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I’m looking for information regarding the resistance spec of the onsemi NSR0530HT1G (Part Number: NSR0530HT1GOSCT-ND). I am using these in charging a 3V lithium coin cell battery on my PCB, but on a couple of boards I have noticed that the coin cells were discharging at a much faster rate than the others. I measured the resistance across the NSR0530HT1G on the boards in question and read around 1-2 Mohm, whereas the diodes on the other boards’ which were not discharging too quickly read 9-10 Mohm. This leads me to believe the NSR0530HT1G diodes are defective, allowing too much leakage current through and discharging the coin cells much quicker than normal, but I cannot find their nominal resistance value on the datasheet. Could you please help me find this?
To start, you can’t really measure resistance directly on a diode because it is dynamic - depending on bias voltage and temperature, so it is not a published spec. You can calculate it in any given scenario by measuring the bias voltage and the reverse current, but measuring such low current can be very challenging.
Selecting a diode for a specific application always involves trade-offs between various specifications. Schottky’s are often preferred for their relatively low forward voltage drop, making them desirable in low-power applications, but they are one of the worst with respect to reverse leakage current.
The NSR0530H is rated to have a typical reverse leakage current of 1.4uA with a 10V reverse bias voltage at 25°C, which calculates to 7.14MΩ. However, as shown in the “Electrical Characteristics” table on page 2 of the datasheet (see below), it is only guaranteed to be less than 10uA under these specific conditions, which is about a 7x difference.
Keeping in mind that potential 7x differential between “Typical” and “Max” values, and looking at the “Leakage Current” graph on page 3 of the datasheet, it shows typical leakage currents for various voltages measured at several temperatures from -40°C up to +125°C.
If you take the worst-case (7x typical) leakage current for 3V at 25°C, it looks to be right around 7uA, which calculates to about 430kΩ for 3V. Note this is just at room temperature. If you start to warm it up a bit, the leakage current grows exponentially, making matters significantly worse.
An important lesson to learn when selecting electronic components is to always consider worst-case specifications, and to go beyond the first page of the datasheet to study the real specs and graphs. The first page is always owned by the marketing types rather than the engineers.
Thank you so much for your detailed response. I was just curious about why just these two diodes in particular are experiencing this higher leakage current/lower measured resistance. The other boards we have made that use the same parts do not experience this fast discharge of the 3V lithium coin cell, and their diodes’ reverse resistance ranges from 8-10Mohm.
Is this due to manufacturer tolerances or did we just get unlucky with these two diodes?
Typically, within the same batch of diodes on a particular day, they would generally tend to be fairly similar, but even then, there will be some variation. If they came from different lots, the chance for variation increases.
Additionally, other things could affect the leakages. It’s possible to have an effect due to differences in the soldering profiles seen between parts. If the reflow process wasn’t tightly controlled, that may cause characteristic changes between parts. A bit of excessive heat can weaken a diode and cause increased leakage.
Another thing could be related to relative conductivity on the surface of the board. Flux residue and other contaminations can increase leakage. Finally, if you notice, as the temperature increases, leakage increases significantly. If measurements were taken with different diode temperatures, the values could differ significantly.
