Advantages of using a non-isolated power supply

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Advantages of using a non-isolated power supply [Copy link]

Do you think that all power supplies of industrial field systems should use isolation solutions to improve reliability? Then you may have fallen into the misunderstanding of power supply use. Perhaps the non-isolated power supply solution is more suitable for you. The original intention of isolating the power supply is to isolate the front-stage equipment from the back-stage equipment of the power supply, so that even if there is a problem with the front-stage equipment, the back-stage equipment will not be damaged. However, it is not meaningful to use isolated power supplies in situations where the working environment is good or the ground of the entire system is shared. At this time, you can use non-isolated power supplies, which have simpler circuit topology, smaller size, extremely high efficiency, and short-circuit protection, undervoltage protection and other functions. The following will introduce this high-performance power supply solution that is easily underestimated. 1. Advantages of non-isolated BUCK power supply topology First, the non-isolated Buck topology circuit has fewer components and a simple circuit. As shown in Figure 1 below, the controller controls the switch MOSFET tube T1 of the Buck circuit and makes it work in the cutoff or saturation region to make the input and output reach volt-second balance, thereby obtaining the desired output voltage. Secondly, its conversion efficiency is higher than that of isolated power supplies. Due to the lack of energy transfer loss of the transformer, the loss is lower than that of the traditional LDO three-terminal regulator that makes the transistor work in the amplification area, as shown in Figure 2 below. The bare metal size of the two is similar, but the LDO linear power supply needs a heat sink due to its low efficiency, while the non-isolated BUCK power supply can be used directly in the circuit without a heat sink. Figure 1 Classic Buck topology circuit Figure 2 Traditional LDO voltage regulation 2. Integrated Buck step-down conversion chip You may wonder why the non-isolated Buck power supply has such an advantage? The reason why the non-isolated Buck power supply has such an advantage (due to the use of a highly integrated Buck chip) is that it uses an integrated Buck step-down conversion chip. The chip uses the Buck topology as a framework to embed various protection circuits into the chip, making the Buck step-down power supply module safer and more reliable. Figure 3 below is a block diagram of the internal circuit of a small-volume buck converter chip of a certain brand. Its size is only 3mm x 2mm in length and width. It has functions such as short-circuit protection, overheat shutdown protection, and undervoltage protection. The circuit loop adopts voltage and current dual-loop control, which makes the system more stable and has good voltage regulation and load regulation. In order to improve the efficiency of light load, this type of IC automatically enters the frequency modulation mode when it is lightly loaded, and improves the efficiency of light load by reducing the switching frequency and loss. Figure 3 Internal block diagram of integrated Buck power conversion chip III. Performance and reliability of Buck non-isolated buck module The non-isolated power module using the integrated Buck buck converter chip is not only smaller in size, but also has superior performance. In addition to considering the basic performance parameters in the design of Buck, what else needs to be considered? We all know that the reliability and service life of electronic equipment are related to the temperature, voltage stress, current stress and ambient temperature of the electronic components in its module. The worse the working environment of the key electronic devices in the module, the higher the working temperature of the electronic devices, the lower the reliability and life. The maximum working junction temperature of the general device is 150℃. The more sufficient the working junction temperature derating is, the higher the reliability of the device. The following table shows the temperature thermal imaging pictures of the key electronic devices from low voltage 19V to high voltage 36V of a power module at room temperature 25℃. It can be seen from the figure that the maximum surface temperature of the key components of the module does not exceed 80℃. After theoretical calculation, its internal junction temperature does not exceed 100℃, which can ensure the reliability of the module. Table of key device temperature [attach] 329062 [/attach] The following table shows the measured voltage stress and current stress of the internal MOSFET and external freewheeling diode of the integrated IC at room temperature 25℃. The voltage and current stress have a certain margin to ensure the reliability of the module. Table of key device voltage and current stress [attach] 329063 [/attach] 4. How to improve the reliability of non-isolated power supply in application After designing a non-isolated power supply, how can we ensure its superior performance in field applications? First of all, it should be clear that although the non-isolated module does not have the characteristics of isolating the front and back stages, as long as its reliability is improved in some applications, it is completely unnecessary to have a power module with isolation function. Therefore, it is extremely important to improve the application reliability of non-isolated power supply. From the user's point of view, reserve sufficient margin. Most power design engineers often cannot leave enough safe working margin for all electronic devices when weighing performance indicators and device costs. In some abnormal situations, if you want to better improve the reliability of the module, the application engineer must not only use it according to the requirements of the data sheet, but also leave more than 30% derating 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 module model E7815OS-500 has a higher efficiency when the input voltage is below 24V, and the efficiency is higher and stable when the load is within 50%~70%. It can be selected to use it with an input voltage of 19V~24V and a load of 50%~70%, which is relatively safe. From the developer's point of view, design a complete reliability test. Of course, in addition to reducing the input voltage and load current, you can also increase the peripheral circuit through electromagnetic compatibility (EMC) related experiments to improve its reliability. Table Efficiency vs. Voltage, Load Relationship Curve [attach] 329064 [/attach]