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Q&A Using FET based followers and design rules

Allow me to address the MOSFET only as a source follower. this has very unpredictable linear use without voltage = current feedback as the RdsOn at threshold has a wide tolerance.(>20%) (Many...

posted 4y ago by TonyStewart‭ · edited 4y ago by TonyStewart‭

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#2: Post edited by

TonyStewart‭ · 2020-11-19T21:51:06Z (about 4 years ago)

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~~Allow me to address rather MOSFET only as a source follower.~~
this has very unpredictable linear use without voltage = current feedback as the RdsOn at threshold has a wide tolerance. (Many wider than the next example). However it is extremely popular in dual N ch. designs for a Half Bridge switching PWM design. The Source Follower is the high side switch and the Common Source is the Low Side PWM switch.
I picked a 1 mohm Nch FET 40V 200A , [IAUA200N04S5N010](https://www.infineon.com/dgdl/Infineon-IAUA200N04S5N010-DS-v01_10-EN.pdf?fileId=5546d462647040d101647051b3671ed1) from Infineon.
Vgs(th)@100uA = 2.8V +/- 0.6V ( Thus Rth = 2.8V/100uA = 28kohm
Then RdsOn= 1.3 mOhm (max) @Vgs=7V and 1.0mOhm (max) @ Vgs=10V
Since your Vg (in) = Vgs + Vs (out) and the output switches towards Vdd Vg must be at least 7V greater than Vdd. This is no problem if Vdd is at least 10V with a diode- cap Boost “voltage doubler” clamped to produce a voltage above Vdd.
Usually in a dual Nch. half bridge you generate this Vgs greater than the Vdd using the low side for PWM so a diode cap can create the well known “boost” above Vdd for the high side source follower.
The other way to remember how this works is that to get a close to spec RdsOn there is a convenient ratio of
Vgs/Vgs(th)=~7V/2.8V=2.5 ratio minimum
-
~~and 3 preferred to meet RdsOn spec.~~
For RF linear design there are many rules which are TL;DR for most.

Allow me to address the MOSFET only as a source follower.
- this has very unpredictable linear use without voltage = current feedback as the RdsOn at threshold has a wide tolerance.(>20%) (Many wider than the next example). However it is extremely popular in dual N ch. designs for a Half Bridge switching PWM design. The Source Follower is the high side switch and the Common Source is the Low Side PWM switch.
I picked a 1 mohm Nch FET 40V 200A , [IAUA200N04S5N010](https://www.infineon.com/dgdl/Infineon-IAUA200N04S5N010-DS-v01_10-EN.pdf?fileId=5546d462647040d101647051b3671ed1) from Infineon.
Vgs(th)@100uA = 2.8V +/- 0.6V ( Thus Rth = 2.8V/100uA = 28kohm
Then RdsOn= 1.3 mOhm (max) @Vgs=7V and 1.0mOhm (max) @ Vgs=10V
Since your Vg (in) = Vgs + Vs (out) and the output switches towards Vdd Vg must be at least 7V greater than Vdd. This is no problem if Vdd is at least 10V with a diode- cap Boost “voltage doubler” clamped to produce a voltage above Vdd.
Usually in a dual Nch. half bridge you generate this Vgs greater than the Vdd using the low side for PWM so a diode cap can create the well known “boost” above Vdd for the high side source follower.
The other way to remember how this works is that to get a close to spec RdsOn there is a convenient ratio of
Vgs/Vgs(th)=~7V/2.8V=2.5 ratio minimum
-
and 3 preferred to meet RdsOn spec. This ratio is **universal** for Vgsth in the 2V range but reduces slightly for sub threshold devices ~ 1V range.
For RF linear design there are many rules which are TL;DR for most.

#1: Initial revision by

TonyStewart‭ · 2020-11-19T21:47:48Z (about 4 years ago)

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Allow me to address rather MOSFET only as a source follower.  

this has very unpredictable linear use without voltage = current feedback as the RdsOn at threshold has a wide tolerance. (Many wider than the next example). However it is extremely popular in dual N ch. designs for a Half Bridge switching PWM design. The Source Follower is the high side switch and the Common Source is the Low Side PWM switch.

I picked a 1 mohm  Nch FET 40V 200A , [IAUA200N04S5N010](https://www.infineon.com/dgdl/Infineon-IAUA200N04S5N010-DS-v01_10-EN.pdf?fileId=5546d462647040d101647051b3671ed1) from Infineon.

Vgs(th)@100uA = 2.8V +/- 0.6V ( Thus Rth = 2.8V/100uA = 28kohm

Then RdsOn= 1.3 mOhm (max) @Vgs=7V and 1.0mOhm (max) @ Vgs=10V 

Since your Vg (in) = Vgs + Vs (out) and the output switches towards Vdd Vg must be at least 7V greater than Vdd. This is no problem if Vdd is at least 10V with a diode- cap Boost “voltage doubler” clamped to produce a voltage above Vdd.

Usually in a dual Nch. half bridge you generate this Vgs greater than the Vdd using the low side for PWM so a diode cap can create the well known “boost” above Vdd for the high side source follower. 

The other way to remember how this works is that to get a close to spec RdsOn there is a convenient ratio of 

Vgs/Vgs(th)=~7V/2.8V=2.5 ratio minimum 
-
and 3 preferred to meet RdsOn spec.


For RF linear design there are many rules which are TL;DR for most.

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