Principle and application of power factor correction controller MSC60028

    Abstract: This article introduces the working principle, performance and characteristics of Motorala's power factor correction controller MSC60028, and importantly analyzes the component parameter design and device selection methods in the active power factor correction control (APFC) circuit. And an actual circuit of MSC60028 in the APFC circuit is given.

    Keywords: active power factor correction, boost converter, pulse width modulation ASC60028

1 Introduction

With the continuous development of power electronics technology, more and more power electronic devices are widely used in various fields. Among them, problems such as harmonic pollution of the power grid and low input power factor have become increasingly prominent. In order to reduce the number of such devices Regarding the harmonic pollution and electromagnetic interference of the power grid, people have proposed corresponding harmonic extraction technology and power factor correction circuit after extensive research and analysis.

Methods to improve the power factor can be summarized into two types: one is the passive power factor correction method, which mainly expands the conduction angle of the input current through circuit design. High-frequency compensation methods can also be used to improve the conduction angle of the input current. pass angle; the other is the active power factor correction method, which inserts a power factor correction device in series between the power grid and the power supply device. Among them, the single-phase BOOST circuit has a simple topology, small current distortion, high efficiency and low cost. It is widely used due to its low advantages and is called active power factor correction (APFC) circuit. Common BOOST circuits include connected current operating mode (CCM) and discontinuous current operating mode (DCM). Because the input end current harmonic distortion of the boost circuit in the continuous current operating mode is small and the peak current is low, it is often used to require harmonic distortion. Small and high power circuits. This article focuses on introducing a new high-performance, continuous-current operating mode power factor control chip MSC60028 and its circuit design method. It also analyzes its design points and typical application circuits in active power factor correction circuit applications.

2 MSC60028 working principle

The MSC60028 power factor controller is available in three package forms: 18-pin dual in-line plastic package, 20-pin dual in-line plastic package with an unused operational amplifier, and 14-pin dual in-line plastic package without operational amplifier. The device contains an oscillator, multiplier, error amplifier, gate driver, and overvoltage and undervoltage protection units. Its internal structure block diagram is shown in Figure 1.

The biggest feature of the MSC60028 power factor controller is the continuous current operating mode. Figure 2 is a block diagram of the active power factor correction circuit composed of the MSC60028 power factor control chip.

As can be seen from Figure 2, the DC output voltage of the boost converter is detected and sent to the error amplifier. Its output is multiplied by the detected voltage at the output end of the input rectifier to obtain a current command. After the current command is corrected by the converter input current, it is then combined with the By comparing the triangular wave signal output by the oscillator, a PWEM signal can be obtained that can maintain the output voltage of the boost converter basically unchanged and make the converter input current and input voltage in phase. Compared with the PWM signal after passing through the gate driver, Directly drives the MOSFET power switching tube.

When the APFC system is started or the load suddenly changes, overvoltage or undervoltage may occur, so the control core is equipped with overvoltage and undervoltage protection circuits. If the voltage on the input exceeds 5.36V, the overvoltage protection circuit immediately shuts down the output of the gate driver. In order to prevent the gate driver from frequently turning on and off when the system is working around the critical point, a 120mV hysteresis voltage is introduced into the circuit, that is, the gate driver will only work again when the voltage drops to 5.24V. If the system is just started and before VDD reaches 8V±3%, the undervoltage protection circuit is in a locked state and the gate driver does not work; when the system is shut down and drops to 7V+3%, the undervoltage protection circuit is in a locked state. status, the gate driver also does not work. This effectively avoids the possibility of the MOSFET switch tube operating in the maximum amplification state, and improves the working life of the switch tube and the reliability of the system. In addition, the SHUTDWN terminal reserves a registered mail input terminal for developers. This terminal does not work when the voltage is lower than 0.8V; and when the voltage is greater than 3.3V (relative to the 5V logic voltage), the gate driver is immediately turned off.

3 APFC design points of MSC60028

The actual application circuit of MSC60028 power factor control chip in APFC circuit is shown in Figure 3. The main technical conditions of this system circuit are:

●Input grid voltage range: AC90V～265V;

●Output DC voltage: DC390C±5%;

●Output power: 400W.

According to the above requirements, the main component parameters of the APFC circuit should be calculated first.

3.1 Design and selection of controller peripheral circuit parameters

a. Design of error amplifier integrating capacitor

The external capacitors at the input and output ends of the error amplifier together with the input impedance of the error amplifier can form an integrating circuit. The value of the capacitor is given by the following formula:

C=1/(2πfR‖A‖)

In the formula: R=200kΩ, which is the input impedance of the error amplifier;

‖A‖=150, the closed gain of the error amplifier, f is the pole frequency, usually the selection range is several Hz.

For example: f=1Hz, it can be calculated from the above formula: C≈4.0(nF), so take C=4.7nF

b. Design of multiplier filter capacitor

The external capacitor at the output end of the multiplier and the output impedance of the multiplier form a low-pass filter. This filter does not attenuate signals around 100Hz. It mainly filters high-frequency PWM components. The value of the capacitor is given by the following formula:

CF=1/(2πfRo)

In the formula, f is the oscillation frequency of the oscillator, usually 10kHz; Ro is the output impedance of the multiplier, usually about 300Ω;

It can be calculated from this: CF=53 (nF)

Take CF=63nF

c. Design of rectified input voltage and DC output voltage detection resistors

Since the detection terminals of these two voltages are both connected to the high-voltage terminal, a voltage dividing circuit is used to sample, and then sent to the MSC60028. The voltage dividing value is generally selected to be ≤VD2/2. If the working voltage is 10V, the divided voltage value should be around 5V. On the premise that the voltage dividing resistor meets the design power requirements, wirewound resistors with higher precision should be selected. In the circuit of Figure 3, R1=R2, R3=R4, and their value relationship is given by the following formula:

U=VmaxR1/(R1+R3)

In the formula: U is the divided voltage, the value should be around 5V; Vmax is the maximum output voltage, and it can be known from the design conditions: Vmax≈400 (V)

If R3=R4=1.0MΩ, then R1=R2≈12658Ω, so R1=R2=12kΩ.

3.2 Design and selection of power components

a. Peak current in inductor L

The peak current in the inductor L can be determined by:

ILP=2(2)1/2Po/ηVAC(L)

In the formula: Po is the required output power; eta is the efficiency of the converter; VAC (L) is the lowest grid input voltage;

If eta=0.92, then: ILP should be 3.7 (A).

Considering that the withstand voltage of the switch tube should be derated by 75%, if the output voltage of the boost converter is 390V, the selected withstand voltage should be at least 500V. In addition, a switch tube larger than the peak inductor current should be selected, and IRF460 can be used.

b. Boost inductor L

When the input grid voltage range is AC 90V ~ 265V, the value of the switching pulse period t is 40μs. At this time: an inductor of about 245μH should be selected.

c. Freewheeling diode D

While the voltage and current of the freewheeling diode connected to the boost inductor meet the above requirements, the reverse recovery time of the switching cycle is less than 1%, so U1550 fast recovery diode is selected as the freewheeling diode.

3.3 Design of auxiliary power supply and input protection circuit

a. Design of auxiliary power supply

In the circuit of Figure 3, adding an auxiliary winding to the boost inductor can provide auxiliary power for the circuit. The number of turns of the auxiliary winding is determined by the number of winding turns of the boost inductor and the output voltage of the boost converter.

Rx in the circuit is the starting resistor. There is no voltage on the boost inductor at the moment of power-on. At this time, Rx provides the starting current to charge the energy storage capacitor. When the voltage at both ends of the energy storage capacitor is charged to a value, the MSC60028 starts to work. , the PFC controller has a PWM signal output, and the auxiliary winding starts to provide power to the MSC60028.

b. Design of input protection circuit

Since the boost converter is followed by a large filter energy storage capacitor, a large lightning current will flow into the power factor correction circuit at the moment the power is turned on, which may cause damage to the rectifier circuit, current detection circuit and control circuit. Damage to the chip. An effective solution is to connect a diode in parallel to the current detection resistor Rs (for different design solutions, an additional diode can be added appropriately). This can prevent excessive inrush current from generating overvoltage on Rs and burning out the control circuit.

4 Conclusion

The author used MSC60028 power factor control chip and MC33157 control/driver chip to design a 400W high-performance, high-power AC electronic ballast. Since the front stage of the system adopts power factor correction technology, its startup and energy-saving effects are ideal, and the waveform distortion is small, thus effectively reducing harmonic pollution to the power grid. In addition, the APFC system composed of the MSC60028 power factor control chip also has the advantages of simple circuit structure, small size, stable and reliable operation, etc., so it has broad application prospects in medium-power APFC circuits.