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PNP for soft switch and reverse battery protection

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.

Reply to
Peabody
Loading thread data ...

There are some ideas here:

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--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin
[snip]

[snip]

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. ...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 to
Jim Thompson

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.

Reply to
Peabody

Yes, I saw that. But it doesn't address using a PNP at all.

Reply to
Peabody

Den torsdag den 9. februar 2017 kl. 20.18.05 UTC+1 skrev Peabody:

use two pfets back to back

or maybe a diode and a voltage regulator with enable

Reply to
Lasse Langwadt Christensen

If your battery is single cell (i.e., up to 4V), the PNP blocks current in both directions (because 4V is below Vebo).

Low-Vce(sat) types usually have quite high hFE in both forward and reverse directions. All you need to do, to make it "turn on" (doesn't matter which direction current flows), is draw base current. The base current goes in from whichever remaining terminal is at a higher voltage, emitter or collector (traditionally, emitter).

With high hFE, the efficiency can be quite good (~99%), but if you aren't controlling base current to keep it proportional to load current, your average efficiency on a variable load will suck.

If your load is highly variable, back-to-back PMOS starts to look really good -- and as a bonus, low voltage MOS has great specs: low Qg, high gain (1.8V logic level!) and low Rds(on).

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

Then use a mosfet!

Or TPS2660, if your budget allows.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

ould

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mA

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not

The resistor will prevent gross damage, but it is known that even a small r everse current through the base emitter junction of a bipolar transistor pr ogressively degrades the junction and the forward current gain of the trans istor. The damage can be annealed out, and there was a Hewlett-Packard func tion generator where one of the routine service procedures involved the ser vice engineer stubbing out his cigarette on a particular output transistor to do just that.

The story dates back to the days when most people smoked ...

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low

The protection diodes will be fine. The microcontroller might not be. The p rotection diodes inject charge carriers into the substrate of the device, a nd they can do unexpected things. The microcontroller can't be relied to pe rform as specified when the protection diodes are conducting.

--
Bill Sloman, Sydney
Reply to
bill.sloman

Bipolar devices are bilateral except that forward and reverse Beta usually differ. Leakage will do you in if you float the base.

PMOS device are not bilateral, unless you swap the body, which I sometimes do ;-)

...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 to
Jim Thompson

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.

Reply to
George Herold

Anyone ever try using a depletion FET in the low side?

Reply to
bitrex

A crude solution...

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 to
Jim Thompson

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 to
Jim Thompson

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 to
John Larkin

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 to
Peabody

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 to
Peabody

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

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.

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!

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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 to
Tim Williams

formatting link

Reply to
Lasse Langwadt Christensen

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.
Reply to
Jim Thompson

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