A simple method to increase the hold-up time of AC-DC power supply

　　0 Preface

　　ACDC switching power supply has an important technical parameter - power-off holding time, which refers to the time difference ⊿t from AC power off to the output voltage dropping outside the accuracy range (usually -2%), as shown in Figure 1. In layman's terms, it is how long the output of the switching power supply can last after there is no input.

　　In many cases, after the system detects AC power failure, it needs to save and transmit data, set the status of the actuator, etc. Therefore, after the AC power failure, the switching power supply needs to continue to provide power to the system for a period of time to ensure that the system is shut down reliably. In addition, in a system with UPS, the switching power supply also needs to maintain normal output during the process of switching from AC power to UPS power supply.

　　Figure 1 Schematic diagram of power-off hold time

　　1 Determinants of power-off retention time

　　The block diagram of the conventional ACDC switching power supply is shown in Figure 2. After the AC input is rectified and filtered, it becomes a DC voltage (with a certain ripple voltage), and then converted into the voltage output we need through the DCDC converter. The control circuit can adjust the duty cycle (PWM mode) according to the input voltage and output load to achieve a stable voltage output. After the AC input is powered off, the energy stored in the input filter capacitor supplies power to the output. In this process, the filter capacitor voltage gradually decreases. The control circuit can still achieve the rated output voltage by adjusting the duty cycle until the capacitor voltage drops beyond the range that the control circuit can adjust, and the output voltage begins to decrease.

　　Figure 2 ACDC switching power supply block diagram

　　Let's use an example to illustrate the factors that determine the power-off hold time. Assume a product with Vo=5V, Po=20W, and efficiency η=0.78, the minimum voltage Vin_min=100V for the DC-DC part to work normally, and the internal input filter capacitor Cin=47uF. Assume that the voltage after the nominal 220Vac input rectification and filtering is a DC voltage (actually with a certain ripple), and its value Vin_nor=308V, according to the energy conservation law, the following formula is obtained:

　　0.5*Cin*(Vin_mor2-Vin_min2)=⊿t*Po/η……(1)

　　Substituting the numerical values, we obtain: ⊿t=77.9ms.

　　From the above formula, we can see that ⊿t is proportional to the input capacitance, AC input voltage, and product efficiency, and inversely proportional to the output power and the minimum operating voltage of the DC-DC part.

　　In actual engineering environments, the input voltage is fixed. For a specific AC-DC power supply product, the internal input filter capacitor Cin, the minimum operating voltage Vin_min of the DC-DC part, and the efficiency η cannot be changed, so the power-off hold time of the power supply itself cannot be changed. The product's Vin_min and η cannot be changed through external adjustments. The only thing that can be adjusted is to connect a rectifier filter at the front end of the power supply. The external filter capacitor is in parallel with the input filter capacitor inside the power supply, which is equivalent to increasing Cin and increasing the power-off hold time.

　　If the filter capacitor added to the front end of the power supply is 100uF/400V, the power-off hold-up time will increase by 165.7ms. If the power supply works under half-load conditions, the power-off hold-up time can be doubled, as shown in Table 1.

　　Table 1 Relationship between power-off hold time, input capacitance and load

　　2 Effect of output capacitance on power-off hold time

　　Under the above conditions, we calculate the effect of extending the power-off holding time by increasing the output capacitor Co. Assume that the output voltage accuracy is ±2%, the output voltage lower limit Vo1=4.9V, Co=40000uF, then:

　　0.5*Co*(Vo2-Vo12)=⊿t1*Po……(2)

　　Substituting the numerical values, we obtain: ⊿t1=1ms.

　　We found that the output capacitor of 40000uF extended the power-off holding time by only 1ms! This shows that increasing the output capacitor has little effect on the power-off holding time.

　　3 Recommended peripheral circuits

　　As shown in Figure 3, the input terminal of the peripheral circuit is connected to the mains, and the output can be equivalent to a DC power supply, which is connected to the L and N of the ACDC switching power supply. C1 should be determined based on the actual load, input voltage and required holding time, and there is no clear recommended value. Due to the addition of C1, the startup impact current will increase, and R1 can be used to reduce the impact current. A 3W winding resistor can be selected, and the resistance value is roughly between 2 and 7.5 ohms. For D1, a 1000V rectifier bridge with a current of 1.5A or above can be selected. In addition, the impact current that the rectifier bridge can withstand (described in the specification) should be greater than the impact current in the actual circuit.

　　Figure 3 Peripheral recommended circuit

　　4 Experimental results and analysis

　　In order to verify the correctness of theoretical analysis and calculation, MORNSUN's LH25-10B05 was selected for testing. Under the conditions of input voltage 220Vac and load 4A, the power-off holding time of LH25-10B05 is 76.8ms, as shown in Figure 5; after connecting an external 100uF/400V input filter capacitor according to the recommended circuit, the power-off holding time of the power supply increases to 249ms, as shown in Figure 6. The test results are consistent with the theoretical calculation results.

　　5 Conclusion

　　This paper theoretically analyzes the power-off process of the switching power supply and derives the calculation formula for the power-off holding time. According to the formula, a way to increase the power-off holding time through an external circuit is found. Through actual tests, the correctness of the theoretical calculation is verified. This method is simple to implement and has a significant effect on increasing the power-off holding time, which has a strong engineering guidance significance.