“A 23-year-old flight attendant of China Southern Airlines was electrocuted while talking on her iPhone5 while it was charging”, the news has attracted wide attention online. Can chargers endanger lives? Experts analyze the transformer leakage inside the mobile phone charger, 220VAC alternating current leakage to the DC end, and through the data line to the metal shell of the mobile phone, and eventually lead to electrocution, the occurrence of irreversible tragedy.
So why does the output of the mobile phone charger come with 220V AC? What should we pay attention to in the selection of isolated power supply? How to distinguish between isolated and non-isolated power supplies? The common view in the industry is:
1. Isolated power supply: There is no direct electrical connection between the input loop and the output loop of the power supply, and the input and output are in an insulated high-resistance state without a current loop, as shown in Figure 1:
2, non-isolated power supply: there is a direct current loop between the input and output, for example, the input and output are common. An isolated flyback circuit and a non-isolated BUCK circuit are taken as examples, as shown in Figure 2.Figure 1 Isolated power supply with transformer
1.The advantages and disadvantages of isolated power supply and non-isolated power supply
According to the above concepts, for the common power supply topology, the non-isolated power supply mainly includes Buck, Boost, buck-boost, etc. The isolation power supply mainly has a variety of flyback, forward, half-bridge, LLC and other topologies with isolation transformers.
Combined with commonly used isolated and non-isolated power supplies, we can intuitively get some of their advantages and disadvantages, the advantages and disadvantages of the two are almost opposite.
To use isolated or unisolated power supplies, it is necessary to understand how the actual project needs power supplies, but before that, you can understand the main differences between isolated and unisolated power supplies:
① The isolation module has high reliability, but high cost and low efficiency.
②The structure of the non-isolated module is very simple, low cost, high efficiency, and poor safety performance.
Therefore, in the following occasions, it is recommended to use isolated power supply:
① Involving possible electric shock occasions, such as taking electricity from the grid to low-voltage DC occasions, need to use isolated AC-DC power supply;
② The serial communication bus transmits data through physical networks such as RS-232, RS-485 and controller local area network (CAN). Each of these interconnected systems is equipped with its own power supply, and the distance between the systems is often far away. Therefore, we usually need to isolate the power supply for electrical isolation to ensure the physical security of the system. By isolating and cutting off the grounding loop, the system is protected from the transient high voltage impact and the signal distortion is reduced.
③ For external I/O ports, to ensure the reliable operation of the system, it is recommended to isolate the power supply of the I/O ports.
The summarized table is shown in Table 1, and the advantages and disadvantages of the two are almost opposite.
Table 1 Advantages and disadvantages of isolated and non-isolated power supplies
2，The choice of isolated power and non-isolated power
By understanding the advantages and disadvantages of isolated and non-isolated power supplies, each has its own advantages, and we have been able to make accurate judgments about some common embedded power supply options:
① The power supply of the system is generally used to improve the anti -interference performance and ensure reliability.
② Power supply of IC or part of the circuit in the circuit board, starting from cost -effective and volume, preferential use of non -isolation schemes.
③ For safety requirements for security, if you need to connect the AC-DC of the Municipal Electricity, or the power supply for medical use, in order to ensure the safety of the person, you must use the power supply. In some occasions, you must use the power supply to strengthen the isolation.
④ For the power supply of remote industrial communication, in order to effectively reduce the effects of geographical differences and wire coupling interference, it is generally used for separate power supply to power each communication node alone.
⑤ For the use of battery power supply, non -isolation power supply is used for strict battery life.
By understanding the advantages and disadvantages of isolation and non -isolation power, they have their own advantages. For some commonly used embedded power supply design, we can summarize the occasions of its choice.
1. Isolation power supply
In order to improve the anti -interference performance and ensure reliability, it is generally used to use isolation.
For safety requirements for security, if you need to connect to the AC-DC of the Municipal Electricity, or the power supply for medical use, and white appliances, in order to ensure the safety of the person, you must use the power supply, such as the MPS MP020, for the original feedback AC- DC, suitable for 1 ~ 10W applications;
For the power supply of remote industrial communications, in order to effectively reduce the effects of geographical differences and wire coupling interference, it is generally used for separate power supply to power each communication node alone.
2. Non -isolation power supply
The IC or some circuit in the circuit board is powered by the price ratio and volume, and the non-isolation solution is preferred; such as the MPS MP150/157/MP174 series buck non-isolation AC-DC, suitable for 1 ~ 5W;
For the case of working voltage below 36V, the battery is used to supply power, and there are strict requirements for endurance, and non -isolation power supply is preferred, such as MPS’s MP2451/MPQ2451.
The advantages and disadvantages of isolation power and non -isolation power supply
By understanding the advantages and disadvantages of isolation and non -isolation power supply, they have their own advantages. For some commonly used embedded power supply choices, we can follow the following judgment conditions:
For safety requirements, if you need to connect to the AC-DC of the Municipal Electricity, or the power supply for medical, in order to ensure the safety of the person, you must use the power supply, and some occasions must be used to enhance the isolation power supply.
Generally, the requirements for module power isolation voltage are not very high, but higher isolation voltage can ensure that the module power supply has a smaller leakage current, higher security and reliability, and EMC characteristics is better. Therefore The general isolation voltage level is above 1500VDC.
3, precautions for the selection of isolation power module
The isolation resistance of the power supply is also called anti-electricity strength in the GB-4943 national standard. This GB-4943 standard is the security standards of information equipment we often say, to prevent people from being physical and electrical national standards, including avoiding avoidance Humans are damaged by electric shock damage, physical damage, explosion. As shown below, the structure diagram of the isolation power supply.
Isolation power structure diagram
As an important indicator of the module power, the standard of isolation and pressure -resistant testing method is also stipulated in the standard. Generally, the equal potential connection test is generally used during simple testing. The connection schematic diagram is as follows:
Significant diagram of isolation resistance
Test Methods:
Set the voltage of the voltage resistance to the specified voltage resistance value, the current is set as the specified leakage value, and the time is set to the specified test time value;
Operating pressure meters start testing and start pressing. During the prescribed test time, the module should be unpattered and free of fly arc.
Note that the welding power module should be selected at the time of testing to avoid repeated welding and damage the power module.
In addition, pay attention:
1. Pay attention whether it is AC-DC or DC-DC.
2. The isolation of the isolation power module. For example, whether the 1000V DC meets the insulation requirements.
3. Whether the isolation power module has a comprehensive reliability test. The power module should be performed by performance testing, tolerance testing, transient conditions, reliability testing, EMC electromagnetic compatibility test, high and low temperature testing, extreme testing, life testing, security testing, etc.
4. Whether the production line of the isolated power module is standardized. The power module production line needs to pass a number of international certifications such as ISO9001, ISO14001, OHSAS18001, etc., as shown in Figure 3 below.
Figure 3 ISO certification
5. Whether the isolation power module is applied to harsh environments such as industry and automobiles. The power module is not only applied to the harsh industrial environment, but also in the BMS management system of new energy vehicles.
4,The perception of isolation power and non -isolation power
First of all, a misunderstanding is explained: Many people think that non -isolation power is not as good as isolation power, because the isolated power supply is expensive, so it must be expensive.
Why is it better to use isolation power than non -isolation in the impression of everyone now? In fact, this idea is to stay in the idea a few years ago. Because the non -isolation stability in previous years has indeed no isolation and stability, but with the update of R & D technology, the non -isolation is now very mature and it is becoming more stable. Speaking of security, in fact, non -isolation power is also very safe. As long as the structure is slightly changed, it is still safe to the human body. The same reason, non -isolation power can also pass many security standards, such as: Ultuvsaace.
In fact, the root cause of the damage to the non -isolation power supply is caused by the surging voltage at both ends of the power AC line. It can also be said that the lightning wave is surge. This voltage is an instant high voltage at both ends of the voltage AC line, sometimes as high as three thousand volts. But the time is very short and the energy is extremely strong. It will happen when it is thunder, or on the same AC line, when a large load is disconnected, because the current inertia will also occur. The isolation BUCK circuit will instantly convey to the output, damage the constant current detection ring, or further damage the chip, causing 300V to pass, and burn the entire lamp. For the isolation anti -aggressive power supply, the MOS will be damaged. The phenomenon is storage, chip, and MOS tubes are burned out. Now the LED -driven power supply is bad during use, and more than 80%are these two similar phenomena. Moreover, the small switching power supply, even if it is a power adapter, is often damaged by this phenomenon, which is caused by wave voltage, and in the LED power supply, it is even more common. This is because the load characteristics of the LED are particularly afraid of waves. The voltage.
According to the general theory, the less components in the electronic circuit, the higher the reliability, and the lower the more component’s circuit board reliability. In fact, non -isolation circuits are less than isolation circuits. Why is the isolation circuit reliability high? In fact, it is not reliability, but the non -isolation circuit is too sensitive to the surge, poor inhibitory ability, and isolation circuit, because the energy enters the transformer first, and then transports it to the LED load from the transformer. The buck circuit is part of the input power supply directly to the LED load. Therefore, the former has a strong chance of damage to the surge in suppression and attenuation, so it is small. In fact, the problem of not isolation is mainly due to the problem of surge. At present, this problem is that only the LED lamps can be seen from the probability that they can be seen from the probability. Therefore, many people have not proposed a good prevention method. More people do not know what wave voltage is, many people. The LED lamps are broken, and the reason can not be found. In the end, there is only one sentence. What this power supply is unstable and it will be settled. Where is the specific unstable, he doesn’t know.
Non -isolation power supply is efficiency, and the second is that cost is more advantageous.
Non -isolation power is suitable for occasions: First of all, it is the indoor lamps. This indoor electricity environment is better and the influence of the waves is small. Second, the occasion of use is a small -voltage and small current. Non -isolation is not meaningful for low -voltage currents, because the efficiency of low -voltage and large currents is not higher than isolation, and the cost is less than much. Third, the non -isolation power supply is used in a relatively stable environment. Of course, if there is a way to solve the problem of suppressing the surge, the application range of non -isolation power will be greatly broaden!
Due to the problem of waves, the damage rate should not be underestimated. Generally, the kind of repaired return, damaging insurance, chip, and MOS’s first one should think of the problem of waves. In order to reduce the damage rate, it is necessary to consider the surge factors when designing, or to quit users when used, and try to avoid surge. (Such as indoor lamps, turn it off for the time being when you fight)
In summary, the use of isolation and non -isolation is often because of the problem of wave surge, and the problem of waves and the electricity environment is closely related. Therefore, many times the use of isolation power and non -isolation power supply cannot be cut one by one. Costs are very advantageous, so it is necessary to choose non -isolation or isolation as LED -drive power supply.
5. Summary
This article introduces the differences between isolation and non -isolation power, as well as their respective advantages and disadvantages, adaptation occasions, and the selection of selection of isolation power. I hope that engineers can use this as a reference in product design. And after the product fails, quickly position the problem.
Post time: Jul-08-2023
