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The charger needs to handle the solar array's output of max 550V, but also efficiently handle lower voltages as weather/etc conditions degrade -- down to roughly 150V. It needs to handle between 15 - 50A. Thats quite the wide range of voltages and current, but is the nature of this particular solar setup. Efficient power conversion is of prime importance. I'll be using an MPPT type algorithm at the MCU with 5V output to the gate driver. PWM frequency will likely depend on the Fet used -- I'm assuming >50KHz. Output power to the batteries will be approx. 120V. Price isn't too much of a concern, hard costs need to come in at under $100USD per unit (but much less is preferable).

One of the options I'm currently looking at is Wolfspeed's 650V SiC Mosfet.

And, on the Si Mosfet side: OnSemi's Super Junction Mosfet.

From what I've read, SiC Mosfets really outperform Si in high power applications and in high frequency PWM drivers. It seems their current market value is in converting much higher voltages, up to 1200V. Those power levels are above the scope of my lowly project of only 550V.

My question is: Are SiC Mosfets an appropriate technology to incorporate into <=550V power switching? Will I gain power conversion efficiencies with them compared to other more standard Si Fets in that power range? Or, do those efficiencies deteriorate at lower voltages -- where Si Mosfets would do just as well?

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    \$\begingroup\$ I don't think you're going to get below US$100 per unit, especially not if you're using SiC. The 50 A requirement also means you'll need a really beefy inductor, and that won't come cheap either. I'd estimate this project would probably cost about $500 per unit, but wouldn't be surprised if it goes higher than that. \$\endgroup\$
    – Hearth
    Commented Mar 28, 2021 at 17:28
  • \$\begingroup\$ If you can afford it, SiC will outperform Si if you select and use it properly, taking advantage of its strengths. It will be much more expensive though. \$\endgroup\$
    – John D
    Commented Mar 28, 2021 at 17:52
  • \$\begingroup\$ depends... SiC is where Si was in the 1960, the production is now ramping up and is almost doubling each year and this will drive the cost down. I spend £350 on a 6switch inverter 3years ago but I can make that out of suitably rated discretes for £80 using Wolfspeeds 4th Gen devices (650V, 60A) in TO-263-HV package. While SiC will be more expensive the systematic wins are huge... heatsinking and cooling is reduced so sometimes it is worth driving cost into one area for it to benefit elsewhere( weight volume other system needs) \$\endgroup\$
    – user16222
    Commented Mar 29, 2021 at 0:10

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My question is: Are SiC Mosfets an appropriate technology to incorporate into <=550V power switching? Will I gain power conversion efficiencies with them compared to other more standard Si Fets in that power range? Or, do those efficiencies deteriorate at lower voltages -- where Si Mosfets would do just as well?

Comparing two devices in the links (Si = FCP125N65S3 and the Cree C3M0120065K), they both have about the same on resistance, max voltage and max current. However, the MOSFET will be slightly more efficient at around 50 kHz but, at speeds probably in excess of 200 kHz the SiC will probably be better due to its lower gate-source capacitance.

The Cree (SiC) will require a more sophisticated protection regime to avoid thermal runaway compared to the regular Si device because most SiCs don't avoid this even in switching applications. Si MOSFETs do avoid this problem rather well.

The gate drive voltages for SiC are about 150% higher than regular Si too. This both causes losses and slows things down a little bit and, designing a SiC gate driver needs a little more thought compared to Si. However, there's still a good chance that SiC will be significantly faster than Si and should be operated at a higher speed.

If the bulk diode is at all utilized for any form of flyback management then the SiC will be better because it has a faster reverse recovery time of 15 ns compared to 339 ns for the silicon but, the diode power losses are far more significant in the SiC compared to the Si.

Remember, this is a quick data sheet-to-data sheet side by side comparison. A true comparison only ever happens when you compare a circuit against another circuit because some parameters will be more important than others.

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    \$\begingroup\$ Just as another note, the energy rated output capacitance of SiC is significantly less than Si for similar RDSon, and that can make a difference in efficiency as well. \$\endgroup\$
    – John D
    Commented Mar 28, 2021 at 18:51
  • \$\begingroup\$ @JohnD very good point. Actually, the Si linked has 62 pF and the SiC linked has 57 pF and that surprised me a little i.e. both being about the same at 400 volts! \$\endgroup\$
    – Andy aka
    Commented Mar 28, 2021 at 18:53
  • \$\begingroup\$ That surprises me too! Usually SiC is substantially lower. \$\endgroup\$
    – John D
    Commented Mar 28, 2021 at 19:14
  • \$\begingroup\$ I don't quite understand the thermal runaway argument. Wouldn't it be the other way around, if you look at the temperature dependence of the on-resistance? \$\endgroup\$ Commented Mar 28, 2021 at 19:43
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    \$\begingroup\$ @JonRB, Thank you for posting a link to St's SiC. With its lower Rds_on it does seem it will have less switching losses than 3rd gen. Although I agree with Andyaka, its good to at least post options outside the strict bounds of the questions: thats how we discover. In my context, I think I will be forced toward using OnSemi's Si because its performance is rather good and at a far lower cost point. Although, I do wish I could move forward on SiC technology. We're just not quite yet at the cost/benefit point for my particular market. \$\endgroup\$ Commented Mar 30, 2021 at 17:45

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