As sensors and communication modules send factory processing intelligence to the edge, routine decisions can be made faster without involving the main processor. The additional intelligence, however, must use the same or even less space on the factory floor. This, in turn, calls for more product functionality in smaller form factors.
Shrinking PCB sizes come with added stress on thermal dissipation. Since heat sinks and fans are out of the question in these designs, the power supply must be highly efficient, delivering more power in a smaller area than ever before. Let’s take a look at how we can design 20W to 30W power supplies with more than 90% efficiency for 24V+ industrial automation systems.
Industrial applications have historically been characterized by a 24V nominal DC voltage bus. For non-critical equipment, the maximum operating voltage is expected to be 36V to 40V, while critical equipment must support 60V. Common output voltages are 3.3V and 5V with currents that vary from 10mA in small sensors to 10s of amps in motion control, CNC, and PLC applications. As such, a step-down (buck) voltage regulator is a clear choice for industrial control applications.
Minimizing Power Dissipation
Non-synchronous buck converters are common step-down architectures, as it’s fairly easy for semiconductor manufacturers to design non-synchronous buck regulators for high voltages. Here, the low-side rectifier diode is external to the IC. In a design with a 24V input and a 5V output, the buck converter works with a duty cycle of about 20%, so the internal high-side transistor (T in Figure 1) conducts only 20% of the time. For the rest of the time (80%), the current is conducted by the external rectifier diode, and this represents most of the power dissipation. Consider, as an example with a 4A load, a Schottky rectifier diode that exhibits a voltage drop of about 0.64V. At 80% duty cycle, the conduction loss is roughly (0.64V) x (4A) x (0.80) = 2W. Using a synchronous architecture, the diode would be replaced by a low-side MOSFET serving as a synchronous rectifier. This way, we can trade off the 0.64V drop across the diode with the drop across the MOSFET transistor’s T2 on-resistance, RDS(ON) (Figure 2), easily reducing the power loss down one order of magnitude.
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Figure 2: Synchronous buck converter
Looking at our example, the MOSFET has a RDS(ON) of just 11mΩ and a package size similar to that of the Schottky rectifier. This leads to a corresponding voltage drop of just (11mΩ) x (4A) = 44mV and a power loss of only (0.044V) x (4A) x (0.80) = 141mW. In short, the MOSFET power loss is about 14x smaller than the Schottky power loss at full load—clear evidence that synchronous rectification minimizes power dissipation.
Learn more about synchronous rectification by reading “Design 20W-30W Power Supplies with Over 90% Efficiency for 24V+ Industrial Automation Systems.”
Nazzareno (Reno) Rossetti, PhD EE at Maxim Integrated, is a seasoned analog and power management professional and published author who holds several patents in this field. His doctorate in electrical engineering is from Politecnico di Torino, Italy.
Ramesh Giri is Director of Product Definition at Maxim Integrated. He has over 25 years of experience in the switched-mode power supply area, and has led power management product definition and applications at Maxim in mobility, server, Datacom, and industrial over the last decade. He is an expert in DC-DC control and has authored several IEEE publications on control of series stacked DC-DC converters. Ramesh holds a US patent for no-opto flyback converters.
Viral Vaidya is an Executive Business Manager for power management products at Maxim Integrated. He has over 10 years of experience in product management for various power management products. He has been at Maxim for more than five years and has served in a similar role for the previous five years. Vaidya has a MSEE from San Jose State University in California.