Using IR2117 MOSFET driver with isolated PV coupler - Page 1
Forum section: electronics / projects

author: Dannyx
author_rank: Regular Contributor
timestamp: April 10, 2025, 04:34:08 pm
content: Good day folks. A while ago, I asked for help regarding various methods of driving an N-channel FET on the high side of a circuit and while the discussion was fruitful and there were many great ideas there, I never actually tried any of them. It's time I revisited that project and focus on one suggestion in particular, which seemed to offer both high-speed switching action and a 100% on time, which is actually the main requirement for this circuit.To keep it as simple as possible, here's what the setup should do: I got a 48v power supply and a battery (two of them, actually to get up to 24v) powering a "load" (a DC-DC converter in this case). Without going into too much detail as to what this does, the end goal is to switch over to the batteries when the supply cuts out - ghetto UPS. The DC-DC converter is capable of operating on both voltages - I checked. I haven't yet decided how to implement the switching scheme - maybe with a Micro or something simpler like a comparator - but I want to discuss if this setup would even work to drive those FETs to begin with.Someone rather cleverly suggested I could use some sort of isolated supply to generate the bootstrap (?) voltage theIR2117required to "pump" the gate of the FETs above the Source voltage, since that's how N-channels work in this config if I understand correctly. The same person also suggested a photovoltaic coupler could act as one, since the required current is very low (hopefully). This is what I came up with.I don't actually have these parts to experiment (plus I don't want to go around busting parts if I can help it, since they're not exactly cheap in the long run), so there's two aspects to this mess:1) Can anIR2117even drive the "back-to-back" FET arrangement I did there2) Can a PV really be used like this. AFDA217was suggested and it's the one I went with, at least on paper, since others are unavailable to me.For (1), I needed two FETs (Q3 and Q4) to block the power supply voltage from over-volting the batteries and I needed Q1 and Q2 to stop the batteries from draining into the power supply when it's off-grid. It WOULD work without them, but would be wasteful.For (2), I guess, in theory, it ought to work. If I understand correctly, taking a look at the internal diagram for the IR2117, the HO pin is switched back and forth between the VB and VS pins based on the "IN" pin. When the "IN" pin is low, HO is connected to Vs, so the FET(s) are off because VGS=0. When the "IN" pin goes high, HO is connected to VB, so now VGS=PV (whatever the Photovoltaic puts out - this should be approximately 12v).Now for the part I'm not sure about: as the FET closest to the power supply starts to conduct, the supply voltage (or battery voltage) passes through the S-D junction and becomes in "series" (?) with the PV and because the PV is floating relative to circuit GND, VGS now becomes PV+Vsup (48v+12v OR 24v+12v), like putting two batteries in series....is this remotely correct, or am I way off ?Thanks.

author: Dannyx
author_rank: Regular Contributor
timestamp: April 10, 2025, 06:46:49 pm
content: Ok, I just realized the IR2117 cannot be powered with more than 20-ish volts on the VCC pin - in my case, it's connected straight to the 48v supply which is not going to work, plus the fully charged battery is also above the maximum rating. A zener or an LDO should go on those VCC pins, but I will sort that out later.It's not a mistake if you catch it first

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 10, 2025, 08:09:10 pm
content: Quote from: Dannyx on April 10, 2025, 04:34:08 pmNot really, you need to watch the currents.The quiescent current of the gate driver far exceeds the PV supply ability.PV optos can directly drive the gate of MOSFETS in SSR usage - ie slow on-off switching.Look at the turn on and turn off times in the data sheets.QuoteA change over scheme like this can be as simple as a 48V SPDT relay, or a beefy Schottky double diode.What current does this operate at ?You should also look at ideal-diode controllers, like AP74700 etc, that has the gate drive done, and includes hot swap abilities, with faster OFF performance when alternating supply.

author: langwadt
author_rank: Super Contributor
timestamp: April 10, 2025, 08:13:51 pm
content: at what current?

author: Dannyx
author_rank: Regular Contributor
timestamp: April 11, 2025, 03:40:05 pm
content: Ok, let's take a step back and analyse:Quote from: PCB.Wiz on April 10, 2025, 08:09:10 pmThis is something that indeed puzzled me when looking at PVs: the parameter they give is the short-circuit current, which is indeed very low - couple of uA at best. Is this the equivalent of the "absolute maximum" current the PV can deliver, considering that it would not be in a "short-circuit" as per the datasheet, but instead driving the Gate of a FET which should be very high impedance ?Quote from: PCB.Wiz on April 10, 2025, 08:09:10 pmYes, I was going to go that way and drive the gates directly with the PV, but the turn-on/off times were much slower compared to a "driver"....and I figured I needed really fast switch times if dealing with an UPS-type application, since we don't know how the load reacts.Quote from: PCB.Wiz on April 10, 2025, 08:09:10 pmThe project DID start off as a very simple idea, but escalated into something more "pretentious": I was planning on using a relay indeed, but the problem is the slow change-over time....well, "slow" compared to what a FET can pull off. I have not experimented, so I can't say for sure if the downstream devices would remain functional during the dead-time when the contacts change over to the other supply (battery or DC supply) - they might restart or lock up. The devices in question are 2 CCTV cameras, a DVR with hard-drive, two network switches, so less than 5A max current. A second issue is the current draw of a relay is higher compared to a MOSFET switch like I imagined it.The diode version is not suitable either: I need to adda chargerto charge the batteries back up, so a diode would cause the charger to push current into the load, since it's in parallel with the batteries, which we don't want. Second, it would not be possible to implement a low-battery cut-off, since there would BE no element to cut off the batteries from the load, so in this respect even the relay version is better.Quote from: PCB.Wiz on April 10, 2025, 08:09:10 pmI did look at several ICs like these, but not all of them are easy to source - I'll take a look again.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 12, 2025, 12:57:51 am
content: Keep it simple, a pair of Schottky diodes is all you need. The loss due to voltage drop is insignificant at those voltages.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 12, 2025, 02:14:04 am
content: Quote from: Dannyx on April 11, 2025, 03:40:05 pmThe PV parts are just like a solar cell, so the SC current and OC voltage define two corners of the curve, and the maximum power point is a soft rectangle, so you might see ~90% of Isc at ~90% of VocQuoteUPS designs are mains cycle time scales, PV+FET can be much lower than that.They give curves for turn on to 5V out, vs IF and load C, and you can shorten the time by pulse drive to 50mA but hold with 5~10mA

author: Dannyx
author_rank: Regular Contributor
timestamp: April 12, 2025, 03:40:26 pm
content: Quote from: NiHaoMike on April 12, 2025, 12:57:51 amDiodes would not allow me to charge the batteries or disconnect them at low voltage, as previously mentioned

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 12, 2025, 08:45:27 pm
content: Quote from: Dannyx on April 12, 2025, 03:40:26 pmThe BMS will take care of the automatic disconnect. And the diode, being reverse biased, will not interfere with the charger.

author: Dannyx
author_rank: Regular Contributor
timestamp: April 12, 2025, 08:55:12 pm
content: True, I guess didn't offer enough details about the batteries: they're two 12v/20Ah SLAs in series, so they won't be using a "BMS" in the true sense of the word, like we're used to seeing on Li-Ion packs, hence the issue remains: any sort of charger, dumb or smart, will need to be in parallel with the batteries, hence it will also push current into the load if I just connect the two "sides" together. It will also not have any control over the over-discharge of the batteries, so a cut-off is still necessary.