A simple method to increase AC-DC power supply power loss and keep time
Abstract: In some special applications, the conventional AC-DC switching power supply power loss holding time can not meet the requirements, and ultra-long power supply holding time switch power supply in the market is very rare. In order to meet the requirements of these special applications, this paper theoretically analyzes the factors that determine the power-down holding time, and proposes a simple method to increase the power-down holding time by using peripheral circuits. Keywords: AC-DC; Module power supply; power-down holding time 0 introduction ACDC switching power supply has an important technical parameter-power-down holding time, it refers to the time difference between AC power loss and output voltage drop to the precision range (usually-2%), as shown in 1. In layman's terms, it is how long the output will last after the power supply is not input. In many cases, after the system detects that the AC power is down, it needs to save and transmit the data and set the status of the actuator. Therefore, after the AC power is down, the switching power supply must be able to provide power to the system for a period of time to ensure reliable shutdown of the system. In addition, in a UPS system, switching from a mains supply to a UPS Power Supply also requires switching the power supply to maintain normal output. Figure 1 power-down Holding time 1 the deciding factor of power-down holding time conventional ACDC Switching Power Supply principle diagram 2 shows that after AC input is rectified and filtered, it becomes a dc voltage (with a certain ripple voltage), and then converts it to the voltage output we need through the dcdc converter. The control circuit can adjust the duty cycle (PWM mode) based on the input voltage and output load to achieve stable voltage output. After the AC input loses power, the energy stored by the input filter capacitor powers the output. In this process, the voltage of the filter capacitor gradually decreases, the control circuit can still realize the rated output voltage by adjusting the duty cycle until the capacitor voltage falls beyond the range that can be adjusted by the control circuit, and the output voltage begins to decrease. Figure 2 ACDC Switching Power Supply principle diagram we use examples to illustrate the factors that determine the power-down retention time. Assume that a Vo = 5 V, po = 20 W, efficiency = 0.78 products, DC-DC part can work properly at the lowest voltage vin_min = 100 V, the internal input filter capacitance of the power supply is CIN = 47 UF. Assume that the voltage after rectification and filtering of the nominal 220vac input is DC voltage (actually there is a fixed ripple), the value of vin_nor = 308 V, according to the conservation of energy has the following formula: 0.5 * CIN * (Vin_mor2-Vin_min2) = snapshot T * Po/ETA ...... (1) The value of substitution can be found at least T = 77.9 Ms. From the above we can see that the voltage T is proportional to the input capacitor, AC input voltage, product efficiency, and the output power, DC-DC part of the Lowest operating voltage is inversely proportional. In the actual engineering environment, the input voltage is fixed. For a specific AC-DC power supply products, internal input filter capacitor Cin, DC-DC part of the Minimum Operating Voltage vin_min, efficiency can not change, so, the power supply's power loss and retention time cannot be changed. Through peripheral adjustment, the vin_min and ETA of the product cannot be changed. The only adjustment is that the first-level rectifying filter is performed at the front end of the power supply, the external filter capacitor is in parallel with the input filter capacitor in the power supply, equivalent to adding CIN and increasing the power loss duration. If the filter capacitor added to the front end of the power supply is 100 UF/400 V, the power loss duration is increased by 165.7 ms based on the previous period. If the power supply is working in half-load conditions, the power-down duration can be doubled, as shown in table 1. Table 1 Relationship between power failure and input capacitance and Load
2. Effect of output capacitor on power-down duration
Under the above conditions, we will calculate the effect of increasing the output capacitor Co to prolong the power-down duration. Assume that the output voltage accuracy is ± 2%, then the output voltage lower limit vo1 = 4.9 V, CO = 40000 UF, there are: 0.5 * Co * (Vo2-Vo12) = rjt1 * Po ...... (2) the substitution values can be: T1 = 1 ms. We found that the output capacity of UF is extended, and the power loss duration is only 1 ms! It can be seen that increasing the output capacitor has little impact on the power-down duration.
3. Recommended peripheral circuit
As shown in 3, the input end of the peripheral circuit is connected to the mains supply, and the output can be equivalent to the DC power supply, connected to the L and N of the ACDC switch power supply. C1 should be determined based on the actual load, input voltage, and the required maintenance time. There is no clear recommendation value. As C1 is added, the impact current increases at startup. R1 can be used to reduce the impact current. You can select a 3 W winding resistance with a resistance of roughly 2 ~ 7.5 ohm. For D1, 1000 V and above can be selected. In addition, the shock current (as described in the Specification) that the bridge can withstand should be greater than the shock current in the actual circuit. Figure 3 recommended peripheral circuit
4. Test Results and Analysis
In order to verify the correctness of theoretical analysis and calculation, the LH25-10B05 of mornsun is selected for testing. When the input voltage is 220vac and the load is 4A, the power-down holding time of the LH25-10B05 is 76.8 ms, and the waveform is 5, the power supply power loss duration is increased to 249 ms, as shown in waveform 6. The test results are consistent with the theoretical calculation results .?
5 conclusion
This paper theoretically analyzes the power-down process of the switching power supply and derives the formula for calculating the power-down holding time. Based on the formula, it finds a way to increase the power-down holding time through an external circuit. The correctness of the theoretical calculation is verified through actual tests. This method is easy to implement and has a significant effect on increasing the power-down holding time. It has a strong engineering guiding significance.
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? Original Reprinted from: http://www.mornsun.cn/news/NewDetail.aspx? Id = 237 & channelid = 132 .?
A simple method to increase AC-DC power supply power loss and keep time
