Dry goods | 64 details that must be paid attention to in switching power supply design

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/ 2019-08-12 / News

1. Transformersdrawings, PCB, schematics The transformer flying line number needs one.

Reason: Safety certification requirements

This is how many engineers apply A problem that will be committed when submitting information for safety certification.

2.XCapacitors bleed resistors need to put two groups.

Reason: UL62368, CCC certification requires disconnecting a set of resistors and then testing the residual voltage of the X capacitors

>

A lot of mistakes that novices make.The revised method can only change the PCB Layout, waste time and purchase proofing.

3. The PCB aperture of the transformer flying wire should take into account the maximum flying wire diameter. It is necessary to reserve two small and small PCB holes.

Reason: Avoid assembly difficulties or over-welding problems

because Safety application certification usually has a series, for example, 24W application for a series, including 4.2V-36V voltage segment, output low voltage 4.2V large current current and high voltage 36V low current flying wire diameter is Different.

The calculation of the diameter of multiple flying leads is as follows:

4. Output The PCB diameter of the DC wire needs to take into account the maximum wire diameter.

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 following form:

5.< u>circuit debugging, the resistance of multiple parallel connected OCP current limiting resistors should be designed to be the same.

Reason: the resistance value The larger the resistance of the larger resistor is.

6. Circuit design, the holes of the heat sink pins are made into a rectangular ellipse (empirical value: 2*1mm).

Reason:Avoid assembly difficulties

Oval-shaped holes allow the radiator to have a moving space, which is very advantageous for assembly and furnace.

7. Circuit debugging, abnormal test, output voltage or OVP design is less than 60Vac (Vpk) / 42.4Vdc (Vrms).

Reason: Security requirements

This newbie is easier to ignore, so Products that apply for certification must be tested with OVP and capture the instantaneous waveform.

8. Circuit design, the explosion-proof hole distance of electrolytic capacitor is more than 2mm, and the horizontal bending foot is 1.5mm.

Reason: Quality improvement

General formal companies have this requirement, The problem of explosion-proof holes is more important in Japan, except in special cases.

9. Circuit debugging, the circuit with LC filter output needs aging to confirm the ripple, if the ripple is abnormal, please adjust the loop.

Reason: Verify product stability

This is important, before me Frequently encountered this problem, the test ripple will become higher after the production line aging, the phenomenon is loop oscillation.

10. Circuit debugging, diode when connected in parallel,An abnormality (including two diodes in the TO-220) should be tested when a diode fault is opened.

Reason: Quality improvement

Small companies generally do not do this Action, an excellent product is to stand any test.

11. Circuit design, if the PCB space is sufficient, please design to kill all safety standards.

Reason: Reduce PCB revisions.

If you have a product that meets the UL60335 standard, and one day the customer wants to meet the UL1310, then you have to change the PCB Layout to take the safety report. If the board you are painting meets various standards,The work behind will be a lot easier.

12. Circuit design, please design ESD to contact ±8KV/air±15KV standard.

Reason: Reduce the number of subsequent rectifications.

Customers like Philips require ESD to be very strict. I heard that Foxconn needs to reach ±20KV, and when there is such a customer request, you have to Busy for a while.

13. Circuit design, when designing the transformer, the VCC voltage at the light load voltage is greater than the IC's undervoltage shutdown voltage value.

determine that the no-load VCC voltage needs to be greater than about 5V of the chip's shutdown voltage.At the same time, it is confirmed that the full load cannot be greater than the overvoltage protection value of the chip.

14. Circuit design, designing the shared transformer must take into account the VCC voltage when using the maximum output voltage. At low temperatures, VCC has a slight NOSIE that will touch the OVP action.

If your product 9V-15V shares a transformer, please confirm the VCC voltage, and the power tube withstand voltage

15. Circuit debugging, Rcs and Ccs values ​​should not be too large, otherwise VDS will exceed the maximum pressure bomber.

LEB front edge blanking time is set shorter, shorter than the peak pulse time, then there is no effect or misjudgment; if Long, the real overcurrent has not played a role in protection.

The RC value of Rcs and Ccs cannot exceed 1NS's Delay. Otherwise, when the output is shorted, Vds will be higher than full load, exceeding MOSFET > Maximum pressure can cause a bomber.

Experience value 1nS Delay is approximately equal to 1K vs. 100PF, which is equal to 100R vs. 102PF

16. When drawing a small board, add a circular hole at the 90 degree corner of the small board pin.

Reason: Easy assembly

Figure:

< p align="center">

The real picture: p>

The actual assembly is as follows:

Doing this can make the board Close to the PCB board, there will be no high phenomenon

17. Circuit design, Schottky's heat sink can be connected The positive line is output so that the Schottky of the iron seal does not need insulating pads and insulating particles.

18. Circuit debugging, 15W or more Do not use 1N4007 for RCD absorption of power. Because 1N4007 is 300uS slower, the voltage drop is also 1.3V. The temperature during aging is very high and it is easy to fail.

19. Circuit debugging, the output filter capacitor's withstand voltage is less than 1.2 times the margin, avoiding damage phenomenon.

Before making this very low-level error, the 14.5V output uses a 16V withstand voltage capacitor, and the mass production has 1% of the capacitor failure.

20. Circuit design, when a large capacitor or other capacitor is made horizontal, if there is a jumper at the bottom, it should be placed at the negative potential, so jumper You don't have to wear a sleeve.

This saves money.

21.Rectifier Bridge heap, diode or Schottky,The wafer size component is acknowledged or described in the BOM, such as 67mil.

Reason: Control suppliers to deliver one-to-one delivery, avoiding suppliers cutting corners and affecting product efficiency

Another annoying thing is that suppliers do their hands and feet, resulting in a whole batch of trial production products can not pass the six-level energy efficiency, because Schottky internal wafers are caused by small.

22. Circuit design, Snubber capacitor, because of the problem of abnormal sound, the priority is to use Mylar capacitor.

One ​​of the ways to handle abnormal sounds

23. Impregnated
TDK
RFinductors and unimpregnated drum differential mode inductors,The noise generated by the immersion paint core is 12dB.

Two ways to deal with abnormal sounds

24. When the transformer is produced by vacuum dipping, it can work at a lower magnetic flux density, and the gap between the three center columns is filled with epoxy black rubber.

Three ways to handle abnormal sounds

25. Circuit design, start resistors, if used before rectification, add a string of several hundred K resistors.

Reason: The IC and MOSFET are not damaged when the resistor is shorted.

26. Circuit design, high voltage and large capacitance and a 103P ceramic capacitor position.

Reason: It has a certain effect on the radiation 30-60MHz.

If space permits, leave a place for PCB Layout to facilitate EMI rectification

27. When testing the EMS project, the maximum program of the product needs to be tested until the product is damaged.

For example, ESD lightning strikes, etc., must be damaged until the product is damaged, and make relevant records to see how much the product has, how to do it. There are several

28. Circuit design, abnormal test, short circuit open component If there is output voltage, LPS test should be carried out, the overcurrent point should not exceed 8A.

More than 8A is not allowed to apply for LPS

29. Safety sample opener, all optional plug-in components to be installed For photographing, the L, N and DC lines are fixed with the white glue of the PCB.

This is a common mistake, often one Sending samples to a third-party organization, and then going back and forth to change it to change

30. Circuit debugging, PSR required for cold machine 1.15 times the current can be turned on, SSR needs 1.3 times current to start, to avoid bad start after aging.

PSR now has a "zero recovery" OCP current for many chips, such as the ME8327N, with "zero recovery" OCP current capability

31. Circuit design, please note that the total capacity of the Y capacitor used cannot exceed 222P because of the leakage current .

For different safety regulations, the leakage current requirements are different. Please pay special attention when setting the time

32. The reverse-strike topological structure, the transformer B value needs to be less than 3500 Gauss, if the transformer is saturated, all actions will be out of control, as shown below, the above picture is Normal, the picture below is saturated.

The magnetic saturation of the transformer must be confirmed, the weight is heavy, this is the first safety performance guarantee,Including magnetic saturation of the overcurrent point, magnetic saturation at the moment of start-up, magnetic saturation of the output short circuit, magnetic saturation at high temperature, and magnetic saturation of high and low pressure.

33.Structure design, heat sink with screw fixing Refer to the following table design, the actual application should increase the amount of 0.5-1mm, refer to the following table:

The screw specifications written on the BOM must be correct, otherwise it will make you uncomfortable in mass production

34.Structural design,AC PIN wire rods need to be hook welded, if not, then the white glue is fixed.

Reason: Safety requirements

Returned samples from third parties , rectification

35. ​​Conductive rectification, segmentation processing experience, as shown below, this is only a method of processing, and some cases are not directly applicable .

36. Radiation rectification,Segmentation processing experience, as shown below, is suitable for some novice engineers, providing a reference direction, some situations are not directly applicable, the most important thing is to understand the mechanism of EMI generation.

37. On the PCB encountered problems, as shown in the figure, why is the 99SE drawing board filled with copper when filling this position? Like a dead copperlike.

D1 has this component The attributes described in the text are placed on top of the foil, as shown in the figure

After placing it on the top screen, it is perfectly solved.

< p align="center">

38. Transformer copper foil shielding is mainly for conduction, and line shielding is mainly for radiation. When conduction is very good, it is possible that you The radiation will be poor. At this time, change the copper foil shield of the transformer into a wire shield, and try to lower the position of 30M drop, so that the radiation will be much faster.

One ​​of EMI rectification techniques

39. When testing radiation, bring more brands of MOS and Schottky. Sometimes when you only have 2 or 3dB, it will be a surprise to change to a different brand.

Two EMI remediation techniques

40.VCC The rectifier diode, this has a great influence on the radiation.

A painful case, a product that has passed EMI, the balance is more than 4dB, mass production many times, one of which EMI found that the radiation is over 1dB, and the defect rate is 50%. After layer inspection, one component is exchanged. The final discovery was a problem caused by the rectifier diode on the VCC. The tube was replaced (remaining the sample) and the margin was 4 dB. For the analysis of the bad pipe, it was found that the internal supplier of the pipe was mirrored.

41. A cold knowledge,How to measure the thickness of the copper foil of the PCB?

Method: Find a smooth and long line on the PCB, measure its length L, measure the width W, and then add DC source The 1A current is measured at both ends of the pressure drop U

based on the resistivity formula, the following formula is obtained:

< Img src="http://file.elecfans.com/web1/M00/A3/0C/pIYBAF1RDamATjmdAAAEwcdyHS0977.png">

Example: Take a piece of PCB copper The foil has a length L of 40 mm and a width of 10 mm. The voltage drop across the 1 A current is 0.005 V. How many um is the thickness of the copper foil?

42. A 36Wadapter EMI rectification case, output 12V/3A, multi-picture comparison, rectification takes 3 weeks.

Transformer winding method 1: Np1→VCC→Ns1→Ns2→copper shielding 0.9Ts→Np2

PCB key layout: Y capacitor ground → large capacitor Ground, transformer ground → Vcc capacitor → large capacitor ground

Note:All outlets of the transformer are not crossed

Figure 1 (115Vac)

Figure 1 It can be seen that the situation at 130-200M is not optimistic;

130-200M is mainly due to PCB layout problems and the Schottky loop on the secondary side. Other places have little effect, and the Schottky set of beads can be completely pressed down, and the figure is forgotten to save.

In order to save costs,The company didn't let me do this, because the magnetic beads affected the cost, and immediately NG dropped the PCB layout, using the key layout of the PCB in Figure 1.

Transformer winding method is unchanged: Np1→VCC→Ns1→Ns2→copper shield 0.9Ts→Np2

PCB key layout: Y capacitor ground → transformer ground → large capacitance ground

Note: There is a cross between the primary and secondary outlets inside the transformer. p>

Figure 1 a (115Vac)

Figure 1a can be seen, change After the PCB layout, the 130M-200M has been completely attenuated, but the 30-130M does not have a good effect. It may be better to have no crossover of the transformer outlet. Carefully observe, this IC has a frequency-jittering function, and some of the peaks are cut off in the conductive part band;

Figure 1 b (230Vac)

Figure 1 b can be seen, the input voltage is 230Vac test, 65M and 83M position is a little top line (red line)

Figure 1 b- 1 (230Vac)

The primary absorption capacitor is increased from 471P to 102P, and the 65M position is pressed down a bit. The back is still a bit high, as shown in Figure 1 b-1. Show;

Figure 1 b-2 ( 230Vac)

Transformer shield changed to line shield (0.2*1*30Ts), fully attenuated behind, as shown in Figure 1 b-2;

Figure 1 b-3 (115Vac)

115Vac input test, the latter 150M is over, fuck! High pressure is good, low pressure is not good, annoyed! It seems that this can't be done;

Figure 1 b-4(115Vac)

Transformer shielding is also replaced by copper foil shielding (the number of turns is changed from 0.9Ts to 1.3Ts), the effect is good, as shown in Figure 1 b-4.

Figure 1 b-5 (230Vac)

115Vac input test, test passed .

Conclusion:

One: Transformer outlets need to be cross-over;

Two: The shorter the Y capacitor circuit trace, the better it will go back to the large capacitor ground through the transformer and not cross the other signal lines.

43. A 48W (36V/1.33A) rectification EMI case, just adjust the Schottky absorption to press 30-40M down .

115Vac low pressure 30M red top line

230Vac high pressure 30M red also top line

After adjusting Schottky absorption:

115Vac low pressure, the chart is very beautiful

230Vac high voltage, trend The picture is very beautiful

44. Safety distance list.

45. Getting started with CAD, PADS problems.

a.PADS drawn PCB exported to DXF The file, after CAD is opened, is a hollow line consisting of two lines, as shown in the figure:

When you don’t start, you can use the L command to describe one by one, sweating.

After using it multiple times, the solution is to use a X command to become a single line

b..CAD drawing line frame Transfer PADS to PCB external block diagram method:

step1. Delete any lines in CAD, leaving only the board frame, other lines can also be deleted. /p>

step2. Tap PE on the keyboard, press Enter, click on one of the mouse points, then press Y, press Enter, then K, press Enter, drag the mouse Select the entire board and press Enter. Press Esc to exit this mode.

step3. Proportional adjustment, SC Press the space to select the entire board.Press the space to click anywhere, the ratio: 39.37, press the space.

46. When drawing the PCB to define the transformer pin, it is necessary to consider whether the incoming and outgoing lines of the transformer will cross because of the The winding has a 45-90 degree intersection at the boundary, and a sleeve is added to the cross-out line to the pin foot.

47. The hotspot area of ​​the PCB must be away from the input and output terminals ,Preventing noise source from stringing to the line causes EMI to deteriorate. When it is necessary, add ground wire or other shielding methods for isolation. As shown in the figure below, a ground wire is added for effective isolation.

Note the safety distance of this ground wire.

48. Drive the resistor as close as possible to the MOS, and the current-sampling resistor should be as close as possible to the chip to avoid other unintended consequences.

PCB layout iron law

49.Share a radiation rectification case, a long strip heat sink has 2 feet, 2 feet are grounded The radiation is hard, but one of the feet is suspended, and the RF section becomes better.The reason behind the analysis is that the two feet are grounded to generate a magnetic field loop. >

This reorganization has cost a lot of money

50. with a fan Power, PCB layout should consider the wind.

Be sure to let the wind out

51. Rod-type inductor between the two legs, remember, remember, remember, prohibit the weak signal routing,Otherwise, you will not find the cause of the accident.

Remember, you have had a big loss on this before

52. Transformer core shape selection summary

a..EE, EI, EF, EEL class, commonly used to make small and medium power transformers, Low cost, simple process

b..EFD, EPC class, commonly used to make products with limited height, suitable for small and medium power class

< p style="text-indent: 2em;">c..EER, ERL, ETD class, commonly used to make large and medium power transformers, especially suitable for making multi-output high-power main transformer, and transformer leakage inductance small,It is easier to comply with safety regulations

d..PQ, EQ, LP class, the middle column of the magnetic core is larger than the general magnetic core, and the leakage inductance of the product is more Small, suitable for small size and high power transformers, the number of output groups can not be too much

e..RM, POT class, commonly used to make communication class or medium and small power High-frequency transformer, its magnetic shielding is very good, it is easy to meet the characteristics of EMC

f..EDR Class, generally used for LED drive, product thickness requirements are thin, transformer manufacturing process is complex

53. u>Components or there may be a higher potential difference between the wires, and the distance between them should be increased to avoid accidental short circuit caused by discharge.

If the distance between D and S of the high-voltage MOS on the primary side is 0.85mm according to the formula 500V, the DS voltage is below 700V. 0.9mm, considering the pollution and humidity, generally take 1.2mm

54. If the TO220 package MOS D string has magnetic beads, need to consider The T foot increases the safety distance.

I have encountered the phenomenon of the bomber before, and the safety distance is solved, because the magnetic beads are easy to get on the residue

55. Send a soil method to verify VCC, put the product in a low temperature environment (refrigerator) for a few minutes, test whether the VCC waveform voltage is triggered to the chip undervoltage protection point.

Small company equipment is not so complete, interested can be a comparison, see how big VCC difference

The design of VCC laps requires consideration of many factors.

56. Add venting holes to the PCB at the bottom of the transformer. Conducive to heat dissipation, the same is true for small boards, considering the wind path.

In the safety certification, this method can be used when the transformer temperature exceeds 2 degrees

57. When there are high voltage components next to the jumper, keep a safe distance, especially for components that are prone to movement or skew.

guarantee the stability of the product in mass production

58. When the output of the large electrolytic bottom must be jumpered, the jumper should be low voltage or Ground wire, in order to prevent over-wave soldering scalding capacitance, generally add casing.

When designing, try to avoid the jumper at the bottom of the capacitor, because increase Costs and hidden dangers

59. When the high frequency switch tube is flat on the PCB, do not place sensitive components such as chips on the other side of the PCB.

Reason: Switch is easy to interfere with the chip on the back when working, causing system instability, and other high-frequency devices are similar.

60.The output DC line should be designed to grow short when the PCB is designed, and the pad hole spacing should be small.

Reason: The length of the tail of the SR is the same length. When the two pad holes are too far apart, it will cause inconvenient production. u>Welding

61. MOS tube and transformer are far away from the AC terminal to improve EMI conduction.

Reason: The high-frequency signal will be coupled through the AC terminal, so that the noise source is detected by the EMI device to cause EMI Problem

62. The drive resistance should be close to the MOS tube.

Reason: Increase anti-jamming capability,Improve system stability

63. A PCB design routing method for a constant voltage constant current with a turn lamp and a failure case.

Please refer to the diagram for PCB design routing:

( a) Layout principle of ground line

As shown by the green line of (1)(2)(3), the ground of R11 and the ground of R14 are connected to the chip. Ground,Connect to the ground of the EC4 electrolytic capacitor. Note that it is not possible to connect 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 is strongly di /dt interference can cause factors such as system instability.

Failed Case:

Cause: When the light is turned on, the red light green light is on, and the red light green flashes alternately.

Corrective action:

Connect the ME4312B chip ground by disconnecting the PCB copper foil using a wire to the output capacitor ground , as shown below:

With the above processing, the light flash problem has been solved, 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欠压保护

0-94mA turn green light 96mA turn red light

turn lamp ratio 94/1043=9%, the turn ratio can be controlled at 3-12%< /p>

64. A SMD Capacitor response to the price increase, the chip capacitors are reserved for a plug-in location, or 104 is changed to 224P, which is relatively cheaper.

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