0 Introduction
An important technical parameter for AC/DC switching power supplies is the power-down hold-up time, which refers to the time difference Δt between the AC power failure and the output voltage dropping outside the accuracy range (usually -2%), as shown in Figure 1. In simpler terms, it's how long the output of the switching power supply can withstand the loss of input.
In many cases, after detecting an AC power failure, the system needs to save and transmit data, set actuator states, and so on. Therefore, after an AC power failure, the switching power supply also needs to be able to continuously supply power to the system for a period of time to ensure a reliable system shutdown. Additionally, in systems with a UPS, the switching power supply also needs to maintain normal output during the switch from AC power to UPS power.
Figure 1. Schematic diagram of power failure retention time
1. Factors determining power failure retention time
The block diagram of a conventional AC-DC switching power supply is shown in Figure 2. The AC input is rectified and filtered to become a DC voltage (with some ripple), which is then converted into the desired output voltage by a DC-DC converter. The control circuit adjusts the duty cycle (PWM mode) according to the input voltage and output load to achieve a stable voltage output. When the AC input is de-energized, the energy stored in the input filter capacitor supplies power to the output. During this process, the voltage of the filter capacitor 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, at which point the output voltage begins to decrease.
Figure 2 Block diagram of ACDC switching power supply
Let's illustrate the factors determining power-down hold-up time with an example. Assume a product with Vo=5V, Po=20W, and an efficiency of η=0.78. The minimum voltage at which the DC-DC section can operate normally is Vin_min=100V, and the internal input filter capacitor Cin=47uF. Assume the nominal 220Vac input voltage after rectification and filtering is a DC voltage (actually, there is some ripple), with a value of Vin_nor=308V. According to the law of conservation of energy, we have the following formula:
0.5*Cin*（Vin_mor2-Vin_min2）=⊿t*Po/η……(1)
Substituting the values, we get: Δt = 77.9 ms.
As can be seen from the above formula, Δt is directly proportional to the input capacitance, AC input voltage, and product efficiency, while it is inversely proportional to the output power and the minimum operating voltage of the DC-DC section.
In real-world 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 section, and the efficiency η are all fixed. Therefore, the power-down hold-up time of the power supply itself cannot be changed. External adjustments cannot alter Vin_min and η. The only adjustable method is to add an additional rectifier and filter stage at the power supply's front end. This external filter capacitor is connected in parallel with the internal input filter capacitor, effectively increasing Cin and thus extending the power-down hold-up time.
If the filter capacitor added to the front end of the power supply is 100uF/400V, the power-down hold-up time will increase by 165.7ms. If the power supply operates under half-load conditions, the power-down hold-up time can be doubled, as shown in Table 1.
Table 1. Relationship between power-down retention time and input capacitance and load
2. The effect of output capacitor on power-down retention time
Under the above conditions, let's calculate the effect of extending the power-down hold-up time by increasing the output capacitor Co. Assuming the output voltage accuracy is ±2%, then the lower limit of the output voltage Vo1 = 4.9V and Co = 40000uF, we have:
0.5*Co*(Vo2-Vo12)=⊿t1*Po……(2)
Substituting the values, we get: Δt1 = 1ms.
We found that adding a 40000uF output capacitor only extended the power-down hold-up time by 1ms! This shows that increasing the output capacitor has a negligible impact on the power-down hold-up time.
3. Recommended peripheral circuits
As shown in Figure 3, the input terminal of the peripheral circuit is connected to AC power, and the output can be equivalent to a DC power supply, connected to the L and N terminals of the ACDC switching power supply. The value of C1 needs to be determined based on the actual load, input voltage, and required hold-up time; there is no specific recommended value. Due to the addition of C1, the inrush current will increase. R1 can be used to reduce this inrush current; a 3W wire-wound resistor with a resistance value approximately between 2 and 7.5 ohms can be selected. For D1, a 1000V rectifier bridge with a current rating of 1.5A or higher can be selected. Furthermore, the inrush current that the rectifier bridge can withstand (specified in the datasheet) should be greater than the inrush current in the actual circuit.
Figure 3 Recommended peripheral circuit
4. Experimental Results and Analysis
To verify the correctness of the theoretical analysis and calculations, the MORNSUN LH25-10B05 was selected for testing. Under the conditions of an input voltage of 220Vac and a load of 4A, the power-down hold-up time of the LH25-10B05 was 76.8ms, as shown in Figure 5. After adding an external 100uF/400V input filter capacitor according to the recommended circuit, the power-down hold-up time increased to 249ms, as shown in Figure 6. The experimental results are in good agreement with the theoretical calculation results.
5 Conclusion
This paper theoretically analyzes the power-down process of a switching power supply and derives a formula for calculating the power-down hold-up time. Based on the formula, a method to increase the power-down hold-up time through external circuitry is found. Practical testing verifies the correctness of the theoretical calculation. This method is simple to implement, significantly improves the power-down hold-up time, and has strong engineering guiding significance.
Original link: http://www.mornsun.cn/news/NewDetail.aspx?id=237
