simulated inductor for dc-dc converter?

[carter tsang]

lm2674 inductor

because the real indcutor can be replaced by the simulated inductor, which consists of op-amps, resistors and capacitors， can I use the simulated inductor to substitute the real inductor in the dc-dc converter, such as buck or buck-boost circuit?
need some suggestions!
thank you very much!

 

[FvM]

lm2674 problem

Simply no. I guess, you're able to find the reason by yourself.

 

[carter tsang]

this simulated inductor can not be used in the situation of power conversion, where a coil is used as energy storage; but, the inductor in the buck circuit is used for energy storage.
Is that right?

and this simulated inductor is usually for grounded inductor, does anyone have some examples of the simulated inductor which is used for floating inductor?because the inductor in the buck converter is not grounded[/quote]

 

[LvW]

................
and this simulated inductor is usually for grounded inductor, does anyone have some examples of the simulated inductor which is used for floating inductor?because the inductor in the buck converter is not grounded

[/quote]

In principle, ungrounded inductances can be simulated with active devices, however, the circuitry is larger by a factor of two, approximately - if compared with grounded devices.

 

[carter tsang]

For example, in buck circuit, I do not think the function of simulated ungrounded inductor is the same as the real inductor which is made of coil, and considering the high frequency which will result in influence on the op amp in the simulated inductor, it is impossible to use the simulated inductor in the buck circuit. But my teacher said the real inductor can be undoubtlessly replaced by the simulated inductor.

 

[LvW]

For example, in buck circuit, I do not think the function of simulated ungrounded inductor is the same as the real inductor which is made of coil, and considering the high frequency which will result in influence on the op amp in the simulated inductor, it is impossible to use the simulated inductor in the buck circuit. But my teacher said the real inductor can be undoubtlessly replaced by the simulated inductor.

What makes an inductor ? Nothing else than a specific frequency dependence connected with a certain phase shift. And this, in principle, can always be realized also by another device containing only amplifiers, resistors and capacitors.
However, the only point is if the active devices still do function properly at the wanted frequencies.

 

[FvM]

...it is impossible to use the simulated inductor in the buck circuit. But my teacher said the real inductor can be undoubtlessly replaced by the simulated inductor.

My first comment wasn't fully correct. In principle, a simulated inductor can replace a real inductor if it is able to operate as an inductor at the respective frequency and can handle the required output voltage.

I read your question however as a practical question rather than a theoretical, cause I'm an engineer and analog designer. And then the answer is simple: It may be possible, but it doesn't make any sense, cause it would consume more energy than it supplies. E. g. in a boost converter, the simulation circuit must have a supply above the boost converter output voltage, you could simply use this supply instead of the boost converter.

Or, as you pointed out, it doesn't actually store energy, it is taking all energy from its power supply.

The OP speed, floatig inductor etc. issues have to be solved in addition, but there are no principal limitations, it's a matter of avaible circuit technology.

There are however applications of simulated inductors in switched mode power supply. Nationals Simple Switcher devices, e. g. LM2674 have it in their control loop.

 

[carter tsang]

are there some methods or articles which introduce the parameters design around the op amp, for example, how to determinate the value of resistors or capacitors around the op amp, or what I should pay attention?
I have some troubles to determine this

 

[LvW]

are there some methods or articles which introduce the parameters design around the op amp, for example, how to determinate the value of resistors or capacitors around the op amp, or what I should pay attention?
I have some troubles to determine this

For your information I attach two documents
1.) A paper containing a GIC structure to realize an active and GROUNDED inductor
2.) A paper showing ONE of several alternatives to build a floating inductor.
Regards

 

[carter tsang]

I use the circuit in the papers to establish a simulated grounded inductor, but it can only simulate the inductor of high value, such as 0.1H, if the value is 100uH, it is tough to determine the parameters of resistors and capacitors. I can not get the 100uH simulated inductor.
the simulation soft ware I used is SIMetrix, in my circuit L=c*(R5/R2)*R3*R4, I add a sine wave signal to the simulated grounded inductor to verify the U-I properties.
[/url]

 

[FvM]

but it can only simulate the inductor of high value

Why?

 

[LvW]

I use the circuit in the papers to establish a simulated grounded inductor, but it can only simulate the inductor of high value, such as 0.1H, if the value is 100uH, it is tough to determine the parameters of resistors and capacitors. I can not get the 100uH simulated inductor.

I suppose you only have chosen equal resistors. That´s not necessary.
In practice you can use different values and - thereby - realize a variety of inductance values.
However, it is recommended to choose R2=R3 (but not necessary). This equality is best for the high frequencies region where the finite opamp transit frequency comes into play.
Regards

One additional comment: Use also ac analysis to verify the voltage-current relationship. By doing this you can see in which frequency range the active replacement works good.

 

[carter tsang]

Why?
I can only get the formula L=c*(R5/R2)*R3*R4, but do not know how to determinate each value in it, so I just let R5=R2=R3=1k or 500 or 10K, and change the value of c and R4 to get the L value that I　want . The only simulated inductor I can get is the situation in the image I attach, and the L value is 0.1uH, and it can be in accordance with the U-I relationship of real inductor. But it is tough for me to get 100uH simulated inductor that I want，what I do is to try different values of R and C again and again

Added after 1 minutes:

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[FvM]

Sorry, I don't get the problem. The 1k resistor level isn't that bad, although already limiting TL082 output voltage. With a 100 pF, you get a 100 uH simulated inductor, showing a good inductive behaviour in a 100 Hz to about 500 kHz frequency range. It's limited by OP DC gain at the lower end and OP GBW product at the upper.

 

[carter tsang]

although it seems to be in accordance with the formula when R=1k and c=100pF then L=100uH, the simulation result is not desirable.
red-sine voltage/10K and peak=12V
green-current

 

[LvW]

Hi carter tsang,

again: You can choose any values for R and C as you like - in accordance with the formula
L=c*(R5/R2)*R3*R4 .

In addition, it is recommended to make R2=R3 for high frequency behaviour, but it´s not absolutely necessary.

The strange behaviour of the curent is supposed to be the result of amplifier saturation (choose a smaller input voltage).
However, why did you follow my recommendation to run an ac analysis ?
In this case you must not be afraid to saturate the amplifier because it is a small signal analysis.

 

[carter tsang]

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[LvW]

ｓｏｒｒｙ，　Ｉ　ｄｏ　ｎｏｔ　ｋｎｏｗ　ｈｏｗ　ｔｏ　ｃｏｎｄｕｃｔ　
ａｃ　ａｎａｌｙｓｉｓ

Ｔｈｅ　ｒｅａｓｏｎ　ｗｈｙ　Ｉ　ｃｈｏｏｓｅ　ｔｈｅ　１２Ｖ　ｉｎｐｕｔ　ｖｏｌｔａｇｅ　ｉｓ　ｔｈａｔ　ｉｎ　ｔｈｅ　ｓｉｔｕａｔｉｏｎ　ｗｈｅｒｅ　Ｉ　ｕｓｅ　ｔｈｅ　ｓｉｍｕｌａｔｅｄ　ｉｎｄｕｃｔｏｒ，　ｔｈｅ　ｖａｌｕｅ　ｏｆ　ｉｎｐｕｔ　ｖｏｌｔａｇｅ　ｉｓ　ａｂｏｕｔ　１０ｖ－（－５ｖ）square wave。 should I change the amplifier?

Please, can you repeat the message ?

 

[carter tsang]

sorry, I do not know how to conduct ac analysis.
And I have try a large number of values to realize the 100uH, but always fail.
The situation where I use this simulated inductor is that the input voltage is 10v-(-5v)square wave, so that is the reason why I choose 12V input sine voltage to verify the U-I relationship.

 

[FvM]

The simulation simply shows, that the present simulated 100 uH inductor can't be used in the intended voltage and current range.

The limitation is by the 1K resistor and by the TL082 internal current limit as well. You'll probably remember, that this point had been mentioned at the beginning of the thread.