Application of quasi resonant PWM controller in po

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Application of quasi resonant PWM controller in power adapter

power adapter not only has a wide variety, but also is widely used in consumer electronic products. Most of the output power provided is less than 100 watts, and the market demand is growing rapidly. Such as electric bicycle battery charger, small-size LCD TV, notebook computer, printer, DVD player and set-top box. Most of the early power adapters used linear transformers. With the requirements of consumer electronic products for high efficiency and wide input voltage range, and the increase of copper, iron and labor costs, most of the original linear transformers in this kind of power adapter were gradually replaced by switching power supply. Thus, the power adapter is changed from a single voltage input of AC 110 V or AC 220 V to a wide voltage range input of AC 85 to 264 V, and the input power frequency is between 47 and 63 Hz

generally speaking, the space volume of the power adapter is relatively small, and large heat sinks cannot be installed. At the same time, the shielding of EMI electromagnetic radiation is also a difficult problem to solve. However, most of the switching power supplies in the power adapters on the market are designed by ordinary hard switching PWM control mode. There are many insurmountable defects, such as high switching noise, complex EMI shielding structure, high material and production cost, low conversion efficiency, large volume of radiator, crowded internal space, large standby power consumption, unable to meet the energy-saving standard, few protection functions, low reliability and so on, Especially in the power adapter with low voltage and high current output above 50W, these defects are more prominent

with the release of various energy-saving specifications in many countries and regions around the world, such as the "Energy Star" certification project, the California Energy Commission (CEC), the EU Code of conduct and the European Organization for energy efficient appliances (geea), all put energy conservation as an important part of environmental protection. For power products that want to be exported to Europe, America and other places, it is very important to meet the internationally enforced standby power consumption regulations and safety certification standards. For example, it is proposed that the standby power consumption is less than 1 watt worldwide; UL safety certification, CE safety certification, etc. On the one hand, the demand for power adapters for consumer electronic products is growing rapidly, and power supply manufacturers must rapidly expand their production capacity. On the other hand, the strict energy-saving regulations issued by many countries in the world make power supply manufacturers face greater pressure in designing "green power supply" and saving energy and reducing consumption, Most power adapters designed with traditional PWM control mode can not meet the design requirements of "green power supply". Therefore, power supply manufacturers must abandon the traditional PWM control mode and adopt new PWM control technology to design their own new power adapter products, so as to meet the growing market demand and the new requirements of switching power supply product specifications and standards. Quasi resonant PWM control technology is a solution that can meet the new design requirements of power adapter. The so-called quasi resonant PWM control is simply a PWM control mode that controls the switch to turn on at the bottom of the voltage Valley, also known as soft switching PWM control mode. The power adapter designed with quasi resonant PWM controller can easily realize many advantages, such as high power conversion efficiency, low noise, low EMI cost, simple structure, low manufacturing cost, low standby power consumption, meeting energy-saving standards, complete protection functions and high reliability. It is especially suitable for the design of small volume and high-power power power power adapter. Ncp1337 is a new high-performance quasi resonant controller launched by ansenmey in order to continuously meet the market demand for smaller, more energy-efficient and lower cost power adapters. It is a good solution for designing high-performance power adapters

taking the latest quasi resonant PWM controller ncp1337 of ansenmey as an example, this paper introduces the design and application of quasi resonant PWM controller in power adapter

Figure 1 Schematic diagram of ncp1337 typical 12V/5A power adapter

performance characteristics of ncp1337 quasi resonant PWM controller

ncp1337 is an enhanced quasi resonant pulse width modulation current mode controller, which combines a real current mode modulator and demagnetization detector to ensure that the power supply can work in discontinuous conductive mode under any load condition. It integrates all the necessary components and functions of a strict and reliable switching power supply (SMPS), requiring only a few external components. It is very convenient to design the power adapter in Free Running Quasi resonant mode. Its main features and advantages are as follows:

1. Discontinuous mode quasi resonant control of free operation. It provides the feasibility for designing a simple and efficient synchronous rectifier circuit with low voltage and high current output, so as to realize the high conversion efficiency of switching power supply. With the change of power supply voltage, it is easy to realize the working frequency jitter, so as to minimize the electromagnetic interference and reduce the device and power consumption of EMI filter

2. Ultra low power consumption. When in standby, it works in the ripple mode with the minimum switching frequency. When the power requirement is lower than the preset level, it enters the ripple mode by monitoring the voltage of the feedback pin, so that the no-load energy consumption of the adapter can meet the energy-saving standard. Because the restart is softened by the internal soft start and the frequency will not be lower than 25kHz, the switching power supply can work without noise under the frequency jump mode of low peak current under no-load,

3. The protection function is perfect. There are many protection functions in the chip, such as automatic restart protection circuit after short circuit, open-loop fault detection and overvoltage locking protection circuit, current limiting protection circuit and overheating protection circuit with hysteresis characteristics. In the case of overcurrent, the output pulse is closed and enters the safe burst mode. Once the fault disappears, the IC will try to restart and automatically return to normal operation

4. Demagnetization detection of internal transformer core. The bottom voltage of the drain of the power switch can be detected automatically without any external signal such as the voltage of the auxiliary winding of the transformer, which makes the circuit design simpler

5. Adjustable overload compensation. It can ensure that the maximum output power provided by the quasi resonant power supply is not affected by the change of input voltage

6. Built in two external failure modes trigger comparators (inhibit and lock). It provides an overall voltage detection function called input undervoltage and overvoltage protection. The protection function of other external faults triggering this prohibition or locking can also be added

7. VCC adopts dynamic self power supply technology. The built-in high-voltage starting current source can directly supply power from 40V to 500V DC without external starting resistance with large loss

8.500ma peak current driving capacity. It can directly drive high-voltage MOSFET power switches with drain current less than 15A

9. The soft start circuit with 4ms inside eliminates the traditional start stress. It can ensure that there is no internal current and voltage overshoot when the switching power supply is started. The current mode controls the peak current of the power switch in each switching cycle, so that the switching power supply has a good dynamic load response speed

10. The maximum working frequency is internally limited to 130KHz, which is lower than the minimum frequency of EMI radiation standard 150kHz

11. The internal front-end blanking and feedback optocoupler can be directly connected with the IC feedback pin without complex peripheral circuit, which effectively reduces the peripheral components controlling the IC

ncp1337 typical 60W power adapter design scheme

◆ basic structure of the circuit

the 60W 12V/5A power adapter designed with ncp1337 adopts the common flyback topology, as shown in Figure 2. F1 fuse and Th1 are the input circuit of power supply composed of negative temperature system resistor and vsr1 varistor, C1, C2 and L1 constitute EMI filter circuit, BD1 is the input stage rectifier bridge, CDC and C4 are the input stage filter capacitors, CB, Rb1, Rb2, RB3 and RB4 constitute the input overvoltage and undervoltage protection and overload compensation circuit, DSN, CSN and RSN constitute the peak voltage absorption circuit, and R7, R8, R10 and RS constitute the MOSFET drive and current sampling circuit, Q1 is a 6A 600V high voltage MOSFET power switch, IC1 is a ncp1337 quasi resonant PWM controller, and DA, RA and Ca constitute the auxiliary power circuit of ncp1337. T1 is pq32/20 ferrite core high frequency power switching transformer. IC2, IC3, R15, R16 and R17 constitute the secondary voltage feedback circuit, T2, DSC, RSC and QSC constitute the secondary rectifier synchronous control circuit, and QR, Co, CP and LP constitute the 12V rectifier output filter circuit. C13 refers to Y1 capacitor of safety regulation

Figure 2 flyback topology circuit diagram

◆ key points of power adapter design

taking Figure 2 as an example, this paper introduces the basic points of designing 60W 12V/5A power adapter with ncp1337

● the power input circuit

F1 is a 3.5 A 250V AC fuse, which is used to prevent large short-circuit current at the power input in case of abnormality inside the power supply. Th1 is a negative temperature coefficient resistance, which is mainly used to reduce the impulse current when the power supply is turned on. Vsr1 is a 470 volt varistor, which is mainly used to absorb the surge voltage on the input line and prevent the devices inside the power supply from being damaged by overvoltage

● EMI common mode suppression circuit

the EMI common mode suppression circuit at the power input is composed of x2 capacitors C1, C2 and common mode inductance L1. Its function is to attenuate the differential mode noise of switching power supply and reduce the radiated energy of EMI. The second function is to suppress the common mode noise amplitude of switching power supply and reduce the conducted interference of switching power supply

● the primary input filter circuit

CDC can choose ordinary high-voltage electrolytic capacitor, which is mainly used to smooth filter the input voltage, filter out the 100 Hz ripple voltage and provide relatively stable DC voltage for the switching power supply. C4 is a high-voltage ceramic capacitor, which is mainly used to filter out high-frequency switching noise and improve the conduction characteristics of the power supply

● input overvoltage and undervoltage protection and overload compensation circuit

the unique input voltage monitoring function of ncp1337 realizes the input overvoltage and undervoltage protection function by detecting the input voltage through Rb1, Rb2, RB3 and CB. Resistor RB4 is used to set the depth of overload compensation. Combined with current sampling resistor RS and transformer primary inductance, it can ensure that the power adapter has a constant overload power point within the full input voltage range. The utility model overcomes the disadvantage that the overload power point of the general power adapter is difficult to keep constant when the input voltage is high or low, so as to avoid the problem that the power adapter will be overheated and damaged due to high power when the input voltage is high or low

● spike voltage absorption circuit

the spike voltage absorption circuit composed of DSN, CSN and RSN is mainly used to absorb the rising edge spike voltage energy generated by MOSFET power switch when turned off, reduce the spike voltage amplitude and prevent overvoltage breakdown of power switch. RSN requires a power resistance of more than 2W, with a resistance value of about 68 kiloohm. CSN requires ceramic capacitors with low equivalent series resistance, with a capacity of about 3300 PI method and a withstand voltage value of more than 400 V. DSN requires the selection of ultrafast recovery diode, and the reverse withstand voltage value is more than 600 volts

● auxiliary power supply circuit

although ncp1337 has internal self power supply system, the energy it provides is very limited by the heat loss capacity of IC itself. When using it to drive high current MOSFET, it is still necessary to increase the auxiliary vehicle lightweight and improve the air quality in the vehicle. It is the trend power supply for the development of automobile industry. Da, RA and Ca are the auxiliary power supplies used to form ncp1337. At the same time, ncp1337 can also control 9 Control experiment mode: the computer automatically controls the experiment process and monitors the output voltage, so as to realize the output overvoltage protection function

● secondary synchronous rectification control circuit

T2, DSC, RSC and QSC constitute the control power of secondary synchronous rectification

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