Latif Ameer
Part one of this blog discussed basic needs of a Limited Power Source (LPS) and provided a brief overview of standards that govern LPS circuits.
To address the needs of protection and limiting, conventional power supplies as illustrated in Figure 1 rely on multiple feedback loops based on two or more optocouplers. These optocouplers transmit the load voltage, load current and open-fault information to the main controller – typically a flyback controller for wattages up to 150W. In addition, the current-sensing circuitry has external current-sense resistors, precision operational amplifiers for amplification and setting the constant current-constant voltage (CC-CV), and constant power feedback loops. Overall, discrete circuits have high component counts, use up valuable real estate on the board and add cost to the system.
Figure 2 shows a proposed design architecture with reduced feedback loops. The architecture uses the UCC28740, a CC-CV flyback controller with optocoupled feedback for voltage and primary-side regulation (PSR) for constant current. The high level of integration of the controls in the UCC28740 controller aids in low component count design and reduced cost.
The two key benefits of this proposed architecture are:
The proposed architecture uses a single optocoupler feedback precision-output voltage control. A PSR circuit provides a redundant voltage-control loop. PSR activates during open-loop conditions such as failure in the optocoupler feedback network. Thus, output voltage is limited and regulated to the value set by as the primary-side feedback components.
The 60W, 24V High Efficiency Industrial Power Supply with Precision Voltage, Current and Power Limit highlights the performance results of precision current and voltage limiting. Figure 3 and Figure 4 show the results of the reference design board for precision current regulation and power foldback.
Explore these industrial power-supply reference designs:
IEC60950-1 Information Technology Equipment - Safety - Part 1: General Requirements
NFPA 70: National Electrical Code (NEC) standard
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