Troubleshooting a hybrid solar inverter overheating.

[Fflint]

I have a (relatively) cheap solar hybrid inverter that is overheating in certain specific conditions (no AC, just charging the battery from the PV input).

Do we have any all-in-one inverter/hybrid inverter experts here?

Specifically it has two mosfets and a diode on a separate small radiator, away from the main airflow path (that is guided using a transparent shroud). That radiator is quickly reaching the temperature limit at about 2.5kW of PV power.

Fans at the bottom of the case are running, but one fan is substantially faster (the other one seems to use different software condition and even stops periodically).

Both these fans are extractors and are right next to eachother.

When AC is plugged however the inverter happily pushes 4.5kW to the grid and the pv radiator doesn't exceed 50C.

Has anyone seen anything like it? I can't feel the difference in airflow. Can these mosfets be driven differently somehow to perhaps lower power consumption which causes them to heat up a lot more?

Rated PV power is 6KW.

[Drjaymz]

I suppose if its getting too hot then either its got more resistance than it should have due to fault in the mosfet or not being driven hard enough or its passing more current that it should be doing.  It sounded like you have more than one mosfet in parallel for current sharing? I suppose one could fail so the other is passing all of the current together with increased resistance would make for more heat than expected whilst still appearing to operate?

[Fflint]

Thanks for the reply. I guess it is possible one of the transistors is damaged, but I thought it unlikely as it handles a lot more power in grid mode. It only overheats in battery mode. This shouldn't really matter for the PV section, but somehow it does. Perhaps specific solar inverter knowledge is needed... So I started this thread.

[indeterminate]

It could be a cheep copy of a good design where they have saved a few cents by substituting parts , it could be counterfit parts or just a bad design.
an easy bodge is to attach a small fan to the heat sink that is overheating.
the alternative is going to involve sum reverse engendering and seeing if the components are fit for purpose and the gates are being driven correctly.

[Drjaymz]

It could be a cheep copy of a good design where they have saved a few cents by substituting parts , it could be counterfit parts or just a bad design.
an easy bodge is to attach a small fan to the heat sink that is overheating.
the alternative is going to involve sum reverse engendering and seeing if the components are fit for purpose and the gates are being driven correctly.

Also a lot of reputable manufactureres were forced to substitute parts in 2020 - 2022 and so that could be a factor - however, they will have tested those parts conform to spec and that the device itself is within spec.  At the end of the day if a FET is getting too hot its either passing more current than it has the ability to disspite , or its not fully turned on (both I2R losses) or faulty - as in only part of the channel is working but thats an unusual failure mode.  IMO they tend to fail short or open usually with a large bang so if still working but getting too hot then suspect the driving circuit or something else downstream is presenting too much load.  I'd usually have a look with a thermal camera but you'd need a good one to compare against (unless something is obviously too hot).  Hybrid solar inverters are not really that complicated and you should be able to identofy the parts against a pretty standard block diagram, at this size they tend to have quite a few parts in parallel for the current capacity.

One final point is that they tend to have a fair number of current measurement points, on the PV / on the Link, Input and output and its possible that its not measuring the current correctly - but for that type of thing you'd need schematics and probably some form of service mode.