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《开关电源》笔记三种基础拓扑（buck boost buck-boost ）的电路基础： 1， 电感的电压公式dtdILV =＝T I L ∆∆，推出ΔI ＝V ×ΔT/L2， sw 闭合时，电感通电电压V ON ，闭合时间t ON sw 关断时，电感电压V OFF ，关断时间t OFF3， 功率变换器稳定工作的条件：ΔI ON ＝ΔI OFF 即，电感在导通和关断时，其电流变化相等。
那么由1，2的公式可知，V ON ＝L ×ΔI ON /Δt ON ，V OFF ＝L ×ΔI OFF /Δt OFF ，则稳定条件为伏秒定律：V ON ×t ON ＝V OFF ×t OFF4， 周期T ，频率f ，T ＝1/f ，占空比D ＝t ON /T ＝t ON /（t ON ＋t OFF ）→t ON ＝D/f ＝TD→t OFF ＝（1－D ）/f电流纹波率r P51 52r ＝ΔI/ I L ＝2I AC /I DC 对应最大负载电流值和最恶劣输入电压值ΔI ＝E t /L μH E t ＝V ×ΔT （时间为微秒）为伏微秒数，L μH 为微亨电感，单位便于计算 r ＝E t /（ I L ×L μH ）→I L ×L μH ＝E t /r →L μH ＝E t /（r* I L ）都是由电感的电压公式推导出来 r 选值一般0.4比较合适，具体见 P53电流纹波率r ＝ΔI/ I L ＝2I AC /I DC 在临界导通模式下，I AC ＝I DC ，此时r ＝2 见P51 r ＝ΔI/ I L ＝V ON ×D/Lf I L ＝V O ＦＦ×（1－D ）/Lf I L →L ＝V ON ×D/rf I L 电感量公式：L ＝V O ＦＦ×（1－D ）/rf I L ＝V ON ×D/rf I L 设置r 应注意几个方面：A,I PK ＝（1＋r/2）×I L ≤开关管的最小电流，此时r 的值小于0.4，造成电感体积很大。
开关电源中的比较常见的双重环路及其应用
工程师都知道，开关电源中离不开环路设计。
环路影响到开关电源的诸多性能指标，譬如输出纹波，动态特性，稳定性，保护特性等。
这篇文章将从下面四个方面讲一讲开关电源中的比较常见的双重环路及其应用：
1.单电压环与单电流环
2.电压环和电流环的双环竞争
3.电压外环电流内环
4.两种双环控制在车载电源产品中的应用
一、单电压环与单电流环
闭环就是通过对被控制变量进行负反馈与设定值进行比较，得到他们之间的偏差，然后通过控制偏差，来实现被控变量稳定在设定值附近。
生活中最常见的一个负反馈闭环就是骑自行车，如果我们想走一条直线，而实际往左偏了，就会将车把手往右调整，如果往右偏了，就往左调整。
最后肯定稳定在这条想走的路线的附近。
如果自行车整个过程一直都是向左偏离一个角度，这个就是静差，也叫稳态误差。
如果自行车稳定在设定路线的左右偏差一点，这个就是误差摆幅，有些场景下也叫纹波峰峰值。
车辆一直行使在设定路线附近，而且偏差小，遇到紧急避让的情况下（动态扰动）也绝不摔倒——这就是好的环路设计。
在比较简单的开关电源中，只需要一个单闭环就可以实现产品的恒压或者恒流输出。
对于恒压源，只需要控制输出电压稳定，对于恒流源只需要控制输出电流稳定。
这里通过最常见的buck电路的单电压闭环和单电流闭环来来分析一下。
以最常见的PI控制作为补偿控制环节。
1）其电压单环的控制闭环框图如下：其中Kadc为采样及反馈环节，Plant。
开关电源反馈环路设计开关电源是一种将输入直流电压转换为所需输出电压的电源装置。
为了实现稳定可靠的输出电压，开关电源需要建立反馈环路进行控制。
开关电源的反馈环路主要包括内部反馈环路和外部反馈环路。
内部反馈环路是指内部电路中的反馈控制电路，用于控制开关管的导通与截止，以维持输出电压的稳定。
外部反馈环路是指从输出端以回路的形式连接到内部反馈电路，通过比较输出电压与参考电压的差异，产生一个控制信号，用于调整开关电源的开关时间和频率，从而调整输出电压。
设计开关电源的反馈环路时，需要考虑以下几个方面：1.选择合适的参考电压源：参考电压源是反馈环路的重要组成部分，它提供一个稳定的参考电压，用作与输出电压进行比较的基准。
一般可选择使用稳压二极管、参考电压芯片或者精密电位器来作为参考电压源。
2.设计错误放大器：错误放大器是反馈环路中的核心部分，它承担着将输出电压与参考电压进行比较的作用，并产生一个误差信号。
常见的错误放大器有比较器、运算放大器等。
在设计选择错误放大器时，需要考虑它的稳定性、带宽、输入阻抗等因素。
3.设计补偿网络：由于开关电源在转换过程中存在一定的延迟、输出的电压下降等因素，所以需要通过补偿网络来减小这些不稳定因素对输出电压的影响。
常见的补偿网络包括零点补偿网络和极点补偿网络。
零点补偿网络主要通过增加相位较大的零点，来提高系统稳定性；极点补偿网络主要通过增加相位较小的极点，来提高系统的相位裕度。
4.设计输出滤波器：开关电源的输出电压通常包含一定的纹波，需要通过输出滤波器来降低纹波，使输出电压更加稳定。
输出滤波器一般由电感、电容和电阻组成，通过调整它们的数值和组合方式，可以实现对纹波的去除或衰减。
在进行开关电源反馈环路的设计时，还需要进行一系列的仿真和实验，包括频率响应的模拟分析、稳态和动态的性能测试等，以确保设计的反馈环路能够实现对输出电压的稳定控制。
总之，开关电源的反馈环路设计是一项复杂的任务，需要综合考虑电源的性能要求、稳定性要求和实际应用需求等因素，通过选择适当的参考电压源、设计错误放大器、补偿网络和输出滤波器等，来实现对输出电压的稳定控制。
最详细的开关电源反馈回路设计开关电源是一种常用的电源供应方式，具有高效率和稳定输出电压的特点。
为了确保开关电源能够稳定工作，需要设计合理的反馈回路。
开关电源的反馈回路是一个闭环控制系统，通过对输出电压进行采样，与参考电压进行比较，计算出误差信号，再经过调整和补偿，使得输出电压稳定在设定值。
首先，需要选择合适的反馈控制策略。
常用的反馈控制策略有电压模式控制（Voltage Mode Control）和电流模式控制（Current Mode Control）。
电流模式控制具有更快的动态响应和更好的稳定性，但需要更复杂的设计和调试，因此在设计中需进行合理选择。
在电压模式控制中，可以使用一个误差放大器进行电压比较，产生误差信号。
误差放大器一般采用差分放大电路，通过输入电压和参考电压的差值乘以一个放大倍数，生成一个调整后的误差信号。
误差放大器的输出信号会经过一个滤波器进行滤波处理，以消除高频噪声。
接下来，需要设计一个比例积分（PI）控制器。
PI控制器可以提供稳定的、无超调的输出响应。
PI控制器的输入是经过滤波器处理后的误差信号，根据误差的大小来调整控制器的输出。
比例增益（Kp）决定了控制器对误差的响应速度，而积分时间常数（Ti）决定了控制器对误差的积分时间，即系统的稳定性。
在设计PI控制器时，可以根据经验公式来选择合适的参数。
通过实际测试和调整，可以优化控制器性能，使得开关电源的输出电压更加稳定。
最后，需要对开关电源进行保护设计。
开关电源反馈回路应具备过压保护、过流保护和短路保护等功能。
过压保护可以避免输出电压过高，过流保护可以防止过大的输出电流，短路保护可以防止输出端短路。
总之，开关电源反馈回路设计需要合理选择控制策略，设计误差放大器和滤波器、PI控制器，并进行参数调整和保护设计。
通过以上步骤，可以设计出稳定可靠的开关电源反馈回路。
最详细的开关电源反馈回路设计开关电源反馈回路设计是个挺有意思的话题。
听起来高深，其实很多细节值得我们好好琢磨。
今天我们就从几个方面聊聊，深入浅出，轻松搞定这些概念。
一、反馈回路的基本概念1.1 什么是反馈回路首先，反馈回路就是把输出信号的一部分送回输入。
这么做的目的是调节输出，使其稳定。
想象一下，开关电源就像一个小孩，时不时需要父母的指导。
没有这些反馈，小孩可能就会偏离轨道，输出的电压也可能出现大起大落。
1.2 反馈类型反馈可以分为两种：正反馈和负反馈。
正反馈就像是推波助澜，鼓励小孩继续做某件事情。
而负反馈则是提醒小孩停下来，纠正错误。
大部分情况下，我们更喜欢负反馈，因为它能帮助系统保持稳定。
通过负反馈，输出电压的波动会被抑制，电源的性能也会更可靠。
二、开关电源的基本结构2.1 开关管的作用开关电源的核心是开关管。
它负责控制电流的开关，调节输出电压。
可以把它想象成一个开关，时而打开，时而关闭。
这个过程中，它的工作频率决定了电源的效率。
频率高了，能量损失就小，输出稳定；频率低了，损失就增加，系统也会变得不稳定。
2.2 变压器的功能变压器在这里也占据重要位置。
它的作用是将输入的高压电压转换为适合的低压电压。
变压器就像是一个聪明的调酒师，能将各种成分混合，调配出最合适的“鸡尾酒”。
这里的鸡尾酒就是我们所需的电压。
2.3 整流与滤波整流和滤波是最后一步，确保我们得到的是平滑的直流电。
整流就像是把粗糙的石头打磨成光滑的宝石。
滤波则是去除电流中的杂音，确保输出的电流干净。
这个过程至关重要，稍有不慎，电源的稳定性就会受到影响。
三、反馈回路设计的要点3.1 控制环路设计设计反馈回路时，控制环路的选择非常关键。
控制环路决定了系统的响应速度和稳定性。
要确保环路的增益合适。
增益太高，系统可能会出现震荡；增益太低，系统反应迟缓。
这里的平衡就像走钢丝，得小心翼翼。
3.2 选择合适的传感器在设计反馈回路时，传感器的选择也不能忽视。
最详细的开关电源反馈回路设计嘿，朋友们！今天咱们聊聊那个让电子世界起舞的秘密武器——开关电源。
想象一下，你正坐在电脑前，眼睛盯着屏幕，手指在键盘上飞快地跳动，而这一切的背后，是那个默默工作的开关电源在为你提供能量。
你得知道，开关电源就像是个超级英雄，它有着强大的“电”力，能够瞬间点亮你的电脑、手机甚至家里的灯泡。
但这个超级英雄可不是随随便便就能出现的哦，它需要经过精心设计和调试，才能成为你最可靠的伙伴。
接下来，咱们来聊一聊开关电源的“电”话。
你得了解它的工作原理。
简单来说，开关电源就是通过控制电流的通断来调节电压的装置。
想象一下，如果你的手机电池电量不足，你会怎么做？当然是去充电啦！开关电源也是一样，它会在你不需要高电压时，自动降低输出电压，让你的设备更省电；在你急需高电压时，又会迅速提升输出电压，让你的设备瞬间充满电。
然后，咱们再来说说开关电源的“电”话。
在这个“电”话中，最重要的一环就是反馈回路的设计。
反馈回路就像是开关电源的“耳朵”，它能够感知到电路中的电流变化，并及时调整输出电压，确保电路的稳定性和可靠性。
那么，如何设计一个既简单又有效的反馈回路呢？你得选择合适的电阻和电容。
这些元器件就像电路中的“音符”，它们能够和谐地演奏出美妙的音乐。
例如，你可以使用一个小电阻作为分压器，将输入电压分成两部分，一部分用于驱动负载，另一部分则用于反馈。
这样，你就可以通过调整反馈电阻的大小来调节输出电压了。
接下来，你得学会读懂电路图。
电路图就像是一张张详细的地图，它能够帮助你快速定位到各个元件的位置和连接方式。
当你遇到问题时，只需仔细阅读电路图，就能找到解决问题的关键所在。
别忘了实践出真知。
理论虽然重要，但实际操作才能真正检验你的设计是否成功。
你可以试着搭建一个简单的开关电源实验台，亲自动手进行调试和优化。
在这个过程中，你可能会遇到各种问题，但只要你保持耐心和信心，就一定能够找到解决问题的方法。
开关电源的“电”话是一门深奥而又有趣的学问。
开关电源的反馈环路设计是开关电源设计的一个非常重要的部分，它关系到一个电源性能的好坏。
要设计一个好的环路，必须要知道主回路的数学模型，然后根据主回路的数学模型，设计反馈补偿环路。
开关电源是一个非线性系统，但可以对其静态工作点附近进行局部线性化，这种方法称为小信号分析法。
以一个CCM模式的BOOST电路为例其增益为：其增益曲线为：其中M和D之间的关系是非线性的。
但在其静态工作点M附近很小的一个区域范围内，占空比的很小的扰动和增益变化量之间的关系是线性的。
因此在这个很小的区域范围内，我们可以用线性分析的方法来对系统进行分析。
这就是小信号分析的基本思路。
因此要对一个电源进行小信号建模，其步骤也很简单，第一步就是求出其静态工作点，第二步就是叠加扰动，第三步就是分离扰动，进行线性化，第四步就是拉氏变换，得到其频域特性方程，也就是我们说的传递函数。
要对一个变换器进行小信号建模，必须满足三个条件,首先要保证得到的工作点是“静”态的。
因此有两个假设条件：1，一个开关周期内，不含有低频扰动。
因此叠加的交流扰动小信号的频率应该远远小于开关频率。
这个假设称为低频假设2，电路中的状态变量不含有高频开关纹波分量。
也就是系统的转折频率要远远小于开关频率。
这个假设称为小纹波假设。
其次为了保证这个扰动是在静态工作点附近，因此有第三个假设条件：3，交流小信号的幅值必须远远小于直流分量的幅值。
这个称为小信号假设。
对于PWM模式下的开关电源，通常都能满足以上三个假设条件，因此可以使用小信号分析法进行建模。
对于谐振变换器来说，由于谐振变换器含有一个谐振槽路。
在一个开关时区或多个开关时区内，谐振槽路中各电量为正弦量，或者其有效成分是正弦量。
正弦量的幅值是在大范围变化的，因此在研究PWM型变换器所使用的“小纹波假设”在谐振槽路的小信号建模中不再适用。
对于谐振变换器，通常采用数据采样法或者扩展描述函数法进行建模。
以一个CCM模式下的BUCK电路为例，应用上面的四个步骤，来建立一个小信号模型。
开关电源反馈回路设计
近年来，随着电子技术的飞速发展，开关电源逐渐成为一种主流
的电源供应方式。
作为一种高效稳定、可靠性高、功率密度大的电源
设计方案，开关电源在电子设备中广泛应用。
其中反馈回路的设计是
开关电源设计的重要组成部分。
反馈回路是开关电源中的关键环节，它决定了整个电源的性能和
稳定性。
在反馈回路设计中，我们需要选择合适的信号采集电路，设
计合理的信号放大电路、比较电路和控制电路。
同时，还需要注意相
邻环节之间的耦合影响以及各种干扰的对设备的影响。
在反馈回路设计时，我们需要重点考虑以下几个方面。
首先，信号恢复的准确性。
在输入信号传输中，一般情况下信号
会受到很多不同的干扰，因此我们需要采用合适的信号采样电路来确
保信号的稳定、精确和信噪比的要求。
据此设计合理的信号放大电路，以保证输出信号的准确性。
其次，反馈电路的稳定性。
反馈回路的稳定性是开关电源设计重
要的考虑因素。
为了保证回路稳定，我们需要合理设计反馈控制电路。
建立合适的模型来对反馈回路进行分析，根据分析结果选择合适的设
计策略，合理的控制反馈环节，调节反馈电路增益。
再者，设计过程中还需要注意尽可能降低功耗，减少热效应对回路的影响。
同时要考虑输出负载的变化对电源的影响，确保电源稳定性不受负载变化的影响。
总之，反馈回路设计是开关电源设计中的重要部分。
通过采用科学的设计方法和完善的技术手段，可以有效地提高开关电源的性能和稳定性，保证电子设备的正常运行。
AND8242/D19 V , 3.0 A Universal Input AC-DC Adaptor Using NCP1271Prepared by: Jon Kraft and Kahou Wong ON SemiconductorINTRODUCTIONThe N CP1271 is one of the latest fixed-frequency current-mode PWM switching controllers with (1) adjustable Soft-Skip t standby operation for low-level audible noise, (2) integrated high-voltage startup for saving standby power, (3) timer based overload fault detection, and (4) internal latch protection features. Table 1 summarizes all the features of an NCP1271 based power supply.This application note presents an example circuit (Figure 1) using the NCP1271 (65 kHz version) in a flyback topology. The design steps and subsequent measurements are also included. An Excel based design worksheet is available at .The measurements show that the 19 V , 3.0 A circuit delivers above 85% across a universal input (85 to 265 Vac).The no load standby consumption is 83 mW at 230 Vac and the light load operation is greater than 75% efficient.Figure 1. Application Circuit SchematicAPPLICATION NOTETable 1. Features of Power Supply Using NCP1271Operation Mode FeaturesTopology CCM/DCM Flyback•Fixed-frequency current-mode control with inherent primary current limitation.•Frequency jittering to soften the EMI signature.•B uilt-in soft-start.•Output short-circuit fault detection independent of the auxiliary winding.•Integrated high voltage startup that minimizes standby power loss.Standby Condition Soft-Skip Operation•Adjustable skip level for optimal standby power consumption.•Proprietary Soft-Skip to reduce the risk of low-frequency audible noise.•Soft-Skip operation is automatically disabled if an abrupt transient load is appliedfrom standby operation. This improves the output response to a transient load.Fault Condition Double HiccupRestart •Double Hiccup operation minimizes the power dissipation in a fault mode and allows the application to auto-recover when the fault is removed.Latch Protection Activated Latch Off•An internal latch makes it easy to add overtemperature protection (OTP) orovervoltage protection (OVP) to any applications.•Latch is reset by unplugging the AC input and allowing V CC to drop below 4 V (typ).The Demo Board SpecificationInput85 to 265 Vac, 50 HzOutput19 Vdc, 3.0 A, IsolatedFeatures•< 100 mW Input Power at 230 Vac•Excellent Light Load Performance•No Audible Noise•> 85% Full Load Efficiency•Short Circuit Protection Activates at < 100Wfor Any Input VoltageA Discontinuous Conduction Mode (DCM) flyback was selected for this application. DCM gives very good stability, small inductor size (lower leakage inductance), and good transient response.Flyback CalculationsSeveral resources are available at to calculate the necessary component values for a flyback supply. In particular, an Excel based design spreadsheet can be found at:/collateral/NCP1271SHEET.xls Additionally, most of the other N CP12xx application notes also apply to the NCP1271. For detailed information on designing a flyback power supply, please visit AND8076/D. Other app notes which may also aid in the design include:AND8069/D Tips and Tricks to Build Efficient CircuitsWith the NCP1200AND8205/D How to Choose a Switching Controller forDesignAND8023/D Implementing the NCP1200 in Low-CostAC/DC ConvertersAND8032/D Conducted EMI Filter Design for theNCP1200AND8076/D A 70 W Low Standby Power Supply with theNCP12xx SeriesBased on the results from the N CP1271 design spreadsheet, the final values for this adapter's key flyback parameters were calculated to be:Np:Ns = 5:1Lp = 180 m HR CS = 0.3 OhmsIpeak(full load) = 3.5 ASwitch Rating = 6 A, 800 VDiode Rating = 3 A, 100 VRsnubber = 100 k WCsnubber = 10 nFSetting the Short Circuit Protection LevelThe current sense resistor (R CS or R8), provides two functions. First it senses the primary current for current-mode PWM operation. Secondly, it provides the maximum primary current limitation according to equation 1:I p(max)+1ĂVR CS(eq. 1) The short circuit protection activates when the I p(max) current is reached for more than 130 ms (typ). This also corresponds to V FB being greater than or equal to 3 V for 130 ms. Therefore, R CS must be set large enough to ensure that the required peak current can always be delivered, but small enough to meet the short circuit protection requirements. A DCM flyback converter has the following relationship:P out+1ń2L p I p2F SW h(eq. 2) Therefore, for an assumed efficiency of 80%, a peak current of 4 A should trigger the short circuit protection circuitry at 80 W. This corresponds to an R CS value of 0.25 W. This change in R CS may also require that the snubber and transformer be re-calculated to handle this level of peak current during the short circuit fault time. A few iterations of the Excel based N CP1271 design spreadsheet should produce a good starting point for the application's design.Over Power CompensationFor this demo board, the short circuit protection is activated with an output load of 76 W at 85 Vac and 93 W at 265 Vac. The variation in short circuit power level withinput voltage is due to the propagation delay (Tprop) of the NCP1271. This propagation delay has a more pronounced effect on the power delivered at high line than at low line as shown in Figure 2.Figure 2. Effect of Propagation Delay on the Maximum Power Delivered at High Line and Low LinePeak Primary CurrentI T propT propThis effect is called “Over Power” because it delivers more power than what is requested by the feedback loop.Specifically, for a DCM flyback system, the total power delivered to the output including the prop delay effect is:P out +1@L p @(I p(max))V bulk ńL p @T prop)2@F sw @h(eq. 3)The NCP1271 has been designed with a very low Tprop (50 ns typ). This minimizes the over power effect. However,if reduced variation is required, then over power compensation can be easily implemented by using one of the circuits shown in Figures 3 and 4.Figure 3. Over Power Compensation by means ofa Resistor to the Bulk VoltageR Figure 4. Over Power Compensation by Modifyingthe Auxiliary Winding TopologyThe circuit in Figure 3 simply modifies the CS setpoint proportional to the HV bulk level. This creates an offset which compensates for the propagation delay. However, this does increase the standby power dissipation. Figure 4 gives another option which results in much lower power dissipation. By altering the position of the Aux winding diode, a new point is created whose voltage is proportional to Vin. The power dissipation is now reduced by a factor of (Np:Naux)2. V alues for Ropp are best found experimentally to give suitable precision for the activation of the short circuit protection.Biasing the ControllerThe NCP1271 includes a high voltage (HV) startup pin (Pin 8) which charges V CC to its operating level. This pin can be directly connected to the high voltage DC bus. Once the device is powered up, an auxiliary winding powers V CC as shown in Figure 5.Figure 5. V CC Biasing SchemeThe range of V CC is from 10 V (min) to 20 V (max). Therefore, the auxiliary winding should be designed to give a level of V CC within this range over all output loads. When the circuit is in standby mode, very few pulses are delivered and the auxiliary level decreases. To provide enough voltage range, a nominal V CC level of 16 V was selected for this application. Additionally, an 18 V (±5%) Zener diode was added externally to protect the controller from abnormally high auxiliary levels. The 16 V bias supply is constructed from a 6:5 turns ratio (19 V:16 V) between the main output and the auxiliary winding.Figure 6 shows the auxiliary supply circuit. A resistor is included to provide the flexibility to redesign the circuit for higher output voltages. Any extra bias voltage greater than 18 V is simply dissipated across the resistor.Figure 6. Auxiliary SupplySoft-Skip AdjustmentWhen the load current drops, the compensation network responds by reducing the peak current. When the peak current reaches the skip peak current level, the NCP1271 enters skip operation to reduce the power consumption. The peak current level at which skip is entered should be set high for good standby power dissipation. However, it also needs to be set low enough that no audible noise occurs during each bunch of skip pulses. To address this need, the NCP1271 has a proprietary Soft-Skip feature which ramps each bunch of pulses. This dramatically lowers acoustic noise and allows a higher skip level to be set for greater power savings. The NCP1271 also allows the designer to select the optimal level of the peak current during skip through a simple resistor from pin 1 to GND. This skip resistor sets the skip level according to equation 4:V skip+I skip R skip(eq. 4) where I skip = 43 m A (typ)The peak current when skip mode is activated can be calculated with equation 5:I peak(skip)+V skip3ĂVI peak(max)(eq. 5) For this demo board, V skip was set to 1.3 V (R skip=30.1 k W). And I peak(max) is 1 V / 0.25 W = 4 A. Therefore,I peak(skip) = 1.7 A.Minimum On Time LimitationThe N CP1271 includes a current sense (CS) Leading Edge Blanking (LEB) filter. The LEB filter blanks out the first 180 ns (typ) of the CS voltage at the beginning of each drive pulse. This helps to prevent a premature reset of the output due to noise. However, this also results in a minimum on time of the device. The duration is equal to the LEB time (180 ns typical) and the propagation delay of logic (50 ns typical). If the application circuit is configured for 0% skip (by connecting Pin 1 to Ground), then that minimum on time duration may result in an abnormally high output voltage during no load conditions. Therefore, it is recommended to set skip to some small value rather than disable it completely. Ramp CompensationThe NCP1271 also incorporates a feature called “ramp compensation.” Ramp compensation is a known mean to cure subharmonic oscillations. These oscillations take place at half the switching frequency and occur only during continuous conduction mode (CCM) with a duty-cycle greater than 50%. To prevent these oscillations, one usually injects between 50 and 75% of the inductor down slope into the CS pin. The NCP1271 generates an internal current ramp that is synchronized with the clock. This current ramp is then routed to the CS pin.Since the flyback design in this app note is well within DCM operation, ramp compensation is not necessary. However, for designs that do run in CCM with the NCP1271, ramp compensation is easy to implement. It only requires one external resistor between Rcs and the CS pin. The value of the ramp resistor to obtain 50% inductor down slope injection can be calculated with the following equation:R ramp+0.50R CSǒ(V out)V f)N PN SǓǒLp F sw100Ăm A0.80Ǔ(eq. 6)Maximum Duty Cycle and Ramp CompensationIf the ramp resistor is set too high, the maximum duty cycle will be reduced. But as a long as R ramp is below 10 k W ,this will not be a problem. A typical graph of the maximum duty cycle verses R ramp is shown in Figure 7. However, it is not recommended to try to reduce the maximum duty cycle by the R ramp value because this relationship is not guaranteed by the production tests of the device.Figure 7. Maximum Duty Cycle Characteristics 010203040506070809005101520253035404550R RAMP , RESISTOR (k W )M A X I M U M D U T Y (%)Optional Output OVP LatchThe NCP1271 includes a feature where if Pin 1 is brought above 8.0 V (typ), the part will safely latch off the controller.The controller is reset by unplugging the AC input. This allows for easy implementation of overvoltage (OVP) or overtemperature (OTP) protection.In order to pull the Pin 1 voltage above the latch threshold,a greater than 8.0 V source is needed. That is usually the bias supply voltage V CC . Therefore, to protect Pin 1, a resistor (R limit ) is connected to limit the current below the maximum allowed level. In addition, the internal ESD diode will limit the maximum voltage on Pin 1 to about 10 V .This latch off feature can be configured in a variety of ways. Some of the most popular include using the auxiliary winding to detect an overvoltage and using an NTC resistor to detect an overtemperature condition. A few variations of these circuits are listed in Figures 8 to 11.Figure 8. Simple Latchoff Circuit by BipolarTransistorsFigure 9. Overtemperature Protection Latch with aNTC ThermistorNTCresistorFigure 10. Output Overvoltage Protection Using theAuxiliary WindingOVPFigure 11. Output Overvoltage Protection Using anOptocouplerIt is important to note that when Pin 1 is open it sets the default skip level to 1.2 V. However, in this mode, pin 1 is internally pulled high to the Vskip-reset level (6.5 V typ). This only leaves about 1.5 V of noise margin before the part latches off. Therefore, if a skip level of 1.2 V is desired, then instead of leaving pin 1 open, it is always recommended to place a 28 k W resistor from pin 1 to GND. Then the skip level becomes 1.2 V (28 k W x 43 m A = 1.2 V), and the pin 1 voltage is also 1.2 V. This gives much better noise immunity and reduces the chance of falsely triggering the latch due to noise or leakage current from the external latch circuitry. Additionally, a small capacitor should be added to pin 1 to further increase the noise immunity.HV Pin Protection CircuitWhen the main power is interrupted in the application, the high voltage DC bus may potentially go negative in a short transient period. Since this is directly connected to pin 8, it could create a reverse current out of the HV Pin and could potentially damage the device. There are two easy solutions to this problem. The first is demonstrated in Figure 12. The inserted diode turns on when the HV Pin voltage goes below the V CC biasing voltage. This eliminates the chance of negative voltage on the HV pin. A second method is shown in Figure 13. Here, the inserted resistor limits the negative current to a low level and protects the HV pin. Either option works well, but for this demo board, a diode between V CC and HV was used.Figure 12. Protection Diode for HV PinFigure 13. Protection Resistor on HV Pin Layout ConsiderationFigures 9-10 show the layout of the design. It is a single-layer PCB. As with any power converter, some care must be exercised with the design and layout. The following are some important guidelines.1.Minimize the high-current loop and locate the ICcontroller outside the high-current loop to preventmalfunctioning of the IC internal logic due tostrong magnetic fields from the high current.2.Locate the decoupling capacitors close to thedevice to improve noise immunity.3.Locate the V CC capacitor very close to the deviceto prevent the circuit from entering a UVLO faultcondition because of noise.4.Locate the output voltage sense resistor close tothe output load points.5.Minimize the current sense trace. It can becomeeasily polluted with noise.6.Minimize the distance between the feedbackopto-coupler and controller because this trace isalso easily polluted.7.Minimize the distance between the MOSFET andcontroller because the PCB trace is high frequencyand high current so it can easily pollute other partsof the circuit.Additionally, there are three pins in the NCP1271 that may need external decoupling capacitors.1.Skip/latch pin (Pin 1) – If the voltage on this pin isabove 8.0 V, the circuit enters latch-off protectionmode. Hence, a decoupling capacitor on this pin isessential to improve noise immunity. Additionally,a resistor should always be placed from this pin toGND to prevent noise from causing the pin 1 levelfrom exceeding the latch-off level.2.Feedback pin (Pin 2) – A small capacitor may benecessary here for improved stability and noiseimmunity.3.V CC pin (Pin 6) – The NCP1271 maintains normaloperation when V CC is above V CC(off)(9.1 Vtypical). If V CC drops below V CC(off), then thecircuit enters UVLO protection and restarts after adouble hiccup. Therefore, if V CC inadvertentlydrops below V CC(off)due to switching noise, thenthe circuit will recognize it as a fault condition.Hence, it is important to locate the V CC capacitorand a ceramic decoupling capacitor as close aspossible to the NCP1271.MeasurementsStandby PerformanceThanks to the features in the NCP1271, the demo board power supply offers excellent no load and light load standby performance. The 230 V ac power consumption of the 57 W circuit is only 83 mW. And the input power at 230 V ac with 500 mW load is only 710 mW. Figure 14 shows the efficiency with output loads from 500 mW to 60 W at 120 Vac and 230 Vac.Figure 14. Efficiency of the NCP1271 Demo Board atNominal Line Voltages P out (W)E F F I C I E N C Y (%)Dynamic StudyFigure 15 shows the startup transient waveforms of the circuit when the input is 110 Vac. A 4 ms soft-start is observed in the drain current. The V FB drops below 3.0 V after 32 ms. Since this is shorter than the 130 ms fault validation time, the circuit does not enter fault condition and starts up normally.Figure 15. Startup TransientV outV CCV fbV CSFigure 16 shows the go-to-standby transition from full load operation. The output voltage (yellow trace) does not consume current and remains at 19 V , but the V CC voltage drops from 16 V to 15 V because the V CC auxiliary winding is not supplying current to the controller. The minimum V CC voltage in the transition can be as low as 12 V . This is why the 16 V biasing voltage was selected to maintain V CC above V CC(off) and prevent a V CC reset.Figure 16. Operating to StandbyV outV CCV CSShort Circuit Protection MeasurementsFigure 17 details the operation of the short circuit protection . The load steps from 60 W to 100 W, causing the peak current to increase to its maximum (1 V) as shown by the blue CS voltage. After approximately 130 ms, the controller shuts the power supply down and enters double hiccup fault operation (Figure 18). This provides very low power dissipation and protects the power components.When the short circuit fault is removed, the application recovers by executing a soft start and bringing the output back to 19 V .Figure 17. Short Circuit Protection is Activated Whenthe Output Load Increases to about 100 WV out V CSV CCFigure 18. The Controller Enters Double Hiccup Fault Operation during a Continuous Short Circuit EventV CSV out V CCConclusionA 57 W flyback power supply featuring over voltage and short circuit protection using the CP1271 was demonstrated to have excellent light load power dissipation and active mode efficiency. The N CP1271's proprietary Soft-Skip t operation offers low-audible-noise and excellent standby performance. The N CP1271 design worksheet, as well as other design aid resources, are available at .Appendix I: Bill of Materials for the NCP1271 19 V/3.0 A Example CircuitDesignator Qty Part Number Description Manufacturer T11E3506-A 3.0 A 508 m H Common-Mode Filter CoilcraftCooper/Coiltronics T21CTX22-17179Custom Transformer 180 m H 30:6:5,2.5 m H Max LeakageIC11NCP1271D65R265 kHz Flyback PWM Controller, SO-7ON Semiconductor IC21TL431AID 2.5 V 1% Voltage Reference, SO-8ON Semiconductor IC3–IC42SFH615AA-X007Optocoupler VishayD1–D441N5406 3.0 A 600 V Diode, Axial 267-05ON Semiconductor D51MMSZ914 1.0 A 100 V Diode, SOD-123ON Semiconductor D61MRA4005T3 1.0 A 600 V Diode, SMA ON Semiconductor D71MURS160 1.0 A 600 V Diode, SMB ON Semiconductor D81MBR3100 3.0 A 100 V Schottky Diode, Axial 267-05ON Semiconductor D101MZP4746A18 V @ 14 mA Zener Diode ON Semiconductor Q11SPP06N80C3 6.0 A 800 V N-MOSFET, TO-220AB InfineonR11P100KW-2BK100 k W 2.0 W, Axial 5%-R21CFR-25JB-10R10 W, 1/4 W Axial YageoR51CRCW12063012F30.1 k W, 1206VishayR61CRCW120610R0F10 W, 1206VishayR71CRCW12065110F511 W 1206VishayR81WSL2512R2500FEA0.25 W 1.0 W 1%Vishay R9, R101CRCW12061691F 1.69 k W, 1206VishayR111CRCW12061582F15.8 k W, 1206VishayR121CRCW12062371F 2.37 k W, 1206VishayC1-C22PHE840MA6100MA040.1 m F X2 Cap 10 mm Pitch Evox RifaC31ECOS2GP820BA,82 m F 400 V Electrolytic PanasonicEETED2G820BA, orEETXB2G820BAC4, C132ECA1EM101100 m F 25 V Electrolytic Panasonic C51630MMB103J10 nF 630 V Film Cap Rubycon C6-C71VJ1206Y122KXXA 1.2 nF 25 V, 1206VishayC9–C103025YXG220M12.5X302200 m F 25 V Electrolytic RubyconC111ERO610RJ4100M 1.0 nF 1.0 kV 5.0 mm Pitch Y2 Cap Evox RifaC121VJ1206Y154KXXA0.15 m F 25 V Ceramic VishayFuse11025TD2-R250 V 2.0 A Tie Delay Fuse Cooper Fuse Heatsink1590302B03600Heatsink for TO-220 Package Aavid Heatsink Insulation14672TO-220 Mica Insulation Keystone AC Connector1770W-X2/10IEC60320 C8 Connector Qualtek DC Connector126-60-4030 or 0096520383-Terminal 3.96 mm Pitch Male Header Molex Standoff44804 K Standoff M/F Hex 4-40 Nyl 0.750”-Heatsink Mechanic130F6984-40 1/4 Inch Screw-Heatsink/Standoff531F21064-40 Screw Nuts-MechanicNylon Washer13049Nylon Shoulder Washer #4-Appendix II: NCP1271 57 W Adaptor LayoutFigure 19. Top ViewFigure 20. Bottom ViewON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer's technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. 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