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Forum Index : Electronics : Simulating a rectifier using MOSFETs
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LadyN Guru  Joined: 26/01/2019 Location: United StatesPosts: 408 |
A rectifier conducts in only one direction, while a MOSFET, when active allows current to flow either way drain-source. While it's possible to address the intrinsic diode conduction with two MOSFETs back to back, is there a way to accurately emulate a rectifier using MOSFETs? The other option could be to use a SCR (I'm also hoping to gate the rectifier from a uC) but I would like to have a lower power dissipation/loss if possible |
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mackoffgrid Guru Joined: 13/03/2017 Location: AustraliaPosts: 460 |
MOSFETs are used in active Active Rectification. Solar Mike has used it in at least one of his projects. |
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| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
Yes, but they still, ordinarily, unless configured in a special way (which is what I'm hoping to figure out), conduct both ways. Normally not an issue unless, for example, say a boost convertor where the "output" has a higher potential than "input" in which case the system will try and feed power back to "input" In my specific case, I want to connect two solar arrays in parallel to each other as well as the grid, where none of them should be feeding power to another or all my electronics projects would be cancelled for the foreseeable future  |
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| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
As a back up plan, I have some RHRG3040 on the way. I thought they were the closest to what I need to rectify the grid with their 400V, 30A nominal rating and they can surge upto 70A Their max Vf = 2.1V but I hope to see half that at our grid level. Even then, for a full bridge, that's a 2v OR over 20W+ power dissipated! Will find out how they hold up in a few weeks time |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
I would have thought ORing the output of the two pv arrays + mains supply via high power diodes would work ok. Providing the PV voltage when loaded is somewhat higher than the mains, power will be taken from the pv until their voltage drops due to high loading or low light, where then the mains takes over in a seamless fashion. If you replace the diodes with active mosfet rectifiers to minimize losses (not warranted here at these low currents?), using a dedicated synchronous driver chip, should still work the same; the chip senses reverse voltages across the mosfet and turns them on\off automatically, where the mosfet substrate diode then blocks reverse voltages as per a normal diode. Mike |
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| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
Mike, what a brilliant idea! I did not know synchronous driver chips existed. How complicated are they to use and are there a few you like to use? This is turning out to be great! ps: any thoughts about the RHRG3040? Maybe by choosing hyperfast (so I could reuse them for dc CONVERTOR PROJECTS) I shot myself in the foot with higher Vf |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
Google IR11672, they are the ones I currently use. That RHRG3040 has quite a high forward voltage drop, why such a fast diode, standard slow diode would have been fine and with a lower voltage drop. Cheers Mike |
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| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
Thank you Mike. Will study it. Andew mentioned you have some designs using it (or similar)? Are they on this forum? Because I didn't know any better and my searches on Mouser and Digikey kept returning SiC diodes. If you have some recommendations (again) that can handle atleast 20A, 300V, I will be much obliged! |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
MOSFET's are much more suitable as a rectifier, when there is more then an amp or so flowing. They are incredibly efficient when used like this, and a tiny MOSFET can pass more current then a simple series diode, AND not get hot and bothered by it.  And then there is the practically zero voltage drop across a MOSFET used as a rectifier vs a diode used as a rectifier. See this excellent video comparing the two methods. Smoke makes things work. When the smoke gets out, it stops! |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
May be easier to get 3 phase rectifier or high power bridge diodes on EBay or Aliexpress at a reasonable cost, single high power diodes seem hard to locate; and just use 1/2 for 3 dc inputs. 3 Phase Rect Cheers Mike |
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| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
Thank you Grogster, Mike! |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Had a look at the video. Think the only pluses are that you get reverse polarity protection and when you drop below the gate turn-on voltage it reverts to a diode. Above the gate turn-on voltage it is not a diode. Around the gate turn-on voltage region it is operating in its "linear region" so watch out for power dissipation issues. The manual for my Electrodacus SBMS-060 says it has ideal diodes so had a closer look. He uses a LM5050-1 to drive a FDTM80060DC NFET. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
I just use a 100 amp bridge rectifier. Not too concerned about the power loss, its going to be less than one percent for a 100v system. But the heat generated by this bridge rectifier is the thing I do not like about it, it runs continuously all day and it requires a surprisingly large heat sink. Still its simple with a lot less to go wrong, and its just one more argument for building a higher voltage lower current system. Cheers, Tony. |
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| LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408 |
I'm not worried about the power loss either but exactly as you said, the heat generated by even a 1V drop at 20+A is severe, specially when it's outside where it could be over 100 F and that seriously worries me. These diodes are the backbone of the whole system and if they fail, there is no source of power to work off of so I want to minimize the chance of failure of this component. The IR11672 helps me trade this concern for complexity. More to learn, read and experiment. PS: I have some 10A10 rectifier diodes on the way. One per array and 3 in parallel to the grid. My assumption is that my grid needs will be minimal, so the bunch of 3 will see very little current through them. Only time will tell! I will start off with the 10A10 and then move onto IR11672 if needed |
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