open loop and closed loop buck converter

[eahmadi]

Hi there,
I'm working on buck converter. I have got confused for differences between open loop and closed loop buck converter. I designed open loop converter.I have been considering a lot of parameters and trades of between them, such as output voltage ripple, inductor current ripple, switching frequency, load current,and like them. I have been spending much time on open loop buck converter, but for example, I can't have a good specific converter. Is it due to design closed loop? Am I right?
I look forward to hearing from you.
Thanks a lot in advance,
Ebi,

 

[phobos1]

In an open loop design the output is not maintained constant as we dont check on its parameters like output current or voltage ......... but in a closed loop the output can be kept constant by increasing or decreasing the swithching freq. duty cycle. to maintain constant output.........

 

[eahmadi]

Thanks phobos1,
I'm a little in dark. Could you please explain clearly?
Thans in advance,
Ebi

 

[mrinalmani]

In open loop converter, you simply have a fixed duty cycle to get the theoretically calculated output voltage. For example if the input is 10V and the output needed is 5 V, the duty cycle is fixed at 50% and there is no means of changing the duty cycle even if the output voltage falls below/above 5V (perhaps due to overloading or some other reason)
On the other hand, in a closed loop converter, the output voltage is continuously measured and fed back to the micro-controller. Depending upon the mismatch between the desired voltage and the actual output voltage, the controller adjusts the duty-cycle continuously. Therefore even if there is a mismatch due to some reason, the duty cycle is adjusted quickly to get the correct output.

 

[BradtheRad]

To grasp the principles behind the buck converter, it helps to experiment with a simulator, especially an animated, interactive type.

This is a tutorial circuit which I constructed, to demonstrate how a buck converter operates. You can control it yourself, or let the automatic clock drive it.



This is done in Falstad's simulator. It can export a link (below) which will:

(1) Open the site www.falstad.com/circuit
(2) Load my schematic into his simulator, and
(3) Run it on your computer.



To start a cycle, press the switch at left with your mouse.
For the second half of a cycle, let up the switch.

Watch the scope trace for the coil. Current rises during switch-On, and falls during switch-Off.

Watch how this action affects the output voltage. Eventually you will find the right tempo to click the switch on and off, so that you obtain a desired output voltage.

You can change values at will. Right-click on a component, and select Edit.

To save a schematic: Select File>Export, copy the text in the window, then paste it into a word processor, and save it from that program.