Isn't the Vbe for that part (at high CE currents) dangerously close to the available 1.7V signal? Seems like that would leave little room for error when selecting a base resistor.

I've used these on a 3V supply with 470 ohms base resistor no problems...

 

I've used these on a 3V supply with 470 ohms base resistor no problems...

Sure, no problems at 3V, but the thread starter says their 555 output is only providing 1.7V. Looking at the TIP121 datasheet, Vbe(sat) for ~2.3A is ~1.7V. That leaves essentially no room for a current limiting base resistor, and no margin for error/parts tolerances, etc.

https://www.onsemi.com/pub/Collateral/TIP120-D.PDF



If you're also suggesting other changes that will improve the 555 output, then that could work, but at 1.7V it looks dubious to me.

 

You could use a sziklai pair to get around both the Vbe and VCEsat problems.
https://www.electronics-notes.com/a...istor/sziklai-compound-complementary-pair.php

 

You could use a sziklai pair to get around both the Vbe and VCEsat problems.
https://www.electronics-notes.com/a...istor/sziklai-compound-complementary-pair.php

A Sziklai pair has its output transistor as an emitter-follower with a voltage loss of about 1.2V at 2.3A plus the saturation voltage loss of its input transistor. Almost half of the 3V supply will be the voltage loss.

 

A Sziklai pair has its output transistor as an emitter-follower with a voltage loss of about 1.2V at 2.3A plus the saturation voltage loss of its input transistor. Almost half of the 3V supply will be the voltage loss.

Here's a circuit from the link:

 

See

 

With low voltage sources, e.g., the 3.0 V the TS is using, the lower voltage drop of a logic-level mosfet can be an advantage. For example, an RDS(on) of 3 mΩ at Vgs = 2.5V will give only a 7 mV drop at 2.9A (25 mW). Moreover, the devices are small and don't generate as much heat as a BJT running at 2.9A.

And did you have a part number in mind when you calc'd those numbers? There's a lot more to calculate for, than just RDS(on) and I'd like to pull the datasheet.

 

And did you have a part number in mind when you calc'd those numbers? I just want to work through the thermal calc to see if the junction temperature will actually handle 2.3A.

At only 25mW (1/40th of a Watt) of dissipation, it would be hard to find a part that *can't* handle it!

 

Here's a real world example - very bread and butter, common, easy: IRL540

https://www.vishay.com/docs/91300/sihl540.pdf


Looking at the graph above, 2.3A is around 0.2Vds (assuming 2.5Vgs, as proposed in @jpanhalt 's example.) That would mean less than 1/2W dissipation, which is easy for a TO220 device.

I'm sure there are plenty of parts with better specs. This was literally the first logic level MOSFET I checked, and it would handle the load just fine with only 2.5V logic, even easier with 3 or 3.3V logic.

 

When minimal voltage drop is a priority, it's often easy to find a MOSFET with low enough Rds to outperform any BJT. For me at least, this comes up pretty often. It has nothing to do with fashion, popularity, or understanding what my options are. The gate properties aren't the only properties to consider when weighing options.

As for this particular project, it's challenging either way you go. If you want to go BJT, that's a lot of current, so you'll need a lot of base current... possibly so much that you'll need two stages (darlington, etc.) If you need a second stage anyway, might be better to use a mix of technologies, or two MOSFETs, one acting like a gate driver of sorts for the other.

*** EDIT:
While I was typing, @jpanhalt beat me to it with a more concise answer.

Disregard, more posts occurred while I was typing. Much appreciated to you both for part numbers. I'm using this more as an educational exercise for myself.

 

And did you have a part number in mind when you calc'd those numbers? There's a lot more to calculate for, than just RDS(on) and I'd like to pull the datasheet.

Here's one: https://www.infineon.com/dgdl/irf6201pbf.pdf?fileId=5546d462533600a4015355e4445919ce

Of course, I am assuming the TS is using a low-power (CMOS) version of the 555 to work at 3 V. The ICM7555 or CSS555 might be good candidates.

 

Here's a real world example - very bread and butter, common, easy: IRL540

https://www.vishay.com/docs/91300/sihl540.pdf

View attachment 181516
Looking at the graph above, 2.3A is around 0.2Vds (assuming 2.5Vgs, as proposed in @jpanhalt 's example.) That would mean less than 1/2W dissipation, which is easy for a TO220 device.

I'm sure there are plenty of parts with better specs. This was literally the first logic level MOSFET I checked, and it would handle the load just fine with only 2.5V logic, even easier with 3 or 3.3V logic.

Where did you come up with the TS wanting a Vds of 0.2? I couldn't find that (stupid bifocals)....

 

TS wanted 2.3A. There was no mention of Vds, but lower is generally better since it means less dissipation and greater efficiency.

Vds at any given gate-source voltage and drain-source current depends on MOSFET specs. In the case of the IRL540 example, given the 2.3A current requirement and the suggested 2.5V or better available for the gate (assuming a switch to CMOS 555 timer,) the 0.2Vds number comes from the IRL540 datasheet, not the TS.

The low Vds number is helpful, because it means much lower power dissipation. Losing 0.2V at 2.3A = 0.46W of dissipation. If you used a Darlington with typical 0.9Vce at 2.3A, you'd get 2.07W of dissipation. Not only is this less efficient, but it probably means needing a heat sink for the Darlington while the MOSFET at the same current is running fairly cool even without a heat sink.

 

This one looks promising. I’m sure it’s expensive. https://www.vishay.com/docs/74400/si8424db.pdf

Rds is 43mOhms at 1.5v Vgs. Can pass 10 amps

Actually only $0.60 at mouser https://www.mouser.com/ProductDetail/?qs=4zFPArRKzl4dDYY3GrU3Pg==

 

TS wanted 2.3A. There was no mention of Vds, but lower is generally better since it means less dissipation and greater efficiency.

Vds at any given gate-source voltage and drain-source current depends on MOSFET specs. In the case of the IRL540 example, given the 2.3A current requirement and the suggested 2.5V or better available for the gate (assuming a switch to CMOS 555 timer,) the 0.2Vds number comes from the IRL540 datasheet, not the TS.

The low Vds number is helpful, because it means much lower power dissipation. Losing 0.2V at 2.3A = 0.46W of dissipation. If you used a Darlington with typical 0.9Vce at 2.3A, you'd get 2.07W of dissipation. Not only is this less efficient, but it probably means needing a heat sink for the Darlington while the MOSFET at the same current is running fairly cool even without a heat sink.

I appreciate your response, and I realize my mistake. It's been a long time since I did much with MosFETs because they are much more complex in terms of gain calculations (at least to me so I tend to avoid them for non-serious projects), and I was looking at Vds wrong (don't know why), which was messing up my thinking. Mentally I kept expecting it to be high, but I know better- it should be low for saturation because of low resistance ideally in saturation.

 

I appreciate your response, and I realize my mistake. It's been a long time since I did much with MosFETs because they are much more complex in terms of gain calculations (at least to me so I tend to avoid them for non-serious projects), and I was looking at Vds wrong (don't know why), which was messing up my thinking. Mentally I kept expecting it to be high, but I know better- it should be low for saturation because of low resistance ideally in saturation.

Happy to help! Yeah, I can see where working out gain calculations would be trickier - so far I've only used them in fairly straightforward on/off switching situations, so never had to think in those terms. Using them in analog controls would undoubtedly be more challenging!

 

... The gate is triggered by 555 timer because i need a delay before switching...

Just an idea: Rather than the 555 how about an 8-pin PIC (or other MCU of your choice) and do the delay in software. I understand though if you don't want to write any code )

 

Just an idea: Rather than the 555 how about an 8-pin PIC (or other MCU of your choice) and do the delay in software. I understand though if you don't want to write any code )

Funnily enough I have just used an 8 pin PIC12HV609 running on a 32kHz crystal for a 100 minute delay. The HV chips include a 5V regulator.