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| Circuit for MosFets in parallel for extra current capacity.(Solved) |
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| spec:
--- Quote from: Jim-0000 on December 30, 2018, 01:36:51 pm ---I will attach a picture of a smaller rating Mosfet switch I have made to fit to another type of aircraft. This plane will be internal combustion engine powered, but will of course have radio control. The radio receivers and servos require 6 volts in this application and I have chosen to use an electronic switch instead of a common mechanical one for reliability reasons. I will also fit this type of switch to my unpowered slope soaring gliders. Some of the higher performance slopers use high power demand servos, so a good reliable supply is essential. This design is by Vollradth and was posted in the link above. --- End quote --- Interesting. That circuit agrees with my calculations which show that 25A per MOSFET is a practical limit. This means that your 100A version would need four parallel MOSFETs to get a good solid reliable design. But if you used some really beefy MOSFETs, the heatsinking requirements would be small. Time allowing, I hope to post a High Side Switch PMOSFET design using four £3UK PMOSFETs, which will hardly need any heatsinking. |
| Jim-0000:
Hi Spec, Thanks for providing the three draft circuits. However, judging by the way the circuitry is arranged, I fear that I have still no described my application and where about in the model power system wiring, this planned switch will go. The motor is 3 phase. The ESC is an electronic commutator. There is no direct connection from battery + or - to the motor. The 3 connections from the ESC go to the motor. The two wires from the battery go to the ESC. The attached diagram might clarify this. The switch will go between the battery and the ESC. It will not go between the ESC and the motor. Edit: The current from the battery to the ESC is DC. While the current from the ESC to the brushless motor is 3 phase AC! . |
| Jim-0000:
This is an oscilloscope display of the waveform to a typical 3 phase brushless motor. The first is at partial throttle. The second at full throttle. I hope this clears up some confusion. Jim. |
| spec:
Thanks Jim-0000 for the information in replies #26 and #27. Yes, that does explain the situation well and means that the only option is high side switching, either with NMOSFETs or PMOSFETs. I would recommend a high side PMOSFET design with beefy PMOSFETs which would need minimum heat sinking: a number of suitable PMOSFETs are available and at a reasonable price too. What do you think? The circuit that I have in mind would be configurable, where every PMOSFET handles 25A, so you just add PMOSFETs to suit the application: 1 x PMOSFET = 25A switching capability, 2x=50A, 3x=75A, 4x=100A, and so on. If you are interested in the theory, it is the RDSS of a MOSFET, when it is turned on hard, that ultimately dictates the current handling capability of a MOSFET. The formula for the power generated in a MOSFET is ID2 * RDSS. Where ID is the drain/source current and RDSS is the resistance between the drain and source (RDSS is often just labeled RDS on data sheets). A beefy PMOSFET can have an RDSS as low as 0.01 Ohms (10 mili Ohms), so at 25A ID the power dissipation would be 252 * 0.01 = 6.25 Watts per PMOSFET. Multiply by four PMOSFETs gives a total power dissipation of 25W, which I suggest would be manageable in your model aircraft. If you are wondering why four beefy PMOSFETs are required to provide a 100A switching capability, rather than just one beefy PMOSFET, here is the answer: 1002 * 0.01 = 100 Watts. |
| Jim-0000:
--- Quote from: spec on December 31, 2018, 06:52:08 am ---Thanks Jim-0000 for the information in replies #26 and #27. Yes, that does explain the situation well and means that the only option is high side switching, either with NMOSFETs or PMOSFETs. I would recommend a high side PMOSFET design with beefy PMOSFETs which would need minimum heat sinking: a number of suitable PMOSFETs are available and at a reasonable price too. What do you think?........... --- End quote --- Spec, I will have to do some reading up on the definitions of the terms your use above. --- Quote ---The circuit that I have in mind would be configurable, where every PMOSFET handles 25A, so you just add PMOSFETs to suit the application: 1 x PMOSFET = 25A switching capability, 2x=50A, 3x=75A, 4x=100A, and so on. If you are interested in the theory, it is the RDSS of a MOSFET, when it is turned on hard, that ultimately dictates the current handling capability of a MOSFET. The formula for the power generated in a MOSFET is ID2 * RDSS. Where ID is the drain/source current and RDSS is the resistance between the drain and source (RDSS is often just labeled RDS on data sheets). A beefy PMOSFET can have an RDSS as low as 0.01 Ohms (10 mili Ohms), so at 25A ID the power dissipation would be 252 * 0.01 = 6.25 Watts per PMOSFET. Multiply by four PMOSFETs gives a total power dissipation of 25W, which I suggest would be manageable in your model aircraft. If you are wondering why four beefy PMOSFETs are required to provide a 100A switching capability, rather than just one beefy PMOSFET, here is the answer: 1002 * 0.01 = 100 Watts. --- End quote --- Yes, I understand that. Thanks. Leave it with me for a few days to do some catch up reading. |
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