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555 timer as PWM controller

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saurav_sdpl

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hii,

I have to design a flyback smps of o/p power 3W.

I want to use 555 timer as PWM oscillator and control the duty cycle from 0 to 50% using control voltage.

1. In what configuration 555 timer will be configured, whether astable or monostable mode

2. How can i limit the duty cycle upto exact 50%

3. How do i calculate the frequency of operation

4. and what other design points i need to take care

Its urgent
 

Is the 555 really that much cheaper than a voltage mode
PWM (35xx series)? At least you'd have things like a voltage
reference and the ability to drive a power MOSFET decently.
Some versions have a 50% max duty, built in.

Why you "want" to use the 555, I don't know. There are so
many options out there that are all pretty cheap.
 

Dear Saurav_sdpl, we have done 555 control of boost and buck circuits for auxiliiary power supplies up to 50W - they work well.
Regards, Orson Cart.
 
[Actually i tried to experiment to vary the duty cycle of 555 timer using the control pin, but i am not able to vary the duty cycle below 50%. Also when i keep the control voltage below 1V. I get a distorted wave shape at o/p.

Please suggest the reason for the same.
I have attached the schematic
 

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  • Simulation file.doc
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555's can be made to work below 50%. A common way to do that is to insert diodes to separate the charge and discharging resistors. AN170 from Philips has a nice discussion of that, but appears not to be available any longer. Here is a portion of Figure 9 from that application note.



Since AN170 is not longer available, I will attach it here too.

John

Edit: Forgot to add that I agree with dick_freebird's analysis above (Post #4).
 

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Last edited:

This is OK , but please have a look at the circuit i have attached in the word file.

I want to get a PWM wave with a duty cycle from 0% to 50% max by varing the voltage at control pin.(2V-4V)

What are the design equation should i follow to achieve this
 

sorry , you are wasting your time and money.

UC3845 or ucc38c45 has 0 to 50% duty cycle and has on-chip driver etc etc.

So do many others.

UC3845 cheaper then 555 timer.

also is NCP12XX range
 

saurav_sdpl said:
[Actually i tried to experiment to vary the duty cycle of 555 timer using the control pin, but i am not able to vary the duty cycle below 50%. Also when i keep the control voltage below 1V. I get a distorted wave shape at o/p.

Please suggest the reason for the same.
I have attached the schematic
Click to expand...

The 555 will naturally run from 50% to about 100% duty cycle. All you have to do is invert the the 555 output to get about 0% to 50% duty cycle.

You may have to change your control signal around to work with that.
 

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