On Fri, 10 Feb 2017 11:19:51 -0800 (PST), Lasse Langwadt Christensen <langwadt@fonz.dk> wrote:>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.jpgI used essentially the above version in my first LiIon charger chip circa 1990 (for California Micro Devices), integrated, that's just one isolation tub... the tub floats and is biased by the most positive side. Here's one I posted some time ago for both over and reverse voltage protection... <http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf> ...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.
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 Friday, February 10, 2017 at 12:38:00 PM UTC-5, Peabody wrote:> 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.Oh, duh.. my mistake... please ignore my post. George h.
Reply by ●February 10, 20172017-02-10
On Friday, February 10, 2017 at 2:19:59 PM UTC-5, Lasse Langwadt Christensen wrote:> 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.jpgHah, Thanks, that would work. George H.
Reply by ●February 10, 20172017-02-10
On Friday, February 10, 2017 at 7:50:26 AM UTC-8, Jim Thompson wrote:> 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.> 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.This PNP transistor will have no problem with Vbe reverse breakdown <https://www.centralsemi.com/get_document.php?cmp=1&mergetype=pd&mergepath=pd&pdf_id=CMPT404A_R1.PDF>
Reply by ●February 10, 20172017-02-10
Jim Thompson says... > R2 causes the emitter-base junction voltage to be too > small in the reversed input voltage mode to leak and > pass large currents. Ok, I breadboarded the PNP circuit in both the normal orientation of the transistor and with the transistor placed backward. Since the base is the middle pin, I just turned the transistor around 180 degrees. The two circuits are shown here: https://s28.postimg.org/8tdthq4il/Peabody_breadboard.jpg In the normal orientation, the 2N3906 functioned normally. Opening the base via J1 turned off the transistor completely, at least to the limit of my meters, at any V+ up to 12V. In the reverse orientation, at low V+ levels, it also functioned normally, but with a bit less output current. This presumably results from the lower beta. However, as I ramped up the V+ power supply, C-E current began to flow regardless of what was happening at the base - even if the base was completely open. Using the best meter I have for measuring very low currents, which is my old analog meter on the 50 uA scale, I found that the needle just began to move when V+ reached 8.03V, and current continued to increase as I increased V+ further, reaching 75 uA with V+ at about 11.9V. This current was minimized with the pullup resistor (your R2, my R1) connected to the collector. "Off" current increased if I connected it to the emitter where you had it. So this is either a characteristic of the 2N3906, or perhaps I have two of them that just happened to fail in the same way, or there's something about the resistor values I used that's causing this result. But if none of those apply, and notwithstanding what Spice may have to say, it's possible that this is a characteristic of all PNP transistors, or possibly all bipolars. And if that's the case, then what I wanted to do, which is use one device for both switching and polarity protection, will work fine at, say, 5V or less, but not if you're using a 9V battery. I guess the good news is that for 3.3V or 5V, this actually works, and you don't even have to turn the transistor around because the reverse voltage is less than the Vebo maximum. You can switch the power on and off with the PNP, and you get free polarity protection to boot. But it appears this doesn't work at higher voltages - unless of course I screwed something up that can be corrected. If anyone wants to try confirming my test, I would be interested in the results. Meanwhile, unless there's a fix I haven't thought of, it looks like the backwards P-channel MOSFET will be needed for polarity protection for the 9V battery.
Reply by ●February 10, 20172017-02-10
Peabody wrote...> > It's the pesky PNP base that I'n worried about.An interesting fact is that many (most?) PNP BJTs actually have quite high Veb breakdown voltages. They may not say so on the specs, but when you measure them, you discover this is true. Consider, as an example, the LM339 comparator's PNP input transistors, Q2 and Q3. They can handle 36 volts of reverse biasing. -- Thanks, - Win
Reply by ●February 10, 20172017-02-10
On 02/10/2017 05:29 PM, Winfield Hill wrote:> Peabody wrote... >> >> It's the pesky PNP base that I'n worried about. > > An interesting fact is that many (most?) PNP > BJTs actually have quite high Veb breakdown > voltages. They may not say so on the specs, > but when you measure them, you discover this > is true. Consider, as an example, the LM339 > comparator's PNP input transistors, Q2 and Q3. > They can handle 36 volts of reverse biasing.Those are laterals, though, which is a bit of a special case--aiui they don't have super high emitter doping. Cheers Phil Hobbsh -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●February 10, 20172017-02-10
In article <20170209-191645.81.0@Peabody.ssl.astraweb.com>, waybackNO584SPAM44@yahoo.com says...> > 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.Use a Pmos as polarity protection and you can soft start the gate. Jamie
Reply by ●February 10, 20172017-02-10
Den lørdag den 11. februar 2017 kl. 00.06.14 UTC+1 skrev M Philbrook:> In article <20170209-191645.81.0@Peabody.ssl.astraweb.com>, > waybackNO584SPAM44@yahoo.com says... > > > > 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. > > Use a Pmos as polarity protection and you can soft start the > gate. > Jamiesoft starting the gate doesn't help much when the body diode is conducting you need two pfets
Reply by ●February 10, 20172017-02-10
In article <20170209-224630.33.0@Peabody.ssl.astraweb.com>, waybackNO584SPAM44@yahoo.com says...> > Jim Thompson says... > > >> 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. > > > That won't keep the load from seeing a reverse voltage. > > > If you could find a small P-Channel device that would be > > your best bet. > > I don't understand. It seems to me the PNP C-E does exactly the > same thing as a p-channel mosfet, which is to block current > from flowing when the battery is connected backward. So > it's the same for the main load with either device. It's > just the pesky PNP base that I'n worried about. The mosfet > doesn't present that problem, but then it can't both switch > and protect in a single device.Ok, I miss understood the problem... First off, you still should use a PChannel for protection because of the body diode in the reverse state and haveing the gate pulled low will force it fully on in the correct state. Otherwise without the gate pulled low, it'll pass voltage with one diode drop worth of loses Put another Pchannel after that one orientated the other way as the circuit switch. The first one can be tied on at all times, the second you simply pull the gate to common when you want it. Basically you get a switch and protection that has very little drop.
