Rectifier

Diodo Rectifier 2N1849

A rectifier is the electronic device that converts alternating current into direct current. This is done using rectifying diodes, whether they are solid-state semiconductors, vacuum tubes, or gas tubes such as those of mercury vapor (currently in disuse).

Depending on the characteristics of the alternating current power they use, they are classified as single-phase, when they are powered by one phase of the electrical network, or three-phase when they are powered by three phases.

Depending on the type of rectification, they can be half wave, when only one of the half cycles of the current is used, or full wave, where both half cycles are used.

The most basic type of rectifier is the single-phase half-wave rectifier, consisting of a single diode between the AC power source and the load.

Uncontrolled single-phase coil

Uncontrolled rectification requires a prior study of the needs, since the rectifying circuit will only work correctly if all the boundary conditions with which the calculation has been made are met. That is, both the input voltage and the RL load must be as specified.

Half Wave Rectifier Circuits

Figure I.- Mid-wave rectifier circuit

It is built with a diode as it can keep the current flow in one direction, it can be used to change a CA signal to one CC. Figure I shows a half-wave rectifier circuit. When the input voltage is positive, the diode is polarized live and can be replaced by a short circuit. If the input voltage is negative the diode is polarized inverse and can be replaced by an open circuit. Therefore when the diode is polarized live, the output voltage through the load can be found through the ratio of a voltage divider. We also know that the diode requires 0.7 volts to polarize, so the output voltage is reduced in this amount (this voltage depends on the material of the diode juncture). When polarization is reverse, the current is zero, so the output voltage is also zero. This rectifier is not very efficient because during the half of each cycle the input is completely blocked from the output, thus losing half the power voltage. The output voltage in this type of rectifier is approximately 0.45 times the effective voltage of the input signal (this 0.45 arises from calculating 12π π ∫ ∫ 0π π U2without θ θ dθ θ =U2π π ≈ ≈ 0.45U{displaystyle {frac {1}{2pi }}}int _{0}^{pi }U{sqrt {2}}}sin {theta }dtheta ={frac {U{sqrt {2}}{pi }}}{approx 0.45U}). The waveform we observe at the exit is shown in figure I.

Full Wave Rectifier Circuits

A full-wave rectifier converts the entire input waveform to a constant polarity (positive or negative) at the output, by inverting the negative (or positive) portions (half-cycles) of the waveform. input. The positive (or negative) portions are combined with the inverses of the negative (positive) portions to produce a partially positive (negative) waveform.

Full-wave rectifier using two diodes with midpoint transformer

Figure 2.- Full K wave rectifier circuit

The circuit, represented in Figure 2, works as follows:

The transformer converts the alternating input voltage into another alternating voltage of the desired value, this voltage is rectified during the first half cycle by diode D1 and during the second half cycle by diode D2, so that the load R reaches a very impure pulsating DC voltage since it is neither filtered nor stabilized.

In this circuit we take the potential value 0 in the intermediate tap of the transformer.

Graetz Double Bridge Full Wave Rectifier

This is a full wave rectifier in which, unlike the previous one, it is only necessary to use a transformer if the output voltage must have a different value from the input voltage.

Figure 3 shows the circuit of a rectifier of this type.

Figure 3.- Full-wave rectifier with Gratz Bridge

In order to facilitate the explanation of the operation of this circuit, we will call the diode located higher up D-1 and D-2, D-3 and D-4 the following ones in descending order.

• During the semi-cycle in which the upper secondary point of the transformer is positive with respect to the lower part of that secondary, the current circulates through the following path:

Top point of secondary --> Diode D-1 --> (+)Load resistance R(-) --> Diode D-4 --> lower point of the secondary.

• In the next half-cycle, when the upper secondary point is negative and the lower positive will do so by:

Lower point of secondary --> Diode D-2 --> (+)Load resistance R (-) --> Diode D-3 --> top of the secondary.

In this case, we see how current flows through the load, in the same direction, in the two half-cycles, with which both are used and a more uniform rectified current is obtained than in the case of the half-wave rectifier, where During one half cycle the flow of current through the load is interrupted.

In both types of full-wave rectifiers, the output rectified current waveform will be that of a pulsating direct current, but with a pulse frequency twice that of the alternating current supply.

Filtering

Filtering

To avoid this inconvenience, we proceed to a filter to eliminate the ripple of the rectified pulsating signal. This is done using RC (resistance-capacitance) or LC (inductance-capacitance) filters, finally obtaining a direct current at the output with a ripple that depends on the filter and the load, so that without any load, there is no ripple. It should be noted that this filter is not linear, due to the existence of the diodes that rapidly charge the capacitors, which in turn slowly discharge through the load.

The ripple voltage (Vr) will be much less than V if the time constant of the capacitor R·C is much greater than the period of the signal. Then we will consider the linear discharge slope and, therefore, Vr = Vpeak T / (R C) Being R·C the constant time of the capacitor, T the period of the signal and Vpeak the peak voltage of the signal.

Controlled single-phase rectification

This is a much more complicated type of regulation to implement, but it provides full control of the load. The scheme of this type of rectifiers would be like the one previously exposed, adding between the load and the rectified output, conceptually, a switch. This 'switch' called thyristors (SCR) would allow the passage of the signal to be cut within a corresponding angle between 0 and 180 degrees of the sine wave, allowing power control within those firing angles.

It should be added that the complexity lies in the design of the control system, where the 'switch' conceptual has to be replaced by a circuit as complicated as the device requires.

Synchronous (or synchronous) rectifier

There are applications in which the forward voltage drop across the diodes (V_F) causes them to have low efficiency, such as some DC-DC converters. A synchronous rectifier replaces the diodes with MOSFET transistors, governed by a control circuit that cuts them off when the voltage enters its negative cycle. This technique has three advantages over diodes:

• There is no V_F in a MOSFET. This behaves like a resistance (R)_ON) so it leads with any voltage value (Vboli0) while a diode needs V/2003/V_Fwhich is of utmost importance in circuits fed to very low voltage.