Timeline for SMPS typical drain current when MOSFET goes to saturation
Current License: CC BY-SA 4.0
18 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Aug 1, 2018 at 10:19 | vote | accept | ObliteratedJillo | ||
| Aug 1, 2018 at 0:18 | comment | added | D.A.S. | onsemi.com/pub/Collateral/UC3844-D.PDF fig16 | |
| Jul 31, 2018 at 23:52 | comment | added | Jack Creasey | @TonyEErocketscientist Which link shows a bjt? ...I don't see it | |
| Jul 31, 2018 at 22:46 | comment | added | D.A.S. | @JackCreasey your link has a BJT output , thus saturation term is correct, but not for FETs. | |
| Jul 31, 2018 at 19:07 | comment | added | Andy aka | I’m not getting anything mistaken. Saturation in a mosfet means channel saturation and constant current and NOT the correct region for lowest on resistance. | |
| Jul 31, 2018 at 18:33 | comment | added | Jack Creasey | @Andyaka You misinterpreting the operation shown in the curves. In the saturation region the device is only able to operate successfully when the current is below the line shown at an particular drive voltage. The area below the line IS in saturation. Look at the Microchip document Page 9 ...I agree that the line shown in a FET Vds/Vgs/Ids graphs is a line of constant current ...but is does not represent thje operating line in an SMPS application where the objective is to achieve the lowest possible RDS(on). Below any particular VGS line is in saturation …..above it is in constant current. | |
| Jul 31, 2018 at 18:07 | comment | added | Andy aka | @JackCreasey MOSFET saturation is not the area that it should be operating in when hard switching on. You are confused in the naming of this region. It is called the triode or linear region. Check on the internet and you'll see I'm right. For a BJT, operating when fully turned-on it's called the saturation region because it is the base that is saturated. For a saturated channel (MOSFET) it behaves as a constant current. | |
| Jul 31, 2018 at 18:03 | comment | added | Jack Creasey | @AndyakaI stand by that point. The FET should absolutely be driven to saturation. Not sure why you think it should not. Now the drive is not instantaneous, so it does have a turn on time ...so it does have a transition (ns) me. But the design intent is to drive the device to its lowest possible on resistance ….by definition this is saturation. | |
| Jul 31, 2018 at 18:00 | comment | added | Andy aka | @JackCreasey you said in your comment "The FET SHOULD ABSOLUTELY be in saturation". That is plainly wrong. If it were a BJT (rather than a MOSFET) then you would be correct. | |
| Jul 31, 2018 at 17:59 | comment | added | Jack Creasey | @Andyaka. The operation in this SMPS is not in the linear mode. Perhaps some reading might help your understanding: microchip.com/stellent/groups/SiteComm_sg/documents/… | |
| Jul 31, 2018 at 17:53 | comment | added | Andy aka | @JackCreasey MOSFET saturation is one thing and MOSFET linear region (where it should be operating) is another. | |
| Jul 31, 2018 at 17:52 | comment | added | Jack Creasey | @Andyaka. I'm not confused about BJT ...are you? This is a stock standard SMPS application with a well understood and documented driver. The FET will need to be in saturation with a low VDS when on. onsemi.com/pub/Collateral/UC3844-D.PDF Look at Figure 16 to understand the current sensing if you are confused about the operating point. | |
| Jul 31, 2018 at 16:22 | history | edited | ObliteratedJillo | CC BY-SA 4.0 |
edited title
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| Jul 31, 2018 at 16:00 | comment | added | Andy aka | MOSFET linear and saturation regions not to be confused with the saturation region of a BJT! @JackCreasey | |
| Jul 31, 2018 at 15:55 | answer | added | Andy aka | timeline score: 3 | |
| Jul 31, 2018 at 15:55 | comment | added | Jack Creasey | Exactly as designed, The FET SHOULD ABSOLUTELY be in saturation. The current rate of rise is limited by the transformer N1 and detected as a voltage developed across R5 to detect the switchoff point. | |
| Jul 31, 2018 at 15:45 | history | edited | ObliteratedJillo | CC BY-SA 4.0 |
added 2 characters in body
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| Jul 31, 2018 at 15:39 | history | asked | ObliteratedJillo | CC BY-SA 4.0 |
