What is the reason for harmonics in dc-dc converter?

[student-engineer]

There is a nonlinear switch in every dc-dc converter.Is it the reason of harmonics in output voltage?

 

[jedishrfu]

I think this describes its function here:

https://books.google.com/books?id=m...y dc-dc converter what is it used for&f=false

https://www.omicsonline.org/open-ac...near-dynamics-2229-8711.1000166.php?aid=38311

Basically it seeks to keep the current levels constant even though devices connected to the converter may be switched on and off.

 

[student-engineer]

Basically it seeks to keep the current levels constant even though devices connected to the converter may be switched on and off.

Yes,I am familiar with the function of switch.Inductor in the circuit maintains a continuous supply of current in the circuit,even when the switch is off.When switch is off,the inductor discharges energy.Because of the switch mode operation of semiconductor switch,the device has a duty cycle.By varying the duty cycle,the output voltage can be varied.
But what is the reason of harmonics in output voltage.Is the nonlinear switch reason of harmonics in dc-dc converter?

 

[donpacino]

have you taken a signals and systems course yet?
have you studied what happens with switched signals?
as a hint... What is the frequency response of a square wave?

 

[student-engineer]

I think this describes its function here:

https://books.google.com/books?id=mZRsBgAAQBAJ&pg=SA9-PA60&lpg=SA9-PA60&dq=nonlinear+switch+in+every+dc-dc+converter+what+is+it+used+for&source=bl&ots=GgThrmPkF5&sig=t8AZur9nj5ztpn61vHdtQJ6qq2o&hl=en&sa=X&ved=0ahUKEwj1-6XU1qDXAhWs6YMKHY8xCYoQ6AEIXDAJ#v=onepage&q=nonlinear switch in every dc-dc converter what is it used for&f=false

https://www.omicsonline.org/open-ac...near-dynamics-2229-8711.1000166.php?aid=38311

The reading material is much informative.Thank you for sharing.

 

[student-engineer]

have you taken a signals and systems course yet?
have you studied what happens with switched signals?
as a hint... What is the frequency response of a square wave?

Yes,I took it.I may not be remembering much about it.The square wave is composed of many harmonics.In power systems too the requirement is to have a perfect sinusoidal waveform.But in reality the wave is distorted due to harmonic components.Harmonics of a signal are analyzed in frequency domain.At fundamental frequency is the dc component.

 

[donpacino]

Yes,I took it.I may not be remembering much about it.The square wave is composed of many harmonics.In power systems too the requirement is to have a perfect sinusoidal waveform.But in reality the wave is distorted due to harmonic components.Harmonics of a signal are analyzed in frequency domain.At fundamental frequency is the dc component.

Well you pointed out that a square wave is made of many harmonics. You also know that a on-off waveform is produced by the switches...

 

[student-engineer]

Well you pointed out that a square wave is made of many harmonics. You also know that a on-off waveform is produced by the switches...

Thank you.But also why does the switch produce a square wave,which is full of harmonics?The reason could might be that the switch used is a nonlinear electronic device.In nonlinear electronics,the current rise is not proportionate with voltage.One reason could might be variation of impedance in non-linear devices.Because of variation of impedance,the voltage is also varied,so the current also varies and this results in nonlinear V-I characteristics of such a device.

 

[Averagesupernova]

It is not possible to have a square wave that is not composed of harmonics. It is part of what defines a square wave. A signals and systems course would explain this.

 

[student-engineer]

It is not possible to have a square wave that is not composed of harmonics. It is part of what defines a square wave. A signals and systems course would explain this.

Yes,I know that a square wave is essentially composed of harmonics which become visible when Fourier transform of the square wave is taken i.e. the harmonics can be seen in frequency domain.The posts on this thread also clarified that the switch in the dc-dc converter produces the square wave voltage waveform when operated in switch mode and the square wave has harmonic content essentially and those harmonics are components of square wave i.e. the square wave is comprised of harmonics,and so the dc-dc converter is a non-linear circuit with harmonics.But why does the nonlinear switch device produce square wave i.e. harmonics?

 

[student-engineer]

The reason that harmonics or square wave is produced by the switch could might be that the switch is a nonlinear device.In a nonlinear device, the impedance is not constant and it varies so a nonlinear characteristic v-i waveform is obtained. Thus harmonics occur in current waveform which cause harmonics in voltage waveform too.
http://www.mirusinternational.com/downloads/hmt_faq01.pdf

 

[donpacino]

stop saying its because of the nonlinear device... its not.
the device produces square waves because that's how it functions

 

[Asymptotic]

But why does the nonlinear switch device produce square wave

Switches don't necessarily create square waves. Square waves are a specific type of pulse waveform characterized by a 50% duty cycle - on half of the time; off half of the time. Switches can create pulses with other duty cycles, and pulse width (duty cycle) control is the idea behind PWM - Pulse Width Modulation.

What other type of waveform other than a pulse can a switch create?

An ideal switch is purely digital. It is either on or off, conducting or non-conducting, true or false - there is no "in-between" - and the resulting waveform is either high or low. If switched at 50% duty cycle a square wave is created.

Another way to create a square wave is to build it using nothing but a collection of sine waves with a large sine wave at the fundamental frequency, and a Fourier series of progressively smaller sine waves at each odd harmonic frequency.

 

[berkeman]

The reason that harmonics or square wave is produced by the switch could might be that the switch is a nonlinear device.In a nonlinear device, the impedance is not constant and it varies so a nonlinear characteristic v-i waveform is obtained. Thus harmonics occur in current waveform which cause harmonics in voltage waveform too.
http://www.mirusinternational.com/downloads/hmt_faq01.pdf

stop saying its because of the nonlinear device... its not.
the device produces square waves because that's how it functions

@student-engineer as @donpacino says, the switch transistor in a DC-DC converter is not necessarily a non-linear device. From your link:

A load is considered non-linear if its impedance changes with the applied voltage.

That is not describing the transistor action in a DC-DC converter...

 

[student-engineer]

@student-engineer as @donpacino says, the switch transistor in a DC-DC converter is not necessarily a non-linear device. From your link:

That is not describing the transistor action in a DC-DC converter...

Yes,but usually a semiconductor switch is used in such a converter.Semiconductor switches such as diode,mosfet,igbt are nonlinear.A dc-dc converter can also be load.In that pdf may be they were mentioning in general terms;I think that it means that a load or a device is nonlinear if its impedance changes.What else could be reason of nonlinearity

 

[berkeman]

diode,mosfet,igbt are nonlinear

The diode has a non-linear V-I characteristic, so it can be used as a "switch" in some applications. The transistor used in a Buck DC-DC converter is not non-linear. It is ON or OFF, at different times. Changing impedance at different times does not make a device non-linear.

The ON-OFF switching action of the transistor in a Buck DC-DC converter helps to create a voltage squarewave which is fed into the inductor. This causes the current through the inductor to vary as a triangle wave. The output smoothing capacitors help to reduce the voltage ripple at the output of the DC-DC converter. In general, that output voltage ripple is at the switching frequency of the transistor, and does not contain much harmonic content.

As I mentioned in my PM to you, please do some more reading about DC-DC converters and how they work, and review the Signals and Systems material from the class you said you took. Then if you have specific questions about that reading, post a link here and ask the *specific* question about the material that you have linked to. That will help us to help you the best. Thank you.

 

[donpacino]

Switches don't necessarily create square waves. Square waves are a specific type of pulse waveform characterized by a 50% duty cycle - on half of the time; off half of the time. Switches can create pulses with other duty cycles, and pulse width (duty cycle) control is the idea behind PWM - Pulse Width Modulation.

Very good point. I started off using a square wave (50% duty cyle) as an example and got locked into a train of thought

 

[donpacino]

Yes,but usually a semiconductor switch is used in such a converter.Semiconductor switches such as diode,mosfet,igbt are nonlinear.A dc-dc converter can also be load.In that pdf may be they were mentioning in general terms;I think that it means that a load or a device is nonlinear if its impedance changes.What else could be reason of nonlinearity

Yes devices such as diodes and mosfet are nonlinear, but they are not used they way. In a DC-DC converter they are used in an on or an off state. If a fet is being used in the linear region, that indicates something is wrong with the design or functionality of the device. It will either be all the way on, or all the way off.

 

[donpacino]

The reason that harmonics or square wave is produced by the switch could might be that the switch is a nonlinear device.In a nonlinear device, the impedance is not constant and it varies so a nonlinear characteristic v-i waveform is obtained. Thus harmonics occur in current waveform which cause harmonics in voltage waveform too.
http://www.mirusinternational.com/downloads/hmt_faq01.pdf

and yes, a non-linear load will impart harmonics onto the system, however like we said above, harmonics will exist with no load or completely linear loads as well

 

[jim hardy]

But why does the nonlinear switch device produce square wave i.e. harmonics?

Doesn't a switch produce a mathematical discontinuity ?

Think simple .
Don't try to force Mother Nature into your preconceived notions.

You keep asking 'what is it about the switch...' when the answer lies elsewhere.

How does a DC to DC converter work ?
It 'chops' the incoming DC into an AC waveform.
Then it changes the amplitude of that AC wave by any of several means. Usually there's inductance involved in that process .
Then it rectifies that modified AC waveform back into DC.
The harmonics you refer to are those of that intermediate AC waveform which might or might not be square..
Realize that 'filtering' back to absolutely pure DC is not possible for in low pass filters the approach to pure DC is asymptotic.

So it's not the switch causing the harmonics , who cares if that's linear or not linear,
It's the transformation to and from AC !
Math says you can't get rid of all vestiges of that AC .

old jim

 

[Tom.G]

How does a DC to DC converter work ?
It 'chops' the incoming DC into an AC waveform

Or, to put Jims explanation in slightly different words:

Consider the Brake Lights of a car. They are ON when the brake pedal is depressed and OFF when the pedal is released.

Same with the Turn Signals on a car. At any given instant they are either ON or OFF. This is because there is a device called a "Blinker" that switches power to the Turn Signals on for a short time then removes power for a short time. In both cases the bulbs get power in pulses. If you do a FT on the voltage to the Brake Lights or the Turn Signals you will see lots of harmonics. That is because of the fast rise and fall times of the pulses, they are either ON or OFF, not hanging somewhere in between.

The 'Switching Transistor(s)" in a DC - DC converter do the same thing as the Blinker does for the turn signals, they are either ON or OFF. Instead of light bulbs, a DC - DC converter uses an Inductor (or a Transformer). In fact, you could replace one of those Turn Signal bulbs with the inductive and output circuit of a DC - DC converter and get an output. (Although it would work very poorly because of the low frequency of turn signals.)

Hope it helps. If not, one of us will try again.

 

[donpacino]

This is a buck converter (reduces the input voltage). When the switch is on the inductor (and capacitor charges). When the the switch is off, the inductor discharges. this switching produces an AC waveform.
The capacitor is what maintains the dc voltage out, "smoothing out" the ac current into DC.

 

[donpacino]

Page 93 is a bridge rectifier, it is not a dcdc converter

 

[donpacino]

What is your background with electronics?

 

[student-engineer]

Page 93 is a bridge rectifier, it is not a dcdc converter

I mean page 93 in adobe acrobat.It makes page 164 of book.

 

[jim hardy]

But how does the dc waveform converts to ac in the circuit of such a converter.

here's a tutorial.
http://www.butlerwinding.com/electronic-transformer-inverter-trans/

The switches can be mechanical as shown in that tutorial or electronic ie transistors or thyristors.Before we had transistors, car radios used mechanical switching to chop 6 volts DC from the car battery into a 6 volt AC 'square' wave for the radio's step up transformer. The transformer made around 300 volts for the tubes.
The vibrator made a background 'hum', audible as soon as you.turned on the radio. They were notoriously unreliable. But the hum gave you hope that the radio would start working after about twenty seconds when the tubes warmed up . Only in the movies did 1930's and 40's radios come on immediately. And the movie soundtrack never has the vibrator hum.

Picture courtesy of http://radioremembered.org/vpwrsup.htm
a full size image that's easier to read is there.

just like i said - chop it with the vibrator, increase its amplitude with the transformer T-1, turn back into DC with the 6X5 rectifier, filter with chokes L6 & L15 and capacitors C-15 & C-16.
"B+" is the electronics term for high voltage supply to tube plate circuits. It originates from early days of broadcasting when most household radios were battery powered. They had two batteries, a 1.5 or 6 volt "A" battery for the tube filaments and a 45 or 90 volt "B" battery for the tube plates.

That's the basics of how it's done. Get your mind around those basic steps before you leap into the math.

Any help ?

It really is that simple.

old jim

 

[jim hardy]

And the movie soundtrack never has the vibrator hum.

I think one movie did , probably a Spielberg. He'd pay that kind of attention to detail. Perhaps "1942" ?

 

[student-engineer]

Thank you all for all the above posts.I was studying buck converter from page 164 (page 93 in acrobat reader) http://dlx.b-ok.org/genesis/462000/e4aa8d24200374a86eb7298713410a10/_as/[Ned_Mohan,_Tore_M._Undeland,_William_P._Robbins]_(b-ok.org).pdf A square wave is given as input.When the switch is on,the diode becomes off and the input voltage appears across the diode.Diode becomes on when switch becomes off.An on switch or diode acts as short circuit.
A square wave is neither ac or dc;it is fluctuating or pulsating direct current, as I read from second answer at https://www.quora.com/Is-square-wave-a-DC-signal-or-AC-signal and https://electronics.stackexchange.com/questions/112223/is-a-square-wave-still-considered-dc
So,square wave comprises dc components as well as a time dependent pulsating component https://en.wikipedia.org/wiki/Pulsed_DC .Inductor can only allow dc to pass,capacitor only allows ac to pass.However,the current in inductor is also a pdc The ac part of inductor current then flows through capacitor,whereas,the dc part goes to the load.

 

[Averagesupernova]

A square wave is always AC. Depending on how it is offset from zero it is often DC as well.

 

[jim hardy]

In the jargon of 1880's, 'alternate current' is flowing current that periodically reverses its direction.

When vacuum tubes came on the scene about twenty years later and 'electronics' was born, it became commonplace to impose an Alternate Current atop a DC one as in radio .

The term AC has come to mean either the original alternate current, or the AC component of a composite current.
DC has come to infer "steady" current, ie one direction with no AC imposed as in a flashlight battery. But still,
top wave is unquestionably AC
bottom wave is unquestionably unidirectional. Strictly speaking that makes it DC

But many folks, in fact most i know, would call it "Fullwave Rectified AC" so as to distinguish it from 'steady current' .
Webster:

You have to filter out the AC component to make it 'substantially constant in value' .This

is AC atop DC, and is unidirectional.

Don't play "Gotcha", take the trouble to completely (edit - or at least adequately) describe your waveform.
This is a square wave centered on zero, ie no offset, and its direction reverses..

This is a square wave with offset Vcc/2 and it's unidirectional.

 

[Asymptotic]

I think one movie did , probably a Spielberg. He'd pay that kind of attention to detail. Perhaps "1942" ?

For those interested in hearing this tell-tale hum check out 5 minutes and 17 seconds into this video.

 

[Averagesupernova]

The radio here: https://www.physicsforums.com/threa...rical-and-measuring-things-etc.755959/page-11
uses a vibrator that has commutator contacts. There is no rectifier diodes to obtain the B+. Was the first time I had ever heard of that but upon researching I found it was not uncommon.

 

[jim hardy]

The radio here: https://www.physicsforums.com/threa...rical-and-measuring-things-etc.755959/page-11
uses a vibrator that has commutator contacts. There is no rectifier diodes to obtain the B+. Was the first time I had ever heard of that but upon researching I found it was not uncommon.

On those radios you have to pay attention to positive or negative ground.

@Asymptotic Great video by the way.

 

[Averagesupernova]

On those radios you have to pay attention to positive or negative ground.

Great video by the way.

Yes you do have to watch the polarity. But I can't take credit for the vid.

 

[jim hardy]

But I can't take credit for the vid.

Well you found it and were kind enough to share it.

EDIT OOPS MY MISTAKE that was @Asymptotic 's video ! Is my face red !

mistaken post now edited for correct attribution... - jh