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PNP for soft switch and reverse battery protection

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  PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-09 13:16 -0600
    Re: PNP for soft switch and reverse battery protection John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-09 11:34 -0800
      Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-09 16:47 -0600
        Re: PNP for soft switch and reverse battery protection John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-09 15:40 -0800
          Re: PNP for soft switch and reverse battery protection bitrex <bitrex@de.lete.earthlink.net> - 2017-02-10 10:47 -0500
            Re: PNP for soft switch and reverse battery protection John Larkin <jjlarkin@highlandtechnology.com> - 2017-02-10 08:30 -0800
        Re: PNP for soft switch and reverse battery protection George Herold <gherold@teachspin.com> - 2017-02-10 07:25 -0800
          Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-10 11:37 -0600
            Re: PNP for soft switch and reverse battery protection Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-10 11:19 -0800
              Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-10 12:48 -0700
              Re: PNP for soft switch and reverse battery protection George Herold <gherold@teachspin.com> - 2017-02-10 12:24 -0800
            Re: PNP for soft switch and reverse battery protection George Herold <gherold@teachspin.com> - 2017-02-10 12:22 -0800
    Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-09 13:21 -0700
      Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-09 16:46 -0600
        Re: PNP for soft switch and reverse battery protection "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-02-09 17:01 -0600
          Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-09 18:59 -0700
            Re: PNP for soft switch and reverse battery protection "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-02-10 13:17 -0600
        Re: PNP for soft switch and reverse battery protection Winfield Hill <hill@rowland.harvard.edu> - 2017-02-10 14:29 -0800
          Re: PNP for soft switch and reverse battery protection Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-02-10 17:58 -0500
            Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-10 16:40 -0700
        Re: PNP for soft switch and reverse battery protection M Philbrook <jamie_ka1lpa@charter.net> - 2017-02-10 18:23 -0500
    Re: PNP for soft switch and reverse battery protection Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-09 14:55 -0800
    Re: PNP for soft switch and reverse battery protection bill.sloman@ieee.org - 2017-02-09 17:31 -0800
    Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-10 08:50 -0700
      Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-10 11:18 -0600
        Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-10 12:41 -0700
          Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-10 16:28 -0600
            Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-10 16:44 -0700
              Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-10 20:49 -0600
                Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-10 20:10 -0700
                  Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-11 11:38 -0600
                    Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-11 11:07 -0700
                      Re: PNP for soft switch and reverse battery protection rickman <gnuarm@gmail.com> - 2017-02-11 18:46 -0500
                    Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-11 11:15 -0700
                      Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-11 18:31 -0600
                        Re: PNP for soft switch and reverse battery protection rickman <gnuarm@gmail.com> - 2017-02-11 19:50 -0500
                          Re: PNP for soft switch and reverse battery protection Clifford Heath <no.spam@please.net> - 2017-02-12 12:44 +1100
                            Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-11 23:26 -0600
                              Re: PNP for soft switch and reverse battery protection Clifford Heath <no.spam@please.net> - 2017-02-12 18:19 +1100
                              Re: PNP for soft switch and reverse battery protection Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-12 08:18 -0800
                              Re: PNP for soft switch and reverse battery protection rickman <gnuarm@gmail.com> - 2017-02-14 01:52 -0500
                        Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-11 18:04 -0700
                          Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-11 23:14 -0600
                            Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-12 08:18 -0700
                            Re: PNP for soft switch and reverse battery protection John Larkin <jjlarkin@highlandtechnology.com> - 2017-02-12 08:29 -0800
      Re: PNP for soft switch and reverse battery protection whit3rd <whit3rd@gmail.com> - 2017-02-10 13:54 -0800
    Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-10 08:53 -0700
    Re: PNP for soft switch and reverse battery protection M Philbrook <jamie_ka1lpa@charter.net> - 2017-02-10 18:08 -0500
      Re: PNP for soft switch and reverse battery protection Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-02-10 15:15 -0800
        Re: PNP for soft switch and reverse battery protection M Philbrook <jamie_ka1lpa@charter.net> - 2017-02-10 18:34 -0500
    Re: PNP for soft switch and reverse battery protection Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-02-11 12:47 -0700
    Re: PNP for soft switch and reverse battery protection Jasen Betts <jasen@xnet.co.nz> - 2017-02-14 06:13 +0000
      Re: PNP for soft switch and reverse battery protection whit3rd <whit3rd@gmail.com> - 2017-02-14 00:52 -0800
        Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-14 22:26 -0600
      Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-14 22:16 -0600
        Re: PNP for soft switch and reverse battery protection John Larkin <jjlarkin@highlandtechnology.com> - 2017-02-14 21:42 -0800
          Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-15 00:21 -0600
            Re: PNP for soft switch and reverse battery protection Jasen Betts <jasen@xnet.co.nz> - 2017-02-15 11:44 +0000
              Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-15 08:15 -0600
                Re: PNP for soft switch and reverse battery protection makolber@yahoo.com - 2017-02-15 09:30 -0800
                  Re: PNP for soft switch and reverse battery protection Peabody <waybackNO584SPAM44@yahoo.com> - 2017-02-15 11:50 -0600
                    Re: PNP for soft switch and reverse battery protection makolber@yahoo.com - 2017-02-15 10:07 -0800
                    Re: PNP for soft switch and reverse battery protection "Tim Williams" <tiwill@seventransistorlabs.com> - 2017-02-15 21:19 -0600
                  Re: PNP for soft switch and reverse battery protection George Herold <gherold@teachspin.com> - 2017-02-15 11:30 -0800
                    Re: PNP for soft switch and reverse battery protection makolber@yahoo.com - 2017-02-15 11:52 -0800
                      Re: PNP for soft switch and reverse battery protection George Herold <gherold@teachspin.com> - 2017-02-15 12:00 -0800
            Re: PNP for soft switch and reverse battery protection John Larkin <jjlarkinxyxy@highlandtechnology.com> - 2017-02-15 10:41 -0800

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#447047 — 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.

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#447055

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.

There are some ideas here:

http://www.ti.com/lit/an/slva139/slva139.pdf



-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

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#447095

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.

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#447107

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.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

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#447156

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?

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#447164

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 

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#447151

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. 

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#447181

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.

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#447190

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

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#447197

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.jpg

I 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.

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#447202

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.jpg

Hah,  Thanks, that would work.

George H. 

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#447201

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. 

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#447063

On Thu, 09 Feb 2017 13:16:44 -0600, Peabody
<waybackNO584SPAM44@yahoo.com> wrote:

[snip]

>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.
>
[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.

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#447091

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.

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#447100

"Peabody" <waybackNO584SPAM44@yahoo.com> wrote in message 
news:20170209-224630.33.0@Peabody.ssl.astraweb.com...
> 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.
>

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 

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#447129

On Thu, 9 Feb 2017 17:01:03 -0600, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>"Peabody" <waybackNO584SPAM44@yahoo.com> wrote in message 
>news:20170209-224630.33.0@Peabody.ssl.astraweb.com...
>> 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.
>>
>
>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

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.

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#447189

"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 

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#447218

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

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#447223

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

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#447236

On Fri, 10 Feb 2017 17:58:24 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>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

Correct!  Discretes, on the other hand, DO have heavily doped
emitters.
		
                                        ...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.

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