Reply by miso July 21, 20122012-07-21
If you can tolerate schottky reverse bias leakage, that would provide a 
better clamp.

You don't indicate if the user is plugging in an external supply, or the 
supply is hard wired. I'm not sure why you would expect a hard wired 
supply to go negative. [OK, maybe from one of those home brew switcher 
designs, which is of course why you should always buy a controller IC.]

I have a break out box I use for mobile applications to distribute 12vdc 
from a marine type battery (AGM). I use a fuse and parallel Schottky in 
the box. Actually a few Schottky diodes. Due to current hogging, I 
expect the diodes to turn on individually, so paralleling them doesn't 
do much in terms of current handling. My guess was the first diodes to 
turn on might fail before the fuse pops, so as each diode fails, a new 
one will take over. I added some junk box standard silicon diodes as 
well. Needless to say I haven't tested this! I just tossed in the parts 
because I had them handy and I couldn't see how it would hurt.

Regarding damage to ICs, you would really have to reach a diode turn on 
voltage to get the chip to draw significant current. The manufacturer is 
just guardbanding this by specing 0.3V.

With static DC, there are two way to damage an IC. Overvoltage will 
break down diode junctions. Reverse voltage will "melt" the metal trace 
in the path of the current.

This leads to a classic problem in IC metal fuse popping. If upon 
popping the fuse the voltage on the test pad spikes upwards, as can 
happen with a sudden di/dt as the fuse pops, the overvoltage can break 
down an internal junction and cause it to leak. Generally is it better 
to set up the fuse popper so it forward biases a junction after the fuse 
is popped. Given that the energy to pop the fuse is stored in a 
capacitor, you can predict how quickly the cap will discharge and thus 
predict the time the metal trace will have to conduct high current. 
Basically you can engineer around the potentially damaging situation. 
Add protection diodes and insure a sufficient metal width to them.
Reply by July 21, 20122012-07-21
On Saturday, July 21, 2012 8:24:17 AM UTC-4, markj...@gmail.com wrote:
> I have a diode 1N5402 connected in parallel reverse biased across the pow=
er supply for reverse polarity protection. The maximum absolute ratings of= the load IC says -0.3V to +30V. If reverse power is applied, anything abov= e diode's forward drop it will short circuit and break the fuse. No iss= ue here. If, let's say, 0.5V is applied in reverse, the diode will not = conduct and reverse supply of -0.5V will go directly to the load IC. Since = it is below the min -0.3V, will it damage the IC?=20
>=20 > -Markj
http://www.ti.com/lit/an/slva139/slva139.pdf
Reply by krw...@att.bizzzzzzzzzzzz July 21, 20122012-07-21
On Sat, 21 Jul 2012 17:49:31 GMT, Mike <spam@me.not> wrote:

>markjsunil@gmail.com wrote: > >> I have a diode 1N5402 connected in parallel reverse biased across the >> power supply for reverse polarity protection. The maximum absolute >> ratings of the load IC says -0.3V to +30V. If reverse power is >> applied, anything above diode's forward drop it will short circuit and >> break the fuse. No issue here. If, let's say, 0.5V is applied in >> reverse, the diode will not conduct and reverse supply of -0.5V will >> go directly to the load IC. Since it is below the min -0.3V, will it >> damage the IC? >> >> -Markj > >Blowing fuses to save an ic is not a very good idea.
Why? I generally have used PTCs instead of fuses and TVSs instead of diodes but the idea is the same.
>Can you put the dide in series with the load instead of in parallel across >the supply?
>That way if the power supply leads are reversed, only the diode leakage >current will flow. This is sped'd at 5uA at room temperature, which should >not harm an ic.
For low currents, this works, as long as you don't need the voltage or regulation. It doesn't protect against over-voltage, though.
Reply by Jim Thompson July 21, 20122012-07-21
On Sat, 21 Jul 2012 11:17:25 -0700, Fred Abse
<excretatauris@invalid.invalid> wrote:

>On Sat, 21 Jul 2012 11:02:04 -0700, Jim Thompson wrote: > >> When I was involved in such things, power supplies were unreliable, and uP >> chips were _very_ expensive. The fuse blower apparatus added pennies to >> the unit cost. > >I still won't incorporate a bought-in PSU in a piece of equipment, without >adding an in-house made crowbar-and-fuse protector. > >I just don't like the idea of my four-, or five-figure gizmo at the mercy >of a two- or three-figure rice bowl. > >The field tech swaps out the PSU, and changes a fuse. I get to sleep >soundly.
Me too ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by Fred Abse July 21, 20122012-07-21
On Sat, 21 Jul 2012 11:02:04 -0700, Jim Thompson wrote:

> When I was involved in such things, power supplies were unreliable, and uP > chips were _very_ expensive. The fuse blower apparatus added pennies to > the unit cost.
I still won't incorporate a bought-in PSU in a piece of equipment, without adding an in-house made crowbar-and-fuse protector. I just don't like the idea of my four-, or five-figure gizmo at the mercy of a two- or three-figure rice bowl. The field tech swaps out the PSU, and changes a fuse. I get to sleep soundly. -- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)
Reply by Jim Thompson July 21, 20122012-07-21
On Sat, 21 Jul 2012 17:49:31 GMT, Mike <spam@me.not> wrote:

>markjsunil@gmail.com wrote: > >> I have a diode 1N5402 connected in parallel reverse biased across the >> power supply for reverse polarity protection. The maximum absolute >> ratings of the load IC says -0.3V to +30V. If reverse power is >> applied, anything above diode's forward drop it will short circuit and >> break the fuse. No issue here. If, let's say, 0.5V is applied in >> reverse, the diode will not conduct and reverse supply of -0.5V will >> go directly to the load IC. Since it is below the min -0.3V, will it >> damage the IC? >> >> -Markj > >Blowing fuses to save an ic is not a very good idea.
When I was involved in such things, power supplies were unreliable, and uP chips were _very_ expensive. The fuse blower apparatus added pennies to the unit cost.
> >Can you put the dide in series with the load instead of in parallel across >the supply? > >That way if the power supply leads are reversed, only the diode leakage >current will flow. This is sped'd at 5uA at room temperature, which should >not harm an ic.
...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by July 21, 20122012-07-21
On Saturday, July 21, 2012 10:08:55 PM UTC+5:30, John Larkin wrote:
> On Sat, 21 Jul 2012 05:24:17 -0700 (PDT), markjsunil wrote: >=20 > &gt;I have a diode 1N5402 connected in parallel reverse biased across the=
power supply for reverse polarity protection. The maximum absolute rating= s of the load IC says -0.3V to +30V. If reverse power is applied, anything = above diode&#39;s forward drop it will short circuit and break the fuse. No= issue here. If, let&#39;s say, 0.5V is applied in reverse, the diode will = not conduct and reverse supply of -0.5V will go directly to the load IC. Si= nce it is below the min -0.3V, will it damage the IC?=20
> &gt; > &gt;-Markj >=20 > -0.5 volts won&#39;t hurt most ICs. >=20 > But if you connect a chunky 30 volt supply backwards, until the fuse > blows the diode forward drop could be several volts. >=20 > What&#39;s your normal supply voltage, and what&#39;s the load current? W=
hat
> IC is it? >=20 >=20 > --=20 >=20 > John Larkin Highland Technology Inc > www.highlandtechnology.com jlarkin at highlandtechnology dot com =20 >=20 > Precision electronic instrumentation > Picosecond-resolution Digital Delay and Pulse generators > Custom timing and laser controllers > Photonics and fiberoptic TTL data links > VME analog, thermocouple, LVDT, synchro, tachometer > Multichannel arbitrary waveform generators
Thanks. The IC is TPA3111. Absolute max Vcc is -0.3 to 30V.
Reply by Mike July 21, 20122012-07-21
markjsunil@gmail.com wrote:

> I have a diode 1N5402 connected in parallel reverse biased across the > power supply for reverse polarity protection. The maximum absolute > ratings of the load IC says -0.3V to +30V. If reverse power is > applied, anything above diode's forward drop it will short circuit and > break the fuse. No issue here. If, let's say, 0.5V is applied in > reverse, the diode will not conduct and reverse supply of -0.5V will > go directly to the load IC. Since it is below the min -0.3V, will it > damage the IC? > > -Markj
Blowing fuses to save an ic is not a very good idea. Can you put the dide in series with the load instead of in parallel across the supply? That way if the power supply leads are reversed, only the diode leakage current will flow. This is sped'd at 5uA at room temperature, which should not harm an ic.
Reply by Jim Thompson July 21, 20122012-07-21
On Sat, 21 Jul 2012 09:38:55 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Sat, 21 Jul 2012 05:24:17 -0700 (PDT), markjsunil@gmail.com wrote: > >>I have a diode 1N5402 connected in parallel reverse biased across the power supply for reverse polarity protection. The maximum absolute ratings of the load IC says -0.3V to +30V. If reverse power is applied, anything above diode's forward drop it will short circuit and break the fuse. No issue here. If, let's say, 0.5V is applied in reverse, the diode will not conduct and reverse supply of -0.5V will go directly to the load IC. Since it is below the min -0.3V, will it damage the IC? >> >>-Markj > >-0.5 volts won't hurt most ICs. > >But if you connect a chunky 30 volt supply backwards, until the fuse >blows the diode forward drop could be several volts. > >What's your normal supply voltage, and what's the load current? What >IC is it?
For sensitive applications, use an SCR fuse blower... and heavy traces on the PCB... I've measured 100A+ during "quick-blowing" a 1A fuse. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by John Larkin July 21, 20122012-07-21
On Sat, 21 Jul 2012 05:24:17 -0700 (PDT), markjsunil@gmail.com wrote:

>I have a diode 1N5402 connected in parallel reverse biased across the power supply for reverse polarity protection. The maximum absolute ratings of the load IC says -0.3V to +30V. If reverse power is applied, anything above diode's forward drop it will short circuit and break the fuse. No issue here. If, let's say, 0.5V is applied in reverse, the diode will not conduct and reverse supply of -0.5V will go directly to the load IC. Since it is below the min -0.3V, will it damage the IC? > >-Markj
-0.5 volts won't hurt most ICs. But if you connect a chunky 30 volt supply backwards, until the fuse blows the diode forward drop could be several volts. What's your normal supply voltage, and what's the load current? What IC is it? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators