CN106054995B - A kind of primary side feedback flyback power supply CCM and the Constant Current Control System of DCM patterns - Google Patents

Abstract

A kind of primary side feedback flyback power supply CCM and the Constant Current Control System of DCM patterns, including current detection module, export feedback module, current calculation module, error calculating module, PID modules, the control system that PWM module and drive module are constituted, the control system connects one closed loop of composition with controlled Switching Power Supply, current detection module, the basic parameter of constant current algorithm is realized in output feedback module collection, current calculation module calculates the average current of primary side inductance in the switch conduction stage, according to the turn ratio relation of input and output, calculate the average current of secondary output current, and obtain output current by carrying out the averaging of time upper integral to the electric current, the output valve performs backoff algorithm in Digital PID module, offset is submitted to PWM control modules, suitable dutycycle is exported through drive module, so as to control the high precision constant current of digital power.

Description

A Constant Current Control System of Primary Side Feedback Flyback Power Supply in CCM and DCM Modes

technical field

The invention relates to the field of switching power supplies, in particular to a constant current control system in CCM (continuous current mode) and DCM (discontinuous current mode) modes of a primary side feedback flyback power supply.

Background technique

The primary side feedback (PSR) eliminates the sampling optocoupler, reduces the number of circuit components, and reduces the complexity of the circuit, so most flyback converters now use PSR control. In order to improve the power application range of the primary side feedback flyback switching power supply, it is necessary to require the switching power supply to have a higher power output. CCM is mainly used for high-power loads, while DCM is mainly used for low-power loads, and the control loop is relatively stable. To achieve high-precision current stability in CCM mode is a top priority and a difficulty.

In order to achieve the stability of the output current, the existing method is to realize the constant current output by means of Pout/Vo=Io, where Pout is the output power of the secondary side, Vo is the output voltage of the secondary side, and Io is the output voltage of the secondary side Output current. In the process of actually adopting this strategy, since the output power Pout of the secondary side is unknown, the input power Pin of the primary side is used to replace the output power Pout of the secondary side, but the calculation of Pin is often too complicated. In addition, to obtain the output power Vout of the secondary side requires An additional sampling circuit is built. The traditional constant current algorithm is too costly to achieve constant current. Because the constant current effect is related to efficiency, the efficiency is greatly affected by the input voltage and load size, but the stability of the current is difficult to achieve high accuracy. .

Realizing constant current through ADC is also a relatively common method. One of the methods is to sample the maximum and minimum values of the current, and then calculate the average value, or use the ADC to sample the current and voltage values at the midpoint of conduction. However, the disadvantage of these two methods is that if the constant current accuracy needs to be improved, the sampling frequency needs to be increased, and the implementation cost for this is often relatively high.

The existence of the above-mentioned technical difficulties requires the establishment of a more efficient calculation model to achieve a higher-precision constant current characteristic.

Contents of the invention

In order to overcome the limitations and deficiencies of the prior art, the present invention proposes a constant current control system in the CCM and DCM modes of the primary side feedback flyback power supply. This method is not only applicable to the DCM mode, but also can be applied to the CCM mode. While improving the constant current precision, the power application range of the circuit can be improved.

In order to achieve the above object, the technical solution adopted by the present invention is: a constant current control system of the CCM and DCM modes of the primary side feedback flyback power supply, which is characterized in that it includes a current detection module, an output feedback module, a current calculation module, an error The control system composed of calculation module, PID module, PWM module and drive module is connected with the controlled switching power supply to form a closed loop;

The current detection module is used to collect the basic parameters to realize the constant current algorithm. The input signal of the current detection module includes the converted peak voltage digital value Vp_dig of the primary peak current Ipeak given by the PWM module, and the switch control signal duty output by the PWM module. And the voltage Vs of the sampling current resistor on the primary side, the output signal of the current detection module includes four time signals ton, t1, t2 and tdelay, and the signal Vcomp1 output to the PWM module, the current detection module includes a DAC unit, two comparators COMP1 And COMP2 and a clock counting unit. The input of the DAC unit is the peak voltage digital value Vp_dig, and the output is the corresponding analog voltage Vp. The positive terminal of the comparator COMP1 is connected to the voltage Vs of the primary side sampling current resistor, and the negative terminal is connected to the primary side peak value. The voltage Vp corresponding to the current Ipeak, the positive terminal of the comparator COMP2 is connected to the voltage Vs of the primary side sampling current resistor, and the negative terminal is connected to the partial voltage Vp_k corresponding to the voltage Vp corresponding to the peak current Ipeak of the primary side, where Vp_k=k*Vp, 0<k <1, k is selected according to when the input voltage is the lowest and the load is the largest, the time length of Vcomp1=0 is 1.5 times to 2.5 times of the time length of Vcomp2=0, the output signal Vcomp1 of the comparator COMP1 and the output of the comparator COMP2 The signal Vcomp2 is an input signal of the time calculation unit;

The output feedback module is used to collect the basic parameters of the constant current algorithm and calculate the duty cycle of the secondary inductor current. The input signal of the output feedback module includes the auxiliary winding sampling voltage Vsense and the switch control signal duty output by the PWM module. The output feedback module The output signal includes the time signal tr value and the pattern recognition signal flag, the output feedback module includes a comparator COMP3, the positive terminal of the comparator COMP3 is connected to the auxiliary winding sampling voltage Vsense, the negative terminal of the comparator COMP3 is grounded, and the output Vcomp3 of the comparator COMP3 is The time signal tr value and the mode recognition signal flag, the DCM mode and the CCM mode are different in the calculation of the tr value. For the DCM mode, when the secondary current is 0, the voltage on the auxiliary winding will resonate. In order to be more accurate To calculate the tr value, the system time corresponding to Vcomp3=1 needs to deduct a quarter of the resonance period to get the accurate tr value. For CCM mode, during duty=0, Vcomp3 is always 1, and duty=0 at this time The period of time is the tr value, flag=1 means CCM working mode, flag=0 means DCM working mode, the basis for judging is whether the voltage on the voltage dividing resistor on the auxiliary winding resonates after the secondary side current returns to 0 , if there is a resonance, it means that it is in the DCM working mode, if there is no resonance, it means that it is in the CCM working mode;

The current calculation module is used to calculate the average value Iav of the primary inductor current during the switch-on phase. The input signals of this module include four time signals t1, t2, ton and tdelay output by the current detection module and the output signal tr of the output feedback module , flag and the output signal Vp_dig of the PWM module, use the flag signal to distinguish whether to use the DCM or the CCM calculation model, and the output signal of the current calculation module is a binary digital quantity Iav_dig;

The error calculation module includes precise digital setting, subtractor, deviation calculation and deviation change rate calculation unit. The positive terminal of the subtractor is connected to the binary digital constant Iref output by the precise digital setting unit. This value is given according to the design index of the system. Subtraction The negative terminal of the device is connected to the binary output current digital quantity Iav_dig output by the current calculation module, and the output of the subtractor obtains the binary voltage deviation digital quantity εμ(t _n ) of the current sampling period through the deviation calculation unit, and passes it through the deviation change rate calculation unit and Subtract the binary voltage deviation digital quantity εμ(t _n-1 ) in the previous sampling period to obtain the binary digital quantity deviation change rate Δεμ(t _n );

The PID module includes four computing units: differential, proportional, integral and summation. The input of the integral and proportional unit is the binary voltage deviation εμ(t _n ), and the input of the differential unit is the binary deviation change rate Δεμ(t _n ). The outputs of the differential, proportional and integral three operation units are summed in the sum operation unit, and the output compensation result of the sum operation module is a binary digital quantity V _PI ;

The input of the PWM module is the compensation result V _PI of the PID module and the Vcomp1 output by the current detection module; through the calculation of the compensation result V _PI and Vcomp1 of the PID module, the information of the switching cycle and duty cycle during normal control is obtained, and the information of the cycle and duty cycle is obtained Finally, the PWM module outputs the switching cycle Ts value and the primary side peak current Ipeak value to the drive module, where the Ts value is the length of the next switching cycle calculated by the PWM module based on the input signal, and Ipeak defines the primary side sampling resistor. Maximum peak current; at the same time, the PWM module uses the V _PI value output by the PID module to output a signal Vp_dig, which is used for information processing of the current detection module and the current calculation module;

The input of the drive module is the signal Ts and Ipeak output by the PWM module. The value of Ipeak limits the maximum current on the primary sampling resistor, and the current is proportional to the conduction time of the power tube, so Ipeak also limits the current of the power tube. On-time, combined with the above two input signals, the drive module outputs a duty cycle waveform, that is, the duty signal, which controls the gate of the power transistor to realize the control of the loop; at the same time, the duty signal is also the current detection module and the output feedback An input signal for the module to process information; repeat the above process to cycle control the switching power supply power tube on and off to make the system more stable and obtain higher constant current accuracy.

Advantage of the present invention and remarkable effect:

1. Through a simple DAC unit and a comparator, the calculation of the average current of the inductor in the conduction phase can be realized, which can be applied to CCM and DCM. The control implementation method of the present invention is simple, flexible, and easy to implement, eliminating the need for output voltage The sampling circuit reduces the power supply development cost. The innovation of the present invention in principle and train of thought is the most fundamental reason that brings this advantage.

2. The present invention can realize high-precision constant current through time compensation calculation. In DCM mode, it has achieved extremely high constant-current precision. Compared with traditional constant-current algorithms, it is better under the constant-current effect of DCM mode.

3. The present invention can be applied to isolated or non-isolated switching power supply circuit structures, and has versatility, reusability and portability;

Description of drawings

Fig. 1 a is the system structural block diagram of control method of the present invention;

Fig. 1b is a structural block diagram of the current monitoring module in Fig. 1a;

Figure 2a is a schematic diagram of the working principle of the output feedback module in DCM mode;

Fig. 2b is a schematic diagram of the working principle of the output feedback module in CCM mode;

Fig. 3a is a schematic diagram of the calculation model in the DCM mode of the output current detection module;

Fig. 3b is a schematic diagram of the calculation model in the CCM mode of the output current detection module;

Fig. 4 is an embodiment of the structure diagram of the closed-loop circuit with the multi-mode control flyback converter of the present invention.

detailed description

The control system for improving the constant current precision of the switching power supply in the present invention is shown in Fig. 1a and Fig. 4, based on the control system comprising a current detection module, an output feedback module, a current calculation module, an error calculation module, a PID module, a PWM module and a drive module. The control system is connected with the controlled switching power supply to form a closed loop. The current detection module and the output feedback module collect the basic parameters to realize the constant current algorithm. Turn ratio relationship, calculate the average current of the secondary output current, and obtain the output current by time-integrating the current, the output value is implemented in the digital PID module compensation algorithm, and the compensation value is submitted to the PWM control module and then passed The drive module outputs an appropriate duty cycle to control the high-precision constant current of the digital switching power supply.

The current detection module, see Figure 1b, includes a DAC unit, two comparators COMP1, COMP2, and a time calculation unit. The input of the DAC unit is the digital value Vp_dig of the peak current given by the PWM module, and the output is the peak current corresponding to the voltage Vp on the sampling resistor. The positive terminal of the comparator COMP1 is connected to the voltage Vs of the primary sampling current resistor, and the negative terminal is connected to the peak value. The voltage Vp corresponding to the current, the output signal Vcomp1 is input to the time calculation unit, the positive terminal of the comparator COMP2 is connected to the voltage Vs of the sampling current resistor, and the negative terminal is connected to the partial voltage Vp_k of the voltage Vp corresponding to the peak current, where Vp_k=k*Vp , where 0<k<1, k is selected according to when the input voltage is the lowest and the load is the largest, the time length of Vcomp1=0 is about twice the time length of Vcomp2=0, and the output signal Vcomp2 is input to the time calculation unit; time The input signals of the calculation unit are the switch control signal duty output by the PWM module, and the output signals Vcomp1 and Vcomp2 of the two comparators. The module calculates the time length of Vcomp1=0 and Vcomp2=0 time period t1 respectively according to the internal clock signal, Vcomp2 =1, Vcomp1=0 time period t2, Vcomp1=1, Vcomp2=1 time period tdelay, and duty=1 time length ton, ton is the switch on time length. The output result of the current detection module is an important parameter for calculating Iav by the current calculation module, and the accuracy of the output parameters of this module will affect the final calculation accuracy.

As shown in FIG. 1 a and FIG. 4 , the output feedback module mainly calculates the duty ratio of the secondary inductor current, and mainly includes a comparator COMP3 (not shown) and a time calculation module. The output feedback here mainly refers to the feedback voltage of the auxiliary winding, rather than the direct feedback of the output voltage. This method of primary side feedback saves the sampling optocoupler, reduces the number of circuit components, and reduces the complexity of the circuit. The positive terminal of the comparator COMP3 is connected to the sampling voltage Vsense on the auxiliary winding voltage dividing resistor, the negative terminal is grounded, the output signal Vcomp3 is input to the time calculation unit, and the switch control signal duty output by the PWM module is also the input of the time calculation unit. The DCM mode and the CCM mode are slightly different in calculation.

The calculation principle of the output feedback module is described in detail below in conjunction with the illustrations. As shown in Figure 2a, this is the working principle diagram of tr calculation in DCM mode. The typical feature in DCM mode is that there is a resonance before the next switch is turned on. In order to accurately calculate the tr value, the duty and Vcomp3 signals can be used for the Vsense signal. Divide three working areas, when duty=1 is the first working area, represented by enable=0, when enable=0, Vcomp3=1, enter the second working area, represented by enable=1, when enable= 1. When Vcomp3 becomes 0 for the first time, it is the third working area, which is represented by enable=2. First calculate the time length of enable=1, which is counted as tr_temp, and then by adding a zero-time variable rise, the first resonant half period t_half after enable=2 can be calculated, at this time tr=tr_temp-t_half/2, calculated using this formula The tr value of t_half is more accurate, and the calculation method of t_half is as follows, rise is always 0 when enable=0 or enable=1, rise remains 0 when enable=2, rise=0, Vcomp3=0, and rise remains 0 when enable=2 , rise=1, when Vcomp3=1, rise becomes 1. In other cases, rise remains 1. In this way, enablele=2 stage, except for the first resonance half cycle rise=0, in other resonance stages, rise is 1, that is, enab1e =2, the time period corresponding to rise=0 is t_half. The initial value of the flag is set to 1, that is, the default is the CCM mode. If the duty is not 0 and Vcomp3=0 occurs, set the flag to 0, indicating that the circuit has entered the DCM working mode. Referring to Figure 2b for the tr calculation method under CCM, the time length of enable=1 is calculated according to the internal clock signal to be the tr value.

The current calculation module mainly calculates the average value Iav of the primary inductor current. The input signal of this module is the output signal t1, t2, ton, tdelay of the current detection module, the output signal tr, flag of the output feedback module, and the output signal Vp_dig of the PWM, use the flag signal to distinguish whether to use the DCM or CCM calculation model, and output the signal Is the digital quantity Iav_dig. The current calculation module is a key module to achieve high-precision constant current. Different calculation models are used for DCM and CCM to ensure high constant-current accuracy in different modes. DCM mode, see Figure 3a, the formula (1) can be deduced by using simple geometric relations

Among them, Iavp is the average current in the conduction phase of the primary side, Ipeak is the peak current of the primary side, ton is the conduction time of the switch tube, and tdelay is the delay caused by the driving module. Considering tdelay can further improve the current accuracy.

For CCM, refer to Figure 3b, and the geometric relationship can also be used to deduce the formula (2)

In this experimental system, the value of k is 0.5.

After obtaining the average current Iav _p in the conduction phase of the primary side, the relationship between the average current Iav _s and Iav _p in the tr phase is shown in formula (3).

Iav _p × n = Iav _s (3)

The average value of the output current Iav can be obtained by taking the average of the secondary current in time, as shown in formula (4)

The current calculation module finally outputs the average value Iav of the primary inductor current.

The error calculation module includes precise digital setting, subtractor, deviation calculation and deviation change rate calculation unit. The positive terminal of the subtractor is connected to the binary digital constant Iref output by the precise digital setting unit. This value is given according to the design index of the system. Subtraction The negative terminal of the device is connected to the binary output current digital quantity Iav_dig output by the current calculation module, and the output of the subtractor obtains the binary voltage deviation digital quantity εμ(t _n ) of the current sampling period through the deviation calculation unit, and passes it through the deviation change rate calculation unit and Subtract the binary voltage deviation digital quantity εμ(t _n-1 ) in the previous sampling period to obtain the binary digital quantity deviation change rate Δεμ(t _n );

The PID module includes four computing units: differential, proportional, integral and summation. The input of the integral and proportional unit is the binary voltage deviation εμ(t _n ), and the input of the differential unit is the binary deviation change rate Δεμ(t _n ). The outputs of the differential, proportional and integral three operation units are summed in the sum operation unit, and the output compensation result of the sum operation module is a binary digital quantity V _PI ;

The input of the PWM module is the compensation result V _PI of the PID module and the Vcomp1 output by the current detection module; through the calculation of the compensation result V _PI and Vcomp1 of the PID module, the information of the switching cycle and duty cycle during normal control is obtained, and the information of the cycle and duty cycle is obtained Finally, output the Ts value and Ipeak value to the drive module, where the Ts value determines the length of the next switching cycle, and Ipeak limits the maximum peak current on the primary sampling resistor; at the same time, the PWM module uses the VPI value to output the signal Vp_dig, This value is mainly used for the information processing of the current detection module and the current calculation module.

The input of the drive module is the output signal Ts of the PWM module and Ipeak. The Ipeak value limits the maximum current on the primary sampling resistor, and the current is proportional to the conduction time of the power tube, so Ipeak also limits the conduction time of the power tube. Combining the above two input signals, the output duty ratio waveform of the drive module, that is, the dutv signal, is connected to the gate of the power transistor to realize the control of the loop. At the same time, the duty signal is also an important input signal indispensable for information processing by the current detection module and the output feedback module. Repeat the above-mentioned process to control the turn-on and turn-off of the power tube of the switching power supply in a cycle, so as to make the system more stable and obtain higher constant current accuracy.

The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments. It cannot be considered that the specific implementation of the present invention is only limited to these descriptions. The present invention described here can have many changes (in other switching power supplies, constant flow algorithm control), this change cannot artificially deviate from the spirit and scope of the present invention. Therefore, all changes obvious to those skilled in the art should be included within the scope of the claims.

Claims (1)

1. A constant current control system of a primary side feedback flyback power supply CCM and DCM mode, characterized in that: comprising a current detection module, an output feedback module, a current calculation module, an error calculation module, a PID module, a PWM module and a drive module The control system composed of the control system is connected with the controlled switching power supply to form a closed loop; The current detection module is used to collect the basic parameters to realize the constant current algorithm. The input signal of the current detection module includes the converted peak voltage digital value Vp_dig of the primary side peak current Ipeak given by the PWM module, the switch control signal duty output by the PWM module, and The primary side samples the voltage Vs of the current resistor, and the output signal of the current detection module includes the conduction time ton of the switch, the time length t1 corresponding to the switch conduction period Vcomp1=0 and Vcomp2=0, the switch conduction period Vcomp1=0 and Vcomp2= 1 corresponds to the time length t2 and the delay tdelay caused by the drive module, four time signals, and the signal Vcomp1 output to the PWM module. The current detection module includes a DAC unit, two comparators COMP1 and COMP2, and a clock counting unit, DAC The input of the unit is the peak voltage digital value Vp_dig, and the output is the corresponding analog voltage Vp. The positive terminal of the comparator COMP1 is connected to the voltage Vs of the primary sampling current resistor, and the negative terminal is connected to the analog voltage Vp corresponding to the primary peak current Ipeak. The positive terminal of the comparator COMP2 is connected to the voltage Vs of the primary side sampling current resistor, and the negative terminal is connected to the partial voltage Vp_k of the primary side peak current Ipeak corresponding to the analog voltage Vp, where Vp_k=k*Vp, 0<k<1, k is According to the lowest input voltage and the largest load, the time length of Vcomp1=0 is selected from 1.5 times to 2.5 times of the time length of Vcomp2=0, the output signal Vcomp1 of the comparator COMP1 and the output signal Vcomp2 of the comparator COMP2 are clocks The input signal of the counting unit; The output feedback module is used to collect the basic parameters of the constant current algorithm and calculate the duty cycle of the secondary inductor current. The input signal of the output feedback module includes the auxiliary winding sampling voltage Vsense and the switch control signal duty output by the PWM module. The output feedback module The output signal includes the time signal tr value and the pattern recognition signal flag, the output feedback module includes a comparator COMP3, the positive terminal of the comparator COMP3 is connected to the auxiliary winding sampling voltage Vsense, the negative terminal of the comparator COMP3 is grounded, and the output Vcomp3 of the comparator COMP3 is The time signal tr value and the mode recognition signal flag, the DCM mode and the CCM mode are different in the calculation of the tr value. For the DCM mode, when the secondary current is 0, the voltage on the auxiliary winding will resonate. In order to be more accurate To calculate the tr value, the system time corresponding to Vcomp3=1 needs to deduct a quarter of the resonance period to get the accurate tr value. For CCM mode, during duty=0, Vcomp3 is always 1, and duty=0 at this time The period of time is the tr value, flag=1 means CCM working mode, flag=0 means DCM working mode, the basis for judging is whether the voltage on the voltage dividing resistor on the auxiliary winding resonates after the secondary side current returns to 0 , if there is a resonance, it means that it is in the DCM working mode, if there is no resonance, it means that it is in the CCM working mode; The current calculation module is used to calculate the average value Iav of the primary inductor current during the switch-on phase. The input signals of this module include four time signals t1, t2, ton and tdelay output by the current detection module and the output signal tr of the output feedback module , flag and the output signal Vp_dig of the PWM module, use the flag signal to distinguish whether to use the DCM or the CCM calculation model, and the output signal of the current calculation module is a binary digital quantity Iav_dig; The error calculation module includes precise digital setting, subtractor, deviation calculation and deviation change rate calculation unit. The positive terminal of the subtractor is connected to the binary digital constant Iref output by the precise digital setting unit. This value is given according to the design index of the system. Subtraction The negative terminal of the device is connected to the binary digital quantity Iav_dig output by the current calculation module, and the output of the subtractor obtains the binary voltage deviation digital quantity εμ(t _n ) of the current sampling period through the deviation calculation unit, which is compared with the previous one through the deviation change rate calculation unit Subtract the binary voltage deviation digital quantity εμ(t _n-1 ) in the sampling period to obtain the deviation change rate Δεμ(tn) of the binary digital quantity; The PID module includes four computing units: differential, proportional, integral and summation. The input of the integral and proportional unit is the binary voltage deviation digital quantity εμ(t _n ), and the input of the differential unit is the binary deviation change rate Δεμ(t _n ). , summing the outputs of the differential, proportional and integral three computing units in the summing computing unit, and the summing computing module outputs the compensation result as a binary digital quantity V _PI ; The input of the PWM module is the compensation result V _PI of the PID module and the Vcomp1 output by the current detection module; through the calculation of the compensation result V _PI and Vcomp1 of the PID module, the information of the switching cycle and duty cycle during normal control is obtained, and the information of the cycle and duty cycle is obtained Finally, the PWM module outputs the switching cycle Ts value and the primary side peak current Ipeak value to the drive module, where the Ts value is the length of the next switching cycle calculated by the PWM module based on the input signal, and Ipeak defines the primary side sampling resistor. Maximum peak current; at the same time, the PWM module uses the V _PI value output by the PID module to output a signal Vp_dig, which is used for information processing of the current detection module and the current calculation module; The input of the drive module is the signal Ts and Ipeak output by the PWM module. The value of Ipeak limits the maximum current on the primary sampling resistor, and the current is proportional to the conduction time of the power tube, so Ipeak also limits the current of the power tube. On-time, combined with the above two input signals, the drive module outputs a duty cycle waveform, that is, the duty signal, which controls the gate of the power transistor to realize the control of the loop; at the same time, the duty signal is also the current detection module and the output feedback An input signal for the module to process information; repeat the above process to cycle control the switching power supply power tube on and off to make the system more stable and obtain higher constant current accuracy.