# How to find out the effect of changing the load in a buck converter?

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#### iVenky

I thought the output voltage in the buck converter is primarily decided by the duty cycle (Vout=DVin)but after simulations I found that it depends on the load resistor (or in other words the load current) too. Is there any formula that tell you the effects of the load?

Thanks a lot.

#### goldsmith

I thought the output voltage in the buck converter is primarily decided by the duty cycle (Vout=DVin)but after simulations I found that it depends on the load resistor (or in other words the load current) too. Is there any formula that tell you the effects of the load?

Thanks a lot.
Hi Ivenky

What kind of buck converter ? open loop or closed loop ? of course for a closed loop buck converter you can be sure that load has no effect on out put voltage .
i.e : vout =D.C(duty cycle *VS)

But if you don't have any feedback path ( amplifying error signal and a simple comparison stage ) your out put voltage will be affected by loading .
Best Wishes
Goldsmith

#### iVenky

hello goldsmith. I actually tried the closed loop. I used this type of circuit.

I actually needed 1 A load current but it couldn't provide that (using 0.5 Ω in that resistor). If I increased the resistance(as done in the above circuit) so the that the load current decreases then I could see the result. In other words only in the second case the output of the op amp is like a square wave whereas in the first one the voltage at the -ve terminal never reaches 0.5 V and hence the output of the op-amp is always high and also the -ve terminal voltage settles at some value below 0.5V

Thanks a lot

#### goldsmith

hello goldsmith. I actually tried the closed loop. Here's the screenshot
Hi again
I afraid but
Your circuit is completely wrong ! why ? because you are driving your series path element as a linear element ! it should be used in on / off region ( as a switch ) aren't you familiar with float drivers ?
I actually needed 1 A load current but it couldn't provide that. If I increased the resistance so the that the load current decreases then I could see the result
i.e a buck converter needs a PWM signal and then an error amplifier and then a comparator to create proper feedback path .
But your circuit is completely linear ! do you know what is the difference between the word linear and the word switch mode ?

All the best
Goldsmith

#### iVenky

I know the difference. So I have to do it like a SMPS

#### goldsmith

I know the difference. So I have to do it like a SMPS
Yes , so now what is your problem ?

with the circuit that you have attached you can't do it .

#### iVenky

I have changed the circuit. Still it's not working. The -ve terminal voltage is not reaching 0.5V.

I have plotted the triangular wave and also the -ve terminal voltage (you can see that it is not equal to 0.5 V ) Thanks a lot

- - - Updated - - -

Waiting for you reply Gold smith

#### goldsmith

Humm
Ivenky
Why frequency of triangle wave is 40 HZ ? why your loop is like that ? why your out put filter is like that ? why that diode is a simple diode ? why and why and why !
May i give you one advise ? try to read some text books with emphasis on SMPS design . if you are interested i can introduce some professional books .

#### iVenky

Just clear me this doubt- I have studied about SMPS but I never realized that the diode and the LC circuit that you use is a buck converter until now. I thought it's just for averaging out the input pulses but anyway both buck and averaging seems to be similar. Am I right in saying that SMPS has a buck converter and that it's similar to averaging circuit?

Thanks a lot

#### goldsmith

Just clear me this doubt- I have studied about SMPS
No you don't have studied ! because SMPS isn't like a linear regulator that after some hour reading , you be able to understand all of it's sections ! SMPS design , is a complete book !
I never realized that the diode and the LC circuit that you use is a buck converter until now
This is not easy to explain it here ! it needs high value of basics !
But just something for clarifying you :
this section plays many roles .
you can justify it's behavior as flows :
1- as a 2nd order power integrator to take average from the amplified PWM signal .
2- to store the energy .
3 - to filter other harmonics ( as an LPF ) i can tell you how it works , but it will be really hard because you don't have required basics .
What is your definition by the word SMPS ? there are gang of arrangements for gang of applications unlike linear regulators !
I really want to help you , and i've tried to show you the path .

##### Super Moderator
Staff member
Hi, mind if I jump in here?

It looks as though your coil's henry value is 1H. In that case 40Hz is too fast a cycle to allow sufficient current to build, and you will not get 1A at the load.

Try a coil value of 10 mH.

Try various switching frequencies and duty cycles.

#### goldsmith

Hi, mind if I jump in here?

It looks as though your coil's henry value is 1H. In that case 40Hz is too fast a cycle to allow sufficient current to build, and you will not get 1A at the load.

Try a coil value of 10 mH.

Try various switching frequencies and duty cycles.
Yes i completely agree with you about improper values !
Best Regards
Goldsmith

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