Home  Articles Advanced Gearbox Tech II: Solid State Switching with MOSFETs
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Advanced Gearbox Tech II: Solid State Switching with MOSFETs |
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Written by Jay
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Tuesday, 10 May 2005 |
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Page 4 of 10 Which MOSFET?
Selecting the right MOSFET for airsoft is a critical step, and it is governed by matching the rated operating specifications of the MOSFET to the expected operating conditions of the mechbox. The specifications of value are maximum Voltage Drain To Source (Vds), maximum continuous Current Drain (Id), maximum Power Dissipation (Pd), and Resistance Drain to Source On (Rds(on)).
Maximum Voltage Drain to Source (Vds):
MOSFETs are extremely sensitive to overvoltage, requiring that the maximum Vds not be exceeded. Since AEG power sources rarely exceed 15 volts even on a full charge, a MOSFET with a Vds rating greater than 15V is required.
Maximum Continuous Current Drain (Id)
Upgraded AEGs often place a heavy current drain on the battery, and a proper MOSFET must be selected which is capable of switching the current. Typical AEGs can exhibit current drains up to about 20 amps, while upgraded AEGs can demand as much as 40 amps. When selecting for the proper Id, it is best to err on the side of caution and utilize a MOSFET with plenty of headroom in Id. It is also important to note that we are interested in the maximum continuous current rating of the MOSFET, not the pulsed rating (Idm), which is typically much much higher.
Maximum Power Dissipation (Pd)
While MOSFETs are highly efficient, there are still inevitable switching losses which must be dissipated by the MOSFET in the form of heat. The maximum Pd rating is the amount of power the MOSFET is designed to handle and properly dissipate without suffering failure, and it is closely related to the external packaging design of the MOSFET. Those which use larger packaging form factors, such as a TO-220AB, are typically able to handle higher power dissipation duties than those using the much more compact I-Pak packaging. Much like the Id rating of the MOSFET, it is best to have plenty of headroom in Pd. Drawing from the experiences of the Japanese airsoft community, it is recommended that a Pd rating of atleast 80 watts is selected. Since there is little space in the typical airsoft rifle for a MOSFET heatsink, it is also recommended that the largest packaging available is selected to help dissipate heat.
Resistance Drain to Source On (Rds(on))
Rds(on) is a measure of the resistance the MOSFET presents to the flowing current during the "on" state. The lower this number is the better, as it means the MOSFET allows more current through with less heat loss.
While there are several MOSFETs which satisfy the criteria above to different degrees, we decided to start with one which has already been tested by the Filipino airsoft community as discussed at this thread at FilAirsoft.com. The MOSFET chosen was the International Rectifier IRF3707:
| Attribute | Value | | Vds | 30V | | Id | 62A | | Pd | 87W | | Rds(on) Max | 12.5 m-ohm |
Full datasheet here: IRF3707 DataSheet
The IRF3707 by International Rectifier can cope with a Vds of 30 volts, well beyond all typical airsoft power sources. The rated continuous current handling capacity of 62 amps also provides plenty of headroom, while the rated 87W power dissipation is right at the recommended value. Lastly, the 12.5 milli-ohms of resistance is relatively small and should be sufficient for airsoft applications.
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