Reversed Biased MOSFETS & BJT's?

[RogerN]

I'm working on a battery charger / discharger design. There is always the
possibility that someone will connect a battery reverse polarity. I don't
want this to destroy the charger. I remember the stuff about correctly
biasing transistors but how will a BJT or MOSTES act if you have Collector
Emitter or Drain Source reverse biased? I'm trying to avoid diodes in the
discharge circuit unless the are required as I want to be able to dicharge
to 1V while measuring current and voltage.

I tried this on simulation and it seems to work (not conduct) but I don't
know if the simulation is realistic with reverse bias. I can also get a 1A
diode to work at 500A in simulation.

Thanks!

RogerN

 

[legg]

On Sun, 12 Oct 2008 08:49:30 -0500, "RogerN" <regor@midwest.net>
wrote:

I'm working on a battery charger / discharger design. There is always the
possibility that someone will connect a battery reverse polarity. I don't
want this to destroy the charger. I remember the stuff about correctly
biasing transistors but how will a BJT or MOSTES act if you have Collector
Emitter or Drain Source reverse biased? I'm trying to avoid diodes in the
discharge circuit unless the are required as I want to be able to dicharge
to 1V while measuring current and voltage.

I tried this on simulation and it seems to work (not conduct) but I don't
know if the simulation is realistic with reverse bias. I can also get a 1A
diode to work at 500A in simulation.

Your model may not include the 'parasitic' diodes that normally exist
in these parts. If in doubt, add them externally in the simulation.

Reverse battery protection at he output of a circuit is the same as
for the input. Googling should provide various suggestions.

RL

 

[Dave Platt]

In article <O-ydncbRR-7ynW_VnZ2dnUVZ_o7inZ2d@earthlink.com>,
RogerN <regor@midwest.net> wrote:

I'm working on a battery charger / discharger design. There is always the
possibility that someone will connect a battery reverse polarity. I don't
want this to destroy the charger. I remember the stuff about correctly
biasing transistors but how will a BJT or MOSTES act if you have Collector
Emitter or Drain Source reverse biased?

Bipolar transistors will often "work" when connected with emitter and
collector reversed (although their speed, gain, and current-carrying
capacity will likely be quite a bit different than in their normal
orientation).

In your situation, whether the BJT will conduct will depend on what
the base voltage is, compared to the "emitter" (i.e. what should be
the collector) - if the base is biased to a voltage which will allow
current to flow through this junction, then the collector/emitter path
will start to conduct as well.

Most power MOSFETs have a "substrate diode" between source and drain.
This diode is reverse-biased during normal operation, but will be
forward-biased (and will start to conduct) if you reverse the source and
drain connections. If your charger/discharger puts the MOSFET across
the battery, with a current limiting resistor, then it's likely that
reversing this battery connection will (in effect) put the battery
into "discharge" mode regardless of what the control logic is doing to
the gate.

I'm trying to avoid diodes in the
discharge circuit unless the are required as I want to be able to dicharge
to 1V while measuring current and voltage.

I tried this on simulation and it seems to work (not conduct) but I don't
know if the simulation is realistic with reverse bias. I can also get a 1A
diode to work at 500A in simulation.

.... until it turns into a NED (noise-emitting diode) for a brief moment.

You might need to use a Schottky diode to isolate the battery from the
charge-discharge circuit, and run a second connection to the battery
(bypassing the diode) to do the voltage sensing.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[ian field]

"RogerN" <regor@midwest.net> wrote in message
news:O-ydncbRR-7ynW_VnZ2dnUVZ_o7inZ2d@earthlink.com...

I'm working on a battery charger / discharger design. There is always the
possibility that someone will connect a battery reverse polarity. I don't
want this to destroy the charger. I remember the stuff about correctly
biasing transistors but how will a BJT or MOSTES act if you have Collector
Emitter or Drain Source reverse biased? I'm trying to avoid diodes in the
discharge circuit unless the are required as I want to be able to dicharge
to 1V while measuring current and voltage.

I tried this on simulation and it seems to work (not conduct) but I don't
know if the simulation is realistic with reverse bias. I can also get a
1A diode to work at 500A in simulation.

Thanks!

RogerN

It may be worth avoiding the issue altogether - on most of the commercial
battery chargers I have, a relay click can be heard which is the reverse
polarity protection. Although most of the chargers contain a
microcontroller, the protection could be implemented much more simply in
your charger with a comparator driving a relay to connect the charger
circuitry to the output leads.

This would have the same shortcoming as the commercial units that if the
battery is discharged dead flat there is no voltage for the comparator to
sense as "correct polarity" I keep a spare current limiting PSU on hand to
get the battery started on these rare occasions.

 

[Phil Allison]

"RogerN"

I'm working on a battery charger / discharger design. There is always the
possibility that someone will connect a battery reverse polarity. I don't
want this to destroy the charger. I remember the stuff about correctly
biasing transistors but how will a BJT or MOSTES act if you have Collector
Emitter or Drain Source reverse biased?

** Mosfets have a diode from source to drain - built in - so as soon as
there is reverse voltage they conduct.

A power BJT will stand maybe 10 volts reverse collector emitter before
conduction begins - but you had better test a few samples before relying on
this.

However, by far you BEST option is to use polarised connectors on the
charger and battery pack to PREVENT reverse connection.



.... Phil

 

[RogerN]

"legg" <legg@nospam.magma.ca> wrote in message
news:l324f45c5rc2cj13qsc5948sa0o82dokfb@4ax.com...

Reverse battery protection at he output of a circuit is the same as
for the input. Googling should provide various suggestions.

RL

For discharge I was planning to have a NPN transistor with a current sense
resistor going to ground. Then apply some base current to get my programmed
discharge current to flow. Perhaps I need the current sense resistor on the
collector so I don't have to figure in base current. If I need to, I could
add in a Schottky diode to prevent current flow in a reverse polarity
situation. But if I don't need the diode, I would prefer not to have the
drop in situations of discharging a single 1.2V cell.

For the charge side, if I have the battery grounded through a current sense
resistor, and the switch on the high side, then a properly connected battery
would have Vplus - Vbatt across the collector to emitter. A reversed
battery would have Vplus + Vbatt across the transistor. I thinking a PNP
common emitter or P channel transistor with base or gate pulled high and
switched lower to turn ON. Seems like the charge side would be correctly
biased and I would just need to use a transistor rated for supply voltage +
battery voltage.

Does this sound right or is there a better way?

Thanks!
RogerN

 

[Fred]

RogerN wrote:

I'm working on a battery charger / discharger design. There is always the
possibility that someone will connect a battery reverse polarity. I don't
want this to destroy the charger. I remember the stuff about correctly
biasing transistors but how will a BJT or MOSTES act if you have Collector
Emitter or Drain Source reverse biased? I'm trying to avoid diodes in the
discharge circuit unless the are required as I want to be able to dicharge
to 1V while measuring current and voltage.

I tried this on simulation and it seems to work (not conduct) but I don't
know if the simulation is realistic with reverse bias. I can also get a 1A
diode to work at 500A in simulation.

Thanks!

RogerN


You could also put a hefty diode in series with an appropriate fuse

(fast fuse) across the battery. If the battery polarity is reversed,
diode conducts and the fuse pops.

Fred


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[ehsjr]

RogerN wrote:

I'm working on a battery charger / discharger design. There is always the
possibility that someone will connect a battery reverse polarity. I don't
want this to destroy the charger. I remember the stuff about correctly
biasing transistors but how will a BJT or MOSTES act if you have Collector
Emitter or Drain Source reverse biased? I'm trying to avoid diodes in the
discharge circuit unless the are required as I want to be able to dicharge
to 1V while measuring current and voltage.

I tried this on simulation and it seems to work (not conduct) but I don't
know if the simulation is realistic with reverse bias. I can also get a 1A
diode to work at 500A in simulation.

Thanks!

RogerN

Sense the battery voltage with a comparator - if correct
polarity, energize a relay that connects the remainder of
the circuit to the battery.

Ed

 

[ian field]

"ehsjr" <ehsjr@NOSPAMverizon.net> wrote in message
news:N%qTk.1442$mi4.1337@nwrddc02.gnilink.net...

RogerN wrote:
I'm working on a battery charger / discharger design. There is always
the possibility that someone will connect a battery reverse polarity. I
don't want this to destroy the charger. I remember the stuff about
correctly biasing transistors but how will a BJT or MOSTES act if you
have Collector Emitter or Drain Source reverse biased? I'm trying to
avoid diodes in the discharge circuit unless the are required as I want
to be able to dicharge to 1V while measuring current and voltage.

I tried this on simulation and it seems to work (not conduct) but I don't
know if the simulation is realistic with reverse bias. I can also get a
1A diode to work at 500A in simulation.

Thanks!

RogerN



Sense the battery voltage with a comparator - if correct
polarity, energize a relay that connects the remainder of
the circuit to the battery.

Ed

It took you over a month to read my reply and pinch my idea!

 

[ehsjr]

ian field wrote:

"ehsjr" <ehsjr@NOSPAMverizon.net> wrote in message
news:N%qTk.1442$mi4.1337@nwrddc02.gnilink.net...

RogerN wrote:

I'm working on a battery charger / discharger design. There is always
the possibility that someone will connect a battery reverse polarity. I
don't want this to destroy the charger. I remember the stuff about
correctly biasing transistors but how will a BJT or MOSTES act if you
have Collector Emitter or Drain Source reverse biased? I'm trying to
avoid diodes in the discharge circuit unless the are required as I want
to be able to dicharge to 1V while measuring current and voltage.

I tried this on simulation and it seems to work (not conduct) but I don't
know if the simulation is realistic with reverse bias. I can also get a
1A diode to work at 500A in simulation.

Thanks!

RogerN



Sense the battery voltage with a comparator - if correct
polarity, energize a relay that connects the remainder of
the circuit to the battery.

Ed


It took you over a month to read my reply and pinch my idea!

Sorry - I didn't see your reply.

Ed