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Hi,
What is the initial inrush into an on-board EV charger at the instant when the wall box applies mains to the charger?

I am speaking about the inrush current into the output capacitors of the on board charger's PFC stage....this can be higher than 80A.

As you know, this happens before the on board charger is actually commanding a charge current to flow.
Do any standards govern this inrush current? (ie, if it should be limited)

Thsi inrush current will damage the relay in the wall box.....i believe we will see millions of damaged wall box relays because of this. (probably its already happening)

Electric car charging wall-switch (16Arms)
https://3814f048-6c69-488c-9f3e-e8d...d/1667b4_c12fc3582f844b2fab75c4ca72218b7a.pdf
 

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I tried to measure this a year or so ago when doing some other measurements on charge points. No real inrush current at all I could see, at least nothing that was out of limits for the charge point contactor. I believe that the very heavy EMC filtering ahead of the rectification and smoothing stage tends to keep the inrush current down, but perhaps some cars may use other methods to reduce this, like NTCs, perhaps. NTCs are pretty much the standard industry solution for limiting inrush with switched mode supplies and inverters, so may well just be a standard part of most OBCs. Either way, it seems to be a non-problem from what I've seen and nothing at all to worry about.
 

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Hi,
What is the initial inrush into an on-board EV charger at the instant when the wall box applies mains to the charger?

I am speaking about the inrush current into the output capacitors of the on board charger's PFC stage....this can be higher than 80A.

As you know, this happens before the on board charger is actually commanding a charge current to flow.
Do any standards govern this inrush current? (ie, if it should be limited)

Thsi inrush current will damage the relay in the wall box.....i believe we will see millions of damaged wall box relays because of this. (probably its already happening)

Electric car charging wall-switch (16Arms)
https://3814f048-6c69-488c-9f3e-e8d...d/1667b4_c12fc3582f844b2fab75c4ca72218b7a.pdf
They are not big capacitors, I suspect probably no bigger than the ones in your TV (if you have a TV).

It's a SMPS in the OBC so it's the switching stage of that which deals with voltage ripple. The ripple from the rectified AC can be then dealt with.

If they are large caps then I'd expect a NTC component to damp the inrush. All chargers are going to be designed different, so there is no one answer.

Put it this way, I had my Ohme set for 16A draw, and it 'occasionally' tripped my 20A RCD, like once in 10 times, so if you were to assume 20% surge on the continuous current draw, then anecdotally it's not really going to be more than this on my Ohme.
 

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Further .. oh I see it is your first post, @cupoftea. Are you trolling and anti-EV, trying to find some problem with them?

Well, there are plenty of problems but you're looking in the wrong place looking for technical complications, we engineers are clever and solved pretty much all of them so far else EVs and chargers would not be a marketed product yet.

The issues around BEVs are now political and if people have the will to do what they need to do or not. Do you?

That's not to say some of 'us' make mistakes .... I'd say most often when mislead by corporate financial management objectives! :devilish:
 

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They are not big capacitors, I suspect probably no bigger than the ones in your TV (if you have a TV).

It's a SMPS in the OBC so it's the switching stage of that which deals with voltage ripple. The ripple from the rectified AC can be then dealt with.

If they are large caps then I'd expect a NTC component to damp the inrush. All chargers are going to be designed different, so there is no one answer.

Put it this way, I had my Ohme set for 16A draw, and it 'occasionally' tripped my 20A RCD, like once in 10 times, so if you were to assume 20% surge on the continuous current draw, then anecdotally it's not really going to be more than this on my Ohme.

The front end capacitors are pretty big, although I couldn't measure any appreciable inrush current when I had a CT and 'scope on the supply to see what happened at start up, as this unit uses controlled rectification to eliminate it, I believe. This is the bank at the front of the Tesla power conversion module, for example:

148282


Inrush current won't be the cause of an RCD trip, either, as they are not over-current devices. The usual cause of nuisance tripping of RCDs when a switched mode supply is connected has nothing at all to do with the inrush to the front end, it's almost always a momentary earth leakage, most often whent he front end EMC filter uses a delta cap, allowing phase current to flow back down the CPC. Not normally an issue as the current should be balanced, but can happen under some switching conditions. For example, the Tesla charger has a normal earth leakage of around 2 mA, but this pulses up to around 20 mA during switch on, I measured. This imbalance is usually too short to cause an RCD to trip (typically they take around 25ms or so).



Edited to add:

Just been back through the notes I made when looking into this a year or so ago (I was really looking at something else). It looks like the way inrush is limited in some OBCs is the same as the way Tesla do it, by combining the task of front end rectification with that of inrush limiting and power factor correction, using switched rectification. Has big advantages, as the same components can do three jobs, and be slightly more efficient as rectifiers, too. Very neat solution, and explains why I saw a really soft start when the Tesla OBCs powered up from the charge point. I feel the need to go and repeat the same work with the I-Pace, so see how that handles this, but my guess is that it will use a similar topology. Interestingly, the Leaf seems to go one stage further, and expand the functionality of the front end rectification and PFC stage so that it can work as a true synchronous rectifier. This may be why the Leaf has an inherent V2G capability, as using true synchronous rectification allows the OBC to be bi-directional.
 

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How long have EV's been around and using wall boxes?

Since 2011 I believe, so where is your evidence that there have been these failures over the last 10years.

Had there been in the early days, manufacturers would have fitted the relevant components to restrict the in rush.
 

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How long have EV's been around and using wall boxes?

Since 2011 I believe, so where is your evidence that there have been these failures over the last 10years.

Had there been in the early days, manufacturers would have fitted the relevant components to restrict the in rush.

Good point. I've been driving plug in cars since 2013, never had an issue. I've measured the inrush current on the Tesla Model 3 and it's not an issue (in fact it ramps up pretty slowly). Also measured the inrush current on our air source heat pump (which is inverted controlled, so in essence near-identical to an OBC front end) and that's the same, the current ramps up slowly.

Seems to be a complete non-problem, perhaps a misunderstanding as to how switched mode supplies are designed by the OP. TBH, the mention of 80 A seemed odd, as typical inrush currents for unprotected simple rectifier - smoothing circuits are only really limited by supply impedance, and this tends to be very low. Initial inrush currents for unprotected rectification front ends tend to be in the few hundred to few thousand amp range in my experience, although with a peak duration that may only be a few tens of µs.
 

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Strange post.
My early GM design Ampera has a slow ramp up to the heady heights of 14.4A over a second or so. Looks like back then the designers had no problems soft starting the charger circuits.
 
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I thought it was standard design practice to put a switched pre-charge resistor around the main contactors if there was a need to prevent any inrush current to large capacitors, thereby avoiding any problem as the contactors close. Does anyone know if this is the case?
 

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Thsi inrush current will damage the relay in the wall box.....i believe we will see millions of damaged wall box relays because of this. (probably its already happening)
I've been using the same contactor for 7 years and 3 different EVs. No sign of trouble.
 

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I thought it was standard design practice to put a switched pre-charge resistor around the main contactors if there was a need to prevent any inrush current to large capacitors, thereby avoiding any problem as the contactors close. Does anyone know if this is the case?
I do not have direct experience of OBCs in EVs, but inverter welders, which are basically large switched-mode power supplies, do have the soft-start circuits that you have described.
Inverter-based Arc, TIG, and MIG welders all have bridge rectifiers at their mains input ( sometimes more than one rectifier in parallel ), and then from 500 to 2,000 uF of smoothing capacitors. Even the cheapest most basic imported units have a pre-charge resistor, or thermistor, which is bypassed by a relay after the initial surge. The resistance value is usually between 20 and 50 Ohms, and it may be before the bridge rectifier or between the rectifier and the capacitors.
 

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The front end capacitors are pretty big, although I couldn't measure any appreciable inrush current when I had a CT and 'scope on the supply to see what happened at start up, as this unit uses controlled rectification to eliminate it, I believe. This is the bank at the front of the Tesla power conversion module, for example:

View attachment 148282

Inrush current won't be the cause of an RCD trip, either, as they are not over-current devices. The usual cause of nuisance tripping of RCDs when a switched mode supply is connected has nothing at all to do with the inrush to the front end, it's almost always a momentary earth leakage, most often whent he front end EMC filter uses a delta cap, allowing phase current to flow back down the CPC. Not normally an issue as the current should be balanced, but can happen under some switching conditions. For example, the Tesla charger has a normal earth leakage of around 2 mA, but this pulses up to around 20 mA during switch on, I measured. This imbalance is usually too short to cause an RCD to trip (typically they take around 25ms or so).



Edited to add:

Just been back through the notes I made when looking into this a year or so ago (I was really looking at something else). It looks like the way inrush is limited in some OBCs is the same as the way Tesla do it, by combining the task of front end rectification with that of inrush limiting and power factor correction, using switched rectification. Has big advantages, as the same components can do three jobs, and be slightly more efficient as rectifiers, too. Very neat solution, and explains why I saw a really soft start when the Tesla OBCs powered up from the charge point. I feel the need to go and repeat the same work with the I-Pace, so see how that handles this, but my guess is that it will use a similar topology. Interestingly, the Leaf seems to go one stage further, and expand the functionality of the front end rectification and PFC stage so that it can work as a true synchronous rectifier. This may be why the Leaf has an inherent V2G capability, as using true synchronous rectification allows the OBC to be bi-directional.
The V2G comment is surely a red herring ?: Nissan have only ever implemented V2X via the DC charging port ( Chademo) with offboard inverter....so far as I know.
 

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I thought it was standard design practice to put a switched pre-charge resistor around the main contactors if there was a need to prevent any inrush current to large capacitors, thereby avoiding any problem as the contactors close. Does anyone know if this is the case?

For AC-DC switched mode front ends, standard practice used to be to fit an NTC and an inrush current limiter. Looking at the way Tesla do it shows that they have used active rectification. That means they can slow start the converter, so limiting the inrush current. Also means they can do on-the-fly power factor correction, I think. I doubt that this is uniquely Tesla approach to designing an OBC. I have a suspicion that the Zoe also uses active rectification, although the Zoe doesn't seem to do PFC, or if it does, it doesn't seem to do it very well.
 

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Discussion Starter · #18 ·
Thanks, it sounds like as you say, the Tesla uses a totem pole PFC , and so can greatly reduce inrush current with that.....What worries us, is that in a year or two, a really el cheapo electric ccar will come to market that has say a 2 Ohm inrush resistor switched out by an inrush relay...and will give >80A of inrush into the Boost PFC output caps before the charger even switches on.........Myself and a friend are bringing a very cheap wall box to market, and we want to be absolutely sure that all EVs have superb inrush current limitation. We cannot find it in the standards as a requirement......so we are worried that an unscrupulous car manufacturer will bring out an OBC with very little inrush limitation.

May i ask do you know if its in the standards.

Also, i appreciate Tesla etc have little inrush, but can we say that for all currently markered EVs?

I hope so, because we want to use a cheap relay and no inrush limitation in our wallbox.
 

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Thanks, it sounds like as you say, the Tesla uses a totem pole PFC , and so can greatly reduce inrush current with that.....What worries us, is that in a year or two, a really el cheapo electric ccar will come to market that has say a 2 Ohm inrush resistor switched out by an inrush relay...and will give >80A of inrush into the Boost PFC output caps before the charger even switches on.........Myself and a friend are bringing a very cheap wall box to market, and we want to be absolutely sure that all EVs have superb inrush current limitation. We cannot find it in the standards as a requirement......so we are worried that an unscrupulous car manufacturer will bring out an OBC with very little inrush limitation.

May i ask do you know if its in the standards.

Also, i appreciate Tesla etc have little inrush, but can we say that for all currently markered EVs?

I hope so, because we want to use a cheap relay and no inrush limitation in our wallbox.

There have been relay failures in one or two charge points that I'm aware of, particularly in units that use PCB mounted relays. I've yet to hear of a proper contactor failure in an EVSE. Also, as it makes sense to combine open PEN fault and DC tolerant earth leakage protection into any new charge point now (as they are mandatory requirements for the majority of all installations now), and as open PEN fault protection requires breaking both live conductors plus the CPC, under full load conditions if there's a fault, then I suspect that inrush current may be far less of a concern than breaking capacity.

Cheaper relays often tend to fail as a result of arcing when breaking, not current surge when making. 99.99% of the time there will be no current flowing when a charge point contactor closes or opens, so it's really just the emergency case of opening under load that's probably going to drive the spec of the device.

For this reason, a proper three pole contactor is probably a better choice than a relay, primarily because contactors are often designed to be able to make and break reactive loads more reliably than some cheaper relays.
 

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Discussion Starter · #20 ·
Thanks, and do you believe we should also put a bidi TVS just downstream of the wall box relay?....so as to freewheel the current in the stray inductance of the charge lead leading off to the EV?....when the relay opens under full load (as you say, to do this PEN check thing)
 
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