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[SOLVED] buck converter with P controller weird performance

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mamech

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hello

I am building currently for the first time a buck converter controlled with a p controller . the problem now is that i notice some weird performance, and I want to ask any expert if what happens is logical or there is something wrong in connections.

buck_circuit.jpg

I give supply to the circuit around 17 volts, I noticed that the volt at point A in the schematic (voltage at source of mosfet) is round 12 volts, which is really weird to me.
4 volts drop across the mosfet??

the other strange thing, is that the maximum output that I can get at point B on schematic is around 7 volts (which means around 5 volts loss after buck circuit).

so the result, is that for supply of 17 volts, I can control the volt only in range of 0-->7 volts only.


can anyone explain to me why this happens?
 

KlausST

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Hi,

First check if connection of the gate driver circuit is ok. Usually there should be a connection to source of fet.
To drive a N ch Fet you need a gate drive voltage about 10V higher than drain voltage. Ofte a bootstrap circuit is used for this. I can't find it.

Please specify your voltage. Is it peak, RMS, or average.

A scope picture could be helpful.

Klaus
 
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mamech

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Please specify your voltage. Is it peak, RMS, or average.

A scope picture could be helpful.

Klaus

It is the volt that I read by multimeter. it is DC voltage

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Hi,

First check if connection of the gate driver circuit is ok. Usually there should be a connection to source of fet.

Klaus

yes, but A connection between source of fet and what? ground? gate? or?
 

KlausST

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Hi,

Pwm is switching. Reading from multimeter is meaningless as long as you don't know how it is calculated.

A 50% duty cycle of 10V (peak) gives 5V avg or 7.1V rms.


Fet/driver connection: read the driver's datasheet.

Klaus
 

schmitt trigger

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Mamech

The circuit is missing.........something. Without going into details, it appears that you have the drain-source connections reversed.
I would suggest to download the controller's data sheet and follow EXACTLY the recommended circuit.

Do you realize that you are showing a n-channel in your diagram?

Also, you don't require the opamp buffer, get rid of it.
 
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mamech

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thank you for your reply


Mamech

The circuit is missing.........something. Without going into details, it appears that you have the drain-source connections reversed.
I would suggest to download the controller's data sheet and follow EXACTLY the recommended circuit.

Do you realize that you are showing a n-channel in your diagram?
they are not reversed, and yes I realize that I am showing n-channel mosfet, because I am using it actually, there is a problem in using n-channel mosfet here?

Also, you don't require the opamp buffer, get rid of it.
[/QUOTE]

I need it to make attenuation for feedback signal without having loading effect on the original load.
 

schmitt trigger

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Of course there is a problem!
As Klaus mentioned, you require one of two things: an auxiliary supply higher than your main supply, or a bootstrap circuit. You have neither.
There is a third alternative, use a P channel mosfet. But you would have to ensure that you don't exceed the Vgsmax rating.

Loading effect on the output?? What divider value do you intended to use which will have an impact on the 100 ohm load?
Besides, the divider's current will help you meet the minimum output current required to keep the inductor in CCM mode. It is beneficial.
 
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ravindragudi

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Agree with comments from Klaus and schmitt trigger.

My suggestion would be to use a P channel mosfet and for the voltage feedback have a resistor divider at node B itself. No need for opamp. The resistors values have to be in hundreds of k ohm to minimize the current consumption.
 

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