Reducing quiescent current of MOSFET pullup/pulldown

Question

I have a device which will run off a 2.7-60V power source. The voltage may go up to 80V during surges.

I have a couple of input protection circuits which use N or P channel MOSFETs and require pullups or pulldowns.

Currently I am using a resistor along with a Zener to prevent exceeding the MOSFET VDS. I believe the resistor needs to be chosen to provide at least as much current as specified in the gate-source leakage characteristic plus the correct biasing current for the Zener diode. However, sizing the resistor for operation at 60V means there is not enough current at 3V. Sizing it for 3V means there is too much current draw at 60V.

I would like to reduce the quiescent current to close to 10uA (gate-source leakage.) Is this possible?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Firstly , I’d like to point out that your Gate & Zener to Source are reversed with Drain.

Second , this is by no means an optimal design but rather a paper napkin type of quick and dirty current limiting voltage clamp tested from 3V with an 80V swing.

A Vgs=1V low RdsOn FET was chosen with almost constant current averaging 8uA total bias current over this supply range with a low side DRAIN rise of 13mV @ 1A. Any zener could be used with low leakage. FET gate leakage must be exceeded.

Simulation

So there must “Better designed” IC solutions that can integrate these functions.

Answer 2

While 80 V LDOs are not available, you can replace R1 with a JFET with gate shorted to source. This acts as a simple current source.

However, 80 V JFETs are also not available, but you can put two 40 V ones in series. As they approach breakdown, they will share the applied voltage. A part like MMBF4117 may be suitable.

If you are not comfortable with them sharing voltage, you can also put 40 V zener diodes in parallel with each one (and another low value R in series with the whole chain).

Answer 3

I use similar circuits often, a kind of "better diode". See if this helps:

I try not to switch GND. It can bring you many troubles with other ground in a system, and EMI related issues.

Attached is one I picked from my circuitry, not for 60V, but to explaining purpose. I know it is not a direct answer, but a solution I can think of.

1. See if you can find a right FET, which has enough high VDS & VGS.
   
2. If you found what you like, get rid of the headache, Zener (D4 in my circuitry). Set R33 + R38 > 600K ohm, that is 10uA at 60V. And, set (60 * R33 / (R33 + R38)) < VGS-MAX.
   

If you find low operating current Zener, that may be added.
Another way to try is adding a current limit / current regulator. A simple BJT can do enough. But, it will be as much difficult as what you are trying to do now, due to the wide dynamic range and low current.
BTW, the zener in my circuitry is at a different side for your purpose.