LT3080 with pass transistor Spice simulation: Different mosfet Voltage offset  - Page 1
Forum section: electronics / beginners

author: BurnedResistor
author_rank: Regular Contributor
timestamp: October 03, 2015, 05:57:49 pm
content: Hey all,I started playing around with LTSpice, and setup a LT3080 voltage regulator with an N-Channel Mosfet pass transistor to increase current handling capabilities. It drives a resistor whose resistance is a function of the voltage drop accros it, making it a 3A current load. I use a similar resistor setup to draw a constant 1ma from the LT3080 as it does not regulate without it. The LT3080 set current is regulated by a transistor whose resistance is stepped up from 0M2 to 3M5 in 20K steps.I have attached the LTspice schematic so y'all can take a look at my masterpieceNow when I was playing around, I noticed that depending on what pass transistor I chose, the current/resistor to get the output voltage to increase changes. Because I don't have much experience with transistors (Still learning...) I am unable to explain this behaviour.What property of the mosfet dictates this behaviour? The effective resistance? The maximum voltage?Attached are two examples of the output voltage right after the pass transistor vs the Resistance of the resistor connecting the set pin of the regulator to ground. Notice how with the IRF6644 the voltage starts rising at around 440K, while the circuit using the IRLR2908 already increases in output voltage at around 200k...---Some more random questions:Is using a mosfet pass transistor the proper way of getting 0-30v 3A out of this circuit/chip? If so, how much heatsinking would I need to dissipate the large amount of energy?Would putting mutiple of the same transistor in parrallel be a good way of spreading the heat? Are there any others?Thank you so much for your help!

author: BurnedResistor
author_rank: Regular Contributor
timestamp: October 03, 2015, 06:02:23 pm
content: Here is a picture of the circuit..!

author: Kleinstein
author_rank: Super Contributor
timestamp: October 03, 2015, 07:17:33 pm
content: The LT3080 is a power regulator, not a driver for a MOSFET. So this may not be a good choice.Also there usually is no separate feedback Pin other than the output. So this circuit would not give a stable output voltage.So its better to use a different chip to drive the output and do the regulation.Using more transistors in parallel to spread the heat is Ok, but you usually need extra resistors at the soure / emitter (possibly base) pin to make the current spread resonably. Here using BJTs is easier than MOSFETs, as MOSFETs have quite some tolerance in threshhold voltage. How much power can be handeld by a transistor depends on the type and voltage. At about 35-40V for a 30 V supply a BJT like 2N3055 can handle up to about 2 A (75 W) if well cooled.With MOSFETs, the power rating a difficult topic. Most types are not specified in linear operation - especially modern low voltage type are not suitable. Something like a IRFP250 might be about as powerfull as the 2N3055.

author: BurnedResistor
author_rank: Regular Contributor
timestamp: October 04, 2015, 06:41:21 am
content: Thank you for you response!I see how the lt regulator could be a problem. What kind of chip/chip and transistor combination would I use for this kind of application? Are there any dedicated chips just for this purpose?Thank you again!

author: newbrain
author_rank: Super Contributor
timestamp: October 04, 2015, 10:24:35 am
content: A couple of possibilities, without extra bjt or mosfet:Use an higher rated IC, such as LT3083 (same as LT3080, but 3A). But if you need the whole 3A, that would be operating at the limit. Not really a good ideaUse several LT308x in parallel, as described in the 3080 DS, page 11.A question from me: has someone tried the 2nd one? Is it very critical for current sharing or stability?

author: BurnedResistor
author_rank: Regular Contributor
timestamp: October 04, 2015, 12:34:46 pm
content: Quote from: newbrain on October 04, 2015, 10:24:35 amThe Lt3080's data sheet gives a max of 1.1A, so to get 3 amps one would need 3 chips. I was toying about with the idea of building a dual rail supply, so that would get rather costly, and does not seem like an elegant solution to the problem!...

author: Kleinstein
author_rank: Super Contributor
timestamp: October 04, 2015, 07:07:28 pm
content: There is the LM723 / uA 723 chip, to make a voltage regulator with external power transistor. There are lots of circuits based on this rather old chip. However it is a little difficult to get the voltage all the way down to zero - but there are ways to do that. It also has a crude current limiting, however nothing precice.The other alternative would be using standard OPs and a voltage reference (starting from LM324 and TL431) - many cheap lab supply are build this way. Usually BJTs are a little easier to handle.

author: BurnedResistor
author_rank: Regular Contributor
timestamp: October 04, 2015, 07:57:19 pm
content: Quote from: Kleinstein on October 04, 2015, 07:07:28 pmJust to get some more practice with analog circuitry, I think I will give the just op amp approach a go. At the very minimum all the circuit does is divide the voltage using a fixed voltage divider, compare it to some sort of reference and if the voltage is to high decrease the current to the bjt... right? I would decrease the voltage by changing the reference. !? Do you have any good recourse on this?Are there any big considerations in terms öf output noise and similar issues that I should be aware off?Thanks for the help!Sent from my SM-N7500Q using TapatalkSent from my SM-N7500Q using Tapatalk