Saturated Inductor and Its Application in Switching Power Supply

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, high current output auxiliary circuit voltage regulation, phase-shifted full-bridge converter, resonant converter and inverter power supply.

1 Classification and physical characteristics of saturated inductors ^[1]

1.1 Classification of saturated inductors

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

1.1.1 Saturable inductor

Its inductance varies with the current passing through it. If the magnetic properties of the core are ideal (for example, rectangular), as shown in Figure 1 (a), the saturated inductor works like a "switch", 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 Saturable Inductor

Also known as controlled saturable reactor, its 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 iron core changes according to the local magnetic loop within one cycle, thus changing the equivalent magnetic permeability of the iron core and the inductance of the coil. If the magnetic characteristics of the iron core are ideal ( the B - H characteristics are rectangular), the controlled saturated inductor is similar to a "controlled switch". In switching power supplies, the application of controlled saturated inductors can absorb surges, suppress spikes, eliminate oscillations, and reduce rectifier losses when connected in series with fast recovery rectifiers. As shown in Figure 1 (b), the controlled 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 controlled saturated inductor 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 inductor can be expressed by formula (1).

E _m = μ VH ² /2 （1）

Where: μ is the critical saturation point magnetic permeability;

H is the critical saturation point magnetic field intensity;

V is the effective volume of the magnetic material.

2) Since the initial magnetic permeability of the saturable inductor is high, the magnetic resistance is small, the inductance coefficient and inductance are both 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 switching element in the circuit.

(a) Ideal magnetic characteristic B=f(H) (b) Saturable inductor B=f(H)

Figure 1 B-H characteristics of saturated inductance

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 gapless saturable inductor and current

The relationship between the gapless saturable inductance L and the current can be expressed by equation (2).

L = (2)

Where: W is the number of turns of the inductor winding;

I is the excitation current;

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

S is the cross-sectional area of ​​the magnetic material;

l of magnetic material is the average length.

1.2.2 Relationship between air-gapped saturable inductor and current

Given any magnetic induction intensity B ₁ in a magnetic conductor's magnetic circuit , the magnetic field intensity H ₁ in the magnetic conductor's magnetic circuit can be calculated from the B = f ( H ​​) curve. The H ₀ value in the air gap can be expressed by formula (3).

H0 ₌ B1 ₍ 3 )

Where: B0 is the air gap magnetic induction intensity _;

a and b are the side lengths of the rectangular cross-sectional area of ​​the magnetic circuit;

l ₀ is the air gap length;

μ0 is _the magnetic permeability of air.

From the magnetic circuit law, we get I = . By changing the value of B and repeating the above steps, we can find the corresponding I and get a set of relationship data between B and I. Let the function corresponding to B and I be B = f ₁ ( I ).

When the leakage inductance is not considered, the calculation formula of the inductance can be expressed by formula (4).

L = W = WS (4)

Where: φ is the magnetic flux of the magnetic circuit.

Then the relationship between the air-gap saturable inductance and current is

L = WSf ₁ ( I ) （5）

2 Application of saturated inductor in switching power supply

2.1 Spike Suppressor

The spike interference in the switching power supply mainly comes from the opening and closing moments of the power switch tube and the secondary side rectifier diode. The saturated inductor, which is easy to saturate and has 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 spike current, and after saturation, its saturated inductance is very small 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 , auxiliary diode D3} and resistor R1 are added to the circuit .

Figure 2 Application of spike suppressor

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 stabilization circuit can be constructed.

There are two types of magnetic amplifier voltage stabilization 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.

Figure 3 Magnetic amplifier voltage type reset voltage regulator circuit

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 tube is forward biased (the other output rectifier tube 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 tube 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).

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 tube is controlled through 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 commutation process, LC resonance is used to make the device zero voltage switch. After the commutation 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 is difficult to meet, especially for the lagging bridge arm switch tube.

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 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 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, the saturated inductor and capacitor are connected in series in the primary of the phase-shifted full-bridge ZVS-PWM switching power supply transformer [5], and the leading arm switch tube operates 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 for the lagging arm switch tube, and realizing the phase-shifted full-bridge ZV-ZCSPWM converter.

2.4 Resonant Converter

A series resonant converter using a series inductor or a saturated inductor [6] is shown in FIG5 . When the resonant inductor current operates in a continuous state, the switch tube is turned off at zero voltage/zero current, but the turn-on is hard-on, 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.

Figure 5 Resonant converter

2.5 Inverter power supply[7]

Inverter power supply is widely used in various fields such as 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 power supply 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 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, 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 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 increasing 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 thus maintaining it at a relatively small value within the load variation range, which naturally improves the dynamic performance.

3 Conclusion

This paper describes in detail 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.