EE05886B1 - Circuit for power factor correction of an AC/DC converter - Google Patents

Abstract

The present invention comprises a power converter with at least one inductor on the input and rectifier system that transform AC voltage to a unipolar DC pulse voltage that filtered with output capacitor. The PFC DC-DC unit can provide PFC and galvanic isolation in one conversion stage. The power converter can implement FET, IGBT, GAN, BJT, JTO and diodes as solid-state switches and solid-state rectifiers to enable the required current consumption. AC current and voltage as well as output voltage are measured by analog to digital converter. AC voltage further is used for forming reference current with PLL block and sine block. For reconfigurable converters a mode selector block defines feedforward gain according to current operation mode.

Description

Technical field

The present invention belongs to the field of control of active rectifiers with power factor correction (PFC) of an AC/DC converter, and more specifically, to the field of active rectifiers with unidirectional or bidirectional smooth switching. The main fields of application of the present invention are charging systems for electric vehicles and power supply systems for electrical appliances.

State of the art

PFC requirements for AC are given in the European standard EN6 1000-3-2. This standard requires a power factor close to unity for power levels greater than 100 W. The PFC function is an integral part of most active rectifiers. It can be implemented as an additional module or as an integrated function in a single-stage active rectifier. Conventional power factor correction algorithms are based on PI (Proportional-Integral) regulators, which are simple but do not always allow the required THD (Total Harmonic Distortion) value to be achieved, causing significant current distortion, especially at zero crossings. The reason for this undesirable phenomenon is both the limited gain of the controller and the delay due to the sampling step. It is known that the sampling delay can be reduced by increasing the switching frequency, but this also increases the switching losses. US 9054597 B2, 09.06.2015, Silergy Semiconductor Technology (CN) describes a boost PFC converter that uses frequency modulation to control a sampling delay. The boost PFC controller includes a non-conducting state signal generator for comparing a choke current with a first control signal, a conducting state signal generator for comparing a second control signal having a first proportionality coefficient, a peak value for comparing an off-time value of said power switch with an external proportionality coefficient of said power switch duty cycle, and a logic circuit connected between the signal generator and

with an external time generator, whereby the logic circuit turns the power switch on when the turn-on signal is active and turns the power switch off when the turn-off signal is active. Simultaneously with the sampling delay, there are distortions in the form of the PFC input current, which are caused by the limited static gain of the PI controller, the evaluation of which is not included in the description of the control method. In addition, setting signals are created based on the values of the analog component, which also transform the form of the current and complicate the digital implementation of the PFC controller.

These distortions can be reduced by post-processing the PI controller signal in a microprocessor as described in US 7821237 B2, 26.10.2010, Cirrus Logic, Inc., where the switch and the choke are combined to form a pulse mode amplifier stage for receiving the rectified line input voltage and generating the connection output voltage; the pulse mode amplifier stage current is measured and a target current is generated that is proportional to the rectified line input voltage, and the desired current value and the measured current value are compared and based on this comparison, a two-level current comparison result signal is output, and in response to the two-level current comparison result signal, a switch control signal is generated that has a cycle adapted to control the switch so that the sensed current is approximately proportional to the rectified line input voltage. The inventive idea of using a state machine to implement PFC control is an equivalent solution to relay control, but due to digital control, a significant response delay is introduced and at the same time it does not allow for flexible modification of the control algorithm, which is potentially possible with microprocessor systems.

Since the proportional gain kp and the integral gain ki depend on the output power of the PFC converter, it is proposed in US 8797004 B2, 5.08.2014, TDK-Lambda UK Limited, to experimentally modify the operation of the regulator and record the mode parameters in a reference table for further use in real-world situations. Due to the variable setpoint in PFC applications, the integral part of PI regulators has limited efficiency and as a result, conventional

Feedback PFC applications have a relatively high distortion factor, which can be reduced through feedforward control. The controller parameters are changed repeatedly with a long transient process, which degrades the dynamic performance of the PFC under operating conditions.

For example, US 2005/0270814 A1, 8.12.2005 describes a feedforward control for improving the duty cycle value during sudden changes in input voltage. This use of the controller is strongly related to the specifics of the FMS7401 microcontroller, where the feedforward block is claimed but not described in the invention, the PI controller parameters are fixed and their values are not estimated.

Pure feedforward control is described in US 2009/0015214 A1, 15.01.2009, Hung-Chi Chen, but it is not efficient because it requires an accurate model of the PFC converter, which is not always possible, and any variation or noise in the measured values can cause a large error.

The closest solution is described in US 7359224 B2, 15.04.2008, International Rectifier Corporation and shown in Figure 1 of said patent. The PFC converter consists of a boost converter having a boost inductance and a PFC switch in series with the boost inductance and the boost inductance and the PFC switch connected through the output of a rectifier that receives AC power from the AC line and the boost converter further comprises a diode connected to a node between the choke and the switch, the output of the diode is connected to an output capacitor, the DC link voltage flows through the output capacitor and the system further comprises a control circuit that receives as an input the rectified AC voltage from the rectifier, a signal proportional to the current through the choke and the DC link voltage through the capacitor, and the control circuit provides a pulse width modulation (PWM) signal for controlling the turn-on time of the power factor PFC switch and comprises in addition: a voltage regulator that provides a voltage signal regulated based on the set voltage and the DC link voltage; a multiplier that is capable of providing a regulated voltage signal

for multiplying the rectified AC input voltage to obtain a current setpoint signal; a current regulator that receives the current setpoint signal and a signal proportional to the choke current, the current regulator further comprising: a different device used to subtract a signal proportional to the choke current from the active setpoint signal, a PI controller adapted to receive the output of the different device and provide a first control signal; a feedforward device that can be used to receive the rectified AC input voltage and generate a second control signal, the second control signal having a smaller dynamic range than the AC voltage; an adder that can be used to add the first control signal to the second control signal to obtain a PWM setpoint signal; a PWM signal generator that can be used to generate a PWM signal for controlling the conduction time of the PFC circuit, the PWM signal generator using the PWM setpoint signal to generate the PWM signal.

The above solution describes a combined feedback-feedback control method used to amplify a PFC converter, improving the converter dynamics and reducing the THD value of the input current.

Conventional control solutions for PFC converters based on PI controllers have significant inertia, since the error reduction does not start immediately after the occurrence of disturbances, but after a significant deviation of the current from the setpoint. In addition, the integral part has a limited effect on the reduction of the error of a variable setpoint signal due to its inertia. US 7 359224 B2 considers that the main disturbing factor is the change in the input voltage νin according to the law vin = Asin(ωt), which is easy to foresee when designing the feedforward connection. This allows calculating the feedforward value of the fill factor DFF as a function of the input voltage vin and the output voltage vout, which can usually be considered a constant value Vout:

DFF =f(vin, vout), (1) where DFF is the fill factor

vin input voltage

vout output voltage,

which allows the error to be significantly reduced.

The residual error was eliminated with a conventional PI controller, but it is not explained how to adjust the gain factors of the P and I parts kp and It is known that the parameters kp and ki have limitations that are limited by the stability conditions and dynamics of the converter. Thus, their nominal values kp(nom) and ki(nom), which satisfy the minimum error, are a function of the converter topology, the value of its parameters and the environmental conditions. Thus, in the case of a converter operating under dynamically changing conditions, for example, a PFC converter, the values kp(nom) and ki(nom) have also changed and should be calculated dynamically. Thus, the use of a PI controller gives modest results for PFC converters with a fixed gain factor.

The essence of the invention

The invention is a circuit for correcting the power factor of an AC/DC converter, wherein the circuit is connected to an alternating current voltage source. The circuit includes a power converter connected to an output capacitor and a current sensor. The circuit includes a control circuit, the input of which is a rectified signal of the alternating current input voltage of the choke current sensor, which is proportional to the current. The signals are fed to an analog-to-digital converter, which is connected to a phase-locked circuit, in which an initial phase for the reference circuit is formed. The reference circuit is connected to the first input of a first output multiplier. A signal is fed to the second input of the first output multiplier from the output of a first PI controller, which forms a control voltage signal and is based on the difference between the output voltage and the set voltage obtained by the first differentiator and the ramp circuit, wherein the set current output signal of the first output multiplier is subtracted from the measured current by the second differentiator and is fed to the second PI controller. A first control signal is formed in the second PI controller, which is fed to the first input of the adder. In the feedforward block, a second control signal is formed based on the input voltage, which is fed to the second input of the adder and then

The control signal is fed to a modulator, where a control signal is formed for the transistors of the power converter. The power converter comprises at least one transistor and one choke. The signal from the second differential device is connected to a model unit, which defines the gain of the proportional and integral parts of the second PI controller based on the parameters of the converter and is limited by the control signal of the input of the second PI controller, the input voltage, the output voltage and the input current. In the feedforward block, a second control signal is additionally formed using the output voltage and the input current, and a sequence of high or low pulses is formed in the modulator depending on the input signal.

According to one embodiment of the invention, the signal from the first output multiplier is corrected by a second multiplier depending on the ambient temperature.

According to one embodiment of the invention, the transfer function of the feedforward block is defined by a model selection block.

According to one embodiment of the invention, the reference current is defined by the signal iref.

In the proposed solution, based on a mathematical model and measured currents, the voltages of the PFC converters and the environmental parameters dynamically determined the maximum value of the proportional part, since this period had the greatest impact on reducing the error when using a variable setpoint:

kp(nom) f=f(vin, vout, iin, L, R, С, T), (2) where

kP(nom) is the nominal gain of the proportional part,

vin is the input voltage,

vout is the output voltage,

iin is the input current,

L inductance,

R resistance,

C capacitance and

T temperature,

whereas the gain factor ki of the integral part is intended to eliminate residual error caused by temperature fluctuations, measurement error and other secondary factors.

For example, a conventional boost PFC voltage converter can be described by the differential equation [7]:

Where

vg(n) is the voltage,

Ts is the temperature,

D(n) fill factor, and

The current iref(n ) represents the choke current iL.

3) a small improvement that introduces the measured current value iL(n) allows for a more robust coupled feed-forward control structure:

(4) The first component is a feedforward block corresponding to the duty cycle of an ideal boost converter and defines the feedforward estimate of the duty cycle DFF: while the second component appears as a change in the conventional proportional part of a PID controller. The difference between the reference current iref(n+1) and the choke current iL(n) in the second part can be defined as the control error e(n+ 1):

e(n) = iref(n+1)-iL(n), (5) and other factors than the nominal gain of the proportional part kp(nom):

Then (4) can be simplified, giving:

D(n) = DFF ( n ) + (7) In reality, the proportional part gain factor may differ from the nominal value kP(nom) and therefore the generalized controller expression is as follows:

D(n) = DFF (n) + kp (n)e(n ),

(8) where kp= kp(nom)·k*, k*∈ [0; ∞ .

An embodiment of the invention is shown in Figure 2 and includes a power converter with at least one choke at the input and a rectifier system that converts alternating current into a unipolar direct current pulse voltage that is filtered by an output capacitor. The PFC DC conversion stage can provide PFC and galvanic isolation in one conversion stage. The power converter can use FET, IGBT, HEMT, BJT, GTO and diodes as semiconductor switches and semiconductor rectifiers for the necessary current consumption. The alternating current and voltage and the output voltage are measured by an analog-to-digital converter 5. The alternating voltage is additionally used to form the control current by a PLL (Phase-locked loop) block 6 and a reference block 16. In addition, the alternating current, alternating voltage and output voltage are used in the feedforward block 14 in the original equations (1) and (2) of the mathematical model of the converter. In addition, the alternating current and output voltage are used in the calculation of the gain factor of the proportional kp and integral ki part in the model unit 18 of the second PI controller 12. To protect against overheating, the current setpoint signal is scaled by the temperature controlled amplifier 17. For reconfigurable converters, the mode selection block 19 defines the feedforward gain factor according to the active operating mode.

Another embodiment of the invention is shown in Figure 3. In this embodiment, the voltage control circuit with the ramp block 7, the first differentiator 8 and the first PI controller 9 is removed and instead the signal iref is used to determine the set current. Such an embodiment is used for charging devices where the output voltage is for constant current mode or for generating a constant current voltage for charging. The correct charging mode is determined by the mode selection block 19.

List of drawings

The invention is described in more detail below with reference to the drawings, in which:

Figure 1 shows a schematic diagram of a boost PFC converter with an integrated feedforward and feedback control circuit based on a PI controller (prior art).

Figure 2 shows a schematic diagram of a combined PFC converter with a combined feedforward and feedback control circuit based on a modeling unit. Figure 3 shows a schematic diagram of a combined PFC converter without a feedback voltage control circuit based on a modeling unit.

Figure 4 describes the timing diagrams of the PFC converter.

Figure 5 describes the network flow shapes.

Examples of carrying out the invention

The operating mode of the proposed invention is considered based on the example of a step-up voltage converter shown in Figure 2.

A circuit for power factor correction of an AC/DC converter, comprising a connection to an AC voltage source 1, and a power converter 2 connected to an output capacitor 3 and a current sensor 4. The circuit for power factor correction (PFC) of an AC/DC converter comprises a control circuit, the input of which is a rectified AC input voltage from a rectifier, which is proportional to the current from a current sensor 4 through a choke. These inputs are connected to an analog-to-digital converter 5, which is connected to a phase-locked circuit 6, which forms the initial phase for a reference circuit 16, which is connected to the first input of the first multiplier 10, the second input of the first multiplier 10 is fed with a signal from the output of the first PI controller 9, which forms a control voltage between the output voltage of the signal and the set voltage obtained by the first differentiator 8 and the ramp circuit 7. The output signal of the set current of the first multiplier 10 is subtracted from the measured current by the second differentiator 11 and is connected to the second PI controller 12, where the first formed control signal is connected to the first input of the adder 13 and the second control signal is formed in the feedforward circuit 14 based on the input voltage and is connected to the second input of the adder 13, and then the control signal

connected to the modulator 15, which forms a control signal for the transistors of the power converter 2.

The circuit for correcting the power factor of an AC/DC converter is characterized in that the power converter 2 includes at least one transistor and one choke, the signal coming from the second differential device 11 is connected to a model unit 18, which is configured to determine the gain factor of the proportional and integral parts of the second PI controller 12 based on the converter parameters and to limit the input control signal, input and output voltages and input current of the second PI controller 12, the feedforward block 14 is configured to additionally form a second control signal together with the output voltage and input current, and the modulator 15 is configured to form a sequence of high or low pulses depending on the input signal.

The signal of the first output multiplier 10 is corrected by the second multiplier 17 depending on the ambient temperature. The transfer function of the feedforward block 14 is defined by the model selection block 19, and instead of the voltage control circuit through the ramp block 7, the first differentiator 8 and the first PI controller 9, the set current is defined by the signal iref.

In such a setup, the analog-to-digital converter reads the input voltage Vin, the input current Iin and the output voltage Vdc. The digital value of the output voltage Vdc_fdb is compared with the reference voltage Vdc_ref, which was created by the ramp block 7 according to Figure 4 a). After comparing these signals, a voltage error signal Δvd ,Vd is obtained, which is sent to the first PI controller 9. The input voltage Vin is used as a reference for a phase locked loop (PLL) to create synchronized input voltage Vin pulses. Unlike the prior art, the invention includes an additional reference block 16 to create a pure normalized sine wave inorm, which is synchronized with the input voltage, which in turn allows for obtaining an ideal control signal for the network current, Figure 4 b). This synchronized sine wave is multiplied by the output signal VAout of the first PI controller 9, which defines the amplitude of the network current and as a result, the network control current IrefPFC is obtained, Figure 4 c). When the power converter 2 heats up, the phase locked loop (PLL) generates pulses synchronized with the input voltage Vin.

in the solution above, the second multiplier 17 limits the set current at the level Iref_lim, Fig. 4 c). This current is compared with the input current Iin by the second differentiator 11 and thereby the current error ΔΔΙΑout is obtained, which is used at the input of the model unit (MU) 18, Fig. 4 d). The model unit limits the error ΔIАout at the levels ΔImax and ΔImin, determines the gain factor of the integral part of the proportional kp and ki based on the input current Iin and the output voltage Vout values, as well as the parameters of the inductance L1 and capacitance C1 of the power converter frequency f, which allow flexible operation of the converter and eliminate unstable effects during the transition. The processed signal is sent to the second PI controller 12 with the modified coefficients kp and k1. After integration and amplification of the output signal of the second PI controller 12, CAout forms the first fill factor D signal component, Fig. 4 e), while the second component is formed in the feedforward (FFD) block

by CFFD 14 based on the power converter control law, Fig. 4 f). The main advantage of the proposed invention is that the FFD feedforward block can change the control parameters using the mode selection block (MS - mode selection) 19 for reconfiguring and bidirectionally changing the structure of the power converter and for smoothly changing the control parameters under changing environmental conditions, for example, temperature. The total signal of the sum of САOut and CFFD is connected to the input of the modulator 15, which forms the power converter control signal uc, which, unlike the prior art device, may have a variable frequency, Fig. 4 j).

Figure 5 shows the timing diagrams for Vin = 230 V, Vout = 415 V, L1 = 700 μΗ, C1 = 440 μF f= 80 kHz, kp = 0.09; ki = 0.016.

The invention discloses a PFC converter control system based on an output voltage feedback control circuit and input current combined feedback and feedforward control circuits. The feedforward control circuit has a reconfigurable structure, while the feedback current control circuit is used with a model unit that defines the integral and proportional terms of a PI controller.

As a result, the control system allows for the reduction of current fluctuations, improving dynamics and increasing stability characteristics.

List of reference numbers

1 - AC voltage source

2 - Power converter

3 - output capacitor

4 - flow sensor

5 - analog-to-digital converter

6- phase lock block

7 -ramp block

8 - first differential gear

9 - first PI controller

10 - first output multiplier

11 - second differential device

12 - second PI controller

13 - adder

14 - relay block

15 - modulator

16 - reference block

17 - second multiplier

18 - model unit

19 - model selection block

Claims (4)

1. A circuit for correcting the power factor of an AC/DC converter, the circuit being connected to an AC voltage source (1) and the circuit comprising a power converter (2) connected to an output capacitor (3) and a current sensor (4), said circuit comprising a control circuit connected as an input through a choke from a current sensor (4) to a rectified signal of an AC input voltage which is proportional to the current, and all such signals being fed to an analog-to-digital converter (5) which is connected to a phase lock block (6) which forms an initial phase for a reference block (16) which is connected to a first input of a first output multiplier (10), and to a second input of the first output multiplier (10) a signal is connected from the output of a first PI controller (9) which forms a control voltage signal and is based on the difference between the output voltage and the set voltage obtained by a first differentiator (8) and a ramp block (7), the set current output signal of the first output multiplier (10) being subtracted measured from the current by a second differential device (11) and is connected to a second PI controller (12), where a first control signal is formed, which is fed to the first input of the adder (13), and in the feed-forward block (14) a second control signal is formed based on the input voltage, which is fed to the second input of the adder (13), and the control signal is fed to a modulator (15), which forms a control signal for the transistors of the power converter (2), characterized in that the power converter (2) includes at least one transistor and one choke, the signal coming from the second differential device (11) is connected to a model unit (18), with which the gain factor of the proportional and integral parts of the second PI controller (12) is defined based on the parameters of the converter and the input control signal, input voltage, output voltage and input current of the second PI controller (12) are limited, in the feed-forward block (14) a second control signal is formed using the output voltage and input current and in the modulator (15) there is a a sequence of high or low pulses formed from an input signal. 2. A circuit according to claim 1, characterized in that the signal from the first output multiplier (10) is corrected by the second multiplier (10) depending on the ambient temperature. 3. A circuit according to claim 1 and 2, characterized in that the transfer function of the feedforward block (14) is a defined model with a selection block (19). 4. A circuit according to claim 1, 2 and 3, characterized in that the control current is defined by the signal iref.