Hi,
I agree with
@Audioguru.
The datasheet says: V_GS_TH = 2...4V for a
tiny current of 250uA.
This is not considered as ON state for a switch.
@danadakk gives good information about: the behviour between a load and switch.
But - in opposite to a regulator - a "switch" always is overdriven to ensure low ohmic Drain-Source even at worst case condition. (temperature, production tolerance... )
No good designer will drive the IRF450 as a "switch" with only V_GS = 4V... and using 3V makes no sense at all. ... even if the load current is below 0.001A
(it also makes no sense to use a 13A rated MOSFET for only 0.001A load current)
Klaus
Hi Klaus
Do agree with your suggested 4V. But for 10Vgs indicated by Audioguru, I hesitates. Perhaps for consumer electronics, 2-3 year lifetime, it works. But for automotive MOSFET, the concern is raised on its durability. You might find the identical MOSFET in said manufacturer automotive catalogue but the gate rating is degraded to 8V, 6V or even lower.
No offensive, I remember those old time when IRF CEO Alex visit his customers and raise MOSFET price by 30%. You know, everyone try to design out IRF parts. IRF played very similar game in its device inside as well. Its datasheet claims logic level but its gate oxide is only 26 nm. Its marketing manager thought the customer is more likely to find the device wear out rather than do a FIB SEM inspection to diagnose the root cause failure (GOX breakdown).
Today, a well know electric drill does not last for 5 year, because they asked the manufacturers to control the lifetime to 2 year. For cost down, the manufacturer used Schottky -MOSFET instead. As we all know that Schottky body diode trade off lds leakage for Rdson and Trr. The consequence is the extra power consumption in the standby mode, and the degradation of the gate oxide. A end user wont recognize it leads to rapid discharged battery until the drill wear out in a few months when the 2 year warranty period expired.
Please check the datasheet of said electric drill MOSFET, its
250uA or 100uA current rate does not exceed. But, is the MOSFET really switched on at 0.25mA. One should define the threshold voltage at the peak of transconductance. In the datasheet, it should be termed as the current density. But, there is no die size mentioned on datasheet, as related to the cost. On the other hand, 250uA Ids is predominately dependent on test and die size. The person who manipulate the datasheet is unlikely to know where and how the 250uA comes from. Should you be interested, you could look into a MOSFET device and try to measure its Ids from active cell, edge termination, and perhaps the leakage current (pA range). The edge termination does also contribute to Ids in particular for a a large die. Please try to completely turn off the device, perhaps by applying a negative voltage on its gate, you might still observe the Ids. It is mainly contributed by the edge. You could find the drain terminal on the die front surface. The leakage current is mainly attributed by the insulate material. Before TI acquire cyclon, its lateral power MOSFET is highly leaking because of poor gate oxidation process.
You are absolutely right, that temperature does affects the threshold voltage. There is no industry standard in the datasheet threshold voltage test. Each manufacture define its own way as to sale its MOSFET. You might teardown the MOSFET in apple iphone has no datasheet. The physical mechanism of the temperature affected Vgst is studied by few companies. Most manufacturers do not will to invest to know how but for shrink the pitch for cost. Most datasheets in market were written by marketing guy who knows nothing about the Vgst, temperature, process variation, DOE, 6 sigma, etc.
For example,
1. you could get the sample from 10 suppliers for the same Vds and Ids rated MOSFET, and please test its BVdss, you might find the 5%-10% higher BVdss measured on US or EU suppliers. 10% for Asian players, Hitachi / Panasonic was 30%. You might observe their test method varies, in some case varies a lot.
2. measure the 250uA Vgst in said 10 devices, you might get Vgst close to 2.4-2.8 for well known players. But their Rdson curves varies a lot.
3. Compare the 75degC and 25degC Vgst curve, you might find significant discrepancies with the datasheet for at least 7 suppliers.
You know much more than in the circuit /PCB design, and certainly I was teaching grandma how to suck eggs. However, there is something, e.g. physics, cannot be bended. It is possible to get low Rdson, Cgd, Trr, or two of them (FOM - Figure of Merit), but not all of them.