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:

• Precise current and power limit, just with primary-side sensing. In the conventional approach shown in Figure 1,The output load current is sensed through discrete operational amplifier circuits and transmitted to main controller through opto-coupler. In some cases an additional opto-coupler is used for feedback redundancy. the output load current is directly sensed and the information is fed back to the flyback controller through a separate optocoupler circuit. The proposed architecture, on the other hand, uses cost-effective and reliable primary-side current sensing. A high degree of precision in the output-current limit is made possible by precision PSR current-sense techniques embedded into the UCC28740. The current limit causes the voltage foldback and ensures tight power limiting.
• Cost-effective open-loop protection and feedback redundancy. The conventional approach shown in Figure 1 uses two feedback loops, both of which are based on optocouplers. The identical nature of these optocouplers and their associated circuitry bring in an additional risk of failure; each of the optocouplers can fail simultaneously under similar stress conditions, which can be detrimental.

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.

Additional Resources

Explore these industrial power-supply reference designs:

• 100W, 24V High Efficiency High PF Industrial Power Supply with Precision Current and Power Limit.
• 60W, 24V High Efficiency Industrial Power Supply with Precision Voltage, Current and Power Limit.

References

IEC60950-1 Information Technology Equipment - Safety - Part 1: General Requirements

NFPA 70: National Electrical Code (NEC) standard