Designing Open Loop Non-Isolated Boost Converter With Adjustable Output Voltage (Part 3/12)

In one of the previous tutorials, open loop boost converter SMPS was designed. In this series, following SMPS circuits are designed –

1. Boost Converters –

a) Open Loop Boost Converter

b) Closed Loop Boost Converter

c) Open Loop Boost Converter with Adjustable Output

d) Closed Loop Boost Converter with Adjustable Output

2. Buck Converters –

a) Open Loop Buck Converter

b) Closed Loop Buck Converter

c) Open Loop Buck Converter with Adjustable Output

d) Closed Loop Buck Converter with Adjustable Output

3. Buck-Boost Converters

a) Open Loop Inverting Buck – Boost Converter

b) Open Loop Inverting Buck – Boost Converter with Adjustable Output

4. Flyback Converter

5. Push-Pull Converter

The open loop boost converter designed in the previous tutorial had a fixed output voltage respective to the input voltage level. The output voltage of the circuit can be made variable by drawing the output through a variable resistor. The output voltage in this circuit still remains unregulated as no feedback is used.

Therefore in this tutorial, an open loop non–isolated boost converter is designed. The boost converter can be designed in two ways-

Open loop boost converter – In open loop boost converter, there is no feedback from output to input contrary to the closed loop which has a feedback circuit. So, the output of an open loop boost converter is not regulated.

Closed loop boost converter – In closed loop boost converter, there is a feedback from the output to the input. So, the output of a closed loop boost converter is regulated.

There are certain design parameters involved in the designing of the boost converter. It is important to understand these design parameters. Any boost canverter can operate in either of the two possible modes of operation. These modes of operation are as follow –

• Continuous Conduction Mode (CCM)- In CCM, the current in the inductor is continuous in the entire cycle of the switching period. So a regulated voltage at the output is obtained but the output is regulated only if the current is drawn within the limits of CCM.

• Discontinuous Conduction Mode (DCM)- In this mode, the current in the inductor is pulsating and it becomes zero for a part of switching time. So a regulated voltage is not received in DCM. But, the voltage can be regulated by connecting a feedback circuit from output to input.

In this tutorial, a non-isolated boost converter is designed which means the input and output share the same ground. The boost converter designed in this project will step up 5V DC to 12V DC with a tolerance limit of +/-0.5V. Once the circuit is designed and assembled, the value of the output voltage and current will be observed using a multimeter. These values will indicate the efficiency of the boost converter designed in the project.

Components Required

Fig. 1: List of Components required for Open Loop Boost Converter

Block Diagram –

Fig. 2: Block Diagram of Open Loop Boost Converter

Circuit Connections –

In this project, an open loop boost converter operating in CCM mode is designed and the component values as per the CCM standard equations are calculated for the desired output.

The boost converter designed in this tutorial will have the following design parameters –

Input voltage, Vin–A Li-ion battery of 3.7V will be used as source. The battery voltage will be the input voltage.

Range of output voltage, Vout – The voltage at the output will be adjustable between 5V and 12V.

Maximum output current, Iout (max)–The maximum limit of output current will be 100 mA. The critical output current limit will be 10mA.

Output ripple voltage (dV) – The maximum output voltage ripple assumed at the output will be 100mV.

Load resistance – In this circuit a resistance at the output will be connected which will act like load for the circuit. The maximum value of resistance can be calculated by ohm’s law which is as follows –

Vout = Iout(max)*RL(max)

RL(max) = Vout/Iout(max)

By putting all the values,

RL = 240E

Now the power rating of the resistance can be calculated as follow – P = (Vout)2/(RL(max))

By putting all the values,

Pout = 2.4W

So, a resistance having 240E value and power rating equivalent or greater than the 2.4W will be used as load at the output for maximum efficiency.

Frequency (Fs)– The frequency of the PWM signal generated by the microcontroller should not be too high or low so a frequency of 10 KHz is selected to operate the switching components of the circuit. The value of the frequency is assumed.

The boost converter has the following circuit blocks –

1. DC source –

A 3.7 V Li-ion Battery is used as the input power source in the circuit. The battery voltage itself is the input voltage in the circuit.

2. Controller and switching mechanism –

An Oscillator is used to generate a Pulse Width Modulated (PWM) signal of a desired frequency. In this boost converter, Arduino UNO is used to generate the PWM signal so, the Arduino board is acting like an oscillator. The PWM signal is a train of the pulse which is used to turn ON and OFF the MOSFET. The MOSFET is used as the switching transistor in the circuit.

For switching purpose, a transistor and a diode are used as a switching components. For the selection of the transistor, MOSFET is chosen as FETs are known for their fast switching speed and low RDS (ON) (drain to source resistance in ON state). So an N-channel FDS7088N3 MOSFET (shown as Q1 in the circuit diagram) is connected parallel to the input DC source which acts like a switch in the circuit as its threshold voltage is very low, around 2V. So it can be easily triggered by a 3.7V battery. In ON state the Vds of FDS7088N3 MOSFET is also very low which reduces the power dissipation of our circuit.

For turning ON and OFF the MOSFET a train of the pulse should be applied to its gate. For this, the controller board generates a Pulse Width Modulated signal of 10kHz. This PWM signal is used to turn ON and OFF the MOSFET. For generating the PWM signal from the controller, an Arduino sketch has been burnt to the board. This Arduino sketch can be downloaded from the code section.

It should be noted that the switching time of the MOSFET and diode should be less than the rise and fall time of the PWM wave. A gate to source resistance must be used to avoid any unwanted triggering of the MOSFET by external noise. It also helps in fast turning OFF the MOSFET by discharging its parasitic capacitance. A low value of the resistor (10E to 500E) should be connected at the gate of the MOSFET. This will solve the problem of ringing (parasitic oscillations) and in rush current in the MOSFET. The voltage level of the PWM signal should be greater than the threshold voltage of the MOSFET. So that the MOSFET can be turned ON fully with minimum RDS (ON ). There should be heat sink mounted with the MOSFET for dissipating the excess heat otherwise the FET can get damaged.

Another switching component used in the circuit is a diode. The switching time of the diode should be less than the rise and fall time of the PWM wave. The Arduino board generates a PWM wave having rise time 110ns and fall time of 90ns. The forward voltage drop of the diode should also be very low otherwise it will dissipate power which will further reduce the efficiency of the circuit. The diode should offer low voltage drop in forward bias and the RDS (ON)of the MOSFET should be low. So in this experiment, a BY399 diode is selected which suits best to the circuit design.

Before generating the PWM signal the switching frequency for the circuit needs to be decided. For this boost converter, a switching frequency of 10kHz is selected which will work fine this converter design.

The duty cycle of the generated PWM signal is another important consideration as it will decide the active state of the MOSFET. The duty cycle can be calculated as follows –

D% = 1- (Vin/Vo)*100

Vo=Desired output voltage, 5V to 12V

Vin =Input voltage, 3.5V

Since the output voltage varies from 5V to 12V so the duty cycle will be calculated as per 5V as well as 12V.

For 5V output,

For Vo(min) = 5V

D(min)% = (1-(3.5/5))*100

D(min)% = 30% (approx.)

and for 12V output,

For Vo(max) = 12V

D(max)% = (1-(3.5/12))*100

D(max)% = 70% (approx.)

A capacitor and resistor of the appropriate value should be used for generating the 10 kHz frequency and 50% duty cycle. The higher is the frequency selected for switching components, the higher are the switching losses. This decreases the efficiency of the SMPS. But high switching frequency reduces the size of the energy storage element and improves the transient response of the output.

3. Energy Storage Element –

An inductor is used for storing the electrical energy in the form of magnetic field. So the inductor acts like an Energy Storage Element. An inductor of 11.5 mH value is used in the circuit. For an inductor, a transformer’s secondary or primary coil, relay coil or any standard inductor can be used which has the desired inductance value.

An inductor of appropriate value must be used in the circuit. The value of the inductor can be calculated as be CCM equation –

Lmin>= Vo(max)/(16* Fs*Io (min))

Io (min) = Critical value of output current to maintain a regulated voltage at the output.

Assuming Io (min) = 10mA

Vo(max) = 12V

By putting all the values in the above equation,

L min>= 7.5 mH

As inductor can be greater than calculated value that is why an inductor of standard value of 11.5 mH is used in the circuit.

4. Output Filtering Element –

As a filtering element, a capacitor (shown as C1 in the circuit diagram) is used at the output of the circuit. In normal operation of Boost circuit, the transistor Q1 turns ON and OFF according to the frequency of the oscillator circuit. This generates a train of the pulse at the inductor L1 and capacitor C1 as well as transistor Q1. As the capacitor is connected with inductor only in the negative cycle of the PWM signal, this makes an LC filter which filters the train of the pulse to produce a smooth DC at the output. The value of the capacitor can be calculated by using following equation of CCM –

For Vout = 12V

C min>= (Io (max) * D(max))/ (Fs*dVo)

Maximum current at the output, Io (max) = 100mA

Duty cycle, D(max)= 0.7

Fs = 10kHz

Desired output ripple voltage, dVo

Assume dVo = 100mV

By putting all the values in the above equation,

C min>= (0.01 * 0.7)/ 10000*0.01

C min>= 70uF

This is the minimum of value of capacitor required. In the circuit, a capacitor of standard value of 100 uF is used.

5. Output Voltage Adjustment –

For varying the output voltage a potentiometer is used at the output of the circuit (as shown in the circuit diagram). The potentiometer gets powered by the battery and then the analog pin of microcontroller sense the voltage of the potentiometer. After sensing the voltage, the microcontroller adjusts the duty cycle as per the desired output voltage. So by turning the knob of potentiometer, the output voltage can be varied as per the requirement.

How the Circuit works –

Any SMPS has some switching components which turn on and off at high frequency and has some storage component which store the electrical energy while the switching components are in conduction state and discharge the stored energy to the output device while the switching components are in non-conduction state.

A simple Boost Converter consists of the inductor(L), a diode(D), a capacitor(C) and a transistor where transistor acts like a switch. In the boost circuit, when switch is closed i.e. the switching component is in conduction state, the inductor starts generating a magnetic field and stores energy. The stored energy in the inductor steps up the output voltage in comparison to the input voltage. When current starts flowing through the switching component as its path is less resistive compared to the path in parallel which contains capacitor and the output load, the inductor generates a positive polarity at its left terminal and negative to right terminal. Due to change in polarity, the diode gets reverse biased. In this condition, the capacitor, which was charged in the previous cycle, provides the current to the load while the switching component goes in non-conduction state or goes open between the ground.

Fig. 3: Circuit Diagram showing ON state of switching Component in Boost Converter

When the switch is open the current is reduced as the impedance increases so the generated magnetic field in the inductor starts collapsing and the polarity of the inductor reverses. This makes the diode forward biased and the capacitor now starts charging with a voltage greater than the input voltage. As the input is now having two sources in series one is the inductor and another is the battery. So the output voltage is always greater than the input voltage.

Fig. 4: Circuit Diagram showing OFF state of switching Component in Boost Converter

So, in ON state, the Diode was in blocking Mode (OFF) and the Transistor was ON. In OFF state, the Diode was in conducting mode (ON) and the Transistor was OFF.

So, it can be said that the Boost Converter is having two switching components – one is the transistor and another one is the diode. At a time only one of the switching component conducts i.e. it is in ON state while the other goes in non-conduction state i.e. it goes in OFF state.

Testing the circuit –

In this circuit, a variable resistor is added in the circuit of open loop boost converter to make the output voltage adjustable. The Arduino sketch is also modified to change the duty cycle of the PWM signal as per the voltage input from the variable resistor.

In the circuit, Battery Voltage, Vbat/Vin = 3.7V

On measuring voltage and current values with different loads at the output when duty cycle is set to 70 percent, following observations were taken –

Fig. 5: Table listing output voltage and current from Open Loop Boost Converter for different loads at 70 percent duty cycle

So, it can be observed that a current of 11.7 mA can be drawn at output with a tolerance limit of +/-0.5V.

On measuring voltage and current values with different loads at the output when duty cycle is set to 30 percent, following observations were taken –

Fig. 6: Table listing output voltage and current from Open Loop Boost Converter for different loads at 30 percent duty cycle

So, it can be observed that a current of 9.1mA can be drawn at output with a tolerance limit of +/-0.5V.

The voltage without a load in both the cases is high. As there is no feedback circuit added in the design which can regulates the output voltage.

As assumed, the maximum output current should be 100mA for 24V and 5V. This drop in voltage is due to the losses in the circuit like switching and conduction losses of diode and MOSFET, losses in the windings that surround the core of inductor , eddy current losses and hysteresis losses in the inductor, capacitor losses due to ESR (equivalent series resistance) and losses due to Rds(on) of N-MOS.

Fig. 7: Prototype of Open Loop Boost Converter designed on a breadboard

The power efficiency of the circuit can be calculated as follow –

For 70% Duty Cycle, maximum output current is 11.7mA

Efficiency % = (Pout/Pin)*100(Output Power) Pout = Vout*Iout

(Output Voltage) Vout = 11.8 V

(Output current) Iout = 11.79 mA

Pout = 139 mW (approx.)

(Input Power) Pin = Vin*Iin

(Input Voltage) Vin = 3.7 V

(Input Current) Iin = 42 mA (measured the input current using ammeter)

Pin = 155 mW(approx.)

By putting all the values,

Efficiency % = 89%

It can be seen that there are certain limitations of this circuit. The output voltage in this circuit is not regulated it varies for different load resistances. This can be improved by adding a feedback circuitry which helps in regulating the output voltage. A Boost Converter with feedback circuit and adjustable output is designed in the next tutorial. Secondly, the efficiency of this boost converter design is 89% due to the power losses in the circuit.

This is an open loop boost converter with non-isolated output and operating in CCM mode. It can be used as switching regulator for LED drivers and as a regulated DC power supply. It can be used for supplying power to low power portable electronic devices. In battery powered applications when there is a space restriction to stack the number of batteries in series for achieving high voltage, this boost converter can be used with less number of batteries to supply the DC power.

This boost converter is simple to design and use cheap components. It can be easily assembled in no time. The circuit has a variable output and can be powered by 3.5V battery. However, the minimum voltage of the battery should not be less than the 3.5V otherwise the microcontroller will not get powered.

Project Source Code

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//Program to 


////// Code for Open Loop Boost converter with variable output voltage ////////

////// Output Voltage Adjustment from 5V to 12V ////////


#define TOP 1599                          // Fosc = Fclk/(N*(1+TOP)); Fclk = 16MHz, Fosc = 10kHz

#define CMP_VALUE_HALF_DUTY 799           // 50% duty cycle

#define Inputpin A4                        // Input pin of potentiometer at A4

#define PWM 9                              // PWM(Pulse Width Modulation) wave at pin 9 

float Mapping_output_voltage();            // function declaration  


float Mapping_output_voltage(){                            
// function definition 

 int Input_Read = analogRead(Inputpin);                     
//reading analog voltage from potentiometer and converting it to digital values in between 0 to 1023

 float Compare_voltage = (((Input_Read*(1.0)/1024)*7)+5);   
// mapping the digital value into analog volatge of 0 to 12

 return(Compare_voltage);                                   
// return the calculated output voltage

}


void setup() {

  // put your setup code here, to run once:

 pinMode(PWM,OUTPUT);                        
//set 9 pin as output 

 pinMode(Inputpin,INPUT);                    
// set A4 pin as input

 TCCR1A = 0;                                 
//reset the register

 TCCR1B = 0;                                 
//reset the register

 TCNT1 = 0;                                   
//reset the register

 TCCR1A |= (1<
 
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