1. Isolated power supply: there is no direct electrical connection between the input circuit and the output circuit of the power supply. The input and output are in an isolated high resistance state, and there is no current circuit, as shown in Figure 1: Figure 1 uses the isolated power supply of the transformer. 2. Non isolated power supply: there is a direct current circuit between the input and output, for example, the input and output are common ground. Take isolated flyback circuit and non isolated BUCK circuit as examples, as shown in Figure 2. Fig. 2 The advantages and disadvantages of non isolated power supply isolated power supply and non isolated power supply can be seen from the above concepts. For common power topology, non isolated power supply mainly includes: Buck, Boost, Buck Boost, etc; The isolated power supply mainly includes various flyback, forward, half bridge, LLC topologies with isolation transformers. In combination with commonly used isolated and non isolated power supplies, we can intuitively draw some advantages and disadvantages of them, which are almost opposite.
When using isolated or non isolated power supply, it is necessary to understand the actual project's demand for power supply, but before this, we can understand the main differences between isolated and non isolated power supply:
The isolation module has high reliability, but high cost and poor efficiency;
The structure of non isolation module is very simple, with low cost, high efficiency and poor security performance.
Therefore, it is recommended to use isolated power supply in the following situations:
The isolated AC-DC power supply is required for occasions involving potential electric shock, such as those where power is taken from the power grid and converted to low-voltage DC;
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 power supply, and the distance between systems is often far. Therefore, we usually need to isolate the power supply for electrical isolation to ensure the physical safety of the system, and cut off the grounding loop through isolation to protect the system from transient high voltage shocks and reduce signal distortion;
For external I/O ports, power isolation is also recommended to ensure the reliable operation of the system. The summary table is shown in Table 1. Their advantages and disadvantages are almost opposite.
Table 1 Advantages and disadvantages of isolated power supply and non isolated power supply:
The selection of isolated power supply and non isolated power supply Through understanding the advantages and disadvantages of isolated and non isolated power supply, we can see that they have their own advantages. For some common embedded power supply options, we can make accurate judgments:
For the power supply of the front stage of the system, the isolated power supply is generally used to improve the anti-interference performance and ensure the reliability;
For the power supply of IC or part of circuits in the circuit board, the non isolated scheme is preferred based on the cost performance and volume;
Where safety is required, such as AC-DC connected to mains or medical power supply, isolation power supply must be used to ensure personal safety, and in some cases, enhanced isolation power supply must be used;
For the power supply of remote industrial communication, in order to effectively reduce the impact of ground potential difference and wire coupling interference, the isolated power supply is generally used to supply power for each communication node separately;
Non isolated power supply is adopted for the occasions where battery power supply is adopted and the endurance is strictly required.
Through understanding the advantages and disadvantages of isolated and non isolated power supplies, we can see that they have their own advantages. For some commonly used embedded power supply designs, we can summarize their selection occasions.
1. Isolate the power supply of the front stage of the power supply system. In order to improve the anti-interference performance and ensure the reliability, the isolated power supply is generally used. Where safety is required, such as AC-DC connected to mains power, or medical power supply and white household appliances, isolated power supply must be used to ensure personal safety, such as MP020 of MPS, which is primary side feedback isolated AC-DC and suitable for 1~10W applications.
For the power supply of remote industrial communication, in order to effectively reduce the impact of ground potential difference and wire coupling interference, the isolated power supply is generally used to supply power for each communication node separately.
2. For power supply of IC or part of circuits in non isolated power supply circuit board, non isolated scheme is preferred based on cost performance and volume; For example, MPS MP150/157/MP174 series buck type non isolated AC-DC is suitable for 1-5W applications. If the working voltage is lower than 36V, the battery power supply is used. If the endurance requirements are strict, the non isolated power supply is preferred, such as MP2451/MPQ2451 of MPS.
Advantages and disadvantages of isolated power supply and non isolated power supply:
Through understanding the advantages and disadvantages of isolated and non isolated power supplies, we can see that they have their own advantages. For some common embedded power supply options, we can follow the following judgment conditions:
Where safety requirements are required, such as AC-DC connected to mains power or medical power supply, isolation power supply must be used to ensure personal safety, and in some cases, enhanced isolation power supply must be used.
Generally, the isolation voltage of module power supply is not very high, but the higher isolation voltage can ensure that the module power supply has less leakage current, higher safety and reliability, and the EMC characteristics are better. Therefore, the isolation voltage level prevailing in the industry is above 1500VDC.
Precautions for the selection of isolated power supply module The isolation and withstand voltage of power supply is also called electric strength in the GB-4943 national standard. This GB-4943 standard is often referred to as the safety standard for information equipment, which is a national standard to prevent physical and electrical injuries to personnel, including the protection from electric shock, physical injury, explosion and other injuries. The following figure shows the structure of isolated power supply. The isolated power supply structure diagram is an important indicator of the module power supply. The standard also specifies the test methods related to isolation and withstand voltage. The equipotential connection test is generally used for simple tests. The connection diagram is as follows: Diagram of isolation and withstand voltage test 1. Test method: set the voltage of the withstand voltage meter to the specified withstand voltage value, set the current to the specified leakage current value, and set the time to the specified test time value; Operate the withstand voltage meter to start the test and pressurize. Within the specified test time, the module shall be free of breakdown and flashover.
Attention shall be paid to the proper temperature for welding the power module during the test to avoid repeated welding and damage to the power module.
In addition, pay attention to:
Pay attention to whether it is AC-DC or DC-DC;
Isolation withstand voltage of isolated power module. For example, whether the isolation 1000V DC meets the insulation requirements;
Whether the isolated power supply module is subject to comprehensive reliability test. The power module shall pass performance test, tolerance test, transient condition test, reliability test, EMC electromagnetic compatibility test, high and low temperature test, limit test, life test, safety regulation test, etc;
Whether the production line of the isolated power module is standardized. The power module production line needs to pass ISO9001, ISO14001, OHSAS18001 and other international certifications;
Whether the isolated power module is used in harsh environments such as industry and automobile. The power module is not only widely used in harsh industrial environment, but also easily used in the BMS management system of new energy vehicles.
The perception of isolated power supply and non isolated power supply first describes a mistake: 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 do you think it is better to use isolated power supply than non isolated power supply? In fact, this idea is stuck in the idea of a few years ago. Because the stability of non isolation in previous years is not as stable as that of isolation, but with the update of R&D technology, non isolation is now very mature and stable.
Speaking of safety, in fact, the non isolated power supply is also very safe now. As long as the structure is slightly modified, it is still very safe for human body. In the same way, the non isolated power supply can also pass many safety standards, such as ULTUVSAACE.
In fact, the source of damage to the non isolated power supply is the surge voltage at both ends of the AC line of the power supply. It can also be said that the lightning surge is an instantaneous high voltage applied to both ends of the AC line of the voltage, sometimes up to 3000 volts, but the time is very short, but the energy is extremely strong, which can occur when lightning strikes, or when a large load is disconnected on the same AC line, 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, breaking the constant current detection ring, or further breaking the chip, resulting in a 300v straight through and burning the entire lamp. For isolated flyback power supply, MOS will be damaged. The phenomenon is that the storage, chip and MOS tube are all burnt out.
At present, the LED driver power supply is broken during use, and more than 80% of them are similar to these two phenomena. In addition, small switching power supplies, even power adapters, are often damaged by surge voltage. In LED power supplies, it is more common because LED load characteristics are particularly afraid of surge voltage.
According to the general theory, the fewer components in the electronic circuit, the higher the reliability, and the lower the reliability of the circuit board of the corresponding more components. In fact, the components of non isolated circuits are less than those of isolated circuits. Why is the reliability of isolated circuits high.
To put it bluntly, it is not reliability, but the non isolated circuit is too sensitive to surges and has poor suppression ability. The isolated circuit, because the energy first enters the transformer, and then is transmitted to the LED load from the transformer. The BUCK circuit is that part of the input power is directly added to the LED load, so the former has strong surge suppression and attenuation capabilities, so the probability of damage when the surge comes is small.
In fact, the problem of not isolating the power supply is mainly the surge problem. At present, because only when LED lamps are used in large quantities, the degree of its solution can be seen from the probability, so many people have not put forward a good prevention method, and more people do not know what the surge voltage is.
The LED lamp was broken, and the reason could not be found. In the end, he could only say that the power supply was unstable, and he did not know exactly where it was unstable.
Non isolated power supply has advantages in efficiency and cost.
Suitable occasions for non isolated power supply:
It is an indoor lamp, which has a good indoor power environment and little surge impact;
It is used for high voltage and small current, and it is meaningless to use non isolation for low voltage and large current, because the efficiency of non isolation for low voltage and large current is not higher than that of isolation, and the cost is not much lower;
Non isolated power supply is used in the environment with relatively stable voltage. Of course, if there is a way to solve the problem of surge suppression, the application scope of non isolated power supply will be greatly expanded!
Because of the surge problem, the damage rate of the isolated power supply cannot be underestimated. In general, the first thing you should think of is the surge problem when you come back to repair and break the insurance, chips, MOS.
In order to reduce the damage rate, surge factors should be taken into account in the design, or users should be warned when using to avoid surge as much as possible. (For example, indoor lamps shall be turned off temporarily in case of thunder)
To sum up, the use of isolation and non isolation is often due to the problem of surge, which is closely related to the power environment. Therefore, in many cases, the use of isolated power supply and non isolated power supply cannot be cut across the board. Non isolated power supply has advantages in energy saving and cost, so it is necessary to scientifically select non isolated or isolated power supply as LED driver power supply.
The article summarized and analyzed the difference between isolated power supply and non isolated power supply, as well as their advantages and disadvantages, adaptability, and the selection precautions of isolated power supply. I hope that engineers can take this as a reference when designing their friends' products, correctly apply power supply in product development, and quickly locate the problem after product failure. Articles are sorted out from the network and are deleted