OPS - Energy Harvesting Name: Alex Phipps Goals: Simulate the direct charge circuit to determine why the efficiency is so low. Assist with presentation preparation. Completely understand Shengwen’s circuits. Key accomplishments. Simulated data for the direct charge circuit. Measured the input and output power with an optimal load. Measured input and output power with varying R1 values. Compared simulated efficiencies with measured ones. Nearly completed the setup of the charge pump circuit. Key issues Simulations still do not match up with the measurements. The sims are very inefficient. Rusty with Cadence and buffer design.

Direct charger efficiency

Model used for simulations

Explanation of direct charger inefficiency The cantilever beam is very non-stable in its parameters. There needs to be a calibration done to find the R1, C1, etc, parameters every time that tests are performed with the beam.

Charge Pump Name: Alex Phipps Goals: Key accomplishments. Key issues Work on first draft of test plan. Conclude the individual simulations of the ring oscillator and charge pump circuit. Combine the two individual components together (oscillator and pump) and simulate the entire circuit. Begin to optimize/size the design so that everything will fit on the chip. Key accomplishments. Individual simulations of the oscillator and charge pump completed. Waveforms behavior is as expected. The simulations of the entire circuit work properly. Key issues New design goal. Small reservoir for quicker charge. Optimization of the circuit is slow due to complexity and inexperience.

Old Design Vbat>1.2V Vbat<1.2V In the original circuit, the charge pump is used to bring the battery voltage up to 1.2V. At this threshold, the controller is turned on, and Shengwen’s power processor takes over. The charge pump allows the voltage driving the circuit to be less than 1.2V. The charge pump is very inefficient, and is turned off once the battery voltage is high enough to power the controller. Vbat>1.2V Vbat<1.2V

New Design Goals The battery acts as a giant capacitor, and the time required to charge it to 1.2V will reflect the initial charge on the battery. When the battery voltage is very low, the charge pump will have to be on for longer periods of time, which lowers the efficiency of operation. The solution is to use a secondary reservoir/capacitor. A smaller capacitor is can be charged up to 1.2V more quickly than the large battery.

New Design Issues There are several control issues that arise due to the second reservoir. We want to use the charge pump as little as possible due to its inefficiency. When the battery voltage is close to 1.2V, the capacitor will only have to power the controller for a short time while the converter charges the battery. Once this occurs the charge pump can be switched off. However, when the battery voltage is closer to 0V, the converter will take longer to charge the battery, and the charge pump will have to maintain the capacitor voltage for an extended period. These issues will have to be addressed with Shengwen, as I am not yet an expert with his control scheme.

Test Plan There are 3 main components of this design: Charge pump, ring oscillator (clock), and reservoir. The first draft test plan is seen below: Explanations for these test points follow.

Ring Oscillator / Buffer The ring oscillator is very sensitive to variations in the load capacitances of each inverter. A buffer is used to minimally load the oscillator and the two clock signals can be taken from the output of the buffer. Measurement directly from the oscillator could cause excessive loading from probe capacitance. A small resistance will be introduced between the buffer and the charge pump because the signal is taken off of the chip. The resistance is from bond pads and from the wire needed to connect the two pins. The buffer design will have to take this parasitic effect into account.

Voutpump Voutpump will allow for the output of the charge pump circuit to be tested independently of the reservoir. The capacitance that is being charged can be varied to examine and debug charge pump circuitry. The possibility exists that the charge reservoir will need to be too large to put on chip. There is a finite capacitance that can be achieved, and this may not be large enough to hold the charge that Shengwen’s circuit needs.

Voutcap Voutcap will be used to measure the amount of energy that can be stored on the reservoir. Connecting it to different loads will allow us to see how the well the charge pump is able to maintain the voltage when energy is being removed.

Simulations Simulations of the entire circuit have been successful. Generic values transistor and capacitor values were used without concern for practical sizing. Design optimization will be needed to determine transistor and capacitor sizing.