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I need to detect small high and low peaks in the battery voltage while in use. The goal is to help estimating the end of the battery charge by its change in impedance, since my circuit uses the battery in predictable cycles of high/low consumption. It's a long term li-ion battery charge (weeks usage) so other methods are unavailable or very very expensive.

So, my idea is to use a variation of the sample & hold peak detector circuit but my reset would use a FET to short the hold capacitor to vbat instead of discharging the capacitor to ground. That would help reducing battery consumption at sample resets since the capacitor only needs a small voltage jump to reset. The largest good capacitor I could find at a reasonable price is 4.7µF.

I will be taking measurements every ten seconds, so my sample and hold circuit must be very low leakage (<3mV/10s would be ideal). The thing is, if I use a MOSFET to reset the capacitor, I'm affraid its body diode's leakage could be of concern, slowly discharging the capacitor in the "sample" phase, thus altering the measurement. Unfortunately, none of the MOSFET datasheets I've read characterize the body diode's leakage, much less over the temperature range (I need 50~60°C).

So, what kind of FET is there that I could drive with 3.3V logic have no body diode, or has a documented reverse leakage below 1µA@60°C when in off state (any direction). Vbat would be in the 5~9V range.

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Even though the body diode in MOSFETs is often drawn as a physical diode next to the MOSFET, it's not actually a discrete building block of a MOSFET. It is a 'parasitic' element that exists inherently because of the silicon build-up of a MOSFET. There are no MOSFETs without body diodes, it's a fact of life.

That also means that this body diode isn't actually a purpose-made diode. It doesn't quite behave like either a signal or power diode and it has fairly poorly defined characteristics. However, it also means that it doesn't have reverse leakage like a traditional power diode. It's part of the entire package, so the datasheet usually defines an overall reverse leakage figure. For instance, completely random power MOSFET datasheet:

http://www.st.com/web/en/resource/technical/document/datasheet/CD00002217.pdf

Here, on page 4/13, it states a 'zero gate voltage drain current', i.e. leakage current. It has a typical and maximum value of 1 and 10µA respectively at Vds=max. That is what you have to design for.

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  • \$\begingroup\$ However, curiously enough, it does have a forward voltage like any normal diode, as seen in figure 11 (page 7) of the same PDF. Nevertheless I was under the impression that other types of FET could be free of this diode, like JFET for instance. Am I wrong? \$\endgroup\$ Commented Aug 14, 2014 at 15:22
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    \$\begingroup\$ The JFET is the only 'exception', but they are utterly useless for everything but some niche signal stuff. Any MOSFET handling power is going to have this inherent diode. Yes, it has a forward voltage like a normal diode and it acts like a silicon diode because it is a parasitic diode; it's an 'accidental' diode that forms because there is P and N doped silicon next to each other. However, you will notice that forward voltage drops and reverse recovery characteristics are worse than most purpose-built diodes, especially for the die sizes. \$\endgroup\$
    – user36129
    Commented Aug 14, 2014 at 20:08
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    \$\begingroup\$ All about body diodes, and more (like other parasitic "components" of a MOSFET) in this amazing two-part article: electronicdesign.com/power/… and electronicdesign.com/power/…. \$\endgroup\$
    – Sz.
    Commented Nov 23, 2015 at 15:44

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