Electronics > Projects, Designs, and Technical Stuff

Choosing an inductor for a boost converter

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aiq25:

--- Quote from: MagicSmoker on September 16, 2019, 02:41:00 pm ---
--- Quote from: aiq25 on September 16, 2019, 02:32:53 pm ---I was considering minimum input voltage of 2.7V. I’m trying to use a TPS1236P, which is minimum current of 6.5A (nominal 8A).

--- End quote ---

You're still okay as far as peak current goes at 2.7V minimum, but the only commonly available lithium chemistry that will tolerate being drained down to that voltage is LiFePO4. Doing this to most other lithium chemistries can cause permanent damage.

--- End quote ---

Yeah from my calculations I should have enough margin with the IC limit being at 6.5A.

Can you explain a bit more on how you came to the 8uH number? I was thinking 3.3uH would be good.

MagicSmoker:

--- Quote from: aiq25 on September 16, 2019, 06:43:44 pm ---Yeah from my calculations I should have enough margin with the IC limit being at 6.5A.

Can you explain a bit more on how you came to the 8uH number? I was thinking 3.3uH would be good.

--- End quote ---

I was assuming a current ripple ratio of 0.4 (that is, peak to peak current is 40% of average input current) and gave you inductance * switching frequency. You didn't specify switching frequency (indeed, you didn't even specify one in your reply above!) so I had to assume one. If you change the switching frequency then the inductance changes in inverse proportion, all else being equal.

Note that operating in DCM (current ripple ration >2)is perfectly reasonable at this power level, but since that requires a peak current >2x the average input current, your chosen IC with a 6.5A limit won't suffice anymore.

aiq25:

--- Quote from: MagicSmoker on September 16, 2019, 07:40:51 pm ---
--- Quote from: aiq25 on September 16, 2019, 06:43:44 pm ---Yeah from my calculations I should have enough margin with the IC limit being at 6.5A.

Can you explain a bit more on how you came to the 8uH number? I was thinking 3.3uH would be good.

--- End quote ---

I was assuming a current ripple ratio of 0.4 (that is, peak to peak current is 40% of average input current) and gave you inductance * switching frequency. You didn't specify switching frequency (indeed, you didn't even specify one in your reply above!) so I had to assume one. If you change the switching frequency then the inductance changes in inverse proportion, all else being equal.

Note that operating in DCM (current ripple ration >2)is perfectly reasonable at this power level, but since that requires a peak current >2x the average input current, your chosen IC with a 6.5A limit won't suffice anymore.

--- End quote ---

Ah ok. Yes I missed that. Switching frequency is 750kHz. 15% current ripple on the input.

splin:

--- Quote from: aiq25 on September 16, 2019, 02:32:53 pm ---I was considering minimum input voltage of 2.7V. I’m trying to use a TPS1236P, which is minimum current of 6.5A (nominal 8A).

--- End quote ---


--- Quote ---Can you explain a bit more on how you came to the 8uH number? I was thinking 3.3uH would be good.
--- End quote ---

Assuming you mean the TPS61236P (google can't find a TPS1236P), you could take a radical approach to the problem - such as reading the datasheet:


--- Quote ---9.2.1.2.3.1 Inductor Selection

Because a 1-μH inductor normally has a higher current rating and smaller form factor than inductors of higher values, the TPS6123x is optimized for 1-μH inductor operation. Inductors of other values may cause control loop instability and so are not recommended.
--- End quote ---

Looks like a good reason not to use a 3.3uH inductor. I used the coilcraft DC-DC inductor selection tool I mentioned earlier:

https://www.coilcraft.com/apps/power_tools/dc-dc

The TPS61236P switches at 1MHz so I entered that and other parameters as shown in the attached. I limited the ripple current to 30% to get the tool to find inductors around 1uH, and used the filter to limit the size to 11mm x 11mm. The attached pdf contains the results.

The cheapest inductor @0.32 is the SD43-102, 29.7mohms, 4.7 x 4.2mm, 549mW total losses of which the AC losses are 34mW. 549mW is 4.5% of the O/P.

The MSS1038-102 @$0.62 is 5.4mohms, 10.5 x 10.2mm, 137mW losses of which 49mW are AC.

All the inductors shown in the coilcraft results have DC losses at least double the DC losses; I guess most similar ferrite inductors won't be very different so I would use DC resistance as the primary selection parameter. Obviously you will have to measure the actual losses of your short listed parts.

The TPS61236P switching frequency varies between 750kHz and 1250kHz so you would need to check the chosen inductor is suitable over the whole frequency range.




MagicSmoker:

--- Quote from: splin on September 16, 2019, 08:16:44 pm ---Assuming you mean the TPS61236P (google can't find a TPS1236P)...

--- End quote ---

Hmm... OP definitely should have mentioned the part number and, ideally, that it wasn't a bog-standard switcher IC to give us some forewarning to check the datasheet ourselves; mentioning that sort of critical detail on post 8 is poor form, but sadly all too common around here.

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