用于信号和能量传递的无铁心印制电路板变压器
Coreless Printed-Circuit Board Transformers for Signal and Energy Transfer

本文介绍了一种最小功率损耗的印制电路板变压器的最佳应用。本文推荐的变压器比标准的脉冲变压器小得多，且适用于信号和能量传递。它可以工作在MHz级频率范围。通过使用调制/解调电路，该变压器可以在直流至300KHz宽的频率范围内进行数字信号传输。本建议可在许多低功率应用中替代铁心变压器。
1、引言
脉冲变压器被广泛地使用在低功率应用的电子设备中。它们用作电隔离，并常常被用于信号和（或）能量传递。变压器如此使用的例子如用作数字数据传输和在功率电子应用中(从直流到几百kHz）作为隔离门驱动电路的去耦变压器。在后一种应用情况，该变压器被用作信号和能量传递，是因为隔离门驱动电路必须提供门信号和为驱动如MOSFET功率器件的能量。在传统情况下，这些脉冲变压器是有铁心的。亦即铁氧体铁心被用作能量传递的磁路。然而，铁心变压器需手工制造绕组，这就增加了劳动成本和由此产生的设备成本。文献[1]提出了一种在双面印制电路板上印制绕组、以去掉手工绕线工序的铁心变压器。无铁心印制电路板变压器的概念没有受到人们很多关注，多半是因为存在误解: （i)平面绕组在印制电路板上只能印制几匝，差不多像是“短路线”; (ii)在变压器结构中，磁心是根本的元件。本文叙述了小功率要求的无铁心印制电路板变压器的最佳使用（详见[2]专利文献）。推荐的变压器有10匝初级和次级绕组，直径约12mm。如图1所示，它比标准脉冲变压器小得多。


2、问题和解决方案
图2为推荐的无铁心印制电路板变压器等效电路模型。使用这种类型变压器的主要问题是磁感应量非常小。仅有几匝短的印制绕组工作差不多像“短路”，除非工作频率很高（通常在MHz频率范围），因此增益低，导电损耗高。然而，传输信号的频率可能是相对低的频率（如从直流到几百kHz)。为克服这些问题，可以将以下两种方法一起使用:
(i)采用如文献[2、3]所述的解调电路，使其能接受MHz范围的高频载频（即变压器的工作频率），在需要的低频信号可以通过解调，并能达到电路输出要求;
(ii)最佳载频可以用‘最大阻抗频率’逼近法选择，所以可使电路的总功率损耗最小。
3、最大阻抗频率
根据等效电路，变压器电路的最大阻抗可以用载频来测定。在电路谐振出现以前，阻抗曲线的第一个峰值符合最大阻抗（除第一个谐振频率以外，变压器的增益将会消失）。在整个变压器电路的输入电流和功率最小时，这个最大阻抗频率应选作最佳工作（负载）频率。当知道连结到无铁心变压器的初级和次级绕组的电路容量时，考虑中的无铁心印制电路板变压器电路模型的第一个计算谐振频率为约9MHz。变压器的阻抗用阻抗分析仪测量。测得的阻抗与计算值有良好的一致性，如图3所示。由图可见，第一个最大阻抗频率8MHz小于第一个为9MHz的谐振频率。这样，8MHz可以选定为系统的载频。对于一个已知的无铁心变压器，其电路的最大阻抗可通过连结到初级和次级绕组的电容器值来改变。当选定的这个最大阻抗频率略小于电路的谐振频率时，该变压器电路具有（i)高增益和（ii）最小功率损耗两特性。

为了确证这个最大的阻抗频率法，载频在传输直流信号（即100%占空系数）的最坏条件下的频率范围内变化。载频通过在次级绕组上接上一个2KW电阻（该电阻连续损耗近似于解调电路）替代解调电路进行估算。对用10V直流电源初级电路的输入电流进行监视，由图4所示。该曲线证实了推荐的变压器电路在最大阻抗频率时消耗的功率最小。利用8MHz相同的载频，整个变压器电路作为在直流到300kHz频率范围600V、16A功率MOSFET的门驱动电路完成了成功地测试。对于门驱动应用来说，无铁心变压器电路的全部功率损耗<0.9W。当用作数字信号传输时，可以达到从直流到300kHz的传输速率。典型的实用的56kHz传输速率的输入和输出数字信号如图5表示。应说明印制绕组的半径r[6mm(或0.006m），同时，而工作载频的任何辐射波长 。就一个单回路天线而言，辐射功率与成正比。文献[4]中，无铁心印制电路板变压器的辐射功率和效率可以忽略不计[因为约为单回路的这样，来自无铁心印制电路板变压器的辐射电磁干扰对低功率应用没有影响。


作者证实了无铁心印制电路板变压器在电子电路中可能用作能量和信号传输的基本原理。提出了最大阻抗法并证明对无铁心变压器的使用表示乐观。无铁心变压器可以比铁心脉冲变压器更小。它的使用可以省去手工绕线和制作磁心的成本。可望它能在许多低功率应用中（诸如数字信号传输和功率电子门驱动电路）替代铁心脉冲变压器。

The authors describe the optimal use of a printed-circuit board transformer with minimum power consumption. The proposed transformer is much smaller than a standard pulse transformer and is suitable for both signal and energy transfer .It can be operated in the megaHertz frequency range.With the use of modulation/demodulation circuits.it can be used for digital signal transmission over a wide frequency range from DC to~300kHz. The proposal can replace core-based dreds of kiloHertz).In the latter case，the transformer is used for both signal and energy transfer because the isolated gate drive circuit has to provide both the gating signal and the energy for driving the power devices such as power MOSFETs.Traditionally，these pulse transformers are core-based.That is，ferrite cores are used tor provide the magnetic paths of the energy transfer .Corebased transformers ，however，need manual winding，which incr-eases the labour cost and hence the cost of the equipment. An attempt [1] has been proposed for core-based transformers to eliminate the manual winding process by printing the windings on double-sided printed-circuit boards(PCBs).The coreless PCB transformer concept has not received much attention，probably because there are misunderstandings:(i) that planar winding with a few turns printed on a PCB is almost like a ’short-circuit’ and (ii) that magnetic core is an essential component in transformer construction. This Letter describes the optimal use of coreless PCB transformers [2] with minimum power requirements.The proposed transformer has 10 turns for both primary and secondary windings，and a diameter of ~12mm .It is much smaller than a standard pulse transformer as shown in Fig.1.
Problems and solutions :Fig.2 shows the equivalent circuit model of the proposed coreless PCB trandformer.The major problem of using this type of transformer is that the magnetising indutance is extremely small.The short printed windings whit only a few turns behave almost like ’ short-circuit paths’ with low gain and high conduction loss unless the operating frequency is very high(usually in the magaHertz range).However ，the frequency of the transmitted signal may be of relatively low frequency(say from DC to a few hundreds of kiloHertz).To overcome these problems .the following two methods can be used together:
(i)The demodulation circuits described in [2，3] can be used so that a high-frequency carrier frequency(i.e.oper
ating frequency of the transformer )in the magaHertz range is adopted while the required low -frequency signal can be demodulated and obtained as the output of the circuit;
transformers in many lowpower applications.i)The optimal carrier frequency can be selected by using a ’maximum-impedance frequency’approach so that the total power comsumption of the circuit is minimised.
Maximum-impedance frequency:Based on the equivalent circuit ，the maximum impedance of the transformer circuit can be plotted with carrier frequency.The first peak of the impedance plot corresponds to the maximum impedance before the circuit resonance occurs(Beyond the first resonant frequency，the gain of the trandsformer will be lost .) This maximum-impedance frequency should be chosen as the optimal operating(carrier)frequen
cy at which the input current and power of the entire transformer circuit can be minimised .With the knowledge of the circuit capacitance conmected to the primary and secondary windings of the corelss transformer，the calculated first resonant frequency of the circuit model of the coreless PCB transformer under consideration is~9MHz.The impdance of the transformer was measurede with an impedance analyser.The measured impdeance agrees well with the calculated value as shown in Fig.3. It can be seen that the first maximum impedance frequency of~8MHz is less than the first resonant frequency of 9MHz.Thus，8MHz can be chosen as the carrier frequency of the system .For a given coreless transformer，the maximum impedance of the circuit can be alterd by the value of the capacitors connected to the primary and secondary windings， By choosing this maximum impedance frequency slightly less than the circuit resonant frequency ，the transformer cirtuit enjoys both(i)high gain and (ii)minimum power consumption.
To confirm this maximum-impedance frequency approach，the carrier frequency is varied over a range of frequencies under the worst-case situation of a transmitted DC signal(i.e.100%duty cycle). This is approximated by replacing the demodulation circuit in the secondary winding by a 2kW resistor (which constantly consumes energy approximately as the demodulation circuit)，The input current of the primary circuit with a 10V DC rail is monitored and shown in Fig.4. THis plot confirms that the proposed transformer circuit consums minimum power at the maximum impedance frequency .Using the same carrier frequency of 8MHz.the entire transformer circuit has been tested successfully as a gate drive circuit for a 600V ，16A power MOSFET over a range of frequencies from DC to~300KHz，For gate drive applications， the entire power consumption of the coreless transformer circuit is <0.9w. When applied for 、结论
Introduction:Pulse transformers are widely used in the electronics industry for low-power applications.They offer electrical isolation and are commonly used for signal and/or energy trancfer.Examples of such uses of transformers are the decouping transformers used for digital data transmission and for isolated gatd drive circuits in power electronics applications (from DC to a few hundigital signal transmission ，a transimission rate from DC to -300kHz can be achieved. Typical practical input and output digital signals at 56kHz transimisson are shown in Fig.5 It should be noted that the radius r of the printed windings is[6mm(or 0.006m)，and the wavelength of any radiated signal at the operating carrier frequency is l =3x108/ 8 x106=37.5m. For a single loop antenna ，the radiated power is porpor tional to ( r/ l)4 The radiation power and efficiency [4] of
the coreless PCB transformer are negligible(since ( r/ l )4 is in the order of 10-18 for a single loop). Thus ，radiated electromagnetic interference from the coreless PCB transformer is not a concerm for low power applications
conclusions: The authors confirm a fundamental concept that coreless PCB transformers can be used for energy and signal transmission in electronic circuits .The maximum impedance approach is proposed and verified to optimise the use of coreless tranformers. Coreless transfroms can be smaller than corebased pulse transformers .Their use can eliminate the costs of the manual windings and magnetics .It is envisaged that they can replace core-based pulse transformers in many low-power applications such as digital signal transmission and power electronics gate drive circuits