"Switched capacitor" power supply from AC current source - Page 1
Forum section: electronics / beginners

author: none
author_rank: Contributor
timestamp: January 10, 2024, 08:20:32 pm
content: I was hoping to simulate a crude inductorless power supply for a magneto (3W 6V hub dynamo -> 0.5A) to about 5 VDC by switching a capacitor to the source using a comparator (/w hysteresis), but I am failing:Can't embed the image:https://imgur.com/a/N6pYWIQThe simulation ends up at MV!The idea was to not load the magneto to minimize losses.Not sure what the load will be, maybe 250 mW.That's in addition to LED bike lights that have built-in voltage limiter (probably zener or shunt regulator).Aside from EMC, is that feasible?

author: Slh
author_rank: Regular Contributor
timestamp: January 10, 2024, 10:04:44 pm
content: I expect that your simulation problem is related to the current source. If i1 is your dynamo then you need to have another look at it. If the dynamo is basically a permanent magnet DC motor then you're better off with a voltage source.Another thing is that you might want proper MOSFET models. I'm not sure if there's a default MOSFET model in ltspice.

author: Benta
author_rank: Super Contributor
timestamp: January 10, 2024, 10:16:39 pm
content: I'm not certain that 1 Farad cap is the right choice.

author: none
author_rank: Contributor
timestamp: January 11, 2024, 07:05:52 pm
content: Thanks, explicitly choosing a FET does help the simulation, though the circuit still doesn't work as intended - I hope I can figure it out soon.From what I've read, a hub somewhat behaves like a current source.Why should the capacitor not be 1F?

author: Slh
author_rank: Regular Contributor
timestamp: January 12, 2024, 07:59:33 am
content: I generally model pm DC motors as a voltage source in series with an inductance and resistance. Then I pay attention to the current to make sure it doesn't get near the saturation current as then the model fails.The problem with the current source is that it need to saturate at a few volts (proportional to the dynamo rpm). A perfect current source will provide current up to infinity volts which is part of why you're getting MV.A simpler thing that you could do if you don't mind having around 6-7V is just have a simple boost converter, using the dynamo as the inductance. You'll probably have to run at a few kHz (rather than 10s or hundreds of kHz) as the dynamo is going to be steel cored.

author: Slh
author_rank: Regular Contributor
timestamp: January 12, 2024, 08:07:57 am
content: Oh, and regarding the 1F cap, there are two issues.1: your time constants are going to be long. It will take a while to charge up.2: they're not that common, are relatively large and tend to have high ESR.

author: none
author_rank: Contributor
timestamp: January 12, 2024, 06:17:03 pm
content: @Slh Thank you for your help!

author: wasedadoc
author_rank: Super Contributor
timestamp: January 12, 2024, 06:32:10 pm
content: A bicycle dynamo (or maybe alternator) is designed to give almost constant current to the lamp (load). The AC voltage (before any commutator or diodes) has both its frequency and voltage proportional to the rotation rate (and hence road speed). Its inductance gives an impedance proportional to the frequency. Result is that if the lamp resistance is low, the current through it and hence its brightness is fairly constant over a wide range of road speeds.

author: none
author_rank: Contributor
timestamp: January 13, 2024, 08:39:51 am
content: So I cobbled together the attached circuit:- voltage source so the simulation runs (current preferred)- fancy comparator without open collector output (what about classics like LM393?)- PMOS high side disconnect switch with NMOS workaround (couldn't get low side switch to work)- arbitrary feedback resistor for a little hysteresis- reference voltage with shunt regulatorIt's supposed to disconnect when the threshold is reached, but now the reference seems to prevent that (sim works better with voltage source instead).I also tried to put the AC current source back in but then the simulation fails with error (sth like "time step too small").Any hints and other help appreciated.

author: none
author_rank: Contributor
timestamp: January 13, 2024, 10:21:17 am
content: TL431 is not only a shunt voltage reference but a comparator and an open collector output.There are a few models linked athttps://ltwiki.org/?title=Components_Library_and_Circuits#Voltage_Regulators...

author: none
author_rank: Contributor
timestamp: January 13, 2024, 11:33:21 am
content: Now if I only get it to fully turn off the NMOS...

author: inse
author_rank: Super Contributor
timestamp: January 13, 2024, 05:57:30 pm
content: Can you show it as picture again, please?I don’t have LTSpice on my smartphone.One more issue, you might have solved in the meantime: how is the regulator meant to start up?If the capacitor is at ca. 0V, the OPAMP has no supply and cannot switch on the MOSFET.If you supply it before the switch, one thing will happen: capacitor is close to zero, dynamo is a current source, so if the OPAMP tries to switch on the MOSFET, the voltage will break down.The circuit might go into some kind of oscillation, a linear mode or fail at all…You will have to implement a current limiting circuit for charging the supercap which is below the capability of the dynamo so that you still have supply voltage „left“ in case the capacitor is discharged.By the way, who cares for EMC on a bike?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 14, 2024, 05:33:24 am
content: Simulation is too early. Can OP explain how is this circuit topology expected to work?

author: none
author_rank: Contributor
timestamp: January 14, 2024, 07:44:58 am
content: Thanks!I've attached the current attempt with TL431 and low-side switch as screenshot.The idea is to keep the supply capacitor charged around the target voltage, i.e. disconnect above some threshold, so the magneto (alternator / generator / dynamo) runs open circuit without load (and losses) at a "high" voltage.To solve the startup/supply issue, the switch must conduct initially and then disconnect when supply voltage is available and the threshold reached.Now my problem seems to be that the TL431 can only pull down to a diode drop (.7 V) and that GS fluctuates with every AC peak so the transistor does not turn off when it's supposed to.Unfortunately I don't see a way around that and everything I've tried (high-side switch, additional transistors) is not working either. :'(

author: none
author_rank: Contributor
timestamp: January 14, 2024, 07:50:20 am
content: attempt at high-side switch

author: inse
author_rank: Super Contributor
timestamp: January 14, 2024, 08:31:47 am
content: I am still not sure whether this circuit will start up properly.Remember, your supercap is a short circuit.Add a resistance to your source, what current flows into your circuit - can’t see the scaling.Start simulation with cap discharged.Maybe simply a 5V LDO linear voltage regulator can do the job?

author: none
author_rank: Contributor
timestamp: January 14, 2024, 11:04:51 am
content: Thanks @inse,I don't know whether the TL431 will cause issues without supply. I assumed the pull-up will cause the low-side MOSFET to conduct and guarantee the initial charge.If the hub behaves like a current source, the cap's "short-circuit" should not be a problem.However, my issue is that the termination does not work, due to the 431 not pulling to ground, and ground being the very thing that is disconnected.So I need to figure out a way to somehow pull the gate down to the negative rail of the rectifier with the lowly .7 V output of the 431.Of course I am open to other solutions (separate comparator and voltage reference).Regarding EMC - I'd just rather not build a transmitter (switching 500 mA via a length of wire)

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 14, 2024, 02:47:25 pm
content: If you want 5V from the dynamo, rectify first, with the diodes and the cap. Then use a buck converter to 5V.The closest thing to your suggested topology is a gated rectifier, used to reduce pass element dissipation in linear bench power supplies. You need a precise timing off of the AC signal zero crossings. You have nothing like that in your schematics, and it would be way more complicated than a post rectifier buck, which you can buy off the shelf (a car cellphone charger)

author: none
author_rank: Contributor
timestamp: January 14, 2024, 06:22:33 pm
content: Yes, an off the shelf switcher would be nice, but the low power AC-DC units I could find are specified for 85 V and above, which may be the no load voltage at higher speed.I'm also not sure how well the switch mode controllers would handle a current source supply.Might not be an issue except for the input voltage range.Could you write a bit about the type of rectifier you mentioned, I did not find immediate matches.

author: inse
author_rank: Super Contributor
timestamp: January 14, 2024, 06:46:43 pm
content: I am afraid that without a model of the dynamo, your simulations are pointless and will lead nowhere.Did you do a measurement of open load voltage, impedance and short circuit current in various speeds?What exactly is it that you want to achieve with your circuit, apart from charging the supercap?

author: none
author_rank: Contributor
timestamp: January 14, 2024, 07:26:10 pm
content: An accurate model would be nice, but I don't have much info other than that to assume it behaves mostly like a current source.However, I also think my issue just boils down to driving the mosfet (UGS) without ground reference, since that is what it's switching.Similarly, while a high side switch does not have the issue with ground, I could not find a way to inherently have it turn on and turn it off later.Another issue arises from the limits on UGS and the high open circuit voltage of the magneto, which means the gate voltage needs to be limited.SSRs use insulated drivers, that may be worth looking into, but I'm still hoping there is a simple solution to the gate drive problem.And yes, keeping the (super)capacitor charged for a low power load without wasting precious energy with a shunt regulator is the sole intention :-)