reverse voltage protection with parallel diode

-Markj Probably not, but why not put the diode in series with the PSU feed ? Or a PFET ?

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?

-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?

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?

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

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.

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'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. Si= nce it is below the min -0.3V, will it damage the IC?=20

hat

Thanks. The IC is TPA3111. Absolute max Vcc is -0.3 to 30V.

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.

...Jim Thompson

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

Why? I generally have used PTCs instead of fuses and TVSs instead of diodes but the idea is the same.

For low currents, this works, as long as you don't need the voltage or regulation. It doesn't protect against over-voltage, though.

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

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.