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I really appreciate all the help thus far, especially from inductorman
I've got the simulations working and want to move on to actually building everything. I just need some help with the design. I'm going to lay the portions of the design down (not expecting anyone to sit and work the math out), where I have questions . I also did take power electronics in undergrad and thoroughly enjoyed it, but it looks like there's a lot we didn't cover! like A LOT regarding non-ideal components
So here we go!
So I'll be building a 4 switch buck boost converter
. The input will be a solar panel with a short circuit current of 8.99A, open circuit voltage of 45V and rated power of 350 W at STC. I'm following this design guide for the buck converter
When the system bucks it just operates as a synchronous buck converter and when it boost, it's just a synchronous boost. I'll be using both modes so that I can step the input through from 0 -> 45V. I don't really care too much about the output.
So here the design starts
- Max input voltage : 45V
- Output voltage : 12V
- Maximum power : 350W
- Switching frequency : 500 kHz
- Inductor current ripple : 30%
- Output voltage ripple : 100mV
Lmin = Vout*(1-D)/(fsw * Iripple)
The maximum output current will be the max power divided by the output voltage = 350/12 = 29.167
Lmin = Vout*(1-D)/(fsw * Iripple) with Duty = minimum Lmin = 12*(1-(12/45))/(500 000 * 29.167* 30%) Lmin = 2.01142 uH
The next step seems to be calculating the peak inductor current:
Ipk = Iout + Irip Ipk = Iout + (Iout*30%)/2 Ipk = 1.15*Iout Ipk = 1.15*29.167 = 33.54205 A
Great ! It seems now that the inductor needs to have a saturation current higher than this and then all is good! So the inductor I selected is this guy which has an inductance of 2.7uH and a saturation current of 47A, which seems like enough leeway
- Although now I have another issue, the application note talks about DCR loss
- They have an inductor that has a DCR of 0.2 milliohm, which is insanely small. I cant understand how these inductors are rated for these currents, but the DCR loss makes them unusable? Do I need to selec another inductor and search for something that can handle this DCR loss? What would an acceptable DCR loss be?
Next I'm going to jump to the output capacitance:
the formula they give is :
Cout = (1-D) /( (Vrip/Vout) * 8 * L * (Fsw*Fsw)) Cout = (1-12/45) /( (0.1/12) * 8 * 3.3uH * (500,000*500,000)) Cout = 13.33 uF The next output capacitor calculation they do is for a load step, although I don't think I need to do this? My load will be fixed with the only time I think this will come into effect is system startup?
I honestly have no idea how to choose these parameters.
I found the following source which seems to be one I agree with.
- Now comes another question. The capacitor ripple current calculation seems to vary a lot from source to source!
You can see the relevant portion here
. Essentially they take the voltage ripple calculated for the capacitor and convert it to an Vrms value.
Vrms = 1 / ( 2 * sqrt(3) )* Vrip Vrms = 1 / ( 2 * sqrt(3) ) ) * 100 = 28.8675 mV
They then use the ESR to determine the ripple current through the capacitor by using ohms law:
Iripple = Vrms / ESR
so if the ESR is say 40 milliohms you'd get a ripple of 28/40 = 0.72 A
Other calculations I've seen have the ripple current as a huuuuuge value, this seems right or should I be using a different formula?
EDIT: Not sure if this might differ depending on the capacitor type...?
Finally I'm not sure how to go about selecting the dead time between the switches? is there some way to determine what the minimum/maximum dead time values should be?
I think my issues are more with the losses in the capacitor and inductor as our undergrad course assumed ideal components. I have some questions on the switching losses but I'll post that in the comments when I finish, mostly on switching times and parasitic components. Lastly the boost converter design will follow this app note
. I'll follow the way they implemented in the sense that the minimum capacitance and inductance values are calculated for both buck and boost and then the selected one should meet both.
I also don't think i need feedback from the output side, as all I want to do is adjust the duty so that the panels voltage slowly steps from 0 up until 45V and at each step apply a sine sweep interpolation on the pwm reference. I'm thus not too worried about the output as it should just act as a "variable load"
Thanks in advance for any help! I've got about 3 books and my notes laid out infront of me and they all ignore this.