Driving a Mosfet hard with low powered op amps - Page 1
Forum section: electronics / projects

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 09:23:12 am
content: Hello all,In the circuit shown above( as an example). In order to have 10 V across R1 the op amp should be supplied with 10v + xV to output the correct voltage to drive the Mosfet.. Is their a way to use 5v op amp + additional circuit to achieve the same result.I am designing a power supply and i dont want my op amps to be powered by 30v in order to achieve high output voltages

author: firewalker
author_rank: Super Contributor
timestamp: December 30, 2014, 09:29:25 am
content: A discrete transistor voltage amplifier.Alexander.

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 10:00:09 am
content: Is this what you mean? Sorry for the hand drawing did it at work?

author: firewalker
author_rank: Super Contributor
timestamp: December 30, 2014, 10:18:41 am
content: I was thinking more of a complementary pair feedback transistor topology.Alexander.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2014, 11:56:50 am
content: I don't see why the MOSFET need to be driven hard. It's operating in the saturation region. This isn't a PWM circuit which needs to switch the MOSFET quickly. It's a linear regulator.

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 12:05:04 pm
content: Quote from: Hero999 on December 30, 2014, 11:56:50 amit is operating in the linear region, what i meant by hard is high voltages in the gate which will give me high voltages in the load.

author: rob77
author_rank: Super Contributor
timestamp: December 30, 2014, 12:20:19 pm
content: Quote from: Asim on December 30, 2014, 12:05:04 pm"driving hard" means provide a lots of current to charge or discharge the gate's capacitance very quickly

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 12:35:57 pm
content: Quote from: rob77 on December 30, 2014, 12:20:19 pmalways good to know a new information

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2014, 12:57:51 pm
content: It looks like a linear current regulator. What exactly are you trying to do?

author: dannyf
author_rank: Super Contributor
timestamp: December 30, 2014, 01:06:00 pm
content: QuoteAlmost: reverse the input pins - the polarity has been reversed by the bjt.

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 02:11:52 pm
content: Quote from: Hero999 on December 30, 2014, 12:57:51 pma power supply

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2014, 03:21:02 pm
content: Quote from: Asim on December 30, 2014, 02:11:52 pmThat's not very descriptive. What sort of power supply?

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 05:05:51 pm
content: Quote from: Hero999 on December 30, 2014, 03:21:02 pmLOL, dual channel (isolated channels) 25V, 2A linear power supply with current limit and 2 digital displays for each channel to display( vout,Iout )when the output is turned on and (Vset,Iset) when the output is off.The design is under process

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2014, 05:18:06 pm
content: The trouble with using a MOSFET is the gate voltage needs to be much higher than the source, so the op-amp will need to be powered from 31V in order to give you 25V out.Why are you using a high voltage MOSFET? They tend to have poor characteristics.

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 05:35:10 pm
content: Quote from: Hero999 on December 30, 2014, 05:18:06 pmumm, what else to use for a linear power supply, other than voltage regulators which are not an option for me ? poor characteristics in terms of what ?

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 05:36:47 pm
content: oh, and this post is about how to drive the mosfet without having to use high voltage supply for the op amps.

author: c4757p
author_rank: Super Contributor
timestamp: December 30, 2014, 05:47:24 pm
content: Quote from: Asim on December 30, 2014, 05:35:10 pm...BJT?Most MOSFETs aren't meant for DC power dissipation (some don't mind - look for SOA - Safe Operating Area - or FBSOA - Forward-Biased Safe Operating Area - in the datasheet), they have a high threshold voltage, they have high gate capacitance (making them slow unless you can, erm, drive them hard), and many have extremely low resistance when saturated (making it easy to have a catastrophic failure if your control loop goes crazy and you have a lot of power available).That said, there's nothingwrongwith using a MOSFET for a linear power supply, you just have to be aware of all of this.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2014, 05:57:47 pm
content: Even if you use a BJT, the op-amp still needs a relatively high supply voltage because the BJT will drop 1V or more, plus the 2V inside the op-amp which has a Darlington pair on the high side of the output.You could add a little voltage gain to your output stage but not too much, otherwise it will be unstable and could oscillate.

author: dannyf
author_rank: Super Contributor
timestamp: December 30, 2014, 06:03:08 pm
content: QuoteUse the opamp to drive a npn which in turn drives a P-ch mosfet. Stability can be an issue, however.

author: dom0
author_rank: Super Contributor
timestamp: December 30, 2014, 06:04:42 pm
content: Your design approach is flawed.Look at the schematics of a well-designed dual channel supply like the HP 6236B. The newer Agilent E3630A uses the same topology.

author: ajb
author_rank: Super Contributor
timestamp: December 30, 2014, 06:11:42 pm
content: In addition to c4757p's points, the gate capacitance of a big power MOSFET will make the control loop trickier to compensate since it will introduce phase shift between the response of the op amp and response of the output. The op amp's output will also likely saturate as it tries to charge and discharge the gate, further degrading its response. If you're going to use a MOSFET in a linear circuit, it'd be a good idea to put a bipolar follower between the op amp and the gate to boost gate drive current, and provide for feedback from the output of the op amp and the output of the follower to control oscillation.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2014, 06:12:45 pm
content: Quote from: dannyf on December 30, 2014, 06:03:08 pmI agree but what about a common emitter amplifier driving an emitter follower? That should be more stable.

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 06:31:46 pm
content: Quote from: 3roomlab on December 30, 2014, 06:23:18 pmNo, gate will be higher than the source but I need it to be done without using high rails for the op amps

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2014, 06:38:10 pm
content: Quote from: Asim on December 30, 2014, 06:31:46 pmThe only way to do that is to have another voltage amplifier after the op-amp.In short, it's not possible to do this. There are no advantages, especially with the MOSFET you've chosen which has a high on resistance (0.85R) which will drop 1.7V at 2A. You might as well use a Darlington pair driven by a common emitter or common base amplifier. It's much simpler and effective.http://www.vishay.com/docs/91070/91070.pdf

author: Asim
author_rank: Regular Contributor
timestamp: December 30, 2014, 06:52:32 pm
content: Quote from: Hero999 on December 30, 2014, 06:38:10 pmthe image I attached was just an example taken from google images, non the less, you are right, Darlington pair will be a better solution just to avoid the hassle with Mosfets.

author: dannyf
author_rank: Super Contributor
timestamp: December 30, 2014, 07:07:17 pm
content: QuoteDepending on what you meant by "flawed" - pretty much all LDOs are collector- / drain-out.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2014, 07:18:36 pm
content: Quote from: Asim on December 30, 2014, 06:52:32 pmGood. That means the op-amp can be powered from 5V.

author: dom0
author_rank: Super Contributor
timestamp: December 30, 2014, 07:38:51 pm
content: Quote from: dannyf on December 30, 2014, 07:07:17 pmThe design is supposed to be used in a dual supply, so it's preferable (- unless one wants to completly separate supplies, just in one casing -) to have only one additional supply for the regulator circuitry, and one central reference element (for all four (U+, I+, U-, I-) regulators). One also usually wants higher output voltages than the ops can sustain (e.g. the quoted HP has 40 V output swing and something like 50 V before the SPE. My home brew build after the schematics of it outputs 70 V and has about 100 V before the SPE. Can't do that if your op needs to reach "around" the output rails.) And you won't get that with this design.For a single supply I'd still very much prefer the most stable configuration (i.e. emitter out).