Designing a power-supply system for industrial automation equipment requires a thorough understanding of the surroundings and conditions that affect the functionality of the wide range of equipment involved. The most important design decision is whether to use linear power supplies or switch-mode power supplies (SMPS).
Today, SMPS are the most popular because of their high efficiency. However, they do have downsides that make linear supplies more desirable. Linear supplies also have their disadvantages, but often turn out to be the best choice for industrial use. Even better is a “have your cake and eat it too” solution: Use a linear supply with low-dropout (LDO) regulators.
Linear vs. SMPS
As mentioned, the primary reason for using switch-mode supplies is their high efficiency. Typical figures vary with the application, but efficiencies of greater than 90% are typical. High efficiency translates into minimum power loss in the supply with its attendant low heat dissipation. In large complex industrial settings with lots of equipment efficiency, power usage and heat dissipation become critical factors in terms of operating costs.
The main disadvantage of the SMPS is its noise generation. High-frequency pulse-width-modulated (PWM) pulses are filtered to form the desired dc level, but the resulting ripple and radiated or conducted noise generated can negatively impact the powered equipment and nearby devices.
Linear supplies with linear regulators don’t have the noise problem. Furthermore, ripple is greatly reduced by large capacitive filters and the feedback action of the regulator circuitry.
1. Often, a traditional linear IC regulator architecture will use a Darlington bipolar series pass transistor.
The big issue with these supplies, though, is that the efficiency is very poor (Fig. 1). A series pass transistor between the dc input voltage and the regulated dc output requires a minimum amount of voltage drop in order to maintain regulation. This voltage, called the dropout or headroom voltage, is usually a minimum of 1.5 to 2.5 V or more.
Since the output current passes through the pass transistor, this device will dissipate a significant amount of power, all of which shows up as heat. As the input or output voltages vary, the feedback senses the change and drives the pass transistor to adjust its conductance to compensate from any variation. This linear operation requires the minimum overhead be observed to maintain regulation. The overall benefit is no noise generation traded off for efficiency.
One way to get the benefits of low noise and good efficiency concurrently is to use a LDO regulator.
LDO Operation and Benefits
One definition of LDO is a regulator that operates with a series-pass-transistor voltage drop of less than 1 V. A dropout voltage of 1 V or more defines a conventional linear regulator.
A dropout of less than 1 V can be achieved with a PNP bipolar series pass transistor, but modern designs generally use a P-type MOSFET with very low dropout voltages of less than 100 mV (in addition to low on-resistance) (Fig. 2). When working with low supply voltages of 3.3 V or less, low dropout levels become a significant percentage of the output, so less is more.