LED now occupies a large position in the power supply market. Its high brightness, low power consumption, long life, fast start-up, low power, no stroboscopic, and less prone to visual fatigue make it a popular product. Key factor. In the power industry, LED isolation and non-isolation are probably the two most commonly heard terms.
Isolated power supply: In some laboratories or high-demand occasions, for the safety of the experimental personnel, the experimental input power supply is generally isolated from the commercial power by a 1:1 power frequency transformer, so that the laboratory experimenter touches the line. There is no danger of electric shock on any of the wires, because the isolated power supply is not connected to the earth.
At present, in the general LED fluorescent tube lighting market, there are non-isolated designs and isolated driving power sources. Non-isolated designs are limited to dual-insulation products, such as replacements for light bulbs, where the LED and the entire product are integrated and sealed in a non-conductive plastic, so the end user does not have any risk of electric shock. The secondary products are all isolated and relatively expensive, but they are essential where the user can access the LEDs and output wiring (usually in the case of LED lighting and street lighting applications). An isolated driver or an electrically isolated LED driver means that the LED can be touched directly without touching the battery. The LED drive power without the isolation transformer can still achieve partial mechanical insulation by means of the protective casing, but the LED fluorescent tube at this time cannot be directly contacted during operation.
The most common is the 1:1 transformer!
In use, the voltage at the user end is the same as the voltage at the primary end, but you can touch a line on the user side without an electric shock. (Note that it is not allowed to grab two lines! It will be an electric shock!!) Because this line is not connected to the earth (the original power ground), the voltage may be high, but it is floating and cannot be generated. Current, so there is no electric shock.
Other isolated power supplies, whether AC or DC, are the same.
The DC-DC isolated power supply mainly stabilizes the output voltage by adjusting the frequency of the output pulse (adjusting the intermittent time of the pulse). The principle of the voltage regulation process is: when the output voltage drops, the R4 voltage, error, LT431 output, optocoupler output current, and R1 voltage are all reduced. When the R1 voltage is less than the 1.5V reference voltage, the LX terminal outputs a low level during the positive half cycle of the modulation pulse. The pulse current of the pulse transformer increases linearly, and D1 reverses the bias. At this time, the secondary pole has no current, and the pulse transformer starts. The energy is stored; and in the negative half of the modulation pulse, the LX terminal outputs a high level, the pulse transformer releases energy, and the induced voltage rises through the D1 (positive conduction) output to increase the output voltage. Thereby the output voltage remains stable. In fact, when the output voltage rises, the voltage and error on the R4 resistor, the output current of the LT431 output to the optocoupler, and the voltage on R1 rise accordingly. When the R1 voltage is greater than the 1.5V reference voltage, During the negative half cycle of the modulation pulse, the LX terminal outputs a high level, and the pulse transformer does not generate a voltage. At this time, the load consumption will cause the output voltage to drop. Thereby the output voltage remains stable.
What is the difference between the two?Mainly from the following aspects:
safetyThe isolated power supply uses a transformer to reduce the voltage to a lower voltage through a transformer and then rectify it into a DC output for power supply. Because the main coil of the transformer is subjected to 220V voltage, the secondary coil only bears the low AC voltage of the output, and the primary and secondary coils are not directly connected, so it is called an isolated power supply. The conversion process of the transformer is: electro-magnetic-electric, not connected to the earth, so there is no danger of electric shock.
The non-isolated power supply is directly input to the electronic circuit with 220V, and the output is output through the electronic components. The input and output are directly connected through the electronic components, so it is called a non-isolated power supply; the two are apparently different from the transformer. LED non-isolated designs are limited to dual-insulation products, such as replacements for light bulbs, where the LED and the entire product are integrated and sealed in a non-conductive plastic, so the end user does not have any risk of electric shock. The secondary products are all isolated and relatively expensive. The non-isolated circuit is directly added to the LED load after the input power is applied through the buck-boost, and there is a risk of electric shock.
Therefore, it is necessary to pass the safety certification, such as 3C, UL, CE, etc., non-isolation is troublesome, the general manufacturer does not have absolute design technical strength, generally not good. Because the insulation and creepage distance are not enough, it can only be designed from the physical structure of the luminaire. The lamp is acceptable or fully plastic. For example, the insulation between the LED and the aluminum heat sink is usually insulated by the film of the printed circuit board of the aluminum substrate. Although this insulating layer can withstand 2000V high voltage, sometimes the burr of the screw hole will cause so-called creepage, making it difficult to pass CE certification.
However, as a complete LED lighting product, the parts that the user can touch on the surface must be isolated and cannot cause electric shock. From the perspective of the entire system, isolation is inevitable, the only difference is the location of the isolation. As a product that can be safely used by end users, the reliability of insulation and isolation must be considered.
Note: Need to be reminded that some manufacturers in order to save costs, the use of direct tapping on the main coil to extract low voltage, this method seems to have a transformer, there is no secondary coil, can not be regarded as isolated power!
In terms of performance, the advantages of isolated power supply are: no threat to the human body, wide voltage performance is very good, non-isolated is now very mature, the voltage range is slightly worse than isolation, the voltage range is between 110V-300V The isolated power supply can achieve 60-300V. The high and low currents are very uniform. The isolated drive is safe but less efficient, and the non-isolated drive is more efficient. The isolated or non-isolated drive should be selected according to the actual requirements.
From the constant current accuracy: the isolation type can be achieved within ±5%, while the non-isolated type is difficult to achieve.
Non-isolated circuits are very sensitive to surges and have poor rejection. Lightning surge, this voltage is instantaneous high voltage, up to several thousand volts, the time is very short, the energy is very strong, this voltage enters the power supply, for the non-isolated BUCK circuit, it will instantaneously convey the output, destroy the constant current detection ring, or Further damage the chip, causing 300v straight through, and burning the entire lamp. In fact, it refers to the non-isolated power supply. When the shipment is in bulk, the repair rate is higher than that of the isolated LED driver, mostly because it is damaged. The probability of isolated power supply bursting is much smaller, and non-isolated is generally around 2% to 3%.
Many grid voltages are unstable and isolation is also a phenomenon. The phenomenon is that the chip, MOS tube, and constant current loop are all burned out, but the isolation is relatively much less.
Note: Therefore, non-isolated surge-proof varistor is essential, and no varistor is guaranteed.
Cost and efficiencyThe cost of electronic products is not only the cost of design and production, but also the cost of maintenance. As far as the LED power supply circuit structure is concerned, the current isolation type scheme is mostly a flyback circuit scheme of AC/DC, so the relative circuit is complicated and the cost is high. The non-isolated type is basically a DC/DC boost or buck circuit, and the relative circuit is relatively simple, so the cost is relatively low.
Non-isolated due to less energy loss of the transformer, the efficiency can generally reach more than 90%, and there are higher power factors. The isolation energy efficiency is generally 88%, depending on the power, so the isolated power supply is also relatively hot.
Non-isolated power supplies are more efficient and cost-effective than isolated power supplies. The isolation power supply efficiency is not easy to be high. If the processing is not good, the heat is very high and the cost is high, especially the LED fluorescent lamp with the built-in lamp. However, non-isolated power supplies, because of the poor ability to suppress lightning surge voltages, will encounter many damages when shipped in large quantities.
Compared with the isolated power supply, the non-isolated power supply mainly reduces the transformer, designs the architecture with the least amount of materials, and achieves the same product function, so the non-isolation cost has a great advantage. This is the reason why non-isolated power supplies are very popular in China.
Note: In low-voltage-powered LED luminaires, the principle of efficiency and cost is preferred, in fact, non-isolated solutions are the best choice.
Use occasionWhether an isolated insulating transformer or an isolated protective cover housing is usually required to be analyzed in many aspects, such as cost and manufacturing process, efficiency and volume, insulation reliability, and safety specifications. Both types of designs will continue to be used, as each design meets different market needs.
From the load range, the output range of the isolated power supply is generally 30-42V, and the non-isolated load range can be 30-84V. Many LED manufacturers choose the power supply for the overall adaptability requires the power supply to adapt to the full voltage 90-265V input, and the load range also requires up to 84V. This choice has certain risks and hidden dangers. At 90V input, the power supply may lose its constant current function, THD. Non-isolated is suitable for high voltage and small current, and the cost of making large current is not cheaper than isolation.
Non-isolated power supply is suitable for: First, indoor lighting, this indoor power environment is better, the impact of small surge, straw hat and patch LED, the use of high voltage and small current. Non-isolation of low-voltage and high-current is meaningless, because the efficiency of non-isolation of low-voltage and high-current is not higher than that of isolation, and the cost is not much lower. It is not suitable for centralized illumination. What is centralized illumination, that is, on the same AC line? There are hundreds of lamps on this line. At this time, because there are too many lamps on one line, the voltage environment on this line is dirty, so the probability of destroying the lamps becomes larger, so the LED lamps are not isolated.
Isolation is inevitable from the entire system of the product, the only difference is the location of the isolation. Some designers use isolated transformer designs, so they can simplify the design of heat dissipation and lampshade (how can the heat dissipation design be simplified? The efficiency of isolation is low, but the heat isolation is great, just simplify the lighting isolation design?). If a non-isolated drive design is used, reliable insulation requirements must be considered for structures such as lamp housings.
Note: As a power driver, both isolated and non-isolated solutions are always present.
Due to the above points, the power supply of these two architectures has their own advantages. The non-isolated power supply focuses on higher power factor and efficiency, which reduces the loss of energy. The isolated power supply emphasizes the safety of life and the overall safety of the fluorescent lamp. In terms of power factor and efficiency, it is slightly inferior to non-isolated power supply. Different power supply types are not mandatory in different occasions and different environments.
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