How to set boost converter output voltage ?

I assume you mean the standard SMPS boost converter? If so and there is a sufficient load then it should 1/(1 - D) where D = duty cycle.

Uh, understand how the stupid thing works, and proceed based on that understanding? That works well for me in most cases. Randomly fiddling with values in SPICE is usually a recipe for disaster.

Are you trying to regulate the output, or just get a sorta-fixed 1:3 output ratio? Are you using a regulator chip, discretes, or re-purposed jelly-bean chips (e.g. 555 timer for PWM generation)? Do you have any requirements for fixed-frequency, output regulation, EMI limiting, etc., etc.?

Get a datasheet of a boost convertor IC. Everything is described in the very details there.

That works well for me in most cases. Randomly fiddling

There is only one efficient cure for imbecility: that is, the patient has to pay for his treatment himself.

You are asking very complicated questions.

VLV

Feedback.

I do understand the basic working of the circuit. Also, no random fiddling with SPICE at all - a discrete component circuit, the power MOSFETs are driven with PWM, and the high output current driver was designed by me as well. The input output ratio is fixed and thus the original question.

One question. The duty cycle you mention here - is it of the PWM pulses driving the power MOSFETs ?

Of course! What else could it be?

s a

It is a bit more complicated with a boost supply tan a buck because at low output currents (the meaning of low is mainly dependent on the inductor value) it will change to discontinuous mode and the voltage ratio is not then determined by the duty cycle. The output voltage rises towards infinity as the output current get lower. Hence feedback is necessary.

kevin

So what you do is work out the duty cycle needed for worst case operation, then use feedback to reduce that duty cycle to get the desired output over a range of conditions -- consider input voltage range and output current range.

Sometimes a minimum load such as an LED indicator and/or resistor can set a known low output current range if the load goes to zero current.

The current limit around the boost switching transistor is for startup and overload conditions, it is not for continuous output current control[1]. Also the current sense may require a small RC to filter the power turn-on transient.

Hope these additions help OP.

Grant.