数控开关电源外文翻译

power supply cookbook

Marty Brown

1．The Role of the Power Supply within the System and Design Program

The power supply assumes a very unique role within a typical system. In many respects, it is the mother of the system. It gives the system life by providing consistent and repeatable power to its circuits. It defends the system against the harsh world outside the confines of the enclosure and protects its wards by not letting them do harm to themselves. If the supply experiences a failure within itself, it must fail gracefully and not allow the failure to reach the system.

Alas, mothers are taken for granted, and their important functions are not appreciated. The power system is routinely left until late in the design program for two main reasons. First, nobody wants to touch it because everybody wants to design more exciting circuits and rarely do engineers have a background in power systems. Secondly, bench supplies provide all the necessary power during the system debugging stage and it is not until the product is at the integration stage that one says “Oops, we forgot to design the power supply!” All too frequently,the designer assigned to the power supply has very little experience in power supply design and has very little time to learn before the product is scheduled to enter production.

This type of situation can lead to the “millstone effect” which in simple terms means “You designed it, you fix it ( forever).” No wonder no one wants to touch it and, when asked, disavows any knowledge of having ever designed a power supply.

1.1 Getting Started. This Journey Starts with the First Question

In order to produce a good design, many questions must be asked prior to the beginning of the design process. The earlier they are asked the better off you are. These questions also avoid many problems later in the design program due to lack of communication and forethought. The basic questions to be asked include the following.

From the marketing department

1. From what power source must the system draw its power? There are different design approaches for each power system and one can also get information as to what adverse operating conditions are experienced for each.

2. What safety and radio frequency interference and electromagnetic interference (RFI/EMI) regulations must the system meet to be able to be sold into the target market? This would affect not only the electrical design but also the physical design.

3. What is the maintenance philosophy of the system? This dictates what sort of protection schemes and physical design would match the application.

4. What are the environmental conditions in which the product must operate? These are temperature range, ambient RF levels, dust, dirt, shock, vibration, and any other physical considerations.

5. What type of graceful degradation of product performance is desired when portions of the product fail? This would determine the type of power busing scheme and power sequencing that may be necessary within the system.

From the designers of the other areas of the product

1. What are the technologies of the integrated circuits that are being used within the design of the system? One cannot protect something, if one doesn’t know how it breaks.

2. What are the “best guess” maximum and minimum limits o f the load current and are there any intermittent characteristics in its current demand such as those presented by motors, video monitors, pulsed loads, and so forth? Always add 50 percent more to what is told to you since these estimates always turn out to be low. Also what are the maximum excursions in supply voltage that the designer feels that the circuit can withstand. This dictates the design approaches of the cross-regulation of the outputs, and feedback compensation in order to provide the needs of the loads.

3. Are there any circuits that are particularly noise-sensitive? These includeanalog-to-digital and digital-to-analog converters, video monitors, etc. This may dictate that the supply has additional filtering or may need to be synchronized to the sensitive circuit.

4. Are there any special requirements of power sequencing that are necessary for

each respective circuit to operate reliably?

5. How much physical space and what shape is allocated for the power supply within the enclosure? It is always too small, so start negotiating for your fair share.

6. Are there any special interfaces required of the power supply? This would be any power-down interrupts, etc., that may be required by any of the product’s circuits.

This inquisitiveness also sets the stage for the beginning of the design by defining the environment in which the power supply must operate. This then forms the basis of the design specification of the power supply.

1.2 Power System Organization

The organization of the power system within the final product should complement the product philosophy. The goal of the power system is to distribute power effectively to each section of the entire product and to do it in a fashion that meets the needs of each subsection within the product. To accomplish this, one or more power system organization can be used within the product.

For products that are composed of one functional “module” that is inseparable during the product’s life, such as a cellular telephone, CRT monitor, RF receiver, etc., an integrated power system is the traditional system organization. Here, the product has one main power supply which is completely self-contained and outputs directly to the product’s circuits. An integrated power system may actually have more than one power supply within it if one of the load circuits has power demand or sequencing requirements which cannot be accommodated by the main power supply without compromising its operation.

For those products that have many diverse modules that can be reconfigured over the life of the product, such as PCB card cage systems and cellular telephone ground stations, etc., then the distributed power system is more appropriate. This type of system typically has one main “bulk” power supply that provides power to a bus which is distributed throughout the entire product. The power needs of any one module within the system are provided by smaller, board-level regulators. Here, voltage drops experienced across connectors and wiring within the system do not bother the circuits.

The integrated power system is inherently more efficient (less losses). The distributed system has two or more power supplies in series, where the overall power system efficiency is the product of the efficiencies of the two power supplies. So, for example, two 80 percent efficient power supplies in series produces an overall system efficiency of 64 percent.

The typical power system can usually end up being a combination of the two systems and can use switching and linear power supplies.

The engineer’s motto to life is “Life is a tradeoff” and it comes into play here. It is impossible to design a power supply system that meets all the requirements that are initially set out by the other engineers and management and keep it within cost, space, and weight limits. The typical initial requirement of a power supply is to provide infinitely adaptable functions, deliver kilowatts within zero space, and cost no money. Obviously, some compromise is in order.

1.3 Selecting the Appropriate Power Supply Technology

Once the power supply system organization has been established, the designer then needs to select the technology of each of the power supplies within the system. At the early stage of the design program, this process may be iterative between reorganizing the system and the choice of power supply technologies. The important issues that influence this stage of the design are:

1. Cost.

2. Weight and space.

3. How much heat can be generated within the product.

4. The input power source(s).

5. The noise tolerance of the load circuits.

6. Battery life (if the product is to be portable).

7. The number of output voltages required and their particular characteristics.

8. The time to market the product.

The three major power supply technologies that can be considered within a power supply system are:

1. Linear regulators.

2. Pulsewidth modulated (PWM) switching power supplies.

3. High efficiency resonant technology switching power supplies.

Each of these technologies excels in one or more of the system considerations mentioned above and must be weighed against the other considerations to determine the optimum mixture of technologies that meet the needs of the final product. The power supply industry has chosen to utilize each of the technologies within certain areas of product applications as detailed in the following.

Linear

Linear regulators are used predominantly in ground-based equipments where the generation of heat and low efficiency are not of major concern and also where low cost and a short design period are desired. They are very popular as boardlevel regulators in distributed power systems where the distributed voltage is less than 40VDC. For off-line (plug into the wall) products, a power supply stage ahead of the linear regulator must be provided for safety in order to produce dielectric isolation from the ac power line. Linear regulators can only produce output voltages lower than their input voltages and each linear regulator can produce only one output voltage. Each linear regulator has an average efficiency of between 35 and 50 percent. The losses are dissipated as heat.

PWM switching power supplies

PWM switching power supplies are much more efficient and flexible in their use than linear regulators. One commonly finds them used within portable products, aircraft and automotive products, small instruments, off-line applications, and generally those applications where high efficiency and multiple output voltages are required. Their weight is much less than that of linear regulators since they require less heatsinking for the same output ratings. They do, however, cost more to produce and require more engineering development time.

High efficiency resonant technology switching power supplies This variation on the basic PWM switching power supply finds its place in applications where still lighter weight and smaller size are desired, and most importantly, where a reduced amount of radiated noise (interference) is desired. The common products where these power supplies are utilized are aircraft avionics,

spacecraft electronics, and lightweight portable equipment and modules. The drawbacks are that this power supply technology requires the greatest amount of engineering design time and usually costs more than the other two technologies.

The trends within the industry are away from linear regulators (except for board-level regulators) towards PWM switching power supplies. Resonant and quasi-resonant switching power supplies are emerging slowly as the technology matures and their designs are made easier. To help in the selection, Table 1–1 summarizes some of the trade-offs made during the selection process.

1电源在系统中的作用和电源的设计过程

电源在一个典型系统中担任着一个非常重要的角色。在某中意义上，可以看做是系统的心脏。电源给系统的电路提供持续的，稳定的能量，使得系统不手外部的干扰，而且可以避免系统对它自己作出伤害。如果电源内部发生鼓掌，不应该造成系统的鼓掌。

然而，电源如此重要的作用却没有得到应有的重视。在设计一个系统的时候，电源系统总是被首先放在一边直到设计的最后才考虑电源的问题。出现这种情况的原因主要有两个。第一，没有人愿意接触这个东西，因为所有的人都想设计更能令人吃惊的东西，而且拥有电源方面知识的工程师本来就很少；第二，在系统调试阶段，一般由通用电源提供系统所需要的电能，只有在产品即将完成的时候才会有人说：“啊，我们忘记设计电源了！“很多时候被派来设计电源的人在电源的设计方面经验很少，并且在产品投入生产之前只会投入很少的时候去学习电源的设计。

这种情况会导致“深远的影响“。简单的说就是“谁设计，谁修理（终身）”。难怪没有人愿意接触电源设计，不愿意接受电源设计的任务。

1．1从一个问题开始探索

为了得到一个好的设计，在开始设计之前，必须首先要提出很多的问题。问题越早被提出来，越有利后面的世界。同样的，这些问题的提出可以避免在设计后期遇到很多的难题，这些难题可能是由于缺少交流和预见而产生的。一些基本的问题如下：

从市场的角度考虑

1．需要提供给系统何等形式的电源？对于每个电源系统来说，有很多不同的设计方法。对应各种方法，都有哪些不利的工作条件。

2．如果在市场上出售，系统必须满足何种安全规格和射频干扰/电磁干扰标准？这不仅仅影响电器方面设计，还会影响结构设计。

3．系统的维护要求是什么？这将决定采用怎么样的保护措施和结构设计以满足要求。

4．产品的工作环境是什么？包括温度范围，环境射频水平，灰尘，湿度，冲击。震动以及其他物理因素。

5．当产品的一部分发生鼓掌的时候，产品性能的降频是多少？这会决定电源供点结构和保护措施。对系统来说是非常必要的。

从产品的其他部分设计者角度分析

1．在设计的系统中，所采用的集成电路技术是什么？如果不知道他们是怎么样损坏的，那么就不能采取有效的措施来保护。

2．负载电流的最大和最小极限是多少？负载电流是不是存在冲击行的特征，例如发动机，视频监视器，脉冲负载等。经验表明，估计值往往要比实际情况要低一些，所以常常需要在估计基础上在加50%的多余。这决定了输出交叉调节和反馈补偿的设计策略。从而满足负载的要求。

3．系统中是否包括对噪音特别敏感的电路？他们包括数摸转换器，摸数转换器，视频监视器等等。这表明电源可能需要额外的滤波措施或者短员的开关频率要与一些敏感电路的工作频率同步。

4．电源的各部分电路是否需要以一定的书许投入运行以实现电源的可靠工作？

5．在整个装置里，分配给电源的空间有多大和是何种形状？一般来说，这部分空间很小。所以应该为电源争取到合理的空间。

6．系统对于电源是否有特殊的接口要求？其中包括鼓掌中断等，几乎所有的电路都有这个需要。

这些问题详细说明了电源的工作环境。为开始设计打下了良好的基础，也是构成电源设计规范的基础。

1．2 电源结构

最终产品中的电源结构必须满足产品的规范。电源系统的作用是将能量有效的分配给产品的各个部分，并满足这些需要。要实现这个目标，在产品中可以采用一个或者多个的的电源系统结构。

对于那些由一个功能模块组成，并且在使用过程中不可分的产品。例如移动电话，阴极射线管监视器，无线电接受器等等，传统的方法是采用集中电源系统。这里，产品包含一个集中电源，并直接输出给产品的各个部分。实际上，一个集中电源系统内部往往包含多路电源输出，从而满足复杂需要的独立电源供电或者满足供电先后顺序的要求。

对于那些由不同的模块组成，并且在使用过程中可以重新装配的产品。比如印制电路板板卡笼型结构，移动电话站等等。选用分布电源系统更为合适。这种类型的系统包含提个住电源，他将能量提供给母线，在由母线分配给产品的各个部分。每个部分需要的电能由小型的板载电源模块提供。这样，在系统的连接器及电压降就不会影响各个部分电路供电电压的质量。

集中电源系统显然具有较高的效率。分布电源系统由两级或多级串联组成，整个电源系统的效率是两级或多级电源变换效率的乘积。例如两级电源变换效率均为80%，电源串联后整个系统的效率是64%。

典型的电源系统往往是这两种电源系统的结合，并可能同时采用开关电源和线形电源的技术。

工程师的座右铭是：“生活是一种折中”‘这句话在这里就能够得到体现。设计一个能够完全满足其他领域的工程师和管理者要求的电源系统几乎是不可能的，这里的要求包括了成本，空间和重量。理想的电源能够满足各种应用要求，功率密度无穷大，同时成本趋与零。显然，实际情况要做出折中的选择。

1．3选择适用的电源技术

一旦电源系统的结构被确定下来，设计者接下来要做的就是为系统中的每一路电路选择适用的技术。在设计的初期阶段，这个过程也许是在选择供电系统结构和电源技术中反复进行。影响这个阶段设计的主要因素是：

1．成本。

2．重量和尺寸。

3．内部会有多少热量产生。

4．输入电源特性。

5．负载的噪声容限。

6．电池寿命（如果产品是便携式的）。

7．需要输出电源的组数及输出特性。

8．产品投向时常的时间。

在电源系统中经常考虑到的三种常见电源技术是：

1．线形电源。

2．脉冲调治（PWM）开关电源。

3．高效率的谐振开关电源。

以上技术都有各自的优势，需要综合考虑上面体积的各种因素，权衡利弊，才能决定满足最终产品形成要的最好技术。电源行业已经将以上每一种技术成功的应用到一定的产品领域，先面进行详细的叙述。

线形电源

线形电源主要应用在对发热和效率不高的应用场合，或者要求低成本及设计周期短的情况。线形电源作为板载电源广泛应用与分布电源系统中，特别是当赔垫电压低于40V时。对于离线式产品，在线形电源的前一级必须能够提供与交流电源可靠的电隔离。线形电源的输出电压只能低于输出电压，并且每个线形电源只能产生一路输出。线形电源的效率是在35%—50%之间。损耗以热的形式耗散。PWM开关电源

PWM开关电源在使用时比线形电源具有更高的效率和灵活性。我们可以在便携式产品，航空和自动化产品，仪器仪表，离线式产品中发现他们的踪影，他们通常应用与要求高效率和多组电源电压输出的场合。开关电源的重量要比线形电源轻的多。因为对于相同的输出功率、，开关电源的散热器要小的多。但是开关电源的成本较高，而且需要较长的开发周期。

高效率的谐振开关电源

高效率的谐振开关电源由基本的PWM开关电源演变而来。他主要应用与需要电源具有更轻的重量和更小的体积，并且对电磁噪声（干扰）有严格要求的场合。通常应用于航空电子设备，航天电子设备，便携式设备及模块电源。谐振开关电源的缺点是需要更长的开发周期，并且比其他的两种电源的成本要高。

除了作为板载电源外，线形电源正逐渐从工业界淡出，超者开关电源方向发展。随着谐振或准谐振开关电源技术的逐渐成熟，他们的设计也变的越来越容易。为了便于选择，表1-1比较了各种电源的技术特点。