Do you think that all industrial field system power supplies should use isolation schemes to improve reliability? Then you may have entered the misunderstanding of power supply, perhaps the non-isolated power solution is more suitable for you, here will be announced for you.
The original intention of isolating the power supply is to isolate the front-end equipment of the power supply from the rear-end equipment. Even if the front-end equipment has a problem, it will not damage the downstream equipment. However, the use of isolated power in applications where the working environment is good or the entire system is shared is not significant. In this case, non-isolated power supplies can be used, and the circuit topology is simpler, smaller, more efficient, and Short circuit protection, under voltage protection and other functions. The following are high-performance power solutions that are easy to watch.
First, non-isolated BUCK power supply topology advantages
First, the non-isolated Buck topology has fewer devices and a simpler circuit. As shown in Figure 1 below, the controller achieves the desired output voltage by controlling the Buck circuit's switching MOSFET T1 and operating it in the off or saturation region to bring the input and output to volt-second balance.
Second, it is more efficient than isolated power conversion. Since the energy transfer loss of the transformer is reduced, the loss is lower than that of the conventional LDO three-terminal regulator that operates the transistor in the amplification region. As shown in Figure 2 below, the bare metal size is similar, but the LDO linear power supply requires low efficiency. Adding a heat sink instead of an isolated BUCK power supply allows direct use of the heat sink in the circuit.
Second, integrated Buck step-down converter chip
You may wonder why non-isolated Buck power supplies have this advantage?
The non-isolated Buck power supply has this advantage (due to the use of a highly integrated Buck chip) due to the use of an integrated Buck buck conversion chip that embeds various protection circuits into the chip in the Buck topology. Make the Buck step-down power module more secure and reliable. Figure 3 below is a block diagram of the internal circuit of a brand of small-volume step-down converter chip. Its dimensions are only 3mm x 2mm, with short-circuit protection, thermal shutdown protection, undervoltage protection, etc. The circuit loop uses voltage and current double loops. Control, so that the stability of the system is better, with good voltage regulation and load regulation, and in order to improve light load efficiency, this type of IC automatically enters the FM mode at light load, and improves the light load by reducing the switching frequency and loss. effectiveness.
Figure 3 Internal block diagram of the integrated Buck power conversion chip
Third, Buck non-isolated buck module performance and reliability
The non-isolated power supply module using the integrated Buck step-down conversion chip is not only smaller in size, but also superior in performance. In addition to considering the basic performance parameters in Buck's design, what else should I consider?
Take the non-isolated step-down open-plate power module of the E7815OS-500 model independently developed by Guangzhou Zhiyuan Electronics Co., Ltd. as an example. The basic parameters of this model are shown in Table 1 below. The parameters are industry-leading and the conversion efficiency is as high as 91% at 10% load.
Table 1 Basic performance parameters of E7815OS-500
In addition to the basic parameters, we need to consider more about its reliability. We all know that the reliability and service life of electronic devices are related to the temperature, voltage stress, current stress and ambient temperature of the electronic components in the module. The harsher the operating environment of the key electronic components in the module, the higher the operating temperature of the electronic device, the lower the reliability and the lifetime. The maximum operating junction temperature of the device is 150 ° C, and the declining working junction temperature is sufficient, the device is reliable. The higher the sex.
Table 2 below shows the temperature thermal imaging picture of the key electronic components of the power module from a low voltage of 19V to a high voltage of 36V at a normal temperature of 25 ° C. It can be seen from the figure that the maximum surface temperature of the key components of the module does not exceed 80 ° C. After theoretical calculation, the internal junction temperature does not exceed 100 °C, which can ensure the reliability of the module.
Table 3 below shows the measured voltage stress and current stress of the integrated IC internal MOSFET and the external freewheeling diode at 25 °C at normal temperature. The voltage and current stress leave a certain margin to ensure module reliability.
How to improve the reliability of non-isolated power supply in application
After designing a non-isolated power supply, how do we ensure that it performs its superior performance in field applications?
The first thing to be clear is that although the non-isolated module does not have the characteristics of isolation before and after the stage, as long as it improves its reliability in some applications, it is completely unnecessary to have a power supply module with isolation function. Therefore, the application reliability of the non-isolated power supply is extremely high. important.
From the user's departure, make sure to reserve sufficient margin. Most power supply design engineers often do not keep the safe working margin of all electronic devices under the trade-off performance index and device cost. In some abnormal situations, they want to improve the reliability of the module better. In addition to the requirements of the data sheet, the engineer must also have a derating of more than 30% when selecting the module. The derating here refers not only to the derating of the output load, but also to the derating of the input voltage. For example, as shown in Table 4 below, the non-isolated power supply module of the E7815OS-500 is more efficient when the input voltage is below 24V, and the load is 50%~70% efficient and stable. 19V~24V, load 50%~70%, the reliability of use is relatively high.
Designed for a complete reliability test from the developer. Of course, in addition to derating the input voltage and load current, it is also possible to increase the reliability of the peripheral circuit through electromagnetic compatibility (EMC) related experiments.
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