Power supplies are ubiquitous in our daily lives. Based on their type, power supplies are divided into isolated power supplies and non-isolated power supplies. To enhance your understanding of power supplies, this article will detail the differences between isolated and non-isolated power supplies, and specifically, the advantages of isolated power supplies over non-isolated power supplies. If you are interested in power supply-related content, please continue reading.
The common perception that isolated power supplies are better than non-isolated ones is outdated. While non-isolated power supplies were indeed less stable a few years ago, technological advancements have made them much more mature and stable. Regarding safety, non-isolated power supplies are also quite safe. With slight structural modifications, they are generally safe for human use. Similarly, non-isolated power supplies can meet many safety standards, such as ULTUVSAACE.
In reality, the root cause of damage to non-isolated power supplies is the surge voltage across the AC power line, which can also be described as a lightning surge. This voltage is a momentary high voltage applied across the AC power line, sometimes reaching 3,000 volts, but the duration is very short, yet the energy is extremely strong. It occurs during lightning strikes, or on the same AC line when a large load is disconnected, due to current inertia. This voltage enters the power supply, and for non-isolated BUCK circuits, it will instantly propagate to the output, damaging the constant current detection loop or further damaging the chip, causing a 300V shoot-through and burning out the entire lamp. For isolated flyback power supplies, it will damage the MOSFET, resulting in the lamp, chip, and MOSFET all burning out. Currently, over 80% of LED driver power supply failures during use are due to these two similar phenomena. Moreover, small switching power supplies, and even power adapters, frequently fail due to this phenomenon, all caused by surge voltage. This is even more prevalent in LED power supplies because LED load characteristics are particularly vulnerable to surge voltage.
Generally speaking, in electronic circuits, fewer components mean higher reliability, and more components mean lower reliability. However, non-isolated circuits actually have fewer components than isolated circuits. So why are isolated circuits more reliable? The truth is, it's not so much about reliability, but rather that non-isolated circuits are too sensitive to surges and have poor surge suppression capabilities. In isolated circuits, energy first enters the transformer and then is delivered to the LED load. In a BUCK circuit, a portion of the input power is directly applied to the LED load, hence the former has strong surge suppression and attenuation capabilities, reducing the probability of damage during a surge. In reality, the main problem with non-isolated power supplies is surge protection. Currently, the extent to which this problem is solved can only be seen probabilistically when LED lights are used in large quantities. Therefore, many people haven't proposed good prevention methods, and many don't even know what surge voltage is. Many people, when LED lights fail, can't find the cause and simply conclude that the power supply is unstable, without knowing the specific reasons for the instability.
Non-isolated power supplies have advantages in both efficiency and cost.
These are advantages compared to isolated power supplies. Isolated power supplies are difficult to make efficient, generate a lot of heat if not handled properly, and are also expensive, especially for LED fluorescent lamps with built-in lamps, where the cost is sky-high. However, non-isolated power supplies have poor suppression of lightning surge voltage, leading to more damage factors when shipping in large quantities. Surge issues always exist, and many isolated power supplies, such as those for outdoor streetlights, fail quickly. Isolated power supplies are often severely damaged by surges. Based on my long-term experience and research in LED power supply shipping and development, I have drawn some conclusions for your reference.
1. High-power LED drivers generally require isolated power supplies. Do not use non-isolated power supplies to save a little cost, otherwise it will be counterproductive.
2. For low-power LED drivers, whether to use isolated or non-isolated power supplies depends on the specific situation. Using isolated power supplies is certainly preferable, but at least two conditions must be met: firstly, the cost must be feasible, and secondly, the heat generated must be manageable. These two issues are challenging for isolated power supplies, while non-isolated power supplies can often be used and often work very well.
3. Suitable applications for non-isolated power supplies: First, indoor lighting fixtures, where the power environment is relatively good and surge impact is minimal. Second, applications involving high voltage and low current. Using non-isolated power supplies for low voltage and high current is pointless because their efficiency is not significantly higher than isolated power supplies, and the cost difference is not substantial. Third, non-isolated power supplies are suitable for environments with relatively stable voltage. Of course, if a solution to surge suppression can be found, the application range of non-isolated power supplies will be greatly expanded!
4. Due to surge issues, the failure rate of isolated power supplies should not be underestimated. When power supplies are returned for repair and the fuse, chip, or MOSFET are damaged, the first thing to consider is the surge problem. To reduce the failure rate, surge factors should be taken into account during the design phase, or users should be advised to avoid surges as much as possible (e.g., temporarily turn off indoor lights during thunderstorms).
