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Inside the insulation design of switching power supply

The current flowing through the body can cause the physiological reaction of the body, the strong reaction depends on the current magnitude, duration, the path through the body.

Only the current of 0.5mA, can impact on the health of the human body, and may cause indirect harm.

More current may cause direct harm to the human body, such as burns or ventricular fibrillation.

Generally speaking, in dry conditions, the peak voltage is less than 40V or 60V DC voltage, usually regarded as not dangerous.

However, the use must be touched or hands exposed parts should be protected or received is to be properly handled.

In order to prevent the body (the operator or repair personnel) shocks, in the design of switching power supply, to comply with the relevant industry safety design standards, such as IEC60950, G4943 and other national standard; in these standards, the switching power supply in different positions the insulation requirements, to ensure the safety of operating personnel.

Two types of insulation

The function (Functional Insulation): the function of insulation insulation purposes, only in maintaining the product to normal operation, does not have any security function.

This insulation is normally used in the same line of two conductors, there is no security isolation requirements section.

For example, PWB green, plastic shell of the electrolytic capacitor is insulation function.

Basic insulation (Basic Insulation): basic insulation is to provide a basic protection against electric shock, to avoid the risk of electric shock, but this kind of insulation can only guarantee the normal state of security, but can not guarantee the transient voltage appears safe, in other words, when the transient voltage occurs, then the basic insulation there may be collapse.

Supplementary insulation (Supplementary Insulation): basic insulation, insulation and additional, is when the basic insulation failure, provide another layer of insulation function.

Generally speaking, as a supplement to the insulation requirements and basic insulation is, so between the two roles are interchangeable.

For example, a cable insulation layer two, we can say that the inner layer is the basic insulation, insulation layer is added, which in turn can be said.

We must pay attention to a little, that is the supplementary insulation must be established in the presence of basic insulation.

Double insulated (Double Insulation): This refers to the basic and supplementary insulation insulation contains both insulation.

As long as you use the right place, the insulation can provide adequate security, there will be danger of electric shock.

(Reinforce Insulation): strengthening insulation which provides insulation degree with double insulation, but double insulation is different, it is not easy to be divided into two parts of basic insulation and insulation, it may be an integral forming spacer, or is composed of many insulating spacer.

Three electrical clearance and creepage distance

Clearance: is the shortest space between two conductive parts or conductive parts and equipment interface measured distance.

This distance measure is not limited by what way.

For example, a product for use in insulating materials when the shell, holes or gaps in this case should be considered as a conductor, a layer of aluminum foil on the shop, still must maintain a certain distance and dangerous parts inside, because these places are easy to reach by the user.

Creepage distance: refers to the shortest distance between two people along the surface of the insulation measured conductive parts or conductive parts and equipment protection interface.

In general, if not the pollution level was 1, is usually greater than the clearance and creepage distance.

Electrical clearance and creepage distance and working voltage, pollution level, altitude, and other relevant insulating materials.

The actual design of the electrical gap on PCB and climbing requirements for electric distance, according to the need of insulation requirements, working voltage, pollution level, altitude, insulation and other details, calculate the look-up table according to the standard requirements.

Altitude: according to the law into a harmonious bath, electric breakdown voltage and gas pressure and the distance between the poles is proportional to the product, when the distance between the poles is fixed, the smaller the pressure is, the breakdown voltage is lower.

The pressure and altitude is inversely proportional to the altitude, it is one of the important factors affecting the spatial distance.

The pollution level: if the space between the two conductor with pollutants, will also affect the insulation degree.

The general pollution standards will be divided into four grades:

The pollution level of pollutants or 1: did not only dry and non pollutant conductivity.

You can use a closed shell, or the surface of the circuit board coating, can meet the level of pollution

Level 2: pollution refers to occasional conductivity of pollutants, such as water vapor condensation.

Only non-conductive pollution, but must take into account the accident due to the transient conductivity caused by pollution.

The pollution level of 3: the conductivity due to pollution, or expected to gel non-conductive pollution becomes conductive.

Pollution level 4: persistent pollution caused by electrical conductivity, such as conductive dust or pollution caused by rain and snow.

Unless otherwise stipulated in the relevant product standard, industrial electrical equipment selection for general pollution level 3 environment, household and similar electrical appliances general selection of environmental pollution grade is 2.

As for the electrical clearance between a circuit and a circuit and the two circuit requirements, IEC60950-2001 is such a requirement.

Four example

Below is a schematic diagram of the key parts of the power supply structure of a typical AC-DC, including part of the rectified AC input, high voltage DC part, DCDC conversion part, the operation may come into contact with the secondary side circuit protection, etc..

Below this figure as an example of a simple switching power supply in practical design, calculation of different parts between the clearance and creepage distance.

For each position in the circuit of the earth, the AC input for L\N, after the rectification of high voltage direct current to the earth, the primary side of the earth, on the earth side, all need to use basic insulation; for a DC-DC line inside the device, because there is no risk of electric shock, only need to use the function of insulation, ensure the normal work circuit, reduce the breakdown, sparking risk; for a circuit on the two circuit, such as E B\C, must adopt reinforced insulation, prevent the personnel to the two circuit in contact, a circuit has the risk of causing electric shocks.