The PV module used is a 366x539x25mm module, built by MULTICOMP, yielding 20W power at 17V. Its maximum current yield is 1.18A. This is used to charge a 12V lead battery with a capacity of 1.2Ah. To avoid the case in which a partially charged battery discharges backwards through the PV module, a diode is needed between the module and the battery. We were able to implement an even better solution, since Prof. Saggini was so kind to lend us a micro-controlled SEPIC converter which already optimizes power transmission from the module to the battery.
From the battery, we have to provide power both for the Cubieboard and the LCD panel. The former works at 5V and supports a maximum current of 2A for a total maximum power consumption of 10W, although it rarely consumes more than 5W. The latter works at 3.3V with a peak current of 900mA for a power consumption of 3W. To provide stable power supply for both, we used a DC-DC dual output buck converter built by Texas Instruments: the TPS54290. This converter, working at a fixed switching frequency of 300kHz, accepts as input a voltage in the 4.5-18V range and provides two outputs up to 90% of the input voltage. One output yields up to 2.5A in current while the other yields up to 1.5A. The converter uses internal switching and current mode control to simplify design for stabilty.
We designed passive circuits for this component as to have the first output provide power for the cubieboard and the second output to power the LCD screen.
We show here how the step-by-step design proceeds for one of the two outputs.
First of all, we have to fix values for the power stage components: if the input is 12V (battery) and the output is 5V (cubie) the duty cycle D is:
If we require the maximum ripple on the inductor current to be 30% of the output maximum current, i.e. 0.3·2.5A = 0.75A, we have to size the inductor to achieve the following:
To be safe, we chose the value of 18μH for L. Next we need to set the value for the output capacitor, this is lower bounded by the value:
Again, to be safe, we use the value C = 22μF. To design the feedback stage components, first of all we have to consider how the internal control works: the output voltage is sensed through a voltage divider and then compared to an internal fixed voltage VREF equal to 0.8V, using a trans-conductance amplifier with gm = 325μS. The output of the error amplifier is then compared with the inductor current, sampled when the high-side switch is in its ON state. The result of this operation acts as a PWM on the duty-cycle of the buck converter.
Fixing the value of R1 to 20.5kΩ, for R2 we get:
Next we have to set the gain of the error amplifier at the cross-over frequency, which is conventionally equal to fsw/10 = 30kHz, as to compute the value of the compensation network components accordingly. To do so, we use equations (5) – (7) from the TPS54290 data-sheet (see references below) and obtain a value of: KEA = 12.124dB. The compensation resistance value is then fixed to:
Instead, for the compensation capacitance we have:
This way we match the frequency of the pole due to the power stage with a zero due to the compensation network.
Texas Instruments provides a proprietary tool (TINA Spice), which allows us, given a model of the TPS54290 component, to validate our design:
Using transient, we get the following waveforms for the two output voltages:
Unfortunately, the model provided by T.I. for our component works only for transient simulations. To check stability we tried to reproduce the whole internal circuitry on SIMETRIX-SIMPLIS. Still, many details of the internal control are not fully explained in the component's data-sheet, making our results fairly approximate.
The subsequent step was to design and print an actual PCB with 12V input from the battery and two outputs: a female USB to provide power to the cubieboard and a 3.3V line output to provide power signals to the LCD panel. Furthermore the input 12V and the ground signal are forwarded to the backlighting pins of the panel.