Like most power MOSFETs the IRF640 (and IRF740) require a fairly high gate voltage to fully turn on. The Rds(on) spec and all the test circuits for both assumes a Vgs of 10v.
A better choice would be a device like the IRLB8721. These are optimized for logic-level gate voltages and achieve a very low Rds(on) by 4.5v. https://cdn-shop.adafruit.com/datasheet ... 721pbf.pdf
Using a MOSFET driver
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