In the product design, if the requirements of the application environment for power isolation are not considered, when the product is applied, the system will be unstable due to improper design, and even the high voltage will damage the downstream load, and there will be hazards. The safety of personal property. Therefore, whether the product design needs to be isolated is very important.

The news that "a consumer was electrocuted while charging his mobile phone" has attracted widespread attention on the Internet. Can chargers also endanger life? Experts analyzed that the internal transformer of the mobile phone charger leaked electricity, and the 220VAC AC leaked to the DC terminal, and was transmitted to the metal shell of the mobile phone through the data cable, eventually leading to electric shock and death, an irreparable tragedy.

Then why does the output of the mobile phone charger have 220V AC? What should be paid attention to in the selection of isolated power supply? How to distinguish whether the power supply is isolated or non-isolated? The general opinion in the industry is:

1. Isolated power supply: There is no direct electrical connection between the input circuit and output circuit of the power supply, and the input and output are isolated high-impedance state, no current loop, as shown in Figure 1:

Figure 1. Isolated power supply with transformer

2. Non-isolated power supply: There is a direct current loop between the input and output, for example, there is a common ground. Take the isolated flyback circuit and non-isolated BUCK circuit as an example, as shown in Figure 2.

Figure 2 Non-isolated power supply

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**Advantages and disadvantages of isolated power supply and non-isolated power supply**

From the above concepts, we can see that for commonly used power supply topologies, non-isolated power supplies mainly include: Buck, Boost, Buck-Boost, etc.; and isolated power supplies mainly include various topologies such as flyback, forward, half-bridge, and LLC with isolation transformers.

Combining commonly used isolated and non-isolated power supplies, we can get some of their advantages and disadvantages intuitively, and the advantages and disadvantages of the two are almost opposite.

To use isolated or non-isolated power supplies, you need to understand what the actual project needs for power supplies, but before that, you can understand the main differences between isolated and non-isolated power supplies:

① The reliability of the isolation module is high, but the cost is high and the efficiency is poor.

② The non-isolated module has a simple structure, low cost, high efficiency, and poor safety performance.

Therefore, in the following situations, it is recommended to use an isolated power supply:

① For occasions involving possible electric shock, such as taking power from the grid and converting it to low-voltage DC, an isolation power supply is required

② The serial communication bus transmits data through physical networks such as RS-232, RS-485 and Controller Area Network (CAN), each of these interconnected systems is equipped with its own Therefore, we usually need to isolate the power supply for electrical isolation to ensure the physical safety of the system, and cut off the ground loop through isolation to protect the system from transient high voltage shocks and reduce Signal distortion;

③ For the external I/O port, in order to ensure the reliable operation of the system, it is also recommended to do power isolation for the I/O port.

As summarized in the table below, the advantages and disadvantages of the two are almost opposite.

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**The choice of isolated power supply and non-isolated power supply**

By understanding the advantages and disadvantages of isolated and non-isolated power supply, they have their own advantages. We have been able to make an accurate judgment on the embedded power supply selection:

① The power supply of the front stage of the system, in order to improve the anti-interference performance and ensure reliability, generally uses an isolated power supply.

② IC or part of the circuit in the circuit board supplies power. In terms of cost performance and volume, non-isolated solutions are preferred.

③ For occasions that require safety, if AC-DC connected to the mains or medical power supply is required, in order to ensure personal safety, an isolated power supply must be used, and in some occasions, an isolated power supply must be used. power supply.

④ For the power supply of remote industrial communication, in order to effectively reduce the influence of ground potential difference and wire coupling interference, an isolated power supply is generally used to supply power to each communication node separately.

⑤ Non-isolated power supply is used for battery-powered occasions with strict endurance requirements.

By understanding the advantages and disadvantages of isolated and non-isolated power supplies, we can see that they each have their own advantages. For some commonly used embedded power supply designs, we can summarize their selection occasions.

**1. Isolated power supply**

For the power supply of the front stage of the system, in order to improve the anti-interference performance and ensure the reliability, the isolated power supply is generally used;

For occasions that require safety, if AC-DC connected to mains power, or medical power supply and white goods are required, in order to ensure personal safety, an isolated power supply must be used, such as MP020 of MPS, which is a primary-side feedback isolated AC-DC power supply. DC, suitable for 1~10W applications;

For the power supply of remote industrial communication, in order to effectively reduce the influence of ground potential difference and wire coupling interference, an isolated power supply is generally used to supply power to each communication node separately.

2. Non-isolated power supply

The IC or part of the circuit in the circuit board supplies power. In terms of cost performance and volume, non-isolated solutions are preferred; MPS MP150/157/MP174 series buck type non-isolated AC-DC, suitable for 1~5W applications;

For occasions where the working voltage is lower than 36V, battery power is used, and battery life is strictly required, it is preferred Use non-isolated power supply, such as MP2451/MPQ2451 of MPS.

Advantages and disadvantages of isolated power supply and non-isolated power supply:

By understanding the advantages and disadvantages of isolated and non-isolated power supply, they have their own advantages. For the selection of embedded power supply, we can follow the following judgment conditions:

In occasions that require safety, if AC-DC connected to mains power or medical power supply is required, in order to ensure personal safety For safety, an isolated power supply must be used, and in some cases, a reinforced isolated power supply must be used.

In general, the isolation voltage of the module power supply is not very high, but a higher isolation voltage can ensure that the module power supply has smaller leakage current, higher safety and reliability, And the EMC characteristics are also better, so the current industry-wide isolation voltage level is above 1500VDC.

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** Notes on selection of isolated power supply module**

Isolation resistance of power supply The GB-4943 national standard is also called the electric strength. This GB-4943 standard is what we often call the safety standard of information equipment. It is a national standard for preventing people from physical and electrical damage, including avoiding people from being injured by electric shocks. , physical damage, explosion and other damage. The following figure is the structure diagram of the isolated power supply.

Isolation power supply diagram

< p> As an important indicator of the module power supply, the standard also stipulates the isolation and withstand voltage related test methods. The equipotential connection test is generally used for simple tests. The connection diagram is as follows:

Schematic diagram of isolation withstand voltage test

Test method:< /p>

Set the voltage of the withstand voltage meter to the specified withstand voltage value, the current to the specified leakage current value, and the time to the specified test time value; Within the specified test time, the module should have no breakdown and no arcing phenomenon.

Pay attention to select the appropriate temperature for welding the power module during the test to avoid repeated welding and damage to the power module.

In addition, pay attention to:

1. Pay attention to whether it is AC-DC or DC-DC.

2. The isolation withstand voltage of the isolated power supply module. For example, whether the isolation of 1000V DC meets the insulation requirements.

3. Whether the isolated power supply module has been fully tested for reliability. The power module should go through performance test, tolerance test, transient condition test, reliability test, EMC electromagnetic compatibility test, high and low temperature test, limit test, life test, safety test, etc.

4. Whether the production line of the isolated power supply module is standardized. The power module production line needs to pass ISO9001, ISO14001, OHSAS18001 and many other international certifications.

ISO certification

5. Whether the isolated power module is used in harsh environments such as industry and automobiles. Power modules are not only widely used in harsh industrial environments, but also in the BMS management system of new energy vehicles.

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** About the perception of isolated power supply and non-isolated power supply**

First, one Myth: Many people think that non-isolated power supply is not as good as isolated power supply, because isolated power supply is expensive, so it must be expensive.

Why is it better to use an isolated power supply than a non-isolated power supply in everyone's impression now, but it is not the case. This kind of thinking is still in the thinking of a few years ago. Because the stability of non-isolation in the past few years is indeed not as stable as isolation, but with the update of research and development technology, non-isolation is now very mature and stable.

Speaking of safety, in fact, non-isolated power supply is also very safe now, as long as the structure is slightly changed, it is still very safe for the human body. In the same way, non-isolated power supply can also pass many safety regulations Standards, such as: ULTUVSAACE, etc.

In fact, the root cause of non-isolated power supply damage is the surge voltage at both ends of the AC line of the power supply. , sometimes as high as 3,000 volts, but the time is very short, but the energy is extremely strong, it will happen when thunder strikes, or on the same AC line, when a large load is disconnected instantly, it will also happen because of the current inertia, This voltage enters the power supply, and for the non-isolated BUCK circuit, it will be transmitted to the output instantaneously, damaging the constant current detection ring, or further damaging the chip, resulting in 300v straight-through, and burning the entire lamp.

For the isolated flyback power supply, it will damage the MOS. The phenomenon is that the storage, chips and MOS tubes are all burned out. At present, more than 80% of the LED drive power supply is broken during use, which are caused by these two similar phenomena. Moreover, small switching power supplies, even power adapters, are often damaged. This phenomenon is caused by surge voltage. In LED power supplies, it is more common, because the load characteristics of LEDs are particularly afraid of surges. surge voltage.

According to the general theory, the fewer components in an electronic circuit, the higher the reliability, and the correspondingly more components, the lower the reliability of the circuit board. In fact, the components of the non-isolated circuit are less than those of the isolated circuit, why the reliability of the isolated circuit is high.

In fact, to put it bluntly, it is not reliability, but non-isolated circuits are too sensitive to surges and have poor suppression capabilities. Isolated circuits, because energy first enters the transformer, and then is transported from the transformer to the LED load . The BUCK circuit is a part of the input power directly added to the LED load, so the former has a strong ability to suppress and attenuate the surge, so the probability of damage when the surge comes is small.

In fact, the problem of non-isolated power supply is mainly the problem of surge. At present, this problem can only be seen from the probability when LED lamps are used in large quantities, so many people There is no good prevention and control method, and more people don't know what the surge voltage is, many people. The LED lamp is broken, and the reason can't be found. In the end, he can only say that the power supply is unstable, so he doesn't know exactly where it is unstable.

The non-isolated power supply has advantages in terms of efficiency and cost.

Suitable occasions for non-isolated power supply: First, indoor lamps and lanterns. This kind of indoor electricity environment is better and the impact of surge is small. Second, the occasion of use is high voltage and small current. It is meaningless to use non-isolation for low voltage and high current, because the efficiency of non-isolation for low voltage and high current is not higher than that of isolation, and the cost is not much lower. Third, use a non-isolated power supply in an environment with relatively stable voltage. Of course, if there is a way to solve the problem of suppressing surges, the application range of non-isolated power supplies will be greatly expanded!

The damage rate of the isolated power supply should not be underestimated because of the surge problem. Generally, the surge problem should be the first thing that comes to mind when the fuse, chip, and MOS are damaged after repairing. In order to reduce the damage rate, it is necessary to take into account the surge factor in the design, or warn the user during use to avoid the surge as much as possible. (Such as indoor lamps, temporarily turn off when there is thunder)

In summary, the use of isolation and non-isolation is often due to the problem of surge, and the problem of surge is closely related to the power environment. Therefore, in many cases, the use of isolated power supply and non-isolated power supply cannot be used across the board. Non-isolated power supply has great advantages in terms of energy saving and cost. Therefore, it is necessary to scientifically choose non-isolated or isolated power supply as the LED drive power supply.

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** Summary**

This article explains the difference between isolated and non-isolated power supplies , as well as their respective advantages and disadvantages, suitable occasions, and the selection precautions for isolated power supplies. I hope engineers can use this as a reference in product design, correctly apply power supplies in product development, and quickly locate problems after product failures where.