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I have a 1 kW three-phase BLDC motor from China, and I was developing the controller myself. At 48 Vdc, the maximum current should be about 25 Amps and a peak current of 50 Amps for short durations.

However when I researched BLDC motor controllers, I came across 24-device MOSFET controllers which have four IRFB3607 MOSFETs per phase (4 x 6 = 24).

The IRFB3607 has an Id of 82 Amps at 25 °C and 56 Amps at 100 C. I can't figure out why controllers will be designed with four times the rated current. Keep in mind that these are cheap Chinese controllers.

Any ideas?

You can see the controllers here, if you need any part of the video translated, please let me know.

https://www.youtube.com/watch?v=UDOFXAwm8_w https://www.youtube.com/watch?v=FuLFIM2Os0o https://www.youtube.com/watch?v=ZeDIAwbQwoQ

Considering heat dissipation, these devices would be operating at 15kHz so about half of the loss would be switching loss.

Keep in mind that these are $25 chinese controllers and each mosfet would cost then about $0.25. I don't think these people care a lot about efficiency or quality. These controllers are warrantied for 6 months to 1 yr max.

BTW in the lay language of the users, Mosfets are called MOS-Tubes. Hence tubes.

Sujoy Bhattacharya
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    You should include a link to an example of mentioned BLDC controller. – Bimpelrekkie Feb 20 '19 at 10:58
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    Mosfets in parallel will reduce the effective Rds_on. Lower power dissipation in the controller and better efficiency. – Peter Karlsen Feb 20 '19 at 11:28
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    _"24 tube Mosfet controllers"_ Tube? – winny Feb 20 '19 at 11:41
  • Stall current is also likely to be about 10x rated current or about 250A. 4 * 82A per phase sounds quite reasonable. –  Feb 20 '19 at 22:07
  • Consider how many MOSFETs are on a typical PC motherboard VRM. A high-end desktop board designed to cope with a heavily-overclocked 16+ core processor pulling upwards of 500W will have eight *high-end* MOSFETs *at minimum*, and possibly 12 to 16. When you look at it this way, a motor that can pull nearly 1 kW continuously needs similarly beefy power delivery. – bwDraco Feb 21 '19 at 02:43

2 Answers2

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The reason to use multiple MOSFETs is to lower power dissipation resulting in a cheaper design.

Yes one MOSFET can handle the current but it will dissipate some power as it does have some resistance, typically 9 mohm for the IRFB3607.

At 25 A that means 25 A * 9 m ohm = 225 mV drop

At 25 A that means 25 A * 225 mV = 5.625 W of power dissipation

A heatsink for that would need to be substantial.

Now let's do the same calculation for 4 IRFB3607 in parallel:

Now 9 mohm is divided by 4 because of 4 parallel devices:

9 m ohm / 4 = 2.25 mohm

At 25 A that means 25 A * 2.25 m ohm = 56.25 mV drop

At 25 A that means 25 A * 56.25 mV = 1.41 W of power dissipation

That 1.41 W is for all MOSFETs together so less than 0.4 W per MOSFET which they can handle easily without any extra cooling.

Above calculation does not take into account that the 9 mohm Rdson will increase when the MOSFETs heat up. That makes the single MOSFET solution even more problematic as an even larger heatsink is required. The 4 MOSFET solution might "just manage" as it still has some margin (the 0.4 W could increase to 1 W and that would still be OK).

If 3 MOSFETs are cheaper than one heatsink (for dissipating 6 Watt) then the 4 MOSFET solution is cheaper.

Also production costs might be slightly lower for placing 4 MOSFETS compared to 1 MOSFET + Heatsink as the MOSFET has to be screwed or clamped to the heatsink, that's manual work so adds cost.

An added benefit is that reliability becomes better as those 4 MOSFETs are by far not "worked" as hard as a the single MOSFET.

Could we use a "4x" bigger, 2.25 mohm MOSFET?

Sure, if you can find it ! 9 mohm is quite low already. It gets increasingly difficult (and more expensive) to get lower as the influence of bonding wires comes into play. Also for sure four "middle of the road" MOSFETs are cheaper than one big fat MOSFET.

Bimpelrekkie
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    Also a saving on the cost of electricity over the lifetime of the system. – Ian Ringrose Feb 20 '19 at 13:22
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    @IanRingrose I doubt the designer cares much about that because they don't pay the electricity bill – Chris H Feb 20 '19 at 13:37
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    You also get more passive cooling from having the power dissipated spread over a larger area (4 parts and their required board space) – W5VO Feb 20 '19 at 14:43
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    @ChrisH but buyer pays electricity bill, and designer cares about his design to sell well. Or at least should care... – Mołot Feb 20 '19 at 14:49
  • @Mołot and how many end-users (even industrial ones) care about a few W of inefficiency? – Chris H Feb 20 '19 at 14:53
  • That increase in Rds(on) with device temperature is a really nice trap, and in general mosfet front page of datasheet ratings should be treated with a huge amount of suspicion (Memember, Marketing get to drool on the front page of datasheets, Engineering seldom has much to do with that part!), do not ever take these at face value. Go to the graphs for the real deal, see figure 4 of the datasheet which says to me that at 100C case you are looking at more like 14mOhm and the best part of 9W dissipation at 25A for a single device. – Dan Mills Feb 20 '19 at 15:18
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    @ChrisH going "green" and educing carbon footprint is fashionable now, so marketing departments of such companies are more and more interested indeed - even if percentage is quite low, it increases. Similar for private users. Don't have any statistics. From my point of view this trend is visible, even if it's negligible overall. – Mołot Feb 20 '19 at 15:22
  • @Molot Over here the trend is not just visible, this climate church is going to be become a chapter in "Extraordinary popular delusions and the madness of crowds". – Unimportant Feb 20 '19 at 15:29
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    Energy efficiency is quite critical for battery powered devices, so even if the heatsink were cheaper there might be good reasons to do this approach. Also, it's a good thing that Rds(on) increases with temperature, so that the hottest mosfet carries the least current. This allows the mosfets to evenly balance their loads, and be placed safely in parallel. – jbay Feb 20 '19 at 16:00
  • Devices with around 1mOhm or less are now quite common (at 20 to 30V Vds) but admittedly they are *very* popular and therefore can be a bit difficult to get. – Peter Smith Feb 20 '19 at 17:06
  • @PeterSmith The low Vds makes it **much** easier to get such a low \$R_{ds,on}\$. For higher voltages a larger drain-source distance is needed to withstand the large voltage when the MOSFET is off. That increases \$R_{ds,on}\$. – Bimpelrekkie Feb 20 '19 at 17:13
  • @Bimpelrekkie I am aware of that :) – Peter Smith Feb 20 '19 at 17:41
  • re "influence of bonding wires comes into play" - to get low resitance and high current throughoutput, semiconducor devices already have multiple bonding wires in parallel. I suppose the "bonding wire issue" when aiming to get even lower rdson is ... not enough space on the die to put more wires.. I couldn't find any modern image right now, but [this shows it well](https://www.electronicdesign.com/power/mosfet-design-basics-you-need-know-part-1) (figure 6th) on old device, we can easily extrapolate.. I remember seeing images where there were tenths of wires in parallel coming out of the die. – quetzalcoatl Feb 21 '19 at 11:18
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    @quetzalcoatl That is exactly what I meant. As the \$R_{ds,on}\$ of the MOSFET itself decreases then the resistance of the bondwires becomes the dominant factor! To counter that indeed more are placed in parallel and that will also increase the current capability. But you can only place bondwires for which there is space. The law of diminishing returns (adding more becomes less effective) comes into play. – Bimpelrekkie Feb 21 '19 at 11:24
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For almost all electrical components, lifetime decreases exponentially with increasing temperature. This is especially true with capacitors, which are found in BLDC motor drivers to decrease electrical noise and high-current peaks.

Let's say that the controller with 4 FETs per phase increased in temperature by 10°C at the rated load. Assuming an ambient temperature of 30°C, the controller would be running at 40°C. At this temperature, even standard-temperature range aluminum electrolytic capacitors could last over 120,000 hours.

If the same controller were to be built with 1 FET per phase instead of 4, the resistance would increase by a factor of 4 and the I^2R losses would also increase by the same amount. With the same heat-sink, the controller would experience 4 times the heating above ambient. It would now be running at 70°C. This would cut the lifetime of the capacitors by around a factor of 10, and would also decrease the life of other components similarly. To counteract this, a larger heatsink would be required, and it would be cheaper (and smaller) to just use more FETs.

Thor Lancaster
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