Simplifying Small Solar Systems Using Hysteretic Controllers

Battery-based solar systems in the 10W-100W range typically use switching regulators to control battery charging. These have the advantage of high efficiency and facilitate peak power point tracking, but at the expense of inductors, circuit complexity, and noise. As a simpler alternative to switching regulators, linear control is available in applications up to about 20W. While simple and quiet, linear charge controllers generate heat that must be removed via a heat sink. The size, cost, and assembly complexity of the heat sink somewhat offset the advantages of a linear charge controller over a switching regulator approach.

A hysteretic controller that simply connects or disconnects the solar panel as needed to limit the battery's state of charge provides an excellent anode, one without the inductor, complexity, noise, and heat dissipation.

Series and parallel hysteretic switching topologies are possible. A series configuration opens the connection to the solar panel when the battery reaches its maximum charge voltage, then reconnects when the battery voltage drops to a lower threshold. The main difficulty with the series configuration is driving the high-side switch, which requires a charge pump for N-channel implementations or a high-voltage, high-side gate drive circuit for p-channel MOSFETs.

The preferred shunt arrangement is shown in Figure 1. In this case, switch (S1) is closed when the battery voltage is below a certain threshold, allowing the solar panel current to charge the battery. When the battery voltage exceeds a second, higher threshold, the switch opens to divert the solar panel current to ground. Diode D1 isolates the battery when S1 shorts the solar panel. The switch can be easily implemented with an N-channel MOSFET, driven directly by the output of a ground-referenced comparator.

Figure 1. A shunt-mode hysteretic switch regulates battery charge in a small solar system.

Figure 2 shows a complete shunt charging controller for a 12V lead-acid battery using an LTC2965 100V micropower voltage monitor as the control element. Although it does not monitor 100V in this application, the LTC2965’s 3.5V to 100V operating range covers the voltage range of a normal 12V battery with ample margin.

Figure 2. Shunt-mode hysteretic regulator. The trip points are temperature compensated over the 0°C to 50°C range.

The LTC2965 contains a ~78M, 10:1 divider that monitors the battery voltage at the VIN pin. The threshold is generated from a precision 2.412V reference via a separate external divider and coupled to an attenuated version of VIN. This arrangement eliminates the need for precise, high value resistors in the main divider.

Hysteresis is set by switching the comparator’s inverting input back and forth between high and low thresholds at the INH and INL pins. These trip points determine the battery voltage at which charging begins and ends.

Other important features include the LTC2965's low power operation (40μA total supply current including Q1 gate driver), built-in 0.5% accuracy reference, and hysteresis for independent threshold adjustment.

Operation is as follows. Initially, with a battery voltage less than 13.7V the comparator output is low and Q1 is off, allowing all available solar panel current to reach the battery through D1 and the load. As the battery charges, its voltage rises and when it reaches the upper charge limit of 14.7V, Q1 turns on, shorting the solar panel to ground. D1 isolates the battery from the shunt path. With Q1 turned on, the rate at which the battery voltage drops depends on the state of charge and the size of the load current. When the battery voltage reaches the floating lower limit of 13.7V, Q1 turns off and the panel current is once again applied to the battery and load.

This charging scheme shares certain attributes of cyclic charging and trickle charging. The initial charge proceeds until the battery voltage reaches 14.7V, after which the circuit begins pulse charging to complete the process.

It is important to properly size the battery and solar panel for a particular application. As a general rule, select the maximum or "peak" panel current equal to 10 × the average load current over a 24 hour period and the battery ampere hour capacity equal to 100 × this same average figure. The peak current for a 36 cell panel is estimated by dividing the panel's claimed "marketing" wattage by 15. A 15W panel can be expected to produce a maximum of ~1A output current under favorable conditions, but this should be verified by actual measurements of the panel under consideration.

These relationships are derived from the assumption that a 4 day run time in Milpitas, California will be provided using no auxiliary battery power, using the panel facing maximum winter sun exposure. In the case of Figure 2, the circuit is designed for a continuous 100mA load (2.4Ah/day), indicating the use of a 1A panel and 10Ah battery. The somewhat smaller battery specified in Figure 2 is undersized, giving approximately 3 days of run time, deprived of any solar input.

The charge threshold is temperature dependent by an NTC thermistor in the 0°C to 50°C range. If operating in a controlled environment, temperature compensation is unnecessary and the thermistor and 150k resistor can be replaced with fixed, 249K units. For applications that wish to eliminate the error introduced by the 1% resistors, Figure 3 shows a simple scheme to trim the charge threshold by ±250mV.

Figure 3. ±250mV trimming scheme. Added to Vref and INH pins in Figure 2.

While solar panels are typically oriented to collect maximum total energy per year, a standalone system must be optimized for operation under a variety of conditions with minimal seasonal insolation, making allowances for coincidental weather patterns. Of primary concern is solar panel orientation, which is a science in itself. Calculations for a theoretical ideal, fixed orientation are relatively simple; however, many non-ideal factors including atmospheric scattering, fog, clouds, shadows, horizon angle, etc. make this science inexact at best.