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Saturated inductor and its application in switching power supply (Part 1)

Introduction

saturated inductor is an inductor with high loop rectangular ratio, high initial permeability, low coercivity and obvious magnetic saturation point within 3-Pack period. It is often used as a controllable time-delay switching element in electronic circuits. Because of its unique physical characteristics, it has been widely used in switching noise suppression of high-frequency switching power supply, high current output auxiliary circuit voltage stabilization, phase-shifting full bridge converter, resonant converter and inverter power supply

1 classification of saturated inductors and their physical characteristics

1.1 experimental data of saturated inductors are accurate and reliable

saturated inductors can be divided into self saturation and controllable saturation

it has many types 1.1.1 saturable inductor

its inductance varies with the passing current. If the magnetic characteristics of the core are ideal (for example, rectangular), as shown in Figure 1 (a), when the saturated inductance works, it is similar to a switch, that is, when the current in the winding is small, the core is unsaturated, and the winding inductance is large, which is equivalent to an 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 a short circuit of the switch

1.1.2 controlled saturable inductor

also known as controlled saturable reactor, its basic principle is that under the DC excitation of AC coil with core, due to the simultaneous AC and DC excitation, the core state changes according to the local magnetic loop within a week, so the equivalent permeability of core and coil inductance are changed. If the magnetic characteristic of the core is ideal (the B-H characteristic is rectangular), the controllable saturated inductor is similar to a controllable switch. In the switching power supply, the application of controllable saturated inductors can absorb surges, suppress spikes, eliminate oscillations, and reduce the loss of rectifier tubes when connected in series with fast recovery rectifier tubes. As shown in Figure 1 (b), the controllable saturation inductor has the characteristics of high hysteresis loop rectangular ratio (br/bs), high initial permeability I, low coercivity HC, obvious magnetic saturation points (a, b) and small high-frequency hysteresis loss due to the narrow area surrounded by its hysteresis loop. Therefore, two remarkable characteristics of controllable saturated inductors in application are

1) due to the small intensity of saturated magnetic field, the energy storage capacity of saturable inductors is very weak and cannot be used as energy storage inductors. The theoretical value of the maximum energy storage em of the saturable inductance can be expressed by formula (1)

em= vh2/2 (1)

where: is the critical saturation point permeability

h is the magnetic field strength at the critical saturation point

v is the effective volume of magnetic material

2) due to the high initial permeability, small magnetic resistance, large inductance coefficient and inductance, the internal initial current of the inductance increases slowly when the external voltage is applied. Only after the time delay of T, when the current in the inductance coil reaches a certain value, the saturable inductance will be saturated immediately, so it is often used as a controllable time-delay switching element in the circuit

(a) ideal magnetic characteristic b=f (H) (b) can display the program running state in real time. B=f (H) Figure 1 B-H characteristic of saturated inductance

1.2 relationship between saturated inductance and current

because the calculation methods of db/di magnetic circuit with and without air gap are different, the two cases are discussed respectively

1.2.1 relationship between air gap free saturable inductance and current

the relationship between air gap free saturable inductance L and current can be expressed by formula (2)

l= (w2s/l) f (wi/l) (2)

where: W is the number of turns of inductive winding

i is excitation current

f is the corresponding function of B ~ H curve of magnetic material for inductance

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