Its function is to convert alternate current to direct current. What we need is a “current steering block” to take this output with its constantly switching polarity, and put it back out in only one single direction. One implementation is to use switches in a diamond configuration, turning them on and off according to our requirement, but implementing this with mechanical switches would be very difficult. We can instead use diodes. A diode is a 2-terminal electronic device, which allows current to pass through it when forward-biased and will block the current when reverse-biased. Thus we can construct what is known as a bridge rectifier.
After rectifying the wave and limiting it to one polarity, we get quite a bumpy waveform ($\lvert\sin{x}\rvert$), so this wave form has to be filtered to allow us to get something which better approximates a pure DC. This is done by filtering, and to do this we attach a capacitor in parallel. The charge-discharge cycle of the capacitor causes the current to not vary as much. There is a bit of a “ripple” still left over, and we select the rating of our capacitor accordingly. Since the ripple is formed by the charge/discharge of the capacitor, we can reduce the ripple by selecting C such that discharge time constant is much greater than the period of our rectified wave. \(R_{L} C >> T; T= \frac{1}{f};\) where $f$ is the frequency of the rectified wave. \(\frac{V_{L}}{I_{Lmax}} >> \frac{1}{f}\) \(C >> \frac{I_{Lmax}}{fV_{l}}\)
While designing this we use \(R_{LC} = \alpha\frac{1}{f}\) where $\alpha$ is our design parameter
Suppose $V_{Lmax} = 5V; I_{Lmax} = 1A$. Then if $f = 100Hz$, \(C = \alpha \frac{1}{(100)(5)}\) Instead, if $f = 50kHz$, \(C = \alpha \frac{1}{(5\times10^4)(5)}\)
So by increasing the frequency, we greatly decrease the capacitance needed to filter our output, something of great significance in appliances like mobile chargers.
For the purpose of regulating our output to be a constant voltage, we take the assistance of a zener diode, a special kind of diode with the amazing property that under reverse bias, it can lock the voltage to a fixed value under ‘zener breakdown’.
We place our zener diode in reverse bias in parallel across our load. We also need a minimum ‘knee current’ across the zener in order to ensure it behaves as a voltage regulator. We used the fact that because of this the voltage across the load is constant and equal to zener voltage, and a simple application of Ohm’s law, to get the useful equations \(I_{L} = I_{S} - I_{Z} = V_{Z}(\frac{1}{R_{L}})\) \(I_{S} = (V - V_{Z})(\frac{1}{R_{S}})\)
