0
\$\begingroup\$

I am trying to use a buck converter to step down a 48v to 33v. The confusing part is that I am trying to simulate it using LTspice and I can’t figure what how to control the nmos with pulse width modulation . Do I need a control circuit for the purpose of simulation or how can I go about it .

\$\endgroup\$

1 Answer 1

0
\$\begingroup\$

If you simulate a synchronous buck converter then the two switches (yes, there are two in a synchronous buck converter) are wired like this: -

enter image description here

Picture above from here.

If SW1 is on then SW2 is off and vice versa. This means that the average output voltage is the input supply voltage (12 volts in the example above) * duty cycle. In the picture above the duty cycle (as a fraction) is: -

$$\dfrac{5}{12} = 41.667 \text{ %} $$

And that leads to an average output voltage of 5 volts. The inductor and capacitor on the output smooth the PWM waveform to a (hopefully) low ripple DC voltage of 5 volts.

I can’t figure what how to control the nmos with pulse width modulation

Use a voltage generator source (set as a square wave of the appropriate duty cycle) to drive the MOSFETs (or switches) just to get a feel for it. Following that, you can develop a PMW signal source based on an analogue input but make sure you get the basics right first.

This usually involves a comparator fed with a reference triangle wave like this: -

enter image description here

Circuit from here.

But you can use an LTSpice voltage source set to produce a triangle wave if you wanted and just use the comparator indicated by U1c in the above.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.