pwm circuit for simple buck converter

[octopus dive]

hi..sorry for interruption..I really need help in finding suitable pwm circuit for buck converter with Vin=15v , Vout=5v? here are the schematic diagram of my buck converter's design..hope to hear from u guys soon..thank u..

 

[prestonee]

The PWM is the more complicated part of the design. you need to implement control theory into the loop, something like PID is usually done as opposed to a linear pwm response. to start your transition instead of using that ideal voltage in pulse you should replace it with a comparitor. then you can start flushing out the requirements to get the comparitor to operate correctly, ie compare which node to what reference. there are IC chips that do this in a much more manner that have built in safety options such as current monitor etc to prevent damage to the inductor.
sorry I know im not giving you a specific answer, just pointing you in a direction.
-Pb

 

[BradtheRad]

The buck converter is able to be driven by hysteresis, built around a comparator.



I have not tested this schematic with hardware but it's a start. Adjust values to suit your purposes.

The sense resistor can be a small value, perhaps under 1 ohm. It only needs to show a difference of a tenth of a volt or so, per cycle.

 

[BradtheRad]

To reach a better grasp on the principles, by experimenting with an interactive animated simulator...

Here is a link that will open the falstad.com/circuit website, load my schematic, and run it on your computer. It is similar to my post #3.

https://tinyurl.com/cjwmsxa

Change values by right-clicking on a component, and select Edit.

Try different loads, Vref, hysteresis feedback resistor value, etc.

I do not know for certain how well this circuit can be made to work with hardware.

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Here is an interactive simulation where you (the user) can click a switch to start the switch-On cycle.
Then let up to see the switch-Off cycle.

http://tinyurl.com/cnnkhcg

You can see how the duty cycle needs to be timed, as you try to obtain 5V at the load.

 

[octopus dive]

i'm really sorry but i don't quite understand about the CLK component in the circuit..it will be hard as i need to build this circuit in hardware form..could u please highlight it in details please by which means its serial name and etc. so that i can find it in any electronic store?

 

[BradtheRad]

CLK is a generic pulse generator. It is the same as V3 in your schematic.
It has to deliver sufficient volt level to turn on the mosfet/transistor.

It might be a 555 timer IC, configured to run in astable mode. There are easy ways to vary its duty cycle.

Or a common astable multivibrator using two transistors, or two logic inverters, etc.



You are undertaking a project which requires that you learn theory at the same time you are building it in hardware. Expect to do a lot of experimenting before you get it right. You may overheat a few components.

You may decide it is easier to use one of the control IC's which are designed to provide PWM to a buck converter (as mentioned in post #2).

 

[octopus dive]

if i wanted to use the first picture, how can i verify to get duty cycle=0.333??is it by changing the value of resistor or by changing the value of the 10v??

 

[RAHEED]

change the capacitor value

 

[octopus dive]

both capacitor or only the top resistor?

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both capacitor or only the top resistor?

 

[BradtheRad]

if i wanted to use the first picture, how can i verify to get duty cycle=0.333??is it by changing the value of resistor or by changing the value of the 10v??

You can vary the duty cycle by applying a voltage to the CTL pin 5:

 

[octopus dive]

the +10v is from a dc voltage source? but then what do u mean applying a voltage to ctl pin 5?is it that i need to use another voltage source or how??really for asking..it is just a bit confusing when i try to do this in simulation and applying it to hardware

 

[BradtheRad]

the +10v is from a dc voltage source? but then what do u mean applying a voltage to ctl pin 5?is it that i need to use another voltage source or how??really for asking..it is just a bit confusing when i try to do this in simulation and applying it to hardware

The schematic is to illustrate a concept. Your supply will be 15V. Looking at the scope trace, it implies that you will obtain 33% duty cycle by applying a volt level to CTL pin 5 which is 1/3 of the supply V.

There are also ways to adjust duty cycle which hook up to the resistor/capacitor network (at the discharge/ trigger/ threshold pins). You may decide to use one of those, if you think it will be easier.

I suppose your output needs to be regulated to 5V? In that case you will take a sense voltage from your output, and use that to apply the correct volt level to CTL pin 5. To this it probably will first need to be shifted/ inverted/ amplified/ attenuated. An op amp might be handy to do that.

Here is a link listing some projects which use a 555 IC to control a switched-coil power supply:

http://www.next.gr/power-supplies/switch-mode/555-timer-as-a-switch-mode-power-supply-l4693.html