Can MOSFET be Used as Pass Element in Linear Regulator? - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 06, 2016, 04:11:05 pm
content: i usually see pass element using power bjt, but... which one will make up a better linear regulator? setup like in the picture. mosfet or bjt? whats the pro and con? thanks.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 06, 2016, 04:23:07 pm
content: my question is also applicable to this setup, from LM317 datasheet. can i change bjt power elements (LM195's) with suitable mosfets? and change 2N2905 with a suitable gate driver?

author: danadak
author_rank: Super Contributor
timestamp: August 06, 2016, 05:09:29 pm
content: This might be of assistance -http://www.ti.com/lit/an/sboa046/sboa046.pdfRegards, Dana.

author: pelule
author_rank: Frequent Contributor
timestamp: August 06, 2016, 05:14:31 pm
content: In your application the MOSFET is used in linear mode.Even a mosfet is designed for switching it can be used also in linear mode (as in many electronic loads).But you have to ensure, the part is allways operaring in its Safer Operating Area (SOA).This SOA will at the end define the part you select.PeLuLe

author: Gyro
author_rank: Super Contributor
timestamp: August 06, 2016, 05:44:24 pm
content: Quote from: Mechatrommer on August 06, 2016, 04:23:07 pmJust take note that this particular circuit is using the current/overtemp limit feature of the LM195 ICs to provide short circuit protection - they are not bog standard bipolar transistors.

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2016, 06:14:09 pm
content: Well, an N-MOSFET on the high side will definitely have a higher drop-out voltage than a Darlington pair, unless a charge pump is used to drive the gate at a higher voltage than the output.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 06, 2016, 06:17:40 pm
content: the reason i ask is because i have a suspicion mosfet is more difficult to drive (driver side) in linear mode like in the setup i showed. and it produces more noise on the regulated output. got more heated up compared to bjt working at the same power dissipation, easily damaged etc... no?Quote from: Hero999 on August 06, 2016, 06:14:09 pmin bjt, its a emitter follower setup, so with the nmosfet, it will be source follower. the linear output of the power nmosfet will be set at much lower than its input, around 5-10Vds, making charge pump unnecessary, yes?

author: MosherIV
author_rank: Super Contributor
timestamp: August 06, 2016, 07:55:48 pm
content: QuoteAs others have pointed out, both are in linear mode of operation.I like to think of them as powerful variable resistors in this scenario. It does not matter which is used, they both have to dissipate the same amojnt of power.Considering FETs are trickier to bias/configure compared to the BJT for the same operation, it is therefore easier to use BJTs.When used in the first schematic, a design trick you can do with BJT is to put a switch in line with the base of the power transistor, the switch can turn off the power transistor by cutting off the base current. This cannot be easily done with FET because disconnecting the gate means it is free to float and therefore continue in linear operation (but now uncontrolled), the FET gate must be pulled to 0V to turn it off.

author: danadak
author_rank: Super Contributor
timestamp: August 06, 2016, 10:00:13 pm
content: In the boosted regulator example the MOSFET has much higher Vth thanthe Vbeon of a bipolar, so the series R to develop turnon has to be muchhigher, thereby wasting power.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2016, 04:24:58 am
content: MOSFETs work fine in linear applications but have some disadvantages:1. At high voltages MOSFETS have lower current density than bipolars so require a larger die making them more expensive.2. Vgs is higher than Vbe and has a lot more variation.3. MOSFETs optimized for switching applications may lack specifications for linear operation.

author: Whales
author_rank: Super Contributor
timestamp: August 07, 2016, 05:37:35 am
content: I tried doing this a few weeks ago with an IRL40B212Notes:I just tied the op-amps output directly to the mosfetEven with the gate shorted to zero, the fet leaked too much current in its fully off state. Output was around 3-5V (of possible ~12V) even with a 1K load.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 07, 2016, 01:40:36 pm
content: thanks all guys, it seems concluded that, with proper control, mosfet can be made as pass element in linear mode regulator just as good as bjt.. i guess, from understanding the replies here... as i'm currently designing my linear regulator psu, i bought TIP142 darlington bjt. its rated 10A, but big TO-247 package. and i also have IRF540 nmosfet rated 28A, at normal (smaller) TO-220 package, so from amperage per surface or footprint area (and i believe cost saving), mosfet is more advantageous, i need 2 of TIP142 for 20A rating which took more space on the PCB. but as my linear psu design already get involved using the bjt, i will continue with bjt, maybe on revision 2 next time, i will consider using mosfet. thanks all.(ps: to be more accurate, my current psu design is following exactly like the LM317 setup above, with paralleled bjt's using 2x TIP142. yes its a normal bjt with no over temp protection, but i think i will take care that in the design such as limiting the power (current) consumption, or maybe temp sensor on the heatsink, fwiw)

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 07, 2016, 01:42:35 pm
content: Quote from: Whales on August 07, 2016, 05:37:35 amthats a 250A mosfet! maybe the excessive leakage is something common with this kind of very high amperage transistor?

author: langwadt
author_rank: Super Contributor
timestamp: August 07, 2016, 02:14:05 pm
content: Quote from: Mechatrommer on August 07, 2016, 01:40:36 pmfor linear operation the current rating isn't very relevant, the limit is temperature rise from power dissipationfor getting the heat into a heatsink TO-247 is better than a to-220

author: VEGETA
author_rank: Super Contributor
timestamp: November 16, 2016, 03:28:26 pm
content: Quote from: Mechatrommer on August 07, 2016, 01:40:36 pmCan you post a simple schematic?I want to design my own PSU but I want it to achieve true 0v output operation, is that possible using your design? Plus, what is the dropout of this design? If it is too small then you won't need any heatsink.