Author Topic: Are logic level gate mosfet slower than regular mosfet?  (Read 5317 times)

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Offline spec

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Re: Are logic level gate mosfet slower than regular mosfet?
« Reply #25 on: November 01, 2018, 10:12:29 am »
doc format?  But... that's almost as bad, what's wrong with plain text? |O

Here's plain text, cleaned up a bit; view with tab size 4.  I'm not sure if that imports into, say, Excel cleanly.

Tim

Don't be too hard on me- I'm just a simple person :)
 

Offline spec

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Re: Are logic level gate mosfet slower than regular mosfet?
« Reply #26 on: November 01, 2018, 10:26:39 am »
BTW, while higher gate voltage means lower Rds(on), I found there is a diminishing return. E.g., a 10V drive was barely better than 7V drive. So, it may not make much sense for charge pumps, etc for a logic-level fet. I suggest do some measurements (but leave some voltage headroom for process variation, temperature effects, etc).

Also datasheets often provide guaranteed Rds(on) at specific voltages, the actual resistance is even smaller (but do not rely on this in mass production).

True, and temperature has a big impact on Rds.

The charge pump gate driver was suggested as a way to use standard MOSFETs, which are more common, cheaper, and tend to have lower parasitic capacitances.

You mention the law of diminishing returns in regard to drive voltage/Rds. As you imply, the secret is to establish the sweet spot in a design :)
 

Offline exe

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Re: Are logic level gate mosfet slower than regular mosfet?
« Reply #27 on: November 01, 2018, 10:32:29 am »
There are also mosfets with integrated drivers. E.g., this one: https://www.onsemi.com/pub/Collateral/NCP5369-D.PDF (just first one from Google).

There are devices with integrated charge pump, over-current, over-voltage (not sure what this exactly means, check datasheet if important) and thermal protection.

Other devices are below. Just google "smart fet", "protected fet", etc.  At least some of them declare switching speed up to 1MHz.

- https://www.infineon.com/dgdl/Infineon-ApplicationNote_v12-AN-v01_00-EN.pdf?fileId=5546d4625b62cd8a015bc8c8e5ae31d1
- https://eu.mouser.com/new/diodes-inc/diodes-zxms-mosfets/
 

Offline exe

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Re: Are logic level gate mosfet slower than regular mosfet?
« Reply #28 on: November 01, 2018, 10:43:21 am »
As you imply, the secret is to establish the sweet spot in a design :)

Yeah, but for a one-off device I'd suggest save time and just get the best part. Sort of overkill, but saves a lot of time... and often board space.

I have CSD17313Q2T (that I'm yet to try), but even at 3V drive it gives less than 50mOhm over entire temperature range (hope it's not a "typical performance") and has very good performance in terms of Rds(on) vs gate charge.

There are inexpensive parts with threshold of less than 1V in sot-23 package with quite low Rds(on). Like AO3414. I bought it because it's cheap (yet to try it, don't have a lab at the moment) :). Just threshold voltage can be as low as 0.4V, so be sure driver can pull that low.
 

Online Ian.M

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Re: Are logic level gate mosfet slower than regular mosfet?
« Reply #29 on: November 01, 2018, 11:14:05 am »
doc format?  But... that's almost as bad, what's wrong with plain text? |O

Here's plain text, cleaned up a bit; view with tab size 4.  I'm not sure if that imports into, say, Excel cleanly.
It doesn't import cleanly.  I've gone back to Spec's list in the .doc, used '_' as the separator then cleaned up out of order columns to get a CSV file that should import OK to any spreadsheet.
 

Offline spec

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Re: Are logic level gate mosfet slower than regular mosfet?
« Reply #30 on: November 01, 2018, 11:53:14 am »
As you imply, the secret is to establish the sweet spot in a design :)

Yeah, but for a one-off device I'd suggest save time and just get the best part. Sort of overkill, but saves a lot of time... and often board space.

I have CSD17313Q2T (that I'm yet to try), but even at 3V drive it gives less than 50mOhm over entire temperature range (hope it's not a "typical performance") and has very good performance in terms of Rds(on) vs gate charge.

There are inexpensive parts with threshold of less than 1V in sot-23 package with quite low Rds(on). Like AO3414. I bought it because it's cheap (yet to try it, don't have a lab at the moment) :). Just threshold voltage can be as low as 0.4V, so be sure driver can pull that low.

Hmm, sounds like you have some interesting parts and plans. I would like to see  any circuits that you design. I'm in the same boat as you- no lab facilities at the moment as I'm living in one house, temporarily, while refurbishing another house which does have a large garage and potential for a lab, but I  need approval from the domestic authority for that. :D
« Last Edit: November 01, 2018, 11:54:49 am by spec »
 

Offline exe

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Re: Are logic level gate mosfet slower than regular mosfet?
« Reply #31 on: November 01, 2018, 03:09:54 pm »
I would like to see  any circuits that you design.

I'm building a four-channel linear lab supply heavily inspired by this one: https://www.eevblog.com/forum/beginners/how-does-blackdog_s-psu-work/ . Basically, I'm taking that design and adding the following features:

1) Digital control
2) Current sinking (but not much, more like 100-500mA)
3) Dual current range (via a switchable shunt)
4) 4 isolated channels
5) graphical display. May be touch control.
6) remote control (probably add this later)
7) very compact, half-rack width
8) switchable output capacitance. This can be a unique feature, haven't seen this before :).
9) probably two channels will be "beefy" (15V 3A), and two will be will low range (both current and voltage), but more precise and with small output capacitor (like 6V, 600mA).

So far I built 6 and half prototypes of power part, now messing up with software part. Since I'm still evaluating different modules, there is no finished schematic yet (don't want to show discarded designs). Also I don't think I'll be able to implement all the features. Current goal is to finish schematic, make one channel evaluate it, learn from mistakes, and will see. Probably, I'll have to give up on some ideas.
 

Offline spec

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Re: Are logic level gate mosfet slower than regular mosfet?
« Reply #32 on: November 01, 2018, 07:01:35 pm »
Wow, quite a project. As I said, keep us informed about progress. :)
 


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