This question came recently on my forum so I decided to answer it with a blog post.
Depending on what device you position on the board, yes, you could in theory reach 70A, as proclaimed by the device vendor. However, lets be realistic. First, this board will not be able to get you to 70A for very long as it is rather small and like you said it does not have the heat sinking needed for such a heinous application. But imagine the board was in fact beefy enough to get you there. Imagine there is a terillion ounces of copper and the board is the size of a football field, still reaching 70A, as well as for how long can you maintain such a current, will be a matter of the laws of physics and in this case, I can quickly summon a nasty equation such as P = I^2*R.
This is an horrendous equation for any device on this planet trying to do 70A worth of continuous current, as regardless of the resistance, we are dealing with a 4900 term (70A * 70A). If the resistance was 1 Ohm, you would be dealing with enough power to cook a meal. Of course, the BTN devices do not have such high resistances. And in fact, the idea behind any motor controller is to have as low a resistance as possible, and in this case, the BTN7970 datasheet tells us that the worst RDSon you can see is about 35.5 milli ohms.
If we now compute the power dissipation on our BTN7970 device running at 70A continuous, it comes up to
4900A^2 * 0.0355Ohms = 149.45W.
This is the amount of power you can use to illuminate a room. Actually, 66% of this is more than plenty to quite illuminate your room. And if you have been brilliant enough to try and exchange a 100W bulb after it has been on for even a few minutes, there is no doubt your fingers are remembering it. 100W is not a trivial amount of power and there is no chip on this planet that can withstand this humongous power dissipation, much less 150W. Do note this is not the amount of power on your motor. Oh no! This is the amount of power that you are miserably wasting in the form of heat at the device’s power elements, which in this case are the two power FETs.
But if a light bulb can take 150W of power, and even much more depending on the bulb, why can’t my device take it? Surely the BTN7970 must be robust enough to take this amount of power and smile about it, right? Well… I don’t think so!
As soon as you hit the 70A mark your die will start to heat up. Now, if we were able to cool the die at the same rate at which the device is getting hot, then yes, you would be able to do 70A of continuous current. However, cooling a device that is generating 150W is not a trivial endeavor. This takes us to thermal impedance, which is a measurement of how easy it is too cool down a device. Or in better words, a measurement of how easy it is for the generated heat to be iradiated away from the heating up device. If all the heat goes away, the device is maintained cool. If heat can not flee away, then it builds up and temperature increases.
I am going to take a number out of my sleeve and say that a board like the AE-7960 has a thermal impedance of about 35C/W. This is based on experience, the fact that the board is 2 layers and because of its size. Add to that quite the amount of wishful thinking, and it takes us to 35 C/W, whereas it can easily be 40 C/W as well. What this number tells us is that at 150W, the device will reach a whooping temperature of 150W * 35C/W = 5250C on top of the ambient, which is usually 25C.
This is of course a big lie as if the device were allowed to run free, the board would burst into flames (or if we are lucky, explode) way long before we get close to a paltry 500C. Of course, the BTN7970 is thermally protected at about 200C, so we don’t get to see cool high current induced AE-7960 based fireworks.
With this information we can now deduce a whole lot of cool conclusions. Or hot, since we are dealing with quite the temperature…
Since we know the maximum temperature the die will tolerate is 200C, we can find the maximum power this board would allow.
P = (200C – 25C)/40C/W = 4.375W
I have chosen 40C/W thermal impedance as I think that is much more realistic of the actual AE-7960 board. Notice I have also decreased the temperature by 25C as the ambient temperature is already there and I can not ignore it. If it was freezing out here then I could assume the 200C temperature rise are allotted to the power device. But since the sun already warmed us up 25C, then all I have left are 175C worth of heating up.
With 4.375W worth of power, what current can I do? We can invert the equation and determine that
I = SQRT(P/I) = SQRT (4.375W/0.035Ohms) = 11.18A.
So, to answer your question, the AE-MDL-7960 board should be able to drive 10A continuous current. This is only true if in fact the Thermal Impedance is 40C/W. If it is more than that, then the current will not be able to be so high. You can always decrease thermal impedance by adding heat sinking or air flow. This board was not designed to accept too much heat sinking, so you may need to get creative. It is an Open Source design so feel free to let us know how to improve it!
But I like to go on and analyze how on Earth can I reach the 70A mark. After all, isn’t this device rated to such currents? That’s what the datasheet says…
Lets assume that in fact I have a continuous 70A load. Sounds like a rather large vehicle of some sort, to be honest, as even wheelchairs and scooters are in the 30A range. If so, then we already know the power is about 150W and the temperature can not be more than 200C, so what thermal impedance would we need?
Theta JA = 200C/150W = 1.33C/W
Unfortunately I am not aware of any cooling system that can give you 1.33C/W other than possibly liquid nitrogen. So if 70A is pretty much unreachable, why is the datasheet selling it as such? The truth of the matter is that the datasheet is never claiming you can go to 70A and stay there forever. What this device can do is take you to 70A for some finite period of time (milliseconds if at all) and then return to a much more realistic current such as 20A or 30A and operate without too much heating, as long as the design has a good enough thermal impedance.
It may then seem like the 70A rating is misleading. However, the truth is that on a large motor application, transients up to 70A are not unheard of. For example, when the motor is starting up it will require a large amount of energy to kick out of its sleeping frenzy and break out of its standing still inertia. The larger the load, the worst it gets. But once the motor is moving, current does not need to be too large. So a 70A capable driver is needed even on an application which will operate at much lesser currents.
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