Here we discuss options for providing power to the circuit itself.
The input from the renewable energy source will be variable and the input voltage will vary. We need to provide power to the microcontroller 'brains' of the device. This requires 5V DC. Hence we need to efficiently step down the voltage from the input to 5V.
The microcontroller and control components all work at 5V dc, but the incoming voltage could be anywhere from 8V to 32V dc (for a 24V battery bank). So we need to step down the variable input voltage to the required 5V.
The microcontroller is low power but still consumes some current. With the MOSFET being driven and the LEDs all at 100% the total current consumed is around 30mA. Power = V * I, so at 5V the power consumption of the circuit is 5 x 0.03 = 0.15W max. The vast majority of the time the current required by the microcontroller is lower, around 5-10mA.
The quiescent current is how much current the power supply uses when it is not providing any useful output power. The power supply itself will consume a small amount of current for its own operation. We need to ensure that this value is as low as possible to ensure a high efficiency.
We could use a linear converter, such as the classic 7805 IC. Linear power supplies step down the voltage by dissipating excess power as heat. The current rating of the output is the current rating of the input. For example: if an output of 100mA at 5V is required, then the input will be the input voltage with the same current (for example 24V at 100mA).
In this example the power dissipated is (24V-5V)*100mA = 1.9W. That makes the efficiency of the power supply to be just 20% of the power in is used as useful power out.
If we keep the input current very low then we can use a linear converter. For example if we are drawing just 1mA in the above example the we are just dissipating 0.019W, although the efficiency is the same.
Here are some available linear converter ICs. These are all through hole devices.
We could also use other techniques to convert the voltage, such as DC to DC conversion, which has the potential to be more efficient.
DC to DC conversion uses some form of switching element to convert from one DC level to another. While more complex, they have the potential to be more efficient as a converter, especially at higher input voltages.
There is an interesting application note on DC-DC converter topolgies from Maxim here (Tutorial 660).
A review of available DC-DC converter designs shows the following as suitable for this application. We need an input voltage of up to 60Vdc (for a fully charged 48V lead acid battery Note: this was the initial design specification, this has since been reduced to 32V DC). The current consumption of the circuit is in the region of 100mA maximum (this needs to be tested and checked), so we must supply 100mA. The DC-DC converter will always step-down (also called a buck converter). Another factor is cost - I am trying to keep the circuit easy to build and inexpensive. Here we review some available ICs which can perform this function and application notes which might be useful. Texas Instruments/National Semiconductors has a design calculator for DC-DC converters available here. This suggested designs based upon LM5574, LM5010A, LM34923 which gave efficiencies greater than 70%.
(Note: Prices checked on 7/2/12, they might not be accurate when you read this)
As you can see there are lots of solutions. The main limiting factor is the high input voltage requirement to run this circuit up to 60V DC. There are only a few options when this is factored in.
The final circuit board will be designed in such that either a 'standard' 7805 linear regulator or the efficient high-voltage DC-DC converter circuit can be used. This might take up a bit more PCB area but allows people to easily get started with no hard-to-source components, but then it can be upgraded to use the converter circuit.
The voltage regulator prototype chosen was a switching regulator based upon the LM2574 0.5A step-down switching regulator. As mentioned before linear regulators are not an efficient way of regulating voltage as they must dissipate any excess voltage drop at the supply current (volts x amps = power) and hence they get hot. This IC comes in two versions, the normal one for up to 40V operation and a high voltage version for up to 60V operation (but costing twice the price). I used the standard configuration circuit for a 5V supply (please check the datasheet). It required 2 capacitors, an inductor and a diode. The total cost of these parts is around £2.50. A bit more information on voltage regulation is on another blog post here.
(1) The parts: LM2574 IC, IC holder, 300uH inductor, 22uF and 220uF capacitor and 1N5819 diode. (2) The completed 7-40V input, 5V output efficient voltage regulator.
The circuit diagram is here:
This was tested at 30V with a 5V 100mA load. The input current was around 20mA (not accurately measured) so the efficiency was:
Power in = 30V x 0.02A = 0.6W
Power out = 5V x 0.1A = 0.5W
Therefore efficiency is around 0.5/0.6 x 100 = 83%, which is pretty good and certainly better than the 20% efficiency that a linear regulator would have.
The main disadvantages of this are the cost of components, the complexity of the circuit and the use of specialist components.
The final design uses the LM2931 linear converter. This was chosen as it was low cost, had the correct voltage and current ratings and had relatively low quiescent current. This can be replaced by any of the ICs above, if this particular device is not available locally.
To ensure that the circuit is efficient, we must also pay attention to the power consumed by the microcontroller.