Why Should A Real Voltage Source BE Considered When Designing A Reliable Power Supply?

For example, a typical input voltage range for a 12V supply voltage is 8V to 16V.

However, when designing a DC-DC converter, it is not enough to consider only the input voltage minimum and maximum values. Figure 1 shows a buck converter with a switch at the positive input. This switch can be turned on or off. The switching speed should be as high as possible so that switching losses are low. However, this causes a pulsed current to flow on the power line. Not every voltage source can provide these pulsed currents without any problems. Therefore, the voltage at the input of the switching regulator drops. To minimize this, a backup capacitor needs to be used at the power input.
Figure 2 shows the circuit from Figure 1, but this time showing both the parasitic elements of the power line and the voltage source itself. The internal resistance of the voltage source, the inductance and resistance of the power supply lines, and any current limiting are key characteristics of the voltage source that must be considered in order for a switching regulator to function properly. In most cases, proper selection of input capacitors will ensure proper circuit operation. The first approach should be to use the recommended capacitor value for CIN in the switching regulator IC data sheet. However, if the voltage source or power line exhibits special characteristics, it is feasible to simulate a combination of a voltage source and a switching regulator.

Figure 3 shows the simulation circuit for the ADP2360 buck converter. A simplified table for generating the input voltage IN using an ideal voltage source is shown here. Since no internal resistors are defined for the voltage source and no parasitic values are provided for the power lines between the voltage source and the switching regulator, the defined voltage is always applied to the VIN pin of the ADP2360. Therefore, no input capacitors need to be added. However, in the real world, switching regulators always require input capacitance because the voltage source and power lines are not ideal. If a simulation environment such as LTspice is also used to examine the behavior of different input capacitances, a voltage source with internal resistance and a power supply line with resistive and inductive parasitics must be used.

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