuit design, high voltage large capacitor and a 103P ceramic capacitor position.
Reason: It has a certain effect on radiation 30-60MHz.
If space permits, leave a spot in the PCB Layout to facilitate EMI correction.
50. When conducting EMS project testing, the product must be tested to its maximum extent until the product is damaged.
For example, ESD lightning strikes, etc., must be struck until the product is damaged, and relevant records should be kept to see how much product margin there is, so that you have a clear idea
51. In circuit design, during abnormal testing, if there is still output voltage when a component is short-circuited or open-circuited, LPS testing should be performed and the overcurrent point cannot exceed 8A.
If you have more than 8A, you cannot apply for LPS
52. For the safety prototype, all optional plug-in components should be installed for photography, and the L, N and DC lines should be fixed to the PCB with white glue.
This is a common mistake. People often send samples to third-party agencies and then make changes back and forth.
53. Circuit debugging: When the machine is cold, PSR needs 1.15 times the current to start up, and SSR needs 1.3 times the current to start up, to avoid poor starting after aging.
Many PSR chips can now achieve "zero recovery" OCP current, such as ME8327N, which has "zero recovery" OCP current function
54. When designing the circuit, please note that the total capacity of the Y capacitors used cannot exceed 222P because of the influence of leakage current.
Leakage current requirements vary according to different safety regulations, so special attention should be paid during design.
55. For the flyback topology, the transformer B value must be less than 3500 Gauss. If the transformer is saturated, all actions will be out of control, as shown below. The upper figure is normal, and the lower figure is saturated.
The magnetic saturation of the transformer must be confirmed. It is the most important safety performance guarantee, including the magnetic saturation of the overcurrent point, the magnetic saturation at the moment of startup, the magnetic saturation of the output short circuit, the magnetic saturation at high temperature, and the magnetic saturation of high and low voltages.
56. Structural design, the heat sink is fixed with screws, refer to the following table for design, in actual application, 0.5-1mm margin should be added, refer to the following table:
59. Radiation rectification and segmented processing experience, as shown in the figure below, is suitable for some novice engineers and provides a reference direction. Some situations cannot be directly applied. The most important thing is to understand the mechanism of EMI generation.
60. Regarding the problem encountered in PCB, as shown in the figure, why can't the 99SE drawing board copper filling fill this position? It seems that there is dead copper
61. The copper foil shielding of the transformer is mainly for conduction, and the wire shielding is mainly for radiation. When the conduction is very good, your radiation may be poor. At this time, change the copper foil shielding of the transformer to wire shielding, and try to lower the 30M drop position. This way, the radiation rectification will be much faster.
EMI rectification technique 1
62. When testing radiation, bring more MOS and Schottky of different brands. Sometimes when the difference is only 2 or 3 dB, you will be pleasantly surprised if you change to a different brand.
EMI Correction Technique 2
63.The rectifier diode on VCC also has a great impact on radiation.
A tragic case: a product that passed EMI had a margin of more than 4dB. It had been mass-produced many times. During one of the mass-production EMI inspections, it was found that the radiation exceeded 1dB, and the defect rate was 50%. After layer-by-layer investigation and component replacement, it was finally found that the problem was caused by the rectifier diode on VCC. The previous tube was replaced (retaining the low sample), and the margin was 4dB. After analyzing the defective tube, it was found that the internal supplier of the tube had done mirror processing.
64. A little-known fact: how to measure the copper foil thickness of PCB?
Method: Find a smooth and long line on the PCB board, measure its length L, then measure its width W, and then use a DC source to add 1A current to measure the voltage drop U at both ends.
According to the resistivity formula, the following formula is obtained:
For example: Take a section of PCB copper foil with a length L of 40mm and a width of 10mm. The voltage drop across the two ends of the copper foil when a current of 1A passes through it is 0.005V. What is the thickness of the copper foil in um?
The author of the post is just a porter and the content is reproduced from the Internet.
This article is really full of details~ But power supply design is not just these. I also saw a series of power supply tips here on EE. If you are interested, you can continue to learn~ The link is below~~ Please add a favorite, like and share it, one-click triple click~~~
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