Subject : Power Electronics
Unit : AC to DC Converters
Low pass (L-C) filter & Two stage Filter
Low Pass (L-C) Filter:
Fig: Low Pass filter
- A passive low pass filter is the ideal choice. But two problems arise; one is the voltage level, the other is the power or current level. All the elements used, L C or R must be properly rated for the voltage or current level as needed.
- A single stage filter (L-C) is used to reduce the harmonic components in both voltage and current waveforms of a single phase full wave diode bridge converter with resistive (R_{L}).
- For the lowest harmonic frequency of 100 Hz, the value of the inductor needed is high, needing an iron-cored coil. The size also may be large, if the power or current level is high. As stated earlier, such that n^{th }harmonic ripple content passes through the filter capacitor (C), the impedance of the series path must be much greater than that of the capacitance, i.e.,
and the effect of load is negligible. As shown, the capacitive reactance chosen is total load impedance divided by a factor of 10
- The advantages are small ripple factor with just a single stage (L-C) used, with higher dc output voltage. The main advantage is poor voltage regulation, also resulting in higher peak anode current and peak inverse voltage rating.
Two-stage Filter:
Fig: Two stage filter
- In a two stage filter, the first one is only capacitive (C_{1}) to reduce the harmonic content in voltage waveform, followed by second stage (R-C), instead of L-C. The size may be reduced as the size of R is smaller than that of L, as given earlier.
- This circuit offers satisfactory operation at light loads, but considerably poor voltage regulation due to drop in R, resulting in higher ripple content, at heavy load.
- If a single capacitor (C_{1}) is used as a single stage one, the ripple factor (RF) is
- For a chosen ripple factor, the value of C_{1 }may be computed, if the values of rated output voltage and current for the converter are known or given.
- These filters may also be used at the output of the dc-dc converter circuits. Only a single stage (L-C) filter may be used, as the frequency in the output voltage in such case is much higher than the harmonic frequency (100 Hz) for the single phase full wave ac-dc converter (rectifier) circuit.