Design of low-cost solar battery charger without MCU

The leakage problem of solar panels can traditionally be solved by using a Schottky diode in series with the solar panel, but the forward voltage drop of the Schottky diode causes it to consume a lot of power under high current conditions. Therefore, expensive heat sinks and delicate layouts are required to keep the Schottky diode at a low temperature. So, is there a low-cost solution? How to solve the problems of "floating voltage control to fully charged battery" and "loading the panel at the optimal power generation point" that most bother designers in solar battery charger design? In the following, Linear power experts will introduce you to the company's latest low-cost solution.

Solar panels have gained widespread acceptance as a practical method of generating electricity in both commercial and residential settings. However, despite advances in technology, solar panels remain expensive. Much of this high cost comes from the panels themselves, where the size (and therefore cost) of the panels increases as the required output power increases. Therefore, it is important to maximize panel performance in order to achieve the smallest, most cost-effective solution.

Generally speaking, energy harvested from a solar panel is used to charge a battery, which in turn provides power for end-application circuitry in the absence of sunlight. To achieve the best design for a solar battery charger, it is necessary to understand the characteristics of the solar panel. First, due to the large bonding area, solar panels will leak, and the battery will discharge through the panel in dark conditions. Also, each solar panel has a characteristic IV curve with a maximum power point, so when the load characteristics do not match the panel characteristics, energy extraction will be reduced. Ideally, the panel would be continuously loaded at the maximum power point to fully utilize the available solar energy and thereby minimize the cost of the panel.

Typically, the panel leakage problem is solved by placing a Schottky diode in series with the panel. Reverse leakage is reduced to a very low value; however, the forward voltage drop of the Schottky diode (which dissipates a lot of power under high current conditions) still causes energy losses. Therefore, expensive heat sinks and careful layout are required to keep the Schottky diode cool. A more effective way to solve this power dissipation problem is to replace the Schottky diode with a MOSFET-based ideal diode. This will reduce the forward voltage drop to as low as 20mV, significantly reducing power dissipation while reducing the complexity, size and cost of the heat sink layout. Fortunately, this goal is easily achieved because some IC suppliers already have ideal diodes with such specifications (for example: the LTC4412 provided by Linear Technology).

However, two issues remain: “float voltage control to a fully charged battery” and “loading the panel at the optimum power generation point.” These issues can often be solved by using a switch-mode charger and a high-efficiency buck regulator.

Linear Technology has developed such a circuit, which consists of the LTC1625 No RESNSE (no sense resistor) synchronous buck controller, the LTC1541 micropower operational amplifier, comparator and reference, and the LTC4412 ideal diode. The circuit is given below for reference:

Figure 1: Peak power tracking buck charger maximizes efficiency

The circuit in Figure 1 is placed between the solar panel and the battery to regulate the battery float voltage. An additional control loop based on the LTC1541 forces the charger to operate at the maximum panel power point. This increase in efficiency reduces the required panel size, thereby reducing the cost of the overall solution. This circuit has important advantages that are particularly evident when there is a mismatch between the peak panel supply voltage and the battery voltage.