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Application analysis of Schottky diode in power management

any asynchronous DC/DC converter requires a so-called freewheeling diode. In order to optimize the overall efficiency of the scheme, Schottky transistors with low forward voltage are usually preferred. Many designs use a diode recommended by the converter design (Networking) tool. This is not always the best choice for diodes. Moreover, if the design tool does not consider the dynamic changes between thermal performance and leakage current, it is very likely that the actual performance is different from the analysis or simulation results of the design tool. This paper will discuss some typical parameters that should be carefully considered when selecting the right diode, and how to use these parameters to quickly determine whether the selection is correct or not

check loss

Figure 1 shows the basic block diagram of asynchronous DC/DC step-down converter. D1 is the required Schottky tube. The left side is the current when switch S1 is closed (time T1), and the right side is the current when switch S1 is open (time T2)

Figure 1: basic block diagram of asynchronous DC/DC step-down converter

when the time is T2, the output current (IOUT) flows through D1. The resulting loss is directly related to the forward voltage (VFW) and output current of D1. PT2 equals iout*vfw. Obviously, we hope to reduce as much as possible to control losses and reduce heating

During the period oft1, D1 was blocked. The only current is the reverse current. This current is relatively weak and is mainly determined by the blocking voltage or input voltage Vin. The power consumption generated by the diode in T1 stage is called pT1, which is roughly equal to ir*vin

for any Schottky diode, there is a trade-off in the design. That is, this device is optimized for low VF or low IR. Therefore, if low VF is selected, IR is higher, and vice versa. In practical application design, it is important not only to observe the value of VF or IR, but also to analyze what results they will produce in actual operation. VF and IR will change with temperature. When the temperature increases, VF will decrease, which reduces the thermal diffusion while the diode heats up. Unfortunately, IR will increase as the diode temperature increases. Therefore, the higher the diode temperature is, the more the leakage current is, and the more the internal power consumption is, which makes the diode temperature higher, thus increasing the leakage current again, and so on

if we insist on using the basic design case of asynchronous DC/DC converter, we might as well make a basic analysis to determine the internal power consumption of diode and the resulting equipment temperature. The operating duty cycle of DC/DC converter is directly related to the ratio of voltage input and output (dc=vout/vin). The lower the ratio of voltage input and output, the longer the time of T2, and the greater the influence of pT2 on the power consumption of the whole diode. Vice versa, the longer T1 (or the higher the ratio of and), the smaller the impact of pT2 on the total power consumption, and the greater the role of pT1

take two DC/DC converters as examples. Both of them are 24V input voltage, but one of them is 18V output voltage and the other is 5V. Use the ratio of VIN and Vout to calculate the duty cycle, and use the VF and IR values in the data sheet to calculate the total power loss in the diode. Then, according to the total power consumption, calculate the diode temperature caused by the implementation of polyurethane waterproof coating and building materials industry standard in China, and find the actual values of VF and IR at this temperature. Finally, the internal power consumption is recalculated according to the new diode temperature. This iterative process can be repeated many times to improve accuracy, but if you only want to roughly show the impact of the different trade-offs between VF and IR, a single iteration is sufficient

the equipment temperature can be calculated by using the basic thermal equation describing the thermal properties, which is no different from the calculation used to describe voltage, current and resistance. Once the internal power consumption (PTOT) of the device is known, it can be multiplied by the thermal resistance from the node to the environment (rtja) to calculate the temperature change at the device node. Add it to the ambient temperature, and you will get the final junction temperature at which the good electromechanical and oil pump of the equipment can reach the power required by the universal experimental machine experiment under this power consumption and ambient temperature

Figure 2 shows the analysis results. The calculations in this example use pmeg3050bep (optimized for low IR) and pmeg3050ep (optimized for low VF) diodes. The output current range is 1~3a. The temperatures of low VF and low IR diodes are compared here. The initial temperature is assumed to be 25 ℃. Ta (first transfer temperature calculation) and TB (second transfer) are shown in the figure at the same time. The left side is the result of the DC/DC converter with 5V output, and the right side is the DC/DC converter with 18V output (the input voltage of both is 24V). When calculating, it is assumed that rtja adopts the basic 200k/w, and then it is adjusted according to the duty cycle. The data sheet of Schottky diode gives the instantaneous thermal effect curve, which allows the designer to determine the actual thermal resistance according to the specific pulse duty cycle (the thermal effect of short pulse current is better than continuous current). Please note that the total thermal resistance of diodes in any application depends on many factors, and layout is the more important one

Figure 2: analysis results of two DC/DC converters

in Figure 2, it can be found that in the above two cases, when TB is transferred at the second temperature, the diode with low VF begins to heat up. The principle is that when the current is constant, the diode becomes hot due to loss at T2. As the diode temperature increases, the leakage current if increases and the forward voltage VF decreases. However, the rate of increase is much higher than the rate of decrease. As a result, the total power consumption in the diode increases rapidly. At higher output current, pT2 is also higher, which makes pT1 increase faster, so the slope is steep under high current cost and benefit

similarly, the effect of input-output voltage ratio can also be seen. The left side shows a 5V output, low duty cycle DC/DC converter. Low duty cycle means that T2 is longer and pT2 is more. Therefore, more initial heat leads to faster increase of IR and higher pT1. The final result is that the diode temperature rises rapidly as the output current increases. At higher current, it can be seen that the temperature has actually exceeded the specified range. The higher 18V output voltage shown on the right leads to a higher duty cycle, which suppresses pT2. Less heating in the diode means less increase in IR. Therefore, pT1 and overall can ensure more efficient production and recycling of waste parts, and the temperature also increases less

it can be concluded that the higher the duty cycle (or the closer the output voltage is to the input voltage), the better the thermal effect of the diode. For example, if the above calculation, the 12V to 2.5V converter can increase the burden of the diode more than the 12V to 5V converter

thermal escape

the effect of increasing with temperature discussed above will bring a common problem, which is called thermal escape. The increased temperature will cause the temperature to rise further until the component is damaged. Therefore, it is strongly recommended that this phenomenon be thoroughly examined in all designs

at present, the common practice is to simulate the power consumption design. You can use standard simulation tools or common simulation tools. It is necessary to check the thermal effect carefully. For the diodes intended to be used, it is very likely that the tools used do not adopt the correct thermal model, or their thermal parameters (which are likely to be related to the layout) do not conform to the design. Obviously, not every diode is exactly the same, so it is absolutely not recommended to use "similar" diodes in analog design, and then assume that their thermal effects (as well as potential electrical effects) are also similar. Although it is not always feasible, it is still recommended to always make prototypes and verify their correct effects. (end)

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