“With the rapid development of the embedded industry, power requirements in various industry applications are getting higher and higher. In order to ensure the stability of the system, isolated power supplies have emerged. But what is the key index in isolated power supply-isolation voltage? What is the relationship with the creepage distance? This article will reveal the principle of isolated power supply for you.
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With the rapid development of the embedded industry, power requirements in various industry applications are getting higher and higher. In order to ensure the stability of the system, isolated power supplies have emerged. But the key indicator in the isolated power supply-what is the isolation voltage? What is the relationship with the creepage distance? This article will reveal the principle of isolated power supply for you.
With the rapid development of the microelectronics industry, the electromagnetic environment where products are used has become more and more complex, and the stability of products has also been greatly affected. Embedded product production companies add various isolation devices or isolation circuits to their products to reduce interference at the work site and enhance equipment stability.
The power supply, as the energy supply part of the embedded device, is the prerequisite for the stable operation of the product. Power isolation is particularly important, and the application of power isolation modules has also become a must for embedded device design.
In industrial equipment, the power supply between the two equipment is required to be isolated. A DC converter with a transformer is used to separate the two power supplies to make them independent of each other, thereby reducing external interference!
1. Introduction of isolation withstand voltage and creepage distance of isolated power supply
1. Introduction to the isolation withstand voltage of isolated power supply
Figure 1 Internal block diagram of isolated power supply
As shown in Figure 1, the internal block diagram of the isolated DC-DC power supply module.
Isolation withstand voltage refers to the highest insulation voltage that two systems that are not directly electrically connected can withstand.
The use of power isolation is different, and the corresponding parameter selection is not necessary. For example: AD-DC power isolation, general industrial occasions require isolation withstand voltage between 3000VAC to 4000VAC; DC-DC power isolation general industrial occasions require 1000VDC to 2000VDC, which may be higher in special industries, and the medical industry requires 6000VDC.
First, distinguish the units of various voltage indicators. The common ones are ADC, VAC and RMS, as shown below.
VAC/VDC refers to AC voltage and DC voltage respectively, but AC and DC cannot be simply converted in isolation withstand voltage. For example, 3000VAC has an amplitude voltage of 4242V, but in practical applications, isolation withstand voltage 3000VAC and 4242VDC is not equivalent . The specific reasons include two points. First, for the isolation module, there is an isolation capacitor between the input and output. For the DC signal, the impedance of the capacitor is infinite, so the size of the isolation capacitor does not have much influence on the DC signal, but for the AC signal There will be a greater impact, manifested in the leakage current will increase, or directly exceed the standard, the system alarms. Second, another difference between AC and DC is that frequency will affect the dielectric constant of the insulating medium, and frequency will cause the dielectric constant of the insulating medium to decrease. Generally, the higher the dielectric constant, the stronger the insulating ability.
RMS refers to the true effective value. In simple terms, it means that AC power is equivalent to the work done by DC power in a unit of time. That is, AC power with a true effective value of 10V and DC power with 10V do the same work for the same load at the same time. This unit is usually not used as a measurement unit for isolation withstand voltage.
2. Introduction to creepage distance of isolated power supply
How to ensure the stability and safety of the isolation withstand voltage and ensure that the isolated power module is not broken down, we must calculate the creepage distance. The shortest spatial distance between two conductive parts, or between a conductive part and the equipment and easily-contacted surface measured along the surface of the insulating material. The distance along the insulating surface is the distance of discharge along the insulating surface, that is, the leakage distance, also called creepage distance, creepage distance = surface distance /The highest voltage of the system varies according to the degree of contamination.
Figure 2 Schematic diagram of creepage distance
In the IEC60950 and GB4943-2011 standards, the minimum safety distance required for different voltage levels is specified, and the safety distance includes electrical distance and creepage distance. For the switching power supply, where the minimum safety distance is mainly required to be guaranteed, there are the following two aspects:
l The safety distance between the primary circuit and the enclosure (protective ground);
l The safety distance between the primary circuit and the secondary circuit.
The advantages and disadvantages of isolated power supply compared to non-isolated power supply are shown in Figure 3.
Figure 3 Comparison of isolated power supply and non-isolated power supply
Second, the application of isolated power supply
Various interferences in the application of embedded products will threaten the stability of the product. In the product design stage, choosing a good isolated power supply module and isolated communication module can effectively shield the interference caused by the use environment and affect the stability of the product. Work brings security.
We often see news reports in the media that mobile phone charging explodes, and the phone is electrocuted while charging. The isolated power module can ensure the stable operation of the product, and more importantly, protect the life and property of the user.
There are two ways to isolate the power supply at present, the device is built separately and the DC-DC power module is used. The comparison is shown below.
1. Scheme selection
After the product performance requirements are a little clear, the next step is to start design and development. The first thing to do is to select the circuit scheme. Here are some more common “negative teaching materials” for everyone.
For example, when designing and developing a mains AC input to DC output, many people think of adopting a power frequency conversion circuit scheme for the first time, because this scheme is relatively simple, a power frequency transformer, plus a rectifier filter can be done, as follows Shown in Figure 4. The efficiency of the products using this scheme is very low, and the volume of the product will be very large, and the application will be accompanied by very disturbing power frequency eddy current sound. The module scheme selects a suitable transformer, and multiple processes ensure the consistency of the transformer product and ensure the final performance of the product.
Figure 4 Comparison of transformer solutions
2. Material selection and PCB design
After the circuit plan is determined, the next step is to design the product performance parameters. The Electronic components in the circuit plan must be parameterized, calculated, and structured material selection. In this link, many trade-offs must be made.
First, the selection of materials. Professional module power supply manufacturers can achieve both. According to the different specification requirements of the product and different application conditions, unnecessary material specifications are discarded and the best required material specifications are selected.
Second, the design of the PCB of the power supply module. Because the module power supply product has the PCB design specification requirements of the module power supply, it must consider the heat dissipation design, EMC design, interference design and production process design, etc. The content involved is not too much, so the PCB design is used in the development process of the module power supply product. One of the most important links to be treated, as shown in Figure 5.
Figure 5 PCB design requirements
ZHIYUAN Electronics isolated DC-DC power supply can provide stable and reliable driving voltage for user systems, and effectively solve the problem of power supply instability caused by static electricity and surges. It is an ideal solution for power supply for sub-systems such as data acquisition and communication on the board. The isolation voltage level covers 1000VDC-6000VDC, meeting the requirements of all working conditions.
Compared with traditional designs, ZHIYUAN Electronics constant-voltage series of isolated power modules adopt the best solutions in the industry from topology, chip solutions to components, with higher integration and reliability, and can isolate user I/O and communication. And other applications provide standard and reliable solutions.
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