To fully understand this topic, we have to talk about the the DC-DC converter. The DC-DC converter is also known as a switch mode converter, where pulses (PWM=pulse width modulation) are used to switch on and off a transistor. They are divided into two which are isolated and non-isolated. The isolated type of converters uses a transformer to isolate the converter from the load, while the non-isolated converter has no transformer. The isolated converters are usually operate with voltages of more than 40 volts. They include flyback converter and forward converter. The isolated converters are used in applications where the voltage is less than 40 volts. They include, buck, boost, buck-boost and cuk converter. This project will only be interested in the isolated type mainly buck and boost converter.
BUCK CONVERTER
The buck converter is a step down converter, its output is a fraction of its input. below is the circuit diagram
The circuit comprises of mainly four components which are transistor (MOSFET) which is driven by a PWM , schottky diode, a coil and a filter capacitor. When the transistor is switched on, the inductor starts to store electromagnetic energy and when it is off, it discharges through the diode. The output of the buck converter is the duty cycle of the PWM times the input voltage. At 100% duty cycle, the input voltage is the same as the output voltage. Therefore varying the duty cycle of the PWM will vary the output voltage.
BOOST CONVERTER
The boost converter is a step up converter its circuit is shown below
The circuit comprises of the same 4 components as the buck converter. The difference is how they are arranged. When the transistor is switched on, current flows from the input source through L and transistor, and energy is stored in the inductor’s magnetic field. There is no current through the diode, and the load current is supplied by the charge in Capacitor. Then when transistor is turned off, L opposes any drop in current by immediately reversing its EMF so that the inductor voltage adds to (i.e., ‘boosts’) the source voltage, and current due to this boosted voltage now flows from the source through the coil, diode and the load, recharging the capacitor as well. Due to this, the output voltage is increased. The output voltage is given as the input voltages divided by (1-the duty cycle).
For this project, the positive buck-boost converter will be used which is a combination of the buck and the boost converter with the capacitor on the buck removed. There is another buck-boost converter, whose output voltage is always inverted. below is the circuit of the inverted buck-boost converter.
