design ideas Simplify Small Solar Systems* with Hysteretic Controller
Mitchell Lee Battery-based solar power systems in the 10W–100W range often use a switching
regulator to control battery charge. These have the advantage of high efficiency
and facilitate peak power point tracking, but only at the cost of an inductor, circuit
complexity and noise. As a simpler alternative to a switching regulator, linear
control is feasible in applications up to about 20W. While simple and quiet, linear
charge controllers generate heat, which must be shed by means of a heat sink.
The bulk, cost and assembly complexity of a heat sink somewhat nullify a linear
charge controller’s perceived advantages 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 anodyne, one devoid of inductors, complexity, noise and heat sinking.
Both series and shunt hysteretic switch
topologies are possible. A series configuration opens the connection to the solar
panel when the battery has reached its
maximum charging voltage, then reconnects when the battery voltage falls to
a lower threshold. The chief difficulty
with a series configuration is driving the
high side switch, which requires either a
charge pump for an n-channel implementation or a high voltage, high side gate
drive circuit for a p-channel MOSFET.
The preferable shunt arrangement is …