1. The transformer drawing, PCB, and schematic diagram of the transformer flying wire number must be consistent.
Reason: Safety certification requirements
This is a mistake that many engineers make when applying for safety certification and submitting materials.
2. There are two sets of bleeder resistors for X capacitors.
Reason: UL62368. CCC certification requires disconnecting a set of resistors and then testing the residual voltage of X capacitors.
A mistake that many novices will make, the only way to correct it is to change the PCB layout again, wasting their own time and purchasing proofing.
3. The PCB aperture of the transformer flying wire should take into account the maximum flying wire diameter, and it is necessary to reserve two groups of PCB holes, one large and one small.
Reason: Avoid assembly difficulties or over-furnace air soldering problems
Because there is usually a series for safety certification, for example, 24W applies for a series, which contains a 4.2V-36V voltage range, and the output of low-voltage 4.2V high current and high-voltage 36V small current flying wire diameter is different.
The diameter of multiple flying wires is calculated in the following table:
4. The maximum wire diameter should be taken into account for the PCB aperture of the output DC cable.
Reason: Avoid assembly difficulties
Because your PCB may be used in different current segments, such as 5V/8A, and 20V/2A, the wires used are different
Refer to the table below:
5. For circuit debugging, the resistance value of multiple parallel connections of OCP current limiting resistors should be designed to be the same.
Reason: The higher the resistance, the more power it can withhold
6. Circuit design, the hole of the heat sink pin is made into a rectangular oval shape (experience value: 2*1mm).
Reason: Avoid assembly difficulties
The oval hole allows the radiator to have a room to move, which is very advantageous for assembly and furnace passing.
7. Circuit debugging, abnormal test, the output voltage or OVP design should be less than 60Vac (Vpk) / 42.4Vdc (Vrms).
Reason: Safety requirements
This novice is easy to ignore, so the product applying for certification must do the OVP test to capture the output of the instantaneous waveform.
8. Circuit design, the explosion-proof hole distance of the electrolytic capacitor is greater than 2mm, and the horizontal bent foot is 1.5mm.
Reason: Quality improvement
Generally, regular companies have this requirement, and Japan attaches more importance to the problem of explosion-proof holes, except in special circumstances.
9. Circuit debugging, the output of the circuit with LC filtering needs to be aged to confirm the ripple, if the ripple is abnormal, please adjust the loop.
Rationale: Verify product stability
This is very important, I have often encountered this problem before, after the production line is aging, the test ripple will become high, the phenomenon is loop oscillation.
10. Circuit debugging, when the diodes are connected in parallel, the abnormality generated when a diode is faulty and open circuit should be tested (including two diodes in TO-220).
Reason: Quality improvement
Small companies generally don't do this action, and a good product has to stand any test.
11. Circuit design, if the PCB space is sufficient, please design it to kill all safety standards.
Rationale: Reduce the number of PCB modifications.
If one of your products meets the UL60335 standards, and one day the customer wants to meet UL1310, then you have to change the PCB layout to take the safety report, if the board you draw meets various standards, the work will be much easier.
12. Circuit design, about ESD, please design to contact ±8KV/air±15KV standard.
Reason: Reduce the number of follow-up rectifications.
Customers like Philips require ESD to be very strict, and I heard that Foxconn also needs to reach ±20KV, and you will have to be busy for a while if there is such a customer requirement.
13. Circuit design, when designing the transformer, the VCC voltage at the light load voltage should be greater than the undervoltage shutdown voltage value of the IC.
The no-load VCC voltage must be greater than about 5V of the chip-off voltage, and the overvoltage protection value of the chip cannot be greater than that of the chip when fully loaded
14. Circuit design, the design of the common transformer needs to take into account the VCC voltage when the maximum output voltage is used, and the VCC will touch the OVP action when there is a slight NOSIE at low temperature.
If your products 9V-15V share the same transformer, please confirm the VCC voltage, and power tube withstand voltage
15. Circuit debugging, Rcs and Ccs values should not be too large, otherwise it will cause VDS to exceed the maximum pressure resistance of the fryer.
If the frontier blanking time is shorter, which is shorter than the time of the spike pulse, then there will be no effect or misjudgment; If it is set for a long time, the real overcurrent will not play a protective role.
The RC value of Rcs and Ccs should not exceed the delay of 1 NS, otherwise the Vds will be higher than when the output is fully loaded when the output is short-circuited, and the maximum withstand voltage of the MOSFET may be blown up.
The Delay of 1 nS is approximately equal to 1K to 100PF and 100R to 102PF
16. When drawing a small board, add a circular drill hole at the 90-degree corner of the pins of the small board.
Reason: Easy to assemble
As shown in the figure:
The actual product is as follows:
The actual assembly is shown in the figure:
This can make the small board and the PCB board fit tightly together, and there will be no floating
17. Circuit design, Schottky's heat sink can be connected to the output positive circuit, so that the iron-sealed Schottky does not need insulating pads and insulating particles
18. Circuit debugging, do not use 1N4007 for RCD absorption of more than 15W, because 1N4007 is 300uS slower, and the voltage drop is also 1.3V, and the temperature is very high during the aging process, which is easy to fail and cause the frying machine
19. Circuit debugging, the withstand voltage of the output filter capacitor needs to meet the margin of 1.2 times, and the mass production is avoided.
In the past, we made this very low-level mistake, using a 16V withstand voltage capacitor for the 14.5V output, and 1% of the capacitors failed poorly in mass production.
20. Circuit design, when the large capacitor or other capacitors are made horizontal, if there is a jumper at the bottom, it needs to be placed at the negative potential, so that the jumper can not wear a sleeve.
This can save costs.
21. Rectifier bridge stack, diode or Schottky, wafer size component approval or BOM list should be described, such as 67mil.
Reason: Control the delivery of suppliers to avoid suppliers cutting corners and affecting product efficiency
What is annoying is that the supplier is manipulating, resulting in a whole batch of trial production products can not pass the sixth level of energy efficiency, the reason is that Schottky's internal wafer is small.
22. Circuit design, Snubber capacitor, because there is a problem of abnormal sound, Mylar capacitor is preferred.
One of the ways to deal with abnormal sounds
23. The noise generated by the impregnated TDK RF inductor and the unimpregnated drum differential mode inductor is 12dB less than that of the impregnated core
The second method of dealing with abnormal sounds
24. Vacuum impregnation paint during transformer production, which can make it work at a lower magnetic flux density, and use epoxy resin black rubber to fill the gaps on the three middle columns
The third method of dealing with abnormal sounds
25. Circuit design, if the starting resistor is used before rectification, a series of several hundred K resistors should be added.
Reason: When the resistor is short-circuited, it does not cause damage to the IC and MOSFET.
26. Circuit design, high voltage large capacitor and a 103P porcelain capacitor position.
Reason: It has a certain effect on radiation 30-60MHz.
If space permits, leave a place in the PCB layout to facilitate EMI rectification
27. When conducting EMS project testing, the maximum program of the product needs to be tested until the product is damaged.
For example, ESD lightning strikes, etc., must be hit until the product is damaged, and make relevant records to see how much the product margin is, so that you know what it is
28. Circuit design, when abnormal test, short circuit open circuit, if there is still output voltage of a component, LPS test should be carried out, and the overcurrent point should not exceed 8A.
Those who exceed 8A cannot apply for LPS
29. Safety open shell prototype, all optional plug-in components should be installed for photography, L, N line and DC line and PCB point white glue fixation.
This is a common mistake, often sending samples to third-party institutions, and then going back and forth to change them
30. Circuit debugging, PSR needs 1.15 times the current to turn on when the machine is cold, and SSR needs 1.3 times the current to turn on to avoid poor start-up after aging
PSR Now many chips can achieve "zero recovery" OCP current, such as ME8327N, which has the function of "zero recovery" OCP current
31. For circuit design, please note that the total capacity of the Y capacitor used cannot exceed 222P, because there is the influence of leakage current
For different safety regulations, the leakage current requirements are also different, so special attention should be paid to the design
32. Flyback topography structure, transformer B value needs to be less than 3500 gauss, if the transformer is saturated, all actions will be out of control, as follows, the above picture is normal, the following figure is saturated.
The magnetic saturation of the transformer must be confirmed, which is the first safety performance guarantee, including the magnetic saturation at the overflow point, the magnetic saturation at the moment of start-up, the magnetic saturation of output short circuit, the magnetic saturation at high temperature, and the magnetic saturation of high and low voltage.
33. Structural design, the heat sink is fixed with screws, refer to the following table design, and the margin of 0.5-1mm should be increased in practical application, refer to the following table:
The screw specifications written on the BOM must be correct, otherwise it will make you uncomfortable during mass production
34. Structural design, AC PIN welding wire needs to use hook welding, if not, it should be fixed with white glue.
Reason: Safety requirements
Samples are often returned by third-party institutions for rectification
35. Conduct rectification and segmentation processing experience, as shown in the figure below, this is just a method of processing, and some situations can not be directly applied
36. Radiation rectification, segmented processing experience, as shown in the figure below, suitable for some novice engineers, to provide a reference direction, some situations can not be directly applied, the most important thing is to understand the mechanism of EMI.
37. Regarding the problems encountered in the PCB, as shown in the figure, why can't the 99SE drawing board be filled with copper when filling this position? It's like there's dead copper
D1 has a textual description of the properties of the component placed on the top layer of copper foil, as shown in the figure
Put it on the top layer after silk screen printing, and solve it perfectly.
38. Transformer copper foil shielding is mainly for conduction, wire shielding is mainly for radiation, when the conduction is very good, it is possible that your radiation will be poor, at this time, change the copper foil shielding of the transformer to a line shield, try to lower the position of 30M drop, so that the rectification of radiation will be much faster.
One of the EMI rectification techniques
39. When testing radiation, bring more MOS and Schottky of different brands. Sometimes there is only 2 or 3dB behind, and there will be surprises when you change to a different brand.
EMI Rectification Techniques No. 2
40. The rectifier diode on VCC also has a great impact on radiation.
A tragic case, a product that has passed EMI, the margin is more than 4dB, and it has been mass-produced many times, among which there is a mass production sampling EMI found that the radiation exceeded 1dB, and the defective rate was 50%. It was eventually found that the problem was caused by the rectifier diode on the VCC, and the previous tube was replaced (leaving the sample low) with a margin of 4dB. The analysis of the defective pipes found that the internal suppliers of the pipes had been mirrored.
41. A trivia, how to measure the copper foil thickness of a PCB?
Method: Find a smooth and long line on the PCB board, measure its length L, measure the width W, and then use the 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, the length L is 40mm, the width is 10mm, and the voltage drop at both ends of the current of 1A is 0.005V, find how much um is the thickness of the copper foil?
42. EMI rectification case of a 36W adapter, output 12V/3A, multi-image comparison, rectification took 3 weeks.
变压器绕法一：Np1→VCC→Ns1→Ns2→铜屏蔽0.9Ts→Np2
PCB key layout: Y capacitance ground→ large capacitance ground, transformer ground→ Vcc capacitor → large capacitance ground
Note: There is no crossover of all outgoing lines of the transformer
Figure 1 (115Vac)
As can be seen in Figure 1, the situation at 130-200M is not optimistic;
The main reason for 130-200M is the PCB layout problem and the Schottky circuit on the secondary side, which has little effect on changing other places, and the Schottky beads can be completely pressed down, and the figure is forgotten to be saved.
In order to save costs, the company did not let me do this, because the beads affected the cost, and immediately NG the PCB layout, and used Figure 1a to trace the key layout of the PCB.
变压器绕法不变：Np1→VCC→Ns1→Ns2→铜屏蔽0.9Ts→Np2
PCB key layout: Y capacitance ground→ transformer ground→ large capacitance ground
Note: There is a cross between the primary and secondary outlets inside the transformer
Figure 1a (115Vac)
As can be seen in Figure 1a, 130M-200M has been completely attenuated after changing the PCB layout, but 30-130M is not as good as Figure 1, and it may be better for the transformer outlet to have no crossover. If you look closely, this IC has a frequency jitter function, and some spikes have been cut off in the conduction part of the frequency band;
Figure 1b (230Vac)
As can be seen in Figure 1b, when the input voltage is tested at 230Vac, there is a bit of a top line (red line) at the 65M and 83M positions
图一b-1（230Vac）
The primary absorption capacitance is increased from 471P to 102P, and the 65M position is pressed down a little, but the back is still a little high, as shown in Figure 1b-1;
图一b-2（230Vac）
The transformer shield is changed to line shielding (0.2*1*30Ts), and the back is completely attenuated, as shown in Figure 1 b-2;
图一b-3（115Vac）
115Vac input test, the back 150M is super again, Fake! The high pressure is good, the low pressure is not good, annoying! It seems that this trick will not work;
图一b-4（115Vac）
The transformer shield is still replaced with copper foil shielding (the number of turns is changed from 0.9Ts to 1.3Ts), and the effect is good, as shown in Figure 1b-4.
图一b-5（230Vac）
115Vac input test, test passed.
Conclusion:
1. The outgoing line of the transformer should not be crossed;
2: The shorter the routing of the Y capacitor circuit, the better, first through the transformer ground and then back to the large capacitance ground, without crossing with other signal lines;
43. A 48W (36V/1.33A) rectification EMI case, just adjusting the Schottky absorption will reduce 30-40M.
115Vac low voltage 30M red top line
230Vac high voltage 30M red also top line
After adjusting Schottky absorption:
115Vac low voltage, the trend chart is very beautiful
230Vac high voltage, the trend chart is very beautiful
44. Safety distance list.
45. Problems that are easy to encounter when using CAD and PADS at the beginning.
a.. The PCB drawn by PADS is exported as a DXF file, and the CAD is opened as a hollow line segment composed of double lines, as shown in the figure:
At first, I didn't.,It's a L command to draw one by one.,Sweat wildly.。。
After using it multiple times, the workaround is to use the X command to turn it into a single wire
b.. How to convert CAD drawing wireframe to PADS to PCB outer block diagram:
step1.Delete the lines in CAD, only the board frame remains, and the other lines can also be deleted without deletion.
step2.Hit PE on the keyboard, press Enter, click on one side of the mouse, then press Y, press Enter, then hit J, press Enter, drag the mouse to select the entire board box, press Enter, press Esc key to exit this mode.
step3.Proportional adjustment, SC press space, select the whole board frame, press space, click anywhere, scale: 39.37, press space.
46. When drawing the PCB to define the transformer pins, it is necessary to consider whether the incoming and outgoing wires of the transformer will cross, because there is a 45-90 degree crossing of the winding between the windings at the boundary, and a bushing needs to be added to the pin pin at the cross outlet.
47. The hot spot area of the PCB must be far away from the input and output terminals, to prevent the noise source from being stringed to the line and causing EMI to deteriorate.
Pay attention to the safe distance of this ground wire.
48. The driving resistor should be as close to the MOS as possible, and the resistor of current sampling should be as close as possible to the chip to avoid other invisible consequences.
The iron law of PCB layout
49. Share a case of radiation rectification, a long heat sink has 2 feet, both feet are grounded, the radiation is hard to complete, and then one of the feet is suspended, and the radiation frequency band becomes better. The reason for the later analysis is that the grounding of 2 feet will generate a magnetic field loop.
This rectification cost a lot of money
50. For power supplies equipped with fans, the PCB layout should consider the air path.
Be sure to let the wind out
51. Between the two legs of the rod type inductor, remember, remember, remember, it is forbidden to walk the weak signal line, otherwise you will not be able to find the reason for the accident.
Remember, I've suffered a lot in this before
52. The shape of the transformer core is selected as a summary
and.. EE,EI,EF,EEL类,常用来制作中小功率的变压器,成本低,工艺简单
b.. EFD, EPC, commonly used to make products with height restrictions, suitable for small and medium-power products
c.. EER, ERL, ETD are commonly used to make transformers of large and medium-sized power, especially suitable for making high-power main transformers with multi-channel output, and the leakage inductance of the transformer is small, and it is easier to comply with safety regulations
d.. PQ, EQ, LP class, the middle column of the core is larger than the general core, the product leakage inductance is small, suitable for small volume and high power transformer, the number of output groups can not be too much
e.. RM, POT class, commonly used to make communication or small and medium-power high-frequency transformers, their own magnetic shielding is very good, easy to meet EMC characteristics
f.. EDR is generally used for LED drivers, the thickness of the product is thin, and the transformer manufacturing process is complex
53. Some components or wires may have a high potential difference between them, and the distance between them should be increased to avoid accidental short circuits caused by discharge.
For example, the distance between D and S of the high-voltage MOS on the primary side of the flyback is 0.85mm according to the formula 500V, and 0.9mm below 700V for DS voltage, and 1.2mm is generally taken considering pollution and moisture
54. If the D pin of the MOS in TO220 package is strung with magnetic beads, it is necessary to consider the T pin to increase the safety distance.
I have encountered the phenomenon of the fryer before, but it was solved by increasing the safety distance, because the magnetic beads are easy to get residues
55. Send a local method to verify VCC, put the product in a low-temperature environment (refrigerator) for a few minutes, and test whether the VCC waveform voltage is triggered to the chip undervoltage protection point.
Small companies don't have so much equipment, so if you are interested, you can make a comparison to see how big the VCC difference is
There are many factors to consider when it comes to the design of VCC turns
56. Add ventilation holes to the PCB at the bottom of the transformer, which is conducive to heat dissipation, and the same is true for the small board, and the air path should be considered.
This method can be used when the temperature of the transformer exceeds about 2 degrees in safety certification
57. When there are high-voltage components next to the jumper, a safe distance should be maintained, especially for components that are easy to move or skew.
Ensure the stability of the product during mass production
58. When the bottom of the output large electrolysis has to take the jumper, the jumper should be a low-voltage or ground wire, in order to prevent the capacitor from being scalded by wave soldering, a casing is generally added.
When designing, try to avoid jumping wires at the bottom of the capacitor, because of increased costs and hidden dangers
59. When the high-frequency switch tube is flat on the PCB, do not put sensitive devices such as chips on the other side of the PCB.
Reason: When the switch tube is working, it is easy to interfere with the chip on the back, resulting in system instability, and the same is true for other high-frequency devices
60. When the output DC line is designed on the PCB, it should be designed to be one length and shorter, and the solder pad hole spacing should be small.
Reason: The tail length of the SR is the same length, and when the two pad holes are too far apart, it will cause inconvenience for production soldering
61.MOS管、变压器远离AC端，改善EMI传导。
Reason: High-frequency signals are coupled through the AC terminal, so that the noise source is detected by the EMI equipment and causes EMI problems
62. The driving resistor should be close to the MOS tube.
Reason: Increase anti-interference ability and improve system stability
63. A constant voltage constant current PCB design wiring method with a rotating lamp and a failure case.
Please see the figure for the PCB design routing method:
(a) 地线的Layout原则
As shown in the green lines (1), 2, and 3, the ground of R11 and the ground of R14 are connected to the ground of the chip, which in turn is connected to the ground of the EC4 electrolytic capacitor. Note that it should not 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 affected by strong di/dt interference and will lead to the instability of the system.
Failure Cases:
Caused problem: When the light is turned on, the red and green lights are on together, and the red light and green flashes alternately.
Corrective measures:
By disconnecting the PCB copper foil, a wire is used to connect to the output capacitor ground, separating the ME4312B chip ground, as shown in the following figure:
Through the above processing, the light flashing 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
CV9V 1.043A
CV8.5V 1.043A
CV8V VCC undervoltage protection
0-94mA to green light 96mA or more to red light
The ratio of turning lights is 94/1043=9%, and the ratio of turning lights can be controlled at 3-12%
64. A small trick to deal with the recent price increase of SMD capacitors, SMD capacitors reserve a plug-in position, or 104 is changed to 224P, which is relatively cheaper.
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