solved: discrete P-FET high side linear regulator? - Page 1
Forum section: electronics / projects

author: incf
author_rank: Frequent Contributor
timestamp: April 24, 2025, 10:32:27 am
content: Is it possible to make a stable P-FET high side linear regulator? Or is it inherently unstable?I (think I possibly) know how to build a 'fairly' stable (but slow) regulator using a N-FET and a TL431.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 24, 2025, 10:49:58 am
content: A P-fet at the high side is how some of the low drop regulators are build.The regulator would likely need similar PID type compensation. With an output stage that is more like a controlled current source the output capacitance would be part of the compensation. So the regulator would be sensitive to the output capacitance - if one has control here this is not a problem, but the output capacitance will matter.

author: moffy
author_rank: Super Contributor
timestamp: April 24, 2025, 11:42:56 am
content: A PNP can also be used, which provides a low dropout because the saturation voltage is fairly low and you don't have to worry about Vbe being added to the dropout voltage like an NPN.

author: xavier60
author_rank: Super Contributor
timestamp: April 24, 2025, 12:10:36 pm
content: This is the schematic of a low current split PSU I built. From memory, the response is about 20us.Edit; fixed error with U3.

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 24, 2025, 01:03:14 pm
content: Obviously not inherently unstable. Possibly a bit harder to stabilize due to larger junction capacitances. It's not like a P-FET will have additional poles or zeros to worry about.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 24, 2025, 03:27:41 pm
content: Interesting!Are you using the LM317/337 solely as current sources?EDIT: noticed that it is actually a pair of LM317.

author: TimFox
author_rank: Super Contributor
timestamp: April 24, 2025, 04:16:34 pm
content: PNP and PFET devices are often used to make low-dropout regulators, since the base or gate does not have to go more positive than the output, as in a NPN-output positive regulator.Note, however, that the output from a NPN pass device is essentially an emitter follower, which already has a low output impedance before applying the overall feedback.A PNP device has the output from its collector, which is inherently high impedance before applying the feedback.The detailed design of the regulator circuit needs to deal with how the load capacitance affects the feedback stability.One simple way of thinking about this: at DC, the closed-loop output impedance of the PNP is low, due to a high loop gain reducing the high collector impedance.As frequency increases, the gain decreases, so the closed-loop output impedance increases with frequency: this may be considered an inductance feeding the load capacitance.This is why some low-dropout devices are unstable when the ESR of the load capacitance is too low (such as a ceramic capacitor instead of an electrolytic).

author: David Hess
author_rank: Super Contributor
timestamp: April 24, 2025, 05:11:41 pm
content: Quote from: incf on April 24, 2025, 10:32:27 amSure it is possible to make a stable p-channel high side linear regulator, just like a stable n-channel low side regulator is possible. Shown below is an example of a full features p-channel high side linear regulator.The difficulty is that having a pass element within the feedback loop which has variable gain that changes with operating point and load, makes frequency compensation more challenging. Miller capacitance in the common emitter/source configuration also becomes important. Notice in the example below that pole-zero compensation is used both around the error amplifier, and in the output divider. An output capacitor with controlled ESR and minimum load current will likely be required.

author: xavier60
author_rank: Super Contributor
timestamp: April 24, 2025, 07:19:30 pm
content: Quote from: UnijunctionTransistor on April 24, 2025, 03:27:41 pmYes, they greatly reduce output ripple.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 24, 2025, 08:09:39 pm
content: Quote from: incf on April 24, 2025, 10:32:27 amOf course, huge volumes of PMOS LDOs are shipped.You can still find regulator controllers for external MOSFETSThis one for PMOShttps://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/MIC5159-Programmable-Current-Limit-microCap-LDO-Regulator-Controller-DS20006789.pdfthis one for NMOShttps://www.richtek.com/Home/Products/Linear%20Regulator/Single%20Output%20Linear%20Regulator/RT9008and a discrete circuithttps://www.analog.com/en/resources/technical-articles/pfet-linear-regulator-has-low-dropout-voltage.htmlthis looks to use dual feedback, with an internal loop lowering the gain of the opamp for stability.and an NMOS one with charge pumphttps://www.ti.com/lit/ds/symlink/ucc2837.pdf

author: David Hess
author_rank: Super Contributor
timestamp: April 25, 2025, 12:19:02 am
content: Quote from: PCB.Wiz on April 24, 2025, 08:09:39 pmIntegrated regulators have an advantage in circuit design. The integrated PNP or p-channel transistor can be tapped to move ESR of the output capacitor to be in series with the output. This provides an AC feedback path internal to the IC which can be used for phase lead making frequency compensation easier.A discrete design can do the same thing by moving the current shunt to be in series with the collector/source, and sometimes this is done in a clever design.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 26, 2025, 01:48:37 pm
content: This will interest you.

author: bdunham7
author_rank: Super Contributor
timestamp: April 26, 2025, 02:53:34 pm
content: Quote from: xavier60 on April 24, 2025, 07:19:30 pmIn that case, is there an error in the schematic? The lower LM317 does not look like a current source like the top one...

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2025, 04:22:03 pm
content: Quote from: bdunham7 on April 26, 2025, 02:53:34 pmThe schematic has an error. It should be configured like the top one with the resistor in series with the output and not the adjustment.

author: xavier60
author_rank: Super Contributor
timestamp: April 26, 2025, 08:02:12 pm
content: Thanks, I have uploaded a corrected schematic to Reply #3.

author: xavier60
author_rank: Super Contributor
timestamp: April 27, 2025, 01:28:22 am
content: Quote from: incf on April 24, 2025, 10:32:27 amThe trick is to use the correct type of compensation for a TL341 controlled regulator depending on the type of pass transistor.Just a capacitor from K to REF for BJT. For MOSFET, series RC is needed.This schematic might seem overkill to supply about 20mA. Needs to be rugged, it powers control circuitry for 74V locomotive diesel transfer pump inverters.I did do a load transient test a long time ago. I don't remember the measurement, but it was more than fast enough with compensation component values just picked out of a hat. And no signs of instability, well over-damped.

author: incf
author_rank: Frequent Contributor
timestamp: April 27, 2025, 02:01:40 am
content: Quote from: xavier60 on April 27, 2025, 01:28:22 amNot overkill at all. I spent a year or two doing aircraft electronics rated for lightening strikes (750V/750A). Nice simple design.I played around with the RC in the simulator a bit.I need to dust off my small signal analysis skills. This thread is a product of me slowly and carefully musing about the possibility of implementing a supply that is equivalent to but more robust than a LM317 and uses less bias current (plus a few other constraints). Not sure I will go for it quite yet - there are a lot of tradeoffs. Even with a bunch of careful design work I doubt I would succeed in making anything that was even 10% as well behaved as a '317 without a great deal of trial and error.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 27, 2025, 04:01:36 am
content: Quote from: incf on April 27, 2025, 02:01:40 amThe LM317 is rather old, and a low bar.The LM317/7805 etc date from when 1Amp loads were common on TTL boards, these days regulators are more turned to modern loads.Here are some examples of better LDOs :https://www.inbisen.com/procate-2-16.htmland many customers want accurate fixed outputs so we have recent offerings likehttps://www.nexperia.com/about/news-events/press-releases/New-series-of-automotive-grade-LDOs-from-Nexperia-offer-high-accuracy-and-ultra-low-quiescent-currentorhttps://www.diodes.com/part/view/AP7369Q

author: xavier60
author_rank: Super Contributor
timestamp: April 27, 2025, 04:29:28 am
content: I did a load transient test on an old protype board at 120mA which has shown up some ringing at the start of the load connection.Changing the compensation RC to 10K/220pF fixed it. This board has a different MOSFET to the IRFP450.

author: xavier60
author_rank: Super Contributor
timestamp: April 27, 2025, 04:58:21 am
content: Wow, something else to be wary of. The ringing was present only because I was testing the board at 30V, no ringing at the normal 74V with the original compensation RC. This is likely due to the change of the MOSFET's capacitance and possibly the change in available Gate drive current.

author: David Hess
author_rank: Super Contributor
timestamp: April 27, 2025, 03:14:08 pm
content: Quote from: xavier60 on April 27, 2025, 04:58:21 amThere are ways to handle that situation, but usually the frequency compensation is adjusted for the worst case operating conditions.Maybe check the MOSFET datasheet to see how much variation in capacitance there should be? Most of the variation should be below 30 volts.Why is the drive changing so much?

author: incf
author_rank: Frequent Contributor
timestamp: April 27, 2025, 06:12:44 pm
content: It turns out when I initially asked this question I had an XY problem.When I modeled a simple regulator in spice the op-amp desperately needed negative feedback to be stable. And the inverting feedback path is totally separate from the (positive feedback path) voltage regulator/PMOS feedback path. I was giving it either no feedback, or positive feedback by accident, so it oscillated no matter what I did. With proper negative feedback in the right place(s), it looks fairly well behaved.

author: xavier60
author_rank: Super Contributor
timestamp: April 27, 2025, 10:35:00 pm
content: Quote from: David Hess on April 27, 2025, 03:14:08 pmThe available Gate drive current will decrease as the voltage across the 33K pull-up resistor drops.I'll just optimize the compensation for a lower input voltage although the small bit of ringing could never be a problem anyway.At one stage I was thinking about changing to a JFET or depletion type MOSFET to avoid the need for the pull-up resistor, just a resistor between Gate and Source.Simple example on page 6. A TL431 could be put in place of the Zener.https://www.infineon.com/dgdl/Infineon-Application_Note_Applications_for_Depletion_MOSFETs-ApplicationNotes-v01_00-EN.pdf?fileId=5546d4624cb7f111014cd63d1a197d94