It is often necessary to increase or decrease DC voltages. The circuits for increasing or decreasing DC voltages are not simple as is the case with AC voltages. Changing the level of DC voltages requires complex circuitry. These circuits are called DC to DC converters. DC – DC converters are the electronic circuits that convert a constant DC voltage into a high voltage level or a low voltage level.
When a circuit increases the DC voltage to a higher level, it is called a Boost Converter. When a circuit steps down the DC voltage to a lower level, it is called a Buck converter. As a boost converter converts DC voltage to a higher voltage level, it is also known as a step-up converter. To increase the voltage signal, a regulator circuit is required that can increase the input voltage signal.
Most electronic devices, such as smartphones and tablets, work with 5V DC. However, for general use, 3.7V batteries are quite common. These batteries can be used to power 5V devices using a boost converter circuit. In this electronic project, the 3.7V lithium-ion battery voltage is increased to 5V DC. The end-of-discharge voltage of the lithium-ion battery can be assumed to be 3.5 V, so this circuit will convert the minimum input voltage of 3.5 V to the level of 5 V. A maximum current of 500 mA can be consumed by this boost converter.
The regulator used to boost the signal in this project is the MC34063AP1, which will increase the input signal to the desired voltage level.
Required components
Figure 1 : List of components required for the DC to DC Boost converter
Circuit connections –
In this project, the boost converter circuit is built using 34063A DC to DC converter IC. The input voltage is supplied through a 3.7 V battery that has its anode connected to pin 6 of the regulator IC and its cathode connected to common ground. A capacitor Cin is connected to pin 6 to remove ripples from the input signal. An additional capacitor C1 is connected in parallel to the capacitor Cin to reduce the overall ESR of the capacitances. The output voltage is taken from pin 5 of the regulator IC through a voltage divider circuit formed by resistors R1 and R2. At pin 7 of the IC, a current limiting resistor Rsc is connected and at pin 8, a resistor R3 is connected to limit the current in the base of the IC's built-in transistor. Pins 2 and 4 of the IC are grounded. At pin 1, an inductor and a diode are connected to increase the input voltage. At pin 3 of the IC, a timing capacitor Ct is connected. There is a Co capacitor connected to the output of the circuit to reduce ripples in the output signal.
How the circuit works –
Before understanding the operation of the 34063 IC based boost converter circuit, it is important to understand how a basic boost converter circuit works. Following is the basic circuit of a boost converter.
Fig. 2: Basic Boost Converter Circuit Diagram
In a boost converter circuit, the output is greater than the input voltage signal. The basic circuit of a boost converter consists of an oscillator to provide the input signal, a diode, a switching component such as transistor and at least one charge storage element (capacitor or inductor).
The oscillator provides a square wave at the input, so during the positive half cycle of the square wave, the inductor stores some energy and generates a magnetic field. During this phase, the left terminal of the inductor has positive voltage. The base of the transistor receives positive voltage and turns on. Therefore, the anode of the diode is at lower potential and acts as an open circuit. Thus, all current from the source flows through the inductor to the transistor and finally to ground.
Fig. 3: Circuit diagram showing the positive cycle in the operation of the Boost converter circuit
During the negative half cycle, the MOSFET is turned off. Due to this, the inductor cannot charge. The current through the inductor generates a reverse emf (according to Lenz's law) which reverses the polarity of the inductor (as shown in the image below). Therefore, the diode is forward biased.
Now the stored charge of the inductor starts discharging through the diode and a higher level voltage is obtained at the output. In this case, the output voltage depends on the charge stored in the inductor. The greater the stored charge, the higher the output voltage. Therefore, if the inductor charging time is longer, the charge stored in the inductor also increases. Thus, they become two input voltage sources – one is the inductor and the other is the input supply. Therefore, the output voltage is always greater than the input voltage.
Fig. 4: Circuit diagram showing the negative cycle in the operation of the Boost converter circuit
To increase efficiency and remove ripples from the boost converter output, some other components need to be added to the basic boost converter circuit.
Designing a boost converter circuit using the 34063 regulator –
In this project, a DC to DC converter is designed using regulator IC 34063. This regulator is an IC specially designed for DC-DC conversion. It provides a constant and regulated output voltage. Internally, this regulator has a transistor with an oscillator where the oscillator provides a square wave frequency of up to 100 kHz.
The input signal for operation of the 34063 regulator can range from 3V to 40V and the output voltage can be adjusted as required using a voltage divider network. The IC can be used in boost converter, buck converter and voltage inverter applications. The IC has 8 pins with the following pin configuration –
Fig. 5: Table listing the IC 34063 regulator pin configuration
The IC regulator comes with the following features –
• Low standby current – consumes much less current when no load is connected to the output.
• It can provide output current up to 1.5 A by changing the external circuit of this boost converter.
• Adjustable output voltage – user can change the output voltage as needed.
• Adjustable frequency up to 100 kHz
Fig. 6: Internal circuit of the IC 34063 regulator
In figure 2 it can be seen that along with the basic components such as oscillator, transistor, diode and inductor that are essentially part of the basic circuit design of a boost converter (figure 1), the 34063 regulator also has additional components. These components are used to provide more features to the user and increase the efficiency of the boost converter circuit.
The following circuit is used to make a boost converter using 34063 regulator –
Fig. 7: Boost Converter Circuit Diagram
The different external components interconnected with the IC regulator serve the following specific functions –
Timing Capacitor CT- There is a capacitor connected to pin 3. Pin 3 serves as a timing capacitor. The capacitor connected at pin 3 defines the switching frequency of the regulator IC.
Current limiting resistor Rsc -There is a current limiting resistor connected at pin 7 of regulator IC. The source current resistance Rsc is connected between pin 7 and the positive terminal of the battery. The resistance Rsc limits the peak current Ipk (maximum internal current flowing from the inductor and diode) of the circuit. This is why when designing the circuit, it is important to choose the right inductor and diode that can allow the maximum current Ipk.
Capacitances Cin, Co and C1 – There are capacitors Cin, Co and C1 connected in the circuit to filter input and output signals. The capacitances Cin and Co are used at the input and output respectively. These capacitors reduce unwanted ripples and noise in input and output signals. The Co capacitor provides a smooth, regulated DC voltage at the output. An additional capacitor C1 of very small value is also used in parallel with the capacitor Cin to reduce the ESR (Equivalent Series Resistance) in the input voltage.
Resistors R1, R2 and R3 – There are feedback resistors R1, R2 and R3 connected in the circuit. The R1 and R2 are the feedback resistors that decide the desired output voltage. The output voltage depends on the feedback resistors by the following equation –
Vout = Vref*(1+(R2/R1))
The voltage Vref is the reference voltage. Internally, the 34063A provides a stable reference voltage of 1.25 V. For a desired output voltage, the values of feedback resistors R1 and R2 can be calculated as follows –
Vout = 1.25*(1+(R2/R1))
5 = 1.25*(1+(R2/R1)) (Since the desired output voltage, Vout = 5V)
Calculating the above equation,
R2 = 3*R1
If R1 is considered 15k ohm
R2 = 3*15000
R2 = 45k ohm This can be rounded to 47K ohm as 47k resistor is easily available.
So in this experiment
R1 = 15k and R2 = 47k
Resistor R3 is used to limit the current flowing to the collector of the transistor that is built into the regulator (see fig. 2)
Inductor L1 and Diode D1 – The inductor and diode are the main components of the basic boost converter circuit. The diode chosen to be used in the circuit is the 1N5822, as this diode has a lower direct voltage drop, high current capacity of up to 3 A and can work at high frequency.
To design a boost converter that converts minimum 3.5V input to 5V output using 34063, the value for different external components should be calculated as shown in Figure 3. According to the datasheet of 34063, for converter elevator the following table can be used to calculate component values. But before calculating the component values it is important to consider the following parameters which are used in the table provided in the datasheet.
(Minimum battery input voltage), Vin (min) = 3.5V
(Required output voltage), Vou = 5 V
(Max output current), Iout(max) = 500 mA
(Transistor saturation voltage), Vsat = 0.5 V (approximate value according to 34063 data sheet)
(Forward voltage drop of the 1N5822 diode), FV = 0.4 (according to the 1N5822 diode data sheet)
(Desired output switching frequency), f = 100 kHz
In the design of this boost converter circuit, the maximum frequency that the 34063AP1 regulator can provide is chosen. Due to the fact that the higher the frequency, the smaller the size of the inductor, this makes the circuit less bulky.
(desired peak-to-peak output ripple voltage), Vripple = 100mV
This is the peak-to-peak ripple voltage that must be considered at the output. The ripple voltage must always be lower for a regulated and constant output.
Table for calculating the values of the Step-Up converter components
Fig. 8: Table used to calculate the values of the Step-Up converter components
For convenience, the following values have been rounded so that the components can be easily assembled.
CT = 150pF, Rsc = 0.22 ohm, Lmin = 10uH, Co = 200uF
Values of other components
Resistance R3- The standard value of resistor R3 is 180 ohm for the step-up converter according to the regulator data sheet 34063. In the circuit it has been rounded to 200 ohms.
Cin Capacitor – In this circuit, a 100 uF capacitor is used for Cin. This is the standard value for the step-up converter according to the regulator's technical data sheet 34063.
Capacitor C1 – The value of capacitor C1 must be smaller so that it can reduce the overall ESR, so the capacitance C1 is considered 0.1uF
After connecting all external components to the regulator IC, the output voltage and current can be measured for practical observations. Measuring different values of voltage and current in the circuit helps to evaluate the efficiency of the boost converter circuit.
Practical battery input voltage, Vin = 3.6 V
Practical output voltage, Vout = 5.35 V
The efficiency of the boost converter circuit needs to be evaluated with different loads. For convenience, resistors of different values are connected as a load to the output for testing. The results obtained during the test are summarized in the following table –
Figure 9: Table listing Boost Converter output voltage and current for different loads
Fig. 10: Graph showing the voltage variation for different loads at the boost converter output
Fig. 11: Graph showing the current variation for different loads at the boost converter output
From practical observations, it can be seen that when the current demand increases, the voltage starts to drop. As with the output voltage of 5 V with a load of 100 ohms, the current consumed at the output is 50 mA. As the voltage output begins to drop below 5V, the current drawn by the load begins to increase. Therefore, the circuit can only supply current up to approximately 50 mA if the output voltage is set to approximately 5V. The efficiency of the circuit can be improved by adding filters and voltage regulators (Zener diodes) to obtain a regulated voltage at the output.
Figure 12: Boost Converter prototype designed on a breadboard
When designing this circuit, it is important that for the stabilized output it is necessary to use a capacitor in the input power and also at the output of the circuit, so that unwanted ripples in the input and output signals can be reduced. A low capacitor value (C1) should also be added in parallel with a high capacitor value (Cin) at the input to reduce the overall ESR. The diode and inductor must be chosen wisely so that they can allow maximum input current (Ipk) through them. Maximizing the output current should be the criteria for choosing the diode and inductor. Input power must be supplied to the 34063 regulator only in its working range. The selection of the diode (D1) must be such that it suffers less direct voltage drop and can work at high frequencies.
Circuit diagrams
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