Saturated Inductor and Its Application in Switching Power Supply

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饱和电感是一种磁滞回线矩形比高，起始磁导率高，矫顽力小，具有明显磁饱和点的电感，在电子电路中常被当作可控延时开关元件来使用。由于其独特的物理特性，使之在高频开关电源的开关噪声抑制，大电流输出辅路稳压，移相全桥变换器，谐振变换器及逆变电源等方面得到了日益广泛的应用。 　　1饱和电感的分类及其物理特性[1] 　　1.1饱和电感的分类 　　饱和电感可分为自饱和和可控饱和二类。 　　1.1.1自饱和电感（Saturableinductor） 　　其电感量随通过的电流大小可变。若铁心磁特性是理想的（例如呈矩形），如图1（a）所示，则饱和电感工作时，类似于一个“开关”，即绕组中的电流小时，铁心不饱和，绕组电感很大，相当于“开路”；绕组中电流大时，铁心饱和，绕组电感小，相当于开关“短路”。 　　1.1.2可控饱和电感（controlledsaturableinductor） 　　又称可控饱和电抗器（controlledsaturablereactor），其基本原理是，带铁心的交流线圈在直流激磁作用下，由于交直流同时激磁，使铁心状态一周期内按局部磁回线变化，因此，改变了铁心等效磁导率和线圈电感。若铁心磁特性是理想的（B－H特性呈矩形），则可控饱和电感类似于一个“可控开关”。在开关电源中，应用可控饱和电感可以吸收浪涌，抑制尖峰，消除振荡，与快速恢复整流管串联时可使整流管损耗减小。如图1（b）所示，可控饱和电感具有高磁滞回线矩形比（Br/Bs），高起始磁导率μi，低矫顽力Hc，明显的磁饱和点（A，B）及由于其磁滞回线所包围的面积狭小而使其高频磁滞损耗较小等特征。为此，可控饱和电感在应用方面的两个显著特点为 　　1）由于饱和磁场强度很小，所以，可饱和电感的储能能力很弱，不能被当作储能电感使用。可饱和电感的最大储能Em的理论值可用式（1）表示。 　　式中：μ为临界饱和点磁导率； 　　　H为临界饱和点磁场强度； 　　　V为磁性材料的有效体积。 　　2）由于可饱和电感的起始磁导率高，磁阻小，电感系数和电感量都很大，在施加外部电压时，电感内部起始电流增长缓慢，只有经过Δt的延时后，当电感线圈中的电流达到一定数值时，可饱和电感才会立即饱和，因而在电路中常被当作可控延时开关元件使用。 　　1.2可饱和电感随电流变化的关系 　　因为，有气隙和无气隙的dB/di磁路的计算方法不同，所以，分别对两种情况进行讨论。 　　1.2.1无气隙可饱和电感与电流的关系 　　无气隙可饱和电感L随电流变化的关系可用式（2）表示。 　　式中：W为电感绕组匝数； 　　　I为激磁电流； 　　　f为电感用磁性材料B～H曲线的对应函数； 　　　S为磁性材料的截面积； 　　　l磁性材料的为平均长度。 　　1.2.2有气隙可饱和电感与电流的关系 　　任意给定一个导磁体磁路中磁感应强度B1，可由B=f(H)曲线求出导磁体磁路中的磁场强度H1。气隙中的H0值可用式（3）表示。 　　式中：B0为空气隙磁感应强度； 　　　a和b为磁路矩形截面积边长； 　　　l0为气隙长度； 　　　μ0为空气磁导率。 　　由磁路定律得改变B值并重复上述步骤，可求出相应的I，得到一组B和I的关系数据。设这个B与I对应的函数为B=f1(I)。 　　在不考虑漏感时，电感的计算式可用式（4）表示。 2饱和电感在开关电源中的应用 　　2.1尖峰抑制器 　　开关电源中尖峰干扰主要来自功率开关管和二次侧整流二极管的开通和关断瞬间。具有容易饱和，储能能力弱等特点的饱和电感能有效抑制这种尖峰干扰。将饱和电感与整流二极管串联，在电流升高的瞬间，它呈现高阻抗，抑制尖峰电流，而饱和后其饱和电感量很小，损耗小。通常将这种饱和电抗器作为尖峰抑制器。 　　在图2所示电路中，当S1导通时，D1导通，D2截至，由于可饱和电感Ls的限流作用，D2中流过的反向恢复电流的幅值和变化率都会显著减小，从而有效地抑制了高频导通噪声的产生。当S1关断时，D1截至，D2导通，由于Ls存在着导通延时时间Δt，这将影响D2的续流作用，并会在D2的负极产生负值尖峰电压。为此，在电路中增加了辅助二极管D3和电阻R1。 　　2.2磁放大器 　　磁放大器是利用可控饱和电感导通延时的物理特性，控制开关电源的占空比和输出功率。该开关特性受输出电路反馈信号的控制，即利用磁芯的开关功能，通过弱信号来实现电压脉冲脉宽控制以达到输出电压的稳定。在可控饱和电感上加上适当的采样和控制器件，调节其导通延时的时间，就可以构成最常见的磁放大器稳压电路。 　　磁放大器稳压电路有电压型控制和电流型控制两种。图3所示为电压型复位电路，它包括电压检测及误差放大电路，复位电路和控制输出二极管D3，它是单闭环电压调节系统。 　　所示为移相全桥ZVS-PWM开关电源磁放大器稳压器[2]。全桥开关电路变压器二次双半波整流各接一个磁放大器SR，其铁心绕有工作绕组和控制绕组。在正半周，当某输出整流管正偏（另一输出整流管反偏），变压器副边输出的方波脉冲加在相应的工作绕组上，使SR铁心正向磁化（增磁）；在负半周，该输出整流管反偏，和控制绕组串联的二极管D3正偏导通，在直流控制电流Ic的作用下，使该SR的铁心去磁（复位）。 　　控制电路的工作原理是：开关电源输出电压与基准比较后，经误差放大控制MOS管的栅极，MOS管提供与输出电压有关的磁放大器SR的控制电流Ic。 　　2.3移相全桥ZVS-PWM变换器 　　移相全桥ZVS-PWM变换器结合了零电压开关准谐振技术和传统PWM技术两者的优点，工作频率固定，在换相过程中利用LC谐振使器件零电压开关，在换相完毕后仍然采用PWM技术传送能量，控制简单，开关损耗小，可靠性高，是一种适合于大中功率开关电源的软开关电路。但当负载很轻时，尤其是滞后桥臂开关管的ZVS条件难以满足。 　　Using saturated inductance as the resonant inductance of the phase-shifted full-bridge ZVS-PWM converter [3] can expand the range of the switching power supply that meets the ZVS condition under light load. Applying it to the arc welding inverter power supply [4] can reduce the loss of additional loop energy and effective duty cycle, expand the load range of zero voltage switching while ensuring efficiency, and improve the reliability of the soft switching arc welding inverter power supply. Connecting 　　the saturated inductance in series with the secondary output rectifier of the isolation transformer of the switching power supply can eliminate secondary parasitic oscillation, reduce circulating energy, and minimize the duty cycle loss of the phase-shifted full-bridge ZVS-PWM switching power supply. 　　In addition, connecting the saturated inductance and the capacitor in series to the primary of the phase-shifted full-bridge ZVS-PWM switching power supply transformer [5], the leading arm switch tube works according to ZVS; when the load current approaches zero, the inductance increases, preventing the current from changing in the opposite direction, creating the ZCS condition of the lagging arm switch tube, and realizing the phase-shifted full-bridge ZV-ZCSPWM converter. 　　2.4 Resonant converter 　　The series resonant converter using series inductance or saturated inductance [6] is shown in Figure 5. When the resonant inductor current works in a continuous state, the switch tube is turned off at zero voltage/zero current, but it is turned on hard, resulting in turn-on loss. The anti-parallel diode is turned on naturally, but there is a reverse recovery current when it is turned off. Therefore, the anti-parallel diode must be a fast recovery diode. In order to reduce the turn-on loss of the switch tube and achieve zero current turn-on, the switch tube can be connected in series with an inductor or a saturated inductor. Before the switch tube is turned on, the saturated inductor current is zero. When the switch tube is turned on, the saturated inductor limits the current rise rate of the switch tube, causing the switch tube current to slowly rise from zero, thereby achieving zero current turn-on of the switch tube, while improving the turn-off condition of the diode and eliminating the reverse recovery problem. 　2.5 Inverter power supply [7] 　　Inverter power supply is widely used in various aspects such as automatic control, power electronics and precision instruments due to its many advantages such as good control performance, high efficiency and small size. Its performance is closely related to the quality of the entire system, especially the dynamic performance of the power supply. Due to the characteristics of the inverter power supply itself, its dynamic characteristics have always been less than ideal. 　　The working principle of the inverter power supply controlled by PWM and PFM determines that in order to obtain a smooth current and voltage waveform, a freewheeling inductor must be added to its output circuit, and this inductor is the main factor affecting the dynamic performance of the inverter power supply. For a constant voltage source, the inductor current is completely inversely proportional to the load; for a controllable constant current source, to make the inductor current change from small to large, it is necessary to take a small load value as a prerequisite. Although it is not a complete correspondence, it can be said that the change in current reflects the change in load to some extent. 　　Therefore, using an inductor that decreases with the increase of current as the output inductor of the inverter power supply can effectively change the time constant T of the power supply output circuit, making it completely inversely proportional to R (T=L/R), and then maintaining it at a relatively small value within the load change range, which will naturally improve the dynamic performance. 　　3 Conclusion 　　This article details the physical characteristics of the saturated inductor and the relationship between the inductance and current. On this basis, it summarizes the application of saturated inductors in spike suppressors, magnetic amplifiers, phase-shifted full-bridge ZVS-PWM converters, resonant converters and inverter power supplies, and briefly analyzes their working principles.