Designing open-loop non-isolated boost converter with adjustable output voltage (part 3/12)

In one of the previous tutorials, SMPS open loop boost converter was designed. In this series, the 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) Buck Open Loop Inverter – Boost Converter

b) Buck Open Loop Inverter – 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 corresponding to the input voltage level. The output voltage of the circuit can be variable by pulling the output through a variable resistor. The output voltage in this circuit still remains unregulated because 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 the output to the input, unlike the closed loop which has a feedback circuit. Therefore, the output of an open-loop boost converter is not regulated.

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

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

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

• Discontinuous conduction mode (DCM)- In this mode, the current in the inductor is pulsating and becomes zero during a part of the switching time. Therefore, a regulated voltage is not received at the DCM. However, the voltage can be regulated by connecting a feedback circuit from the output to the 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 from 5V DC to 12V DC with a tolerance limit of +/-0.5V. Once the circuit has been 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.

Figure 1: List of components required for the Open Loop Boost Converter

Fig. 2: Block diagram of the Open Loop Boost converter

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

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

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

Output voltage range, Vout – The output voltage will be adjustable between 5V and 12V.

Maximum output current, Iout (max.) – The maximum output current limit 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 will be connected to the output that will act as a load for the circuit. The maximum resistance value can be calculated using Ohm’s law, which is as follows –

Vout = Iout(max)*RL(max)

RL(max) = Vout/Iout(max)

Putting all the values,

RL = 240E

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

Putting all the values,

Pout = 2.4W

Thus, a resistance with a value of 240E and a power equivalent to or greater than 2.4W will be used as a 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, therefore a frequency of 10 KHz is selected to operate the switching components of the circuit. The frequency value is assumed.

The boost converter has the following circuit blocks –

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

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 acts as an oscillator. The PWM signal is a pulse train used to turn the MOSFET on and off. MOSFET is used as switching transistor in the circuit.

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

To turn the MOSFET on and off, a pulse train must be applied to its gate. To do this, the controller board generates a 10kHz pulse width modulated signal. This PWM signal is used to turn the MOSFET on and off. To generate the controller's PWM signal, an Arduino sketch was written 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 must be less than the rise and fall time of the PWM wave. A gate-to-source resistance must be used to prevent any unwanted triggering of the MOSFET by external noise. It also helps to quickly turn off the MOSFET by discharging its parasitic capacitance. A low resistor value (10E to 500E) must be connected to the MOSFET gate. This will solve the ringing (eddy oscillations) and spike current problem in the MOSFET. The PWM signal voltage level must be greater than the MOSFET threshold voltage. So that the MOSFET can be turned ON completely with minimum RDS (ON). There must be a heat sink mounted with the MOSFET to dissipate excess heat, otherwise the FET may be damaged.

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

Before generating the PWM signal, the switching frequency of the circuit needs to be decided. For this boost converter, a switching frequency of 10kHz is selected, which will work well in 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, 3.5 V

Since the output voltage varies from 5V to 12V, the duty cycle will be calculated for both 5V and 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 appropriate value must be used to generate the 10 kHz frequency and 50% duty cycle. The higher the frequency selected for the switching components, the greater the switching losses. This decreases the efficiency of the SMPS. But the high switching frequency reduces the size of the energy storage element and improves the transient response of the output.

An inductor is used to store electrical energy in the form of a magnetic field. Therefore, the inductor acts as an energy storage element. An inductor of value 11.5 mH is used in the circuit. For an inductor, a secondary or primary coil of a transformer, a relay coil, or any standard inductor that has the desired inductance value can be used.

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

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

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

Assuming Io (min) = 10mA

Vo(max) = 12V

Putting all the values ​​in the above equation,

Lmin>= 7.5 mH

Since the inductor may be larger than the calculated value, that is why a standard value inductor of 11.5 mH is used in the circuit.

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 the Boost circuit, transistor Q1 turns on and off according to the frequency of the oscillator circuit. This generates a pulse train in inductor L1 and capacitor C1, as well as transistor Q1. Since the capacitor is connected to the inductor only in the negative cycle of the PWM signal, this forms an LC filter that filters the pulse train to produce a smooth DC at the output. The value of the capacitor can be calculated using the following CCM equation –

For Vout = 12V

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

Maximum output current, Io (max) = 100mA

Duty cycle, D(max)= 0.7

F = 10kHz

Desired output ripple voltage, dVo

Suppose dVo = 100mV

Putting all the values ​​in the above equation,

Cmin>= (0.01 * 0.7)/ 10000*0.01

Cmin>= 70uF

This is the minimum capacitor value required. A capacitor with a standard value of 100 uF is used in the circuit.

To vary the output voltage, a potentiometer is used at the output of the circuit (as shown in the circuit diagram). The potentiometer is powered by the battery and then the analog pin of the microcontroller senses the voltage of the potentiometer. After detecting the voltage, the microcontroller adjusts the duty cycle according to the desired output voltage. Then, by turning the potentiometer knob the output voltage can be varied as required.

Any SMPS has some switching components that turn on and off at high frequency and has some storage component that stores the electrical energy while the switching components are in conducting state and discharges the stored energy to the output device while the switching components are not driving. state.

A simple boost converter consists of the inductor (L), a diode (D), a capacitor (C) and a transistor where the transistor acts as a switch. In the boost circuit, when the switch is closed, i.e., the switching component is in a conducting state, the inductor starts generating a magnetic field and stores energy. The energy stored in the inductor increases the output voltage compared to the input voltage. When current begins to flow through the switching component, as its path is less resistive compared to the parallel path containing the capacitor and output load, the inductor generates a positive polarity at its left terminal and negative at its right terminal. . Due to the change in polarity, the diode becomes reverse biased. In this condition, the capacitor, which was charged in the previous cycle, supplies current to the load while the switching component goes into a non-conducting state or opens between ground.

When the switch is open, the current is reduced as the impedance increases, so the magnetic field generated in the inductor begins to collapse and the polarity of the inductor reverses. This makes the diode forward biased and the capacitor now starts charging with a voltage higher than the input voltage. Since the input now has two sources in series, one is the inductor and the other is the battery. Therefore, the output voltage is always greater than the input voltage.

Fig. 4: Circuit diagram showing the OFF state of the switching component in the Boost converter

Therefore, in the ON state, the Diode was in Blocking Mode (OFF) and the Transistor was ON. In the OFF state, the Diode was in conduction mode (ON) and the Transistor was OFF.

So, it can be said that the Boost Converter has two switching components – one is the transistor and the other is the diode. At a time, only one of the switching components conducts, that is, it is in the ON state, while the other enters the non-conducting state, that is, it enters the OFF state.

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

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

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

Thus, it can be seen that a current of 11.7 mA can be consumed at the output with a tolerance limit of +/-0.5V.

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

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

Thus, it can be seen that a current of 9.1mA can be consumed at the output with a tolerance limit of +/-0.5V.

The no-load voltage in both cases is high. As there is no feedback circuit added in the design that can regulate the output voltage.

As assumed, the maximum output current should be 100mA for 24V and 5V. This voltage drop is due to losses in the circuit such as switching and conduction losses of the diode and MOSFET, losses in the windings surrounding the inductor core, 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: Open Loop Boost Converter prototype designed on a breadboard

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

For 70% duty cycle, the maximum output current is 11.7mA

Efficiency% = (Pout/Pin)*100(Power output) 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 (measure input current using ammeter)

Pin = 155 mW (approx.)

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 circuit that helps regulate 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 a switching regulator for LED drivers and as a regulated DC power supply. It can be used to supply power to low-power portable electronic devices. In battery powered applications, when there is space constraint to stack number of batteries in series to achieve high voltage, this boost converter can be used with less number of batteries to supply DC power.

This boost converter is simple to design and uses inexpensive components. It can be easily assembled in a short time. The circuit has a variable output and can be powered by a 3.5V battery. However, the minimum battery voltage should not be less than 3.5 V, otherwise the microcontroller will not be powered.

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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 voltage from 0 to 12

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