Solar panel feeding switching device? - Page 1
Forum section: electronics / projects

author: Peabody
author_rank: Super Contributor
timestamp: January 01, 2022, 02:57:15 am
content: Most of the time it seems the power source for a DC-DC converter is pretty low impedance. But a solar panel has relatively high impedance. So I wondered - if you connect a scope to the panel outputs while it's powering some switching device, what would you see? Would it be a flat line, or would the mosfet switching be reflected on the scope? If it's flat, is that because of the high switching frequency?

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 01, 2022, 03:28:09 am
content: I would expect it to wander about a little as MPPT does its thing. Most likely small steps up and down at fixed timing intervals since that's the most common way MPPT is implemented.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 01, 2022, 09:29:48 am
content: Yes, MPPT adjusts the converter's effective input resistance, to match the source's. Note that it's matching the source's average resistance (V / I), not the incremental (dV / dI) or AC resistance. The incremental resistance need not be anything much, in particular, anyway; when LTI* conditions apply, we have the maximum power transfer theorem, but these need not match in general otherwise.*Linear time-invariant. So, basic junk like resistors, capacitors, inductors, CV and CC sources. Not PV panels and power supplies!As for switching, you may see some ripple, depending on how well filtered the switching device is. "Switching device" is rather generic so it's not obvious whether a raw switch was intended, or a converter proper, as in the first sentence?The only thing that does nail down a converter, is the filter -- as an LTI element, it must have a characteristic impedance, and some resistance to terminate it (which might be provided by the control circuit, source/load, or internally as damping resistance). This forces the terminal impedance towards this value, near its cutoff frequency. At frequencies below filter cutoff, the impedance can diverge from that value, all the way to 0 or +/-infty at DC (i.e., CV or CC, or anywhere inbetween). This is true regardless of converter type (current- or voltage-sourcing inverter). (What about high frequencies? Typically a reflective filter is used, i.e., impedance goes to zero/infty by way of an outward-facing shunt capacitor/series inductor (respectively).)Tim

author: Peabody
author_rank: Super Contributor
timestamp: January 01, 2022, 04:53:18 pm
content: Thanks, but at the simplest level, I think you have a solar panel feeding a mosfet switch which lets the panel current flow through an inductor. When the switch closes, current will rise, and when it opens, current will fall. Since a panel's voltage changes when current changes, it seems you would see those oscillations when scoping the panel voltage. On Youtube videos pretty much all I ever see on a scope is circuit ouput. And the few times I see the input displayed, they are feeding the circuit from a bench power supply. But that's not the same as a solar panel. So I just wondered what you would see with a solar panel in real life. I'm not asking about large changes as in MPPT, but what you see when the mosfet is switching at 1Mhz or whatever.

author: Simon
author_rank: Unknown
timestamp: January 01, 2022, 05:29:33 pm
content: I'm not sure why you ask. Pretty much every single solar panel on earth is connected to a DC/DC or at least a switchmode converter such as a MPPT and grid tie inverter.This is why all switch mode converters have a bypass capacitor on their input. Look at any GTI and it is rammed full of capacitors. Read the datasheet for any all in one DC/DC converter chip and there will be calculations for that input capacitor to maintain certain voltage ripple limits on the input and there will be specifications around the ESR for that capacitor often with suggestions for a minimum ceramic capacitor size with the rest left to electrolytic capacitors preferably of the polymer type. This is all to maintain the input voltage and avoid undesirables happening.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2022, 01:20:25 am
content: Ah well you're missing about three other things! There's a switch into an inductor yes, and then when the switch turns off, there is a catch diode recirculating its current through the output; it's not simply free hanging! Meanwhile the input current doesn't suddenly stop (and remember, if the source has any wiring length, it also has inductance, so that current has to go somewhere in the mean time), there is a bypass capacitor or other filtering, which carries the current inbetween switch pulses.Tim

author: Peabody
author_rank: Super Contributor
timestamp: January 02, 2022, 05:49:29 am
content: Ok, so the panel voltage will stay pretty level, at least in the short term. Thanks very much for the explanations.

author: Simon
author_rank: Unknown
timestamp: January 02, 2022, 09:45:23 am
content: Yes, the panel voltage will be a constant in the short term. The input capacitors have milli ohm's of ESR so any current will be sourced from them holding up the voltage. This is the same concept for any application of a bypass capacitor. For example microcontrollers will always require a small ceramic as close as possible. This is because like with a SMPS the current will be drawn in surges, usually as the chips internal switches all change state together on a clock pulse. So I have seen in the past a device someone made with a little 8 pin PIC. He omitted the bypass capacitor. The result was that the 5V rail would spike up and down in sync with the clock by up to 0.5V on a 5V rail. This was an extreme case due to the designers stupidity and that he also put the bypass capacitor for the voltage regulator too far from the regulator on a thin trace.It's really just like a tank with water flowing in slowly and water being gulped out in bursts. The actual average flowrate of the bursts over time is the same as the slow flowing feed to it, the tank provides the water for the peak demands and is constantly replenished from the source. This stops the thing doing the gulping being starved when it wants a lot in one go but there is no reserve to take from so it all goes to pot.The important thing to understand in electronics about components is their parasitic's. The things you don't want them to have but they just do because it is real life. When looking at a part think more about the properties it is not renowned for. If it's a capacitor, well you want to know it's capacitance, but that is not the whole story, it has resistance and inductance too, these will have as much a bearing on it's behaviour as the thing you care about, it's capacitance, well you should also be caring about the parasitics, that is what makes the whole component.A length of wire is not just a near 0 resistance conductor, it does have resistance and this does matter, it also has inductance and capacitance with respect to other thhings, for example a pair of wires carrying power will have capacitance. I once investigated why a thermostat was being damaged in a 24V air conditioning system, turned out it was back EMF from the fans being turned off. So despite the power rail being at 24V at the thermostat power input it could hit -400V! on the same wire as the 24V supply, why? because of the resistance and the inductance of the wire meant that at the frequency the back emf pulse was happening that wire had a high impedance (AC resistance) so you could have at the same time a very different voltage at each end of a 1.75m wire.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 02, 2022, 11:53:51 am
content: Here is an example of voltage ripple on one of my own solar arrays, the ripple is in fact the MPPT perterb & observe algorithm that can be seen tracking a passing cloud. You may think this voltage ripple from MPPT rather large but it is automatically chosen to ensure sufficient SNR on the measurement channels in a noisy environment with low cost in mind.High frequency switching ripple is entirely dependant upon the amount of input capacitance provided between the PV panels & switching stage and that is a compromise between cost of such capacitance and loss of PV efficiency due to ripple, increased EMI & cost of EMI filtering, and even SNR in the input measurement channels so there are many compromises to be made.

author: Simon
author_rank: Unknown
timestamp: January 02, 2022, 12:02:11 pm
content: But are those oscillations the ripple due to switching or the MPPT varying it's current draw to find the peak power point and hovering around it to help track it? I suspect the latter.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 02, 2022, 12:11:30 pm
content: Amount of high frequency noise can't be arbitrarily high, because the inverter has to pass EMI regulations.On the other hand, it can't be zero either because inverters use power switching devices at high frequencies, and perfect filtration is impossible because any filter topology "leaks" due to circuit parasitics if nothing else (also, good filters cost money).So expectsomehigh frequency ripple, and then slower changes due to MPPT algorithm doing it's job.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 02, 2022, 12:15:55 pm
content: Quote from: Simon on January 02, 2022, 12:02:11 pmAs stated " the ripple is in fact the MPPT perterb & observe algorithm"

author: Simon
author_rank: Unknown
timestamp: January 02, 2022, 12:45:46 pm
content: ah right. I think the OP question was more about the really small time stuff of ripple due to switching cycles in a SMC

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2022, 01:55:43 pm
content: Quote from: Siwastaja on January 02, 2022, 12:11:30 pmDepending on where it is... saw one manufacturer that flat out said they neither need nor intend to (this was gear for fixed installations). Granted, such an application could tolerate quite a lot (I would think the much more relaxed FCC Part 18 would apply, but IANAL).Even for residential application, wasn't it Dave had an inverter box emitting quite a din?Tim

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 02, 2022, 02:18:06 pm
content: Quote from: Simon on January 02, 2022, 12:45:46 pmI was trying to show the OP a REAL PV waveform as he seemed to be complaining all he could find were powered by bench power supplies. It also shows that for all intents & purposes the HF ripple is of no consequence assuming a normally designed converter with some modicom of input capacitive filtering.I would add for those interested a useful resource is LTspice for visualising such things prior to construction, it is perfectly feasible to make models of selected PV cells etc so such effects can be observed at different levels of insolation.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 02, 2022, 02:22:09 pm
content: Quote from: T3sl4co1l on January 02, 2022, 01:55:43 pmFor area's of high density living like most of the UK modern electronic power equipment can be a real pain, not only blotting out radio's but also broadband and sometimes television. I know I am not the only one to have had those experiences!

author: Peabody
author_rank: Super Contributor
timestamp: January 02, 2022, 05:10:44 pm
content: Quote from: fourtytwo42 on January 02, 2022, 02:18:06 pmThanks very much, 42. While Simon's example of the microprocessor's switching clock has a regulated supply, a solar panel does not provide a regulated supply, and I had just never seen what a panel's voltage looks like on a scope when feeding any kind of switching regulator or converter. If the ripple in your image is indeed P&O stuff, then it's clear the switching doesn't present any problems, with enough capacitance. But it occurs to me that the capacitance might also mess up the P&O. I mean, you don't want to damp everything too much when you really need to see the variations. I guess it's just figuring out what the Goldilocks capacitance is.With respect to simulating solar cells in LTSpice, I would like to know how you would go about simulating a panel so that it has specific maximum Isc and Voc values. I found a Youtube video on simulating a panel, and followed that. But I was looking at a specific panel that Digikey sells, and ended up making it come out right only by changing the parameters to completely unrealistic values. I don't know whether the rest of the curve is at all right. Anyway, I'll post the .asc I ended up with. If you have a better way, I'd like to see it.https://www.digikey.com/en/products/detail/anysolar-ltd/SM531K08L/9990469And finally, I'm trying to simulate a Microchip opamp (MCP6041) as part of a solar charger, but the Pspice simulation provided by Microchip doesn't work in LTspice. I keep seeing that Spice is Spice, and if it works in one it should work in another. But I certainly haven't found that to be the case. Please let me know if you know of a way to make this work in LTspice.

author: Simon
author_rank: Unknown
timestamp: January 02, 2022, 05:50:22 pm
content: Quote from: Peabody on January 02, 2022, 05:10:44 pmYou have totally missed the point. What you are asking about is universal. Don't you always put a bypass capacitor next to any chip? why do you do this? and if you don't why not? Any device that may desire a sudden burst of current needs bypassing. Ever a switching converter fed by polymer bateries will need an input cap if the converter is not right on top of the batteries. All wires, never mind the solar cells will add source resistance and inductance that has to be countered with a local buffer. It is for switchbode converters the inductance that is more of a problem as at the frequencies they work at the wire will appear to be an inductor NOT a wire or even resistor. If you have panels on your roof and an inverter downstairs the wires to the panels at the switching frequency will be a higher source impedance than the panels.What 42 showed you is an MPPT tracker at work where the voltage varies over seconds and the converter is constantly moving it's current draw around purposely to get the panel voltage to change so that it can measure the power and see which direction it needs to track in. Over these seconds of time the bypass capacitance is invisible as the bypass capacitance is there to deal with the MHz of switching.You need to think about how the system behaves in the sub microsecond timescale. The microcontroller example I gave had nothing to do with the voltage regulator, the regulator was so far away that it had no bearing, even 3 alkaline batteries in series could have shown some peaking droops at the clock switch point. While the clock was 1MHz the disturbance waveforms did not last 1µs, the lasted mere ns, to see one waveform in any detail it was impossible to see more than one at a time on the scope screen they were so fast. This was not about the 1MHz clock but the moment, than infinitesimally small amount of time right after the clock switched and the rate of change in current draw, even though at mere mA it happened so fast that the PCB trace leading to the regulator had a very high impedance and the voltage divider formed by the impedance of the trance and the micro controller was such that the microcontroller only got 90% the voltage it was meant to for mere ns. If I had the clock signal going on the scope with the power line it would have been impossible to see one clock cycle and the disturbance on the power line, it would have been se fast that dt would be the thinnest of lines if at all.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 02, 2022, 07:57:18 pm
content: Quote from: Peabody on January 02, 2022, 05:10:44 pmMy apologies to Simon (and I agree with everything you say) I am only trying to answer what I think the OP wants.Peabody, 1st of all that example is a 1Kw array running at about 600W at the time of the scope shot, the attached boost converter is running at 125Khz using a 100uH inductor and 3.3uF Polypropylene (very low ESR) input capacitor, small as you say to enable fast reaction in the MPPT algorithm although that is a matter of choice / tradeoff. Unfortunately I don't have any shots of the resulting HF noise although it is extremely easy to simulate, I do use an additional input noise filter (common mode choke) to reduce EMC as long solar wiring makes a good aerial! I should also mention the switch gate is slugged to reduce di/dt again to help with EMI but this does increase power dissipation (another compromise).As for simulation of cells I use an extremely simple model so I can simulate hundreds of them without chewing to much sim time, it's not better than yours, just different.cell3.asc(0.97 kB - downloaded 54 times.)Many components found online either don't simulate at all or take an age to do so, a good trick I use with op-amps is find an LT/AD equivalent (same characteristics) and use that instead from the built in libraries.Good luck in your endeavours

author: Simon
author_rank: Unknown
timestamp: January 03, 2022, 10:49:37 am
content: And that 100µH inductor at 1MHz has an impedance of 628 ohms, never mind your panel impedance and don't forget the wiring will add impedance. But the 3.3µH capacitor is a local source of near 0 impedance supply and will trickle charge at a low frequency and discharge at the rate the converter requires power - not totally discharge that is just supply that instantaneous demand. The feed to the capacitor is at DC so really 0Hz so there will only be resistive losses, as far as the capacitor is concerned it is connected to the solar panels and these with the wiring and the inductor is a low resistance (resistance not impedance) source. But the switch mode converter cannot see the panels due to the 628 ohms of input impedance just from the switching frequency, but it can see the capacitor and will draw power from that.If you want to be even more realistic you need to remember that it's not actually about the switching frequency but the change in current or di/dt, so the converter switch will switch n 1/10th of a cycle or less so actually that current spiking is at at least 10MHz, so now you are looking at over 6k ohm of impedance between the converter and the panels before you worry about what the panel impedance is. But it still does not matter because the bulk capacitor is there to provide a low impedance source instead and as the capacitor is charging at DC is just sees the mere ohms of wire and panel source impedance. This is slightly simplified because in reality there will be some peaking between the capacitor and the panels and when the capacitor is suddenly at a lower voltage than the panels it will want to charge more quickly than at DC so there will be a frequency component there but that is no something to worry about at this stage. You can of course just model this if you like.Power source, resistance in series, inductor in series and a resistor being turned on and off by a mosfet. Look at the voltage at the mosfet and the current draw, it will be pretty bad, now add a capacitor at the mosfet and enjoy.