Designing an open loop isolated flyback converter (Part 11/12)

The Flyback Converter is one of the topology of SMPS. 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

A Flyback Converter is like a Buck-Boost DC to DC converter which provides the output voltage either less than or greater than the input supply voltage. It is one of the topologies of the Switched Mode Power Supply (SMPS) so it works on the principle of switching regulators.

Flyback converter is similar to Buck-Boost converter with an exception that inductor used in Flyback converter has isolated winding. Due to isolated winding, the output voltage is non-inverting in the Flyback converter. So, a positive output voltage is obtained.

Fig. 1: Circuit Diagram of Typical Buck-Boost Converter

Fig. 2: Circuit Diagram showing Isolated Winding in Flyback Converter

Fig. 3: Circuit Diagram showing Change in Diode Connection in Flyback Converter

Contrary to Linear Regulators, Flyback Converters use switching regulators. That is why, the voltage is not altered by heat dissipation and the law of power conservation applies to these converter circuits. According to the law of power conservation, input power must be equal to output power.

Pin = Pout

Vin*Iin = Vout*Iout

In Boost Mode of Flyback Converter, Input voltage is less than output voltage, so the output current is less than the input current. While in Buck Mode of Flyback Converter, Input voltage is greater than output voltage, so the output current is greater than the input current. Therefore, SMPS can also Buck or Boost the output current along with voltage which increases its efficiency as compared to linear regulators.

The Flyback Converter can be designed in two ways –

Open loop Flyback Converter – In open loop Flyback 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 flyback converter is not regulated.

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

There are certain design parameters involved in the designing of the Flyback converter. It is important to understand these design parameters. Any Flyback converter 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, an isolated Flyback converter is designed which means the input and output have different ground. The Flyback converter designed in this project will have an adjustable output between 12V DC and 5V DC. 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 flyback converter designed in the project.

Components Required –

Fig. 4: List of Components required for Open Loop Isolated Flyback Converter

Block Diagram –

Fig. 5: Block Diagram of Open Loop Isolated Flyback Converter

Circuit Connections –

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

The Flyback converter has the following circuit blocks –

1. DC source –

A 12V Battery is used as the input power source in the circuit.

2. Oscillator and Switching Mechanism –

For switching purpose a transistor and a diode are used as switching components.

The switching components has to operate on a specific frequency. This frequency is generated through an oscillator circuit. In this project, Arduino UNO is used to generate a PWM signal which provides the required frequency. Any other Arduino Board like Arduino Mega can also be used. In fact any microcontroller or microcontroller board which can output PWM can be used in the circuit. Arduino is chosen as it is the most popular prototyping board and can be easily programmed. Due to large community support, it is easy to learn and work on Arduino. 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 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). In this configuration, the MOSFET is connected in high-side configuration. As in high side, the N-channel MOSFET requires a Bootstrap Circuitry or a Gate Driver IC for its triggering, this makes the driver more complicated. A P-channel MOSFET (shown as Q1 in the circuit diagram) is used in the circuit as it does not require a Gate Driver in its high side but it has high Rds (On) as compared to N-MOS. This results in more power loss. The MOSFET used in the circuit has its threshold voltage around 10V to 12V.

The switching time of the MOSFET and diode should be less than the rise and fall time of the PWM wave. The diode should offer low voltage drop in forward bias and the RDS (ON)of the MOSFET should be low. Always a gate to source resistance should 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) can be used at the gate of MOSFET. This will solve the problem of ringing (parasitic oscillations) and inrush 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 ) .

The MOSFET cannot be triggered by the microcontroller as the microcontroller can generate a 5V PWM signal only. Therefore an additional IC IR2110 is used in the circuit to generate a PWM signal of 12V and the input to the IR2110 is provided by the microcontroller. IR2110 is a high and low side driver. It is a high speed (operational at high frequency) power MOSFET and IGBT driver with independent high and low side referenced output channels. The floating channels can operate up to 500V or 600 V. The IC is 3.3V logic compatible that is why it can be used with any microcontroller. The IC comes in a 14 Lead PDIP package. IR2110 has the following pin configuration –

Fig. 6: Table listing pin configuration of IR2110 IC

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 flyback converter, a switching frequency of 10kHz is selected as the transformer used in the circuit will start saturating beyond this frequency.

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 –

Vout = Vin *(D/(N*(1-D)

Where,

Desired output voltage, Vout = 5V

Input Voltage, Vin = 12V

N = turns ratio,

Ns= Number of transformer secondary turns

Np= Number of transformer primary turns

Np/Ns= 9/1

D = duty cycle

By putting all the values,

D = 0.8 or 80%

For generating 10 kHz PWM signal having duty cycle of 80%, the Arduino board is programmed. The Arduino sketch required for generating the desired PWM output is attached in the tutorial. It can be downloaded and burnt to an Arduino board for use.

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 –

A transformer is used for storing the energy in the form of the magnetic field with Np/Ns = 9/1. So the transformer acts like an Energy Storage Element. The peak current in the primary of the transformer needs to be calculated for its selection. It can be calculated as follow –

Ipeak = (4*(Io(max))/(3*N*(1-D))

Io(max) = maximum output current

Considering Io(max) = 50mA

By putting all values,

Ipeak = 36mA (approx.)

A transformer of with Np/Ns = 9/1 and higher current rating from calculated above is used in the circuit. The transformer primary current rating should be greater than peak current so that the desired current at the output can be obtained. Such transformer should be chosen which can sustain the desired switching frequency. Otherwise, the transformer will start saturating and cause power losses 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 Flyback converter 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 in both negative and positive 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 –

Cmin >= (Irms)/(8*Fs*DVo)

Where,

Cmin = minimum value of capacitor

Switching frequency, Fs = 10 kHz

DVo = Output ripple voltage

Assuming DVo = 100mV

Irms = RMS current

Irms = (D*(Ipeak)2)1/2

Irms = (0.8*(0.036)2)1/2

Irms = 32mA (approx.)

Now by putting all the value,

Cmin >= 4uF

As it is the minimum value of capacitor required, so in the circuit a capacitor of standard value is used which can be easily available, so, capacitor of 10 uF is taken.

The value of the capacitor should be greater than or equivalent to the calculated value. So that it is able to provide the desired current and voltage at the output. The capacitor used in the circuit must be of higher voltage rating than the output voltage. Otherwise, the capacitor will start leaking the current due to the excess voltage at its plates and will burst out. It is important that all the capacitors should be discharged before working on a DC power supply application. For this, the capacitors should be shorted with a screwdriver wearing insulated gloves.

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.

The Fly-Back converter consists of a transformer(TR1), a capacitor(C1), a transistor which acts like a switch and diode (D1) which acts like second switch. When the switch is closed then the transformer’s primary coil is directly connected to the input source and starts accumulating energy. This generates a voltage in the secondary of the transformer but of opposite polarity. This makes the diode reverse biased. In this state, the output capacitor provides the current to the load which was previously charged in OFF state.

Fig. 7: Circuit Diagram showing ON state of switching Component in Flyback Converter

When the switch is open, the source gets disconnected from the circuit and the current in the primary winding starts decreasing and becomes zero. The stored energy in the magnetic core of the transformer starts releasing to the secondary winding. This makes the diode forward biased and the secondary winding now provides the current to the load through diode D1. When the stored charge in inductor starts reducing then the output voltage starts falling. But now the capacitor acts as a current source and keeps providing the current to the load until the next cycle i.e. until commencement of ON state.

Fig. 8: Circuit Diagram showing OFF state of switching Component in Flyback Converter

In this configuration of Fly-Back Converter, the duty cycle (D) and the turns ratio of the transformer are the factors which decides whether the converter steps up or step down the input voltage.

Testing the Circuit –

This Flyback converter has an adjustable output between 12V and -5V DC.

Fig. 9: Prototype of Open Loop Flyback Converter designed on a breadboard

In this circuit, Input Voltage, Vin = 12V

Practically, Battery Voltage, Vin = 11.8V

On measuring voltage and current values with different loads at the output, following observations were taken –

Fig. 10: Table listing output voltage and current from Open Loop Flyback Converter for different loads

So, it can be observed that a current of 9.54 mA can be drawn at 4.77 V output with a tolerance limit of +/-0.5V.

The power efficiency of the circuit at maximum output current of 9.54 mA for 5V output can be calculated as follow –

Efficiency % = (Pout/Pin)*100

(Input Power) Pin = Vin*Iin

(Output Power) Pout = Vout*Iout

(Output Voltage) Vout = 4.77V

(Output current) Iout = 9.54mA

Pout = 46mW(approx.)

(Input Voltage) Vin = 11.8V

(Input Current) Iin = 6.3mA (measured input current using ammeter)

Pin = 74mW

By putting all the values,

Efficiency % = 62%

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. There are switching and conduction losses of diode and MOSFET, losses in the windings that surround the core, eddy current losses and hysteresis losses in the inductor, capacitor losses due to ESR (equivalent series resistance), losses due to high Rds(on) of P-MOS.

This is an open loop flyback converter with non-isolated output and operating in CCM mode. It can be used as a low loss current source to drive LEDs or power portable self-powered devices. It can also be used as an interface between battery and components in CPU or notebooks where voltage demand is lower than battery voltage.

Project Source Code

###

//Program to 


/*

Code for Flyback with Input voltage = 12V and Output voltage = 5V

 This code will generate a PWM (Pulse Width Modulation)signal of 10kHz with 20% duty cycle

 but desired duty cycle is 80%. 

 As in our experiment we are using a P-channel MOSFET, PMOS is triggered by negative voltage.

 So we need to set duty cycle of PWM signal to 20% so that duty cycle of the MOSFET can be 80%


 */

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

#define DUTY_CYCLE  320              // OCR1A value for 20% duty cycle 

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


void setup() {

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

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

TCCR1A = 0;                                 //reset the register

TCCR1B = 0;                                 //reset the register

TCNT1 = 0;                                   //reset the register

TCCR1A |= (1<
###