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Problems Design DC-DC Step Down Converter

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macmmt

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Hey,

Im designing a step-down dc-dc converter. The input voltage is 7V and I want an output voltage of 2V. The control is done through PWM with an IC 555. The switching frequency is 300kHz.

I have tried to make it work, but it doesnt. I would like, you help me checking it to find any error. Please let me know what is happening with this circuit. I have attached the circuit design.

Thank you very much for your help.
 

Two comments:
- You can't drive a NMOS-FET this way. It needs 5 to 10 V Vgs to turn it on. Switchers with NMOS output stage
typically use a bootstrap circuit to drive the FET. Consult existing circuit examples.

- A buck converter with unregulated PWM gives very poor output voltage stability. With no load, the output voltage
rises up to the input level.
 

Hi,
You do not have any feedback loop so your output voltage is not regulated. It will just continue to rise up to Vin.
You need the feedback connection to control the duty cycle of your PWM signal.
 

How would you do the feedback connection with a 555 to regulate de output voltage?
 

Hi Macmmt,

Why do you have to use a 555 timer?
You can buy a step down controller IC instead of using 555 timer.
 

This circuit give some feedback contol to alter the on time and some regulation.

* C5 was added to keep the simulator convergence happy.
 

Hi Skowe26

It is a project, and I have to use the 555 timer as the controller of the stepdown circuit.
 

Thanks for the circuit diagram. I have one more question. Why do u have to use an optocoupler?
 

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