OnSemiNCP1562 High Efficiency 350WATX Power Supply Reference Design

/a>ATX power supply” title=”ATX power supply”>ATX power supply” title=”ATX power supply”>ATX power supply. This article introduces the main features of the NCP1562x series, the block diagram and the main indicators of the high-efficiency 350W ATX power supply reference design, detailed circuit diagram and corresponding Materials list.

The NCP1562x is a family of voltage mode controllers designed for dc–dc converters requiring high–efficiency and low parts count. These controllers incorporate two in phase outputs with an overlap delay to prevent simultaneous conduction and facilitates soft switching. The main output is designed for driving a forward converter primary MOSFET. The secondary output is designed for driving an active clamp circuit MOSFET, a synchronous rectifier on the secondary side, or an asymmetric half bridge circuit.

The NCP1562 family reduces component count and system size by incorporating high accuracy on critical specifications such as maximum duty cycle limit, undervoltage detector and overcurrent threshold. Two distinctive features of the NCP1562 are soft–stop and a cycle skip current limit with a time threshold. Soft–stop circuitry powers down the converter in a controlled manner if a severe fault is detected. The cycle skip detector enables a soft–stop sequence if a continuous overcurrent condition is present.

Additional features found in the NCP1562 include line feed–forward, frequency synchronization up to 1.0 MHz, cycle–by–cycle current limit with leading edge blanking (LEB), independent under and overvoltage detectors, adjustable output overlap delay, programmable maximum duty cycle, internal startup circuit and soft–start.

Main features of NCP1562x series:

Dual Control Outputs with Adjustable Overlap Delay

>2.0 A Output Drive Capability

Soft–Stop Powers Down Converter in a Controlled Manner

Cycle–by–Cycle Current Limit

Cycle Skip Initiated if Continuous Current Limit Condition Exists

Voltage Mode Operation with Input Voltage Feedforward

Fixed Frequency Operation up to 1.0MHz

Bidirectional Frequency Synchronization

Independent Line Undervoltage and Overvoltage Detectors

Accurate Programmable Maximum Duty Cycle Limit

Programmable Maximum Volt–Second Product

Programmable Soft–Start

Internal 100 V Startup Circuit

Precision 5.0 V Reference

These are Pb–Free Devices

NCP1562x series applications:

Telecommunications Power Converters

Low Output Voltage Converters using Control Driven Synchronous Rectifier

Industrial Power Converters

42 V Automotive System

ATX Power Supplies
OnSemiNCP1562 High Efficiency 350WATX Power Supply Reference Design
Figure 1. NCP1562x Family Block Diagram

High Efficiency 350W ATX Power Supply Reference Design

ON semiconductor was the first Semiconductor company to provide an 80 PLUS open reference design for an ATX Power Supply in 2005. This 1st generation reference design, was certified and met all the requirements of the 80 PLUS program. Following on this successful 1st generation design, ON Semiconductor is introducing its improved 2nd Generation reference design. This 2nd generation design utilizes newer ICs from ON Semiconductor that enable this design to exceed 80% efficiency starting at 20% load across different line conditions with ample margin to spare.

This reference document provides the details behind this 2nd generation design. The design manual provides a detailed view of the performance achieved with this design in terms of efficiency, performance, thermals and other key parameters. In addition, a detailed list of the bill-of -materials (BOM) is also provided. ON Semiconductor will also be able to provide technical support to help our customers design and manufacture a similar ATX power supply customized to meet their specific requirements.

The results achieved in this 2nd generation design were possible due to the use of advanced new components from ON Semiconductor. These new ICs not only speeded up the overall development cycle for this new design, but also helped achieve the high efficiencies while at the same time keeping a check on the overall cost. With the use of these new ICs, ON Semiconductor has proven again that the emerging requirements for high efficiency desktop power supplies can be met and further, can be optimized to meet specific performance vs. cost goals.

This 2nd generation design consists of a single PCB designed to fit into the standard ATX enclosure along with a fan. Figure 1 below presents the overall architecture employed in this design – detailed schematics are included later in this design manual. As seen in figure 1, this design employed an Active Clamp forward topology using the new, highly integrated Active Clamp Controller IC from ON Semiconductor – NCP1562. A Continuous Conduction Mode (CCM) Power Factor Correction (PFC) IC was employed for the active PFC circuit. This IC, the NCP1653 provides an integrated, robust and costeffective PFC solution. The standby controller, NCP1027, is an optimized IC for the ATX power supply and incorporates a high-voltage MOSFET. On the secondary side, this architecture employs a post regulator approach for generating the 3.3 V output. This is an alternative approach to the traditional magnetic amplifier (Mag Amp) approach.

Though ON Semiconductor believes that this post regulator approach provides the highest efficiency amongst the different means of generating these outputs in the power supply, it is important to note that if the customer desires to use a different approach, that is possible – ie a similar design can be developed that utilizes all the other pieces of this architecture without the post regulator and still achieve very good results.

With the introduction of this 2nd generation, high-efficiency ATX Power Supply, ON Semiconductor has shown that with judicious choice of design tradeoffs, optimum performance is achieved at minimum cost.

Figure 2. ATX Power Supply Reference Design Block Diagram
Main indicators of high-efficiency 350W ATX power supply:

Figure 3. Outline drawing of 350W ATX power supply

Figure 4. PFC controller circuit diagram

Figure 5. EMC board circuit diagram

Figure 6. Active Clamp Controller Board Circuit Diagram

Figure 7. Supervisory and 3.3V Post-Regulator Controller Board Circuit Diagram

Figure 8. Main board PFC and standby circuit diagram

Figure 9. The circuit diagram of the active clamp part of the main board

Figure 10. The circuit diagram of the main board after 3.3V voltage regulator

Motherboard bill of materials:

Active Clamp Parts Bill of Materials:

PFC part list of materials:

EMC part list of materials:

List of materials for post regulator part:

For details, see:

The Links:   NL12880BC20-05KD NL6448BC26-26F