Hi,
I've been dabbling with a small sensor project with a battery backed power supply. In short I want 5V power from a USB supply on the regular with a single cell LiPo as the backup. Simple enough, a charger chip, a couple of diodes and a mosfet should do the trick. So, i sketched one up, built it up on a breadboard, tested for
several minutes and felt rather confident with myself. A couple of weeks later and the PCB shows up in the mail, solder it up and things are no longer as rosy as on the breadboard...
My problem is that I get battery voltage on the USB port when USB is disconnected (no 5V input). To be presise, I measure 3.45V on the USB connector when the battery is connected (no USB power), which is about a diode drop less than the 4.1V charge voltage of the battery. There is a Schottky diode in circuit to prevent this so I started to do some digging into other possible reasons for how this "reverse" voltage manages to get to the 5V rail. Both diodes was soldered in correctly (not reversed). I suspected maybe the charger chip could have some strange quirks and leaking back to Vdd, but nope, it is not the culprit. Turns out the Schottky diodes I got from DigiKey must be dodgy.
More on why I believe that in a bit.
Attached is the schematic of the power circuit.
As is evident, D1 will prevent the battery voltage from reaching Vdd (5V rail) so the charge chip will be unpowered and no battery voltage on the USB connector when the battery is in use. One could say that there is a pull-down resistor missing on the gate of Q3, but that is taken care of by R1 and R2 (the voltage divider for measuring the USB voltage) so pulling down the gate to enable the mosfet is covered. D2 protects the battery from getting 5V through the integral diode of Q3. Q1 powers the charge indicator and Q2 is added to the voltage divider for measuring battery voltage so that it does not drain when not being measured.
For my breadboard tests I used a BAT42 as D1 and D2, it's only rated for 300mA, but that is more than sufficient for my load.
For the actual PCB I selected
Toshiba CUHS20S30H3F as it has a very low forward voltage (250mV@500mA).
After eliminating all the reasons I could think of as causes for my issue, I started testing the CUHS20S30 diodes and sure enough, when applying 5V in reverse (5V on the cathode), I get 5V on the anode -
not what I expected for a 30V reverse rated diode.
So, on to testing with some meters where things got interesting... My bench multimeter (Rigol DM3058) shows me a forward voltage of 65mV and OPEN in the reverse direction. As expected and the same was the story with my other multimeters, EXCEPT the "disposable" little Aneng AN8009 that I keep in a little mobile toolbox. That little sucker shows me a forward voltage of 80mV, and
in the reverse direction it shows 2.9V And, I did test several diodes.
Now, this is supposed to be a 30V Schottky diode, not 3V so did I get the wrong part?
Nope, not according to the datasheet which states that the marking on the diode should be "86", which under the microscope is correctly marked on the diodes I received from DigiKey.
Links to DigiKey and the datasheet at Toshiba:
https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/CUHS20S30-H3F/9749634?s=N4IgTCBcDaIMIFUASBlMAGFBmdIC6AvkAhttps://toshiba.semicon-storage.com/info/docget.jsp?did=63606&prodName=CUHS20S30So, anything obvious I am missing or misinterpreting, or have I simply got parts from a dodgy batch of Toshiba diodes?
BR,
Knut