Analysis and design of quasi-resonant power supply based on NCP1337

0 Introduction

As the global industry's requirements for power efficiency are getting higher and higher, power supplies including ATX power supplies and consumer electronics products require higher energy-saving requirements. Traditional power supply designs all work in fixed frequency mode, which makes the power tube consume a lot of instantaneous power during high voltage switching on and off, and also causes certain electromagnetic interference. Quasi-resonant technology and soft jump cycle technology can solve this problem. ON Semiconductor's NCP1337 controller is an excellent representative for quasi-resonant switching power supplies.

1 Quasi-resonance principle

The principle of quasi-resonant conversion is to reduce the conduction loss of the power switch in the topology. The MOSFET of a general flyback switching power supply has a fixed on/off time and operates at a fixed frequency. As shown in Figure 1, we can see that during the magnetic reset process, due to the existence of the transformer inductance and the parasitic capacitance on the power tube, the voltage drop on the switch tube oscillates. However, it can be found that point A in the voltage oscillation curve is the first minimum value (or valley value) of the MOSFET drain-source voltage. If we turn on the MOSFET tube at this time, the current spike during conduction will be minimal. Under certain conditions, zero voltage switching (ZVS) can even be obtained. This is done by adjusting the operating frequency of the power supply. Regardless of the load or line voltage at the time, the MOSFET always remains on at the bottom of the valley. Compared with the discontinuous and continuous operating modes of the flyback converter, the quasi-resonant switch provides lower conduction losses, thereby improving efficiency and reducing device temperature.

2 Introduction to NCPl337

NCP1337 is an enhanced quasi-resonant pulse width modulation current mode controller, which combines a true current mode modulator with a demagnetization detector to ensure that the power supply can operate in discontinuous conduction mode under any load conditions. It integrates all the necessary components and functions to build a rigorous and reliable switching power supply (SMPS). Figure 2 is the pin layout diagram.

Its main features:

1) Free-running boundary/critical mode, quasi-resonant control;

2) Minimum switching frequency (25 kHz) skip cycle mode;

3) Self-recovery short-circuit protection independent of auxiliary voltage;

4) Built-in two external failure mode trigger comparators (disable and lock);

5) Built-in 4.0ms soft start;

6) 500mA peak current driving capability;

7) Maximum operating frequency 130kHz;

8) Internal front end blanking, internal over-temperature protection;

9) Dynamic self-powering technology (DSS) between 12 and 10V.

There are two important features in NCP1337. First, the soft jump cycle technology is used to control the peak current and remove some switching pulses, thereby controlling the switching loss and achieving excellent high-efficiency performance under no-load and light-load conditions. It can also effectively remove noise when the transformer enters the jump cycle operation. Second, in order to ensure that it can be turned on at the bottom value at any time and realize quasi-resonant operation, the coil-free demagnetization detection technology is used.

2.1 Soft-jump cycle technology

Under light load or standby mode, NCP1337 enters soft jump cycle mode: when the FB setting point is 20% lower than the maximum peak current (Ucs at 100mV), the output pulse stops. When the FB loop forced setting point is higher than 25% (Ucs at 130mV), the switch conversion starts again, and each start is internally softened, that is, soft start, so that the frequency will not be lower than 25kHz. When this situation occurs at low peak current, soft start, TOFF is clamped, even with a low-cost transformer, it can work without noise. As shown in Figure 3.

2.2 Coil-free demagnetization detection technology

In order to obtain the quasi-resonant operation mode, the optimal point should correspond to the "valley point" of the drain voltage, which also corresponds to the lowest energy storage point of the total drain capacitance. ON Semiconductor's specific power MOSFET driver, hybrid MOS and bipolar mechanism detects the negative gate current, so that the negative gate current is not conducted through the bottom end, but through the path of the positive VCC voltage. In this way, the detected current flows from VCC to the gate through a very simple compensation mechanism, forming an active negative voltage clamp. Therefore, the negative gate current can be converted into a positive current that is easy to handle. Subsequently, a simple comparator can detect the zero current gate crossing and provide a "valley point" signal. Therefore, the valley voltage of the drain of the power switch tube can be automatically detected without any external signal such as the transformer auxiliary winding voltage, making the circuit design simpler.

3 15V/60W Power Supply Design Example

The quasi-resonant switching power supply designed by this paper using NCP1337 is the power supply part used in the industrial sewing machine controller, replacing the traditional linear power supply. Due to its small size, it can save space and the output voltage is relatively stable. Because the voltage of the industrial power grid fluctuates greatly, the output voltage fluctuates greatly when using a transformer, and the control reliability decreases. The switching power supply does not have high requirements for the input waveform, and the input voltage change has little effect on the output voltage. Its technical requirements are: AC input voltage 90~265V, DC output voltage 15V. The circuit schematic is shown in Figure 4. Here, the power input circuit is simplified, and only Uin is used to represent the DC input after AC rectification, and the secondary winding output is 15V DC, and its maximum power can reach 60W. As can be seen from the figure, this switching power supply part has few peripheral components and no external resonant capacitor, making the design simpler and more convenient. The following is a brief description of the circuit:

1) Peak voltage absorption circuit

VD2, C4 and R17 form a peak voltage absorption circuit, which mainly absorbs the rising edge peak voltage energy generated when the MOSFET power switch tube is turned off, reduces the peak voltage amplitude, and prevents the power switch tube from overvoltage breakdown.

2) Input undervoltage protection and overload compensation circuit

The chip has a unique input voltage monitoring function, which detects the input voltage through R1, R2, R3, and C2 to achieve input over-voltage and under-voltage protection. Resistor R4 is used to set the depth of overload compensation.

3) Auxiliary power supply circuit

Although NCP1337 has an internal self-powered system, it still needs to add an auxiliary power supply to drive high-current MOSFET. VD3, R9, C6, VD5, VT2 and VD1 constitute the auxiliary power supply circuit.

4) Secondary rectifier filter circuit

Since there is no strict EMI requirement, VD4 and C7 are simply used as the rectifier control circuit, and C8, C9 and L1 are used to form a 15V rectifier filter circuit.

5) Isolation voltage feedback sampling circuit

U2, U3, R12, R13, R14, C5, R15, and R16 form a secondary voltage feedback circuit, and resistors R15 and R16 are used to set the voltage size, and the specific value is 2.5×(1+R15/R16).

6) Other devices

R7, R8, R10 and R11 form the MOSFET drive and current sampling circuit, VT1 is a 2.5A 900V high voltage MOSFET power switch tube, U1 is a NCP1337 quasi-resonant PWM controller. T1 is a PQ32/20 ferrite core high frequency power switch transformer. C9 is a safety standard Y1 capacitor.

7) Experimental results

According to the above schematic diagram, an actual switching power supply is made, and different load conditions are analyzed and studied. The waveforms are shown in Figures 5, 6 and 7. Figure 5 is a no-load operation diagram. It can be seen that the chip works in the soft jump cycle mode, and the startup is internally softened, that is, soft start operation. Figure 6 is a 20W load waveform diagram, and its switching frequency is 124kHz. Figure 7 is a 30W load waveform diagram, and its switching frequency is 127kHz. By comparison, it is found that when the load changes, its switching frequency also changes accordingly, and the number of resonances decreases, but the switch can be turned on at the bottom value, so that the power supply works in quasi-resonance mode. In this way, the conduction loss of the switching power supply will be reduced, reducing EMI interference.

4 Conclusion

Through the research and use of NCP1337, it is shown that the switching loss and standby power of the quasi-resonant technology are very outstanding, and it can achieve the design requirements of small and medium power supplies such as high efficiency, low power consumption and low cost.