Consider realistic voltage sources when designing power supplies

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What you will learn:

  • The importance of the input voltage range in power supply design.
  • Using simulation to ensure that the switching regulator is working properly.

The power source, in actual use, is never ideal. Real behavior, including interference, must be taken into account in building a reliable electrical system. When using power supplies, we make sure that a DC to DC converter, such as a switching regulator, can withstand a certain input voltage range and generate the required output voltage with sufficient current.

The input voltage is often specified as a range because it is usually not regulated exactly. For a power supply to operate reliably, however, there must always be an input voltage within the allowable range available for the switching regulator.

For example, a typical input voltage range for a 12 V supply voltage might be 8 to 16 V. Figure 1 shows a buck converter (buck topology) that generates 3.3 V from a nominal voltage of 12 V.

However, when designing the DC-DC converter, it is not enough to consider only the minimum and maximum values ​​of the input voltage. Figure 1 shows that the buck converter has a switch at its positive input. This switch is on or off. The switching speed should be as high as possible so that only low switching losses occur, but this causes a pulsed current to flow through the power line.

Not all voltage sources can supply such pulsed currents without any problems. As a result, voltage drops occur at the input of the switching regulator. To minimize this, backup capacitors are needed directly at the input of the power supply. This type of capacitor is presented as CIN in Figure 1.

Figure 2 represents the circuit of Figure 1, but this time with the parasitic elements of the power line and the voltage source itself. The internal resistance of the voltage source (RSERIES), the inductance and resistance of the power line (power line R, L) and any current limitation are key characteristics of the voltage source that must be taken into account to ensure trouble-free operation of the switching regulator.

For the most part, the correct selection of input capacitors can ensure proper circuit operation. The first approach should be to take the recommended capacitance value for CIN of the technical sheet of a switching regulation circuit. However, if the voltage source or the power line has special characteristics, it makes sense to simulate the combination of the voltage source and the switching regulator. figure 3 shows a simulation performed with the LTspice simulation environment from Analog Devices.

A simulation circuit for the ADP2360 buck converter is shown in a simplified form in figure 3—The input voltage IN is generated with an ideal voltage source. Since no internal resistance is defined for the voltage source and no parasitic value is specified for the supply line between the voltage source and the switching regulator, the defined voltage is always applied to the VIN pin of ADP2360. Therefore, it is not necessary to add an input capacitor (CIN).

In the real world, however, an input capacitor is still needed with a switching regulator because the voltage source and power line are not ideal. If a simulation environment such as LTspice is also used to verify behavior with different input capacitors, a voltage source with internal resistance, and a power line with parasitic values ​​for resistance and inductance (Fig. 2, again) must be used.

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Rosemary C. Kearney