反激的典型波形

1、反激變換器的例子Analysis of basic waveforms 基本波形分析The analysis of the basic waveforms will be done on a simulated example of a flyback converter operating in discontinuous conduction mode. Typical drain-source voltage waveform of the primary side switch is shown in Fig. 16.在電感電流斷續(xù)模式下運(yùn)行的反激變換器的典型一次側(cè)漏源極開(kāi)關(guān)電壓波形見(jiàn)

2、圖。Fig. 16 Typical drain-source voltage of the MOSFET in a flyback圖反激變換器的典型漏源極電壓These drain-source voltage waveforms can be theoretically distinguished into typical elements. Different physical phenomena influence the waveform at given time interval. Fig. 17 and Tab. 4 demonstrate the main elements o

3、f the voltage waveform. The superposition of all these elements results in a typical drain-source voltage shown in Fig. 16.這些漏源極電壓波形能用典型的理論來(lái)描述。各個(gè)時(shí)間段有不同物理現(xiàn)象影響這些波形。圖和平臺(tái)描述了電壓波形的主要原理。把這些原理按時(shí)序整合呈現(xiàn)出圖所示的典型漏源極電壓。Fig. 17 Main elements of the drain-source voltage圖漏源極電壓的主要原理原理：開(kāi)通期間的電壓下降過(guò)程原理：在開(kāi)通期間因寄生震蕩產(chǎn)生的電流尖刺原理

4、：關(guān)斷期間的電壓上升原理：緩沖電路的鉗位電壓原理：鉗位過(guò)程結(jié)束后主要由場(chǎng)效應(yīng)晶體管輸出電容和變壓器漏感引起的寄生振蕩原理：磁芯存儲(chǔ)磁能釋放完畢后主要由場(chǎng)效應(yīng)晶體管輸出電容和變壓器電感引起的寄生振蕩原理：反激變換器釋放磁能期間的反射電壓原理：與直流母線電壓等幅的主要方波Tab. 4 Main elements of the drain-source voltage平臺(tái)漏源極電壓的主要原理The spectrum of the whole drain-source waveform (Fig. 16) is presented in Fig. 18.圖所示的漏源極電壓呈現(xiàn)的電磁干擾頻譜見(jiàn)圖。Fig

5、. 18 Spectrum of the drain-source voltage (as shown in Fig. 16)圖圖所示的漏源極電壓呈現(xiàn)的電磁干擾頻譜The spectra of the main elements of the drain-source voltage can be found in Fig. 20. Fig. 19 is exactly the same as Fig. 17 and has been repeated here for better under-standing.圖描述了漏源極電壓主要原理產(chǎn)生的電磁干擾頻譜。為便于理解，將圖映射成圖。Fig.

6、 19 Main elements of the drain-source voltage (repeated, same as Fig. 17)圖漏源極電壓的主要原理（正確重復(fù)圖）Fig. 20 Spectra of the main elements of the drain-source voltage圖漏源極電壓主要原理產(chǎn)生的電磁干擾頻譜This method allows associating certain parts of the spectrum with their root causes, i.e. the peak at 20 MHz in the spectrum o

7、f the drain-source voltage is caused by the parasitic oscillation due to the output capacitance of the MOSFET and the leakage inductance of the transformer.這種方法可以確定電磁干擾頻譜中某些頻點(diǎn)的來(lái)源，也就是說(shuō)漏源極電壓產(chǎn)生的電磁干擾頻譜中的兆赫茲峰點(diǎn)是鉗位過(guò)程結(jié)束后主要由場(chǎng)效應(yīng)晶體管輸出電容和變壓器漏感引起的寄生振蕩產(chǎn)生的。The analysis of the drain current of the primary switch wi

8、ll be done in the same way. Fig. 21 demonstrates a typical drain current in a DCM flyback.對(duì)一次側(cè)開(kāi)關(guān)的漏極電流進(jìn)行分析采用相同的方法。圖展示出一個(gè)工作于電感電流斷續(xù)模式反激變換器的典型漏極電流。Fig. 21 Typical drain current in a flyback圖反激變換器的典型漏極電流This waveform can be presented as a superposition of the following elements (Fig. 22 and Tab. 5). The

9、superposition of all these elements results in a typical drain current shown in Fig. 21.這個(gè)波形可以被看作是下列原理的疊加（圖和平臺(tái)）。全部這些波形的疊加整合結(jié)果變成圖所示的典型漏極電流。Fig. 22 Main elements of the drain current圖漏極電流的主要原理原理：漏極電流的主要三角波形原理：在開(kāi)關(guān)開(kāi)通期間因寄生分布電容引起的電流尖刺原理：鉗位過(guò)程結(jié)束后主要由場(chǎng)效應(yīng)晶體管輸出電容和變壓器漏感引起的寄生振蕩原理：磁芯存儲(chǔ)磁能釋放完畢后主要由場(chǎng)效應(yīng)晶體管輸出電容和變壓器電感引起的

10、寄生振蕩Tab. 5 Main elements of the drain current平臺(tái)漏極電流的主要原理The spectrum of the whole drain current waveform (Fig. 21) is presented in Fig. 23.全部漏極電流波形產(chǎn)生的電磁干擾頻譜（圖）呈現(xiàn)在圖。Fig. 23 Spectrum of the drain current (as shown in Fig. 22)圖漏極電流產(chǎn)生的電磁干擾頻譜（與圖相同）The spectra of the main elements of the drain current can

11、 be found in Fig. 25. Fig. 24 is exactly the same as Fig. 22 and has been repeated for better understanding.漏極電流主要原理產(chǎn)生的電磁干擾頻譜見(jiàn)圖。圖和圖相同。Fig. 24 Main elements of the drain current圖漏極電流的主要原理Fig. 25 Spectra of the main elements of the drain current圖漏極電流主要原理產(chǎn)生的電磁干擾頻譜As in case of drain-source voltage this

12、 method allows to associate the elements of the drain current waveform with its contribution to the whole spectrum. For example, the peak at 20 MHz in the spectrum is caused by the parasitic oscillation due to the output capacitance of the MOSFET and the leakage inductance of the transformer.就象漏源極電壓

13、的例子那樣，用這種方法也可以找出漏極電流的哪一部分對(duì)電磁干擾頻譜產(chǎn)生影響。舉例說(shuō)明，兆赫茲的峰點(diǎn)是鉗位過(guò)程結(jié)束后主要由場(chǎng)效應(yīng)晶體管輸出電容和變壓器漏感引起的寄生振蕩產(chǎn)生的。This method of separating the waveform in time domain into its main elements helps to find out what part of the spectrum in frequency domain caused by what related physical phenomena. The separation into main eleme

14、nts should be done in respect of reasonable events in the power circuit like on and off slopes, oscillations, clamping, snubbering, reflected voltage, etc.這種在時(shí)域里對(duì)主要原理進(jìn)行拆分的方法有助于找出產(chǎn)生電磁干擾頻段的干擾源。這種離析主要原理的手法有助于合理審視電源電路里諸如變化速率、振蕩、鉗位、緩沖、反射電壓等過(guò)程。In this flyback example only the primary switch has been analy

15、zed as active source of electrical noise. There are also others, like secondary side diodes or synchronous rectifier, control IC (especially its gate drive), etc. In order to obtain more complete analysis all these interference sources have to be analyzed.在這個(gè)反激變換器里只對(duì)一次側(cè)開(kāi)關(guān)進(jìn)行電磁噪聲產(chǎn)生的分析。但是還有其他的部分，象二次側(cè)的二

16、極管或同步整流器、控制集成電路（尤其是它們的柵極驅(qū)動(dòng)）等等。按順序分析將獲得更完善的關(guān)于這些電磁干擾源的解析。However, it is impossible to predict the conducted EMI spectrum using this approach due to the fact, that only interference sources are considered. There is no analysis of the spreading paths of the interference in this method.然而，這種方法不可能預(yù)知用頻譜反映的

17、電磁干擾的實(shí)際行為，僅僅是干擾源被重視起來(lái)。在那里沒(méi)有對(duì)分布參數(shù)產(chǎn)生的干擾進(jìn)行分析的方法。Nevertheless, the association of harmonics root cause with the respected physical phenomena will reduce the efforts of EMI reduction. The impact of the identified root cause can be reduced not only by filtering, but also by means of influencing the root c

18、ause itself.不過(guò)，重視物理現(xiàn)象并不能成就電磁干擾的降低。降低干擾并不僅僅是濾波，也同樣意味著干擾源自身的影響。Operation modes of discontinuous flyback converter 電感電流斷續(xù)工作反激式變換器的運(yùn)行模式The flyback converter running in discontinuous conduction mode can be operated in hard switching or quasi resonant (or valley switching, or ZVS) mode regarding the prima

19、ry side switch. The difference between a hard switching and quasi resonant flyback converter is the turn on time point of the primary switch. In a hard switching mode the turning on of the MOSFET is not synchronized with the drain-source voltage value. This type of converters runs mainly in fixed fr

20、equency mode.電感電流斷續(xù)工作的反激式變換器一次側(cè)開(kāi)關(guān)可工作于硬開(kāi)關(guān)或準(zhǔn)諧振（或谷值開(kāi)關(guān)或零電壓開(kāi)關(guān)）模式。硬開(kāi)關(guān)和準(zhǔn)諧振反激變換器之間的差異在于一次側(cè)開(kāi)關(guān)的開(kāi)啟時(shí)間點(diǎn)。在硬開(kāi)關(guān)里場(chǎng)效應(yīng)晶體管的開(kāi)啟波形拐點(diǎn)并不和漏源極電壓值同步。這種變換器大體上運(yùn)行于固定頻率模式。In a quasi resonant mode the resonant circuit determined by the output capacity of the MOSFET and the inductance of the transformer will be utilized to switch on

21、 at lowest possible value of the drain-source voltage. This circuit starts to oscillate at the end of the current flow through the secondary side of the transformer, hence at the end of the flyback phase. The MOSFET will be turned on at the minimum of this oscillation. The quasi resonant approach us

22、es this oscillation to achieve minimum voltage switching during turn on for the MOSFET. This operation mode runs at a variable frequency.在準(zhǔn)諧振模式里，由變壓器電感和場(chǎng)效應(yīng)晶體管輸出電容引起的諧振促使開(kāi)關(guān)的開(kāi)通時(shí)刻發(fā)生在漏源極電壓的最小值上。這種電路在電流從變壓器二次側(cè)流盡以后（反激回掃過(guò)程結(jié)束）開(kāi)始振蕩。場(chǎng)效應(yīng)晶體管將在振蕩幅值的最小值開(kāi)啟（谷值開(kāi)通）。這種運(yùn)行模式工作在可變的頻率上。Higher amplitude of the oscillation

23、results in lower drain source voltage level at which the MOSFET turns on correspondingly lower switching losses and higher efficiency of the system.更高幅值的振蕩導(dǎo)致場(chǎng)效應(yīng)晶體管更低的漏源極開(kāi)通電壓幅值來(lái)產(chǎn)生更低的開(kāi)關(guān)損耗和更高的系統(tǒng)效率。To achieve high oscillation peaks, the design of the transformer has to be set to high reflected voltage.

24、This increase of the reflected voltage results in a higher drain-source voltage blocking MOSFET and longer duty cycles.要達(dá)到比較高的振蕩電壓峰值，變壓器的反射電壓必須設(shè)置的比較高。增加的反射電壓導(dǎo)致使用更高漏源極擊穿電壓的場(chǎng)效應(yīng)晶體管和更大的開(kāi)關(guān)占空比。Comparison of three different flyback solutions has been made. All of them have been operation at 300 kHz, bus vo

25、ltage of 400 V, output power of 120 W, output voltage of 16 V. These design included different modes of operation and different values of reflected voltage, resulting in different MOSFETs voltage ratings:比較現(xiàn)有的三種反激變換器。它們都工作在千赫茲，直流母線電壓伏特，輸出功率瓦特，輸出電壓伏特。這些設(shè)計(jì)包含不同的運(yùn)行模式和反射電壓等級(jí)，因此使用不同電壓等級(jí)的場(chǎng)效應(yīng)晶體管：l 

26、0;    Hard switching flyback with CoolMOS 600V, reflected voltage of 100V l      硬開(kāi)關(guān)反激變換器使用伏特CoolMOS，伏特反射電壓l      Quasi resonant flyback with CoolMOS 600V, reflected voltage of 100V l      準(zhǔn)諧振反激變換器使用伏特CoolMOS，伏

27、特反射電壓l      Quasi resonant flyback with CoolMOS 800V, reflected voltage of 390V l      準(zhǔn)諧振反激變換器使用8伏特CoolMOS，39伏特反射電壓The clamping snubber circuit was set to the rated breakdown voltage of the MOSFET (600 V and 800 V respectively).鉗位緩沖電路被設(shè)定在場(chǎng)效應(yīng)晶體管的額定擊穿

28、電壓上（分別為伏特和伏特）。Flyback in hard switching mode with 600V MOSFET 使用伏特場(chǎng)效應(yīng)晶體管的硬開(kāi)關(guān)反激變換器The hard switching approach (as shown in Fig. 26) doesnt consider the minimum drain-source voltage. The MOSFET will be turned on hard, in this case at a voltage level of 500 V (at time point 3.3 s). The discharge of cir

29、cuits parasitic capacitances leads to a high current spike during turning on.硬開(kāi)關(guān)（圖所示）幾乎不考慮漏源極電壓的最小值。場(chǎng)效應(yīng)晶體管開(kāi)通應(yīng)力大，在這個(gè)例子里，開(kāi)通電壓在伏特（在.微秒的時(shí)間點(diǎn)）。由寄生電容引起的泄放電流在開(kāi)通時(shí)產(chǎn)生很高的電流尖刺。Fig. 26 Drain-source voltage and drain current of hard switching 600V flyback圖伏特硬開(kāi)關(guān)反激變換器的漏源極電壓和漏極電流Flyback in quasi resonant mode with 60

30、0 V MOSFET 使用伏特場(chǎng)效應(yīng)晶體管的準(zhǔn)諧振反激變換器The drain-source voltage (Fig. 27) starts oscillating at the end of the flyback phase and reaching the minimum of 300 V when the MOSFET turns on. 漏源極電壓（圖）在反射過(guò)程結(jié)束后并減小到伏特時(shí)場(chǎng)效應(yīng)晶體管導(dǎo)通。The duty cycle is lower compared to an 800 V solution due to a lower reflected voltage of 10

31、0V. Shorter duty cycle for the same output power results in higher peak currents on the primary side.因?yàn)榉氐姆瓷潆妷?，比較伏特解決方案它有更小的占空比。小占空比實(shí)現(xiàn)同樣的功率輸出必須使用更高的一次側(cè)峰值電流。Fig. 27 Drain-source voltage and drain current of quasi resonant 600V flyback圖伏特準(zhǔn)諧振反激變換器的漏源極電壓和漏極電流Flyback in quasi resonant mode with 800 V MOS

32、FET 使用伏特場(chǎng)效應(yīng)晶體管的準(zhǔn)諧振反激變換器The drain-source voltage (Fig. 28) starts oscillating at the end of the flyback phase and reaching the minimum of 100V when the MOSFET turns on. The turning on current spike is low. 漏源極電壓（圖）在反射過(guò)程結(jié)束后并減小到伏特時(shí)場(chǎng)效應(yīng)晶體管導(dǎo)通。開(kāi)通電流尖刺比較低。The duty cycle is higher compared to a 600V solution

33、due to a higher reflected voltage of 390V. Longer duty cycle for the same output power results in lower peak currents on the primary side.因?yàn)橛蟹氐姆瓷潆妷?，所以有比伏特解決方案更大的占空比。更大的占空比實(shí)現(xiàn)同樣的輸出功率可以使用更低的一次側(cè)峰值電流。Fig. 28 Drain-source voltage and drain current of quasi resonant 800V flyback圖伏特準(zhǔn)諧振反激變換器的漏源極電壓和漏極電流Comparison of spectra 干擾頻譜比較The spectra of the drain-source voltages for corresponding flyback design (Fig. 26Fig. 27 and Fig. 28) are shown in Fig. 29.相應(yīng)設(shè)計(jì)的反激變換器（圖、圖和圖）的漏源極電壓干擾頻譜如圖所示。Fig. 29 Spectra of the drain-source voltage (simulated)圖漏源極電壓的頻譜（仿真）As it can be seen th