Designing the positive buck boost converter

PWM CIRCUIT
The PWM pulse that will be used, will be generated from the 555 timer IC. The timer is connected in astable mode where the output is taken from pin 3 and the duty cycle is controlled by varying the voltage going into pin 5. The circuit diagram is shown below.

The frequency of the timer is set by R1, R2 and C3 using the formula
F= (1.44)/((R1 + 2R2)*C3)
Capacitor C1 and C2 are used to filter out noise. More details on this can be found in the 555 timer datasheet.

POSITIVE BUCK - BOOST CONVERTER

The PBB converter was built using 2xIRF510 power MOSFET, 2xBY229 rectifier diode, one inductor and one capacitor.  The circuit diagram is shown below

From the circuit above it can be seen that this converter can have 4 possible states which are shown in the table below

However for simplicity this project will only use state 00 and 11. The output voltage from this circuit can rise from zero to the desired value you want depending on the inductor size and the duty cycle of the PWM. Also it should be noted that the larger the inductor size, the lesser the current consumed and the lesser the output voltage.

FEEDBACK DESIGN

The purpose of the feedback circuit is to ensure that the DC motor rotates at a constant speed irrespective of the load changes. In theory we know that one of the way to vary the speed of a DC motor is to vary the voltage applied to the terminals of the DC motor. Also we know that when the load a DC motor increases, the speed reduces which means the voltage at its terminal reduces and it posses more torque. To maintain this motor back to its original speed, the duty cycle of the PWM has to be increased thereby increasing the overall output voltage. Below is the circuit for the feedback control

The circuit above behaves as an amplifier and as a comparator. The circuit ensures that the voltage at the inverting pin is equal to that of the non-inverting pin in so doing the the motor is maintained at a constant speed. If the inverting pin is greater than the non-inverting pin, the output voltage is low and vice versa.

RPM COUNTER

The RPM counter is to be designed using a reflective object sensor (QRD1114) and a 2n3904 NPN transistor. The motor shaft is black with a strip of white line. When the sensor detects a white surface it gives a high voltage and when it detects  a black surface it gives low voltage, thereby generating a pulse signal. The 2n3904 transistor will form a darlington pair with the photo transistor thereby generating enough voltage that can be detected. Below is the circuit diagram

The controller that will be used to count the number of pulse will be the PICAXE 20M2 micro-controller. The micro-controller has the ability to sample the number of pulses that enters into any of its input pin. The value counted is multiplied by 60 to find the RPM.

Complete system working
System Simulation

Hardware implemented 

Overview of DC-DC converters

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.