Realizing Power Factor Correction Using DSP-EEWORLD

Collect

1 Introduction

According to the provisions of the national "Regulations on the Administration of Compulsory Product Certification", "Regulations on the Administration of Compulsory Product Certification Marks", and "Catalogue of the First Batch of Products Implementing Compulsory Product Certification", from May 1, 2003, products listed in the "Catalogue" that have not obtained a compulsory product certification certificate and have not been affixed with the China Compulsory Product Certification Mark shall not be sold, imported, or used in business activities. Air conditioners are also included in the "Catalogue". In order for variable frequency air conditioners to pass the China Compulsory Certification (abbreviated as "3C"), the harmonic current must meet the requirements of the national standard GB17625.1-1998 "Limits of harmonic currents emitted by low-voltage electrical and electronic equipment (equipment input current per phase ≤ 16A)". At present, most of the variable frequency air conditioners in China use the method of adding inductance to the DC path and connecting capacitors in parallel. This method improves the power factor, but the harmonic current still cannot meet the requirements of the national standard. The use of active power factor correction (APFC) technology can effectively solve this problem. This article introduces the use of digital signal processors (DSP) to achieve full digital control of APFC.

As we all know, since the current at the power input end of the bridge rectifier capacitor filter circuit is no longer a sine wave, but a discontinuous spike pulse with a high amplitude, its fundamental component is very small, while the high-order harmonics are very rich, the total harmonic content is considerable, and the power factor of the circuit is very poor. The country is increasingly restricting the entry of low power factor equipment into the market. In the past decade, many companies have developed many special components to solve the APFC problem, which will increase the cost of the equipment. Now, a low-cost DSP has been developed. DSP has many functions, such as ADC (analog-to-digital converter), PWM (pulse width modulator), PIO (programmable input and output port), so that a lot of real-time control can be completed and the above-mentioned special PFC components can be replaced. In other words, the entire solution uses digital control, saves components, provides more flexibility, and can achieve nonlinear and complex control. And all of this does not require hardware changes.

2 Power Factor

Line power factor is defined as the ratio of active power to apparent power, that is,
PF=P/S=P/Vrms×Irms
, where P is active power (W) and S is apparent power (VA)
. The PF value is between 0 and 1. If the load is a purely resistive load, the active power is equal to the apparent power, and PF is equal to 1; otherwise, PF is less than 1. Power factor control is to find a way to make PF equal to 1.

The input power of the electronic device
Pin(t)=∫Vin(t)×Iin(t)×dt
If Vin(t)=Vi×Sin(ωt), and PF=1, then
Iin(t)=Iin×Sin(ωt)
Where Iin=iL=γio·Vin (γio is a ratio), then
Iin(t)=(γio·Vin)·Sin(ωt)

3 DC-DC Converter

For PFC, there are many different circuit topologies. Each DC/DC converter has two basic operating modes: DCM (discontinuous operation mode) and CCM (continuous operation mode). In discontinuous operation mode, the inductor current will reach zero in each PWM cycle; while for continuous operation mode, the inductor current never reaches zero in each PWM cycle. Discontinuous mode converters are suitable for low power applications, while continuous mode converters are suitable for medium and high power applications. In our PFC application, the boost converter adopts average current mode control.

In order to control PFC, a suitable PFC model is required to complete the control algorithm. Here is a simple model of a boost converter operating in continuous mode. The results are as follows:
Gid(s)=iL(s)/d(s)≈Vo/(S·L)
iL(s) is the change in inductor current d(s) is the change in duty cycle
Vo is the output voltage L is the inductor size

Before entering CCM, the boost converter operates in the DCM stage. The transfer functions of the two stages are different, but due to the low gain, there will be no problems before reaching stability. Therefore, Gid represents the useful transfer function to adjust the current control loop. For the voltage control loop, the feedback compensation network of the output voltage is as important as the feedback current loop. Now, an equivalent, small signal analysis is performed for a complete half cycle of the input sine wave voltage. Assuming that the input current is controlled, tracking the input voltage waveform after voltage division, the result is as follows:
Gvo(s)=vo/vc=gc·(ro//Ro)/(1+S·Co·ro//Ro)

Therefore, Gvo(s) represents a useful transfer function to regulate the voltage control loop.

[page]4 Digital Control

Digital control uses two PI (proportional integral) controllers, one for current loop control and one for voltage loop control. As long as there is a current loop, it must have a current reference signal
Iref(t)=|γio·Vi·Sin(wt)|

Iref(t) The power frequency is 2ω (100Hz).

In order to achieve the purpose of PFC, the current loop control bandwidth is more suitable for 1.5KHz~2.5KHz. Here, there are two issues worth noting. First, the value from the current sensor does not need to be low-pass filtered, because this will cause an unacceptable lagging phase shift and may cause unstable operation. Secondly, before entering CCM, the boost converter operates in DCM mode. The two modes have different transfer functions, that is, two different dynamics. However, due to the lower gain in DCM, there is no instability problem. Based on this idea, a PI controller is sufficient for the current control loop, and it is recommended to implement the integral part, so the dynamic characteristics are improved. The selection of proportional gain and integral gain must ensure that a bandwidth of 1.5KHz to 2.5KHz is obtained.

Finally, from the measured input current and voltage waveforms, it can be seen that the current and voltage are consistent in phase and magnitude, so the power supply load appears to be purely resistive (PF→1), which satisfies Iin=iL=γio·Vin.

The current control loop must track the current reference waveform, and the obtained inductor current iL must be as close as possible to Iref. As for the voltage control loop, a bandwidth of 10Hz to 30Hz must be guaranteed.

The control algorithm described above has been implemented on the variable frequency air conditioner. The relevant electrical parameters are as follows:

input voltage Vin=160～260Vrms, output voltage Vout=385V, maximum output power Po=2400W, boost inductor L=0.6 mH, output capacitor Co=1000 uF, switching frequency f=20KHz.

ADMC328 is a fixed-point DSP launched by AD company, with a single-cycle instruction execution time of 50ns. ADMC328 has three-phase 16-bit PWM for motor control, two independent 8-bit auxiliary PWM, frequency range 39KHz ~ 10MHz, the frequency value of auxiliary PWM is set by a dedicated register. In PFC application, we choose the switching frequency as 20KHz, AD converter to collect current value (SHUNT), sampling frequency 4.2KHz, 8bit accuracy. What is special about this system is that there is no association between ADC and auxiliary PWM, in other words, ADC and auxiliary PWM are completely independent in operation. Therefore, we can separate the main program (such as motor control) and execute the PFC program separately. The PFC program is executed by each PWMSYNC interrupt. The current value and voltage value are obtained by the AD converter. All these signals are sufficient to control the entire PFC. And the price of ADMC328 is less than 3 US dollars.

5 Separate parts instead of modules

At present, there are two solutions for implementing PFC circuits: modular and separate parts. In the modular solution, the driver part is packaged together with the boost diode and MOSFET (or IGBT), which has small stray inductance, small switching peak voltage, and high reliability, but the cost is also high. In addition, due to the lack of a buffer circuit, the switching loss is large and the EMI is also relatively large. Considering the low cost of variable frequency household appliances, we adopt a separate component solution and a lossless buffer circuit composed of LC passive components and fast recovery diodes (as shown in Figure 1). This LC circuit changes the switching transition process of the switch tube, so that the change of the switching voltage and current is not abrupt (i.e. hard switching) but slow (i.e. soft switching), thereby significantly reducing the switching loss and improving the reliability of the circuit.
DSP

Figure 1 Lossless buffer power factor correction circuit

参考文献：

1 Analog Devices INC.,ADMC328 28-Lead Rom-Based DSP Motor Controller,Data Sheet,(1999)

2　De Mari Yves,Motion Control Group,Milan Lab,EASY POWER FACTOR CORRECTOR USING A

Reference address：Realizing Power Factor Correction Using DSP

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