CN102307017A - Control method applied to active-clamp flyback miniature photovoltaic grid-connected inverter device - Google Patents

Abstract

The invention relates to a control method applied to an active clamp flyback micro photovoltaic grid-connected inverter device. The active clamp flyback micro photovoltaic grid-connected inverter device includes a flyback converter and a power frequency pole gender conversion circuit. In this device, the current reference is used to control the peak current of the primary side of the flyback, so that it outputs a half-wave sinusoidal current, and its output voltage is clamped by the grid voltage. When the instantaneous power is small, the flyback control method adopts the combination of constant frequency current discontinuous and variable frequency current critical continuous. When the flyback converter works in the variable frequency current critical discontinuous mode, the auxiliary switch tube is turned on for a period of time after the secondary current of the flyback converter reaches zero. And it can realize leakage inductance energy feedback and soft switching of the main switching tube under different instantaneous powers. Efficiency at full load conditions can be greatly improved.

Description

A control method applied to an active clamp flyback miniature photovoltaic grid-connected inverter device

technical field

The present invention is a control method applied to an active clamp flyback miniature photovoltaic grid-connected inverter device

Background technique

For photovoltaic power generation systems, there are two potential markets: megawatt-level large-scale photovoltaic power plants and small-scale building and household photovoltaic power generation systems. Micro photovoltaic grid-connected inverters have broad application prospects in the latter.

The main factors affecting the efficiency of the micro photovoltaic grid-connected inverter using flyback topology on the DC side are the leakage inductance of the transformer and the loss of the main switching tube, so further optimization of the two is required.

To deal with the leakage inductance energy of the transformer, passive snubber circuits and active snubber circuits can be used. For passive snubber circuits, such as RCD snubber circuits or LC snubber circuits, when the flyback converter works in the critical continuous conduction mode controlled by frequency conversion, the parameters of the passive components in the snubber circuit are difficult to tune.

When the input and output of the flyback converter are both DC and work in the constant frequency mode, the traditional active clamping technology can not only realize the absorption and feedback of leakage inductance energy, but also realize the zero-voltage switching of the switching tube. However, in the flyback micro photovoltaic grid-connected inverter, the output voltage of the flyback converter is clamped by the grid voltage, so it has a wide range of output voltage and works in variable frequency mode. In this case, the clamping circuit controlled by analog has been difficult to achieve the required effect.

Contents of the invention

The technical problem to be solved by the present invention is to provide a control method applied to an active-clamp flyback miniature photovoltaic grid-connected inverter device, which can realize clamping under the conditions of wide-range output voltage and instantaneous power changes. The zero-voltage switching of the main switching tube on the primary side of the clamp flyback micro-photovoltaic grid-connected inverter device, the energy feedback absorption of the transformer leakage inductance and the efficiency improvement at light load. For this reason, the present invention adopts following technical scheme:

The active clamp flyback miniature photovoltaic grid-connected inverter device includes a flyback converter, a power frequency polarity conversion circuit, and a controller, and the controller is used to control the flyback converter and a power frequency pole the operation of sex switching circuits;

The flyback converter includes:

An input port to accept the DC input voltage of the photovoltaic side;

One output port, which outputs a sine half-wave current, and its output voltage is clamped by the grid voltage;

A transformer, the excitation current of the primary side of the transformer is controlled by peak current, the peak value of the excitation current follows the current reference, and the output average current of the secondary side of the transformer is modulated into a sine half wave shape;

A primary side main switch tube, the primary side main switch tube is connected in series with the primary winding of the transformer, when the primary side main switch tube is turned on, the transformer stores energy; when the primary side main switch tube When it is turned off, the energy stored in the transformer is coupled to the secondary side, and released to the power grid through the power frequency polarity conversion circuit;

An active clamp circuit is provided in the flyback converter, and the active clamp circuit includes an auxiliary switch tube, an auxiliary diode and a clamp capacitor, and after the auxiliary switch tube is connected in parallel with the auxiliary diode Then form a series branch with the clamping capacitor, and connect the series branch in parallel at both ends of the primary side winding of the transformer in the flyback converter or at both ends of the primary side main switch tube;

Power frequency polarity conversion circuit, a full-bridge inverter circuit composed of four switching tubes, processes the sine half-wave current output by the flyback converter through polarity conversion, and incorporates it into the power grid to realize output sine wave grid current.

Further, the excitation current of the transformer works in the mode of combining discontinuous state and critical continuous state within half a power frequency cycle, that is, the working mode of the flyback converter combines discontinuous conduction mode and critical continuous conduction mode .

Further, the instantaneous value of the output voltage and input voltage of the active clamp flyback micro photovoltaic grid-connected inverter device at the current moment, the electrical angle of the grid voltage at the current moment, and the zero-crossing signal of the grid voltage are obtained, and combined with the current reference The peak value, thus obtaining the current reference, including the current reference of discontinuous conduction mode and critical continuous conduction mode:

Among them, I _o is the peak value of the current reference, V _g is the instantaneous value of the grid voltage (that is, the instantaneous value of the output voltage of the zero-voltage switch flyback micro photovoltaic grid-connected inverter device), and V _PV is the clockwise value of the photovoltaic side voltage (that is, the instantaneous value of the input voltage of the zero-voltage switch flyback micro photovoltaic grid-connected inverter device), θ is the electrical angle at the current moment, N is the transformation ratio of the transformer, and f _D is the discontinuous conduction mode The switching frequency of the main switching tube on the primary side.

Further, given a switching frequency threshold of the primary main switching transistor, when the switching frequency of the primary primary switching transistor is higher than the threshold, the flyback converter enters the discontinuous conduction mode, that is, the transformer The exciting current works in a discontinuous state, and adopts the current reference of the discontinuous conduction mode; when the switching frequency of the main switching tube of the primary side is lower than the threshold value, the flyback converter enters the critical continuous conduction mode, and current reference using the discontinuous conduction mode.

Further, in the discontinuous conduction mode, the turn-on of the main switch on the primary side is determined by a fixed switching period, and the turn-off of the main switch is determined by the current reference in the discontinuous conduction mode;

In the critical continuous conduction mode, the turn-on of the main switch on the primary side is determined by the turn-off of the auxiliary switch, that is, after the auxiliary switch is turned off, the main switch on the primary side is turned on after a dead time. The turn-off of the primary primary switch is determined by the current reference in the critical conduction mode.

Further, in the critical continuous conduction mode, the auxiliary switching tube is turned on for a period of time after the secondary current of the flyback converter reaches zero, and the turn-on time is calculated by the following formula:

${\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; aux</span>}}<span\; class="notranslate">\; \&Greater\; Equal;</span>\frac{\sqrt{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}}{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}<span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; PV</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; ]</span>$

Wherein, L _k is the primary side leakage inductance of described transformer, C _r is the drain-source parasitic capacitance of described primary side main switching tube, K _p is the ratio of described transformer primary side excitation inductance and primary side leakage inductance;

The controller obtains the moment when the secondary side current of the flyback converter reaches zero, gives the turn-on signal of the auxiliary switch tube, calculates and counts the conduction time at the same time, and gives the Auxiliary switch off signal.

Further, the controller obtains the grid voltage zero-crossing point, and provides the power frequency driving signals of the four switch tubes in the power frequency polarity conversion circuit; the pair of tubes are driven in the same phase, and the upper and lower tubes are driven in reverse phase.

The controller can adopt a digital chip and its peripheral circuit, which controls the flyback converter and the power frequency polarity conversion circuit, so that the flyback converter and the power frequency polarity conversion circuit work according to the above method.

The control method applied to the active-clamp flyback miniature photovoltaic grid-connected inverter device of the present invention is aimed at the special application under the condition of wide-range output voltage and constant change of instantaneous power. When the instantaneous power is small, the frequency of the main switching tube on the primary side will become very high, and the combination of the fixed frequency current discontinuous conduction mode and the variable frequency current critical continuous conduction mode greatly improves the efficiency at light load; When the flyback converter works in the variable frequency current critical continuous conduction mode, the auxiliary switch tube is turned on for a period of time after the secondary current of the flyback converter reaches zero. The leakage inductance energy feedback and the soft switching of the main switching tube of the primary side can be realized under different voltage and instantaneous power. Efficiency at full load conditions can be greatly improved. The parasitic oscillation caused by the leakage inductance is suppressed, and the EMI characteristics of the circuit can also be improved.

Description of drawings

Fig. 1 is the structural representation of the active clamp flyback miniature photovoltaic grid-connected inverter device applying the control method of the present invention;

Figure 2 is a waveform diagram of each signal in the power frequency cycle;

Fig. 3 is each signal wave diagram of high-frequency cycle;

Detailed ways

The present invention will be described in detail below in conjunction with the accompanying drawings.

FIG. 1 is a schematic structural diagram of an active clamp flyback miniature photovoltaic grid-connected inverter applying the control method of the present invention. 10 is an active clamp flyback photovoltaic grid-connected inverter device; 101 is a solar panel; 102 is a flyback converter; 1021 is an active clamp circuit in a flyback converter; Figure 103 is a power frequency pole Figure 104 is the grid side filter circuit; Figure 105 is the FPGA control circuit, that is, the controller, that is, the above; V _PV - photovoltaic side DC input voltage; V _g - grid side voltage; T _r - Transformer of the flyback converter; S _m - primary switch tube; S _a - auxiliary switch tube; D _a - auxiliary diode; C _clamp - clamping capacitor; D _s - output rectifier diode on the secondary side of the flyback converter; S ₁ ~ S ₄ - the switch tube of the polarity conversion circuit; G _sm - the drive signal of the primary switch tube output by FPGA; G _{sa -} the drive signal of the auxiliary switch tube output by FPGA; G _s1 ~ G _{s4 -} the drive signal output by FPGA The driving signal of the switching tube in the polarity conversion circuit.

The active clamp flyback miniature photovoltaic grid-connected inverter device mainly includes a solar photovoltaic array 101 , a flyback converter 102 , a polarity conversion circuit 103 , a grid-side filter circuit 104 , and an FPGA control circuit 105 .

The switch tubes in the polarity conversion circuit 103 may use four thyristors, or two thyristors and two semiconductor field effect transistors.

In the flyback converter 102, the terminal with the same name of the primary winding of the transformer T _r is connected to the positive pole of the DC input voltage of the photovoltaic side, and the non-identical terminal of the primary winding of the transformer T _r is connected with the drain of the main switch S _a of the primary side, The source of the main switching tube S _a on the primary side is connected to the negative pole of the DC input voltage on the photovoltaic side, the non-identical end of the secondary winding of the transformer T _r is connected to the anode of the secondary output rectifier diode D _s , and the secondary winding of the transformer T _r The end with the same name of the grid side filter circuit 104 is connected, the cathode of the rectifier diode D _s is connected to the other input terminal of the grid side filter circuit 104, the source of the auxiliary switching tube S _a is connected to the primary winding of the transformer T _r The terminal of the same name is connected, the drain of the auxiliary switch S _a is connected to one end of the clamp capacitor C _clamp , and the other end of the clamp capacitor C _clamp is connected to the drain of the primary switch S _a .

The FPGA control circuit 105 respectively collects the instantaneous value of the output voltage and input voltage of the zero-voltage switch flyback micro photovoltaic grid-connected inverter device at the current moment, the electrical angle of the grid voltage at the current moment, and the zero-crossing signal of the grid voltage, combined with the MPPT The peak value of the current reference obtained by the algorithm, according to the formula

The obtained digital signal of the current reference is output by the FPGA control circuit 105 and sent to the flyback converter after digital-to-analog conversion. The obtained current reference waveform is shown in FIG. 2 . The current reference is synchronized with the grid voltage to ensure that the grid-connected current can be in phase with the grid voltage, eliminating the need for a current loop and simplifying control.

For primary side current acquisition, as shown in Figure 1, the collected signal is used as the positive input of the comparator, and the current reference signal output by the FPGA control circuit 105 is used as the negative input of the comparator. The turn-off signal of S _m is shown in Fig. 2 as the waveform diagram. The turn-on of the primary switch S _m is determined by the following two conditions:

(1) The FPGA control circuit 105 counts the switching cycles of S _m of the main switching tube on the primary side, and when the switching frequency exceeds the maximum allowable switching frequency (threshold), the flyback converter 102 enters the constant-frequency current discontinuous conduction mode , at this time, the switching cycle of the flyback converter 102 is limited to a fixed value, and the FPGA controller 105 counts the switching cycles of the main switching tube S _m of the primary side, until the specified switching cycle is counted, the main switching tube S _m of the primary side conduction.

(2) When the switching frequency of the main switching tube S _m on the primary side is lower than the maximum allowable switching frequency (threshold), the flyback converter 102 works in the frequency conversion current critical continuous conduction mode. After the auxiliary switching tube S _a is turned off, delay After a period of dead time, the main switch S _m is turned on.

The switching point of the two working modes of the flyback converter 102 changes with the change of the input power of the photovoltaic side, which is programmed by the FPGA controller 105 .

When the flyback converter 102 works in the variable frequency current critical continuous mode, the auxiliary switching tube S _a starts to work, and the sampling secondary current is compared with zero. When the secondary current reaches zero, the corresponding input signal of the FPGA controller 105 is set to "1". , FPGA controller 105 receives the set "1" signal, turns on the auxiliary switch S _a , the turn-on time of the auxiliary switch S _a is calculated by the FPGA, counts the turn-on time T _au , and turns off the auxiliary switch S _a . The calculation formula of the turn-on time T _au of the auxiliary switch tube S _a is as follows

${\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; aux</span>}}<span\; class="notranslate">\; \&Greater\; Equal;</span>\frac{\sqrt{{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; k</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}}}}{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; p</span>}}}<span\; class="notranslate">\; [</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; PV</span>}}}{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; g</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&theta;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; ]</span>$

The FPGA control circuit 105 collects the grid voltage zero-crossing point, and provides the power frequency driving signals of the four switch tubes in the power frequency polarity conversion circuit 103, driving the pair of tubes in the same phase, and driving the upper and lower tubes in reverse phase.

When the flyback converter 102 works in the critical continuous conduction mode, the active clamp circuit starts to work to suppress the drain voltage spike caused by parasitic parameters, absorb and feed back leakage inductance energy, and realize zero-voltage switching of the main switching tube, thereby The efficiency of the system is improved, and the EMI characteristics of the circuit can also be improved. The specific working process is shown in Figure 3.

At time t ₀ , the main switching tube S _m is turned on, the auxiliary switching tube S _a is in the off state, the primary current rises, and the energy is stored in the excitation inductance of the transformer.

At time _t1 , the primary current reaches the current reference, and the main switch S _m is turned off. At this time, the primary current charges the resonant capacitor C _r . Because the resonant capacitor is small, the drain-source voltage of the main switch increases linearly.

During the period from t ₂ to t ₃ , the anti-parallel diode of the auxiliary switch tube S _a is turned on, and the energy stored in the leakage inductance L _k is absorbed by the clamp capacitor C _clamp , which suppresses the drain voltage spike and improves the EMI of the circuit characteristic. When the voltage at both ends of the excitation inductance L _m reaches the voltage V _g sin(ωt)/N that the secondary side voltage is coupled to the primary side, the secondary side rectifier diode D _s is turned on.

At time _t3 , the primary current drops to zero, and the energy stored in the transformer is fully coupled to the secondary side of the flyback converter.

At time t ₄ , the secondary current of the flyback converter drops to zero. At this time, the auxiliary switch is turned on, the clamp capacitor voltage reversely excites the leakage inductance, and the secondary winding is forward biased, and the secondary rectifier diode continues When it is turned on, the leakage inductance energy absorbed by the clamp capacitor is coupled to the secondary side. At this stage, both the leakage inductance current and the excitation inductance current increase in reverse, making preparations for the zero-voltage turn-on of the main switch.

At time _t5 , the auxiliary switching tube is turned off, and the resonant capacitor C _r is discharged with the help of the negative leakage inductance current. At this stage, the leakage inductance current is smaller than the excitation inductance current, so the secondary rectifier diode is still in the conduction state.

From t ₆ to t ₇ , because the leakage inductance is very small, the leakage inductance current rises to the excitation inductance current, the secondary rectifier diode is turned off, and the resonant capacitor is discharged through the leakage inductance and excitation inductance.

At time _t7 , the drain-source voltage drops to zero, and the anti-parallel diode of the main switch is turned on, and the zero-voltage turn-on of the main switch can be realized by turning on the main switch before the polarity of the excitation inductor current changes.

Claims (7)

1. control method that is applied to the miniature photovoltaic combining inverter device of active-clamp inverse-excitation type; The miniature photovoltaic combining inverter device of said active-clamp inverse-excitation type comprises an anti exciting converter and power frequency polarity switching, controller; Said controller is used to control the work of an anti exciting converter and a power frequency polarity switching, it is characterized in that:

Said anti exciting converter comprises:

An input port is accepted photovoltaic side DC input voitage;

An output port, output half-sinusoid electric current, its output voltage is by the line voltage clamper;

A transformer, the former limit exciting curent of said transformer adopts peak current control, and its exciting curent peak value is followed said current reference, and transformer secondary output average current is modulated to the half-sinusoid shape;

A former limit main switch, the former limit windings in series of said former limit main switch and said transformer, when the main switch conducting of said former limit, said transformer stored energy; When said former limit main switch turn-offed, the energy of said transformer storage was coupled to secondary, discharges to electrical network through said power frequency polarity switching;

Be provided with an active clamp circuit in the said anti exciting converter; Said active clamping circuir comprises an auxiliary switch; A booster diode and a clamp capacitor; Said auxiliary switch with constitute series arm with said clamp capacitor again after said booster diode is parallelly connected, said series arm is connected in parallel on winding two ends, the former limit of transformer or main switch two ends, former limit in the anti exciting converter;

The power frequency polarity switching by the full bridge inverter that four switching tubes constitute, is handled the half-sinusoid electric current that said anti exciting converter is exported through the method for polar switching, and it is connected to the grid, and realizes the sinusoidal grid-connected current of output.

2. the control method that is applied to the miniature photovoltaic combining inverter device of active-clamp inverse-excitation type according to claim 1; It is characterized in that: the exciting curent of said transformer works in the mode that on-off state combines with critical continuous state in half power frequency period, and promptly said anti exciting converter mode of operation combines discontinuous conduction mode and critical continuous conduction mode.

3. the control method that is applied to the miniature photovoltaic combining inverter device of active-clamp inverse-excitation type according to claim 1 and 2; It is characterized in that: obtain output voltage and the instantaneous value of input voltage of the miniature photovoltaic combining inverter device of active-clamp inverse-excitation type of current time, the electrical degree and the line voltage zero cross signal of current time line voltage; Peak value in conjunction with said current reference; Thereby obtain said current reference, comprise the current reference of discontinuous conduction mode and critical continuous conduction mode:

Wherein, I _oPeak value for current reference; V _gBe the line voltage instantaneous value, i.e. the instantaneous value of the output voltage of the miniature photovoltaic combining inverter device of Zero-voltage switch flyback-type; V _PVBe photovoltaic side voltage up time value, the i.e. instantaneous value of the input voltage of the miniature photovoltaic combining inverter device of Zero-voltage switch flyback-type; θ is the electrical degree of current time, and N is the no-load voltage ratio of said transformer, f _DSwitching frequency for said former limit main switch under the discontinuous conduction mode.

4. according to claim 2 and the 3 described control methods that are applied to the miniature photovoltaic combining inverter device of active-clamp inverse-excitation type; It is characterized in that: the switching frequency threshold value of a given said former limit main switch; When the switching frequency of said former limit main switch is higher than threshold value; Said anti exciting converter gets into discontinuous conduction mode; The exciting curent that is said transformer works in on-off state, and adopts the current reference of said discontinuous conduction mode; When the switching frequency of said former limit main switch was lower than threshold value, anti exciting converter got into critical continuous conduction mode, and adopted the current reference of said discontinuous conduction mode.

5. the control method that is applied to the miniature photovoltaic combining inverter device of active-clamp inverse-excitation type according to claim 2 is characterized in that:

Under the discontinuous conduction mode, opening by fixing switch periods of said former limit main switch determines that the shutoff of main switch is by the current reference decision of said discontinuous conduction mode;

Under the critical continuous conduction mode; The shutoff of opening by said auxiliary switch of said former limit main switch determines; Promptly, auxiliary switch has no progeny when closing; Through opening former limit main switch behind the Dead Time, the shutoff of said former limit main switch is determined by the current reference under the said critical conduction mode.

6. the control method that is applied to the miniature photovoltaic combining inverter device of active-clamp inverse-excitation type according to claim 2; It is characterized in that: under the critical conduction mode; Said auxiliary switch is opened a period of time at the secondary current of anti exciting converter after zero, this ON time is calculated by following formula:

$T_{\mathrm{aux}}\&GreaterEqual;\frac{\sqrt{L_k\&CenterDot;C_r}}{K_p}[\frac{N\&CenterDot;V_{\mathrm{PV}}}{V_g\&CenterDot;\sin \left(\mathrm{\&theta;}\right)}+1]$

Wherein, L _kBe the former limit leakage inductance of said transformer, C _rBe the drain-source utmost point parasitic capacitance of said former limit main switch, K _pBe the ratio of the former limit of said transformer magnetizing inductance with former limit leakage inductance; The secondary current that controller obtains said anti exciting converter provides the signal of opening of said auxiliary switch to zero constantly, calculates simultaneously and counts said ON time, count down to the cut-off signals that provides said auxiliary switch after the said ON time.

7. based on the described control method that is applied to the miniature photovoltaic combining inverter device of active-clamp inverse-excitation type of claim 1; It is characterized in that: controller obtains the line voltage zero crossing, and the power frequency that provides four switching tubes in the said power frequency polarity switching drives signal; Pipe is driven homophase, and pipe drives anti-phase up and down.

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