Linear voltage limiter - what is this function/circuit called? Examples? - Page 1
Forum section: electronics / general

author: wb0gaz
author_rank: Frequent Contributor
timestamp: June 29, 2021, 09:27:38 pm
content: I need a linear (not switching) DC circuit that would (ideally) dissipate in a series pass element (P-FET?) any voltage above a given level as heat. This is sort of a DC series voltage regulator, but when voltage is below threshold, the series pass element would act as a minimal resistance (such as Rds ON for a FET.)The use case is to prevent a fully charged battery from damaging it's attached application (such as a LiIon battery whose full-charge voltage is somewhat above safe limit for the load, but most of the discharge cycle is safe for the load (discharge would continue then until the LiIon battery is properly depleted.) Example application - threshold voltage might be 14.0V, peak input voltage might be 16.8V (decreasing as the battery discharges), so that at 1 ampere load, initial dissipation would be 16.8-14.0 = 2.8*1 = 2.8 watts; once the battery declines to about 14.0V in this case, the residual dissipation would be (for example) RdsON of the FET * I^2.Thanks for any advice of what this function would be called (so that I can search for examples), or guidance on how to approach designing this circuit.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: June 29, 2021, 10:02:44 pm
content: Thanks for the helpful reply, it helps clarify the need.As for dissipating excess current, the idea I had in mind was that the pass element would only dissipate current to the extent it was demanded from the load. For example, if the battery were 16.8V and the threshold were 14.0V and the load were drawing 1 microamp, the intended thermal waste would be 16.8-14.0=2.8*1uA = 2.8 microwatts. If the load current exceeds the capability of any component in the supply chain, that would be "left to the reader" (=not concerned about that eventuality at this stage.)From my prior (limited) experience with linear/LDO regulators, I've not found one that has pass behavior below it's minimum input voltage (for example, a 7805 linear regulator works when the supply voltage is 8 volts (and the current is within limits), but I don't believe it has a specified behavior below some lower limit (example, 4 volts may just result in the regulator being an open circuit between source and load.) If there is a part that you (or anyone reading) is aware of that would help towards my objective, I'd like to know!As for partially charging the battery - it's a nice idea I hadn't thought of, however, I'm constrained to work with commercial charger where the cut-off point is pre-determined.Another constraint I left off my initial posting - all voltages would be >0 (that is, no negative voltages, and no condition where the pass element would need to deal with negative voltage when the threshold is positive.)And yes, switch mode device would be a more general solution, however, I need to preclude that option.As for a series element controlled by an op-amp, that was somewhat the direction I was headed, but I was hoping that there would be a name or more concise description for this sort of circuit function.So, thanks for helping me think through this!Dave

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2021, 02:03:01 am
content: An LDO does that, with a well-defined voltage limit. Be sure to check ground pin current, as this can be quite nasty on some (mostly older PNP pass?) types.For a looser, few-component design, depletion MOS comes to mind:https://electronics.stackexchange.com/questions/237566/typical-use-of-depletion-mosfetGood for overvoltage, especially transient e.g. surge or load dump protection.Also the non-answer: why can't the circuit be redesigned to tolerate the higher voltage? It's just a few volts extra, it's not like LiIon should ever deliver load dump surges. Or the battery designed to use 3 instead of 4 cells, of somewhat larger size (higher mAh).Tim

author: wb0gaz
author_rank: Frequent Contributor
timestamp: June 30, 2021, 02:32:43 am
content: Hi Tim,Thanks for the response!I'll look at depletion mode FET design per the link you sent.Seems I should be able to use op amp on the load side of a series pass depletion mode FET drive the gate of the FET in a way that holds the voltage on the load side of the FET constant (so when the voltage drops below a given threshold, the op amp would drive the depletion mode FET into full conduction by pulling the gate way down). Anyway, that's one thought such as it is!As for the application device voltage limitation - an existing piece of radio equipment that's got a supply voltage spec arising from use in a vehicle (typically 13.8V +/- 10-15%) which is between the discharge range for a 3S LiIon and 4S LiIon battery (3S spends a fair bit of the discharge curve below lower limit; 4S spends an early part of the discharge curve above upper limit, which is what I'm thinking of "chopping off"). Being radio equipment is also why I'd like to avoid a switching converter - I'm willing to waste the surplus potential during the early stage of discharge.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2021, 03:04:59 am
content: It's uh, not backwards... just offset. Don't need any opamps or anything, it's more-or-less fully on at Vgs = 0 and turns off at Vgs < 0 (for N-ch).Are you sure it can't tolerate higher voltages? Automotive standard is something like 15V nominal high, with swells to much more (24V jumpered, load dump over 50V). If it can be inspected internally, that might give confidence into its operating range, rather than making an overly safe assumption.I've used switchers plenty of times with radios; they're hardly insurmountable. You do need good filtering and layout, and may need shielding.Tim