OnSemiNPC1562100W，48V，DC/DC转换器参考设计方案

OnSemi公司的NCP1562是一款高效、外围器件少的DC/DC电压模式转换控制器芯片。控制器包含两同相输出，通过一个重叠延迟来保护同时导通，有力于软开关。主输出主要驱动初级前向转换MOSFET。次输出主要驱动有源钳位电路MOSFET，次级主要由一个同步整流器或由一个半桥电路组成。NCP1562与众不同的特性是软停止和时间阀值内循环跳跃电流限制。软停止控制方式是在检测到有严重的故障时，软停止电路系统停止对转换器供电。循环跳跃电流限制方式是在连续出现过流故障时，循环跳跃检测器使能连续软停止。本文主要介绍了该芯片的主要特性及功能强大的保护特性，并给出芯片内部框图、方案介绍、方案原理图、方案PCB图及元器件列表。

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.

NCP1562主要特性：

· 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

NCP1562应用：

· Telecommunications Power Converters
· Low Output Voltage Converters using Control Driven Synchronous Rectifier
· Industrial Power Converters
· 42 V Automotive System
· ATX Power Supplies


图1. NCP1562内部方框图

NCP1562-100WGEVB方案介绍：

The NCP1562 PWM controller contains all the features and flexibility needed to implement an active clamp forward dc?dc converter. This IC operates from an input supply up to 150 V, thus covering the input voltages usually found in telecom, datacom and 42 V automotive systems. One can also note that the NCP1562 can be used in mains related applications (e.g. desktop, server, flat TVs) as it can be supplied by an auxiliary power supply.

The NCP1562 is the ideal choice for new generation isolated fixed switching frequency dc?dc converters using the active clamp topology with synchronous rectification to achieve extremely high conversion efficiency. This controller will help designers cope with their daily challenge, “small form factor highly protected module” through the following features:

· Dual Outputs with Adjustable Overlap Delay:
provide design flexibility. Output 1 (OUT1) drives the main switch in a forward or flyback converter topology.Output 2 (OUT2) can be used to drive an active clamp/reset switch, a synchronous rectifier switch, or both. OUT2 has an adjustable overlap delay to prevent simultaneous conduction of the switching elements.

· Soft?Stop:
discharges the active clamp capacitor prior to turn off to eliminate unwanted oscillations.

· An Internal Startup Regulator:
provides power to the NCP1562 during startup. Once the system powers up, the regulator is disabled, thus reducing power consumption. The regulator can be powered directly from the input line.

· Soft?Start:
allows the system to turn on in a controlled manner and reduce stress on system components.

· Adjustable Maximum Duty Ratio:
allows the design to be optimized without a penalty on drain voltage. Duty ratio is controlled within ±5%.

· Adjustable Volt-second Limit:
prevents transformer saturation and improve transient response.

· Line Feedforward:
adjusts the duty ratio inversely proportional to line voltage, allowing the controller to respond in the same cycle to line voltage changes. It provides the controller some advantages of current-mode control, while eliminating noise susceptibility, low power jitter and the need for ramp compensation.

· Dual Mode Overcurrent Protection Circuit:
handles momentary and continuous overcurrent conditions differently to provide the best tradeoff in system performance and safety.

· Line Under/Overvoltage Detector:
circuits enable the device when the line voltage is within the pre-selected voltage range. A resistor divider from the input line biases the under and overvoltage detectors. The accurate UV limit allows the converter to operate at high duty ratio without creating additional component stresses.

NCP1562-100WGEVB方案原理图：

图2. NCP1562-100WGEVB方案原理图

NCP1562-100WGEVB方案材料清单：




PCB元件布局图：