Switched bulk capacitance - Page 1
Forum section: electronics / projects

author: foxtrot
author_rank: Contributor
timestamp: November 13, 2023, 10:19:49 pm
content: I have an application in which I've got a buck converter for which I need to tightly control inrush power consumption, and also need relatively high bulk output capacitance (1mF). I feel like I've seen this before, but has anybody here come up with (presumably) a inrush-controlled means of switching in bulk capacitance? I was thinking a low-side switch on one or more big caps w/configurable inrush control would be good, but there's nothing off the shelf that seems to work nicely at my voltage range (24V). Thoughts, napkin sketches, and links to reference designs would be much appreciated 🙏

author: langwadt
author_rank: Super Contributor
timestamp: November 13, 2023, 11:01:46 pm
content: just current limit the buck?

author: jbb
author_rank: Super Contributor
timestamp: November 13, 2023, 11:40:43 pm
content: Quote from: langwadt on November 13, 2023, 11:01:46 pm+1For bonus points, a buck withinputcurrent limit can be the most effective thing for charging those caps up fast.——If your load equipment doesn’t like a slow voltage rise time, you could add a power switch between the buck converter (incl 1mF cap) and the load.——1 mF is alotof output capacitance. It might be absolutely necessary, but can you get it down somehow? For ripple, maybe a multiphase buck could improve the situation? Or go 0.5V high and use a linear LDO for cleanup?If you need big cap bank for a pulse discharge, I suggest that you really don’t want to add extra resistance to the cap bank in the form of some extra switch

author: twospoons
author_rank: Frequent Contributor
timestamp: November 14, 2023, 01:54:51 am
content: If your buck has a soft-start function, just set that up to be long enough to keep the inrush low.

author: foxtrot
author_rank: Contributor
timestamp: November 14, 2023, 02:07:20 am
content: Quote from: twospoons on November 14, 2023, 01:54:51 amThat was my original thought; however, I'm a little worried that a long-enough soft start (maybe half a second?) will cause problems (i.e. the device thinking it has some sort of fault).Quote from: langwadt on November 13, 2023, 11:01:46 pmAre you thinking basically a current sensing amplifier tied to the buck's feedback pin? I haven't designed something like that before :/Quote from: jbb on November 13, 2023, 11:40:43 pmYeah; there's a class D amplifier attached to this thing – as well as a few other bucks for various elements in the system. Suggestions are always appreciated.

author: mariush
author_rank: Super Contributor
timestamp: November 14, 2023, 05:50:51 am
content: NTC/PTC resistor in series with the voltage, whichever version has high resistance when cold and low resistance when hot.A timer or something that would activate a mechanical relay or something equivalent to short out the resistor after a few seconds... this is common in computer power supplies

author: jbb
author_rank: Super Contributor
timestamp: November 14, 2023, 07:47:53 pm
content: On soft start: I don’t remember seeing ‘soft start too slow’ in buck controller data sheets. Maybe you could experiment?On current limiting: theoretically you can put a current shunt and sense amp etc down yourself, but I recommend picking a buck controller with current limiting built in.Quote from: foxtrot on November 14, 2023, 02:07:20 amHmm. Perhaps a dumb question: could you throw out the buck converter and run your Class D amp straight off the original power supply?Traditional linear audio amplifiers are known for liking great big DC caps because they need to ride though the nulls in the incoming AC power. Have you experimented with different sizes of cap to see what’s necessary for you? And if the sound quality is impaired, do you know how/why?I haven’t worked with Class D before, but I imagine 2 things to look out for are “the DC rail dips too much for bass”, “the switching noise from the buck makes undesirable tones in the outputs.”If it’s the latter, have you looked into synchronising the buck converter and Class D amplifier switching frequency? This can (as I understand it) help you place weird frequency mixing products outside the audible range.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 14, 2023, 07:58:46 pm
content: Every buck converter in existence, which does not self-destruct, already has this current limiting built-in. This includes basically every buck control IC on the market. You are solving an imaginary problem.Worst offenders from 1980's do it with "soft start" circuitry (which just makes the voltage reference ramp up slowly) alone, but it's still there; if it wasn't, the inductor just saturates and the MOSFETs go bang on first power-up, with much less capacitance than 1mF. Most sensible modern controllers add some way of more or less approximate current sensing (e.g., Vds drop sensing of integrated mosfet, or connections to explicit current sense shunt resistor), and cycle by cycle limitation.

author: David Hess
author_rank: Super Contributor
timestamp: November 15, 2023, 01:36:50 am
content: Quote from: foxtrot on November 14, 2023, 02:07:20 amFor current mode regulators, it works better to current limit by applying feedback to the regulator's compensation pin which itself controls the peak inductor current. This is also how soft start should be implemented.QuoteIt seems odd that such a large output capacitance is required. Switching regulators are usually more limited by the impedance of the output capacitor rather than the capacitance itself.

author: the_cake_is_a_lie
author_rank: Contributor
timestamp: November 15, 2023, 03:46:16 am
content: Quote from: foxtrot on November 13, 2023, 10:19:49 pmYeah I haven't seen more than 220uF bulk capacitance on the output of a switching regulator. I'd question if 1mF were really necessary.

author: foxtrot
author_rank: Contributor
timestamp: November 15, 2023, 05:16:49 pm
content: Quote from: Siwastaja on November 14, 2023, 07:58:46 pmThis specific application is powered by an upstream buck that has a very precise power limit. If I exceed this power limit, the upstream buck will shutdown for a period and reset. I have no control over this, so I have to design my buck to play by its rules. This is – at least, in my mind – the reason that I need to soft start: charging that much output capacitance is going to cause me to exceed my power limit. Is this assumption somehow incorrect?Quote from: the_cake_is_a_lie on November 15, 2023, 03:46:16 amYeah, I would love if it weren't. I may try to get around it by distributing bulk capacitance along the output rail behind inrush-controlled load switches.Quote from: jbb on November 14, 2023, 07:47:53 pmSadly, this was relayed to me by TI's engineers as their concern. So I guess it's something they just don't consider relevant enough to put in their datasheets. 😂Quote from: jbb on November 14, 2023, 07:47:53 pmNot a dumb question! I have a wide V_in range that's roughly double my output voltage. The Class D amp operates *very* efficiently and with minimal THD at this output voltage – which is why I chose it as the primary rail voltage. Unfortunately (as you see elsewhere in this post) there's a buck upstream of this one that is *very* tightly power-limited, with a circuit-breaker behavior. If I exceed the power limit, I get shut off. Not my choice, and out of my control.Quote from: jbb on November 14, 2023, 07:47:53 pmI don't have a lot of room to experiment with this one; the datasheet of my amp does a lot of heavy lifting, since it clearly specifies max ripple, its effect on THD, etc. From the perspective of my device, I've got a DC power source.Quote from: jbb on November 14, 2023, 07:47:53 pmI suspect you're right re: DC rail dipping for too much bass. Luckily, my Class-D will change its equalization on the fly if it detects that its supply rail is browning out, but the overall goal of sound quality is compromised in this case – which is also something to avoid. I'm sizing for 1.5x RMS current.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 15, 2023, 05:23:09 pm
content: Quote from: foxtrot on November 15, 2023, 05:16:49 pmBut you still have "your buck" - so just use a low enough current limit. Practically, maybe this means looking at buck controller IC which actually have inputs for current sense resistor, this way it can be more accurate.Input current limiting, as mentioned by some others, would do the whole charge a bit faster, but it is a more specialized feature, while output current limiting exists in every sensible buck converter, and even more accurate versions are notthathard to find. Also look for an IC where you can disable the hickup/fault mode so that it can run in current-limited mode for unlimited time.QuoteIt is incorrect, because the charging power is something you can choose, and as I said, it is something every buck converter already controls to some extent. Just use lower current, and if your application requires especially exact regulation, then just find a controller which allows more exact control; those with explicit shunt resistor inputs are better, and true CC-CV parts are even better.

author: foxtrot
author_rank: Contributor
timestamp: November 15, 2023, 05:50:35 pm
content: Quote from: Siwastaja on November 15, 2023, 05:23:09 pmI've had somewhat of a difficult time finding them.. Perhaps you'd be willing to take a quick look? My constraints are as follow:V_in = 36-60VV_out = 21VV_out_ripple = 200mV p-pI_out = 3.8A

author: mariush
author_rank: Super Contributor
timestamp: November 15, 2023, 06:33:30 pm
content: Maybe have a look at ICs like SIC461 / SIC462SIC461 (max 10A out) :https://www.digikey.com/en/products/detail/vishay-siliconix/SIC461ED-T1-GE3/7682210SIC462 (max 6A out) :https://www.digikey.com/en/products/detail/vishay-siliconix/SIC462ED-T1-GE3/6574659FAN65004B may also work :https://www.digikey.com/en/products/detail/onsemi/FAN65004B/8636353maybe renesas raa211651 ?https://www.digikey.com/en/products/detail/renesas-electronics-corporation/RAA2116514GNP-HA0/15196000

author: Siwastaja
author_rank: Super Contributor
timestamp: November 15, 2023, 06:37:09 pm
content: Quote from: foxtrot on November 15, 2023, 05:50:35 pmI remember using LM25117 once, this has shunt resistor connections, and datasheet has an example circuit to feedback the current for accurate CC mode if the crude one doesn't suffice (it probably does). It has pins to disable hickup/fault mode, too, so it will happily output against current limit for unlimited time; I used it for CC-CV battery charging. Voltage range does not suffice for you but maybe it helps to know I found the part using TI's WEBENCH which has this "IC FEATURES" filter more capable than those at distributor sites.

author: foxtrot
author_rank: Contributor
timestamp: November 15, 2023, 07:18:57 pm
content: Quote from: Siwastaja on November 15, 2023, 06:37:09 pmYeah; V_IN(max) of the LM25117 is insufficient. The TI portfolio is pretty limited in this regard.Quote from: mariush on November 15, 2023, 06:33:30 pmThis may be a good option. I'm going to dig into it further! Thanks for the suggestion.