1, switching power supply.
At the moment when the switching power supply is turned on, the capacitance voltage of the filter capacitor connected in parallel with the load cannot change suddenly, so a large charging current will be generated, which is what we often call starting surge current. When the filter capacitor is initially charged, it generates starting surge current, and its magnitude depends on the input voltage value when it is started and the total internal resistance of the loop composed of bridge rectifier, electrolytic capacitor and other components.
Calculation of surge current caused by capacitance;
For example, when the input voltage V is 220Vac and the internal resistance of the whole circuit (including rectifier bridge and filter capacitor) is Rs = 1ω, if the power supply is turned on just when the input waveform of the power supply reaches 90 degrees, the peak value of the surge current will reach: I = (220×1.414)/kl. Although this surge current has a short time (mS level), if it is not suppressed, it will shorten the life of the input capacitor and rectifier bridge, and may also cause the voltage of the input power supply to decrease, so that other power supply equipment using the same input power supply will be cut off instantly and interfere with the normal work of adjacent equipment.
? When we choose the power NTC thermistor to protect the surge current caused by the filter capacitor, what parameters should we choose? There are some knowledge points. We suggest that the parameters of power NTC thermistor should be selected from R25 resistor, maximum steady-state current and maximum allowable capacitance (Joule energy). ?
? A. selection of resistance value of power NTC thermistor R25.
The maximum starting current allowed by the circuit determines the resistance of the power NTC thermistor.
Assuming that the rated input of the power supply is 220VAC, the internal resistance is1Ω, and the maximum allowable starting current is 60A, the minimum resistance of the selected power supply NTC in the initial state is:
rmin =(220× 1.4 14/60)- 1 = 4.2(ω)
For this application, we suggest choosing R25 resistance of power NTC thermistor ≧ 4.2Ω.
B. Selection of the maximum steady-state current of power NTC thermistor. ?
The principle of selecting the maximum steady-state current should meet the following requirements: the actual working current of the circuit
Many power supplies are designed with wide voltage (AC 90V-240V), but the power of the product is fixed, so it should be noted that the working current at low voltage input is much higher than that at high voltage input.
According to the formula: w = v I, under the same power condition, for example, the input voltage is 90V, and the working current is 2.7 times that of the input voltage of 240V. Therefore, the actual working current of the circuit is calculated at the lowest voltage.
C. Selection of maximum allowable capacitance (Joule energy) of power NTC thermistor.
For a certain type of power NTC thermistor, the size of the filter capacitor allowed to be connected is strictly required, and this value is also related to the maximum rated voltage.
Starting surge is caused by capacitor charging, so the capacity of NTC thermistor at a given voltage is usually used to evaluate its ability to withstand surge current.
For NTC thermistor with specific power, the maximum Joule energy it can bear has been determined.
Joule energy calculation formula of power NTC thermistor: E= 1/2 CV2.
As can be seen from the above formula, the allowable capacitance value is inversely proportional to the square of the rated voltage. Simply put, the larger the input voltage, the smaller the maximum capacitance allowed to access, and vice versa.
The specifications of power NTC thermistor products usually define the maximum capacitance allowed to be connected at 220V/AC.
Assuming that the maximum rated voltage of the application condition is 420VAC and the filter capacitance is 200 μ F, according to the above energy formula, the equivalent capacitance at 220VAC can be converted as follows:
200×(420)2/(220)2=729μF Therefore, when selecting the type, it is necessary to select the type of NTC thermistor that allows access to power with a capacitance greater than 729μF at 220VAC.
2. DC transformer.
There are large-capacity DC transformers in some electrical appliances. When the transformer is energized, if the sine wave of the power supply is at zero, the transformer will generate a large surge current (inrush current), which is 8-30 times of the rated current of the transformer.
In this case, there will be two problems: 1, transformer overcurrent damage, which will affect transformer life; 2. It interferes with the power grid, thus affecting other electrical appliances used in the power grid. Connecting the power NTC thermistor in series in the power supply circuit can effectively eliminate this influence, as shown in the following figure:
The power of the two transformers in the above figure is 1000W, and the total power is 2000W W w. ..
A. How to select the energy of power NTC thermistor for transformer protection?
? Energy required for NTC thermistor = 1/2I2.
Transformer impedance (ω) XL = input voltage/surge current?
? = 220 V /564 A
? ≈0.390(Ω)
According to XL=2πfL?
Transformer inductance L=0.390/(2*3. 14)/50? ≈ 1242 (er)
Energy required for NTC thermistor (Joule) = 1/2I2.
? = 1/2*( 1242* 10-6)*5642
≈ 198 (Joule)
Among them, the peak surge current 564A is measured by an oscilloscope.
Input voltage =220VAC, working frequency =50Hz.
According to E= 1/2 CV2? It can be calculated that C=6875uF at 240V, which means that the maximum impact capacitance of NTC should be ≥6875uF.
Assume that the efficiency of the transformer is 90% and the minimum input voltage is 90V.
? I steady state = transformer power/(transformer efficiency * minimum input voltage)
=2KWA/(0.9*90V)=24.70A ≈ 25A
? That is, the steady-state current of power NTC thermistor should be ≥25A.
C. The maximum starting current allowed by the circuit determines the resistance of the power NTC thermistor.
? In this way, after calculating the maximum impulse capacitance, steady-state current and resistance value of the required power NTC thermistor, the model of the power NTC thermistor can be conveniently selected from the specification.