On Thursday, February 9, 2017 at 5:48:02 PM UTC-5, Peabody wrote:> John Larkin says... > > >There are some ideas here: > > > >http://www.ti.com/lit/an/slva139/slva139.pdf > > Yes, I saw that. But it doesn't address using a PNP at all.Right, use a Pfet instead. You could do your slow start thing by adding a cap from drain to gate and a resistor from the negative rail to the gate. Reverse protection and slow start, three components. (I assume someone here will yell at me if the RC idea is bad for some reason.) George H.
PNP for soft switch and reverse battery protection
Started by ●February 9, 2017
Reply by ●February 10, 20172017-02-10
Reply by ●February 10, 20172017-02-10
On 02/09/2017 06:40 PM, John Larkin wrote:> On Thu, 09 Feb 2017 16:47:38 -0600, Peabody > <waybackNO584SPAM44@yahoo.com> wrote: > >> John Larkin says... >> >>> There are some ideas here: >>> >>> http://www.ti.com/lit/an/slva139/slva139.pdf >> >> Yes, I saw that. But it doesn't address using a PNP at all. > > Then use a mosfet! > > Or TPS2660, if your budget allows. > >Anyone ever try using a depletion FET in the low side?
Reply by ●February 10, 20172017-02-10
On Thu, 09 Feb 2017 13:16:44 -0600, Peabody <waybackNO584SPAM44@yahoo.com> wrote:>I'm working on a small circuit that will be powered by a 9V battery. I would >like to combine the functions of a soft power switch and reverse battery >protection in a single device. I think the answer may be to use a PNP >transistor on the high side, with a base resistor to ground - actually to >ground through a physical momentary switch, or indirectly to ground via an >NPN transistor controlled by a microcontroller output pin. But for purposes >of this post, just think of a base resistor to ground. I only need a few mA >of current, and the base resistor needed will be 100K. > >My question concerns reverse polarity protection. I believe I understand >correctly that current will not flow backwards from collector to emitter of >a PNP so long as the transistor's voltage rating is not exceeded. So the >"main" power flow will be protected. > >But the absolute maximum base-emitter reverse voltage for this transistor >(BC560C) is 5V. So if 9V were to be applied directly to the base, with the >emitter at ground, I think the transistor would be blown. The question is >whether the 100K resistor limits current enough so that the transistor is not >damaged. I just don't have any experience with this situation, and don't >knpw what actually works. There is also a separate question as to whether >the limited reverse current flowing through the resistor would damage the >microcontroller (max 3.6V Vcc) since all of that current would actually flow >through it via its protection diodes. > >I guess if it's clear that, at 9V, 100K is gonna keep anything from losing >its smoke, I would just go with that. But I'm more than a little goosey >about that. The easiest alternative I can think of is to just insert a diode >between the base and resistor. That would prevent any reverse current >flowing into the PNP base up to the rating of the diode, which will be way >above 9V, while not really affecting how the transistor functions. In >particular, it would have no effect on the main E-C voltage drop in normal >operation. > >So what do you think? Am I right at least about the theoretical risk? If >so, does the 100K resistor cover me, or do I need the diode? Or is there a >better way? Well, I guess there's always a better way. As I said, I want to >combine a soft switch with polarity protection. A P-channel MOSFET can't do >both because the body diode has to be one way to function as a switch, and >the opposite way to function as polarity protection. The PNP looks like the >right solution. > >Thanks for any suggestions.A crude solution... <http://www.analog-innovations.com/SED/Peabody_Switch.png> Runs the PNP inverted, so low Beta, burns extra bias current to make up for the low Beta, but collector junction is exposed to 9V instead of emitter junction. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
Reply by ●February 10, 20172017-02-10
On Thu, 09 Feb 2017 13:16:44 -0600, Peabody <waybackNO584SPAM44@yahoo.com> wrote:>I'm working on a small circuit that will be powered by a 9V battery. I would >like to combine the functions of a soft power switch and reverse battery >protection in a single device. I think the answer may be to use a PNP >transistor on the high side, with a base resistor to ground - actually to >ground through a physical momentary switch, or indirectly to ground via an >NPN transistor controlled by a microcontroller output pin. But for purposes >of this post, just think of a base resistor to ground. I only need a few mA >of current, and the base resistor needed will be 100K. > >My question concerns reverse polarity protection. I believe I understand >correctly that current will not flow backwards from collector to emitter of >a PNP so long as the transistor's voltage rating is not exceeded. So the >"main" power flow will be protected. > >But the absolute maximum base-emitter reverse voltage for this transistor >(BC560C) is 5V. So if 9V were to be applied directly to the base, with the >emitter at ground, I think the transistor would be blown. The question is >whether the 100K resistor limits current enough so that the transistor is not >damaged. I just don't have any experience with this situation, and don't >knpw what actually works. There is also a separate question as to whether >the limited reverse current flowing through the resistor would damage the >microcontroller (max 3.6V Vcc) since all of that current would actually flow >through it via its protection diodes. > >I guess if it's clear that, at 9V, 100K is gonna keep anything from losing >its smoke, I would just go with that. But I'm more than a little goosey >about that. The easiest alternative I can think of is to just insert a diode >between the base and resistor. That would prevent any reverse current >flowing into the PNP base up to the rating of the diode, which will be way >above 9V, while not really affecting how the transistor functions. In >particular, it would have no effect on the main E-C voltage drop in normal >operation. > >So what do you think? Am I right at least about the theoretical risk? If >so, does the 100K resistor cover me, or do I need the diode? Or is there a >better way? Well, I guess there's always a better way. As I said, I want to >combine a soft switch with polarity protection. A P-channel MOSFET can't do >both because the body diode has to be one way to function as a switch, and >the opposite way to function as polarity protection. The PNP looks like the >right solution. > >Thanks for any suggestions.The diode isn't needed. Should have checked that before posting :-[ ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
Reply by ●February 10, 20172017-02-10
On Fri, 10 Feb 2017 10:47:52 -0500, bitrex <bitrex@de.lete.earthlink.net> wrote:>On 02/09/2017 06:40 PM, John Larkin wrote: >> On Thu, 09 Feb 2017 16:47:38 -0600, Peabody >> <waybackNO584SPAM44@yahoo.com> wrote: >> >>> John Larkin says... >>> >>>> There are some ideas here: >>>> >>>> http://www.ti.com/lit/an/slva139/slva139.pdf >>> >>> Yes, I saw that. But it doesn't address using a PNP at all. >> >> Then use a mosfet! >> >> Or TPS2660, if your budget allows. >> >> > >Anyone ever try using a depletion FET in the low side?A depletion fet could be used as a current limiter, with maybe a shunt diode downstream. I often use a polyfuse and a shunt diode or transzorb. OP could just use a series schottky for reverse polarity protection. Losing a few tenths of a volt, out of 9, shouldn't matter. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●February 10, 20172017-02-10
Jim Thompson says... > A crude solution... > <http://www.analog-innovations.com/SED/Peabody_Switch.png> > Runs the PNP inverted, so low Beta, burns extra bias > current to make up for the low Beta, but collector > junction is exposed to 9V instead of emitter junction. Thanks very much. And I saw your followup about the diode not being needed. But I don't understand R2. Shouldn't it be tied to the collector? It seems that with your placement the transistor would be always-on. Or if it's off, then when the load side voltage drops below 8.4V, it would begin to oscillate. After posting yesterday, I got out my breadboard and some 2N3906's, and got some surprising results which I need to investigate further. Basically, with the battery connected backward so that C is positive and E is negative, I get some current flowing from C through to E, pretty much regardless of what the base is doing - even if it's open. I didn't think that was supposed to happen. And that's on two different transistors, so not just a bad part. Well, I've always just used transistors in the normal orientation, and with the power supply connected properly, but have never looked at what happens when you hook them up backwards. So I have some experimenting to do - including on your inverted orientation of the PNP. Thanks again.
Reply by ●February 10, 20172017-02-10
George Herold says... >> > http://www.ti.com/lit/an/slva139/slva139.pdf >> Yes, I saw that. But it doesn't address using a PNP at >> all. > Right, use a Pfet instead. You could do your slow start > thing by adding a cap from drain to gate and a resistor > from the negative rail to the gate. Reverse protection > and slow start, three components. Please excuse my ignorance, but does "Pfet" refer to a P-channel mosfet, such as shown in Figure 3 in the TI pdf, or a depletion-mode p-channel JFET? Figure 3 works fine for reverse polarity protection, but can't also be used to switch the current on and off. The body diode would always pass current to the load even when the transistor is off.
Reply by ●February 10, 20172017-02-10
"Jim Thompson" <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in message news:fh7q9cpt854po4r3codj5bl8u813h26pmo@4ax.com...> Bipolar devices are bilateral except that forward and reverse Beta > usually differ. Leakage will do you in if you float the base.Yeah, which makes the low-Vce(sat) types interesting because their betas are surprisingly close. I measured something like 250 forward, 150 reverse, for one particular part, and I think most perform similarly well.> PMOS device are not bilateral, unless you swap the body, which I > sometimes do ;-)That's great for you, Mr. IC-Pants, but if you're not making 10k+ of them at a time..... ...There's one MOSFET with separate substrate pin still on the market, for this purpose! http://ww1.microchip.com/downloads/en/DeviceDoc/mic94050.pdf If you don't mind being locked into a special part, it's a slam-dunk for the application. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Reply by ●February 10, 20172017-02-10
Den fredag den 10. februar 2017 kl. 18.38.00 UTC+1 skrev Peabody:> George Herold says... > > >> > http://www.ti.com/lit/an/slva139/slva139.pdf > > >> Yes, I saw that. But it doesn't address using a PNP at > >> all. > > > Right, use a Pfet instead. You could do your slow start > > thing by adding a cap from drain to gate and a resistor > > from the negative rail to the gate. Reverse protection > > and slow start, three components. > > Please excuse my ignorance, but does "Pfet" refer to a > P-channel mosfet, such as shown in Figure 3 in the TI pdf, > or a depletion-mode p-channel JFET? > > Figure 3 works fine for reverse polarity protection, but > can't also be used to switch the current on and off. The > body diode would always pass current to the load even when > the transistor is off.http://images.cnitblog.com/blog/268182/201308/24170317-e690cdfaee344442afb26f3146bcfdfe.jpg
Reply by ●February 10, 20172017-02-10
On Fri, 10 Feb 2017 11:18:43 -0600, Peabody <waybackNO584SPAM44@yahoo.com> wrote:>Jim Thompson says... > > > A crude solution... > > > <http://www.analog-innovations.com/SED/Peabody_Switch.png> > > > Runs the PNP inverted, so low Beta, burns extra bias > > current to make up for the low Beta, but collector > > junction is exposed to 9V instead of emitter junction. > >Thanks very much. And I saw your followup about the diode >not being needed. > >But I don't understand R2. Shouldn't it be tied to the >collector? It seems that with your placement the transistor >would be always-on. Or if it's off, then when the load side >voltage drops below 8.4V, it would begin to oscillate. > >After posting yesterday, I got out my breadboard and some >2N3906's, and got some surprising results which I need to >investigate further. Basically, with the battery connected >backward so that C is positive and E is negative, I get some >current flowing from C through to E, pretty much regardless >of what the base is doing - even if it's open. I didn't >think that was supposed to happen. And that's on two >different transistors, so not just a bad part. > >Well, I've always just used transistors in the normal >orientation, and with the power supply connected properly, >but have never looked at what happens when you hook them up >backwards. So I have some experimenting to do - including >on your inverted orientation of the PNP. > >Thanks again.R2 causes the emitter-base junction voltage to be too small in the reversed input voltage mode to leak and pass large currents. ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | STV, Queen Creek, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | Thinking outside the box... producing elegant solutions.
