Feedback circuit for a Buck Converter

Thread Starter

14GHz

Joined Jul 27, 2022
2
Hi all!
I am trying to design my own buck converter and have done some research on the subject.
So the typical ICs used are some like the LM2569S for example, but it is limited to 3A output.

So I want to create my own buck converter using a high power MOSFET instead.
Now my issue is when it comes to the feedback loop, I need to be able to adjust my PWM duty cycle to keep my output constant when a load is applied etc.

But I am having trouble finding any information about such feedback circuits.
I was looking at using a 555 timer but how would i control my duty cycle with my output voltage?

Can anyone lend a helping hand or point me in the right direction?
 

Papabravo

Joined Feb 24, 2006
18,968
Hi all!
I am trying to design my own buck converter and have done some research on the subject.
So the typical ICs used are some like the LM2569S for example, but it is limited to 3A output.

So I want to create my own buck converter using a high power MOSFET instead.
Now my issue is when it comes to the feedback loop, I need to be able to adjust my PWM duty cycle to keep my output constant when a load is applied etc.

But I am having trouble finding any information about such feedback circuits.
I was looking at using a 555 timer but how would i control my duty cycle with my output voltage?

Can anyone lend a helping hand or point me in the right direction?
There are a couple of problems involved in this project. This is not a straightforward weekend project, nor is it a cut and paste exercise. How much do you know about control theory? This will be required if you intend to compensate the control loop. Second, how much do you know about how to control both current and voltage in a buck converter? Third, how much do you know about sub-harmonic oscillation and slope control? Fourth, how much do you know about layout techniques for high power circuits? Last are you willing to spend the time required to learn about these things? We could be talking about a substantial investment of time and money.

Knowledge of how to use a simulator will be helpful in regards to all of this. The following may give you some insight:
 

crutschow

Joined Mar 14, 2008
30,420
Best is to find a switching regulator IC that can power an external MOSFET to get the current you need (how much current and voltage?).
Those have a built-in feedback circuit to control the PWM duty-cycle and maintain a constant output voltage.

Trying to kludge one with a 555 is above your pay grade, I'm afraid.
 

Ian0

Joined Aug 7, 2020
5,819
Hi all!
I am trying to design my own buck converter and have done some research on the subject.
So the typical ICs used are some like the LM2569S for example, but it is limited to 3A output.

So I want to create my own buck converter using a high power MOSFET instead.
Now my issue is when it comes to the feedback loop, I need to be able to adjust my PWM duty cycle to keep my output constant when a load is applied etc.

But I am having trouble finding any information about such feedback circuits.
I was looking at using a 555 timer but how would i control my duty cycle with my output voltage?

Can anyone lend a helping hand or point me in the right direction?
Here is an excellent guide to feedback:
https://www.ti.com/seclit/ml/slup340/slup340.pdf

Almost every switched-mode topology can be made using a 555. To vary the pulse width disconnect Ra from the power supply and connect it to the output of the error amplifier. The higher the error voltage the faster the capacitor charges and the shorter the on time.
Rb controls the Off time, and Rb doesn't vary, so you have made a constant-off-time regulator.
 

crutschow

Joined Mar 14, 2008
30,420
Almost every switched-mode topology can be made using a 555.
I think recommending a 555 for making a switching power supply to someone who is a newbie about control circuit feedback and stability is asking for likely failure.
Why do that when there are many IC switching regulator circuits that will do the job much simpler?
 

Ian0

Joined Aug 7, 2020
5,819
I think recommending a 555 for making a switching power supply to someone who is a newbie about control circuit feedback and stability is asking for likely failure.
Why do that when there are many IC switching regulator circuits that will do the job much simpler?
Recommending? I was answering: "I was looking at using a 555 timer but how would i control my duty cycle with my output voltage?"
Some would criticise contributors for recommending other parts when the TS wanted to use a 555.
This is a forum, so there should be scope for all points of view.
 

Papabravo

Joined Feb 24, 2006
18,968
There are many pathways to enlightenment on the subject of Switch-Mode Power Supplies and the TS should pursue the most comfortable of them. Hopefully he will have the luxury of pursuing all of them.
 

k1ng 1337

Joined Sep 11, 2020
626
I made several DC-DC topologies with two 555 timers and a comparator. There are many issues with this yet a stable output voltage was obtained. Expect noise on the output, low efficiency and a small range of operating currents.

It was a good learning exercise but that's about it. Refer to section: 8.2.2 Pulse-Width Modulation to get started. Similar 555 designs can be found on the internet.
 

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