Saturation inductance is an inductance with high hysteresis loop square ratio, high initial permeability, low coercive force, and obvious magnetic saturation point. It is often used as a controllable delay switching element in electronic circuits. Due to its unique physical characteristics, it has been increasingly widely used in high-frequency switching power supply switching noise suppression, high-current output auxiliary circuit voltage stabilization, phase-shifting full-bridge converters, resonant converters and inverter power supplies.

1. Classification of saturated inductance

Saturated inductors can be divided into two categories: self-saturation and controllable saturation.

1.1 Self-saturated inductance

Its inductance varies with the magnitude of the current passing through it. If the magnetic properties of the iron core are ideal (for example, rectangular), as shown in Figure 1(a), the saturated inductance works like a "switch", that is, the current in the winding is small, the iron core is not saturated, and the winding inductance is large , which is equivalent to "open circuit"; when the current in the winding is large, the iron core is saturated, and the winding inductance is small, which is equivalent to "short circuit" of the switch.

1.2 Controllable saturated inductance

Also known as a controllable saturable reactor, its basic principle is that under the action of DC excitation, the AC coil with iron core is excited at the same time as AC and DC, so that the state of the iron core changes according to the local magnetic loop within one cycle, so the equivalent of the iron core is changed. Magnetic permeability and coil inductance. If the magnetic properties of the iron core are ideal (the B-H characteristic is rectangular), the controllable saturated inductance is similar to a "controllable switch". In switching power supplies, the application of controllable saturated inductance can absorb surges, suppress peaks, eliminate oscillations, and reduce the loss of rectifiers when connected in series with fast recovery rectifiers. As shown in Figure 1(b), the controllable saturated inductor has a high hysteresis loop squareness ratio (Br/Bs), high initial permeability μi, low coercive force Hc, and obvious magnetic saturation point (A, B ) and due to the small area surrounded by its hysteresis loop, it has small high-frequency hysteresis loss and other characteristics. For this reason, the two notable features of the controllable saturated inductance in application are

1) Since the saturation magnetic field strength 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).

Em=μVH2/2 (1)

In the formula: μ is the permeability of the critical saturation point;

H is the magnetic field strength at the critical saturation point;

V is the effective volume of the magnetic material.

2) Due to the high initial permeability of the saturable inductor, small reluctance, large inductance and inductance, when the external voltage is applied, the initial current inside the inductor increases slowly. Only after the delay of Δt, when When the current in the inductor coil reaches a certain value, the saturable inductor will be saturated immediately, so it is often used as a controllable delay switching element in the circuit.

                                                                                          (a) Ideal magnetic characteristics B=f(H)           (b) B=f(H) of saturable inductance

                                                                                                              Figure 1 B-H characteristics of saturated inductance

2. Application of saturated inductance in switching power supply

2.1 Spike suppressor

The peak 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 inductance with the characteristics of easy saturation and weak energy storage capacity can effectively suppress this kind of spike interference. The saturated inductance is connected in series with the rectifier diode. When the current rises, it presents high impedance and suppresses the peak current. After saturation, its saturated inductance is very small and the loss is small. This saturable 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 inductance Ls, the amplitude and rate of change of the reverse recovery current flowing in D2 will be significantly reduced. Thus, the generation of high-frequency conduction noise is effectively suppressed. When S1 is turned off, D1 is turned off, and D2 is turned on. Since there is a turn-on delay time Δt in Ls, this will affect the freewheeling effect of D2, and will generate a negative peak voltage at the negative pole of D2. For this reason, auxiliary diode D3 and resistor R1 are added in the circuit.

                                                                                                                                  Figure 2 Application of spike suppressor

2.2 Magnetic Amplifier

The magnetic amplifier uses the physical characteristics of the turn-on delay of the controllable saturated inductance to control the duty cycle and output power of the switching power supply. The switching characteristic is controlled by the feedback signal of the output circuit, that is, the switching function of the magnetic core is used to realize the pulse width control of the voltage pulse through a weak signal to achieve the stability of the output voltage. Add appropriate sampling and control devices to the controllable saturated inductance, and adjust the turn-on delay time to form the most common magnetic amplifier voltage regulator circuit.

There are two types of magnetic amplifier voltage regulator circuits: voltage control and current control. Figure 3 shows the voltage-type reset circuit, which includes voltage detection and error amplification circuit, reset circuit and control output diode D3, it is a single closed-loop voltage regulation system.

                                                                                                                    Figure 3 Magnetic amplifier voltage-type reset regulator circuit

Figure 4 shows the phase-shifted full-bridge ZVS-PWM switching power supply magnetic amplifier regulator. The secondary double half-wave rectification of the full-bridge switching circuit transformer is respectively connected to a magnetic amplifier SR, and its core is wound with a working winding and a control winding. In the positive half cycle, when one output rectifier is forward-biased (another output rectifier is reverse-biased), the square wave pulse output by the secondary side of the transformer is added to the corresponding working winding to make the SR core positively magnetized (magnetized); In half a cycle, the output rectifier tube is reverse-biased, and the diode D3 connected in series with the control winding is forward-biased, and under the action of the DC control current Ic, the core of the SR is demagnetized (reset).

                                                                                      Figure 4 Phase-shifted full-bridge ZVS-PWM switching power supply magnetic amplifier regulator

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 transistor is controlled by error amplification, and the MOS transistor provides the 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 both zero-voltage switching quasi-resonant technology and traditional PWM technology, and has a fixed operating frequency. During the commutation process, the LC resonance is used to make the device zero-voltage switch, and after the commutation is completed, it remains Using PWM technology to transmit energy, simple control, low switching loss and high reliability, it is a soft switching circuit suitable for large and medium power switching power supplies. But when the load is very light, especially the ZVS condition of the switch tube of the lagging bridge arm is difficult to meet. Using the saturated inductance as the resonant inductance of the phase-shifted full-bridge ZVS-PWM converter can expand the range where the switching power supply meets the ZVS condition under light load. Applying it to the arc welding inverter power supply 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 soft-switching arc welding inverter power supply. reliability. Connecting the saturated inductance in series with the secondary output rectifier tube of the isolation transformer of the switching power supply can eliminate the secondary parasitic oscillation, reduce the circulating energy, and minimize the duty cycle loss of the phase-shifted full-bridge ZVS-PWM switching power supply. In addition, the saturated inductance and capacitance are connected in series to the phase-shifted full-bridge ZVS-PWM switching power supply transformer once, and the super-forearm switching tube works according to ZVS; when the load current approaches zero, the inductance increases to prevent the current from reversing The change creates the ZCS condition of the lagging arm switching tube and realizes the phase-shifted full-bridge ZV-ZCSPWM converter.

2.4 Resonant Converter

A series resonant converter using series inductors or saturated inductors is shown in Figure 5. When the resonant inductor current works in a continuous state, the switch tube is turned off with zero voltage/zero current, but it is hard to turn on, and there is a turn-on loss. The anti-parallel diode is naturally turned on, but there is a reverse recovery current when it is turned off. Therefore, the anti-parallel diode must use a fast recovery diode. In order to reduce the turn-on loss of the switch tube and realize zero-current turn-on, the switch tube can be connected in series with an inductance or a saturated inductance. Before the switch tube is turned on, the saturated inductor current is zero. When the switch tube is turned on, the saturated inductance limits the current rising rate of the switch tube, so that the current of the switch tube rises slowly from zero, thereby realizing the zero current turn-on of the switch tube, improving the turn-off condition of the diode, and eliminating the reverse recovery problem .

                                                                                                                                         Figure 5 Resonant converter 

2.5 Inverter power supply

The inverter power supply is widely used in various aspects such as automatic control, power electronics and precision instruments because of its good control performance, high efficiency, small size and many other advantages. 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 not been 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 inductance must be added to its output circuit, and this inductance 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, in order to increase the inductor current from small to large, a small load value must be taken as the premise. Although it is not a complete correspondence, it can be said that Changes in current reflect changes in load to some extent. Therefore, using the inductance that decreases with the increase of current as the output inductance of the inverter power supply can effectively change the time constant T of the power supply output circuit so that it is completely inversely proportional to R (T=L/R), and then the load Maintaining a relatively small value within the range of variation will naturally improve dynamic performance.