need help with choosing an inductor for a buck converter.

need help with choosing inductor for a buck conveter.



I am designing a very simple buck converter but the simulator is showing that the inductance average power is 3 watt in only first 150 microseconds then it goes to to mW the rest of the time, if i used the usual 0.5 watt available inductance will the circuit be ok?

Re: need help with choosing inductor for a buck conveter.

You are showing the instantaneous power, it doesn't create losses. You'll rather look for avg(I²R) and peak flux.

Re: need help with choosing inductor for a buck conveter.

Hi,

I think there's something more fundamental we need to clarify. Let's get cleared with the spec first.

With D=0.4, your converter output voltage will be about 4V. With 1kohm load resistance you'll get 4mA of output current. Is that your spec?

Re: need help with choosing inductor for a buck conveter.

@Akanimo I am only designing this to learn I will not use it in a real application so I trying to lower the current for the safety of me and the components.

@FvM, as I said the average for the first 150 microseconds, is about 3 watt but the instantaneous values reach to 30 watts.

I asked a friend and he said it's because the capacitor is acting as a short circuit in the first microsecond until it's charged then the current go lower and that why i find the high power dissipated in the inductor at first and this may also damage the mosfet and he says that the solution is making some kind of soft starter by manipulating the pwm to the mosfet in first 3 sec so it's not open completly until this transit period ends, what do you think of this opinion?

Re: need help with choosing inductor for a buck conveter.

The calculated power value is no meaningful parameter and doesn't tell anything about the inductor load. Inductor respectively transistor current is much more interesting.

I understand that the fixed duty cycle buck converter control is used as simplified model. A real buck converter uses current limiting and/or soft start.

Re: need help with choosing inductor for a buck conveter.

Actually, to choose a proper inductance, you have to specify the maximum ripple current that you want. In this case, since your load current is 4mA your ripple current will be too small for the purpose of learning and the analysis will be less meaningful. Reduce the load resistance to say 10ohm so you have 400mA of load current and we will start from there.

And 25V pulse input is not a realistic value. You have 10V at the input to your converter.