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Limiting initial charge current into supercap from a boost converter?

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wb0gaz:
(edited to clarify operating use case)

I need to charge a supercap (1.6F, 16V) from a small boost converter (can deliver about 500 mA, and does not have onboard current limiting).

I am looking for simple(st) way to avoid overloading the boost converter while the supercap is charging.

The use case is this - the boost converter is sufficient to supply all of the energy the application requires over it's life cycle, however, there are occasional peak current demands (see clarification below) from the application which I intend to satisfy with the supercap, which would then be recharged from the boost converter. Voltage stability is not required (that is, the supercap's voltage would drop after discharge and would be (relatively slowly) replenished from the boost converter.

(CLARIFICATION - the peak current period is very brief and will reduce voltage across the super capacitor only modestly; this problem is focused on managing initial charge-up of the super capacitor after power-up where the super capacitor would pose a short- or near-short circuit load to the preceding boost converter - so losing some energy - perhaps from heat dissipated in a pass element - is OK as this energy loss would be a one-time event after power-up; after initial power-up the voltage across the super capacitor will be high enough that it won't short-circuit the preceding boost converter)

The boost converter is based on ME2149 IC.

The supply power to the boost converter is not specifically limited, however, I believe that limiting current prior to the boost converter may not be successful as the boost converter (which works in a 2-6V input range and will nominally be supplied with 4-5VDC) does not operate fully with supply voltages below 2V (hence, while the supercap is charging, the supply and boost converter will be working into a very low ESR, particularly at initial start-up.)

Suggestions?

Can I clarify the requirements or circumstances or use case further?

Thank you!

Dave

sandalcandal:
Would be helpful if you can post a schematic of your current (planned) implementation.

Edit: This response is incorrect. See below

Normally in this situation you'd be doing some sort of soft-start with the boost converter running at low duty cycle to limit current, possibly with some current limiting feedback control. Not sure from the datasheet but it looks like it might be possible to do an extended soft start using a capacitor on the CE pin slowly charged up by a resistor to Vcc (might also need a parallel resistor to discharge this capacitor). Bypassing R1 in the feedback divider with a parallel capacitor could also have a similar effect.

You could do a more complicated network with actives on the feedback pin to do more controlled current limiting but then it is much less cheap or simple.

Frequency is fixed and it looks like it has an internally limited maximum duty cycle so you might be able to just limit current to safe levels by picking an appropriate inductance.

sandalcandal:
Worst case current (from the basic inductor equation) would be
\[ I_{max} = \frac{V_{max} D_{max}}{L f_{min}} \]
Subbing in max recommended inductance and other values from this datasheet with your given max supply voltage:
\[ I_{max} = \frac{6*0.78}{4.7*10^{-6}*0.8*10^6}= 1.24 A\]
Which is within the datasheet current rating for all the non SOT23-5 versions it seems? Also not clear to me from the datasheet  how the PFM works which might result in \$f_{min}\$ going lower and raising current.

Edit: corrected values and improved wording.

wb0gaz:
Thank you sandalcandal for the early reply.

I'm not able to post a schematic right now, however, the entire end-to-end solution looks just like the ME2149 data sheet typical application circuit, except that Rl is in parallel with a 1.6F 16V capacitor with low (and unspecified) ESR.

The boost converter is exactly represented by the datasheet page 2 ("typical application circuit"). I do not know right now the parameters of the inductor L (I'll be working with a small module sourced online), however, it seems that there would be risk to the inductor (at least) when the converter is working into a short circuit (which it would see - at least approximately - at cold start.) From what I've been able to gather, this type of circuit is not intended to work into a direct short (i.e., supplying some finite current into a short circuit.) I am uncertain of this scenario (hence my posting.)

Assuming that the boost converter can't work into a short circuit (posing some in-range current to the supply on the left, and delivering current into an approximately zero-ohm load on the right), I  was wondering if I could identify some circuit that would pose a low voltage drop (unspecified) when current flow is below some threshold (500 mA in my example) and increases when current flow tries to exceed the threshold (so that the total current flow does not exceed 500 mA), that seems like it would solve the problem. The element would dissipate the energy it absorbs during the start-up phase, which is OK.

Thanks again,

Dave

wb0gaz:
One sentence in the datasheet caught my eye (from page 8, section 3, part of "External parts selection for DC/DC converter"):

"However, a higher capacitance is recommended if the output voltage is high or the load current is large."

I don't know if this means "higher capacitance" would include 1.6 Farads? Even a few tens of uF would be a high load for a tiny period of time, however, this capacitance would be ~100K times that envisioned in the datasheet.

In this application, output voltage will be 12-14 volts and load current of the application net of the super capacitor (although extremely low duty cycle) could be several amperes; load current into the super capacitor would ideally be limited to a safe level for the boost converter (500 mA in this example.)

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