Since I fried the speed controller portion of the receiver module on my Super Cub, I figured I would see if I could fix it first before I just buy a whole new unit. There’s a single TO-220 package FET on the board which is marked with the numbers 07N03L. Doing a Google search on that number reveals a part made by a company called Infinion for which the datasheet is readily available. You can download it here. So let’s just see what this little IC is made of, and maybe we can figure out why we cooked it and what we can replace it with.
Vds = 30V: Drain-to-source voltage. This is good, it means a lipo 11.1V pack alone isn’t going to burn up the chip.
Rds = 6.6mohms: Drain-to-source resistance is 0.0066 ohms. This is a decent number, but for real high-current applications you like to see one more zero in there. It’s still lower than those mini Tamiya plugs. =)
Id = 50A: Drain current is 50 amps. So by design the chip can carry 50A. That may seem good, but it doesn’t mean it can drive a motor that draws 50A, because there’s still power dissipation and heat to worry about. The manufacturer assumes the user is going to allow for adeqaute power dissipation by heat sinking the case and thereby maintaing a reliable operating temperature. So it’s up to the end user how many amps the device can ultimately carry. In the case of the Hobbyzone unit, there is no heat sink at all. And there is no airflow. These two factors alone are enough to drop the actual current rating to at most 25% of the rated current. Given that derating factor, we might expect to get a usable 12-13 amps out of the ESC. I’ve seen other data sheets claim their parts can operate at 50% their rating current without heat sinking, but it’s still pushing the thermal limits of the device, which will ultimately lead to failure.
International Rectifier make a whole line of HEXFET Power MOSFETs which are designed to be used in motor-control applications (in automobiles for example) requiring extremely high current and/or voltage capabilities. In a TO-220AB package they make a part that can carry a whopping 260A. That’s the first part I’ll be picking up. There’s an ongoing thread over at RCUniverse by a guy named khalsans who is extremely knowledgable with respect to electronics, specifically he’s done a lot of investigation into the workings of these receiver/ESC units. He replaced this very same FET in his plane with a IRL3803 MOSFET from International Recitifier. He popped a heat sink on there and says the unit works much better and is able to handle the load of the bigger motors they’re using. That part is rated for 140A continous and Digikey has it for $3.90. But, they have several more units with even higher current-carrying capability but otherwise whose specs are nearly identical that I’d like to try out. One of them being the IRL3713 which is a 30V N-Channel MOSFET with a 260A Id rating. I should be able to get away without using a heat sink and not reaching the thermal limits of the part. It’s most likely overkill, which is why I’m going to buy a few others as well that range from 140A to 210A. The on-resistance is only 0.003 ohms too, which is good, it means more voltage at the motor compared with the original FET which was twice as high.
While I had my receiver/ESC apart I thought I’d better measure the gate voltage from the control chip on the PCB to see if it fits within the specs on the new Power MOSFET I was picking out. I measured a analog increase from 0V to 4.4V while throttling up on the transmitter. Half throttle measured 2.50V. 1/4 throttle measured about .7V. Of course this signal is pulsed, so it’s not pure DC except at full throttle. While the DMM reads 2.2V for example, it just means we’re looking at a square wave with 4.4V peaks and about 50% duty cycle. The stock FET was rated at min=1.2V and max 2.0V. The IRL3713 is rated at 1V and 2.5V for min and max. It looks like it should be a good replacement, since it has the low turn-on threshold that we need. And it only costs $2.70 and I only have to buy 1. Thanks Digikey! But no thanks on the $5 handling fee for orders under $25. That always happens to me, and I can never find enough parts to buy at once to not just pay the $5. Of course a week later I find something I need and by then I just end up paying the $5, again. But it’s better than having to buy 100 parts just to meet the minimum buy, right?
A few hours later – Order has been placed with Digikey. I bought 5 different FETs including two made by Infineon that appear to be exact replacements to the original one, but with twice the current and power dissipation. The 100A one was only $1.86, so what could it hurt to try out? Also at the recommendation of khalsans I picked up some 1N4005 diodes to replace the shunt diode across the motor. And lastly I bought 10 0.1uF caps for 19 cents each. A couple more of those on the motor might clean up my radio glitching problems. Now I just need to find a good 3S lipo for cheap…