0
\$\begingroup\$

schematic

simulate this circuit – Schematic created using CircuitLab

When it comes to zener diodes I find it hard to get information about minimal currents needed for it to breakdown so I thought I would ask here.

In my project I have a 12S Li-ion battery, whose voltage can vary between 50V and 36V. This is quiet a big voltage difference to layout a resistor divider, so I decided to put a zener between gate and source to limit the voltage to maximum 16V.

The capacitor is there to limit in inrushcurrent for capacitve load.

About the zener diodes I can't find information in the datasheet about the minimum current needed for it to breakdown.

For the worst case I assume a Vbatt voltage of 36V, Vgate would then be 20V (Vs-Vz). The current through the diode is then 20V / 100k = 200uA.

Would that be enough current to limit the Vgs to 16V ? If not I'd have to use lower value resistors, a bigger cap and have bigger static power consumption which is not ideal for battery operated systems.

The diode I'm planning to use is MMSZ5221BT1.

Is there some information I'm missing in the datasheet or some general rule how much current a zener needs to breakdown?

\$\endgroup\$
2
  • \$\begingroup\$ I think this answers your question. electronics.stackexchange.com/questions/80145/… \$\endgroup\$
    – Big6
    Aug 31, 2019 at 1:42
  • \$\begingroup\$ In your datasheet you have both Izk and Izt. I tend to use a value between those two. If you want better regulation, may want to move close to Izt, otherwise shift more toward Izk \$\endgroup\$
    – Big6
    Aug 31, 2019 at 1:46

1 Answer 1

Reset to default
1
\$\begingroup\$

The information is in the datasheet, you just didn't know it.

The MMSZ5221BT1 has a Zener voltage of only 2.4V nominal. Its likely that the FET won't turn on at that gate voltage. If you want the max gate voltage to be 16V then you need the MMSZ5246BT1G.

The big table on page 2 gives you the info you need. IZt the Zener current in the column heading is the test current (7.8mA in this case) for establishing the Zener voltage Vz (16.8V max). IR is the maximum current (1uA) at test voltage VR (12V).

From that info we know that the diode will draw 1uA at 12V and 7.8mA at 16.8V. So if your diode current is between 1uA and 7.8mA then the diode voltage will be between 12V and 16.8V.

If you are just using the diode to protect the gate then the exact voltage won't be that important as long as it significantly above the gate threshold and well below the max rated gate voltage. 12V to 16.8V seems like it would be.

From practical experience I would bet that at 200uA the Zener voltage would be very close, but a little below 16V.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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