1 Introduction For modern electronic systems, even the simplest electronic system composed of single chip microcomputer and single I/O interface circuit, its power supply voltage is generally composed of +5v, 15V or 12V, while for more complex electronic systems, the actual power supply voltage is even more. At present, it is mainly composed of the following voltages: ++3.3V, +5v, 15V, 12V, -5v, 9v, ++ 18V, ++24V, ++27V, 60V,+135v,+35v. Different electronic systems not only have strict requirements for the above voltage combinations, but also have strict requirements for many electrical characteristics of these power supply voltages, such as voltage accuracy, voltage load capacity (output current), voltage ripple and noise, start-up delay, rise time, recovery time, voltage overshoot, power-off delay time, step load response, step linear response, cross adjustment rate, cross interference and so on. 2 Multi-channel output power supply For power users, it is generally hoped that the power supply products they choose are "fool-like", that is, as long as the load does not exceed the maximum power supply voltage, no matter how the load characteristics of the system change, each power supply voltage is still accurate. Only in this respect, most multi-output power supplies are not satisfactory at present. In order to further illustrate the characteristics of multi-output power supply, firstly, the block diagram of multi-output switching power supply shown in figure 1 is started. As can be seen from the figure 1, only the main circuit Vp really forms closed-loop control, and other auxiliary circuits such as Vaux 1 and Vaux2 are out of control. According to the control theory, no matter how the input and output change (including voltage change, load change, etc.), only Vp can ensure a fairly high accuracy (generally better than 0.5%) under the action of closed-loop feedback control. ), that is to say, Vp largely depends only on the reference voltage and sampling ratio. For Vaux 1 and Vaux2, its accuracy mainly depends on the following aspects: 1)t 1 the turns ratio of the main transformer, and mainly depends on the load of NP 1: NP2 or NP 1: NP32) auxiliary circuits. 3) Load of main circuit. Note: If the above three points are set, the influence of the change of input voltage on the auxiliary circuit is very limited. The above three points, as a specific switching power converter, the turn ratio of the main transformer has been set, so the biggest factor affecting the output voltage accuracy of the auxiliary circuit is the load of the main circuit and the auxiliary circuit. In switching power supply products, there are special technical indicators to explain and standardize this characteristic of power supply, that is, cross-load regulation rate. In order to better explain this problem, the following introduces the calculation method of cross-load adjustment rate. 2. 1 calculation steps of multi-output cross-load adjustment rate of power converter 1) The connection of test instruments and equipment is shown in Figure 2. 2) Adjust the input voltage of the tested power converter to the nominal value, close the switches S 1 and S2…Sn, adjust the output current of each channel of the tested power converter to the rated value, measure the output voltage Uj of J channel, and measure the output voltages of other channels in the same way. 3) Adjust the output load currents of all paths except the j-th path to the minimum value, and measure the output voltage ULj of the j-th path. 4) According to the formula (1), calculate the cross-load adjustment rate SIL of the J-th road. Where: δUj is the absolute value of the difference between Uj and other output voltages ULj when the load current is minimum; Uj is the output voltage of the j-th path when the output current of each path is rated. According to the above test and calculation method, the cross-load regulation rate can be understood as the percentage of the influence of step load change (100%-0%) of all other output circuits on the output voltage accuracy of this circuit. 2.2 Multi-output switching power supply The actual switching power supply consists of the principle of figure 1. The main control circuit only feeds back the main output voltage, and other auxiliary circuits are completely released. At this time, it is assumed that the power ratio of the main and auxiliary circuits is 1: 1. From the actual measurement, the cross-load adjustment rate of the main circuit is better than 0.2%, and the cross-load adjustment rate of the auxiliary circuit is greater than 50%. Neither designers nor users of switching power supply can accept adjusting the cross load to more than 50%. The most direct way to reduce the cross-load regulation rate of the auxiliary circuit is to add a linear regulator (including three-terminal regulator and low dropout three-terminal regulator) to the auxiliary circuit, as shown in Figure 3. As can be seen from Figure 3, due to the introduction of the linear regulator V, a part of power loss is increased in the auxiliary circuit, and the power loss is P=. In order to make the cross-load regulation rate of the auxiliary circuit small, it is necessary to consciously improve the voltage difference of the linear regulator, that is, consciously improve it, which has the disadvantages of increasing the power loss of the power supply and reducing the power efficiency. When designing and applying the power supply according to the principles in Figure 1 and Figure 3, we should pay attention to the following principles: 1) The actual current used by the main circuit should be at least 30% of the maximum output current; 2) The voltage accuracy of the main circuit should be better than 0.5%; 3) The power of the auxiliary circuit is preferably less than 50% of the power of the main circuit; 4) The cross load adjustment rate of auxiliary circuit shall not be greater than 10%. 2.3 In many applications, the improved multi-output switching power supply requires the power of two outputs to be basically equal, such as 12V/0.5A,15v/1a.. Through years of practice, we have designed the circuit as shown in Figure 4, which can better achieve the purpose of improving the cross-load regulation rate. The core of the circuit design idea in Figure 4 has the following two points. 1) The positive and negative output filter inductors L 1 and L2 are wound on the same magnetic core, and the inductance of L 1 and L2 is exactly the same by double-wire winding method. And pay attention to the actual connection phase relationship (differential mode method). The connection mode of the filter inductor makes the changes of the two output currents induce each other, which greatly improves the cross-load regulation rate of the two outputs to some extent. 2) As can be seen from Figure 4, the sampling comparators Rs 1 and Rs2 are not connected to the main circuit Vp as shown in Figure 1, but directly connected to the output terminals of the positive and negative power supplies, and the logic "ground" is not the output terminal of the power supply, but takes the negative voltage output terminal as the logic "ground" potential for sampling comparison and reference voltage. In this way, the sampling error will reflect the voltage accuracy change of the positive and negative outputs at the same time, and also have feedback effect on the positive and negative outputs, which can greatly improve the cross-load adjustment rate of the two outputs. Taking 15V/ 1A power supply as an example, the measured bidirectional cross-load regulation rate is better than 2% by using the circuit design in Figure 4. When designing and applying the power supply based on the principle of Figure 4, the principles that should be paid attention to are: 1)2 channels are preferably symmetrical outputs (power symmetry and voltage symmetry), and there is no obvious difference between the main and auxiliary circuits, such as 12V and 15V. 2) The accuracy of the two output voltages is not too high, about1%; 3) The cross adjustment rate of the two outputs is relatively high, about 2%. The following is a universal three-way power supply design scheme, as shown in Figure 5. As can be seen from fig. 5, the main +5v output and the auxiliary Vout (which can be 15V or 12V) output circuits are not only independent in feedback, but also independent in PWM (pulse width modulator), power conversion and transformer. The three power supplies can be regarded as the independent combination of 1+5V power supply and 1 Vout power supply. In order to further reduce the mutual interference between them and the peak-to-peak value of their respective output voltage ripple, the input reflection ripple of each independent power supply should be further reduced (generally, the peak-to-peak value of ripple should be less than 50mV, and the effective value of ripple should be less than 10mV) and the synchronous operation mode should be adopted. 2.4 High-frequency magnetic amplifier regulator In multi-output power supply, the output circuit often adopts high-frequency magnetic amplifier regulator, which has been widely used in multi-output regulated power supply because of its low cost, high efficiency, high voltage stabilization accuracy and high reliability. Magnetic amplifier can accurately control the switching power supply, thus improving its stability. The magnetic core of a magnetic amplifier can be made of permalloy, ferrite or amorphous, nanocrystalline (also called ultramicrocrystalline) materials. Amorphous and nanocrystalline soft magnetic materials have high permeability, high rectangular ratio and ideal high temperature stability. When they are applied to magnetic amplifiers, they can provide unparalleled output adjustment accuracy and achieve higher work efficiency, so they are favored. In addition to the above characteristics, amorphous and nanocrystalline cores also have the following advantages: 1) low saturation permeability; 2) low coercivity; 3) the recovery current is small; 4) the magnetic core loss is small; The output voltage regulator of magnetic amplifier does not use thyristor or semiconductor power switch tube, but connects a saturable choke coil in series at the output end of rectifier tube (as shown in Figure 6), so its loss is very small. As can be seen from fig. 6, the key of magnetic amplifier regulator is controllable saturation inductance Lsr and reset circuit. The controllable saturation inductance consists of a rectangular b? H-type loop magnetic core and its winding play the role of working winding and control winding. Reset refers to the demagnetization process after the magnetic flux reaches saturation, so that the magnetic flux or magnetic flux density returns to the initial working point, which is called flux reset. Due to the characteristics of the magnetic core material used in the magnetic amplifier regulator (good rectangular B? H-ring and high permeability), so that the controllable saturated inductor presents high impedance to the input pulse when the magnetic core is not saturated, which is equivalent to an open circuit, and the impedance of the controllable saturated inductor is close to zero when the magnetic core is saturated, which is equivalent to a short circuit. At present, the working frequency of switching power supply has been mentioned above several hundred kHz, and the wide application of magnetic amplifier in switching power supply puts forward higher requirements for soft magnetic materials. At such a high frequency, the resistivity of permalloy is very low (about 60 μ ω? Cm) leads to excessive eddy current loss, which leads to higher temperature and lower efficiency. Although the ultra-thin belt and ultra-thin belt can be improved, the cost will be greatly increased; Ferrite has high resistivity (greater than 105μ ω? Cm), but its Bs is too low and Curie point is too low. Due to the harsh working environment, there are strict requirements for stress sensitivity and thermal stability of materials, and the above materials are difficult to meet the requirements. The appearance of amorphous alloys has greatly enriched soft magnetic materials. Among them, Co-based amorphous alloy has moderate saturation magnetic induction intensity, and ultrafine alloy has high saturation magnetic induction intensity, both of which have extremely low saturation magnetostrictive coefficient and magnetocrystalline anisotropy. Co-based amorphous and ultrafine crystallites can have low high frequency loss while maintaining a high square ratio. When used in high-frequency magnetic amplifier, it can greatly improve power efficiency and greatly reduce weight and volume, so it is an ideal core material for high-frequency magnetic amplifier. 3 Typical application of high-frequency magnetic amplifier output regulator The multi-channel output power supply shown in circuit diagram 7, its main circuit is closed-loop feedback PWM control mode, and the auxiliary circuit is magnetic amplifier regulated power supply. Because the input voltage waveform of the auxiliary magnetic amplifier is controlled by the ratio of the main and secondary windings of the transformer and the working state of the main circuit (the output voltage of the main circuit and the load of the main circuit, etc.). ), the cross-load regulation rate of the auxiliary circuit is still not ideal. Fig. 8 shows a multi-output regulated power supply designed entirely by using the voltage regulation technology of a magnetic amplifier. The front stage of the power supply is a double transformer self-excited power conversion circuit, and the multi-channel outputs of the back stage are all magnetic amplifier voltage stabilizing circuits. In addition, there is no relationship between channels, no feedback between front and back stages, and no pulse width modulator (PWM). The advantages of this circuit are: 1) The circuit structure is simple, the number of components used is small, except for two power tubes, all other components are permanent or semi-permanent, with high reliability and convenient manufacture; 2) There is no isolated feedback amplifier in the circuit, so it is extremely easy to adjust, and once it is adjusted, it does not need maintenance. The conversion power of the previous stage depends on the total output power of the latter stage; 3) The output characteristics of each channel are independent of each other, and the voltage regulation is adjusted independently, and there is no distinction between the main circuit and the auxiliary circuit. Therefore, the load regulation rate of each output circuit is very ideal, less than 0? 5%; 4) The magnetic amplifier is in an "open circuit" state at the moment when the power supply is turned on. At this time, the conduction current of the power tube approaches zero, so the loss is minimized, which is beneficial to the high-frequency efficiency of the converter; 5) Because the front-stage power converter is a pure square wave with an adjustable width and the rear-stage power converter is connected with a magnetic amplifier, the peak-to-peak value of the output ripple can be greatly reduced. The output ripple of ordinary PWM power supply is about 65,438+0% of the nominal output voltage. The peak-to-peak ripple can be easily reduced to about 0.65,438+0% by using a rectifier circuit with a magnetic amplifier. The comprehensive electrical characteristics of the above large regulated power supply with magnetic amplifier are incomparable to other PWM isolated negative feedback multi-channel power supplies. Especially for the practical application of multi-channel power supply, the internal characteristics of power supply and the load characteristics of electronic system can be ignored and there is no problem in use. However, there are still some problems to be solved in modern large-scale regulated power supply with magnetic amplifier. The circuit form of 1) needs to be further improved (especially the power conversion circuit in the previous stage of power supply), and overvoltage and undervoltage protection, overcurrent and short circuit protection and power supply enabling terminal should be added. 2) Further increase the working frequency to reduce the volume. 3) further improve the efficiency and reduce the magnetic loss. Conclusion To sum up, users of multi-channel power supply can put forward the characteristic parameters of power supply more truly according to the power consumption of electronic system. For multi-channel power supply designers, they can learn more about the current multi-channel power supply design methods, reduce the development cycle of new products, and get twice the result with half the effort.