Power Supply Units and Electronic Oscillator Circuits
Power Supply Units (PSUs) and Operation
Defining Power Supply Units (PSUs)
A PSU is an electronic device capable of transforming the AC voltage from the industrial distribution network (50 Hz) into voltage suitable for powering electronic devices.
Switched Power Supplies vs. Linear Sources
A Switched Power Supply is an electronic device that converts electrical energy by switching transistors. While a voltage regulator uses transistors biased in the active region of amplification, switching power supplies use the same switch, turning on and off at high frequencies (typically 20–100 KHz) between cut (open) and saturation (closed).
Disadvantages of Switched Power Supplies
Compared with linear sources, switched power supplies:
- Are more complex.
- Generate high-frequency electrical noise that must be carefully minimized to avoid causing interference to nearby equipment.
Regarding Size and Weight, the linear transformer is usually heavier than the switched one. Output voltage [characteristics may also differ].
Linear Power Supplies
The structure of linear power supplies follows this pattern:
- Transformation
- Rectification (Correction)
- Filtering
- Regulation
1. Transformation
The transformer is used to reduce the voltage taken from the AC power distribution network (220 V RMS, 50 Hz) and to isolate the power assumption from the rest of the circuit. It has two windings, primary and secondary, on the same magnetic core, so that there is a power transmission by electromagnetic induction.
2. Rectification (Correction)
Rectification is the process by which the secondary AC current is transformed into unidirectional (or pulsed) current. Rectification may be:
- Half-wave: Secondary current passes in one direction only through the diode. Thus, current flows always in the same direction through the load.
- Full-wave: Consisting of four diodes. The current through the load $R_L$ always circulates in the same direction, and there are no zero conduction intervals as in the half-wave rectifier.
3. Filtering
Once the AC signal is rectified, a filtering stage is included to eliminate alternate components and obtain a continuous signal. This is achieved by a capacitor in parallel with the load, $R_L$.
4. Regulation
Regulation is done to provide the source with low output impedance. At the same time, it decreases the ripple.
Types of Regulators
- Zener diode as voltage reference.
- Power transistors to allow large output currents.
- Regulators with operational amplifiers.
- Integrated regulators.
Oscillators and Their Classification
What is an Oscillator?
An oscillator is a circuit that is capable of converting DC current into a current that varies periodically in time (periodic current). These oscillations may be sinusoidal, square, triangular, etc., depending on the form given to the wave produced. It is basically an amplifier whose input signal is taken from its own output through a feedback loop.
Coupled Transistor Oscillator
This is a typical oscillator circuit for RF receivers integrated into one chip.
Phase Shift Oscillators
- Phase Shift Oscillator (Transistor): Uses a BJT transistor as the active element of the amplifier stage.
- Phase Shift Oscillator (FET): Uses an FET transistor as the active element of the amplifier stage.
Low Frequency Oscillators
The Wien-Bridge Oscillator
The Wien-bridge oscillator circuit is a typical oscillator used for small to moderate frequencies in the range of 5 Hz to 1 MHz. It is used mostly in commercial audio generators.
High-Frequency Oscillators (LC Circuits)
There are two classical models of oscillator for communications with a single active element, which can be a BJT or a MOSFET.
Colpitts Oscillator
Although the Wien bridge oscillator is excellent at low frequencies, it is not appropriate when operating at high frequencies (above 1 MHz). An alternative is an LC oscillator, a circuit that can be used for frequencies between 1 and 500 MHz.
Hartley Oscillator
In the Hartley oscillator, the feedback voltage is generated by an inductive voltage splitter, consisting of $L_1$ and $L_2$.
Crystal Oscillators
A crystal oscillator is a harmonic oscillator whose frequency is determined by a quartz crystal or a piezoelectric ceramic. Because accuracy and stability of the oscillation frequency are important, a quartz crystal oscillator is often used. Crystal oscillators are commonly employed in communication transmitters and receivers.