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Selection and Calculation of Capacitance in Power supply(IV)

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Selection of capacitors.


1. Capacity selection:

Large capacitance, the heavier the load, the stronger the ability to absorb current, the larger the capacity of this large capacitance;

Small capacitance, based on experience, can generally 104.

The experience of others.

1. The capacitance to ground filter needs a small capacitance parallel to the ground, which provides a ground to ground path for the high frequency signal.

2. In the power filter, the capacitance should be as close to the ground as possible.

3. Theoretically, the bigger the capacitance is, the better, the larger the capacitor is, the better is the high frequency wave.

4. The reliable method is to parallel a large capacitance and a small capacitor, generally require a difference of more than two orders of magnitude, in order to obtain a larger filter band.

Specific case: AC220-9V after the full bridge rectifier, the need to add the filter capacitance is how big? How big is the capacitance to be added after passing through 78LM05?

The former capacitance should be more than 15V, capacitance capacity should be more than 2000 microtransmitters. The latter capacitance should be more than 9 V, capacity should be more than 220 microtransmitter.


2. A single-phase bridge rectifier circuit with a capacitor filter, the output voltage is 24V and the current is 500mA.

1) Select rectifier Diode 2) Select filter Capacitor 3) another: is Capacitance filter reduced or supercharged?

Because the bridge type is full wave, the current of each diode is only half of the load current, so the maximum current of the diode is more than 250 Ma; the output voltage of the capacitor filter bridge rectifier is 1.2 times the effective value of the input AC voltage. So your circuit should enter an AC voltage RMS of 20V, and the diode will bear a maximum backvoltage of 2 times the root sign of this voltage, so the diode voltage should be more than 28.2V.

(2) the filter capacitance: 1, voltage > 28.2V / 2, the size of C: formula RC ≥ 3- 5) × 0.1 seconds, R=24V/0.5A=48 in this question, we can get C ≥ 0.00625-0.0104F, that is, the value of C should be more than 6250 μ F.

Capacitor filtering is an increase in voltage.

Selection principle of filter Capacitor.

In the design of power supply, the selection principle of filter capacitance is: C ≥ 2.5T / R;

Where C is the filter capacitance in UFU, T is the frequency, and R is the load resistance in Ω.

Of course, this is only a general principle of selection, in practical applications, such as conditions (space and cost) allow, select C ≥ 5T/R.


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