“Multiple output switching power supplies are widely used in various complex low-power Electronic systems. As far as multiple output is concerned, usually only the one with low output voltage and large output current range is used as the main circuit for feedback adjustment control to ensure that the input voltage And keep the output voltage stable when the load changes. Due to the influence of the leakage inductance and winding resistance between the various windings of the transformer, the auxiliary output voltage changes with the change of the output load. Generally, when the main output is fully loaded and the auxiliary output is lightly loaded, the auxiliary output The voltage will increase, and when the main output is lightly loaded and the auxiliary output is full, the auxiliary output voltage will decrease, which is the load crossing of multiple outputs
Multiple output switching power supplies are widely used in various complex low-power electronic systems. As far as multiple output is concerned, usually only the one with low output voltage and large output current range is used as the main circuit for feedback adjustment control to ensure that the input voltage And keep the output voltage stable when the load changes. Due to the influence of the leakage inductance and winding resistance between the various windings of the transformer, the auxiliary output voltage changes with the change of the output load. Generally, when the main output is fully loaded and the auxiliary output is lightly loaded, the auxiliary output The voltage will increase, and when the main output is lightly loaded and the auxiliary output is full, the auxiliary output voltage will decrease. This is the load cross regulation problem of the multiple output. A multiple output switching power supply is designed based on the TOPSwitch-GX series. A good solution to the problem of load cross regulation rate of multiple outputs, the power supply can output stably under various working conditions, the main output voltage ripple is less than 3%, the auxiliary output ripple of each channel is less than 5%, and the load cross regulation rate Less than 5%. In an AC servo system, the power supply supplies power to the control part and power part of the system. The performance of the switching power supply directly affects the servo characteristics of the AC servo system. How to design a stable performance, low crossover adjustment rate, and voltage ripple The switching power supply with small wave is a problem that needs to be solved urgently.
Performance comparison with TOPSwitch-FX
The series single-chip switching power supply is a new type of high-frequency switching power supply launched by American Power Integration in the mid-1990s. It is the abbreviation of Three Terminal Off Line PWM Switch (Three Terminal Off Line PWM Switch) and is known as the “top switching power supply” . It is characterized by integrating the PWM controller and MOSFET power switch in the high-frequency switching power supply on the same chip, which is a two-in-one device. TOPSwitch-GX is the fourth-generation series of products launched by the company. In addition to having all the advantages of the TOPSwitch-FX series, it also increases the maximum output power from 75W to 290W, which is suitable for forming high-efficiency, isolated switching power supplies with high and medium power. ; Increasing the switching frequency to 132KHz will help reduce the volume of the high-frequency transformer and the entire switching power supply. It is suitable as the main control device of the on-board power supply of the servo motor control board. When the load of the switching power supply is very light, it can automatically The switching frequency is reduced from 132KHz to 30KHz (in half-frequency mode, from 66KHz to 15KHz), which can reduce switching loss and further improve power efficiency. The new energy-saving technology called EcoSmart is adopted, which significantly reduces the remote on/off The power consumption in mode, when the input AC voltage is 230V, the power consumption is only 160mW.
2.2 The working principle of TOPSwitch-GX
The internal part of the TOPSwitch-FX series is mainly composed of 18 parts. The main difference from the third-generation TOPSwitch-FX series is that some improvements have been made on the basis of this series. 3 new unit circuits have been added to the original 5 components, and the current limit The regulator also adds a soft-start output terminal; increases the switching frequency generated by the frequency jitter oscillator to 132KHz (full frequency mode) or 66KHz (half frequency mode); adds a “stop logic” (STOP LOGIC) to the frequency jitter oscillator ) The circuit makes the work more reliable. TOPSwitch-GX uses the feedback current Ic to adjust the duty cycle D to achieve the purpose of voltage stabilization. When the output voltage U0 decreases, the feedback current Ic is reduced through the optocoupler feedback circuit, the duty cycle increases, and the output voltage rises accordingly. High, and ultimately make U0 remain unchanged. Similarly, when the output voltage Uo rises, through internal adjustment, U0 can also remain unchanged.
Power supply main circuit design scheme
Power requirements for control board and power board
The power supply supplies power to the control board and power board of the AC servo system, and takes the form of on-board power supply as part of the control board and power board. The system requires the on-board power supply to have a voltage ripple of less than 5% and multiple outputs Voltage cross regulation rate is less than 5%, stable output under various working conditions, to meet the power supply requirements of the system, the control board and the power board power ground are isolated from each other to avoid electromagnetic interference, and the power supply is required to be soft-started to avoid instantaneous high voltage Have an adverse effect on the system. According to the system requirements, the output voltage, current and function of the multi-output on-board power supply are designed as shown in Table 1. Among them, the +5V and +15V outputs share the same ground, and +3.3V has nothing to do with it. The +5V two outputs share the same ground, and the two sets of grounds are electrically isolated from each other.
Power main circuit
The main circuit of the onboard power supply is shown in Figure 1. The power supply adopts a single-ended flyback topology structure and selects the TOPSwitch-GX series circuit. When the power supply input current is 85V-265V, the AC voltage U passes through an electromagnetic interference (EMI) filter, an input rectifier filter and a series of soft-start circuits to obtain a DC High voltage DCP, DCP is connected to the L terminal through R68, which can make the limit current decrease with the increase of DCP. Use clamp diode and blocking diode D1 to replace the more expensive TVS (transient voltage suppressor), which is used to absorb the TOP244Y switch. The peak voltage generated by the leakage inductance of the high-frequency transformer during the period protects the drain. The secondary voltage is rectified and filtered into multiple outputs. The 15V power output and auxiliary output use fast recovery diodes. Other outputs The Schottky diode is used to reduce the loss of the rectifier tube.
Circuit design scheme of multi-output switching power supply realized by TOPSwitch-GX single chip microcomputer
The power supply circuit uses TOP244Y, PC817 photocouplers and LMV431A adjustable precision shunt regulator tubes. In order to reduce the volume of the high-frequency transformer and enhance the degree of magnetic field coupling, the secondary winding adopts a stacked winding method. The internal reference voltage of LMV431A is 2.495V. The output voltage is divided by potentiometer and R65. The adjustable voltage is between 2.5V (reference value) and 37V (maximum value). R66 and C75 form the frequency compensation network of LMV431A. Except for the 3.3V main output, the other outputs have no feedback. The output voltage is determined by the turns ratio of the high-frequency transformer. In addition, in order to minimize electromagnetic interference, a common mode choke is connected to the output side of the switching power supply. , To improve electromagnetic noise.
Multi-channel output cross adjustment rate
For multiple outputs, if each output voltage is required to have high accuracy, the coal stove should have an independent closed-loop voltage stabilizing circuit. If only one output is a heavy load, the auxiliary load of the other output is lighter. For the output voltage accuracy If the requirements are not very strict, you only need to add a feedback control loop to the loop where the heavy load is located. Among the 4 outputs of this module, since the +3.3V output is the most important load, the output current (maximum 3A), +5V, +5V (HVDD) + 15V is the power supply of the integrated circuit, allowing the voltage to vary within 10% , The current is small, so only a closed-loop voltage regulator circuit is used in the +3.3V output loop. In the loops of +5V, +5V (HVDD), +15V, the 7805 and 7815 integrated voltage regulators, because the flyback transformer itself is a coupled Inductor, the transformer with this high-frequency flyback topology can improve Multi-channel output cross adjustment rate.
In the design process of this kind of switching power supply, because the parameters of the high-frequency transformer have a great influence on the performance of the switching power supply, the design of the high-frequency transformer is very important. For multiple output power supplies, the output impedance directly determines the change of the output voltage. , The output impedance is proportional to the leakage inductance between the output windings, and the coupling degree of the primary and secondary windings also has a great influence on the output impedance, so the design of a multi-output high-frequency transformer should make the output windings tightly coupled, and the output The winding (main output winding) with a large current variation range should be best coupled with the primary winding in order to improve the cross adjustment rate. Through experiments and analysis, for the transformer used in this system, the best winding sequence of the windings is the original winding first. Then wind the secondary winding on half of the side, and finally wind the other half of the primary side and the bias winding.
Leakage inductance will cause a spike in the voltage of the transformer. For a flyback transformer, the spike directly causes the output voltage to rise when the auxiliary output is lightly loaded. If the clamping voltage can be kept slightly higher than the secondary reflected voltage, the cross regulation rate of the multi-output flyback switching power supply can be greatly improved. Here, a clamping diode and a clamping diode are connected in parallel on the primary side of the transformer. The clamp circuit method composed of fast recovery diodes, this clamp circuit can limit the spike voltage rise and absorb the energy of the spike voltage, the secondary reflected voltage is 135V, and the 200V P6KE clamp diode and FR1107 fast recovery diode are used.
After taking the above measures, the cross regulation rate of multiple output voltages has been greatly improved, the main output voltage ripple is less than 3%, the auxiliary output voltage ripple of each channel is less than 5%, and the load cross regulation rate is less than 5%.
Design of high frequency transformer
The single-ended flyback high-frequency switching transformer is the key to the switching power supply. This transformer is essentially a coupled inductor. It must have the functions of energy storage, voltage transformation, and energy transfer. The author uses the area product method to design the high frequency transformer.
Design known parameters
These parameters are determined by the designer according to user needs and circuit characteristics, including input voltage Vin, output voltage Vout, power Pout of each output, efficiency η, switching frequency fs (or period T), and withstand voltage Vmos of the main switching tube of the line .
In a flyback converter, the reflected voltage of the secondary side, that is, the sum of the flyback voltage Vf and the input voltage, cannot be higher than the withstand voltage of the main switching tube, and a certain margin (here assumed to be 150V) is required. The excitation voltage is determined by the following formula:
In the formula, VinDCMax is the minimum DC voltage input from the front end of the transformer.
After the flyback voltage is determined, the turns ratio of the primary and secondary sides can be determined by formula (2), namely
In the formula, Np is the number of turns of the primary winding and the number of turns of the secondary winding.
The maximum duty cycle DMax of the flyback power supply appears in the state of the lowest input voltage and maximum output power. Dmax can be obtained according to the magnetic balance of the transformer in the steady state, namely
When the maximum duty cycle is set, when the switch tube is turned on, the primary side current is Ip1; when the switch tube is turned off, the primary side current rises to the sewing current Ip2. If Ip1 is 0, it means that the transformer is working in discontinuous mode, otherwise it is working in continuous mode. From the law of conservation of energy, the following formula can be obtained:
(Ip1+Ip2) DMaxVinDCMax=Pout/η (4)
In the general continuous mode design, set Ip2=3Ip1, so that the primary current of the transformer can be calculated, and the primary inductance Lp can be obtained from the following formula:
For continuous mode, ΔIp=Ip2-Ip1=2Ip1; for discontinuous mode, ΔIp=Ip2.
The required iron core can be obtained by the area product AwAe method according to formula (6):
In the above formula, Aw is the window area of the magnetic core, Ae is the cross-sectional area of the magnetic core, Bw is the working magnetic induction intensity of the magnetic core, and K0 is the effective use coefficient of the window, which is determined according to the requirements of safety regulations and the number of output channels, generally 0.2-0.4 , Kj is the current density coefficient, generally 395A/cm2.
Choose a suitable magnetic core according to the obtained AwAe value, and generally try to choose a magnetic core with a relatively large window length and width, so that the effective use coefficient of the magnetic core window is higher and the leakage inductance can be reduced at the same time.
After the magnetic core is determined, the number of turns of the primary side can be calculated according to the following formula:
Then calculate the number of turns of the secondary side according to the relationship of the turns ratio between the primary side and the secondary side. Sometimes the number of turns required is not an integer. At this time, some parameters should be adjusted to make the number of turns of the primary side and the secondary side appropriate.
In order to avoid saturation of the magnetic core, an appropriate air gap lg should be added to the magnetic circuit, the value of which is calculated by the following formula:
At this point, the main parameters of the single-ended flyback switching power supply transformer have been determined. After the design is completed, it is necessary to check whether the window area is large enough, and whether the loss and temperature rise of the transformer meet the requirements.
The known parameters in this power system are input current and voltage 85: 265V, η is 0.8 (determined by experience), fs=132KHz, Vmox=700V.
The circuit adopts intermittent working mode. After repeated calculations and experiments, the main parameters of the designed high-frequency transformer are as follows:
Primary side inductance Lp=390μH; magnetic core adopts E125 ferrite core; primary and secondary winding Np=63, Ns1=2, Ns2=2, Ns3=3, Ns4=9, NB=7; winding adopts sandwich core Winding method, in which the primary winding and the secondary winding are insulated with 3 layers of nylon insulating material, and the secondary windings are insulated with one layer between each layer. Because the secondary winding has a large current, the skin effect of the current is taken into account , So the secondary winding adopts multi-strand parallel winding.
Experimental data and conclusions
(1) Voltage regulation rate: Under the condition of rated load, when the input AC voltage changes between 85VAC-256VAC, the voltage regulation rate of the measured circuit is shown in Table 2.
(2) Cross regulation rate: Under the rated input voltage (220VAC), when the load changes from 10% to 100% of the rated value, the cross regulation rate of the measured circuit is shown in Table 3.
(3) Load regulation rate: Under the rated input voltage (220VAC), when the load changes from 10% to 100% of the rated value, the load regulation rate of the measured circuit is the same as the cross regulation rate.
(4) Efficiency: Under the condition of rated input voltage and rated load, the efficiency of the measured circuit is η=84%.
Under the conditions of rated input voltage and rated load, the entire servo system can operate stably. The measured main output of the power supply is shown in Figure 2. The attenuation of 10 times is achieved during the measurement. It can be seen from Figure 2 that when the system is working normally , The ripple peak voltage is 0.06V, and the ripple is less than 3%. As the auxiliary power supply of the servo system, the power supply has been practically applied in the gate servo system and the spindle servo system, and it works reliably.
The series of single-chip switching power supplies have outstanding features such as monolithic integration, simple peripheral circuits, best performance indicators, no power frequency transformers, and complete electrical isolation, which greatly simplifies the design of switching power supplies below 150W and the development of new products. Greatly shorten the development cycle of switching power supply. Multiple output power supplies are widely used as auxiliary power supplies in motor control systems and other power electronic equipment. The designed multiple output power supplies have proven to be reliable through practical applications, and can be applied to other control systems and circuits with a little modification.