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Schottky Diode Leakage Current  (February 19, 2011)

I got a call from a company who was scratching their head about the short battery life of their 3v lithium battery powered product.  Their engineers said that the battery would last for years but testing had shown that the battery only lasted a few months.  They needed my help to figure out where the excess current was going.
The company sent me a few working units and a documentation package.  I looked over the circuit schematic and saw right away the problem.  The product was a low power device, which spent most of its time in a standby mode.  Its microprocessor would wake up periodically, collect some data, spit out a RF message, then go back to sleep. It was powered by a 3v lithium coin type cell.
To prevent damage to their circuit, should the battery be installed backwards, the company used a classic diode across their 3v battery input as shown below.  If the battery polarity were reversed, the diode would be forward biased, clamping the voltage to about 0.4v.  The series resistor would limit the current in this condition.  I took one look at this circuit and knew right away where some of the excess current was going.  They were using a schottky diode instead of a signal diode in this circuit.  Many engineers make the mistake, believing that a schottky diode would operate much like a signal diode in a reversed mode.  Well, it does, to a point.  Schottky diodes do have a lower forward voltage drop but they leak a lot more current when reversed biased.  As shown in the graph below, at a voltage of just 3v and at a 25C room temperature, the diode will leak about 10 microamps of current.  On a hot 40C day, the leakage current could get up to 50 microamps. For most applications, this sort of current is nothing but for this super low power product, it is enough to kill a battery before its time.
Reversed Battery Voltage Clamp Schottky Diode Reversed Bias Leakage Current
The solution is to not use a schottky diode.  A signal diode, like a 1N4148, with a high enough current rating will have a much lower leakage current as shown below.  Note that its reverse leakage current is a lot lower.  However, its forward drop is much higher when the current reaches about 50ma.  Based on the curve, the voltage could reach 0.8v volts at 50ma. On some circuits, the maximum negative supply voltage really needs to be less than -0.6v but in this particular product the slightly higher negative voltage is not a problem. 
1N4148 Reverse Bias Leakage Current 1N4148 Forward Voltage
In conclusion schottky diodes are great devices but their higher reverse bias leakage current can cause problems when used in ultra low power circuits.   

March 2011     Issue 15

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