In-line switch for energy harvester

Question

I have a seemingly simple problem.

I am developing a system that harvests energy from a piezo-electric vibration source. The harvesting circuit works well and I am happy with that, but I also want to use the vibration source as a sensor to measure vibrations, and the harvester is interfering with that because it draws a current from the high-impedance source, distorting the signal from the vibration.

I thought to include a switch before the harvester circuit so that I can switch it off from a microcontroller temporarily, disconnecting the harvester from the source. The concept is depicted in the diagram.

The question is now, how to implement this.

The difficulty lies in:

• the unknown voltage level at the source input
• the relatively low voltage at the output of the MCU (1.8 V)
• the requirement that the switch must be closed when the MCU is off, as the harvester needs to be able to cold-start the system

I came up with a somewhat complicated solution, using a depletion mode JFET, that cuts off the voltage if the gate-source voltage is positive. This still has the drawback that, as I see it, the JFETs have a fairly high on-resistance if V_GS is zero (around 200 Ω, which is very considerable for the harvesting application).

Furthermore, I need two additional transistors to achieve the highest possible Vgs, when I want to switch the JFET off.

Am I missing some easier solution here? It seems like a not-so-uncommon problem to me, but I couldn't really find something fitting here. Are there ICs that you know of that would serve the purpose?

Answer 1

after quite some searching, I found this excellent IC (FSA515) from OnSemi. It is a depletion mode switch, conducting with low Ron (<1Ohm), when no supply voltage is applied and isolating when there is a supply voltage.

Didn't know that existed, but that definitely solves my problem.

I would still be interested in other approaches if you have some.