Research on multi-channel flyback converter based on NCP1200A

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Research on multi-channel flyback converter based on NCP1200A [Copy link]

Abstract: The principle of the pulse width modulation current mode controller NCP1200A for low power general offline power supply is introduced, and the advantages of this controller are further illustrated by the developed multi-channel isolated flyback converter test prototype and its test results.

Keywords: current mode control; flyback; leading edge blanking; dynamic self-power supply

　

0 Introduction

The flyback converter has the characteristics of simple circuit structure, input and output electrical isolation, wide voltage regulation range, and easy multi-channel output, so it is suitable as an auxiliary switching power supply in power electronic equipment.

Current-controlled pulse width modulation is a novel control technology that overcomes the shortcomings of traditional voltage-controlled pulse width modulation technology, making the switching power supply system have the characteristics of fast transient response, high stability, simple overload and short-circuit protection, etc. NCP1200A is a current-mode pulse width modulation controller for low-power general-purpose offline power supplies. It represents a major leap in the direction of ultra-small switching power supplies. It includes timing components, feedback devices, low-pass filters and self-power supply. This paper studies the functions of this chip and successfully designs and manufactures a prototype of a multi-channel isolated output flyback converter.

1 Introduction to Current Controlled Pulse Width Modulator NCP1200A

NCP1200A adopts the standard current model system, and the drive off time is determined by the peak current setting point. This control chip has the following features:

——No auxiliary power winding is required;

——There is an output short-circuit protection circuit inside;

——Extremely low no-load standby power consumption;

——Current mode with cycle skipping function;

——There is a 250ns leading edge blanking circuit inside;

——250mA peak source/sink current capability;

——Internally fixed frequency is 40/60/100kHz;

——Direct connection via optocoupler.

Its internal structure is shown in Figure 1, and the functions of each pin are as follows:

——Pin 1 is used to adjust the level at which the jump cycle occurs;

-- Pin 2 Connect the optocoupler to this pin to adjust the primary side peak current set point according to the output power requirement;

-- Pin 3 detects the primary current and sends the detected value to the internal comparator through the leading edge blanking circuit;

——Pin 4 is grounded;

——Pin 5 output drives the external MOSFET switch tube;

——Pin 6 is connected to an external large-capacity capacitor with a typical value of 10μF;

-- Pin 7 is used to ensure proper leakage current distance;

——Pin 8 is connected to the high voltage DC mains, which injects a constant current into _the Vcc bulk capacitor.

Figure 1 NCP1200A internal structure

2 Design features and precautions

2.1 Dynamic Self-Powering (DSS)

Dynamic self-powering technology makes the designed flyback converter without auxiliary power winding and can be directly powered by the high-voltage DC mains. The principle of DSS is based on the charging and discharging of _the Vcc bulk capacitor from a low level to a higher level. Its working process is as follows:

When the power supply is turned on, if V _cc < V _ccH , the current source is turned on and there is no pulse output; if V _cc drops and is greater than V _ccL , the current source is turned off and the output is pulsating; if V _cc rises and is less than V _ccH , the current source is turned on and the output is pulsating; the typical value is V _ccH = 12V, V _ccL = 10V, and the schematic diagram of its working process is shown in Figure 2.

Figure 2 Schematic diagram of charging and discharging of _Vcc capacitor

2.2 Cycle skipping working mode

Since the switching power supply has good efficiency under normal load conditions, its efficiency will begin to decline when the output power decreases. The NCP1200A has an embedded skip cycle function. When the output power demand decreases below a given value, it automatically skips the switching cycle. This is achieved by monitoring the FB pin. When the load demand decreases, the internal loop requires a smaller peak current. When this set value reaches a certain level, the integrated circuit will prevent the current from continuing to decrease and begin to blank the output pulses to achieve the skip cycle mode. When the FB skip threshold value (default value 1.2V), the peak current will not exceed 1V/ R _sense . When the integrated circuit enters the skip cycle mode, the peak current will not be less than V _pin1 /3.3. At this time, the user can adjust the peak current of the skip cycle by changing the voltage value of pin 1. In this way, under no-load conditions, the total standby efficiency of the NCP1200A can easily meet the design requirements.

2.3 Short circuit protection function

By continuously monitoring the feedback loop, the NCP1200A detects a short circuit and immediately reduces the output power to protect the entire circuit. Once the short circuit disappears, the controller can return to normal operation. Therefore, for a given constant output power supply, this protection function can be easily disconnected.

2.4 Implementation of soft start

The time required for the Vcc voltage to drop from 12V to 10V determines whether the system can start normally. The duration of the corresponding transient fault caused by the output capacitor charging must be less than the _time for _{the Vcc voltage to drop from 12V to 10V, otherwise the feedback monitoring loop will treat it as a short circuit, making the power supply unable to start normally. The voltage on Vcc is} related to the external capacitor value C of pin 6. Assuming that the time required for the system to reach the rated load is 6ms, _{the fall time of Vcc} must be greater than 6ms, which is set to 10ms here. The current consumption of the integrated circuit including the MOSFET driver is 1.5mA. From the formula C = i ·Δ t /Δ V , the required capacitance value can be obtained. According to the conditions set here , C = 7.5μF, and C = 10μF is used in the actual circuit to achieve the soft start function.

2.5 Chip pin protection against negative voltage spikes

This chip can prevent sensitive pins from being damaged by negative voltage spikes by adding peripheral circuits. Negative signals can cause great damage to control chips that only have positive bias, making them unable to work stably. If the primary inductance of the transformer and the external capacitor of pin 6 resonate due to improper design, negative pulses will be generated on pin 6, causing chip damage. In order to effectively prevent this from happening, the two circuits shown in Figure 3 can be used to protect the chip.

Figure 3 Circuit to prevent negative voltage pulse

2.6 Built-in Leading Edge Blanking (LEB) Function

In general, control chips must perform low-pass filtering on the detected current waveform before the current detection input on the primary side to prevent transient overcurrent caused by the switching spike of the switch tube. NCP1200A has a 250ns leading edge blanking circuit inside, so there is no need to add an external low-pass filter network to process the detected current, making the circuit design simpler.

3 Test results

This article further introduces the working principle of NCP1200A and its simplicity in practical design through the test prototype. This test device is a flyback converter with 48V DC input and three isolated outputs. The various parameters are set as: V _in =48V, V _{cc =} V _cc1 = V _cc2 = 15V, V _EE =-15V, switching frequency f =40kHz, transformer primary inductance L =330μH, rated output power P =14W, duty cycle D =0.4 (to prevent D >0.5, which makes the circuit unable to work stably, and no slope compensation is required). The structural principle of the circuit is shown in Figure 4, and an RC D passive absorption network is used to prevent the MOSFET from being burned.

Figure 4 Structural principle of the circuit

The developed test prototype is shown in Figure 5. Its specific dimensions are 105mm×30mm×23mm. It adopts a vertical plug-in structure, which is convenient for connection with the main equipment.

Figure 5: Test prototype

The actual test waveforms measured by the developed test prototype are shown in Figures 6 to 8. Figure 6 shows the charge and discharge waveforms on _the Vcc capacitor, which well realizes the dynamic self-power supply of the chip and ensures the soft start of the device.

Figure 6 Vcc _capacitor charge and discharge waveform

(a) Primary side detection current waveform

(b) Voltage waveform between drain and source of switch tube

Figure 7 i _cs and v _ds waveforms

Figure 8 Output pulses at different powers ( P ₁ > P ₂ )

Figure 7 (a) and (b) are the current waveform on the primary sampling resistor and the voltage waveform between the drain and source of the MOSFET when the circuit is working in the continuous current state (CCM) and the discontinuous current state (DCM). The current and voltage glitches in the test waveform are caused by the leakage inductance of the primary transformer, the parasitic capacitance of the MOSFET, and the reverse recovery of the secondary diode. Since NCP1200A has a 250ns LEB function, the high-frequency pulse in the detection current will not affect the stability of the circuit.

Figure 8 is a comparison of the driving waveforms of the prototype under rated output power and light load conditions, from which we can see the chip's cycle skipping working state, thereby effectively reducing power consumption and achieving high efficiency requirements.

4 Conclusion

NCP1200A is a current-controlled pulse width modulator with excellent performance, suitable for ultra-small switching power supply design. It has functions such as dynamic self-power supply, soft start, low-loss operation with skip cycle, short-circuit protection, etc. It can realize multi-channel isolated output and high-efficiency small flyback converter without almost any peripheral discrete devices. Therefore, this chip is widely used in AC-DC adapters, offline battery chargers, auxiliary power supplies, etc.

　

About the Author

Zhang Xiaofeng (1980-), male, master, his main research direction is power electronics power converter and motor digital control technology.

Lv Zhengyu (1957-), male, professor, doctoral supervisor, senior member of IEEE, his main research directions are electromagnetic compatibility in power electronics, intelligent control, power converters and power electronic devices.