Paralleling two MOSFETS to handle more current.
photomankc
Posts: 943
So I am working on a soft-switch for my robot project. It involves a PICAXE and a P-Chan MOSFET used to switch the high-side of my main power supply based on a user button press or the RaspberryPi sending a power-down signal. I would like to have plenty of head-room in current capacity and I have two options on hand:
- One is giant-sized SMT MOSFETs that are rated to 25A or something stupid high like that. I don't think I could reasonably even make traces to handle that but it would run most any project I have without a sweat.
- The other is an SOT-23 package that is rated to 3.5A peak.
What I thought would be an ideal match is to parallel two of the smaller ones for 7A combined and that would be 75% headroom over what my peak draw is right now.
My question is.... is it really that simple? Can you just parallel the MOSFETs and drive the two gates the same? I know often times that does not work out as intended as one component is ever so slightly different and ends up taking on all the load, fails and then the next in line repeats it's demise. Reading some posting seems to suggest that this is not the case with MOSFETs but I wanted to ask directly before I start working up a PCB.
- One is giant-sized SMT MOSFETs that are rated to 25A or something stupid high like that. I don't think I could reasonably even make traces to handle that but it would run most any project I have without a sweat.
- The other is an SOT-23 package that is rated to 3.5A peak.
What I thought would be an ideal match is to parallel two of the smaller ones for 7A combined and that would be 75% headroom over what my peak draw is right now.
My question is.... is it really that simple? Can you just parallel the MOSFETs and drive the two gates the same? I know often times that does not work out as intended as one component is ever so slightly different and ends up taking on all the load, fails and then the next in line repeats it's demise. Reading some posting seems to suggest that this is not the case with MOSFETs but I wanted to ask directly before I start working up a PCB.
Comments
Can you use an IRF3708? They're rated at 62 Amps. They are N-channel, however. Does that make a difference for your application?
http://www.digikey.com/product-search/en?x=15&y=12&lang=en&site=us&KeyWords=irf3708
And, instead of traces, could you just wire the heavy current lines?
About the only reason you might see the practice disappear is that MOSfet have continued to grow larger and larger in capacity and what once required 5 in parallel can now be done by one.
http://www.mouser.com/Semiconductors/Discrete-Semiconductors/Transistors/MOSFET/_/N-ax1sf?P=1z0jobfZ1yzoflnZ1yzv5fzZ1z0j3zpZ1z0iw8zZ1yzuz45Z1z0j40aZ1yzuz33Z1yzuz1sZ1yzrbixZ1z0iwq9Z1z0iw8tZ1yzuyi2Z1z0j3zuZ1z0iw94Z1z0j40gZ1yzv5geZ1z0j40lZ1z0jobkZ1z0y3dtZ1yzrsvvZ1yzvps3Z1yzv6f0Z1yzv6ezZ1z0w5gtZ1yzxj3qZ1yzn6neZ1z0w1xcZ1yzvtbuZ1z0w63oZ1yzvtbdZ1yzn6nwZ1z0w9xnZ1z0w0pcZ1yzro8wZ1yzt5o2Z1yzuaysZ1yzv6f1Z1yzxlnu&Keyword=mosfet+p+channel&FS=True
I strongly prefer to switch the high side for this application. I don't like to have a circuit (robot that I'm playing with) that is live just waiting for an accidental ground to be found when it's supposed to be off. I don't think there is any other compelling reason beyond that and the fact I prefer it conceptually.
I can go on and use the bigger part I guess. Might not save a whole ton of real-estate vs the tracing out parallel SOT-23s anyway. No plated via's in Kyle's home PCB lab. 25A is plenty for this application.
Pretty much that simple, but you should also do an inrush and thermal budget, and often that selects the device more than rated currents.
For Thermal you use i * i * R powers, to check what temperature rise you can expect.
Quite often designs use FETS rated on paper > 10x current, to get the heat under control.
Inrush budgets involve asking "Who will be the fuse?" and checking startup/surge/stall current levels, which can be significant on any Motor load.
There are high side Switch devices out there, that include a FET ( P, or N+Charge Pump) and thermal and current sense elements.
Sometimes we do funny things, though. I recall an Aussie ham radio operator who wanted a power amplifier for his 6M transmitter (50MHz). He boldly went forth and paralleled a bunch of commodity MOSFETS of the sort one might find at Radio Shack. Worked great. IIRC, he ended up with a power output around 250 W.
Edit: I found the article here. He used IRF510 MOSFETs.
You would never run a MOSFET at its rated current continuously unless prepared to spend money on big heatsink and fan or liquid cooling,
the max current rating is almost invariably the abs max power rating expressed as a current. Peak loads that really are occasional peaks are
OK. I've seen 25A MOSFETs that can handle 25A if dissipating 100W - not my idea of a sensible way to do things though!
Always use the Rds(on) to calculate the heat dissipation for your load current and decide that way - you need to get a handle on how much
heat the package can dissipate with various heat-sinking options and take that into account too. And then decide if paralleling up devices
is worthwhile (doubling up halves overall power dissipation, reduces package dissipation by 4). Sometimes just getting the right device
is simpler (A nice 3 milliohm Rds(on) FET can solve a lot of problems, but it still won't handle 25A without a heatsink - typically such devices are
rated for 170A, but that's 90W of dssipation!)
Don't go for devices with over-generous voltage ratings (Vds), since they have much higher Rds(on) for the same money. Pick Vds about
twice your supply for a good safety margin but no more.
[ edit: take the mighty IRFS3107-7P, rated at 2.1 milliohm and 260A - yet AFAICT the bonding leads to the chip melt at 240A - the datasheet calls
this "package limited" !! ]
[edit: to answer the original question - so long as you parallel devices of the same part number there shouldn't be issues]
As has been noted, 25 amps is not a large current for a single power rated mosfet, so you certainly don't need to go multiple devices.
If you do want to use multiple smaller mosfets then there is a little trick with the gate control that should be observed. The turn-on threshold voltage is a little different for each mosfet. If the gates are all shorted together, just like the sources and drains are, then the one with the earliest threshold will take the higher initial switching load.
So to keep the switching currents more evenly distributed it's advised to use a low value series resistor on each gate. Basically the same as what is done on a single mosfet but having a resistor on each individual mosfet gate.
I know that you can not operate at the absolute max current on anything and expect longevity but was not aware that 7 to 10 times could be needed. I thought 2 to 3 would be reasonable. I was thinking that the 25A would be good for 5A to maybe to 8A without extraordinary measures. However, reality is the RaspberryPi has a nominal 500 to 700mA draw and I can't see any extra's I'd add topping double that so I think the 25A I have on hand would handle that with little heating. Then the main power switch-on could trigger relays to power up the seperate H-bridges for the motors.
You need to also consider thermal budgets. Some numbers :
If we take a '25A' fet as being a nominal 80 milli-ohms, these are the power points
8A => 8*8*.08 = 5.12W << serious heatsink needed, and ~640mV of DC drop may be a problem
3A => 3*3*.08 = 0.72W << Still needs large PCB copper, but might save a heatsink.
2A => 2*2*.08 = 0.32W << getting down to SMD parts, without excess PCB area
So you can see why a designer might choose a '25A' FET, but run it at under 2A
This is the large guy I have on hand:
http://www.fairchildsemi.com/ds/ND/NDP6020P.pdf
Vds -20V
Vgs +/-8V
Ids (cont) -24A
Ids (pulse) -70A
Rds @ -4.5V 0.050
So following your calculations:
1A ==> 1 * 1 * 0.05 = 0.05W Easy
2A ==> 2 * 2 * 0.05 = 0.20W Ok
4A ==> 4 * 4 * 0.05 = 0.80W Harder
8A ==> 8 * 8 * 0.05 = 3.20W Hard
12A => 12 * 12 * 0.05 = 7.20W Holy Smile!
I'm working up the Schematic now.
Sir you said that for 5.12W, there will be 640mv voltage drop. Can you explain me , is any mathematical calculation behind that voltage drop.
Yes, the "mathematical calculation" is based on Ohm's Law, and is the 8A*8A*.08Ω I wrote above.
More can be found here
https://www.evilmadscientist.com/2012/basics-power-dissipation-and-electronic-components/
You can use a 2N3055 transistor and a heat sink along with a few additional components to build a simple adjustable constant current circuit that makes a good adjustable load. Got one kicking around somewhere that I have used occasionally for many years.