TESLA SOLAR has developed Maximum power point tracking (MPPT) charge controllers, which tracks the maximum power point to charge solar batteries availing maximum efficiency from one or more photovoltaic devices(solar panels). Solar cells have a complex relationship between solar irradiation, temperature and total resistance that produces a non-linear output efficiency. It is the purpose of the MPPT system to sample the output of the cells and apply the proper resistance (load) to obtain maximum power for any given environmental conditions. MPPT devices are typically integrated into an electric power converter system that provides voltage or current conversion, filtering, and regulation for driving various loads, including power grids, batteries, or motors.
Two-phase inter-leaved switching MPPT
The power delivered by solar panels depend upon the current drawn, environmental conditions like temperature, solar radiation etc. Maximum Power Point Tracking (MPPT) is a method of extracting maximum available power from Solar Panel to load, batteries or devices like inverters etc.
How it works?
Let us consider in a solar panel if we do not draw any current (i.e I = 0A), the voltage across the panel will be 22V. If we short circuit this panel, the current will be 10A and voltage across the panel will become 0V. In both the cases, we will get 0W from the panel.
i.e, 22V × 0A = 0W
and 0V × 10A = 0W
Note – The following values are not actual, these values are given only for understanding the principle of MPPT.
If we draw 0A from the panel, the panel voltage will be 22V. Output = 0W
If we draw 1A from the panel, the panel voltage will drop to 20V. Output = 20W
If we draw 2A from the panel, the panel voltage will drop to 18V. Output = 36W
If we draw 5A from the panel, the panel voltage will drop to 17V. Output = 85W
If we draw 8A from the panel, the panel voltage will drop to 16V. Output = 128W
If we draw 8.5A from the panel, the panel voltage will drop to 12V. Output = 102W
If we draw 9A from the panel, the panel voltage will drop to 2V. Output = 18W
If we short circuit the panel, current will be 10A and the panel voltage will drop to 0V.
Output = 0W
So, if you draw less current the panel voltage will be more and if you draw more current the panel voltage will be less. But the product (panel voltage × panel current) is more at a certain point. This point is called Maximum Power Point. This point will vary according to the intensity of the sunlight and the type of the panel.
MPPT action in MEDI’s two phase interleaved MPPT solar charger is done by shifted phase modulation in interleaved switching. The DSP will vary PWM duty cycle of interleave switching for obtaining the Maximum Power Point. The DSP will continuously multiply voltage and current from the panel while varying the duty cycle of interleave switching. At a particular duty cycle the maximum wattage (V × I) is obtained. The DSP will track this point continuously and ensures the working of MPPT Charger is always at this Maximum Power Point.
Since it is two-phase interleave switching, the Maximum Power Point Tracking will be more efficient than the conventional switching MPPT chargers.
The circuit is based on DSP. The DSP will intelligently calculate at what impedance the maximum power is transferred from the panel to the battery. The DSP will scan the impedance (i.e transformation ratio between the panel and battery) of the circuit from maximum to minimum and find out at which point the maximum power is delivered. Then the DSP fixes the impedance at that point.
With this method we get 20 – 30 % extra energy than the panel directly connected to the battery.
DSP based two phased interleaved switching.
Control device – DSP
Power topology – two phased interleaved switching
Switching element – MOSFETs
Charging algorithm – Four-level charge with MC, CV, Release and Trickle.
Isolation – No isolation, positive is common
Switching frequency in each phase is 18KHz, resultant switching frequency in the final output 36KHz.
Low ripple current in input and output due to inter-leaved switching.
Panel reverse protection
Battery full charge cut-off
Battery reverse protection (optional)
Power consumption from battery during night (when no panel voltage available) < 0.02W
SPECIFICATION OF 10KVA EIGHT PHASED INTER-LEAVED SWITCHING MPPT
DSP based eight phased interleaved switching.
Control device – DSP
Power topology – eight phased interleaved switching
Switching element – MOSFETs
Isolation – No isolation, positive common
Switching frequency in each phase is 18KHz, resultant switching frequency in the final output 144KHz.
Very low ripple current in input and output due to inter-leaved switching.
MCCV battery charger with temperature compensation – Maximum Current Constant Voltage (MCCV) battery charging method. When the battery is discharged or not fully charged, the MPPT action will track the panel impedance and give the maximum current to charge the battery. When the battery voltage is close to full charge voltage, it will reduce the current to keep the battery voltage constant. The current will become zero when the battery voltage is equal or more than the full charge voltage.
Temperature compensated charging -22mv to -30mV per Deg C per battery (settable).
Internal monitoring and recording of data like panel current, panel voltage, battery current, battery voltage is done using RTC and flash memory with year, month, date, day, hour, minute with leap year correction.
Every 55 micro-second the unit will read the data and display on the LCD. Average of these samples will be taken every fifteen minutes which will be used for data logging. Data can be stored up to one year which can be monitored through RS485 / USB.
Efficiency – because of inter-leaved switched high efficiency can be achieved, >99%
Total input voltage range: 48V to 220V, Total output voltage range: 40V to 150V