Safety design of power supply

Safety design of power supply

Link: Safety Design of Power Supply

1. The meaning of safety regulations

Realize the safety of electricity in application in the form of regulations;

It is an abbreviation for safety regulations (laws and regulations) and safety standards.

2. The purpose of implementing safety regulations

Reduce various injuries of electrical products to personal and property,

Including electric shock, fire, overheating, mechanical damage, etc.,

Reduce the risk of running the company.

3. Possible dangers of electronic products and equipment

1) electric shock;

2) Energy hazard (such as a short circuit between the positive and negative poles of the battery);

3) on fire;

4) Hazards related to heat;

5) Mechanical hazards;

6) chemical hazards;

7) Radiation hazard.

4. Basic principles of safety

Under normal use and single fault conditions, the device

prevent personal injury and fire hazards;

The manufacturer should provide sufficient information,

Describe all necessary conditions,

To ensure that the user will not cause danger when using the equipment according to the manufacturer's regulations;

safety-related components,

shall be certified by a recognized testing agency,

Or as an integral part of the equipment, subjected to relevant tests;

Avoid repairs caused by equipment during operation, installation, maintenance, transportation or storage,

The manufacturer shall provide the necessary instructions.

Instructions and equipment markings related to safety shall use words and logos recognized by customers.

5. The basic idea of ​​security protection

The way to provide double protection between the hazardous location and people is to adopt reasonable insulation measures:

1. Working (functional) insulation (F)

2. Basic insulation (B)

3. Supplementary insulation (S)

4. Double insulation (D)

5. Reinforced insulation (R)

6. Protective earth (PE)

Remove threats to personal safety

1. Electric shock

2. Fire

3. Scald

4. Radiation

5. Mechanical damage

6. The harm of electric shock to people

Human's Response to Electricity

1. Skin impedance: Human skin impedance changes according to the humidity of the skin. When it is dry, it reaches 3 megaohms, and when it is wet, it is about 500 ohms. The impedance of the skin will also change with the ambient temperature, fatigue, air humidity, shock, anxiety and other factors. Change.

2. Fatal current: The time of limit current that 99% of healthy men's heart can pass is: (that is, fatal current and time).

7. Dangerous voltage

1. IEC 60950-1 office environment 60Vdc, 42.4Vpeak, 30Vac

2. IEC 60950-22 outdoor environment 30Vdc, 21.2Vpeak, 15Vac

8. People's stimulation performance to temperature

1. Muscle tissue will be destroyed at 50°C; nerve tissue will be destroyed at lower temperature;

Brain tissue will be destroyed at 42°C;

2. When the skin temperature reaches 44°C, after 6 hours, the body's thermoregulation function will be lost and the cells will be destroyed.

When the temperature is between 44°C and 51°C, every 1°C increase will double the speed of cell destruction. When it reaches 80°C, it only takes 1 second to completely destroy the cells.

3. Common temperature of electrical products: PN junction = 150°C, PCB material 130°C;

4. When the wire and cable are overloaded (3 times the rated current), the metal surface temperature is about 400°C;

5. 0.5A current can maintain the arc, the temperature is 2000℃~3000℃, and the maintenance voltage is 20V;

9. According to the environment in which the equipment is located, determine its pollution level

Pollution degree 1 - sealed environment or similar;

Pollution degree 2 - office environment or similar;

Pollution degree 3 - factory environment or similar;

10. Determination of safety distance

Safe distance includes

Clearance (space distance), creepage distance (creep distance) and insulation penetration distance

1. Cl-clearance: The shortest distance measured along the air between two adjacent conductors or a conductor and the surface of the adjacent motor casing.

2. Creepage distance (cr-creepagedistance): The shortest distance between two adjacent conductors or a conductor and the surface of the adjacent motor casing measured along the insulation surface.

Determination of electrical clearance:

According to the measured working voltage (peak working) and insulation level, the distance can be determined

Generally speaking, cr≥cl, the requirements of both must be met when wiring (that is, the distance of the surface and the distance of the space must be considered), **
slotting (slot width should be greater than 1mm) can only increase the surface distance That is, the creepage distance cannot increase the electrical clearance, so when the electrical clearance is not enough, slotting cannot solve this problem. When slotting, pay attention to whether the position and length of the slot are appropriate to meet the requirements of the creepage distance.

The relationship between electrical clearance and altitude and atmospheric pressure:
electrical clearance altitude correction coefficient

Creepage distance: mainly determined by effective value or DC working voltage;

1. If there is no slot between parts: according to the working voltage, pollution level, overvoltage level,

  材料类别、绝缘类型进行查表；即为相应的电气间隙和爬电距离；     

2. There are slots between parts: slots have no effect on electrical clearances, but only affect creepage distances;

Determination of electrical clearance:

According to the measured working voltage and insulation level, the distance can be determined

Usually:
AC part of the primary side: L-N ≥ 2.5mm in front of the fuse, LN PE (earth) ≥ 2.5mm,
there is no requirement after the fuse device, but keep a certain distance as far as possible to avoid short circuit damage to the power supply.

Primary side AC to DC part ≥ 2.0mm

Primary side DC ground to ground ≥ 2.5mm (primary side floating ground to ground)

Part of the primary side to the part of the secondary side ≥ 4.0mm, components connected between the primary and secondary sides

The electric gap of the secondary side part should be ≥0.5mm

Secondary side ground to ground ≥ 1.0mm

Note: Before determining whether the requirements are met, a force of 10N should be applied to the internal parts, and a force of 30N should be applied to the outer shell to reduce the distance, so that the
space distance still meets the regulations in the worst case.

Determination of creepage distance:

usually:

(1) AC part of the primary side: L-N ≥ 2.5mm in front of the fuse, LN ground ≥ 2.5mm,
there is no requirement after the fuse, but try to keep a certain distance to avoid short circuit damage to the power supply.

(2), primary side AC to DC part ≥ 2.0mm

(3), primary side DC ground to ground ≥ 4.0mm such as primary side ground to earth

(4) Primary side to secondary side ≥ 6.4mm, such as optocoupler, Y capacitor and other components with a pitch of ≤ 6.4mm to be slotted.

(5) ≥ 0.5mm between the secondary side parts

(6), the secondary side to the ground ≥ 2.0mm or more

(7), between two stages of transformer ≥ 8.0mm or more

The isolation distances on components and PCB boards are as follows: (the following values ​​do not include margins)
a. For AC-DC power supplies (take no PFC circuit and the input rated voltage range is 100-240V~ as an example)
electrical clearance creepage distance
L line-N line (before the fuse) 2.0mm 2.5mm
input-ground (before the rectifier bridge) 2.0mm 2.5mm
input-ground (behind the rectifier bridge) 2.2mm 3.2mm
input-output (transformer) 4.4mm 6.4mm
input- Output (except transformer) 4.4mm 5.5mm
input-core, output-core 2.0mm 2.5mm

b. For AC-DC power supply (take PFC circuit and input rated voltage range of 100-240V~ as an example), the
clearance creepage distance
L line-N line (before the fuse) 2.0mm 2.5mm
input-ground (rectifier bridge Front) 2.0mm 2.5mm
input-ground (rectifier bridge) 2.2mm 3.2mm
input-output (transformer) 5.2mm 9.0mm
input-output (except transformer) 4.4mm 6.4mm
input-core, output-core 2.2 mm 3.2mm

c. For DC-DC power supply (take the input rated voltage range of 36-76V as an example), the
creepage distance of the air gap
(DC+)-(DC-) (before the fuse) 0.7mm 1.4mm
input-ground (before the fuse) 0.7mm 1.4mm
input-ground (after the fuse) 0.9mm 1.4mm
input-output (considered as basic insulation) 0.9mm 1.4mm
input-output (considered as reinforced insulation) 1.8mm 2.8mm
input-magnetic core, output-magnetic Core 0.7mm 1.4mm

A more accurate method is to check the table according to the safety regulations: working voltage, pollution level, overvoltage level, material category, and insulation type;

The clearance and creepage distances are then calculated according to the altitude correction factor;

11. Requirements for safety devices - X, Y capacitors

The X and Y capacitors must meet the voltage requirements used;

X, Y capacitors must meet the temperature requirements

The X and Y capacitors used on the primary side of AC/DC must be safety capacitors

1. Definition of safety capacitor

The discharge of safety capacitors is different from ordinary capacitors. Ordinary capacitors will retain their charge for a long time after the external power supply is disconnected. If you touch them with your hands, you will be charged, but safety capacitors do not have this problem.

At the input end of the AC power supply, it is generally necessary to add 3 safety capacitors to suppress EMI conduction interference.

2. Classification

Safety capacitors are divided into x-type and y-type. AC power input is divided into 3 terminals: live wire L/neutral wire N/ground wire G, (L=Line, N=Neutral, G=Ground). Between "LN", that is, between "live wire-neutral wire" is the X capacitor; across "LG/NG", that is, between "live wire-ground wire or neutral wire-ground wire" is Y capacitance. A capacitor connected between the live wire and the neutral wire is like an "X", and a capacitor connected between the live wire and the ground wire is like a "Y". These are not classified according to the material.

X capacitor: Since the location of this capacitor connection is also critical, it also needs to meet relevant safety standards. The X capacitor is also one of the safety capacitors. According to actual needs, the capacitance of the X capacitor is allowed to be larger than that of the Y capacitor, but at this time a safety resistor must be connected in parallel at both ends of the X capacitor to prevent the power cord from being damaged due to the charging and discharging process of the capacitor. Cause the power cord plug to be charged for a long time. The safety standard stipulates that when the power cord of a working machine is unplugged, within two seconds, the voltage (or ground potential) at both ends of the power cord plug must be less than 30% of the original rated working voltage.

As one of the safety capacitors, X capacitors are also required to obtain certification from safety testing agencies. X capacitors are generally marked with safety certification marks and withstand voltage AC250V or AC275V, but their real DC withstand voltage is as high as 2000V or more. Do not use ordinary capacitors with a nominal withstand voltage of AC250V or DC400V as substitutes. Usually, X capacitors use polyester film capacitors with relatively large ripple currents. This type of capacitor has a large volume, but it allows a large current for instantaneous charging and discharging, and its internal resistance is correspondingly small.

X capacitors are divided into X1, X2, X3, the main difference is:

1. X1 high voltage resistance greater than 2.5 kV, less than or equal to 4 kV,

At present, the demand for this kind of capacitor is not too much, and it is expected to become a demand trend in the future.

2. X2 high voltage resistance is less than or equal to 2.5 kV,

This kind of X2 capacitors currently has different certification standards in international applications, but they are all in line with domestic and foreign approvals. Please rest assured to use them, and you cannot indicate that other safety certificates cannot be used due to wrong guidance. The main reasons for this are:

Because the AC output and input of some countries are different,

For example: 110VAC in Japan, 220VAC in China and so on.

The current nominal voltages in the industry are: 250VAC/275VAC/280VAC/300VAC/305VAC/310VAC/315VAC/330VAC,

There are also Farah's 440VAC, etc.,

This is an industry international standard stipulated according to different needs;

There are 21-day and 56-day implementation standards for steady-state humid heat, both of which are in line with international standards;

There are C-level and B-level flame retardant grades, which are also in line with international test grade standards;

The temperature standard for capacitors is 100 degrees and 110 degrees, which is also in line with the international temperature standard for performing tests.

3. X3 high voltage resistance is less than or equal to 1.2 kV

At present, there are relatively few safety capacitors using X3 in various industries, and the X2 standard is generally used.

Y capacitor:

The connection position of these two Y capacitors is more critical, and must comply with relevant safety standards to prevent leakage of electronic equipment or electrification of the casing, which is likely to endanger personal safety and life.

They are all safety capacitors, which require that the capacitance value cannot be too large, and the withstand voltage must be high.

In general, for machines working in the subtropical zone, the ground leakage current should not exceed 0.7mA;

For machines working in temperate zones, the ground leakage current should not exceed 0.35mA.

Therefore, the total capacity of Y capacitors generally cannot exceed 4700PF (472).

The capacitance of the Y capacitor must be limited,

So as to achieve the control under the action of rated frequency and rated voltage,

The magnitude of the leakage current flowing through it and the purpose of affecting the EMC performance of the system.

GJB151 stipulates that the capacity of the Y capacitor should not be greater than 0.1uF.

In addition to complying with the corresponding grid voltage withstand voltage, the Y capacitor

It is also required that such capacitors have sufficient safety margins in terms of electrical and mechanical properties,

Avoid breakdown and short circuit phenomenon under extremely harsh environmental conditions,

The withstand voltage performance of the Y capacitor is of great significance to the protection of personal safety.

Specially point out:

As a safety capacitor, the Y capacitor must be certified by a safety testing agency.

The appearance of Y capacitors is mostly orange or blue.

Generally, they are marked with safety certification marks (such as UL, CSA, etc.) and the words AC250V or AC275V.

However, its true DC withstand voltage is as high as 5000V or more.

It must be emphasized that the Y capacitor should not be replaced by ordinary capacitors such as AC250V or DC400V with a nominal withstand voltage.

3. Why can safety capacitors achieve rapid discharge?

The current that allows instantaneous charging and discharging is also very large, and its internal resistance is correspondingly small. The X capacitor is usually plastic-packed with an outer square and an inner high-voltage OPP (metallized polypropylene material is wound and processed). Because of the small internal resistance, the OPP material not only has better electrical performance, but also can effectively reduce the impact of high-frequency pulses on the power supply by connecting it in parallel with the input end of the power supply.

12. Safety Capacitor

1. Definition of safety capacitor

   安规电容器是行业对抑制电源电磁干扰用固定电容器的俗称，因为该类电容符合安全规范、且通过安全规范测试认证，同时其本体印刷有多个国家的安全认证LOGO标志，故而称为安规电容器。此类电容在实际应用中的“安规”表现在：即使电容器失效后，也不会导致 电击，不危及人身安全；此外，它采用阻燃材料制造，顶多会爆炸（只是炸裂，没有火产生，只产生气体），然后就是短路，不会导致火灾发生。 

figure 1

   安全规范（简称“安规”）安全规范对产品的装置与电子组件有明确的陈述及指导，以避免由于设计不良或使用不当而导致电击、能量（打火/拉弧/爆炸）、火灾、辐射、机械与热/高温危险、化学危险等事故和灾害，要求生产厂商尽可能给终端使用者提供具有安全、高品质的产品，保护使用者与操作者生命、财产安全。

2. Function & Classification

   安规电容通常只用于抗干扰电路中的滤波作用。它们用在电源滤波器里，起到电源滤波作用，分别对共模，差模干扰起滤波作用。例如：当在电源跨线电路中使用电容器来消除噪音时，不仅仅只有正常电压，还必须考虑到异常的脉冲电压（如闪电）的产生，这可能会导致电容器冒烟或者起火。所以，跨线电容器必须使用安规电容。通常，出于安全考虑和EMC考虑，一般在电源入口建议加上安规电容。

   交流安规电容在电路的位置及作用：﹙1﹚Across-the-line接线间；﹙2﹚Antenna-coupling天线耦合；﹙3﹚Line-bypass  旁路。 

  根据IEC60384-14，电容器分为X电容和Y电容，如（图2）所示：X电容是指跨于L-N之间的电容；Y电容是指跨于L-G/N-G之间的电容。

figure 2

  注：（L=LINE    N=NEUTRAL     G=GROUND）     

  即：火线-零线间的是X电容，火线或零线与地线间的是Y电容。

  备注说明：一般作安规测试时，元件需同时通过X电容和Y电容的测试标准。例  如：X1/X2代表元件同时符合X电容和Y电容的测试标准。

2.1 X capacitance

 根据安全等级，X电容可以分为3类：

X capacitors are generally marked with safety certification marks and withstand voltage AC250V or AC275V, but it can be seen from the above table that its real DC withstand voltage is at least 2500V (X2). So when using it, don't use ordinary capacitors with a nominal withstand voltage of AC250V or DC400V as substitutes.

   通常，X电容多选用纹波电流比较大的聚脂薄膜类电容。这种类型的电容体积较大，但其允许瞬间充放电的电流也很大，而其内阻相应较小。普通电容纹波电流的指标都很低，动态内阻较高。用普通电容代替X电容，除了电容耐压无法满足标准之外，纹波电流指标也难以符合要求。

  根据实际需要，X电容的容值允许比Y电容的容值大。但此时必须在X电容的两端并联一个安全电阻，防止拔掉电源线时由于该电容的放电过程太慢而致电源线插头长时间带电。安全标准规定，当正在工作之中的机器电源线被拔掉时，在两秒钟内，电源线插头两端带电的电压（或对地电位）必须小于原来额定工作电压的30%。

2.2 Y capacitance

According to the safety level, Y capacitors can be divided into 4 categories:

Y capacitors are mostly orange or blue in appearance, and are generally marked with safety certification marks (such as UL, CSA, etc.) and withstand voltage AC250V or AC275V. However, as can be seen from the above table, its true DC withstand voltage is as high as 5000V (Y2). It must be emphasized that the Y capacitor should not be replaced by ordinary capacitors such as AC250V or DC400V with a nominal withstand voltage.

   Y电容的电容量必须受到限制，从而达到控制在额定频率及额定电压作用下，流过它的漏电流的大小和对系统EMC性能影响的目的。GJB151规定Y电容的容量应不大于0.1uF。一般情况下，工作在亚热带的机器,要求对地漏电电流不能超过0.7mA；工作在温带机器，要求对地漏电电流不能超过0.35mA。因此，Y电容的总容量一般都不能超过4700PF（472）。

   Y电容除符合相应的电网电压耐压外，还要求这种电容器在电气和机械性能方面有足够的安全余量，避免在极端恶劣环境条件下出现击穿短路现象，Y电容的耐压性能对保护人身安全具有重要意义。

   Y1属于双绝缘Y电容，用于跨接一二次侧；Y2则属于基本单绝缘Y电容，用于跨接一次侧对地保护即FG线。

2.3 The difference between X capacity and Y capacity

   X电容抑制差模干扰，Y电容抑制共模干扰。

  基于漏电流的限制，Y电容值不能太大，一般X电容是uF级，Y电容是nF级。

   X电容常采用金属化聚丙烯薄膜电容器（MPX）；通常，X电容多选用耐纹波电流比较大的聚脂薄膜类电容。这种类型的电容体积较大，但其允许瞬间充放电的电流也很大，而其内阻相应较小。X电容采用塑封、方形结构和内部高压OPP材料（金属化聚丙烯材料卷绕加工而成），OPP材质不但有更好的电气性能，而且与电源的输入端并联可以有效的减小高频脉冲对电源的影响。Y电容常采用高压瓷片的。

3. Precautions for using safety capacitors

   X、Y电容必须满足使用的电压要求，对于超过标称电压的，可以选择串联使用来均压，但是必须使用完全一致的电容串联。

   X、Y电容必须满足温度要求。对于Y电容，UL认证时的温度最高只能为85度，这是由于UL标准最高只进行85度的测试；但是欧洲认证的温度往往较高，目前UL同意采用欧洲认证的Y电容温度作为最高的使用温度。

   考虑电路输入端的可能的脉冲电压的大小，不同的电容承受脉冲电压的大小不同，电容的耐压不能小于电路中的脉冲电压的峰值。例如：UPS的输入端，考虑过电压等级III，一般来说脉冲电压峰值为4000V，所以输入端只能选择X1、Y1、Y2电容。

   Y1、Y2电容作为安全电容的X、Y电容，要求必须取得安全检测机构的认证。电容外观一般都标有安全认证标志（如UL、CSA 等标识）和耐压AC250V 或AC275V字样字样。必须强调，X、Y 电容不得随意使用标称耐压AC250V 或者DC400V之类的普通电容来代用。

4. Selection of safety capacitors

    对于逆变器中X、Y电容的选择，我们需要考虑所要求的绝缘要求，如基本绝缘、附加绝缘、双重绝缘、加强绝缘等。在不同的绝缘要求中，电容的选择也是存在区别的。

   根据电路的工作电压，过电压等级选择合适的安规电容。

5. Silk screen reading of safety capacitors

Silkscreen

annotation:

1 is the manufacturer's trademark (LOGO)

2 is the manufacturer's type body (MKP) and capacitor certification category (X2 level)

3 is the nominal capacity of the capacitor (for RC components, the nominal resistance value is required)

● E6 number series is preferred for safety capacitor capacity:

 1、1.5、2.2、3.3、4.7、6.8的及其10进倍数

● E12 number system can also be selected:

 1、1.5、1.8、2.2、2.7、3.3、3.9、4.7、5.6、6.8、8.2

4 is the capacity tolerance

●K grade (±10%); J grade (±5%); M grade (±20%)

(The default tolerance of IEC60384-14 standard is ±20%, and K level is generally selected)

5 is the rated voltage and the nature of the power supply (the AC voltage can be represented by the symbol ~)

● Rated voltage values ​​for general use: 250VAC, 275 (280) VAC, 305 (310) VAC, 440VAC, 760VAC.

● The rated voltage of the capacitor should be equal to or greater than the connected power supply voltage, and the design of the capacitor should consider that the system voltage is 10% higher than its nominal voltage

6 for climate categories (40/105/21)

● The lower limit temperature is -40; the upper limit temperature is +105, and the duration of the steady-state damp heat test is 21 days

According to the IEC60384-14 standard, the duration of the steady-state damp heat test is divided into 21 days and 56 days, and the default is 21 days. The general climate category is followed by a letter to indicate the flame retardant level. The flame retardant grade is carried out by needle flame burning test, which is divided into three grades: A\B\C, and IEC60384-14 standard allows C grade by default.

7 is the safety certification that the capacitor complies with.

8 Implementation standards for the manufacture of this capacitor

6. Appendix: common safety signs