1
\$\begingroup\$

I'm a hobbyist and I've designed and built (fabbed a small circuit board, schematic attached) a circuit to power a brushless DC motor using a solar panel which also charges a stand-by battery and powers other support circuitry.

The small 2S Li-Po battery will source about 3A current when needed by the motor and can tolerate about 150-200mA when charging, up to about 8.4V. The solar panel can source up to about 3A of current to supplement the battery to power the motor and, at low current demand, can reach over 9V. When not powering the motor, the Solar_Motor voltage and current can be controlled to charge the battery at a low rate. When the voltage difference (Solar voltage - Battery voltage) is less than about 0.75V I can PWM the signal to the NPN transistor to control the FET to limit the current and voltage presented at the battery for charging. I can turn the transistor off and no measurable current flows from solar to battery. Note that the D1 diode has a lower forward voltage than the protection diode in the FET for when the motor demands large current from the battery, so I don't think I have damaged that protection diode or the FET.

But I think I've done something naïve or ignorant with the circuit and here is my problem: When the solar voltage exceeds about 0.75V above the battery voltage, I can reduce the PWM duty cycle on the base of the transistor to 0 and current still flows. I need to control the current in order to protect the battery while charging. In fact, if I hard-code the output labeled DIO_Low to LOW (near 0V potential) and increase the 'solar' (a bench supply for testing) voltage difference to 0.75V or more, I see the current flow into the battery slowly increase as I increase the voltage difference.schematic of circuit

The problem seems to be related to the relative voltage between Solar_Motor and VBatt, not the absolute voltage of Solar_Motor because I have tested with different states of charge on VBatt with varying Solar_Motor voltage and the problem only occurs when the difference exceeds about 0.75 Volts.

I have tested resistance when the circuit is unpowered and am fairly confident there aren't any unintended solder 'whiskers' bridging any parts of the circuit. I think the crux of the problem may lie in a subtlety of the semiconductors that I don't fully appreciate. Or maybe I have just done something dumb (?). Thanks, if you can help.

\$\endgroup\$
4
  • \$\begingroup\$ It's likely not the MOSFET - the circuitry around it looks okay. My guess is that there's current flowing through the signal wires going to the BLDC controller. Try desoldering that MOSFET entirely and see if the unwanted current is still flowing. (Or unplug the BLDC controller's signals.) \$\endgroup\$
    – Jonathan S.
    Commented Apr 28 at 20:32
  • \$\begingroup\$ Maybe some leakage at Q5? Q6 have a very low threshold of 0.65V worst case to start to turn on. \$\endgroup\$
    – Tyassin
    Commented Apr 28 at 20:38
  • \$\begingroup\$ Thanks, I could have slightly cooked the Q5 when I was stuffing the board and now it is leaky. I'll probably try to remove, test, replace with new both of those components and see if anything changes. The thing is, a leak at Q5A seems like it would only be dependent on the Solar_Motor voltage, not the difference from the battery. I did add the inductor and D2 diode after the initial build so it is also possible I overheated the 2 pin on the MOSFET. \$\endgroup\$
    – OCPatch
    Commented Apr 28 at 23:47
  • \$\begingroup\$ Okay, I've removed Q5A (and B) from the circuit so Q6 gate is (presumably) always pulled high. I've created a way to quickly connect and disconnect the D2/L1 part of the circuit from the Q6 pin 2 connection. I've replaced the Q6 part on the board; I was careful to quickly hand-solder the part down. When Q6 is connected and the voltage difference source-to-drain across the Q6 part is greater than about 0.75V current starts to flow. When it is disconnected, current does not flow into the system or battery. The Q6 part is very small (3-pin SC-70) so I wonder if I am still damaging it or (?). \$\endgroup\$
    – OCPatch
    Commented Apr 29 at 20:32

1 Answer 1

Reset to default
0
\$\begingroup\$

And the answer is: I did do something dumb/careless/naïve. I grabbed a symbol and footprint for the p-channel enhancement mode MOSFET but didn't carefully verify the pin numbering. As a result, I had a footprint that swapped the source pin for the drain pin on the part. It was confusing because I could PWM control the current flowing through the MOSFET up to a point. Then, what I thought was some sort of leakage current was actually the protection diode in the MOSFET conducting at about 0.7-0.75V forward bias.

I flipped the (very tiny) SC-70 3-pin MOSFET part on its back, rotated it 120 degrees to make sure the base pin still lined up, then soldered that pin and the source pin (actually pin 2 on the PMF170XP) down, added a wire from the drain pin to the inductor. Voila! It worked for the range of solar voltages and different battery states of charge under different intermittent loads for the motor that I can encounter for my application.

So don't be like me. Don't assume all 3-pin MOSFETs, even those of the same type, will have the same pin assignments.

\$\endgroup\$

Not the answer you're looking for? Browse other questions tagged or ask your own question.