Controlled input cap charging - Page 1
Forum section: electronics / projects

author: johofz
author_rank: Contributor
timestamp: August 27, 2022, 09:02:11 pm
content: Hello dear EEVblog-Community,I'm working on an ac-dc converter for a bicycle dynamo hub. One tricky thing about it is, that if not loaded, the voltages can get very high. Voltages above 150V peak to peak ac at speeds above 30km/h are what I'm dealing with. This leads to a few problems like huge input caps and finding switching regulators with high enough input voltage capability.My idea to counter the problem was to use a high side p-channel mosfet in combination with a n-channel gate driver in combination with a mcu. I need the mcu anyways and I also need a zero-crossing detection. Therefore my plan is to switch on the p-channel mosfet around the zero crossing points of the rectified input voltage. I want to use a very large double layer cap that feeds the switching regulator aswell as a linear regulator for the low energy mcu.The benefits are that the input caps can be smaller since they don't have to withstand the high voltages. It's also beneficial to be able to detach the dynamo hub when not needed. I plan to keep the voltage after the p-channel high side mosfet below 10V. If a load is attached, the voltage drops way below 10V.One thing that I don't like about the circuit is, that it won't startup unless the mcu has power. I need a way to trigger either the p-channel or the n-channel mosfet at the beginning, that can be deactivated later by the mcu. Since the input cap is so big, the mcu has enough time to boot and then regulate the voltage. Batteries are not an option.I'd like to hear your thoughts on this. I don't do ac-dc that often, so I might have overlooked an easy solution.Here is the circuit modeled in LTSpice:Eingangsspannungsbegrenzung.asc(6.4 kB - downloaded 44 times.)

author: johofz
author_rank: Contributor
timestamp: August 29, 2022, 11:52:50 am
content: After thinking more about the circuit, I think I should use a n-channel Mosfet in combination with a gate driver over a p-channel mosfet. It's probably a bad idea to use a p-channel mosfet for low voltages under 10V since I wold need to supply negative v_gs since the source voltage can be as low as 0V and will never rise above 10V when a load is attached.I probably should also rephrase my question, since the problem is more about the startup of the circuit.Is there a solution that satisfies these conditions:1. It needs to provide a way to charge the input cap when the mcu is not booted yet. Either by supplying the v_gs to the n-channel mosfet or by shorting it.2. It must not interfere with the circuit, once the mcu is booted and does it's job of controlling the charging of the input cap via the n-channel mosfet.Here is the circuit with some annotations

author: Hiemal
author_rank: Regular Contributor
timestamp: August 29, 2022, 09:03:38 pm
content: One could do something similar to how inrush current limiting is done with SMPS... a resistor, or NTC in parallel with a relay is usually how it's done I believe! Would be a lot simpler and less finicky.

author: Benta
author_rank: Super Contributor
timestamp: August 29, 2022, 09:18:56 pm
content: First, it can't be a dynamo. A dynamo would supply DC.But it's apparently an alternator of some sort, A permanent-magnet type wouldn't deliver voltages as high as what you mention, so it seems to have a sort of unregulated field winding. If you could regulate that, the result would be much better.Can you provide some more details on the generator/alternator? Or even disassemble one with photos?

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2022, 09:27:26 pm
content: Quote from: Benta on August 29, 2022, 09:18:56 pmBicycle dynamos are really alternators. They used to really be dynamos but they changed to alternators, presumably because they don't need brushes. Unfortunately the name dynamo stuck in common usage, even though it's technically incorrect.A bicycle dynamo really does output a high voltage, when unloaded. I think this is because it's cheap to make them with a high leakage inductance, which didn't matter, when the load was just an incandescent lamp.Quote from: johofz on August 29, 2022, 11:52:50 amWhy not simply use a mains powered switched mode power supply? It can be made to work at lower voltages, by replacing the bridge rectifier with a voltage multiplier. A larger filter capacitor might need to be fitted so it works at lower speeds.

author: Benta
author_rank: Super Contributor
timestamp: August 29, 2022, 09:43:11 pm
content: Quote from: Zero999 on August 29, 2022, 09:27:26 pmI'm fully with you here, the old bottle-type "dynamos" driven at the top directly from the tire had a cylindric PM rotor inside a stator winding. But they rotated quite fast, and were designed for 4.5 or 6 V bulbs.But this is about a hub type, which is slow. So either it's got a rotor with alotof magnets, or it's more like an automotive alternator with slip rings.I'm stumped myself, I've never taken one apart, but 150 V seems unlikely to me.Educate me, please.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2022, 09:45:45 pm
content: Quote from: Benta on August 29, 2022, 09:43:11 pmOr it has a small planetary gearbox to increase the speed of the rotor? Either way, I'd expect the output to be similar.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 29, 2022, 09:46:35 pm
content: Can a shunt power rated zener be added, say 25% higher than the voltage when your load has started?You would need to know the current it would deliver at the zener voltage maybe by testing.

author: Benta
author_rank: Super Contributor
timestamp: August 29, 2022, 10:09:37 pm
content: Found some references.Indeed, it's PM synchronous generators (just like the "bottle dynamos"), but with typically 16 pole pairs on the rotor. Typical load 6 V, 3...5 W. Rotor is external, stator in the centre.I still find the 150 V implausible, but not impossible.OP, how much current do you need for your circuit?

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 29, 2022, 10:17:33 pm
content: I remember the bottle type could deliver an unhealthy wallop when spun open circuited.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2022, 11:00:23 pm
content: Quote from: Benta on August 29, 2022, 10:09:37 pmThe original poster said 150Vp-p, which is 53VRMS and is plausible.

author: johofz
author_rank: Contributor
timestamp: August 30, 2022, 12:07:25 am
content: I work for a german manufacturer of "dynamo" hubs. It's a common therm in the industry (at least in Germany) to refer to them as dynamos.They are almost always claw pole generators and have no gear boxes since they don't go well with the shaky nature of claw pole generators and the losses are too high.Without a load, the voltage goes well above 70V RMS at about 50km/h with a 28'' wheel. When current is drawn, it generates a field that opposes the magnets and the voltage reaches an equilibrium that usually lies below 10V due to (very old) regulations. Basically the load determines the voltage limit by the current it draws.The goal is to build a phone compatible charging circuit that can supply at least 5W. Anything above that means 10W of mechanical power that needs to be produced by the driver, which already is about 10% of an average persons power output.Pole pairs vary between 13-20.I'm not sure if this actually works or not, but could I simply use a 100k pull-up resistor from drain to gate and just "over-write" it with a sufficient gate-driver? I tend to overcomplicate things so this came to my mind a bit latehere is the circuit:and here is the response to a "cold start" with the PWM pin floating: