This is a simple solar boost converter and voltage limiter circuit that charges a 12V battery from a 6V solar panel. It also demonstrates MPPT (Maximum Power Point Tracking) capability. When we think of MPPT, we generally think of microcontrollers and complex power computing algorithms, but such computing power is not actually required. Note that this concept may be new to the world.
Two schematics are provided. The first simply illustrates how the boost switching converter topology functions, while the second is a workable DIY schematic. This is recommended for the more advanced experimenter who has an oscilloscope at his disposal. It is also a great experiment for students and those who wish to expand their minds a little.
Photos of the prototype
Boost converter theory
Per the boost converter topology sketch, inductor L1 charges when Q1 turns on. When Q1 turns off, L1 discharges into the battery via D1. Performing this simple operation thousands of times per second results in appreciable output current. It is also called inductive discharge. For this to function, the input voltage must be lower than the output voltage. Also, with a solar panel source, energy storage in the form of a capacitor (C1) is required so that the solar panel may continue to output current between cycles.
Boost converter circuit schematic
The circuit consists of essentially three sections including a 555 MOSFET gate driver, 555 PWM modulator and op amp voltage limiter. The 555 with its totem pole output can source as well as sink roughly 200mA and makes a great low power gate driver. The 555 PWM modulator is the classic 555 oscillator circuit. To regulate the C3 discharge time (inductor charge time), pin 5 is held at a regulated 5V.
Op amp U1A integrates the battery voltage signal when the divided set point voltage is compared with the 5V reference. When the voltage exceeds the setting, the output integrates in the negative direction thus reducing the repetition rate of the PWM generator and limiting any subsequent charging. This effectively prevents overcharging.
Circuit power sourced by the solar panel
To prevent unnecessary battery discharge when the sun is not shining, all circuitry is powered via the solar panel. The one exception is the voltage feedback divider that draws approximately 280uA.
Logic level MOSFET
Since the circuit must operate at low voltages (this one works down to about 4V input) a logic level MOSFET is required. It turns on at fully at 4.5V. The device I actually used was the MTP3055.
D2 voltage clamp
In this circuit, the battery MAY NOT BE DISCONNECTED or the MOSFET will self-destruct when it turns off. Since this is too much to be expected, 24V zener D2 performs a safety clamp function. Without this, I, myself would have destroyed many MOSFETS.
As the solar panel voltage /current increases, the PWM generator increases its repetition rate thus resulting in increased output current. At the same time, additional voltage is applied to the inductor thus increasing its charge current. As a result, the boost regulator really digs in as the voltage increases, or lets up as the voltage diminishes. To achieve maximum transfer of power with full sunlight, potentiometer R8 is adjusted so that the battery charging current is maximized –this is the maximum power point. If the circuit is operating properly, there will be a very shallow peak as R5 is rotated. Diode D3 makes the automatic MPPT adjustment function more sensitive by subtracting a fixed voltage from the voltage difference between the battery and the average voltage across C3. Under lower light conditions, you will find that R3 is not exactly at optimum, but it will not be significantly off. Note that intelligent MPPT controllers can do a better job across the full range, but such improvement is very marginal.
This circuit is tuned for a 9V, 3W solar panel. Boost regulators tend to be finicky and will not operate over a wide range of conditions –if your system uses a different solar panel power rating, expect problems. The only items that need adjustment are the inductance of L1 and the value of C3. I was surprised that the repetition frequency turned out so low (approx 2kHZ). I started with a 100uH inductor, but it just seemed to work a whole lot better with the 390uH inductor –originally, I wanted about 20kHZ. For best operation, plan to charge the inductor to about 5 to 10 times the solar panel current, and then allow an extended period of time (3X) for the inductor to completely discharge. This allows for acceptable operation when the source gets close to the battery voltage. Note that low resistance inductors offer the best efficiency. The greatest loss actually occurs in the schottky diode, and lowest loss is what these diodes are noted for.
High frequency operation is generally preferable in order to minimize the inductor size. However, for experimentation, use what works best.
Suggested components are indicated on the schematic. Of course, the charger may be scaled for actual requirements.
For the future
Undocumented words and phrases –for our ESL friends
dig in –idiomatic phrase –literally to dig a hole –in electronics, it indicates extreme effort