A series commutated SCR makes a very unusual Solid State Switch in this solar charge regulator control. Advantages include simplicity and robustness. The SCR performs (3) functions: switch, latch and reverse polarity diode (reverse blocking thyristor). The SCR conducts the charging current from the solar panel to the battery while a series MOSFET performs the function of commutating (turning off) the SCR current at the end of the conduction period. All circuitry consists of readily available discrete components.
This circuit is applied in fashion similar to this previously published circuit:
Parallel forced-commutation of SCRs is relatively common, but the series commutation technique described here may be new to the world. Check out the previously published forced commutation circuit: http://www.electroschematics.com/10783/scr-based-sss-solar-charge-control/
Programmable Unijunction Transistor Gate Driver
The SCR has its gate driven by a PUT (Programmable Unijunction Transistor) gate driver. The advantage is high gate drive current (100mA peak), fast pulse rise-time and Schmitt Trigger type of operation. Q1 is wired as a free-running oscillator that outputs pulses every 6seconds (0.1HZ). Resistors R3 & R4 bias the gate of the PUT at mid voltage. When C1 charges to one junction drop above the gate voltage, it fires and the charge in C1 is dumped into the SCR gate. R5 limits the peak current and extends the pulse duration. When the PUT anode voltage drops below the threshold current, the PUT turns off and the cycle repeats.
Operation is simple –Q1 keeps firing the SCR while the voltage regulator keeps commutating the SCR via turning off the normally conducting MOSFET (Q2). When the SCR is Off, LED D3 is illuminated. When the battery is low, the LED does not light –it lights only when the battery voltage setting has been reached. When the battery is fully charged, the SCR conducts for only a short period of time in each 6sec oscillator period.
A TL431 is strapped to regulate at its minimum voltage of 2.5V. R10 biases it at about 1mA. It is referenced to the positive battery terminal. This is far superior to a low voltage zener because it has very low dynamic resistance and low thermal temperature coefficient (sharp threshold).
Q4 is a PNP transistor that performs the function of voltage comparator. It compares the output of the feedback voltage divider from the battery with the 2.5V reference (plus the Vbe junction voltage drop). As the battery voltage increases beyond this threshold, collector current begins to flow and subsequently turns on Q3. When the collector voltage of Q3 drops to zero, the MOSFET turns Off, subsequently commutating the SCR. Positive feedback is provided by capacitor C2 –it couples the change in Q3 collector voltage back to the base of Q4. This allows for clean oscillation-free switching. Resistor R14 reduces the positive feedback in order to prevent oscillation.
While a bipolar transistor has a Vbe that is temperature dependent, it tends to track the battery voltage that is also temperature dependent. No attempt was made for accurate temperature compensation.
SCR turn-off time
The S2800A has a 50uS turn-off time specification. The minimum commutation period that I observed is about 1mS –overkill.
D2 is a zener that prevents the gate voltage of Q2 from exceeding 10V. Otherwise, it could float up to the open circuit voltage of the solar panel and be subject to damage. D1 prevents reverse current through LED D3 when there is no solar activity.
Connecting two power devices in series (SCR1 & Q2) causes additional conduction loss. In order to minimize the subsequent conduction voltage drop, I selected a MOSFET with low Rds on. As a result, both SCR1 and Q2 require heatsinks. Conduction voltage drop is about 1.4V. This is not all that bad considering that a reverse polarity rectifier is not required –this is inherent in the reverse-blocking thyristor.
For the future:
Self-commutated SCR resonant link voltage regulator for 12V to 5V conversion (very exciting prospect –topology is new to the world)