Research and simulation of flyback converter based on NCP1205

Publisher:科技飞翔Latest update time:2012-03-15 Source: 电源在线网Keywords:NCP1205 Reading articles on mobile phones Scan QR code
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.OverviewNCP1205

is a single-ended PWM controller newly launched by ON Semiconductor. It integrates a current control regulator and a demagnetization detector to ensure that the circuit can work in discontinuous mode (DCM) under any load and any linear condition in quasi-resonant mode and can make the switch tube achieve valley switching (that is, the loss is minimized when it is turned on). Through its inherent variable frequency mode (VFM), the controller can reduce its operating frequency and maintain its current peak value unchanged when the output power demand decreases. By applying this unique frequency change technology, NCP1205 has become a pioneer in low-power integrated switching power supplies. The unique technology of valley switching ensures the minimum switching loss and the lowest power loss when no load. NCP1205 has two structures: 8-pin and 16-pin. The 8-pin structure is selected as the introduction object in this article. Figure 1 and the table below are its pin names and function descriptions.


2. Characteristics of NCP1205

1. When working in quasi-resonance mode, natural valley switching is beneficial to reduce EMI
2. Smooth frequency conversion at light load is beneficial to reduce standby power and ripple
3. Maximum switching frequency is adjustable
4. The internal leading edge blanking (LEB) of the current sensing input has a 200ns delay, which greatly improves the anti-interference ability of the chip;
5. The driver output source current/mirror current are both 250mA
6. The operating voltage range is wide from 8 to 36V (up to 45V)
7. The undervoltage lockout (UVLO) range is wide from 7.2 to 15V
8. Automatic recovery short-circuit protection (OCP)
9. Built-in 3mA startup current source
10. Current control mode
11. Adjustable overvoltage level
12. There are two package types: 8-pin and 14-pin

3. Working principle of NCP1205

At rated output power, the circuit works in the traditional current mode, at which time the current peak is determined by the feedback signal. While maintaining the switching frequency constant, each cycle of startup begins at the end of primary demagnetization (Valley Switching). At this time, the system operates in a critical mode (BCM) between continuous (CCM) and discontinuous (DCM). When the output power decreases, the switching frequency naturally increases, which will inevitably increase the switching loss and reduce the efficiency of the switching power supply. To solve this problem, NCP1205 clamps its maximum switching frequency at 125KHz. When the switching frequency reaches its clamping value in BCM mode, the control of the switching frequency is taken over by the voltage-controlled oscillator (VCO) inside the NCP1205, and the switching frequency begins to decrease. At this time, the circuit operates in variable frequency mode (VFM). Regardless of BCM or VFM mode, the controller performs valley switching on the switch to ensure minimum loss. It is worth noting that during the transition period before and after the VCO takes over, the peak current is not fixed, but decreases as the output power decreases. When the peak current decreases to the minimum value (250mA/Rsense), the switching frequency continues to decrease until 0Hz (the IC stops working at this time). Therefore, NCP1205 has become the best chip recommended by the International Energy Association for standby power.

4. Determination of operating frequency

To obtain the resonant operating frequency, we must first determine the switching cycle. The switching cycle mainly consists of three parts: turn-on time Ton, turn-off time Toff and delay time Tw.

[page]5. Design Example

This paper uses QR technology to design a 120W AC/DC switching power supply, and its technical requirements are: AC input voltage 85~240V, DC output voltage 20V. The traditional snubber circuit uses RCD and RC peak absorption circuits. Although it can reduce the withstand voltage requirements of the power switch tube, due to the large loss of the snubber circuit and severe heat generation, high-power resistors and high-voltage, high-frequency and large-capacity capacitors are required, which in turn increases the temperature of the local area in the machine, affects the surrounding semiconductor devices, and reduces the working reliability. Therefore, this paper adopts a new type of soft snubber circuit, which uses an added capacitor to form a resonant commutation circuit. It combines current-type PWM control and uses the simplest passive soft snubber method, the least additional components, and less additional loss to complete the transient snubber of the electronic switch high-speed shutdown inductive load and eliminate the circuit defects caused by the poor reverse recovery of the output diode. This experiment uses SIMetrix software for simulation, and its simulation circuit is shown in the figure.

6. Conclusion

Figure 2 shows that after the high level appears in the demagnetization detection, the MOSFET is triggered to turn on after a slight delay. Each trigger occurs after the demagnetization is detected to be successful. This can prevent the transformer from saturating due to incomplete demagnetization. Figure 3 shows that after the MOSFET is turned off, the voltage across the transformer is reversed, and the transformer transfers the energy stored on the primary side to the secondary side of the transformer. When the transformer is demagnetized, the circuit works in a resonant state, and the waveform of Vds is close to sine. The lowest point of the sine is called the valley. If it is turned on at the center of these valleys, we can minimize the circuit's turn-on loss. If it is properly selected, it is turned on when Vds just drops to 0, and we can achieve zero voltage conversion, namely ZVS. Different input and output conditions determine different recovery times. NCP1205 provides smooth frequency conversion to ensure that the circuit can achieve valley turn-on under different conditions. Reduce turn-on loss, thereby improving efficiency.


References [1] Zhang Zhansong, Cai Xuansan, Principle and Design of Switching Power Supply, Beijing: Electronic Industry Press, 1999
[2] Wang Yang, Lin Haiqing, Chang Yue, Determination of Operating Frequency and Development of Flyback Quasi-resonant Switching Power Supply, Power Electronics Technology, 2005
[3] Mao Xingwu, Liu Yongliang, Single-ended PWM Controller NCP1205 Characterized by QR Operation and Soft Frequency Bending and Its Application, Foreign Electronic Components, 2003
[4] Wang Zhiqiang, High Frequency Switching Power Supply, School of Electric Power, South China University of Technology, 2002
[5] Lin Zhoubu, Zhang Wenxiong, Lin Yuanzun, Soft Buffering Technology of Flyback Switching Power Supply, Power Electronics Technology, 2001
Keywords:NCP1205 Reference address:Research and simulation of flyback converter based on NCP1205

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