Application of saturable inductor in switching power supply

1 Introduction

Saturable inductors are inductors with high hysteresis loop squareness ratio, high initial permeability, low coercive force and obvious magnetic saturation point. They are often used as controllable delay switching elements in electronic circuits. Due to their unique physical properties, saturable inductors have also been increasingly widely used in high-frequency switching power supply switching noise suppression technology and large current output auxiliary circuit voltage stabilization technology.

2 Basic physical characteristics of saturable inductors

图1(a)和图1(b)分别是普通铁氧体电感和可饱和电感的磁滞回线。从两者的对比中可以明显看出可饱和电感具有高磁滞回线矩形比（Br/Bs）、高起始磁导率mi、低矫顽力Hc、明显的磁饱和点（A，B）等特怔。此外，由图1(b)还可以看出，可饱和电感的磁滞回线所包围的面积狭小，所以可饱和电感的高频磁滞损耗相应也较小。由于可饱和电感通常是由微晶、非晶、坡莫合金等铁磁性材料制造的，所以可饱和电感一般都具有很高的起始磁导率mi和很高的饱和磁感应强度Bs。由于物理特性上的差异,可饱和电感在应用方面与普通铁氧体电感有两个显著的不同之处：

(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 the following formula:

Em = m·H2·V/2 (1)

Where: m: critical saturation point magnetic permeability; H: critical saturation point magnetic field intensity; V: effective volume of magnetic material.

(2) Since the initial magnetic permeability of the saturable inductor is high and 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 the delay time DT, when the current in the inductor coil reaches a certain value, the saturable inductor will immediately saturate. It has a very obvious switching characteristic, so it is often used as a controllable delay switching element in the circuit. This switching characteristic of the saturable inductor is not possessed by ordinary ferrite inductors.

The curves of the current intensity I of the common ferrite inductor and the saturable inductor changing with time t under the action of the DC voltage Vdc are shown in FIG2(a) and FIG2(b).

3 Switching noise suppression effect of saturable inductors

Switching power supplies usually operate in the frequency range of tens of kilohertz to hundreds of kilohertz. The rectifier diodes on the secondary side of the power supply will flow through a large reverse recovery current during the high-frequency shutdown process. Therefore, the power supply conduction spike noise formed is an important part of the switching power supply noise. Trying to reduce the reverse recovery current of the rectifier diodes, thereby reducing the conduction and radiation noise of the switching power supply is an important aspect of switching power supply design.

In the forward power supply circuit shown in Figure 3(a), when the primary power tube V1 starts to conduct, the secondary rectifier diode D1 starts to conduct, and D2 starts to cut off, due to the charge storage effect of the diode PN junction, a reverse recovery spike current i with a large current change rate di/dt flows through D2, causing corresponding spike currents to flow through V1 and D1. Under the combined effect of leakage inductance, line inductance and other factors, high-frequency conduction noise will be generated at the power supply output. The greater the reverse voltage applied to D2, the longer the reverse recovery time of D2, the greater the reverse recovery current change rate di/dt, and the greater the power supply output noise. A large reverse recovery spike current will not only generate power supply noise, but also easily damage the rectifier device.

When the primary power tube V1 starts to cut off, the secondary rectifier diode D1 starts to cut off, and D2 starts to conduct and continue current, due to the same reason, high-frequency shutdown noise will also be generated at the output end of the power supply. The shutdown noise is usually much smaller than the conduction noise and is generally not considered as a key point in power supply design.

In order to effectively reduce the reverse recovery current of the rectifier diode and suppress the reverse recovery current change rate, the measures usually taken in power supply design are: select Schottky diodes with no PN junction charge storage effect and very short reverse recovery time, or select rectifier diodes with small reverse recovery current change rate and soft recovery characteristics as secondary rectifier devices; add RC buffer circuits at both ends of the rectifier diode, or connect a small inductor in series with the rectifier diode to soften the change rate of the switching voltage or reverse recovery current. Since the saturable inductor has the characteristics of large inductance, easy saturation, and weak energy storage, it is very suitable to be connected in series with the rectifier diode as a current limiting inductor, so that the amplitude and change rate of the reverse recovery current of the rectifier diode are limited to a reasonable range.

在图3（b）所示电路中，当V1开始导通，D1开始导通，D2开始截止时，由于可饱和电感Ls的限流作用，D2中流过的反向恢复电流的幅值和变化率都会显著减小，从而有效地抑制了高频导通噪声的产生。在二极管D2导通、关断、导通的过程中，Ls中磁感应强度的变化如图4所示。D2中的电流由正向电流、零电流、最大反向电流再到零电流的反向恢复过程中，Ls中相应的磁感应强度位于图4中的A、B、C、D各点。在二极管D2续流导通后，相应的磁感应强度将会由D点重新过渡到A点。在D2由截止变为导通续流时，由于Ls存在着导通延时时间DT，这会影响D2的续流作用，并会在D2的负极产生负值尖峰电压，在电路中增加辅助二极管D3及电阻R1，可以较好地解决这一问题。

4 Application of saturable inductor in magnetic amplifier voltage regulation technology

Magnetic amplifier voltage regulation is a method of controlling the duty cycle and output power of a switching power supply and stabilizing the output voltage of the power supply by using the physical characteristics of the conduction delay of a saturable inductor. By adding appropriate sampling and control elements to the saturable inductor and adjusting its conduction delay time, the most common magnetic amplifier voltage regulation circuit can be formed. Figure 5 is a dual-tube forward switching voltage regulator with an input of 110Vac~220Vac/50HZ and an output of 5Vdc/20A and 12Vdc/10A, in which 5V is the main control circuit. The auxiliary 12V output current is relatively large, and there are high requirements for voltage regulation accuracy and load stability. The voltage regulation effect of ordinary methods is not very ideal. If the magnetic amplifier voltage regulation technology is used, this problem can be satisfactorily solved, making the structure of the voltage regulation circuit simple, the power dissipation small, and the voltage regulation accuracy of the millivolt level can be achieved.

After the 110Vac~220Vac/50HZ input is boosted by the active power factor correction circuit, a 400Vdc DC voltage is output and added to the primary of the power module. The secondary winding N1 outputs a square wave voltage with a peak value of 20V and a duty cycle of about 25%.

The output peak voltage of N2 is 50V. Before adding the magnetic amplifier voltage stabilization circuit (the dotted box in Figure 5), the output voltage V2»50V´0.25=12.5V at the 12V of the auxiliary circuit. When the 5V is fully loaded and the 12V is unloaded, the actual output voltage will be much higher because the auxiliary circuit has no dead load discharge circuit. After adding the magnetic amplifier voltage stabilization circuit, due to its regulation function, the auxiliary circuit can obtain an ideal 12V output voltage under any load condition. The working principle of the magnetic amplifier voltage stabilization circuit is analyzed below. The magnetic induction intensity change curve of the saturable inductor Ls during the voltage stabilization process can still be represented by Figure 4.

When the primary power tubes V1 and V2 are turned off, the secondary diode D1 is reversely turned off, the diode D2 is turned on for freewheeling, the energy storage inductor L1 releases energy, and the auxiliary circuit of the power supply is in the off state. At this time, a small current If of the milliampere level flows into the saturable inductor Ls through Q1 and D3, and an additional magnetic induction intensity B0 is generated in Ls, and B0 is located at point V of the hysteresis loop. After the power tubes V1 and V2 are turned on again, due to the current limiting effect of the inductor Ls, the current in D1 can only increase slowly, D2 still plays the role of freewheeling, and the auxiliary circuit of the power supply is still in the off state. Only after the delay time DT, when the current in D1 increases to a certain value and the magnetic induction intensity in Ls reaches the saturation magnetic induction intensity BS (point A), Ls will be saturated immediately, the current in D1 will increase rapidly, D2 will be turned off quickly, the energy storage inductor L1 will enter the energy storage stage, and the auxiliary circuit of the power supply will enter the on state.

From the above analysis, we can see that due to the insertion of the saturable inductor Ls, the on-time T'ON and duty cycle a' of the auxiliary circuit are smaller than those of the main circuit. By adjusting the duty cycle a', the purpose of output voltage regulation of the auxiliary circuit is finally achieved. a' has a large adjustment range. When the auxiliary circuit is completely unloaded, a' can be adjusted to almost 0, so that the auxiliary circuit has a high load stability and voltage regulation accuracy. The duty cycle a' can be calculated according to the following formula:

a'= T'ON/T＝(TON-DT)/T (2)

Where: TON: the on-time of the main circuit; T: the switching cycle of the power supply; DT: the on-delay time of Ls.

The turn-on delay time DT of the saturable inductor is determined by the additional magnetic induction intensity B0, current If, etc. B0 and If are automatically adjusted by the sampling control circuit according to the output voltage. DT is given by the following formula:

DT = "N" ´ Ae ´ (BS-B0) / Vin (3)

In the formula: BS: saturation magnetic induction intensity of the saturable inductor; B0: between ±BS, determined by the operating current If;
Ae: cross-sectional area of ​​the saturable inductor; N: number of turns of the saturable inductor coil;
Vin: voltage applied across the two ends of the saturable inductor.

When B0=-BS, DT has a maximum value DTmax=2N´Ae´BS/Vin; when B0=Br, DT has a minimum value DTmin= N´Ae´ (BS-Br)/Vin. If is generally designed to be tens of milliamperes, and the approximate value of If is given by the following formula:

If »（HC- B0/mi）´lm/N (4)

Where: HC: coercive force of saturable inductor; mi: initial magnetic permeability; lm: equivalent magnetic path length of saturable inductor.

Based on the same principle as the auxiliary output voltage regulation of the forward power supply, two sets of magnetic amplifier voltage regulation circuits can form the auxiliary output voltage regulation unit of the push-pull power supply and bridge power supply. In addition, the magnetic amplifier can also be conveniently used to form the auxiliary output voltage regulation unit of the forward and reverse mixed power supply, etc., which will not be described here one by one.

5 Conclusion

This article introduces the basic physical characteristics of saturable inductors and the switching noise suppression effect of saturable inductors, focuses on analyzing the working principle of magnetic amplifier voltage regulator circuits, and proposes several theoretical calculation formulas. In the actual engineering design of power supplies, since suppliers usually provide empirical design formulas for saturable inductors and magnetic amplifiers, the actual design work is relatively simple and easy. The requirements for circuit parameters of the magnetic amplifier voltage regulator circuit are not very strict, and under normal circumstances, very good voltage regulation effects can be achieved.