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# Dingfeng Capacitor---Capacitor's Influence on Keeping Time

Jun 18,2016

ACDC switching power supply is an important technical parameters, electric holding time, refers to from the AC power to the output voltage drop to outside the accuracy range (usually 2%) lag ⊿ t, as shown in figure 1.Informally, switching power supply with no input, output how much longer can hang on.

In many cases, the system is detected after AC power lost, need for data storage, transmission, to set the state of the actuator, and so on.After the AC power switch power supply, therefore, also need to can continue to give the system to provide power for a period of time, to ensure the reliable system shut down.In addition, with the help of UPS system, from the mains switch to the process of UPS power supply, also need to switch power supply can maintain normal output.

Figure 1 off electricity to keep time
1 determinants of hold time when power supply drop
Conventional ACDC principle of switch power supply block diagram as shown in figure 2, the AC input after rectifying filter into a dc voltage ripple voltage (a), then through the DCDC converter converts the voltage output of what we need.Control circuit can be adjusted according to the input voltage and output load duty ratio (PWM mode) to achieve the stable voltage output.In AC input power, the input filter capacitor stored energy to the output of the power supply, in this process, filter capacitor voltage drops gradually, by adjusting the duty ratio control circuit can still achieve the rated output voltage, until the capacitor voltage drop to outside the scope of control circuit can adjust the output voltage began to decline.

Figure 2 ACDC switching power supply principle block diagram
Through examples to illustrate how we decided to keep the time factor when power supply drop.Assuming a Vo = 5 v, Po = 20 w, the efficiency for eta = 0.78 products, the minimum voltage DC - DC part working Vin_min = 100 v, the power supply within the input filter capacitor Cin = 47 uf.Assuming that nominal 220 vac input rectifier filter after the voltage of dc voltage ripple) (the actual to a certain extent, its value Vin_nor = 308 v, according to the energy conservation has the type:
Cin * (Vin_mor2 - Vin_min2) = 0.5 * ⊿ t * Po/eta...(1)
In numerical available: ⊿ t = 77.9 ms.
As you can see, from the type ⊿ t and input capacitance, AC input voltage, is proportional to the product efficiency, and output power and DC - DC part is inversely proportional to the minimum operating voltage.

In the actual engineering conditions, the input voltage is fixed.Specific to a particular model of AC - DC power supply products, internal input filter capacitor Cin, DC - DC part minimum working voltage Vin_min, efficiency eta has been unable to change, so the power supply itself off electricity to keep time to change.Through peripheral adjustment, also can't change the product Vin_min and eta, only can adjust the power front-end rectifier filter after level, an external filter capacitance and power within the input filter capacitor is parallel relationship, equivalent increased the Cin, increase the hold time when power supply drop.

Dingfeng Capacitor---Capacitor's Influence on Keeping Time