Nokia DCV-15 Desktop Stand
In order to get the charging board to fit the stand, some slight modification to the Nokia stand was necessary. There is a solid piece of metal, probably copper, about one quarter of and inch thick that is attached to the inside of the stand with screws in the area where the charging board was to be added. This metal is most likely a counter-weight for the stand to make it heavier and more resistant to capsizing when the phone is in the cradle. Without this metal, the stand functions normally. The stand weighs less without it, but this is of no concern in this phase of testing. Once this weight was removed, there was sufficient room in the upper area of the stand for a PCB. The dimensions of this area were obtained using calipers.
2. PHONE:
The design aspect of this project is focused on the receiving side. For this stage of research, of which the goal is to prove that the wireless battery charger idea is feasible, it was decided to incorporate the energy harvesting circuitry and antenna in some sort of base station or charging stand. The Nokia 3570 was the first phone that was received for the research. This phone comes standard with a battery and an AC/DC travel charger. The battery included with the phone has a voltage range from 3.2V - when the phone shuts off - to 3.9V when fully charged. This battery only takes about 2 hours to charge when plugged into the wall through the travel charger supplied with the phone. This charger has an unloaded, unregulated direct current (DC) output voltage of 9.2V. When connected to the phone, the charging voltage goes to the battery voltage, appro 3.6V, and then slowly increases until it saturates at 3.9V. This charger regulates the current to around 350mA.
3. ANTENNA
The antenna plays a very important role. To charge a battery, a high DC power signal is needed. The wireless battery charger circuit must keep the power loss to the minimal.
The considerations of choosing the appropriate antenna are:
1. Impedance of the antenna
2. Gain of the antenna
Quarter-wave Whip Antenna
4. CHARGE PUMP
At this point, it is necessary to explain what exactly a charge pump is, and how it works. A charge pump is a circuit that when given an input in AC is able to output a DC voltage typically larger than a simple rectifier would generate. It can be thought of as a AC to DC converter that both rectifies the AC signal and elevates the DC level. It is the foundation of power converters such as the ones that are used for many electronic devices today. These circuits typically are much more complex than the charge pumps used in this thesis.
Power converter circuits have a lot of protective circuitry along with circuitry to reduce noise. In fact, it is a safety regulation that any power-conversion circuits use a transformer to isolate the input from the output. This prevents overload of the circuit and user injury by isolating the components from any spikes on the input line. For this thesis, however, such a low power level is being used that a circuit this complex would require more power than is available, and it would therefore be very inefficient and possibly not function. In that case, it is necessary to use a simple design. The simplest design that can be used is a peak detector or half wave peak rectifier. This circuit requires only a capacitor and a diode to function. The schematic is shown in Figure 4.1. The explanation of how this circuit works is quite simple.
The AC wave has two halves, one positive and one negative. On the positive half, the diode turns on and current flows, charging the capacitor. On the negative half of the wave, the diode is off such that no current is flowing in either direction. Now, the capacitor has voltage built up which is equal to the peak of the AC signal, hence the name. Without the load on the circuit, the voltage would hold indefinitely on the capacitor and look like a DC signal, assuming ideal components. With the load, however, the output voltage decreases during the negative cycle of the AC input, shown in the figure 4.2.
Figure 4.1: Peak Detector
Figure 4.2: Half-wave Peak Rectifier Output Waveform
This figure shows the voltage decreases exponentially. This is due to the RC time constant. The voltage decreases in relation to the inverse of the resistance of the load, R, multiplied by the capacitance C. This circuit produces a lot of ripple, or noise, on the output DC of the signal. With more circuitry, that ripple can be reduced.
The next topology presented in Figure 4.3 is a full-wave rectifier. Whereas the previous circuit only captures the positive cycle of the signal, here both halves of the input are captured in the capacitor. From this figure, we see that in the positive half of the cycle, D1 is on, D2 is off and charge is stored on the capacitor. But, during the negative half, the diodes are reversed, D2 is on and D1 is off. The capacitor doesn’t discharge nearly as much as in the previous circuit, so the output has much less noise, as shown in Figure 3.4. It produces a cleaner DC signal than the half-wave rectifier, but the circuit itself is much more complicated with the introduction of a transformer. This essentially rules this topology out for this research because of the space needed to implement it.
Figure 4.3: Full-wave Rectifier
Figure 3.4: Full-wave Rectifier Output Waveform
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