GT3.0 Input MOSFET limit?

System · May 2007

I vaguely remember reading a thread that indicated the max input voltage limit for MOSFETs in the GT but I cannot find it again. Is there such a limit? If so, what is it? Thanks. Tom

mike95490 · May 2007

Re: GT3.0 Input MOSFET limit?

I suspect it's about 600V.
MPPT range is 195 - 550V

http://www.xantrex.com/web/id/172/p/968/pt/23/product.asp

BB. · May 2007

Re: GT3.0 Input MOSFET limit?

600 Volts is also the typical maximum voltage per NEC for "normal" wiring... And 600 volts is the maximum allowed input voltage for the GT series inverters.

Solar Guppy (one of the GT design engineers) said that, typically, running higher input DC voltages on the inverters would cause them to be a little bit less efficient because of the parasitic capacitance of the MOSFETs (the wider voltage you charge and discharge a capacitor, the more power you toss away).

-Bill

System2 · May 2007

Re: GT3.0 Input MOSFET limit?

Thanks for the info about 600V. I'm hoping that is not the absolute max before damage because if I look at the xantrex inverter sizing tool, even with 6 Sanyo 195s, the Max Voc @ Min Temp is nominally 444.3V. These panels are rated +10%, so I assume theoretically I could get a string that has all at 110% nominal, so does that mean theoretically I could get 444.3V *110% = 488.7V at min temp? Add the "edge of cloud" effect that I've been reading about, and somewhere I think I read that can add 1.3x irradiance, so that gives me 635V? I thought I read in some forum thread that the absolute max is 800V, but I can't find that thread again.

Solar Guppy · May 2007

Re: GT3.0 Input MOSFET limit?

You mis-understanding things ...

Power and Edge of Cloud boost the current NOT the voltage. The Only effect to voltage is tempature. Just use the Xantrex sizing tool as it check all the parameters your trying to learn about.

FYI, No inverter handle more than 600V, so if that WAS your design issue, you would have to change the panel configuration, which you don't.

BB. · May 2007

Re: GT3.0 Input MOSFET limit?

600 VDC is the rated maximum (I don't remember reading 800 VDC anywhere wrt the GT inverter, so I can't help with that)--normally, one would expect tests to push the limits by 10-20% during design verification--but in the end, if you exceed 600 VDC, the warranty is void and you run the risk of having your home owners insurance not covering fire damage (not that I would expect a fire). I don't know about the Xantrex GT, but the Outback MX 60 specifically says that they log the maximum input voltage if it exceeds design specifications.

From what I understand--the +/- 10% -30% power output change would more be rated on the output current than expecting higher than normal output voltage... The voltage is, more or less, based on physics--so once you have reached a minimum watts / sq ft, any additional light produces more current. (Note; output voltage is directly proportional to temperature).

And, since your maximum design voltage is based on Open Circuit voltage (panel connected to an inverter, but no power being drawn)--I would not expect a +10% increase in Open Circuit Voltage during an edge of could effect... I would expect Open Circuit Voltage, once you have good sun, would be related to a cold panel getting exposed to full sun. And once the inverter starts loading down the panels, the true output voltage will drop quite a bit as it seeks the MPPT (Maximum power point tracking) to satisfy P=V*I .

In any case, the typical recommendation is to ensure that the minimum operating voltage is >200 VDC and that ensure that your in your coldest expected sunny weather that open circuit voltage does not exceed maximum open circuit rating.

It is a balancing act in the sense that inverters are typically more efficient when operated near their minimum input voltage (lowest MOSFET switching losses at low input voltages) and wiring is more efficient when operated at near maximum voltage and minimum current (takes more copper to carry current and Power Loss = R*I^2--you double the current you get 4x the heating loss in the wiring due to wiring)...

-Bill

PS: I see Solar Guppy has typed faster than I--and listen to him--he is a design engineer for the Xantrex GT.

System2 · May 2007

Re: GT3.0 Input MOSFET limit?

BB & Solar-Guppy,

Thanks!

Tom

mike95490 · May 2007

Re: GT3.0 Input MOSFET limit?

BB wrote:

It is a balancing act in the sense that inverters are typically more efficient when operated near their minimum input voltage (lowest MOSFET switching losses at low input voltages)

Ah Ha ! Does this explain the differences seen between different installation efficiencies, of inverters at 90% and others, with comparable wattage loads, at 96% ??

BB. · May 2007

Re: GT3.0 Input MOSFET limit?

Mike,

Solar Guppy is the design engineer on these things... But my answer would be--it depends... There are so many variables in the design of any inverter that efficiencies are a combination of how much money the company wants to put in an inverter (thicker wire, more expensive MOSFETS) and how much labor (more expensive to hand wrap a toroid type transformer than the standard type, etc.

And you have fixed losses of running the conversion circuitry (electronics that controller the inverter and switch the MOSFETs) which look great at full load, but really kills the efficiencies at low power (50 watt load at 3kWatts is great, 50 watt load at 100 watts is terrible).

And you can have almost 100% efficient little PWM inverters (very low loss when fully turned on during bulk charging and not much power required to run electronics) but still be losing a lot of power from the solar panels (say panels could be outputting 10 amps at 20 VDC, but the PWM inverter is passing 10 amps at 14 VDC to the battery; 200 watts vs 140 watts in this example).

That is why for the larger systems a MPPT controller is better for charging batteries because they can "match" the IV curve of the load (batteries) with the IV curve of the solar panels.

-Bill

RCinFLA · July 2007

Re: GT3.0 Input MOSFET limit?

Worse case for maximum input voltage is the coldest temp the panels will be subjected to with no load.

A bright, very cold day, where for whatever reason, your batteries are fully charged is when it will happen. (assuming you don't have an auxilary load dump)

Take max unloaded max voltage spec (usually spec'd at 25 deg C) and raise it by 2.5 mV per deg C below the spec'd temp, deg C.

GT3.0 Input MOSFET limit?

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