No current sense resistor required for MOSFET switch protection
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From: John Larkin on 8 Apr 2010 11:48 On Thu, 8 Apr 2010 08:35:46 -0700 (PDT), "oparr(a)hotmail.com" <oparr(a)hotmail.com> wrote: >>That's certainly true, but keeping those secrets safe usually involves >>maintaining a low profile and not blabbing about them in public. > >So this is your crusade to keep a tight lid on this one? > >>"Loose lips sink ships", you know, so, again, what was the purpose of >>the original post? > >Maybe I'm a life jacket salesman. > More likely an amateur with a silly circuit. John
From: Jim Thompson on 8 Apr 2010 12:08 On Thu, 08 Apr 2010 08:31:47 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 08 Apr 2010 10:36:03 -0400, Phil Hobbs ><pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >>Jim Thompson wrote: >>> On Wed, 07 Apr 2010 17:42:45 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>> >>>> On 4/6/2010 12:31 PM, John Larkin wrote: >>>>> On Mon, 5 Apr 2010 17:23:01 -0700 (PDT), "oparr(a)hotmail.com" >>>>> <oparr(a)hotmail.com> wrote: >>>>> >>>>>> No relatively expensive high side current monitor, comparator or latch >>>>>> ICs either. You're wasting your time and money using these items if >>>>>> all you need is a fast electronic "fuse". A single BJT, in conjunction >>>>>> with the MOSFET, is all that is required for UVLO, overcurrent and >>>>>> short circuit protection. Throw in a voltage reference, like the >>>>>> TL431, if the switch has to cover a wide voltage range. >>>>> I sort of doubt it. >>>>> >>>>> John >>>>> >>>> He can do most of that (if he's prepared to be crappy enough) by using >>>> one of those beta-dependent BJT circuits of yours--stick a BJT in series >>>> with the source of the MOSFET, and give it just enough base current to >>>> almost saturate at the highest current you want to allow. >>>> >>>> NOT how you'd want to do it in real life, but it'd limit the current and >>>> so on. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> Yep. Larkin's magic bag of tricks can certainly give one heartburn >>> ;-) >>> >>> ...Jim Thompson >> >>I've got a lot of mileage out of sleazier tricks than that one! >> >>Cheers >> >>Phil Hobbs > > >Is designing to the min/max specs on a datasheet a "sleazy trick"? > >Is adhering to a rule somebody taught you 50 years ago a virtue? > >John That's fine. In my non-pick-and-select world, beta generally can range over 8:1 when you account for process corners and temperature (and sometimes Early effect). ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Jim Thompson on 8 Apr 2010 12:09 On Thu, 08 Apr 2010 08:48:50 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 8 Apr 2010 08:35:46 -0700 (PDT), "oparr(a)hotmail.com" ><oparr(a)hotmail.com> wrote: > >>>That's certainly true, but keeping those secrets safe usually involves >>>maintaining a low profile and not blabbing about them in public. >> >>So this is your crusade to keep a tight lid on this one? >> >>>"Loose lips sink ships", you know, so, again, what was the purpose of >>>the original post? >> >>Maybe I'm a life jacket salesman. >> > >More likely an amateur with a silly circuit. > >John Silly and doesn't work. Pray for flame. It can be quite educational :-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Phil Hobbs on 8 Apr 2010 12:10 John Larkin wrote: > On Thu, 08 Apr 2010 10:36:03 -0400, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> Jim Thompson wrote: >>> On Wed, 07 Apr 2010 17:42:45 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >>> >>>> On 4/6/2010 12:31 PM, John Larkin wrote: >>>>> On Mon, 5 Apr 2010 17:23:01 -0700 (PDT), "oparr(a)hotmail.com" >>>>> <oparr(a)hotmail.com> wrote: >>>>> >>>>>> No relatively expensive high side current monitor, comparator or latch >>>>>> ICs either. You're wasting your time and money using these items if >>>>>> all you need is a fast electronic "fuse". A single BJT, in conjunction >>>>>> with the MOSFET, is all that is required for UVLO, overcurrent and >>>>>> short circuit protection. Throw in a voltage reference, like the >>>>>> TL431, if the switch has to cover a wide voltage range. >>>>> I sort of doubt it. >>>>> >>>>> John >>>>> >>>> He can do most of that (if he's prepared to be crappy enough) by using >>>> one of those beta-dependent BJT circuits of yours--stick a BJT in series >>>> with the source of the MOSFET, and give it just enough base current to >>>> almost saturate at the highest current you want to allow. >>>> >>>> NOT how you'd want to do it in real life, but it'd limit the current and >>>> so on. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> Yep. Larkin's magic bag of tricks can certainly give one heartburn >>> ;-) >>> >>> ...Jim Thompson >> I've got a lot of mileage out of sleazier tricks than that one! >> >> Cheers >> >> Phil Hobbs > > > Is designing to the min/max specs on a datasheet a "sleazy trick"? > > Is adhering to a rule somebody taught you 50 years ago a virtue? > > John > I don't think it's particularly sleazy, no--since it's guaranteed by the data sheet, it ought even to be OK for Joerg's crowd. I build all kinds of things that won't work if the beta is out of spec, though usually it's noise and offset currents that I have to worry about rather than big bias shifts. My only teensy worry about yours is what its behaviour looks like at the corner cases--it has only 300 mV V_CE nominal, which is pretty near saturation. Saturation makes the beta go through the floor, so the collector current could drop pretty badly if the LM1117 were near its lower spec limit for output voltage. (I haven't read the datasheet, but I sort of doubt that the beta is guaranteed at 300 mV V_CE.) The point I was making in my first post wasn't that it was a bad plan for biasing a voltage reference as in your circuit, just that nobody in his right mind would do it in the source of a power MOSFET in a SMPS. It does meet oparr's description, though, and I wouldn't be surprised if his circuit were something like that--the TL431 would be to keep a constant voltage across the base bias resistor. He's cagey enough about it that either (a) he thinks it's a genuine crown jewel or (b) he doesn't want to get his tailfeathers singed by bragging about a kluge. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: oparr on 8 Apr 2010 12:55
>He's cagey enough about >it that either (a) he thinks it's a genuine crown jewel or (b) he >doesn't want to get his tailfeathers singed by bragging about a kluge. I haven't the faintest idea as to what my "circuit" is being likened to. This thread is a followup (sort of) to an earlier thread "High side current monitor with latch" in which Joerg gives me some advice (albeit old school) regarding MOSFET protection. After throughly investigating the characteristics of MOSFETs, it dawned on me that they are probably the easiest of all transistors to protect. It also dawned on me that others, including Joerg, were probably aware ot this already but have decided to keep this to themselves. Hence this thread. It is my opinion, based on both characteristics and test results, that a MOSFET can be operated very close to its absolute maximum thresholds, without damage, when used in conjunction with a single small signal BJT. Not being one to ever give anything on a silver platter (too expensive), that's about all you need to know from me. Furthermore, I'm convinced that "regulars" like Joerg etc. are the ones who should be imparting this knowledge. I'm just an occasional visitor and already a "John Fields" seems to be on the verge of "death threats" over this. On Apr 8, 12:10 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > |