Pardon a post about electronic design. In a recent Dave Jones EEVBlog video nr 1561 ... <https://www.youtube.com/watch?v=FKC6lnTKH-c> ... he shows his USB powered variable bench power supply. At about 28:10 the regulator schematic is shown. I am baffled why he uses two series pass transistors; one for current, one for voltage. I cannot recall ever seen the functionality split into two power transistors in series before. Is that laziness or brilliance? Does anyone have ideas about the design reasoning? Diode D4 in the voltage regulator does not inspire confidence. piglet
Two series pass devices?
Started by ●August 30, 2023
Reply by ●August 30, 20232023-08-30
On Wednesday, August 30, 2023 at 5:11:38 PM UTC-4, piglet wrote:> Pardon a post about electronic design. In a recent Dave Jones EEVBlog > video nr 1561 ... > > <https://www.youtube.com/watch?v=FKC6lnTKH-c> > > ... he shows his USB powered variable bench power supply. At about 28:10 > the regulator schematic is shown. I am baffled why he uses two series > pass transistors; one for current, one for voltage. I cannot recall ever > seen the functionality split into two power transistors in series > before. Is that laziness or brilliance? Does anyone have ideas about the > design reasoning? > > Diode D4 in the voltage regulator does not inspire confidence.He has a forum. You could just go there and ask him. I'm sure he would love to discuss it with you. -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
Reply by ●August 30, 20232023-08-30
piglet wrote:> Pardon a post about electronic design. In a recent Dave Jones EEVBlog > video nr 1561 ... > > <https://www.youtube.com/watch?v=FKC6lnTKH-c> > > ... he shows his USB powered variable bench power supply. At about 28:10 > the regulator schematic is shown. I am baffled why he uses two series > pass transistors; one for current, one for voltage. I cannot recall ever > seen the functionality split into two power transistors in series > before. Is that laziness or brilliance? Does anyone have ideas about the > design reasoning? > > Diode D4 in the voltage regulator does not inspire confidence.A pertinent previous post: <https://www.eevblog.com/forum/projects/usupply-usb-rev-b-update/> contains a link to this schematic: <https://eevblog.com/files/uSupplyUSBrevB.pdf> Does your SED post pertain to pass transistors Q5 and Q6 shown in the schematic? Danke, -- Don, KB7RPU, https://www.qsl.net/kb7rpu There was a young lady named Bright Whose speed was far faster than light; She set out one day In a relative way And returned on the previous night.
Reply by ●August 30, 20232023-08-30
On Wednesday, August 30, 2023 at 5:11:38 PM UTC-4, piglet wrote:> Pardon a post about electronic design. In a recent Dave Jones EEVBlog > video nr 1561 ... > > <https://www.youtube.com/watch?v=FKC6lnTKH-c> > > ... he shows his USB powered variable bench power supply. At about 28:10 > the regulator schematic is shown. I am baffled why he uses two series > pass transistors; one for current, one for voltage. I cannot recall ever > seen the functionality split into two power transistors in series > before. Is that laziness or brilliance? Does anyone have ideas about the > design reasoning?They use similar in HP benchtops. It's necessary if you want a precision programmable current limit. Until it limits, it's just a saturated FET putting most of the voltage on the voltage regulation FET.> > Diode D4 in the voltage regulator does not inspire confidence.Note on schematic says D4 prevents interference by input diodes ( of error opamp ) on the voltage measurement, meaning at R22+ R23 junction. If I could read the pn of the op amp it would be clear, but it's not readable as is. Maybe it's some quasi-Norton amp. Whatever it is the input is acting on about a diode drop below the voltage at external (+) input. Schematic symbols of resistors look like inductors. FETs look like RJ-45 connectors.> > piglet
Reply by ●August 30, 20232023-08-30
Nevermind. The Youtube video finally downloaded. The pass transistors you ask about are Q1 and Q2. Danke, -- Don, KB7RPU, https://www.qsl.net/kb7rpu There was a young lady named Bright Whose speed was far faster than light; She set out one day In a relative way And returned on the previous night.
Reply by ●August 30, 20232023-08-30
On Wed, 30 Aug 2023 22:11:28 +0100, piglet <erichpwagner@hotmail.com> wrote:>Pardon a post about electronic design. In a recent Dave Jones EEVBlog >video nr 1561 ... > ><https://www.youtube.com/watch?v=FKC6lnTKH-c> > >... he shows his USB powered variable bench power supply. At about 28:10 >the regulator schematic is shown. I am baffled why he uses two series >pass transistors; one for current, one for voltage. I cannot recall ever >seen the functionality split into two power transistors in series >before. Is that laziness or brilliance? Does anyone have ideas about the >design reasoning? > >Diode D4 in the voltage regulator does not inspire confidence. > >pigletThat's the craziest design I've ever seen. It must be a deliberate parody.
Reply by ●August 30, 20232023-08-30
John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote:> On Wed, 30 Aug 2023 22:11:28 +0100, piglet <erichpwagner@hotmail.com> > wrote: > >> Pardon a post about electronic design. In a recent Dave Jones EEVBlog >> video nr 1561 ... >> >> <https://www.youtube.com/watch?v=FKC6lnTKH-c> >> >> ... he shows his USB powered variable bench power supply. At about 28:10 >> the regulator schematic is shown. I am baffled why he uses two series >> pass transistors; one for current, one for voltage. I cannot recall ever >> seen the functionality split into two power transistors in series >> before. Is that laziness or brilliance? Does anyone have ideas about the >> design reasoning? >> >> Diode D4 in the voltage regulator does not inspire confidence. >> >> piglet > > That's the craziest design I've ever seen. It must be a deliberate > parody. > >Disclaimer: I find EEVblog practically unwatchable, so I haven’t looked at the schematic. One reason to separate the current limit and voltage regulator functions is to sharpen up the current limiter by putting it inside the voltage regulator loop. I commonly do that with LND150 depletion FETs in HV bias supplies for APDs and SiPMs. They look like ~1k resistors below about 1.5 mA, which is enough to hurt the gain linearity, but the feedback loop effectively knocks that down to way under an ohm. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
Reply by ●August 31, 20232023-08-31
On Thu, 31 Aug 2023 02:23:59 -0000 (UTC), Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote: >> On Wed, 30 Aug 2023 22:11:28 +0100, piglet <erichpwagner@hotmail.com> >> wrote: >> >>> Pardon a post about electronic design. In a recent Dave Jones EEVBlog >>> video nr 1561 ... >>> >>> <https://www.youtube.com/watch?v=FKC6lnTKH-c> >>> >>> ... he shows his USB powered variable bench power supply. At about 28:10 >>> the regulator schematic is shown. I am baffled why he uses two series >>> pass transistors; one for current, one for voltage. I cannot recall ever >>> seen the functionality split into two power transistors in series >>> before. Is that laziness or brilliance? Does anyone have ideas about the >>> design reasoning? >>> >>> Diode D4 in the voltage regulator does not inspire confidence. >>> >>> piglet >> >> That's the craziest design I've ever seen. It must be a deliberate >> parody. >> >> > >Disclaimer: I find EEVblog practically unwatchable, so I haven�t looked at >the schematic.It's a small lab power supply powered by USB-C. It has several microprocessors, layers of isolators and switching regs and linear regs, a custom enclosure with custom-machined connectors, a custom planar transformer, a custom heat sink, a zillon layer PCB, and what must be tens of KLOCs. It's apparently been in design for about 5 years. Squeaky talks about it in an almost bearable tone. Really, it's funny enough to have a look at. Skip the blather and look at the schematic pages. (I had hernia surgery today and am still goofy from the drugs, so this sort of nonsense fit right in.)> >One reason to separate the current limit and voltage regulator functions is >to sharpen up the current limiter by putting it inside the voltage >regulator loop. > >I commonly do that with LND150 depletion FETs in HV bias supplies for APDs >and SiPMs. They look like ~1k resistors below about 1.5 mA, which is enough >to hurt the gain linearity, but the feedback loop effectively knocks that >down to way under an ohm. > >Cheers > >Phil Hobbs
Reply by ●August 31, 20232023-08-31
On 30/08/2023 22:40, Ricky wrote:> On Wednesday, August 30, 2023 at 5:11:38 PM UTC-4, piglet wrote: >> Pardon a post about electronic design. In a recent Dave Jones EEVBlog >> video nr 1561 ... >> >> <https://www.youtube.com/watch?v=FKC6lnTKH-c> >> >> ... he shows his USB powered variable bench power supply. At about 28:10 >> the regulator schematic is shown. I am baffled why he uses two series >> pass transistors; one for current, one for voltage. I cannot recall ever >> seen the functionality split into two power transistors in series >> before. Is that laziness or brilliance? Does anyone have ideas about the >> design reasoning? >> >> Diode D4 in the voltage regulator does not inspire confidence. > > He has a forum. You could just go there and ask him. I'm sure he would love to discuss it with you. >Thanks, good call, I'd completely forgotten! piglet
Reply by ●August 31, 20232023-08-31
On 30/08/2023 23:50, Fred Bloggs wrote:> On Wednesday, August 30, 2023 at 5:11:38 PM UTC-4, piglet wrote: >> Pardon a post about electronic design. In a recent Dave Jones EEVBlog >> video nr 1561 ... >> >> <https://www.youtube.com/watch?v=FKC6lnTKH-c> >> >> ... he shows his USB powered variable bench power supply. At about 28:10 >> the regulator schematic is shown. I am baffled why he uses two series >> pass transistors; one for current, one for voltage. I cannot recall ever >> seen the functionality split into two power transistors in series >> before. Is that laziness or brilliance? Does anyone have ideas about the >> design reasoning? > > They use similar in HP benchtops. It's necessary if you want a precision programmable current limit. Until it limits, it's just a saturated FET putting most of the voltage on the voltage regulation FET. > >> >> Diode D4 in the voltage regulator does not inspire confidence. > > Note on schematic says D4 prevents interference by input diodes ( of error opamp ) on the voltage measurement, meaning at R22+ R23 junction. If I could read the pn of the op amp it would be clear, but it's not readable as is. Maybe it's some quasi-Norton amp. Whatever it is the input is acting on about a diode drop below the voltage at external (+) input. > > Schematic symbols of resistors look like inductors. FETs look like RJ-45 connectors. > >> >> pigletThe opamp is OPA2180 zero drift/auto zero type with inverse parallel input protection diodes. D4 looks mega kludgy - if it were mine I'd investigate using a different opamp or taking the error signal from the voltage follower used for metering. Still I guess in real life the voltage drop error across D4 at nanoamp opamp input bias current is near negligible? piglet
