Quasi-resonant soft switching power supply with positive and negative excitation

   Early switching power supplies operated by forcibly turning on or off the excitation tube. The switching noise and switching loss were large, and the working efficiency was difficult to further improve. Soft switching technology uses LC resonance to adjust the current or voltage value at the switching moment to minimize the switching loss. It is superior to hard switching power supplies in terms of switching noise and working efficiency. Therefore, resonant switching power supplies will develop rapidly. There are many types of chips that realize soft switching, and the working principles are different. For example, the quasi-resonant reverse controller UCC28600 chip works by turning on the excitation tube after the reverse excitation voltage drops to the minimum value, the excitation current reaches the peak value, or the excitation tube is turned off at a fixed time. It has a unipolar output and its switching frequency changes with the output power. It is generally used for low-power power supplies; the resonant mode controller UCC25600 has a basic fixed resonant frequency. It uses feedback to automatically adjust the switching frequency so that the circuit can be adjusted between resonance and detuning, changing the effective excitation power. It has a bipolar output and is generally used for 100 W to 1 kW power supplies. This article is based on the UCC28600D chip to study the design points of this type of soft switching power supply.
1 UCC28600D chip working characteristics
    The UCC28600D chip is a multi-mode quasi-resonant reverse controller with low power consumption, only 8 ports, and simple circuit connection. The chip is equipped with an oscillator with a variable oscillation frequency, which does not directly determine the output pulse frequency. Its pulse output and pulse closing mode are determined by the external circuit state of the chip: when the potential of the voltage state detection protection terminal 7 drops to the lowest value (voltage valley point), the output pulse is turned on; when the current flowing out of the 7-port reaches 450 μA (the terminal potential is 0 V at this time) or the voltage of the 7-port exceeds 3.75 V, it enters the overvoltage protection state; according to the detected 3-port potential value, the output pulse is turned off or the pulse is turned off at a time, which is 0.4 V to 0.8 V in the quasi-resonant mode or discontinuous mode. In the foldback mode, the 3-port potential is fixed at 0.4 V, and the excitation current is no longer detected, and the pulse is turned off at an internal time. The pulse frequency of the chip is always automatically adjusted between 40 kHz and 130 kHz through the potential of port 2, and the potential of port 2 is adjusted in a closed loop by the power supply output parameters (preset voltage or current value): 4.0 V to 5.0 V, it works in the intermittent state of the quasi-resonant mode; 2.0 V to 4.0 V, it works in the continuous state of the quasi-resonant mode (130 kHz); 1.4 V to 2.0 V, it works in the frequency foldback mode (40 kHz to 130 kHz); 0.5 V to 1.4 V, it works in the low-frequency energy-saving mode. The higher the pulse frequency, the smaller the output power, which is a feature of the anti-excitation circuit. Therefore, UCC28600D is suitable for anti-excitation working mode.
2 Soft switching power supply constructed by UCC28600D
    The power supply works in the anti-excitation mode, and the output power can be changed by adjusting the pulse frequency. For the positive excitation mode, the output power needs to be adjusted by changing the pulse duty cycle, and the frequency conversion function of the UCC28600D chip itself does not play a direct role. The working mode of the anti-excitation power supply is to first convert electrical energy into magnetic field energy and store it in the magnetic circuit or magnetic core material, and then convert the magnetic field energy into electrical energy for output in the next time period. The amount of magnetic field energy stored per unit time determines the output power of the anti-excitation power supply. The available energy storage size of the magnetic core material can be calculated by the following formula [1]:
    
where μr is the relative magnetic permeability of the material, V is the volume of the magnetic core material (in mm3), and Bm is the maximum working magnetic induction intensity (in T). In addition to being proportional to its volume, the energy storage capacity of the magnetic core material is also proportional to the maximum magnetic induction intensity and inversely proportional to the relative magnetic permeability. Taking the EC2828 ferrite core transformer as an example, its core volume is about 5,800 mm3, the maximum magnetic induction intensity can only be 0.4 T, and the maximum available magnetic induction intensity is only about 0.2 T (the value is related to the operating frequency) [2], and the relative magnetic permeability is about 2,000. When the magnetic core is tightly combined, the maximum energy storage is 46 μJ. Calculated at a pulse frequency of 100 kHz, the maximum output power is about 4.6 W, while the output power of the magnetic core of the same specification working in positive excitation mode is above 50 W. If an air gap is set in the magnetic circuit, although the energy storage can be increased, the leakage magnetic flux will increase. It can be seen that simply using the reverse excitation method is not the best solution, and it is difficult to exert the power supply capacity that should be possessed. In this design, a unipolar positive and reverse excitation sharing method based on positive excitation is adopted, so that the power supply can provide as much power as possible. At the same time, the power supply time in each cycle is more balanced, which is conducive to reducing the pulsation of the output voltage.
    The circuit is shown in Figure 1. In addition to the AC voltage input filter rectifier circuit, it also contains 6 functional modules. (1) Pulse generation and excitation circuit, mainly composed of IC1 and the primary winding of transformer T1, controls the energy conversion of the transformer; (2) Leakage inductance energy absorption consumption and resonant circuit, absorbs the energy stored in the leakage inductance of the transformer, limits the reverse excitation voltage on the excitation tube, and forms a demagnetizing resonance with the transformer excitation winding; (3) The chip power supply network is divided into a resistor current limiting power supply and a transformer T1 feedback power supply, providing a chip operating voltage between 13 V and 15 V; (4) Line voltage and reverse excitation voltage sampling protection circuit to detect overvoltage; (5) Positive and reverse excitation output and filtering circuit; (6) Voltage limiting feedback network to stabilize the output voltage.

3 Determination of the main circuit parameters
    (1) Parameters of the leakage magnetic energy absorption and resonant circuit using varistor
    The leakage magnetic energy absorption and resonant circuit consists of R23, R2, C3, C20, D3 and the primary coil of the transformer, which absorbs the leakage magnetic flux energy during the reverse excitation period. R2 is a varistor to limit the voltage on C3 to basically not exceed 330 V, so that the drain potential of the excitation tube basically does not exceed 630 V, protecting the excitation tube from breakdown due to excessive voltage. R23 is a damping resistor, which consumes part of the transferred energy. Capacitor C20 has two functions: one is to play a buffering role for diode D3 at the beginning of reverse excitation; the other is to form a resonant circuit after the transformer is demagnetized.
    The primary side of the transformer and the energy absorption circuit form a resonant circuit. After the transformer energy storage is basically released, the drain potential of the excitation tube decreases in a resonant process. The speed at which the drain potential of the excitation tube decreases is determined by capacitor C20 and the primary inductance of the transformer. The larger the capacity of C20, the faster the drain potential of the excitation tube decreases. Figure 2 is the potential curve when C20 is 100 pF, where the larger amplitude is the drain potential curve of the excitation tube, and the smaller amplitude is the transformer output voltage curve. Of course, the resonant period is also related to the transformer distributed parameters.

    The resonance that the quasi-resonant mode of UCC28600D relies on is generated after the transformer energy storage is released, and the amplitude of the decline is related to the amplitude of the drain reverse excitation voltage. The higher the drain reverse excitation voltage, the greater the amplitude of the drain potential decline, which is basically symmetrical to the drain line potential. Therefore, the reverse excitation voltage should be slightly higher than the line voltage, so that the drain potential of the excitation tube drops to near 0 V through the resonance process, eliminating as much loss as possible when the excitation tube is turned on. This is significantly different from the design of other reverse excitation switching power supplies. The reverse excitation voltage in Figure 2 is 200 V, and the corresponding reverse excitation voltage in Figure 3 is 300 V, which is significantly lower than the resonant low potential of the former, reaching an ideal state close to 0 V.

    (2) Switching power transformer parameter design
    Switching power transformer parameter design is one of the key contents in switching power design. Here, the limited power design method [3-4] is adopted, with 60 W as the basic design parameter and the maximum output voltage of 29 V. The minimum switching frequency corresponding to the maximum output power is 40 kHz. When the required output power decreases, the driver chip will automatically increase the switching frequency to reduce the excitation power.
    In the limited power design, the core transformer specifications are first determined according to the installation space and rules. Here, EC2828 horizontal structure and PC40 material are selected. The center magnetic column load area Ae is about 78.5×10-6 m2, the maximum magnetic induction intensity Bmax at 100 kHz can only be taken as 0.4 T, and the relative magnetic permeability is about 2 000. The positive excitation voltage is 260 V~300 V, and the reverse excitation voltage is 300 V. Both positive and reverse excitations output electrical energy. According to the voltage-time balance equation Upositive t1=Unegative t2, the positive excitation time is close to 12 μs in the minimum 40 kHz and maximum 25 μs cycle, the maximum reverse excitation time is about 12 μs, and there is at least 1 μs of resonant half-cycle time. The smaller the number of winding turns, the smaller the inductance, the faster the current rises in a fixed time, and it is easy to enter magnetic saturation. In order to prevent magnetic saturation, the excitation winding is limited by a minimum number of turns. The minimum number of turns of the excitation winding is calculated by the following formula:
    
In which, the line voltage is substituted according to the maximum value of 300 V, and the maximum magnetic induction intensity Bmax is taken as 0.36 T, which leaves a certain margin compared with 0.4 T. Considering that the number of turns of the output winding is an integer, the positive excitation output voltage at the lowest input should reach 40 V. It is more appropriate to set the turns ratio to 260:40. The positive excitation output winding N3 is set to 20 turns, and the actual N1 can be taken as 130 turns. The positive excitation turns ratio is 6.5.
    According to the 60 W input power, if the reverse excitation output is completely, the required excitation current peak is about 1 A. Reverse excitation is the output of energy stored in the magnetic core. Generally, the energy storage of the magnetic core is limited, and this current value cannot be achieved by excitation alone. If the reverse excitation output energy accounts for 20% of the total energy, the required maximum excitation current Im is:
    
    In the current-limited power working mode of UCC28600D, if the maximum excitation current is limited to 1 A, the insufficient reverse excitation current is supplemented by the positive excitation current, then the maximum positive excitation current should reach 0.7 A, which is controlled by the output filter inductor. The above is the limit value calculated based on the maximum cycle. If the switching frequency is increased, the chip itself will change the current limit value accordingly. For the output voltage of 29 V, considering that there is still a certain voltage drop in the rectifier diode, the voltage of the reverse excitation output winding should be preset to 30 V. The reverse excitation turns ratio is 10:1. According to the voltage turns ratio relationship, the 30 V reverse excitation output winding needs to be wound with 13 turns. Similarly, the 16 V feedback voltage winding also uses reverse excitation output and needs to be wound with 7 turns.
    (3) Determine the power limiting resistor.
    The excitation circuit driven by the UCC28600D chip limits the output power by limiting the excitation current. For the excitation current limiting type power supply, whether the positive excitation or the reverse excitation method is used, the output power is basically the same. Therefore, the current limiting value can be calculated by referring to the simple reverse excitation excitation current, and then the power limiting resistor R12 can be calculated, see Figure 1. According to the above calculation, the simple reverse excitation excitation current should reach about 1 A, and the current limiting resistor corresponding to the power limiting voltage of 0.8 V should be 0.8 Ω. If it is taken as 1 Ω, its actual output power will be reduced.
    (4) Determine the positive excitation filter inductance.
    The output rectifier filter circuit includes positive excitation output rectification and filtering, reverse excitation output rectification and filtering. The positive excitation output winding and the reverse excitation output winding are set independently and rectified independently. L2 and D2a are used for positive excitation rectification and filtering, and D2b is used for reverse excitation rectification. The two share the filter capacitor. If the inductance of the positive excitation filter inductor L2 is too small, the filtering effect is not good. If it is too large, the current rise rate is low, and the current increment reflected on the excitation coil is insufficient, which may cause the switching frequency to decrease. The principle of determining L2 is that the excitation current plus the positive excitation coupling current should reach 1 A within 12 μs. According to the volt-ampere relationship of the inductor, under the action of the standard line voltage of 280 V, the current of the 9 mH excitation winding increases to 0.37 A after 12 μs, and the coupling current should reach 0.63 A. The coupling turns ratio of the positive excitation is 6.5, and the current increment on the filter inductor within 12 μs needs to be 4.1 A. Under normal positive excitation conditions, the voltage applied to the filter inductor is 10 V, and only a filter inductor of 29 μH can increase the current by 4.1 A within 12 μs. Therefore, the inductance of the filter inductor L2 should be controlled at about 30 μH, and its value does not need to be too strict.
    According to the above design, a high-performance low-power power supply can be obtained. When the output is 42 W, the AC component of the voltage at the output port of the power supply is measured using an oscilloscope low-interference measurement method, as shown in Figure 4. As can be seen from the waveform, the output voltage has only a very small noise voltage component, and the amplitude of the noise voltage and the pulsating voltage caused by the switching cycle are both less than 5 mV, which is impossible for traditional switching power supplies. The noise voltage is no longer the main problem in the use of power supplies. After adopting the quasi-resonant working mode, the loss of the excitation tube is small. When the output power is 42 W, the overall working efficiency of the power supply is measured to be 85%. The maximum loss is in the output rectifier, transformer, and leakage magnetic energy absorption circuit. If synchronous rectification technology [5] is adopted, the working efficiency can be further improved.

References
[1] Chen Tingxun. Realization of low electromagnetic interference switching power supply [J]. Modern Electronic Technology, 2009, 9(18): 201-204.
[2] Electronic Transformer Professional Committee. Electronic Transformer Handbook [M]. Shenyang: Liaoning Science and Technology Press, 1998.
[3] Chen Tingxun. Unidirectional design method of high-frequency transformer for switching power supply [J]. Journal of Zhejiang Ocean University (Natural Science Edition), 2009, 27(3): 358-360.
[4] Sun Xiaolin, Li Guoyong, Wang Zhihai. Design analysis of high-frequency switching power supply transformer [J]. Automation Technology and Application, 2008, 27(6): 53-56.
[5] Hu Zongbo, Zhang Bo. Study on bidirectional conduction characteristics and rectification loss of MOSFET in synchronous rectifier [J]. Proceedings of the CSEE, 2002, 3(3): 88-93.