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.

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.

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  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).

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 .

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.