How does Buck Converter work? | DC-DC Converter - 1

Summary

This video script delves into the workings of a Buck converter, a device that efficiently steps down DC voltage levels. It explains the use of a PWM signal to control a MOSFET, the role of an inductor and capacitor in stabilizing voltage, and the importance of feedback for load adjustments. The script also covers the generation of the PWM signal through an op-amp and comparator, and the use of P-channel MOSFETs for efficient voltage regulation. The summary highlights the closed-loop operation of the Buck converter, emphasizing its ability to maintain a consistent output voltage despite varying loads.

Takeaways

• 🔌 A buck converter is used to step down DC voltage efficiently.
• 🕒 The operation of a buck converter is based on the concept of Pulse Width Modulation (PWM) and duty cycle.
• 🔄 The average voltage output is determined by the duty cycle of the PWM signal, which can be adjusted to reduce voltage.
• 🛠️ An electrical switch, such as a MOSFET, is used in place of a simple mechanical switch for better control and efficiency.
• 🧲 An inductor is added to the circuit to smooth out the voltage spikes and maintain constant current flow.
• 🔁 The inductor opposes changes in current flow, helping to stabilize the voltage after the switch opens.
• 💡 A Schottky diode is included to provide a path for electrons when the switch is open, preventing damage from high negative voltage.
• ⚡ High-frequency PWM signals are used to minimize voltage and current instability caused by the inductor.
• 🔋 A capacitor is used in parallel to further smooth the output voltage by storing and releasing energy as needed.
• 🔄 Feedback mechanisms are crucial for adjusting the PWM signal in response to load changes, ensuring a stable output voltage.
• ⚙️ The complete buck converter circuit includes an op-amp for error amplification, a comparator for PWM signal generation, and a P-channel MOSFET for efficient switching.

Q & A

What is the purpose of a buck converter?

A buck converter is used to step down DC voltage efficiently, converting a higher input voltage to a lower output voltage as required by the load.

How does a PWM signal relate to the operation of a buck converter?

A Pulse Width Modulation (PWM) signal controls the switching of the MOSFET in a buck converter. The duty cycle of the PWM signal determines the average voltage output, which in turn steps down the input DC voltage.

What is the role of an inductor in a buck converter?

The inductor in a buck converter helps to maintain a constant current flow by changing the voltage across itself. It also stores energy in its magnetic field during the switch-on phase and releases it during the switch-off phase, smoothing out the output voltage.

Why is a Schottky diode used in a buck converter?

A Schottky diode is used to provide a path for the electrons when the inductor tries to maintain the current flow as the switch opens, preventing high voltage spikes that could damage the components.

What is the function of a capacitor in the buck converter circuit?

A capacitor in a buck converter circuit is used to smooth out the output voltage by changing the current through it. It stores energy in its electric fields during the switch-on phase and supplies current when the switch is off, helping to maintain a stable output voltage.

How does the feedback mechanism in a buck converter work?

The feedback mechanism in a buck converter adjusts the duty cycle of the PWM signal based on the voltage across the load. It uses a voltage divider to reduce the output voltage and feed it into an operational amplifier (op-amp), which compares it with a reference voltage and adjusts the PWM signal accordingly.

What is the significance of the duty cycle in a PWM signal?

The duty cycle of a PWM signal is the proportion of time the signal is in the active (on) state during one period. It directly affects the average voltage output in a buck converter, with a higher duty cycle resulting in a higher average output voltage.

Why is a P-channel MOSFET used instead of an N-channel MOSFET in a buck converter?

A P-channel MOSFET is used because it turns on when the gate voltage is below the source voltage, which is suitable for the buck converter's design where the gate voltage needs to be lower than the source voltage to turn on the MOSFET.

How does the operational amplifier (op-amp) function in the buck converter's feedback loop?

The op-amp in the buck converter's feedback loop acts as an error amplifier, amplifying the difference between the input voltage (which is a fraction of the output voltage) and a reference voltage. It adjusts its output to minimize this difference, effectively controlling the duty cycle of the PWM signal.

What is the purpose of the triangle wave generator in the buck converter circuit?

The triangle wave generator provides a varying voltage input to the comparator in the buck converter circuit. It interacts with the error amplifier's output to generate the PWM signal, with the comparator outputting a high voltage when the triangle wave is higher than the error amplifier's output and a low voltage otherwise.

How can the output voltage of a buck converter be adjusted?

The output voltage of a buck converter can be adjusted by changing the value of the feedback resistors in the voltage divider or by using a potentiometer to alter the feedback voltage. This change in feedback affects the error amplifier's output and, consequently, the PWM signal's duty cycle.

Outlines

00:00

🔌 Understanding Buck Converters for DC Voltage Step-Down

This paragraph introduces the concept of stepping down DC voltage using a buck converter. It explains the basic mechanism involving a switch that is turned on and off periodically to create a Pulse Width Modulation (PWM) signal, which controls the average voltage output. The role of the duty cycle in determining the output voltage is highlighted, along with the use of a MOSFET to replace the mechanical switch for efficiency. The paragraph also covers the function of an inductor in the circuit to maintain current continuity and the addition of a Schottky diode to manage voltage spikes. The importance of a capacitor for smoothing out the voltage and the challenges of load variation on output voltage are also discussed, leading to the necessity of feedback mechanisms in the design of a buck converter.

05:03

🔧 Design and Operation of a Closed-Loop Buck Converter

The second paragraph delves into the design and operation of a closed-loop buck converter. It describes the function of an operational amplifier (op-amp) as an error amplifier, comparing the output voltage to a reference and adjusting the duty cycle of the PWM signal accordingly. The use of a triangle wave generator and a comparator to produce the PWM signal that controls the MOSFET is explained. The choice of a P-channel MOSFET over an N-channel MOSFET is justified based on the gate voltage requirements and the circuit's operation. The paragraph also discusses the role of pull-up resistors and voltage regulators in establishing reference voltages, and the importance of feedback in maintaining a stable output voltage despite load changes. Design considerations, such as ensuring the error amplifier's input is below the reference voltage and the proper selection of resistor values for the feedback divider, are highlighted to prevent potential issues in the converter's operation.

Keywords

💡Step-down

Step-down refers to the process of reducing voltage levels in electrical circuits. In the context of the video, it is specifically about reducing AC or DC voltage levels. The script mentions using a transformer for AC and a buck converter for DC voltage step-down, which is central to the video's theme of explaining voltage regulation.

💡Buck converter

A buck converter is a type of power converter used to step down DC voltage efficiently. It is the main focus of the video, as it explains the design and working principles of this device. The script describes how a buck converter uses a switch controlled by a PWM signal to regulate voltage, illustrating its function with the example of reducing 12 volts to 6 volts.

💡PWM (Pulse Width Modulation)

PWM is a method used to encode a message into a pulse signal whose duty cycle can be varied. In the video, PWM is used to control the switching of the MOSFET in the buck converter, thereby controlling the output voltage. The script explains how the duty cycle of the PWM signal affects the average voltage output by the converter.

💡Duty cycle

The duty cycle is the proportion of time in which a digital signal is active compared to the total period of the signal. In the context of the buck converter, the script explains how varying the duty cycle can adjust the average voltage output, with a 50 percent duty cycle resulting in half of the input voltage.

💡MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor)

A MOSFET is a type of transistor used for high-frequency switching applications. In the script, it is mentioned as the electrical switch used in the buck converter, controlled by the PWM signal to regulate the voltage. The script also discusses the necessity of using a P-channel MOSFET for the converter's operation.

💡Inductor

An inductor is an electrical component that resists changes in electric current and stores energy in its magnetic field. The script describes its role in the buck converter to smooth out voltage spikes by changing voltage across itself when current flow is initiated or ceased.

💡Schottky diode

A Schottky diode is a type of semiconductor diode with a low forward voltage drop. In the video script, it is used to provide a path for electrons when the inductor's current falls, preventing high negative voltage spikes that could damage components.

💡Capacitor

A capacitor is an electrical component that stores energy in an electric field. The script explains how a capacitor is used in the buck converter to further smooth out the voltage by changing the current through it when the switch is opened or closed, maintaining a stable output voltage.

💡Op-amp (Operational Amplifier)

An op-amp is a high-gain electronic voltage amplifier with differential input and single-ended output. In the video, it is used as an error amplifier and comparator in the buck converter's feedback loop to adjust the PWM signal based on the output voltage.

💡Feedback loop

A feedback loop is a process in a system where the output is passed back through the system to control the input. In the context of the buck converter, the script describes how a feedback loop is used to adjust the duty cycle of the PWM signal in response to changes in the load, ensuring a stable output voltage.

💡Potentiometer

A potentiometer is a type of resistor with a sliding contact that forms a variable resistor. In the script, it is mentioned as a component that can be adjusted to change the feedback voltage, thereby altering the output voltage of the buck converter.

Highlights

A transformer is used to step-down AC voltage, while a buck converter is used for stepping down DC voltage.

Buck converter design involves a switch controlled by a PWM signal to lower DC voltage efficiently.

The duty cycle is the proportion of time a digital signal is in the active state within a period.

A 50% duty cycle results in an average voltage half of the input, demonstrating voltage reduction with a simple switch.

An electrical switch, such as a MOSFET, is used for efficiency, controlled by a PWM signal.

An inductor is added in series with the load to smooth out the voltage spikes created by the switch.

Inductors oppose changes in current, creating a counter voltage when the switch is closed.

A Schottky diode is used to prevent damage from high negative voltage spikes.

The frequency of the PWM signal is increased to maintain stability in voltage and current.

A capacitor is added in parallel to smooth out voltage fluctuations further.

Capacitors store energy in electric fields and act as an open switch when fully charged.

Feedback is necessary to adjust the PWM signal's duty cycle according to load changes.

An operational amplifier (op-amp) is used as an error amplifier to compare input voltages and adjust the output.

A triangle wave generator and comparator are used to create the PWM signal controlling the MOSFET.

P-channel MOSFETs are used because they turn on when the gate voltage is below the source voltage.

A pull-up resistor is added to ensure the gate voltage is sufficient to turn on the P-MOSFET.

Voltage regulators create reference voltages for the circuit without drawing current or losing power.

The closed-loop buck converter maintains a constant output voltage despite load changes by adjusting the PWM signal.

Design considerations include ensuring the error amplifier's voltage is always below the reference and selecting appropriate resistor values.

The comparator's input arrangement affects the feedback voltage range and the high voltage output direction.

Transcripts

To step-down AC voltage we use a transformer, but, how do you step down DC voltage?

We use a buck converter.

What is a buck converter?

In this video, we will be exploring the design process and working of a Buck converter which

is used to lower the DC voltage efficiently.

If you have a source, let say DC, and a switch, that is turned on and off periodically,

you get a PWM signal.

The amount of time a digital signal is in the active state relative to the period of

the signal is called its duty cycle.

If the switch is on for a long duration, the duty cycle increases,

and if it's on for a short duration, the duty cycle decreases.

Now, if you calculate the average of a cycle, with duty cycle 50 percent,

it's half of the input voltage.

That is, we have reduced the voltage from 12 volts to 6 volts with just a switch.

For efficiency reasons, we will replace it with an electrical switch, or a MOSFET.

This MOSFET is controlled by a PWM signal.

How it is generated and controlled will be explained later in the video.

But, This is just the amplified PWM signal and it has high voltage peaks.

To smooth this we add an inductor in series with the load.

This is an inductor.

Inductor wants to keep the current constant through itself and for that it will instantly

change the voltage across itself.

As the switch closes, the current starts to flow.

To resist this flow, the inductor drops the voltage at another end to zero, creating equal

and opposite voltage to the battery.

This is possible due to the magnetic fields which are generated in the inductor.

But it can't resist it for a long time, thus the current starts flowing and voltage at

the other end starts rising.

Also, the inductor starts storing the energy in its magnetic fields.

After some time the magnetic field stabilizes and the inductor will act as a closed switch,

allowing maximum current to flow.

Now, If you open the switch, then there is no source to supply current and thus current

starts falling.

But as you know current through the inductor cannot change instantly.

Hence, now the inductor act as a battery supplying current but it slowly runs out of energy.

As this end is open, the electrons accumulate here creating a high negative voltage.

This may damage the components.

Thus we add a low voltage drop, Schottky diode to create a path for electrons.

But the voltage at the load still has high voltage spikes.

Hence, we increase the frequency of the PWM signal, such that the voltage and current

by the inductor remains somewhat stable.

This is also the reason why the switch mode power supply uses high frequencies.

To further smooth-out them we add a capacitor in parallel.

This is a capacitor.

Capacitor wants to keep the potential difference constant across itself and for that it will

change the current through it.

As the switch closes, the voltage increases to 5 volts.

To resist this, the capacitor flows current through itself, raising the other terminal

to 5 volts.

This is possible due to the electric fields which are generated in the capacitor.

But it can't flow current for a long time as the plates get charged, thus the current

reduces, and voltage at the other end starts dropping to the ground.

Also, the capacitor starts storing the energy in its plates.

After some time the plates get fully charged, thus no current can flow, and the capacitor

act as an open switch.

Now, If you open the switch, then there is no source to supply voltage and thus voltage

starts falling.

But as you know voltage across the capacitor cannot change instantly.

Hence, now the capacitor act as a battery supplying current.

But it slowly runs out of energy and thus the flow of current reduces and

after some time it stops, thus the voltage drops.

And we have created is the buck converter.

But, there are some issues, as the load changes the voltage across the load also changes,

so we need to create feedback to change the duty cycle of the PWM signal with respect

to the load, also, how is this PWM signal getting generated?

This is the complete circuit.

The voltage at the output is reduced by a voltage divider and it is fed into an op-amp

which acts as an error amplifier.

This component is known as an Operational amplifier, or an Op-amp in short.

It has two inputs and one output, the other two terminals are for the supply voltage.

It is used to amplify the difference between the inputs.

It compares both the input.

If the voltage at the non-inverting input or the Positive terminal is greater than the

inverting one, then the output is the positive supply voltage.

Or, if the voltage at the inverting input or the Negative terminal is greater than the

non-inverting one, then the output is the negative supply voltage.

In this configuration, The op-amp wants to keep both of its inputs at the same voltage

and for that, it will change its output.

As the feedback is to negative input, the output varies according to the voltage at the negative input.

This can be seen as a see-saw.

When the input voltage falls, the output rises.

And when the input rises, the output falls.

This is how the differential amplifier works.

This is a Triangle wave generator.

The output of the error amplifier and wave generator is fed to another Op-Amp.

Which acts as a comparator and outputs the PWM signal which controls the MOSFET.

Here the negative input is a constant voltage from the differential amplifier,

and the positive input is the wave form triangle wave generator.

Hence, when the triangle wave is higher than the constant voltage,

The output is the positive supply voltage.

Here it is plus five volts.

And, when the constant voltage is higher than the triangle wave,

the output is the negative supply voltage.

Here it is ground or zero volts.

This is how the comparator works.

If the constant voltage increases, then the width of On-time decreases,

and if the voltage decreases, the duty cycle of PWM also decreases.

This is a P-channel MOSFET.

An N-channel MOSFET will turn on when the gate voltage is above the source voltage,

and a P-channel MOSFET will turn on when the gate voltage is below the source voltage.

So when the PWM is high the MOSFET is off and when the PWM is low MOSFET is on.

Hence, the duty cycle of PWM and MOSFET is the opposite.

As one increases other decreases.

But you may think, why not use an N channel MOSFET, because, it is on when the gate is high,

and off when the gate is low?

Because the voltage at the gate required to turn on the n-MOSFET should be higher than the source voltage,

And during operation, the drain and source voltage will be almost the same.

Thus we need higher than Vcc at the gate to turn on the MOSFET.

Hence, we use a P-MOSFET.

Also, we add a pull-up resistor between source and gate.

This is the positive supply from the battery.

These voltages are created with the help of voltage regulators.

As these voltages are used only for references, thus there is no current drawn from them and no power is lost.

In real cases, due to non-ideal components, some power is lost as they draw a small amount of current.

Now, Let's look at the working of the complete circuit.

We will have an input voltage of 12 Volts, and we want the output of 5 Volts irrespective of the load.

Now, If we increase the load, the current increases, but the PWM is the same as before,

hence the voltage falls.

This causes the voltage at the reference to fall and thus increasing the difference between

this and reference at the differential amplifier.

This increases the voltage at the output and then the duty cycle of PWM decreases from

the comparator thus increasing the on-time of MOSFET and increasing the voltage at the load.

Also, we can change the voltage by changing the value of the potentiometer.

This change in resistance will affect the feedback voltage, thus it will change the

output of the error amplifier and then change the PWM signal.

Now, while designing this some care should be taken.

First, the voltage at the error amplifier should always be below this reference voltage.

Hence, we select the feedback voltage divider such that the voltage does not exceed 2 volts.

Thus the resistor value turns out to be 100 Kilo ohm and 20 k trimmer.

But, this will reduce the voltage to a minimum of 6 volts.

Hence, we use a 100-kilo ohm trimmer.

For this arrangement of the op-amp and the value of the resistor, the minimum output

voltage is nearly 4.5 volts.

For these values, If the load is reduced by a large amount, the reference voltage may

rise above 2.5 volts as the energy storage components release their energy.

This isn't an issue with this analog circuit as the error amplifier will output zero volts

and this will turn off the MOSFET.

But, for a digital PWM controller, this may cause some problems or damage them.

The second is the inputs of the comparator.

In this arrangement, we get a range from 12 to 5 volts for this feedback resistor values.

And, If the inputs of the comparator are reversed then the range becomes 8 to almost zero volts.

Also, the high voltage output is towards the high end of the trimmer.

This is how the closed-loop buck converter works.

The voltage and current from the source are converted to lower voltage and higher current at the output.