APPLICATION NOTE 005
Benjamin Bacon, Owner, Plane Space Design
The POWIM-PD models that are designed to have the voltage source connected to their PWR1 input be greater than the voltage connected to their PWR2 input are not provided with a means to ensure that this condition is enforced. The external circuit can provide for the maintenance of this condition, but if that is not done the chance that PWR2 will become greater than PWR1 exists.
The plot shown above is the results of a PSPICE simulation that represents the case where the source connected to PWR2 becomes greater than the source connected to PWR1. In the plot the curve labels and the POWIM-PD circuit connection points that they represent are as follows.
| POWIM-PD CIRCUIT ELEMENT | PLOT LABEL |
|---|---|
| PWR1 | V(n001) |
| PWR2 | V(n006) |
| Current from PWR1 | Ix(Q5:D) |
| Current from PWR2 | Ix(Q4:D) |
| POWIM- Output Voltage | V(load) |
The test circuit was simulated as if the Pulsed Driver was replaced by the impedance of the power resistor in the pulsed driver circuitry. The simulation proceeded as follows. The PWR2 input voltage climbs from 10.0 V to 13 V in Piece-Wise Linear steps. The PWR1 input voltage stays fixed at 12.5 V throughout the simulation. The time length of the simulation is 30 milliseconds.
The results of the simulation showed that the output current from PWR1 persists on the load until the PWR2 voltage rises to equal the POWIM- Output Voltage. At this point the PWR2 current starts to increase rapidly and the PWR1 current starts to decrease rapidly. As the voltage on PWR2 continues to increase more current comes from this source and less current comes from PWR1. The sources pass through a current sharing point where they both provide the same magnitude of current to the load. This point is reached in less than 1 millisecond.
By the time the voltage at PWR2 rises to equal the voltage at PWR1 virtually all of the load current comes from PWR2. Eventually, as PWR2 becomes greater than PWR1, the current from PWR1 to the load stops.
Some key points to notice are
As soon as the voltage at PWR2 equals the voltage at the POWIM- Output, the Output voltage begins to track the voltage connected at PWR2.
The current sharing point is rapidly reached as the initial current changes are fast; and it is not long afterwards that PWR2 is the primary current provider.
Also note that for the same voltage levels, PWR2 supplies more current to the load than PWR1. That is because PWR1 also has the task of providing current to a network of resistors that controls the switching behavior of the circuit. PWR2 does not have this additional requirement and therefore puts all of its current into the load.
The channel that hosts PWR1 is not off, but is only suppressed by the PWR2 channel in terms of its share of the current sharing responsibilities. This means that PWR1 does not have to rise to its switch-on voltage to be restored to dominance. That only needs to happen if the PWR1 channel is switched off by falling below its switch-off voltage (12 V in this case). Since PWR1 remains at 12.5 V, which is above its switch-off voltage, for the entire duration of the simulation it will resume sourcing power to the load if or when the voltage on PWR2 falls below the voltage on PWR1.
The behavior that was obtained for the circuit in this simulation is reversible. That is, as the voltage at PWR2 decreases along the same path of values that it increased on, the currents and load voltage will reverse along their same paths as well and will assume the same values as they did before.