Saturated Inductor and Its Application in Switching Power Supply

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Saturated Inductor and Its Application in Switching Power Supply [Copy link]

[Author: Li Jinpeng, Yin Huajie, Hou Congling Reposted from: Polaris Electric Technology

The classification and basic physical characteristics of saturable inductors are introduced, and the applications of saturable inductors in spike suppressors, magnetic amplifiers, phase-shifted full-bridge ZVS-PWM converters, resonant converters and inverter power supplies are summarized. 　　 Saturation Inductor and Its Application in Switching-mode Power Supply LI Jinpeng,YIN Huajie,HOU Congling　　 (Astec Lab. South China University of Technolgy,Guang zhou Guang dong 510640 China) 　 　　Abstract : The class and basic physical characteristic of the saturation inductor, and its applications in peak suppresser, magnetic amplifier, shift phase full bridge,resonant converter and inverter supply are introduced. 　　Keywords : saturation inductor;peak suppressor; magnetic amplifier; shift phase full bridge; resonant converter; inverter supply. 　 0 Introduction 　　Saturated inductor is an inductor with high hysteresis loop squareness ratio, high initial permeability, small coercive force and obvious magnetic saturation point. It is often used as a controllable delay switch element in electronic circuits. Due to its unique physical properties, it has been increasingly widely used in high-frequency switching power supply switching noise suppression, large current output auxiliary circuit voltage regulation, phase-shifted full-bridge converter, resonant converter and inverter power supply. 1 Classification of saturated inductors and their physical properties [1] 1.1 Classification of saturated inductors 　　Saturated inductors can be divided into two categories: self-saturated and controlled saturated. 1.1.1 Self-saturated inductors (Saturable inductors)　　The inductance varies with the current passing through. If the magnetic properties of the core are ideal (for example, rectangular), as shown in Figure 1 (a), the saturated inductor is similar to a "switch" when working, that is, when the current in the winding is small, the core is not saturated, the winding inductance is large, which is equivalent to an "open circuit"; when the current in the winding is large, the core is saturated, the winding inductance is small, which is equivalent to a "short circuit" switch. 1.1.2 Controlled saturated inductors (controlled　　saturable inductors) are also called controlled saturated reactors. The basic principle is that when an AC coil with an iron core is excited by DC, due to the simultaneous excitation of AC and DC, the state of the core changes according to the local magnetic loop within a cycle, thereby changing the equivalent magnetic permeability of the core and the inductance of the coil. If the magnetic characteristics of the core are ideal (B-H characteristics are rectangular), the controllable saturated inductor is similar to a "controllable switch". In the switching power supply , the application of controllable saturated inductor can absorb surges, suppress spikes, eliminate oscillations, and reduce the loss of the rectifier when connected in series with the fast recovery rectifier. As shown in Figure 1 (b), the controllable saturated inductor has the characteristics of high hysteresis loop rectangular ratio (Br/Bs), high initial magnetic permeability μi, low coercive force Hc, obvious magnetic saturation point (A, B) and small high-frequency hysteresis loss due to the small area surrounded by its hysteresis loop. For this reason, the two significant features of the controllable saturated inductor in application are: 　　1) Since the saturation magnetic field intensity is very small, the energy storage capacity of the saturable inductor is very weak and cannot be used as an energy storage inductor. The theoretical value of the maximum energy storage Em of the saturable inductor can be expressed by formula (1). 500)this.style.width=500;" border=0> Where: μ is the critical saturation point permeability; 　　　H is the critical saturation point magnetic field intensity; 　　　V is the effective volume of the magnetic material. 　　2) Since the initial permeability of the saturable inductor is high, the magnetic resistance is small, the inductance coefficient and inductance are large, when the external voltage is applied, the initial current inside the inductor increases slowly. Only after a delay of Δt, when the current in the inductor coil reaches a certain value, the saturable inductor will immediately saturate. Therefore, it is often used as a controllable delay switch element in the circuit. 　　 500)this.style.width=500;" border=0> 1.2 Relationship between saturable inductance and current 　　Because the calculation methods of dB/di magnetic circuits with and without air gaps are different, the two cases are discussed separately. 1.2.1 Relationship between saturable inductance and current without air gap 　　The relationship between the saturable inductance L without air gap and current can be expressed by formula (2). 　　500)this.style.width=500;" border=0> Where: W is the number of turns of the inductor winding; 　　　I is the exciting current; 　　　f is the corresponding function of the B-H curve of the magnetic material used for the inductor; 　　　S is the cross-sectional area of the magnetic material; 　　　l is the average length of the magnetic material. 1.2.2 Relationship between the saturable inductor and the current with 　　an air gap. For any given magnetic flux density B1 in a magnetic conductor magnetic circuit, the magnetic field intensity H1 in the magnetic conductor magnetic circuit can be calculated from the B=f(H) curve. The H0 value in the air gap can be expressed by formula (3). 　　500)this.style.width=500;" border=0> Where: B0 is the magnetic flux density of the air gap; 　　　a and b are the side lengths of the rectangular cross-sectional area of the magnetic circuit; 　　　l0 is the air gap length; 　　　μ0 is the air permeability. 　　According to the magnetic circuit law , we can get the corresponding I by changing the value of B and repeating the above steps, and get a set of relationship data between B and I. Let the function corresponding to B and I be B=f1(I). 　　When the leakage inductance is not considered, the calculation formula of inductance can be expressed by formula (4). 2 Application of saturated inductance in switching power supply 2.1 Spike suppressor The spike interference in the switching power supply mainly comes from the turn-on and turn-off moments of the power switch tube and the secondary side rectifier diode. The saturated inductor with the characteristics of easy saturation and weak energy storage capacity can effectively suppress this spike interference. When the saturated inductor is connected in series with the rectifier diode, it presents high impedance at the moment of current increase, suppressing the peak current, and its saturated inductance is very small after saturation, and the loss is small. This saturated reactor is usually used as a spike suppressor. 　　In the circuit shown in Figure 2, when S1 is turned on, D1 is turned on and D2 is turned off. Due to the current limiting effect of the saturable inductor Ls, the amplitude and rate of change of the reverse recovery current flowing through D2 will be significantly reduced, thereby effectively suppressing the generation of high-frequency conduction noise. When S1 is turned off, D1 is turned off and D2 is turned on. Since Ls has a turn-on delay time Δt, this will affect the freewheeling effect of D2 and generate a negative peak voltage at the negative electrode of D2. For this reason, an auxiliary diode D3 and a resistor R1 are added to the circuit.　　 　　 500)this.style.width=500;" border=0> 2.2 Magnetic 　　Amplifier The magnetic amplifier uses the physical characteristics of the controllable saturated inductor conduction delay to control the duty cycle and output power of the switching power supply . The switching characteristics are controlled by the feedback signal of the output circuit, that is, the switching function of the magnetic core is used to achieve voltage pulse width control through weak signals to achieve output voltage stability. By adding appropriate sampling and control devices to the controllable saturated inductor and adjusting its conduction delay time, the most common magnetic amplifier voltage regulator circuit can be constructed. 　　There are two types of magnetic amplifier voltage regulator circuits: voltage type control and current type control. Figure 3 shows a voltage type reset circuit, which includes a voltage detection and error amplifier circuit, a reset circuit and a control output diode D3. It is a single closed-loop voltage regulation system. 500)this.style.width=500;" border=0> 　　FIG4 shows a phase-shifted full-bridge ZVS-PWM switching power supply magnetic amplifier regulator [2]. The secondary double half-wave rectifier of the full-bridge switching circuit transformer is connected to a magnetic amplifier SR, whose core is wound with a working winding and a control winding. In the positive half cycle, when a certain output rectifier is forward biased (the other output rectifier is reverse biased), the square wave pulse output by the secondary side of the transformer is added to the corresponding working winding, so that the SR core is forward magnetized (magnetized); in the negative half cycle, the output rectifier is reverse biased, and the diode D3 connected in series with the control winding is forward biased and turned on. Under the action of the DC control current Ic, the core of the SR is demagnetized (reset). 500)this.style.width=500;" border=0> 　　The working principle of the control circuit is: after the output voltage of the switching power supply is compared with the reference, the gate of the MOS tube is controlled by error amplification, and the MOS tube provides a control current Ic of the magnetic amplifier SR related to the output voltage. 2.3 Phase-shifted full-bridge ZVS-PWM converter 　　The phase-shifted full-bridge ZVS-PWM converter combines the advantages of zero voltage switching quasi-resonant technology and traditional PWM technology. The operating frequency is fixed. During the phase change process, LC resonance is used to make the device zero voltage switch. After the phase change is completed, PWM technology is still used to transmit energy. It is simple to control, has low switching loss and high reliability. It is a soft switching circuit suitable for large and medium power switching power supplies . However, when the load is very light, the ZVS condition of the lagging bridge arm switch tube is difficult to meet. 　　Using the saturated inductor as the resonant inductor 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 [4] can reduce the loss of additional loop energy and effective duty cycle. On the basis of ensuring efficiency, it expands the load range of zero voltage switching and improves the reliability of the soft switching arc welding inverter. Connecting 　　the saturated 　　inductor 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, by connecting the saturated inductor and the capacitor in series on the primary side 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 [6] using a series inductor or a saturated inductor 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. 500)this.style.width=500;" border=0> 2.5 Inverter [7] 　　Inverter is widely used in automatic control, power electronics and precision instruments due to its good control performance, high efficiency and small size. Its performance is closely related to the quality of the whole system, especially the dynamic performance of the power supply. Due to the characteristics of the inverter itself, its dynamic characteristics have always been less than ideal. 　　The working principle of the inverter 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. For a constant voltage source, the inductor current is completely inversely proportional to the load; for a controllable constant current source, in order to make the inductor current change from small to large, a small load value must be used as a prerequisite. Although it is not a completely corresponding relationship, 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 can effectively change the time constant T of the power 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 paper details the physical characteristics of saturated inductors and the changing relationship between inductance and current. On this basis, it summarizes the applications 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. References [1] Cai Xuansan. Principle and design of saturable reactor [M]. Beijing: Tsinghua University Press, 1978. [2] Cai Xuansan. Magnetic amplifier output voltage regulator in switching power supply [J]. Power World, 2001, (3): 1-6. [3] Hua GCh. An Improved Fullbridge Zero-voltage switched PWM Converter Using a Saturable Inductor [J]. IEEE Transon PE, 1993, 8: 530-534. [4] Chen Shujun, Yin Shuyan. Improving the performance of soft-switching arc welding inverter power supply using saturated inductor [J]. Journal of Welding, 1998, (12): 46-49. [5] Hu Xiaoguang, Cai Weizheng. A new type of ZVZCS full-bridge PWM converter [J]. Journal of Electrical Engineering, 2002, (7): 4-7. [6] Ruan Xinbo, Yan Yangguang. Soft switching technology of DC switching power supply [M]. Beijing: Science Press, 2000. [7] Wang Xinzhi. Research on improving the dynamic characteristics of inverter power supply using saturated inductance [J]. Power Electronics Technology, 1998, 32(1): 26-28.
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