How to design a DC-DC to let output voltage follow the input voltage?

How to design DC-DC to let output voltage follow input voltage?

Hi, all

Could you please give me some hint? I'm totally new in this DC part.
I want to design a circuit:
DC input, voltage changes from 0 to 4V
DC output, voltage follow input to change. For example,
input=0， output=0
input=1, output=1
input=2,output=0
input=3,output=1
input=4,ouput=0

Like below, xaxis is Vin, y axis is Vout. Which kind of circuit or architecture should I use?

Re: How to design DC-DC to let output voltage follow input voltage?

Better to use a micro controller having ADC and fix the voltage tolerance (for 1 volt consider 0.9V to 1.1V)

Re: How to design DC-DC to let output voltage follow input voltage?

This requires a more complete specification.
Voltages available, frequency response, power-handling needs...

Re: How to design DC-DC to let output voltage follow input voltage?

Ignoring any other requirements that may be there the easiest way is to just short the output to the input :smile: but I assume you can't do that since you need to drive something from the output which the input cannot do. For that the easiest way is to use an opamp in the voltage follower mode like shown here http://www.learningaboutelectronics.com/Articles/Voltage-follower. Now you may be able to get away with this simple solution if your specs allow you to choose a suitable opamp that can satisfy them.

Re: How to design DC-DC to let output voltage follow input voltage?

Are we to assume that between the points you have
declared, it's linear interpolation as the graphic shows?

Simplest could be ADC, cal-map xyzROM, DAC (but this
will be "steppy" at lower bit-widths) reading VIN and
driving VREF with appropriate feedback network.

You might also be able to use a VIN "thermometer"
comparator cascade, and analog switches across
subsections of the feedback network and perhaps
a VREF source network, to make the ups and downs.
Poor-boy version of ADC (comparators) and DAC
(switches), and the ROM is you.

Re: How to design DC-DC to let output voltage follow input voltage?

I am a bit confused to what is required.

Are you talking about a function like this below?

Re: How to design DC-DC to let output voltage follow input voltage?



Using a difference amplifier you can create either a rising slope or falling slope as shown in the image. All you need to do is detect in which region does vi lie and then connect the right signals for v1 and v2 of the difference amplifier. If the change boundaries are 1,2,3 volts then you need 3 comparator and some simple logic to switch the right voltages to the input of the difference amplifier.

Re: How to design DC-DC to let output voltage follow input voltage?

aryajur, I agree with your proposed method, and my plots were obtained with a similar circuit. We still need to get clarification if this is the wanted transfer function.

Re: How to design DC-DC to let output voltage follow input voltage?

I think a Schmitt Trigger will serve with a carefully selected threshold voltage.

Re: How to design DC-DC to let output voltage follow input voltage?

Below is the circuit, I used to generate the function. I am sure it could be simplified, but I didn't bother with it any further once it worked.

Re: How to design DC-DC to let output voltage follow input voltage?

Thanks a lot！！！！！！ Please give me some time to understand what you say.

Re: How to design DC-DC to let output voltage follow input voltage?

JWHassler said:

This requires a more complete specification.
Voltages available, frequency response, power-handling needs...

Click to expand...

Supply voltage is 2V, I want to get positive voltage and negative voltage which are as large as possible. The output voltage can change following the change of the input voltage. Vout could be similar to the picture, it can also be sine wave or cos wave.
Frequency response and power issue are not required. maybe there is some requirement, but for me the first priority is to have a basic idea about which kind of structure I can use. So I don't consider frequency response or power yet.

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aryajur said:

Ignoring any other requirements that may be there the easiest way is to just short the output to the input :smile: but I assume you can't do that since you need to drive something from the output which the input cannot do. For that the easiest way is to use an opamp in the voltage follower mode like shown here http://www.learningaboutelectronics.com/Articles/Voltage-follower. Now you may be able to get away with this simple solution if your specs allow you to choose a suitable opamp that can satisfy them.

Click to expand...

Thanks! I think I forget to draw something. Is it possible to generate +/- voltage based on input?

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E-design said:

I am a bit confused to what is required.

Are you talking about a function like this below?

Click to expand...

Sorry to let you confused. I don't draw it clearly. It should be like this below. The output voltage can be triangle, sine or cos wave, it doesn't matter. Anyway, Vout can follow Vin to change to positive or negative value.


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dick_freebird said:

Are we to assume that between the points you have
declared, it's linear interpolation as the graphic shows?

Simplest could be ADC, cal-map xyzROM, DAC (but this
will be "steppy" at lower bit-widths) reading VIN and
driving VREF with appropriate feedback network.

You might also be able to use a VIN "thermometer"
comparator cascade, and analog switches across
subsections of the feedback network and perhaps
a VREF source network, to make the ups and downs.
Poor-boy version of ADC (comparators) and DAC
(switches), and the ROM is you.

Click to expand...

Thanks a lot! I am trying to understand what you say. May I ask is it possible to generate positive and negative voltage using these circuits?

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aryajur said:

View attachment 142414

Using a difference amplifier you can create either a rising slope or falling slope as shown in the image. All you need to do is detect in which region does vi lie and then connect the right signals for v1 and v2 of the difference amplifier. If the change boundaries are 1,2,3 volts then you need 3 comparator and some simple logic to switch the right voltages to the input of the difference amplifier.

Click to expand...

Thanks aryajur!!! May I ask does this circuit can generate positive and negative voltage based on , for example [from 0 to 2V] supply voltage?

Re: How to design DC-DC to let output voltage follow input voltage?

May I ask does this circuit can generate positive and negative voltage based on , for example [from 0 to 2V] supply voltage?

Click to expand...

Not impossible, but what's the purpose at all?

The non-linear, non-monotonic Vin to Vout function requested in post #1 has been understood as analog circuit design problem in the first place. As the circuit elaboration by E-design clarifies, this design aspect is already quite challenging. You need in fact a dedicated comparator or equivalent circuit function block for each of the three break points in the Vin/Vout characteristic and additional circuit elements to transfer the comparator result to the output, e.g. the suggested analog switches.

The thread title "let output voltage follow input voltage" is mostly misleading, I fear.

Asking for a specific power supply, e.g. DC/DC up-conversion or polarity inversion is an additional requirement, it would be usually implemented independent of the non-linear voltage characteristic.

In case the term "supply voltage" is meaned as another word for "Vin" in the original diagram, I'd say it's still possible, but even more complicated.

Re: How to design DC-DC to let output voltage follow input voltage?

It is not difficult to offset the output of the current circuit to go positive and negative, but as FvM stated anything beyond this gets messy with just analog design tools.

I would also like to know what the purpose of all this is? There may be other ways of achieving your goal.

Re: How to design DC-DC to let output voltage follow input voltage?

FvM said:

Not impossible, but what's the purpose at all?

The non-linear, non-monotonic Vin to Vout function requested in post #1 has been understood as analog circuit design problem in the first place. As the circuit elaboration by E-design clarifies, this design aspect is already quite challenging. You need in fact a dedicated comparator or equivalent circuit function block for each of the three break points in the Vin/Vout characteristic and additional circuit elements to transfer the comparator result to the output, e.g. the suggested analog switches.

The thread title "let output voltage follow input voltage" is mostly misleading, I fear.

Asking for a specific power supply, e.g. DC/DC up-conversion or polarity inversion is an additional requirement, it would be usually implemented independent of the non-linear voltage characteristic.

In case the term "supply voltage" is meaned as another word for "Vin" in the original diagram, I'd say it's still possible, but even more complicated.

Click to expand...

Thanks，FvM! I repeat my understanding based on your reply.
1. Voltage comparator is required to decide the breaking point of Vin. For example, when Vout begins to increase, when it begins to derease,when it go to negative voltage. All these breaking points need comparators to decide.
2. For positive Vout, E-design's sch is a good start point to learn
3. I search the website and found a lot of circuit about transfering positive voltage to positive and negative voltage at the same time. But they are all fixed output voltage. I really don't have idea about how to generate changing negative voltage which are also a function of Vin.
4. There are no requirement of "a specific power supply, e.g. DC/DC up-conversion or polarity inversion".
5. The supply voltage is 2V, which is fixed. The input signal is a changing voltage,maybe 0-1V or 0-2V, it doesn't matter. The output signal has different function of Vin in different range of Vin, as I draw.

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E-design said:

It is not difficult to offset the output of the current circuit to go positive and negative, but as FvM stated anything beyond this gets messy with just analog design tools.

I would also like to know what the purpose of all this is? There may be other ways of achieving your goal.

Click to expand...

Thanks,E-design. This is my assignment which don't have practical purpose. Because I am a new EE in DCDC, I have no idea about how to do the first step or what I should search in google. Because I search "generate negative voltage", but I can only find "generate fixed positive and negative voltage" but not this kind of changing positive and negative voltage.