TWI653810B - Control device and control method - Google Patents

Abstract

The present invention relates to a control device and a control method, which are applied to a flyback converter. The flyback converter includes an auxiliary switch. The control device includes: an on-time setting unit for setting an on-time threshold according to a reference value of the excitation negative current and an output voltage of the flyback converter; and an on-time control unit for outputting a control signal to control the auxiliary switch Turn on, turn off the auxiliary switch when the on time of the auxiliary switch reaches the on time threshold. In this case, the primary-side switch of the flyback converter can be turned on at zero voltage under different output voltages.

Description

Control device and control method

This case relates to the field of power electronics technology, and in particular, to a control device and a control method applied to a flyback converter.

At present, quasi-resonant flyback converters are the most popular circuit topology applied to small power switching power supplies. The quasi-resonant flyback converter can realize the primary-side power switch when the low-voltage input (V _bus <nV _o , where: V _bus is the input voltage; n is the turns ratio of the primary winding of the transformer; V _o is the output voltage) Zero voltage turn-on (ZVS), when the high-voltage input (V _bus > nV _o ), can achieve the valley turn-on of the primary-side power switch, so that the switching loss can be significantly reduced. However, with the development of high frequency, although the quasi-resonant flyback converter can achieve valley turn-on at high-voltage input, the turn-on loss has become larger and larger, which seriously affects the efficiency of the converter. In order to solve the problem that the quasi-resonant flyback converter cannot fully realize the zero-voltage turn-on (ZVS) of the primary-side power switch at the high-voltage input, the prior art scheme proposes new control methods such as delayed conduction of the secondary-side synchronous rectifier and New circuit topologies such as source clamp flyback converters.

However, the prior art solution is only applicable to a case where the output voltage is constant, and in the case of a variable output voltage application, it cannot be guaranteed that the zero-voltage turn-on of the primary-side power switch can be achieved under all operating conditions.

It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of the case, and therefore may include information that does not constitute the prior art known to those of ordinary skill in the art.

The purpose of this case is to provide a control device and a control method.

According to an aspect of the present invention, a control device is provided, which is applied to a flyback converter, and the flyback converter includes an auxiliary switch. The control device includes: an on-time setting unit, which is configured to: Output voltage to set the on-time threshold; and an on-time control unit for outputting a control signal to control the conduction of the auxiliary switch, and turn off the auxiliary switch when the on-time of the auxiliary switch reaches the on-time threshold.

In an exemplary embodiment of the present application, the flyback converter is an RCD clamped flyback converter or an active clamped flyback converter.

In an exemplary embodiment of the present application, the auxiliary switch is a synchronous rectifier tube, a clamp tube, a switch connected in parallel to the secondary-side rectifier unit of the flyback converter, or a switch connected in series to the auxiliary winding of the flyback converter.

In an exemplary embodiment of the present application, the on-time control unit is configured to output a control signal according to a timing start signal.

In an exemplary embodiment of the present invention, the operating mode of the flyback converter is a discontinuous mode or a critical continuous mode.

In an exemplary embodiment of the present application, the on-time control unit includes a timer and an auxiliary switch controller. The timer receives a timing start signal and starts the timer according to the timing start signal. Perform timing and generate timing signals; the auxiliary switch controller receives timing signals and generates control signals according to the timing signals.

In an exemplary embodiment of the present application, the auxiliary switch controller turns on the auxiliary switch according to the timing start signal.

In an exemplary embodiment of the present application, when the timing signal is greater than or equal to the on-time threshold, the auxiliary switch controller turns off the auxiliary switch.

In an exemplary embodiment of the present application, the timer further resets the timer according to a reset signal.

In an exemplary embodiment of the present application, in the discontinuous mode, a timing start signal is obtained by detecting an on signal of an auxiliary switch; in the critical continuous mode, a timing start signal is obtained by detecting a zero-crossing point of a negative excitation current.

In an exemplary embodiment of the present application, the zero crossing of the exciting negative current is detected by a current transformer, a sampling resistor or the internal resistance of the auxiliary switch.

In an exemplary embodiment of the present application, the reset signal is obtained by detecting an off signal of the auxiliary switch.

In an exemplary embodiment of the present application, the on-time setting unit includes: a negative excitation current setting unit for generating a reference value of the negative excitation current; and an on-time calculation unit for calculating the reference value of the negative excitation current and the flyback converter. The output voltage is calculated to obtain the on-time threshold.

In an exemplary embodiment of the present application, the excitation negative current setting unit is configured to set a reference value of the excitation negative current based on the input voltage of the flyback converter.

In an exemplary embodiment of the present application, the excitation negative current setting unit is configured to set a reference value of the excitation negative current based on the input voltage and the output voltage of the flyback converter.

In an exemplary embodiment of the present application, the output voltage of the flyback converter is variable.

In an exemplary embodiment of the present invention, the output voltage of the flyback converter is 5V, 9V, 15V, or 20V.

According to an aspect of the present invention, there is provided a switching power supply including a control device according to any one of the above.

According to an aspect of the present invention, a control method is provided, which is applied to a flyback converter. The flyback converter includes an auxiliary switch. The control method includes: (a) detecting an output voltage of the flyback converter, and based on the output voltage and a negative magnetizing current; The reference value sets the on-time threshold; (b) controls the conduction of the auxiliary switch according to a control signal, and turns off the auxiliary switch when the on-time of the auxiliary switch reaches the on-time threshold.

In an exemplary embodiment of the present application, the flyback converter is an RCD clamped flyback converter or an active clamped flyback converter.

In an exemplary embodiment of the present application, the auxiliary switch is a synchronous rectifier tube, a clamp tube, a switch connected in parallel to the secondary-side rectifier unit of the flyback converter, or a switch connected in series to the auxiliary winding of the flyback converter.

In an exemplary embodiment of the present invention, step (b) includes: outputting the control signal according to a timing start signal.

In an exemplary embodiment of the present invention, the operating mode of the flyback converter is a discontinuous mode or a critical continuous mode.

In an exemplary embodiment of the present invention, step (b) includes: starting a timer to perform timing according to a timing start signal to generate a timing signal; and generating a control signal according to the timing signal.

In an exemplary embodiment of the present invention, the auxiliary switch is turned on according to the timing start signal.

In an exemplary embodiment of the present application, when the timing signal is greater than or equal to the on-time threshold, the auxiliary switch is turned off.

In an exemplary embodiment of the present invention, step (b) further includes: resetting the timer according to a reset signal.

In an exemplary embodiment of the present application, the timing start signal is obtained by detecting the on signal of the auxiliary switch in the discontinuous mode; and the timing start signal is obtained by detecting the zero crossing of the negative excitation current in the critical continuous mode.

In an exemplary embodiment of the present application, the zero crossing of the exciting negative current is detected by a current transformer, a sampling resistor or the internal resistance of the auxiliary switch.

In an exemplary embodiment of the present application, the reset signal is obtained by detecting an off signal of the auxiliary switch.

In an exemplary embodiment of the present application, step (a) includes: calculating the on-time threshold based on the output voltage and the reference value of the exciting negative current through a division operation.

In an exemplary embodiment of the present invention, the control method further includes: (c) after the auxiliary switch is turned off, the primary-side power switch of the flyback converter is realized by resonance of the excitation inductance and the parasitic capacitance in the flyback converter The zero voltage of the tube is turned on.

In an exemplary embodiment of the present application, step (a) further includes: setting a reference value of the exciting negative current based on the input voltage of the flyback converter.

In an exemplary embodiment of the present application, step (a) further includes: setting a reference value of the negative excitation current based on the maximum value of the input voltage of the flyback converter.

In an exemplary embodiment of the present application, step (a) further includes: setting a reference value of the exciting negative current based on the input voltage and the output voltage of the flyback converter.

In an exemplary embodiment of the present application, the output voltage of the flyback converter is variable.

In an exemplary embodiment of the present invention, the output voltage of the flyback converter is 5V, 9V, 15V, or 20V.

According to the control device and control method of the exemplary embodiment of the present invention, the on-time threshold is set according to the reference value of the exciting negative current and the output voltage of the flyback converter, and a control signal is output to control the conduction of the auxiliary switch. The auxiliary switch is turned off at the on-time threshold. On the one hand, through the reference value of the exciting negative current and the output voltage of the flyback converter that is monitored in real time, the on-time thresholds under different voltage states can be set in real time; on the other hand, the control signal of the auxiliary switch is adjusted instantly according to the on-time threshold , Used to make the on-time of the auxiliary switch follow the on-time threshold, so that the zero-voltage turn-on of the primary-side power switch in the flyback converter at different output voltages can be achieved.

It should be understood that the above general description and the following detailed description are merely exemplary and explanatory, and should not limit the case.

S ₁ ‧‧‧Primary power switch

S ₂ ‧‧‧Clamp

S _R ‧‧‧Synchronous Rectifier

I _s ‧‧‧ secondary current

t0-t5‧‧‧time

L _m ‧‧‧ Excitation inductance

V _o ‧‧‧ output voltage

C _EQ ‧‧‧ Parasitic capacitance

I _{m_n} (t) ‧‧‧ amplitude of negative excitation current

S _aux ‧‧‧ Switch in parallel with diode D1

W _aux ‧‧‧ auxiliary winding

S _{aux_VCC} ‧‧‧ Switch in series with auxiliary winding

600, 1100, 1200, 1400, 1500‧‧‧ control devices

610, 1110, 1210, 1410, 1510‧‧‧Flyback converter

620‧‧‧on time setting unit

630, 1130, 1230, 1430, 1530‧‧‧ On time control unit

640, 1140, 1240, 1440, 1540‧‧‧ Excitation negative current setting unit

650, 1150, 1250, 1450, 1550‧‧‧ on-time calculation unit

1480, 1580‧‧‧ input voltage detection unit

810‧‧‧Timer

820‧‧‧Auxiliary switch controller

I _{m_N} ‧‧‧ Reference value of negative excitation current

t _set ‧‧‧on time threshold

T‧‧‧Transformer

C _o ‧‧‧ output capacitor

V _bus ‧‧‧ input voltage

R ₁ ‧‧‧first resistor

R ₂ ‧‧‧Second resistor

(a), (b), (c) ‧‧‧ steps

FIG. 1 schematically illustrates a circuit diagram of an active clamp flyback converter in a technical solution.

FIG. 2 schematically illustrates a discontinuous mode control waveform diagram of an active clamp flyback converter in a technical solution.

FIG. 3 schematically shows a circuit diagram of an RCD clamp flyback converter in a technical solution.

FIG. 4 schematically illustrates a critical continuous mode control waveform diagram of an RCD clamp element flyback converter in a technical solution.

FIG. 5 schematically shows a circuit diagram of an RCD clamp flyback converter in another technical solution.

FIG. 6 schematically illustrates a control principle block diagram of a control device according to an exemplary embodiment of the present invention.

FIG. 7 schematically illustrates a control principle block diagram of a control device according to another exemplary embodiment of the present invention.

FIG. 8 schematically illustrates a circuit diagram of an on-time control unit according to still another exemplary embodiment of the present invention.

FIG. 9 schematically illustrates a discontinuous mode control waveform diagram of an RCD clamp element flyback converter according to still another exemplary embodiment of the present invention.

FIG. 10 schematically illustrates a critical continuous mode control waveform diagram of an active clamp flyback converter according to still another exemplary embodiment of the present invention.

FIG. 11 schematically illustrates a specific embodiment of an on-time control method of an RCD clamp flyback converter according to still another exemplary embodiment of the present invention.

FIG. 12 schematically illustrates a specific embodiment of an on-time control method of an active clamp flyback converter according to another exemplary embodiment of the present invention.

FIG. 13 schematically illustrates a specific embodiment of a method for setting a reference value of an exciting negative current of an RCD clamp flyback converter according to another exemplary embodiment of the present invention as a function of an input voltage.

FIG. 14 schematically illustrates a specific embodiment of a method for setting a reference value of an exciting negative current of an active clamp flyback converter according to an input voltage according to another exemplary embodiment of the present invention.

FIG. 15 schematically illustrates a flowchart of a control method according to still another exemplary embodiment of the present invention.

Some typical embodiments embodying the features and advantages of this case will be described in detail in the description in the subsequent paragraphs. It should be understood that the present case can have various changes in different aspects, all of which do not depart from the scope of the present case, and that the descriptions and diagrams therein are essentially for the purpose of illustration, rather than limiting the case.

Figure 1 shows a circuit diagram of an active clamp flyback converter in a technical solution. The active clamp flyback converter can realize the zero-voltage turn-on (ZVS) of the primary-side power switch S _1. The existing control method is: control the clamp S _{2 to} be turned on only before the primary-side power switch S _{1 is} turned on. Set time, this set time is t2-t3 in the control waveform diagram shown in Figure 2.

FIG. 3 shows a circuit diagram of an RCD clamp flyback converter in a technical solution. RCD clamp flyback converter by delaying conduction quasi-resonant flyback converter secondary-side synchronous rectifier S _R to achieve the primary side power switch S ₁ of the zero-voltage (ZVS), secondary-side synchronous rectifier prior the method of controlling the conduction delay is S _R: S _R to control the synchronous rectifier continues to conduct after the set time of the secondary-side current I _s to zero, t1-t2 waveform diagram for controlling the set time of 4 as shown in FIG.

The above two methods for achieving the zero voltage turn-on (ZVS) of the primary-side power switch S ₁ are both achieved by controlling the set time of the synchronous rectifier S _R or the clamp S ₂ to be turned on. The situation is applicable.

However, with the development of power adapters, especially the popularization and popularization of USB-PD Type-C, the application of variable output voltage has become increasingly popular. For the application of variable output voltage, the above control method is no longer applicable, because: whether it is an RCD clamp flyback converter or an active clamp flyback converter, it achieves zero-voltage turn-on of the primary-side power switch. The basic principle of (ZVS) is as follows: before the primary-side power switch S _{1 is turned on} , a negative excitation current I _{m_n} is generated on the excitation inductance L _m of the transformer, and the primary-side power switch is realized by the help of the negative excitation current I _{m_n} . The zero voltage turn-on (ZVS) of the tube S ₁ is determined by the following formula:

Where: L _m is the excitation inductance value of the transformer, n is the turns ratio of the transformer, V ₀ is the output voltage value of the converter, I _{m_n} (t) is the amplitude of the negative excitation current, and t is the on-time of the auxiliary switch ( For the synchronous rectifier of the quasi-resonant flyback converter, it refers to the on-time after the secondary side current I _s drops to zero, and for the clamp of the active clamp flyback converter, it refers to the primary ON time before the power switch is turned on).

It can be seen from the above formula that for a fixed design, L _m and n are fixed. If the output voltage Vo is fixed, it can be known from the formula (1) that the fixed on-time t means a fixed magnitude of the negative excitation current. Therefore, by controlling the synchronous rectifier tube S _R or the clamp tube S _{2 to be} turned on for a set time t It is suitable for the application of fixed output voltage. If the output voltage is variable, a fixed on-time means that the magnitude of the negative magnetizing current will change as the output voltage V _o changes. Taking the application of USB-PD Type-C as an example, the minimum output voltage is 5V and the maximum output voltage is 20V. If a fixed on-time control method is used, it will cause one of the following two results:

A: If the set on-time exactly meets the condition of zero-voltage turn-on (ZVS) of the primary-side power switch when the output voltage is 5V, then when the output voltage is 20V, the amplitude of the negative excitation current generated will be the output voltage of 4 times the amplitude of the negative excitation current at 5V. Excessive negative excitation current will introduce additional losses and affect the efficiency of the converter.

B: If the set on-time exactly meets the condition of zero-voltage turn-on (ZVS) of the primary-side power switch when the output voltage is 20V, then when the output voltage is 5V, the amplitude of the negative excitation current generated will only be the output voltage of 1/4 at 20V, too small excitation negative current amplitude will cause the primary-side power switch to achieve zero voltage turn-on.

Based on the foregoing, in this exemplary embodiment, a control device is first provided. The control device is used to control a flyback converter 610, where the flyback converter 610 includes an auxiliary switch. Referring to FIG. 6, the control device 600 may include an on-time setting unit 620 and an on-time control unit 630. Wherein: the on-time setting unit 620 for setting a conduction time threshold value t _set according to the exciting negative current reference value and the output voltage V _o; and on-time control unit 630 is turned on to output a control signal to control the auxiliary switch, turned on of the auxiliary switch When the time reaches the on-time threshold _tset , the auxiliary switch is turned off. For example, the control signal may be obtained in accordance with the timing t _set start signal and on-time threshold.

According to the control device of this exemplary embodiment, on the one hand, through an excitation negative current reference value and the output voltage of the flyback circuit monitored in real time, on-time time thresholds under different voltage states can be set in real time; on the other hand, according to the on-state The time threshold adjusts the on-time of the auxiliary switch in real time to make the on-time of the auxiliary switch follow the on-time threshold, so that the zero-voltage turn-on of the primary-side power switch in the flyback converter at different output voltages can be achieved.

In this example embodiment, the flyback converter further includes a primary-side switching unit, a secondary-side rectifying unit, a transformer, and an output capacitor, wherein the primary-side switching unit includes a primary-side power switching tube, and the secondary-side rectifying unit includes a first Terminal and second terminal, the first terminal and the second terminal are electrically connected to the transformer and the output capacitor, respectively. In order to adapt to the application of variable output voltage, to achieve zero voltage turn-on (ZVS) of the primary-side power switch in the full input voltage range (such as 90 ~ 264Vac) and full load range under different output voltages, the auxiliary switch needs to be directly controlled On time. According to the following formula (2):

From the above formula (2), it can be known that, for a set excitation negative current reference I _{m_N} , the on-time threshold t _set and the output voltage _Vo are inversely related. Adjusting the on-time threshold of the auxiliary switch according to different output voltages, and then adjusting the on-time of the auxiliary switch, can achieve the purpose of controlling the negative excitation current. Therefore, the basic principle of this case is that before the primary-side power switch is turned on, by controlling the on and off of the auxiliary switch, a negative excitation current is generated in the flyback converter. First, the conduction of the auxiliary switch is controlled so that the conduction time of the auxiliary switch reaches the conduction time threshold value t _set . Then, the auxiliary switch is controlled to be turned off. After the auxiliary switch is turned off, the primary side power switch tube is realized by the resonance of the excitation inductance L _m and the parasitic capacitance C _EQ of the primary line by using the negative excitation current at this time as the initial value. Zero voltage turn-on (ZVS). In this case, by setting the on-time threshold of the auxiliary switch reasonably, the zero-voltage turn-on (ZVS) of the primary-side power switch tube can be achieved in the full input voltage range and the full load range of different output voltages. In this embodiment, the parasitic capacitance C _{EQ is} composed of the parasitic capacitance of the primary-side power switch S1 and the parasitic capacitance of the primary-side coil of the transformer T.

It should be noted that, in this exemplary embodiment, the output voltage of the flyback converter is variable. For example, the output voltage of the flyback converter may be 5V, 9V, 15V, or 20V, which is not specifically limited in this case.

In addition, in the present exemplary embodiment, the flyback converter 610 may be an active clamp flyback converter as shown in FIG. 1 or an RCD clamp flyback converter as shown in FIGS. 3 and 5. , But the flyback converter in the example embodiment of the present case is not limited to this. Correspondingly, in this exemplary embodiment, the auxiliary switch of the flyback converter 610 may be the clamp tube S ₂ shown in FIG. 1 or the synchronous rectifier tube S _R shown in FIG. 3. The auxiliary switch in the exemplary embodiment is not limited thereto. For example, the secondary side shown in Figure 5 is a diode-rectified RCD clamp flyback converter. The auxiliary switch may be a switch S _aux connected in parallel to the diode D1, or its auxiliary switch may be connected in series to the auxiliary winding. W _aux switch S _{aux_VCC} .

It should be noted that, in this exemplary embodiment, the working mode of the flyback converter may be a discontinuous mode or a critical continuous mode, which is not specifically limited in this case.

Further, as shown in FIG. 7, in this exemplary embodiment, in order to reasonably set the reference value of the negative excitation current and the threshold value of the on time, the on time setting unit 620 may further include: a negative excitation current setting unit 640 and on time calculation Unit 650. The excitation negative current setting unit 640 is configured to set the excitation negative current reference value I _{m_N} based on an input voltage or / and an output voltage of the flyback converter. The on-time calculation unit 650 is configured to set the on-time threshold t _set according to the negative excitation current reference value I _{m_N} and the output voltage V _{o of the} flyback converter.

In an embodiment, the on-time calculation unit may include a multiplication or division circuit, but is not limited thereto. The multiplication or division circuit receives the reference value of the excitation negative current I _{m_N} and the output voltage V _{o of the} flyback converter, and according to the parameters of the circuit itself, such as the value of the excitation inductance L _m and the turns ratio n of the transformer, after formula (2) Calculate to set the on-time threshold t _set .

In this exemplary embodiment, the on-time control unit 630 may be implemented in various ways. FIG. 8 illustrates an embodiment of the on-time control unit 630 according to the present case. As shown in FIG. 8, the on-time control unit includes a timer 810 and an auxiliary switch controller 820. The timer 810 is configured to start timing according to a timing start signal and generate a timing signal. The auxiliary switch controller 820 is configured to generate a control signal according to the timing signal.

In this exemplary embodiment, the auxiliary switch controller 820 turns on the auxiliary switch according to the timing start signal; the timing signal gradually increases after the timer 810 starts counting, and when the timing reaches the on-time threshold t _set , the auxiliary switch controller 820 turns off the auxiliary switch.

In the present exemplary embodiment, for the intermittent operation mode, the timing start signal of the timer 810 may be obtained by an on signal of an auxiliary switch. As shown in FIG. ₂ , the rising edge transition signal of the S ₂ driving signal at time t2 is an on signal of the auxiliary switch; or, as shown in FIG. 9, the rising edge transition signal of the S _R driving signal at time t 2 To turn on the auxiliary switch, you can get the timing start signal by detecting the rising edge transition signal. It should be noted that the timing start signal can be synchronized with this rising edge transition signal, or can be obtained by a certain delay from the rising edge transition signal.

Further, in this exemplary embodiment, for the critical continuous mode, the timing start signal of the timer can be obtained by detecting the zero-crossing point of the negative excitation current (such as time t1 in FIG. 4). Specifically, the current transformer, sampling resistor or internal resistance of the power device such as the The internal resistance is used to detect the zero crossing of the exciting negative current.

In one embodiment, the timer 810 is also reset according to a reset signal. Further, in this exemplary embodiment, the reset signal of the timer can be obtained by the off signal of the auxiliary switch. For example, the reset signal of the timer can be synchronized with the off signal of the auxiliary switch, or by the off signal Do get delayed. As shown in Figure ₂ , the falling edge transition signal of the S ₂ drive signal at t3 is the off signal of the auxiliary switch; as shown in Figure 9, the falling edge transition signal of the S _R drive signal at t3 is the auxiliary switch. Or as shown in Figure 10, the falling edge transition signal of the S ₂ driving signal at t2 is the off signal of the auxiliary switch, and the reset signal can be obtained by detecting the falling edge transition signal. It should be noted that the reset signal can be synchronized with the falling edge transition signal, or can be obtained by a certain delay from the falling edge transition signal.

In this example embodiment, the on-time of the auxiliary switch is controlled to control the negative excitation current. There are many different methods for different flyback converters. The following describes the RCD clamped flyback converter in discontinuous mode and The active-clamp flyback converters in discontinuous mode are illustrated separately.

Fig. 11 shows a specific embodiment of a control device. As shown in FIG. 11, the control device 1100 is used to control the flyback converter 1110. The control device 1100 includes an on-time control unit 1130, an excitation negative current setting unit 1140, and an on-time calculation unit 1150. The flyback converter 1110 is an RCD clamped flyback converter, which includes a primary-side switch unit, a secondary-side rectifier unit, a transformer T, and an output capacitor C _o , wherein the primary-side switch unit includes a primary-side power switch S ₁ . The stage-side rectifier unit includes a synchronous rectifier S _R , and the secondary-side rectifier unit is electrically connected to the transformer T and the output capacitor C _o respectively.

In this embodiment, the on-time calculation unit 1150 obtains the on-time threshold t _set according to the output voltage signal V _o monitored in real time and the excitation negative current reference value I _{m_N} output by the excitation negative current setting unit 1140, and sets the on-time threshold t _{set is} sent to the on-time control unit 1130; the control device 1100 obtains the timing start signal through the second on-on signal of the synchronous rectifier tube S _R (such as the S _R driving signal at time t2 in FIG. 9); the on-time control unit 1130 An on-time threshold t _set and a timing start signal are obtained, and are used to output a control signal to turn on the synchronous rectifier S _R , and turn off the synchronous rectifier S _R when the on-time of the auxiliary switch reaches the on-time threshold t _set . At the same time, the on-time control unit 1130 implements a reset according to a reset signal generated by an off signal of the synchronous rectifier S _R.

Fig. 12 shows another specific embodiment of a control device. As shown in FIG. 12, the control device 1200 is used to control the flyback converter 1210. The control device 1200 includes: an on-time control unit 1230, an excitation negative current setting unit 1240, and an on-time calculation unit 1250. The flyback converter 1210 is an active clamp flyback converter, which includes a primary-side switching unit, a secondary-side rectifying unit, a transformer T, and an output capacitor C _o , wherein the primary-side switching unit includes a primary-side power switch S ₁ and The clamp tube S ₂ , the secondary-side rectifier unit includes a synchronous rectifier tube S _R , and the secondary-side rectifier unit is electrically connected to the transformer T and the output capacitor C _o respectively.

In the embodiment, the on-time calculation unit 1250 obtains the on-time threshold t _set according to the output voltage signal V _o monitored in real time and the excitation negative current reference value I _{m_N} output by the excitation negative current setting unit 1240, and sets the on-time threshold t _set delivered to the on-time control unit 1230; 1200 by the control means opening the clamp transistor S ₂ signal to obtain timing start signal.

The on-time control unit 1230 obtains a timing start signal and an on-time threshold t _set for outputting a control signal to turn on the clamp S ₂ , and turns off the clamp S when the on-time of the auxiliary switch reaches the on-time threshold t _set . ₂ . At the same time, the on-time control unit 1230 generates a reset signal according to the turn-off signal of the clamp tube S ₂ to realize resetting.

In addition, in each exemplary embodiment of the present application, a negative excitation current setting unit is included to set a reference value of the negative excitation current I _{m_N} . Regarding the setting of the reference value of the negative excitation current, it is known through research that at the low-voltage input (V _bus <nV _o ), the zero-voltage turn-on (ZVS) of the primary-side power tube can be achieved without the help of the negative excitation current; at the high-voltage input (V _bus > nV _o ), in order to achieve zero voltage turn-on (ZVS) of the primary-side power tube, the minimum amplitude of the negative excitation current must meet:

Among them: I _{m_N} is the reference value of the negative excitation current, V _bus is the input voltage, V _O is the output voltage, and n is the turns ratio of the transformer; L _m is the inductance of the magnetizing inductance; and C _EQ is the parasitic capacitance value.

According to the above formula (3), for a specific circuit design, n, L _m, and C _EQ are fixed. In order to achieve zero voltage turn-on (ZVS) of the primary-side power tube, the reference value of the excitation negative current and the input voltage V _{bus is} related to the output voltage V _O. Therefore, the excitation negative current setting unit can adjust the excitation negative current reference value in real time based on the input voltage and the output voltage of the flyback converter.

However, with the above method, in order to adjust the excitation negative current reference value I _{m_N} in real time, two variables need to be monitored in real time: the input voltage V _bus and the output voltage V _O. This will increase the complexity of control. Further studies found that: a flyback transformer in the case of working the high voltage input (V _bus> nVo) when, negligible influence on the output voltage of the negative magnetizing current reference value, i.e., the negative exciting current reference value is only related to the input voltage, This greatly simplifies the setting of the reference value of the exciting negative current. Then, the above formula (3) can be simplified into the following formula (4):

Thereby, the excitation negative current setting unit can set a reference value of the excitation negative current based on the input voltage of the flyback converter.

In this embodiment, there are two methods for setting the reference value of the exciting negative current:

Fixed reference value setting method: In order to achieve zero voltage turn-on (ZVS) of the primary-side power switch in the full input voltage range, the reference value of the negative excitation current is set according to the maximum input voltage, that is:

Where: V _{bus_max} is the maximum input voltage.

For the fixed reference value setting method, when the input voltage is at the maximum value, it can just meet the zero voltage turn-on (ZVS) of the primary-side power switch; but when the input voltage is low, the negative excitation voltage generated by this control method The current amplitude is larger than the amplitude of the negative excitation current required to achieve zero voltage turn-on (ZVS) of the primary-side power tube, which will cause additional losses and is not conducive to efficiency optimization. In applications where efficiency requirements are not very high, a fixed reference value setting method can be used.

For applications with high efficiency requirements, the setting method of the reference value changing with the input voltage can be used to optimize the efficiency of the converter. Therefore, the reference value of the negative excitation current can be set as:

Among them: I _{m_N} (V _bus ) is the reference value of the negative excitation current.

For a specific circuit design, L _m and C _EQ are fixed. From the above formula (6), it can be known that the reference value of the excitation negative current is proportional to the input voltage V _bus , and the excitation negative current setting unit can detect according to the input voltage detection unit. The input voltage value V _{bus is} directly calculated as the reference value of the excitation negative current I _{m_N} .

Fig. 13 shows still another specific embodiment of a control device. For example, FIG. 13 is similar to the structure in FIG. 11, but FIG. 13 further includes a specific example of the excitation negative current setting unit. Shown in FIG. 13, the control device further includes an input voltage detection unit 1480, in the present embodiment, the input voltage detection unit 1480 comprises a first resistor R ₁ and a second resistor R _2, and first and second resistors R ₁ through Two resistors R ₂ divide the voltage to detect the input voltage V _bus . The input voltage detection unit 1480 inputs the input voltage V _bus to the excitation negative current setting unit 1440 to set the excitation negative current reference value I _{m_N} , and sends the excitation negative current reference value I _{m_N} to the on-time calculation unit 1450, and the on-time calculation unit 1450 Calculate the on-time threshold t _set based on the reference value of the negative current I _{m_N} and the output voltage V _{o of the} real-time monitoring, and input the on-time threshold t _set to the on-time control unit 1430; Fig. 9 ( _SR driving signal at time t2) to obtain a timing start signal to enable the on-time control unit 1430; the on-time control unit 1430 obtains an on-time threshold t _set and a timing start signal for outputting a control signal to turn on synchronous rectifier S _R, and the auxiliary switch to the on-time to reach the on time threshold t synchronous rectifier off when the _set S _R. At the same time, the on-time control unit 1430 implements a reset according to a reset signal generated by an off signal of the synchronous rectifier S _R.

Fig. 14 shows a further specific embodiment of a control device. FIG. 14 is similar to the structure in FIG. 12. The main difference is that the auxiliary switch in FIG. 14 is the clamp tube S ₂ on the primary side of the active clamp flyback converter.

In addition, in this exemplary embodiment, a control method is also provided. The control method can be applied to the flyback converter as shown in FIG. 6 to FIG. 14. The flyback converter includes an auxiliary switch. As shown, the control method may include the following steps: step (a): detecting the output voltage of the flyback converter, and setting the on-time threshold based on the output voltage and the reference value of the exciting negative current; steps (b): Turn on the auxiliary switch according to the control signal, and turn off the auxiliary switch when the on-time of the auxiliary switch reaches the on-time threshold.

On the one hand, the on-time time thresholds under different voltage states can be set in real time through a reference value of the exciting negative current and the output voltage of the flyback converter that is monitored in real time; on the other hand, the on-time of the auxiliary switch is adjusted according to the on-time threshold The time is used to make the on-time of the auxiliary switch follow the on-time threshold, so that the zero-voltage turn-on of the primary-side power switch in the flyback converter at different output voltages can be achieved.

Further, in this exemplary embodiment, the auxiliary switch may be a synchronous rectifier tube, a clamp tube, a switch connected in parallel on the secondary side rectifier unit of the flyback converter, or a switch connected in series with the auxiliary winding of the flyback converter. .

Further, in this example embodiment, in the discontinuous mode, the timing start signal can be obtained by detecting the on signal of the auxiliary switch; and in the critical continuous mode, the timing start can be obtained by detecting the zero crossing of the negative excitation current. signal.

In addition, in this example, step (a) may further include: calculating the on-time threshold value based on the output voltage and the reference value of the exciting negative current through a division operation.

In addition, in this exemplary embodiment, the control method may further include: (c) after the auxiliary switch is turned off, the primary-side power switch of the flyback converter is realized by resonance of the excitation inductance and parasitic capacitance in the flyback converter The zero voltage of the tube is turned on.

Since the steps in the control method in this exemplary embodiment correspond to the functions of the units or modules of the control device one by one, they will not be repeated here.

This case can be modified in various ways by those skilled in the art, but none of them can be protected by the scope of the patent application.

Claims (35)

A control device is applied to a flyback converter. The flyback converter includes an auxiliary switch. The control device includes: an on-time setting unit for determining a reference value of an exciting negative current and the flyback converter. Output voltage to set a conduction time threshold; and a conduction time control unit for outputting a control signal to control the conduction of the auxiliary switch, and turning off the auxiliary switch when the conduction time of the auxiliary switch reaches the conduction time threshold, wherein the conduction time The control unit is configured to output the control signal according to a timing start signal. The control device according to claim 1, wherein the flyback converter is an RCD clamped flyback converter or an active clamped flyback converter. The control device according to claim 1, wherein the auxiliary switch is a synchronous rectifier tube, a clamp tube, a switch connected in parallel to the secondary side rectifier unit of the flyback converter, or an auxiliary device connected in series to the flyback converter. Winding switch. The control device according to claim 1, wherein the flyback converter operates in a discontinuous mode or a critical continuous mode. The control device according to claim 1, wherein the on-time control unit includes a timer and an auxiliary switch controller, the timer receives a timing start signal, and starts the timer for timing according to the timing start signal To generate a timing signal; the auxiliary switch controller receives the timing signal and generates the control signal according to the timing signal. The control device according to claim 5, wherein the auxiliary switch controller turns on the auxiliary switch according to the timing start signal. The control device according to claim 5, wherein when the timing signal is greater than or equal to the on-time threshold, the auxiliary switch controller turns off the auxiliary switch. The control device according to claim 5, wherein the timer further resets the timer according to a reset signal. The control device according to claim 5, wherein in the discontinuous mode, the timing start signal is obtained by detecting an on signal of the auxiliary switch; in the critical continuous mode, the timing is obtained by detecting a zero-crossing point of the negative excitation current. Start signal. The control device according to claim 9, wherein the zero crossing of the exciting negative current is detected by a current transformer, a sampling resistor, or an internal resistance of the auxiliary switch. The control device according to claim 8, wherein the reset signal is obtained by detecting an off signal of the auxiliary switch. The control device according to claim 1, wherein the on-time setting unit includes: an excitation negative current setting unit for generating the reference value of the excitation negative current; and an on-time calculation unit for the reference value of the excitation negative current And the output voltage of the flyback converter to calculate the on-time threshold. The control device according to claim 12, wherein the excitation negative current setting unit is configured to set the reference value of the excitation negative current based on the input voltage of the flyback converter. The control device according to claim 12, wherein the excitation negative current setting unit is configured to set the reference value of the excitation negative current based on an input voltage and an output voltage of the flyback converter. The control device according to claim 1, wherein the output voltage of the flyback converter is variable. The control device according to claim 15, wherein the output voltage of the flyback converter is 5V, 9V, 15V, or 20V. A switching power supply includes the control device according to any one of claims 1 to 16. A control method is applied to a flyback converter, the flyback converter includes an auxiliary switch, wherein the control method includes: (a) detecting an output voltage of the flyback converter, and based on the output voltage and an excitation negative A current reference value to set an on-time threshold; (b) controlling the conduction of the auxiliary switch according to a control signal, and turning off the auxiliary switch when the on-time of the auxiliary switch reaches the on-time threshold, wherein The start signal outputs the control signal. The control method according to claim 18, wherein the flyback converter is an RCD clamped flyback converter or an active clamped flyback converter. The control method according to claim 18, wherein the auxiliary switch is a synchronous rectifier tube, a clamp tube, a switch connected in parallel to the secondary side rectifier unit of the flyback converter, or an auxiliary device connected in series to the flyback converter. Winding switch. The control method according to claim 18, wherein the flyback converter operates in a discontinuous mode or a critical continuous mode. The control method according to claim 18, wherein the step (b) comprises: starting a timer to perform timing according to a timing start signal to generate a timing signal; and generating the control signal according to the timing signal. The control method according to claim 22, wherein the auxiliary switch is turned on according to the timing start signal. The control method according to claim 22, wherein when the timing signal is greater than or equal to the on-time threshold, the auxiliary switch is turned off. The control method according to claim 22, wherein the step (b) further comprises: resetting the timer according to a reset signal. The control method according to claim 22, wherein in the discontinuous mode, the timing start signal is obtained by detecting the on signal of the auxiliary switch; and in the critical continuous mode, the zero-crossing point of the exciting negative current is obtained Timing start signal. The control method according to claim 26, wherein the zero-crossing point of the exciting negative current is detected by a current transformer, a sampling resistor, or an internal resistance of the auxiliary switch. The control method according to claim 25, wherein the reset signal is obtained by detecting an off signal of the auxiliary switch. The control method according to claim 18, wherein the step (a) comprises: calculating the on-time threshold based on the output voltage and the reference value of the exciting negative current through a division operation. The control method according to claim 18, wherein the control method further comprises: (c) after the auxiliary switch is turned off, the resonance of the magnetic field inductance and the parasitic capacitance in the flyback converter is used to realize the The zero voltage of the primary-side power switch is turned on. The control method according to claim 18, wherein the step (a) further comprises: setting the reference value of the exciting negative current based on the input voltage of the flyback converter. The control method according to claim 31, wherein the step (a) further comprises: setting the reference value of the exciting negative current based on a maximum value of the input voltage of the flyback converter. The control method according to claim 18, wherein the step (a) further comprises: setting the reference value of the exciting negative current based on the input voltage and the output voltage of the flyback converter. The control method according to claim 18, wherein the output voltage of the flyback converter is variable. The control method according to claim 34, wherein the output voltage of the flyback converter is 5V, 9V, 15V, or 20V.