Constant current control method of primary side feedback flyback converter

Abstract

This invention discloses a constant current control method for a primary-side feedback flyback converter, relating to the field of isolated converter technology. The method includes: when the primary-side feedback flyback converter operates in continuous current mode, determining the transformer demagnetization duration, cycle duration, and primary winding current valley value for the nth switching control cycle; obtaining the switching turn-off duration for the (n+1)th switching control cycle based on the transformer demagnetization duration, cycle duration, primary winding current valley value, a set reference primary current peak value, and the desired target current value; controlling the switching turn-on duration for the (n+1)th switching control cycle based on the reference primary current peak value, and controlling the switching turn-off duration for the (n+1)th switching control cycle to be the calculated turn-off duration. This method avoids low-frequency current ripple by eliminating the need for PID modulation calculations and can eliminate subharmonic oscillations at low input voltages.

Description

Technical Field

[0001] This invention relates to the field of isolated converter technology, and in particular to a constant current control method for a primary-side feedback flyback converter. Background Technology

[0002] With technological advancements, switching power supplies are widely used in low-to-medium power applications. Isolated switching power supplies achieve electrical isolation between input and output, offering advantages such as safety isolation and high reliability. Flyback converters, with their simple structure, high reliability, and low cost, are widely used in low-to-medium power consumer electronics. Among these, flyback converters based on primary-side feedback technology have a simpler circuit structure compared to traditional flyback converters based on secondary-side feedback technology. Furthermore, they eliminate the need for nonlinear components such as optocouplers, further improving reliability, lifespan, and integration.

[0003] The current method for achieving constant output current in primary-side feedback flyback converters is to control the average current of the output diodes to equal the target current. When the system is stable, the output load current equals the average current of the output diodes, thus achieving the target current. Based on the average current of the output diodes, the on-time of the switching transistors or the peak current of the primary winding can be controlled using PID modulation to stabilize the output current at the designed target current value. However, with PID modulation, the closed-loop bandwidth of the system will be much lower than the switching frequency, inevitably introducing low-frequency current ripple into the output current. Secondly, in continuous current mode, when using peak current control, a switching duty cycle higher than 0.5 with a low input voltage will introduce subharmonic oscillation problems, potentially causing abnormal primary-side current sampling or output current deviation. Low-frequency current ripple degrades output current ripple performance, while the instability and current deviation problems introduced by subharmonics must be avoided.

[0004] There are two common approaches to eliminating subharmonic oscillations. One approach is to use a slope compensation method, which superimposes a slope voltage with a certain negative slope onto the peak current. This method requires a reasonable design of the slope voltage. A slope that is too large or too small will fail to eliminate subharmonics or make the output unstable at the target value. Moreover, the generation of the slope voltage increases the complexity and cost of the circuit. The other approach is to sample the current at a specific moment and adjust the control variables by calculation to eliminate subharmonics. This approach usually requires a high-precision, high-speed analog-to-digital converter, which brings additional cost and power consumption. Summary of the Invention

[0005] To address the aforementioned problems and technical requirements, the inventors have proposed a constant current control method for a primary-side feedback flyback converter. This method eliminates the need for PID modulation, directly calculates the control parameters of the flyback converter, avoids the low-frequency current ripple introduced by PID control, and prevents subharmonic oscillation effects.

[0006] The technical solution of the present invention is as follows:

[0007] This application provides a constant current control method for a primary-side feedback flyback converter, including the following steps, under the nth switching control cycle:

[0008] When the primary-side feedback flyback converter operates in continuous current mode, determine the transformer demagnetization time T for the nth switching control cycle. r (n), period duration T s (n), Primary winding current valley value I pv (n), the valley value of the primary winding current I pv (n) is the minimum current flowing through the primary winding during the conduction of the switching transistor;

[0009] Based on the transformer demagnetization time T r (n), period duration T s (n), Primary winding current valley value I pv (n) and the set reference primary current peak value I ppREF And the target current value I to be achieved oREF Obtain the turn-off duration T of the switching transistor in the (n+1)th control cycle. off (n+1);

[0010] Based on the reference primary current peak value I ppREF The on-time T of the switching transistor during the (n+1)th control cycle on (n+1), and control the turn-off duration of the switching transistor in the (n+1)th switching transistor control cycle to be T. off (n+1).

[0011] A further technical solution is that the method also includes:

[0012] Determine the operating mode of the primary-side feedback flyback converter;

[0013] When the primary-side feedback flyback converter operates in discontinuous current mode, determine the transformer demagnetization time T for the nth switching control cycle. r (n), Switch conduction time T on (n);

[0014] Based on the transformer demagnetization time T r (n), Switch conduction time T on(n) and the set reference primary current peak value I ppREF And the target current value I to be achieved oREF Obtain the turn-off duration T of the switching transistor in the (n+1)th control cycle. off (n+1);

[0015] Based on the reference primary current peak value I ppREF The on-time T of the switching transistor during the (n+1)th control cycle on (n+1), and control the turn-off duration of the switching transistor in the (n+1)th switching transistor control cycle to be T. off (n+1).

[0016] The further technical solution involves determining the operating mode of the primary-side feedback flyback converter, including:

[0017] During the turn-off period of the switch in the nth control cycle, when the comparison result between the voltage divider signal of the primary auxiliary winding and the zero voltage is constant at an effective level, it is determined that the primary feedback flyback converter is operating in continuous current mode.

[0018] During the turn-off period of the switching transistor in the nth switching control cycle, when there is a level change in the comparison result between the voltage divider signal of the primary auxiliary winding and the zero voltage, it is determined that the primary feedback flyback converter is operating in discontinuous current mode.

[0019] The voltage divider signal of the primary auxiliary winding is taken from the voltage divider terminal signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

[0020] The further technical solution is to determine the demagnetization time T of the transformer. r (n), Switch conduction time T on (n) and the set reference primary current peak value I ppREF And the target current value I to be achieved oREF Obtain the turn-off duration T of the switching transistor in the (n+1)th control cycle. off The expression for calculating (n+1) is:

[0021]

[0022] Where, n ps This is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding of a transformer.

[0023] The further technical solution is to determine the demagnetization time T of the transformer. r (n), period duration T s (n), Primary winding current valley value I pv (n) and the set reference primary current peak value I ppREF And the target current value I to be achievedoREF Obtain the turn-off duration T of the switching transistor in the (n+1)th control cycle. off The expression for calculating (n+1) is:

[0024]

[0025] Where, n ps This is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding of a transformer.

[0026] A further technical solution is to determine the transformer demagnetization time T during the nth switching control cycle when the primary-side feedback flyback converter is operating in continuous current mode. r The methods of (n) include:

[0027] During the turn-off period of the nth switching control cycle, the time length T during which the comparison result of the voltage divider signal of the primary auxiliary winding and the zero voltage is found to be at an effective level is obtained. r_temp The demagnetization duration T in this mode r (n), the voltage divider signal of the primary auxiliary winding is taken from the voltage divider signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

[0028] A further technical solution is to determine the transformer demagnetization time T during the nth switching control cycle when the primary-side feedback flyback converter is operating in discontinuous current mode. r The methods of (n) include:

[0029] During the switch-off period of the nth switch control cycle, the time length at which the comparison result between the voltage divider signal of the primary auxiliary winding and the zero voltage is effective at the moment of switch-off is defined as T. r_temp The time length when the comparison result is invalid is defined as t. valley ; Calculate [T] r_temp -(t valley / 2)] is the demagnetization time T in this mode. r (n), the voltage divider signal of the primary auxiliary winding is taken from the voltage divider signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

[0030] A further technical solution is to determine the on-time T of the switching transistor in the nth control cycle when the primary-side feedback flyback converter is operating in discontinuous current mode. on The methods of (n) include:

[0031] During the switching period of the nth switching control cycle, the duration for which the comparison result between the voltage divider signal of the primary auxiliary winding and the zero voltage is invalid is taken as the switching duration T. on(n), the voltage divider signal of the primary auxiliary winding is taken from the voltage divider signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

[0032] Its further technical solution is to, based on the reference primary current peak value I ppREF The on-time T of the switching transistor during the (n+1)th control cycle on (n+1) methods include:

[0033] When the switching transistor is turned on, the switching transistor control signal remains at an active level until the primary current sampling voltage signal rises to the set primary voltage peak value. When the primary current sampling voltage signal rises to the set primary voltage peak value, the switching transistor control signal is switched to an inactive level, and the switching transistor is turned off.

[0034] The primary-side current sampling voltage signal is the voltage of the current sampling resistor connected in series between the source terminal of the switch and the input ground terminal; the set primary-side voltage peak value is the reference primary-side current peak value I. ppREF The product of the current sampling resistor connected in series between the source terminal and the input ground terminal of the switching transistor.

[0035] A further technical solution is to determine the primary winding current valley value I during the nth switching control cycle when the primary-side feedback flyback converter is operating in continuous current mode. pv The methods of (n) include:

[0036] During the conduction period of the switching transistor, the primary current sampling voltage signal is compared with the first voltage V. p1 Second voltage V p2 For comparison, the primary-side current sampling voltage signal is defined as being higher than the first voltage V. p1 Or the second voltage V p2 The comparison result at that time is the effective level, compared with the first voltage V. p1 The comparison result is the first comparison result, compared with the second voltage V. p2 The comparison result is the second comparison result; the time length between the moment the switch is turned on and the moment the second comparison result becomes effective is recorded as t. a The time length between the moment when the second comparison result becomes valid and the moment when the first comparison result becomes valid is recorded as t. b Based on the linear increase of the primary-side current sampling voltage signal during the switching transistor's conduction period, the valley value V of the primary-side current sampling voltage signal during the switching transistor's conduction period is calculated. pv (n) is represented as:

[0037]

[0038] Then the valley value V pv (n) corresponds to the primary winding current valley value I pv(n) is calculated using the following formula:

[0039] I pv (n)=V pv (n) / R p ;

[0040] Wherein, the primary-side current sampling voltage signal is the voltage of the current sampling resistor connected in series between the source terminal of the switching transistor and the input ground terminal; the first voltage V p1 Second voltage V p2 Given a voltage, and V p1 >V p2 ;R p This is a current sampling resistor connected in series between the source terminal of the switching transistor and the input ground terminal.

[0041] The beneficial technical effects of this invention are:

[0042] (1) This application adopts the primary-side feedback control method. Under the current switching control cycle, the switching turn-off time of the next switching control cycle is directly calculated based on the calculated primary-side current parameters and related time parameters. This is used as the control parameter of the converter to realize the constant current control of the load output. There is no need to use PID modulation, which can avoid the influence of the low-frequency current ripple introduced by the traditional PID modulation method on the primary-side feedback flyback converter.

[0043] (2) This application adopts a method that combines primary-side peak current control and turn-off duration control to control the switching transistor. In continuous current mode and when the duty cycle is large, the fixed peak current control is achieved by adaptively changing the conduction duration of the switching transistor. The disturbance in the current switching transistor control cycle returns to zero at the end of this cycle. Therefore, the disturbance will not be transmitted to the next cycle and will not cause unstable problems such as subharmonic oscillation when the input voltage is low.

[0044] (3) The method proposed in this application is applicable to isolated or non-isolated switching power supply circuit structures and has universality, reusability and portability. Attached Figure Description

[0045] Figure 1 This is a flowchart of the constant current control method for the primary-side feedback flyback converter provided in this application.

[0046] Figure 2 This is a schematic diagram of the closed-loop connection between the main topology circuit and the control circuit provided in this application.

[0047] Figure 3 The switching control signal duty and the primary current sampling voltage signal v provided in this application are... p Primary auxiliary winding voltage distribution signal v FB and output diode current i dThe diagram shows the working waveforms; where (a) is the working waveform of the related variables in discontinuous current mode, and (b) is the working waveform of the related variables in continuous current mode.

[0048] Figure 4 This application provides a functional circuit diagram of the time detection module and schematic diagrams of the working waveforms of various variables related to time calculation; wherein: (a) is the working waveform of the relevant time variables in discontinuous current mode, (b) is the working waveform of the relevant time variables in continuous current mode, and (c) is a schematic diagram of a time detection module.

[0049] Figure 5 The primary current sampling voltage signal v provided in this application p Peak V during switch-on period pp With valley value V pv The waveform diagrams are as follows: (a) is a waveform diagram using the direct sampling method, and (b) is a waveform diagram using the indirect measurement method based on the comparator circuit.

[0050] Figure 6 This application provides a schematic diagram of the working waveforms of various variables related to the calculation of converter control parameters; wherein: (a) is the working waveform of the relevant current variables in discontinuous current mode, and (b) is the working waveform of the relevant current variables in continuous current mode.

[0051] Figure 7 This is a waveform diagram of the switching transistor control signal control method provided in this application. Detailed Implementation

[0052] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0053] Please refer to Figure 1 As shown, this embodiment provides a constant current control method for a primary-side feedback flyback converter, applied to a primary-side feedback flyback converter structure, whose main topology circuit is as follows. Figure 2 As shown, it includes: input DC voltage V in Negative terminal connected to input ground G ND1 Input DC voltage V in The positive terminal is connected to the primary winding W of the transformer. p The same-named terminal, the primary winding W p The opposite terminal of the switch transistor M p The drain terminal of the switching transistor M p Source and input terminals G ND1 A small-value current sampling resistor R is connected in series between them. p The voltage across the resistor is defined as the primary current sampling voltage signal v. p One end of the clamping circuit is connected to the switching transistor M via the input diode DC.p One end is the drain terminal, and the other end is connected to the input DC voltage V. in Positive terminal. Transformer secondary winding W s The output diode D is connected to the opposite terminal. s The positive terminal, the secondary winding W s The same name is connected to the output ground terminal G. ND2 Output diode D s The negative terminal is connected to the output capacitor C. L Positive terminal, output capacitor C L Negative terminal connected to output ground G ND2 Output load R L With output capacitor C L Parallel connection. An auxiliary winding W is also added to the primary side of the transformer. a Its corresponding terminator is connected to the input ground terminal G. ND1 Furthermore, a resistor voltage divider circuit (i.e., a circuit formed by resistors R1 and R2 connected in series) is connected between the same-named and opposite-named terminals to divide the voltage at the opposite-named terminals of the winding. The voltage signal at the voltage divider terminal is defined as the voltage divider signal v of the primary auxiliary winding. FB .

[0054] For the main topology circuit described above, the relevant parameters are given: the switching transistor control signal duty, and the primary-side current sampling voltage signal v. p Primary auxiliary winding voltage distribution signal v FB and output diode current i d The working waveform diagram is as follows: Figure 3 (a) and Figure 3 As shown in (b), the start time of the nth switching control cycle is first defined as the moment when the duty signal switches from "1" to "0", i.e., the starting time of switching transistor M. p The diode D switches from the on state to the off state at time t0. s When the diode is turned on, the output diode current i d From peak I sp The current begins to decrease linearly; at time t1, the output diode current i d The signal linearly decreases to zero; at time t2, the duty signal switches from "0" to "1", i.e., the switching transistor M... p The circuit switches from the off state to the on state; after time t2, the primary winding current i p Linearly increase, when i p Reaching the set primary-side current peak value I pp_set At that time, i.e., the primary side current sampling voltage v p The set primary-side voltage peak value V is reached. pp_set At that time, the switching transistor M pTurning off, i.e., the duty signal switching from "1" to "0", marks the end of the current cycle and the start of the (n+1)th switching control cycle. The time length from t0 to t1 is the output diode D of the nth cycle. s The conduction time is denoted as the transformer demagnetization time T. r (n); The time length from t0 to t2 is the switching transistor M. p The shutdown duration is denoted as T. off (n); The time length from t2 to t3 is the switching transistor M. p The conduction duration is denoted as T. on (n). In discontinuous current mode, the demagnetization time T r Less than the shutdown duration T off And the primary winding current valley value I pv Equal to zero; in continuous current mode, the demagnetization time T r With shutdown duration T off The values ​​are equal, and the valley value I of the primary winding current is... pv Greater than zero. Wherein, the primary winding current i... p For the flow through the primary winding W p The current, the valley value of the primary winding current I pv During the conduction period of the switching transistor, the current flows through the primary winding W. p The minimum current.

[0055] The constant current control method proposed in this embodiment specifically includes the following steps:

[0056] In the nth switching control cycle:

[0057] Step 1: Determine the operating mode of the primary-side feedback flyback converter, including:

[0058] like Figure 4 As shown in (a), during the switch-off period of the nth switch control cycle (i.e., when the duty signal is "0"), when the voltage divider signal v of the primary auxiliary winding... FB The comparison result with zero voltage 0V is S comp3 When there is a level change, it is determined that the primary-side feedback flyback converter is operating in discontinuous current mode.

[0059] like Figure 4 As shown in (b), during the switch-off period of the nth switch control cycle, when the voltage divider signal v of the primary auxiliary winding... FB The comparison result with zero voltage 0V is S comp3 When the effective level is constant, it is determined that the primary-side feedback flyback converter is operating in continuous current mode.

[0060] In this embodiment, the effective level is defined as high level, and the ineffective level as low level. v is defined. FBComparison results S at values ​​above 0V comp3 If the signal is high, it is recorded as state "1"; otherwise, the comparison result S is... comp3 When the level is low, it is recorded as state "0".

[0061] Step 2a: When the primary-side feedback flyback converter is operating in continuous current mode, determine the transformer demagnetization time T for the nth switching control cycle. r (n), period duration T s (n), Primary winding current valley value I pv (n).

[0062] Among them, the transformer demagnetization time T of the nth switching transistor control cycle is determined. r The methods of (n) include:

[0063] like Figure 4 As shown in (b), during the turn-off period of the switch in the nth control cycle, the voltage divider signal v of the primary auxiliary winding is acquired. FB The comparison result with zero voltage 0V is S comp3 The duration T when the level is active r_temp The demagnetization duration T in this mode r (n) is also used as the turn-off time T of the switching transistor in this mode. off (n), i.e., T r (n)=T off (n). Set the duty signal to low level and compare the result S. comp3 When the level is high, the corresponding state variable enable is denoted as "1".

[0064] Among them, the period duration T of the control cycle of the nth switching transistor is determined. s The methods of (n) include:

[0065] Period duration T s (n) represents the on-time T of the switching transistor. on (n) and the off-time T off The sum of (n). For example... Figure 4 As shown in (b), during the switching period of the nth switching control cycle, the voltage divider signal v of the primary auxiliary winding is acquired. FB The comparison result with zero voltage 0V is S comp3 The duration of the invalid level is taken as the on-time T of the switch. on (n). Set the duty signal to high level and compare the result S. comp3 When the state variable enable is low, it is denoted as "0". Primary-side feedback flyback converters where the state variable enable only switches between "0" and "1" operate in continuous current mode.

[0066] Among them, the valley value I of the primary winding current in the nth switching control cycle is determined. pv The methods of (n) include:

[0067] Based on primary side current sampling voltage signal v p The valley value V during the conduction period of the switching transistor. pv (n), the valley value of the primary winding current I is calculated. pv (n). Similarly, the peak value of the primary current I can also be calculated. pp (n), the relation satisfies equation (1).

[0068] I pp (n)=V pp (n) / R p I pv (n)=V pv (n) / R p (1)

[0069] Among them, V pp (n), V pv (n) can be obtained from different methods. Figure 5 (a) is direct sampling V pp V pv The waveform diagram of the method shows that when the duty signal switches from "0" to "1", the current and voltage v of the sampling resistor are sampled at this time. p , denoted as V pv Before the duty signal switches from "1" to "0", the current and voltage v of the sampling resistor are sampled at this time. p , denoted as V pp .

[0070] Figure 5 (b) is a V based on a comparator circuit. pp V pv Indirect measurement method. The primary-side current is sampled during the switching transistor's conduction period, along with the voltage signal v. p Respectively with the first voltage V p1 Second voltage V p2 Compare and define the primary-side current sampling voltage signal v. p Higher than the first voltage V p1 Or the second voltage V p2 If the comparison result is valid, it is recorded as state "1"; otherwise, if the comparison result is low, it is recorded as state "0". This is compared with the first voltage V. p1 The comparison result is the first comparison result S. comp1 , with the second voltage V p2 The comparison result is the second comparison result S. comp2 Record the switching transistor's on-time (rising edge of the duty signal) to the second comparison result S. comp2Time to become effective level (S) comp2 The time length between the rising edges of the signal is t. a Record the second comparison result S. comp2 From the moment it becomes an effective level to the first comparison result S comp1 The time length between the moment the signal becomes effective is t. b The first comparison result S comp1 The duration of time when it is "1" is t c Based on the primary-side current sampling voltage signal v p It increases linearly during the conduction period of the switching transistor, i.e., v p The upward slope remains unchanged, as shown in equation (2).

[0071]

[0072] Based on equation (2), the peak value V of the primary side current sampling voltage signal during the switching transistor's conduction period is calculated. pp (n), Valley Value V pv (n) is shown in equation (3). Substituting into equation (1), the peak value I of the primary winding current can be obtained. pp (n), Valley Value I pv (n) is shown in equation (4).

[0073]

[0074]

[0075] It should be noted that this embodiment uses fixed primary-side peak current control, that is, I pp (n) is set to a constant peak primary current I. pp_set , and I pp_set The value is the set peak value of the primary current I. ppREF , while I pv (n) can be obtained by choosing one of the two methods given in this step.

[0076] Step 3a: Based on the transformer demagnetization time T r (n), period duration T s (n), Primary winding current valley value I pv (n) and the set reference primary current peak value I ppREF And the target current value I to be achieved oREF Obtain the turn-off duration T of the switching transistor in the (n+1)th control cycle. off (n+1), including:

[0077] Figure 6 The average current I of the output diode dA schematic diagram related to the calculation method is shown in the figure, which displays the primary winding current i. p Excitation current i m and output diode current i d The waveform, i p As shown by the solid line, i m As shown by the dashed line. During the conduction phase of the switching transistor, i... p The number of i increases linearly, at which point i... m with i p Coincidence, i d Equal to zero; in i d When i is greater than zero m Greater than i p Output diode current i d Proportional to i m with i p The difference is shown in equation (5), where n ps It is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding of the transformer.

[0078] i d =n ps (i m -i p (5)

[0079] Figure 6 (a) and Figure 6 (b) shows the relevant waveform diagrams in discontinuous current mode and continuous current mode, with the average current I of the output diode. d It can be described as i d The average value of the integral over time is denoted as I. d I d It is proportional to the area of ​​the shaded region in the diagram. It can be derived from i. m and i p In T r The average value is calculated based on the stages and the time integral, taking into account the time interval from stage t0 to t1. p The primary current peak value I is equal to zero. pp_set Equal to the peak value of the fixed reference primary current I ppREF , then I d It can be made by I ppREF I pv T r and T s The result is shown in equation (6).

[0080]

[0081] In the nth switching control cycle, at T off (n) stage, i mThe decreasing slope k2 depends only on the output capacitor voltage, which can be considered approximately constant. Therefore, the turn-off duration T in the (n+1)th switching control cycle... off (n+1), i m The descending slope is still k2, and k2 can be determined by T. r (n), I ppREF and I pv (n) is obtained, as shown in equation (7). Similarly, the primary winding current i p The rising slope k1 is only related to the input voltage. In the nth and (n+1)th periods, the primary winding current i p The upward slope k1 is approximately constant, and k1 can be determined by T. on (n), I ppREF and I pv (n) is obtained, as shown in equation (7).

[0082]

[0083] like Figure 6 As shown in (b), T r (n)=T off (n), the primary-side feedback flyback converter operates in continuous current mode, I pv (n+1) and I pv (n) are non-zero and not necessarily equal, assuming T off When (n+1) is known, consider T in continuous current mode. r (n+1) and T off (n+1) are equal, I pv (n+1) can be based on the excitation current i m The descending slope k2 and T off (n+1) is obtained, as shown in equation (8). Therefore, T on (n+1) can be further derived from I pv (n+1) and k1 are obtained as shown in equation (9). Based on equation (6), in order to make the average value of the output diode current I d (n+1) equals the target current value I oREF To achieve a constant output load current, as shown in equation (10). Combining equations (7), (8), and (9), I... pv (n+1) and T on Substituting (n+1) into equation (10), we can obtain the result when T off When (n+1) satisfies equation (11), I d (n+1) equals I oREF .

[0084] I pv (n+1)=I ppREF-k2T r (n+1)=I ppREF -k2T off (n+1) (8)

[0085]

[0086]

[0087]

[0088] When the average current I of the output diode d It remained stable at the target current value I oREF When the load is stable, the output load current is equal to I. oREF This achieves constant current control of the output.

[0089] Step 4: Based on the reference primary current peak value I ppREF The on-time T of the switching transistor during the (n+1)th control cycle on (n+1), and control the turn-off duration of the switching transistor in the (n+1)th switching transistor control cycle to be T. off (n+1), including:

[0090] like Figure 7 As shown, each time the switching control signal duty changes from high to low (from "1" to "0"), the timer counter is reset to zero and starts counting. This is the start time of the nth switching control cycle. The counter reaches the switching off time T of the nth switching control cycle. off When the duty signal changes from low to high (from "0" to "1"), the switching transistor turns on. When the duty signal is at an active level, the primary-side current sampling voltage signal v... p Rise to the set primary-side voltage peak v pp_set Previously, the duty signal remained at an active level, and the primary side current sampling voltage signal v p Rise to the set primary-side voltage peak v pp_set When the duty signal is switched to an invalid level, the switching transistor is turned off, the nth cycle ends, and the start time of the duty signal for the (n+1)th switching transistor control cycle begins. The above control method is repeated to control the switching transistor's on and off in each switching transistor control cycle. The set primary-side voltage peak value V... pp_set For reference, the peak value of the primary current I ppREF The product of the current sampling resistor value and the resistance value.

[0091] Step 2b: When the primary-side feedback flyback converter operates in discontinuous current mode, determine the transformer demagnetization duration T for the nth switching control cycle. r(n), Switch conduction time T on (n).

[0092] Among them, the transformer demagnetization time T of the nth switching transistor control cycle is determined. r The methods of (n) include:

[0093] like Figure 4 As shown in (a), during the switch-off period of the nth switch control cycle, the voltage divider signal v of the primary auxiliary winding is defined at the switch-off time. FB The comparison result with zero voltage 0V is S comp3 The duration of the effective level is T. r_temp Define S comp3 The duration of the invalid level is t. valley Calculate [T] r_temp -(t valley / 2)] is the demagnetization time T in this mode. r (n), the voltage divider signal of the primary auxiliary winding is taken from the voltage divider terminal signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

[0094] The comparison result S corresponding to the falling edge time of the duty signal comp3 The state variable enable, when high, is recorded as "1". The duty signal is set to low, and the comparison result S... comp3 When the signal is low, the corresponding state variable enable is recorded as "2". The duty signal is set to low, and the comparison result S... comp3 The state variable enable that changes from low to high level is denoted as "3".

[0095] Among them, the on-time T of the switch in the nth control cycle is determined. on The methods of (n) include:

[0096] like Figure 4 As shown in (a), during the conduction period of the switching transistor in the nth switching control cycle, the voltage divider signal v of the primary auxiliary winding is acquired. FB The comparison result with zero voltage 0V is S comp3 The duration of the invalid level is taken as the on-time T of the switch. on (n). Set the duty signal to high level and compare the result S. comp3 The state variable enable corresponding to a low level is denoted as "0". The primary-side feedback flyback converter, whose state variable enable switches sequentially from "0" to "1" to "2" to "3", operates in discontinuous current mode. The state switching conditions are shown in Table 1.

[0097] Table 1. State variable switching conditions under discontinuous current mode

[0098]

[0099] In discontinuous current mode, the switching off time T of the nth switching control cycle is... off (n) represents the sum of the time lengths of the state variable enable "1" + "2" + "3", and in this mode, T r (n) < T off (n).

[0100] Step 3b: Based on the transformer demagnetization time T r (n), Switch conduction time T on (n) and the set reference primary current peak value I ppREF And the target current value I to be achieved oREF Obtain the turn-off duration T of the switching transistor in the (n+1)th control cycle. off (n+1), including:

[0101] Based on the same description, formulas (5), (6), and (7) for step 3a are obtained, and will not be repeated here. Figure 6 As shown in (a), T r (n) < T off (n), the primary-side feedback flyback converter operates in discontinuous current mode, I pv (n+1) and I pv (n) are equal and all are zero. Therefore, T r (n+1) and T r (n) are equal, T on (n+1) and T on (n) are equal. Therefore, T needs to be adjusted. s The value of (n+1) makes the average current I of the output diode in the (n+1)th control cycle of the switching transistor to be controlled so that... d (n+1) equals the target current value I to be designed. oREF Substituting equation (7) into equation (6), we obtain equation (12). Based on equation (12), in order to make I d (n+1) equals I oREF Then T off (n+1) needs to satisfy equation (13).

[0102]

[0103]

[0104] When the average current I of the output diode d It remained stable at the target current value I oREF When the load is stable, the output load current is equal to I.oREF This achieves constant current control of the output.

[0105] Execute step 4 to achieve, in discontinuous current mode, based on the reference primary current peak value I ppREF The on-time T of the switching transistor during the (n+1)th control cycle on (n+1), and control the turn-off duration of the switching transistor in the (n+1)th switching transistor control cycle to be T. off (n+1).

[0106] This embodiment employs a primary-side feedback control method, which involves adding an auxiliary winding to the primary side of the transformer and connecting a small-value current sampling resistor R in series between the source terminal of the switching transistor and the input ground terminal. p Extract the voltage divider signal v from the auxiliary winding respectively. FB and current sampling resistor voltage v p In the current switching control cycle, the off-time of the switching transistor in the next control cycle is directly calculated based on the calculated primary-side current parameters and related time parameters. This off-time is used as the control parameter of the converter, thereby generating the switching control signal "duty," which can achieve a constant output current in both modes. This method does not require PID modulation, thus avoiding the impact of low-frequency current ripple introduced by traditional PID modulation methods on the primary-side feedback flyback converter. Furthermore, this embodiment uses a combination of primary-side peak current control and off-time control to control the switching transistor's on / off state. In continuous current mode and with a large duty cycle, the on-time of the switching transistor adaptively changes to achieve fixed peak current control. Disturbances in the current switching control cycle return to zero at the end of this cycle, so the disturbances will not propagate to the next cycle, and no instability problems such as subharmonic oscillations will occur at low input voltages.

[0107] Based on the same inventive concept, this embodiment also provides a constant current control system for a primary-side feedback flyback converter. The solution provided by this system is similar to the solution described in the above method. Therefore, the specific limitations in the constant current control system embodiment provided below can be found in the limitations of the constant current control method above, and will not be repeated here.

[0108] like Figure 2 As shown, the system includes a primary-side feedback flyback converter main topology circuit and a control circuit. The main topology circuit samples the primary-side current and voltage signal v. p The voltage divider signal v of the primary auxiliary winding and the primary side FB Feedback is given to the control circuit, which then uses the input signal v as a basis for its operation. FB v p Loop control generates a duty signal for the switching transistor M, which is then fed back to the switching transistor M in the main topology circuit. pThis forms a closed-loop connection to achieve constant current control of the load output.

[0109] The control circuit includes a primary-side current detection module, a time detection module, a control parameter calculation module, and a PWM drive module. Specifically:

[0110] The primary-side current detection module is used to determine the valley value I of the primary-side winding current during the nth switching control cycle when the primary-side feedback flyback converter is operating in continuous current mode. pv (n); also used to output the valley value of the primary winding current I. pp (n). In this embodiment, a fixed primary-side peak current control is used, i.e., I... pp (n) is set to a constant peak primary current I. pp_set , and I pp_set The value is the set peak value of the primary current I. ppREF .

[0111] The time detection module is used to determine the transformer demagnetization time T during the nth switching control cycle. r (n), period duration T s (n), Switch conduction time T on .

[0112] The control parameter calculation module is used to calculate the transformer demagnetization time T when the primary-side feedback flyback converter is operating in continuous current mode. r (n), period duration T s (n), Primary winding current valley value I pv (n) and the set reference primary current peak value I ppREF And the target current value I to be achieved oREF Obtain the turn-off duration T of the switching transistor in the (n+1)th control cycle. off (n+1); It is also used when the primary-side feedback flyback converter is operating in discontinuous current mode, based on the transformer demagnetization time T. r (n), Switch conduction time T on (n) and the set reference primary current peak value I ppREF And the target current value I to be achieved oREF Obtain the turn-off duration T of the switching transistor in the (n+1)th control cycle. off (n+1).

[0113] The PWM drive module is used to drive the primary current I based on the reference peak current. ppREF The on-time T of the switching transistor during the (n+1)th control cycle on (n+1), and control the turn-off duration of the switching transistor in the (n+1)th switching transistor control cycle to be T. off(n+1), forming the switching control signal duty for the (n+1)th switching control cycle.

[0114] like Figure 4 As shown in (c), the time detection module contains two sub-units: a comparator unit and a time calculation unit. The comparator unit calculates the time from the signal v. FB The comparison result is S, which is compared with zero voltage (0V). comp3 The data is input to the time calculation unit, which calculates the relevant time variables and passes them to the control parameter calculation module.

[0115] The primary-side feedback control system provided in this embodiment is based on the same principle as the primary-side feedback control method described above. It can achieve a constant output current of the load in both continuous current mode and discontinuous current mode, avoiding the low-frequency current ripple of PID modulation by eliminating the need for PID modulation calculations, and also eliminating subharmonic oscillation problems at low input voltages. This system is applicable to isolated or non-isolated switching power supply circuit structures, and possesses versatility, reusability, and portability.

[0116] The above descriptions are merely preferred embodiments of this application, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.

Claims

1. A constant current control method for a primary-side feedback flyback converter, characterized in that, The method comprises, in the first n switching tube control period: When the primary-side feedback flyback converter operates in continuous current mode, determine the first... n Transformer demagnetization time per switching control cycle T r ( n ), cycle duration T s ( n Primary winding current valley value I pv ( n The primary winding current valley value I pv ( n It is the minimum current flowing through the primary winding during the conduction of the switching transistor; Based on the transformer demagnetization time T r ( n ), cycle duration T s ( n Primary winding current valley value I pv ( n and the set reference primary current peak value. I ppREF and the target current value to be achieved I oREF Get the first n +1 switching control cycle switching transistor turn-off time T off ( n+ The calculation expression for 1) is: ； wherein, n ps is the ratio of the number of turns of the primary main winding to the number of turns of the secondary winding of the transformer. Based on the peak value of the reference primary current I ppREF Control the first n +1 switching control cycle switching transistor on-time T on ( n+ 1), and control the first n The turn-off time of the switching transistor for +1 switching control cycle is T off ( n+ 1).

2. The constant current control method of a primary side feedback flyback converter according to claim 1, wherein, The method further includes: Determine the operating mode of the primary-side feedback flyback converter; When the primary-side feedback flyback converter operates in discontinuous current mode, determine the first... n Transformer demagnetization time per switching control cycle T r ( n ), Switch conduction time T on ( n ); Based on the transformer demagnetization time T r ( n ), Switch conduction time T on ( n ) and the set reference primary current peak I ppREF and the target current value to be achieved I oREF Get the first n +1 switching control cycle switching transistor turn-off time T off ( n+ 1); Based on the peak value of the reference primary current I ppREF Control the first n +1 switching control cycle switching transistor on-time T on ( n+ 1), and control the first n The turn-off time of the switching transistor for +1 switching control cycle is T off ( n+ 1).

3. The constant current control method of a primary side feedback flyback converter according to claim 2, wherein, The method for determining the operating mode of the primary-side feedback flyback converter includes: In the first n When the comparison result of the voltage division signal of the primary side auxiliary winding and zero voltage is always valid during the switch-off period of the switch control cycle, it is determined that the primary side feedback flyback converter works in continuous current mode. In the n During the turn-off period of the switching transistor in each switching control cycle, when there is a level change in the comparison result between the voltage division signal of the primary auxiliary winding and the zero voltage, it is determined that the primary feedback flyback converter is operating in discontinuous current mode. The voltage divider signal of the primary auxiliary winding is taken from the voltage divider terminal signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

4. The constant current control method for a primary-side feedback flyback converter according to claim 2, characterized in that, The method based on transformer demagnetization time T r ( n ), Switch conduction time T on ( n and the set reference primary current peak value. I ppREF and the target current value to be achieved I oREF Get the first n +1 switching control cycle switching transistor turn-off time T off ( n+ The calculation expression for 1) is: ； wherein, n ps is the ratio of the number of turns of the primary main winding to the number of turns of the secondary winding of the transformer.

5. The constant current control method for a primary-side feedback flyback converter according to claim 1, characterized in that, When the primary-side feedback flyback converter operates in continuous current mode, determine the first... n Transformer demagnetization time per switching control cycle T r ( n The methods include: In the n The length of time during which the comparison result of the voltage divider signal of the primary auxiliary winding and the zero voltage is effective during the switch turn-off period of each switch control cycle. T r_temp The demagnetization duration in this mode T r ( n The voltage divider signal of the primary auxiliary winding is taken from the voltage divider terminal signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

6. The constant current control method of a primary side feedback flyback converter according to claim 2, wherein, When the primary-side feedback flyback converter operates in discontinuous current mode, determine the first... n Transformer demagnetization time per switching control cycle T r ( n The methods include: In the n During the switch-off period of each switch control cycle, the time length at which the voltage divider signal of the primary auxiliary winding is compared with the zero voltage at the moment of switch-off is defined as: T r_temp The time length when the comparison result is an invalid level is defined as . t valley ;calculate[ T r_temp -( t valley / 2)] is the demagnetization time in this mode. T r ( n The voltage divider signal of the primary auxiliary winding is taken from the voltage divider terminal signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

7. The constant current control method of a primary side feedback flyback converter according to claim 2, wherein, The method comprises the following steps of: determining the on duration of the first switch control period when the primary side feedback flyback converter works in the discontinuous current mode n T on ( n )​ In the n During the switching period of each switching control cycle, the duration for which the comparison result between the voltage divider signal of the primary auxiliary winding and zero voltage is invalid is taken as the switching duration. T on ( n The voltage divider signal of the primary auxiliary winding is taken from the voltage divider terminal signal of the resistor voltage divider circuit connected between the two ends of the auxiliary winding of the primary side of the transformer.

8. The constant current control method of a primary side feedback flyback converter according to claim 1 or 2, characterized in that, The reference primary current peak value I ppREF Control the first n +1 switching control cycle switching transistor on-time T on ( n+ 1) The methods include: When the switching transistor is turned on, the switching transistor control signal remains at an active level until the primary current sampling voltage signal rises to the set primary voltage peak value. When the primary current sampling voltage signal rises to the set primary voltage peak value, the switching transistor control signal is switched to an inactive level, and the switching transistor is turned off. Wherein, the primary-side current sampling voltage signal is the voltage of the current sampling resistor connected in series between the source terminal of the switching transistor and the input ground terminal; the set primary-side voltage peak value is the reference primary-side current peak value. I ppREF The product of the current sampling resistor connected in series between the source terminal and the input ground terminal of the switching transistor.

9. The constant current control method of a primary side feedback flyback converter according to claim 1, wherein, When the primary-side feedback flyback converter operates in continuous current mode, determine the first... n The primary winding current valley value during each switching control cycle I pv ( n The methods include: During the conduction period of the switching transistor, the primary current sampling voltage signal is compared with the first voltage. V p1 Second voltage V p2 The comparison is made by defining the primary-side current sampling voltage signal as higher than the first voltage. V p1 Or the second voltage V p2 The comparison result at that time is the effective level, compared with the first voltage. V p1 The comparison result is the first comparison result, and it is compared with the second voltage. V p2 The comparison result is the second comparison result; the time length between the moment the switch is turned on and the moment the second comparison result becomes effective is recorded as follows: t a The time length between the moment when the second comparison result becomes valid and the moment when the first comparison result becomes valid is recorded as follows: t b Based on the linear increase of the primary-side current sampling voltage signal during the switching transistor's conduction period, the valley value of the primary-side current sampling voltage signal during the switching transistor's conduction period is calculated. V pv ( n ) is represented as: ； Then the valley value V pv ( n The corresponding primary winding current valley value I pv ( n It is calculated by the following formula: I pv ( n ) = V pv ( n ) / R p ； Wherein, the primary-side current sampling voltage signal is the voltage of the current sampling resistor connected in series between the source terminal of the switch and the input ground terminal; the first voltage V p1 Second voltage V p2 Given the voltage, and V p1 > V p2 ; R p This is a current sampling resistor connected in series between the source terminal of the switching transistor and the input ground terminal.

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