Design and Implementation of On-board Electrical Power ... - OUFTI-1

Figure 5.37: Schematics of the first dissipation system.The advantage of this implementation is that nearby all the power can be dissipated ina resistor in series with the transistor. The disadvantage is that the transistor acts as aswitch which can only be open or closed. When it is closed, the voltage of the batteries busmay suddenly drop and the control opens the switch. The circuit then enters in oscillation.To avoid this, a feed-back resistor is inserted on the comparator, introducing an hysteresis.This fixes a problem to bring another one: the dissipation system will partially discharge thebatteries each time it is activated.The circuit was tested and failed: the field effect transistor had a too high gate thresholdvoltage. Moreover, the oscillations occurred (test report: [1]).It was discarded and a second version was designed.5.4.3 Second versionThis version is closer to the example of figure 5.36. A TL1431 is used instead of the Zenerdiode, for a better precision in the regulation. The TL1431 has three PIN: the cathode, theanode and the reference. The functional block diagram of the TL1431 is shown on figure 5.38.Figure 5.38: Functional block diagram of TL1431.It looks similar to the first dissipation circuit, but it uses a operational amplifier insteadof the comparator and a bipolar transistor instead of the FET. This is an analogical circuit.When it is activated, the circuit will find an equilibrium, and the amount of power dissipated inthe shunt will exactly be the amount of exceeding power. The batteries will not be dischargedby the circuit.80