Apart from the sun,
solar power systems cannot work without a reflow protection diode
between the solar panel and the energy store. When current flows into
the store, there is a potential drop across the diode which must be
written off as a loss in energy. In the case of a Schottky diode, this
is not less than 0.28 V at nominal current levels, but will rise with
higher ones. It is clear that it is advantageous to keep the energy loss
as small as possible and this may be achieved with external circuitry
as shown in the diagram. The circuit is essentially an electronic switch
consisting of a high precision operational amplifier, IC1a, a Type
OP295 from Analog Devices, and a MOSFET, T1.
This arrangement has the advantages over a Schottky diode that it
has a lower threshold voltage and the lost energy is not dissipated as
heat so that only a small heat sink is needed. When the potential at the
non-inverting input of the op amp, which is configured as a comparator,
rises above that at the inverting input, the output switches to the
operating voltage. The transistor then comes on, whereupon
light-emitting diode LD1 lights. Diode D3 clamps the inputs of IC1a so
that the peak input voltage cannot be greater than half the threshold
voltage, provided the values of R3 and R4 are equal.
The op amp provides very high small-signal amplification, a small offset voltage, and consequent fast switching. The MOSFET changes from on to off state and vice versa at drain -source voltages in the microvolt range. In the quiescent state, when UDS
is 0 V, the transistor is on, so that LD1 lights. The operating voltage
(C–A) may be between 5 V (the minimum supply for the op amp and the
input control potential, UGS, of the transistor) and 36 V (twice the zener voltage of D1). Zener diode D1 protects the MOSFET
against excessive voltages (greater than ±20 V). Diode D3 and resistors
R3 and R4 halve the potential across the inputs of the op amp.
This ensures that operation with reversed or open terminals is harmless. The substrate diode of the MOSFET is of no consequence since it does not become forward biased as long as the forward voltage, USD, of the transistor is held very low. The on -resistance, RSD,
of the transistor is only 8 mΩ and the transistor can handle currents
of up to 75 A. When the nominal current is 10 A, the drop across the
on-resistance is 80 mV, resulting in an energy loss of 0.8W. This is low
enough for a SUB type with a TO263-SMD case to be used without heat sink. When the current is 50 A, however, it is advisable to use a SUP type with a TO220 case and a heat sink since the transistor is then dissipating 12.5 W.
Even then, the voltage drop, USD = 0.32 V
is significantly lower than that across a Schottky diode in the same
circumstances. Moreover, owing to the high precision of IC1a, a number
of transistors may be used in parallel. The circuit proper draws a
current of 150 µA when only one of the op amps in the OP295 is used. An
even lower current is drawn by the alternative Type MAX478 from Maxim.
However, the differences between these two types are only relevant in
the low current and voltage ranges. Both have rail-to-rail outputs that
set the control voltage accurately even at very low operating voltages.
This is important since the switch-on resistance of MOSFETs
is not constant: t drops significantly with increasing gate potentials
and decreasing temperature. A experimental circuit may use an LM358 op
amp and a Type BUZ10 transistors, but these components do not give the
excellent results just described.