Active Clamp Flyback Converter, Feedback Voltage Generation Circuit and Method Thereof

Abstract

The present application discloses an active clamp flyback converter, a feedback voltage generation circuit and a method for an active clamp flyback converter. The active clamp flyback converter includes: a transformer, a main power switch, an auxiliary switch, an auxiliary capacitor, a feedback voltage generation circuit and a control circuit. The active clamp flyback converter realizes precise sampling of output voltage information.

Description

Technical Field

[0001] The present invention relates to an electronic circuit, and more particularly, to an active clamp flyback converter and a method thereof. Background Art

[0002] The active clamp flyback circuit has been widely used in the field of electrical energy isolation conversion due to its high efficiency and low EMI (electromagnetic interference). The so-called active clamp flyback circuit is to add an auxiliary switch and a snubber capacitor in addition to the main power switch on the primary side of the flyback circuit, as Figure 1 shown. The active clamp flyback circuit includes a main power switch S1 and an auxiliary switch S2 coupled to the primary winding T1 of the transformer T. The auxiliary switch S2 is not turned on during the conduction period of the main power switch S1 and remains turned on during the off period of the main power switch S1 (i.e., the two are complementary in conduction), or is turned on for a period of time during the off period of the main power switch S1. When the main power switch S1 is turned off, the leakage inductance energy of the transformer T1 is transferred to the snubber capacitor C1 for storage; since the auxiliary switch S2 is turned on (or turned on for a period of time), the snubber capacitor C1 reversely charges the leakage inductance after the primary current drops to zero, so that the energy absorbed by the leakage inductance is re-stored in the leakage inductance or released to the load, thereby improving the efficiency.

[0003] Due to the input-output isolation of the flyback circuit, in the case of primary side control (i.e., the main control circuit is arranged on the primary side), the output voltage is usually fed back to the primary side through an optocoupler, an isolation capacitor, an auxiliary winding, etc. However, these methods are costly and have a short service life. In addition, there are some methods that directly sample the voltage across the primary winding and calculate the output voltage through the turns ratio of the primary and secondary windings. However, this method does not take into account the leakage inductance of the transformer, and the obtained output voltage information is not accurate enough, resulting in poor load regulation, especially under light load. Summary of the Invention

[0004] According to an embodiment of the present invention, there is provided an active clamp flyback converter, including: a transformer that receives an input voltage, the transformer including a primary winding and a secondary winding, and the transformer further having a leakage inductance that is equivalent to being connected in series with the primary winding; a main power switch coupled between the primary winding and the primary reference ground, wherein the common connection point of the main power switch and the primary winding is a switching node; an auxiliary switch and an auxiliary capacitor coupled between the switching node and the input voltage; a feedback voltage generating circuit that generates a feedback voltage according to a current signal representing the current flowing through the primary winding, the input voltage, the switching node voltage, and an auxiliary control signal for controlling the auxiliary switch; and a control circuit that generates a main control signal and the auxiliary control signal according to the feedback voltage for respectively controlling the main power switch and the auxiliary switch.

[0005] According to an embodiment of the present invention, a feedback voltage generation circuit for an active clamp flyback converter is further provided. The active clamp flyback converter includes: a transformer having a primary winding, a secondary winding, and a leakage inductance; a main power switch coupled between the primary winding and a primary reference ground; an auxiliary switch and an auxiliary capacitor coupled between a common connection point of the main power switch and the primary winding and an input voltage. The feedback voltage generation circuit includes: a first delay circuit that delays an auxiliary control signal for controlling the auxiliary switch for a first time duration to generate a first delayed signal; a second delay circuit that delays the auxiliary control signal for a second time duration to generate a second delayed signal; a first sample-and-hold circuit that samples and holds a current signal representing the current flowing through the primary winding in response to the first delayed signal to obtain a first held signal; a second sample-and-hold circuit that samples and holds the current signal in response to the second delayed signal to obtain a second held signal; an arithmetic circuit that performs a multiplication operation on the difference between the first held signal and the second held signal, the time difference between the first time duration and the second time duration, and the inductance value of the leakage inductance, and performs an addition operation on the product of the three and the voltage at the common connection point of the main power switch and the primary winding and a subtraction operation on the product of the three and the input voltage to obtain a feedback voltage.

[0006] According to an embodiment of the present invention, a method for an active clamp flyback converter is further provided. The active clamp flyback converter includes: a transformer having a primary winding, a secondary winding, and a leakage inductance; a main power switch coupled between the primary winding and a primary reference ground; an auxiliary switch and an auxiliary capacitor coupled between a common connection point of the main power switch and the primary winding and an input voltage. The method includes: the primary winding receives an input voltage, and the input voltage is transferred to the secondary side by periodically turning on and off the main power switch to obtain an output voltage; during the period when the main power switch is turned off, the auxiliary switch is turned on; the current flowing through the primary winding, the voltage at the common connection point of the main power switch and the primary winding, and the input voltage are sampled; a feedback voltage is generated in response to the current flowing through the primary winding, the input voltage, the voltage at the common connection point of the main power switch and the primary winding, and an auxiliary control signal for controlling the auxiliary switch; a main control signal and the auxiliary control signal are generated in response to the feedback voltage to respectively control the main power switch and the auxiliary switch.

[0007] According to the above-mentioned active clamp flyback converter, feedback voltage generation circuit, and method according to various aspects of the present invention, accurate sampling of the output voltage is achieved. Description of the Drawings

[0008] Figure 1 is a circuit structure schematic diagram of a traditional active clamp flyback circuit;

[0009] Figure 2 is a circuit structure schematic diagram of an active clamp flyback converter 200 according to an embodiment of the present invention;

[0010] Figure 3 For the Figure 2 Schematic circuit diagram of the feedback voltage generation circuit 104 in the active clamp flyback converter 200 according to an embodiment of the present invention;

[0011] Figure 4 Schematic circuit diagram of the operational circuit 405 in the feedback voltage generation circuit 104 according to an embodiment of the present invention;

[0012] Figure 5 Schematic circuit diagram of the feedback voltage generation circuit 104 according to another embodiment of the present invention;

[0013] Figure 6 Schematic circuit diagram of the control circuit 105 according to an embodiment of the present invention;

[0014] Figure 7 Schematically shows a method flow chart 700 for an active clamp flyback converter according to an embodiment of the present invention. Detailed implementation manners

[0015] The specific embodiments of the present invention will be described in detail below. It should be noted that the embodiments described here are only for illustrative purposes and do not limit the present invention. In the following description, a large number of specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to those of ordinary skill in the art that: It is not necessary to employ these specific details to practice the present invention. In other instances, well-known circuits, materials, or methods have not been specifically described in order to avoid obscuring the present invention.

[0016] Throughout the specification, the mention of "an embodiment", "embodiments", "an example", or "examples" means that: The specific features, structures, or characteristics described in connection with the embodiment or examples are included in at least one embodiment of the present invention. Thus, the phrases "in an embodiment", "in embodiments", "an example", or "examples" appearing throughout the specification do not necessarily all refer to the same embodiment or example. Furthermore, the specific features, structures, or characteristics may be combined in any suitable combination and / or sub-combination in one or more embodiments or examples. In addition, those of ordinary skill in the art should understand that the drawings provided herein are for illustrative purposes only and the drawings are not necessarily drawn to scale. It should be understood that when an element is referred to as being "coupled to" or "connected to" another element, it may be directly coupled or connected to the other element or there may be intervening elements. In contrast, when an element is referred to as being "directly coupled to" or "directly connected to" another element, there are no intervening elements. The same reference numerals indicate the same elements. The term "and / or" used herein includes any and all combinations of one or more of the associated listed items.

[0017] Figure 2 Schematic diagram of the circuit structure of the active clamp flyback converter 200 according to an embodiment of the present invention. In Figure 2 the illustrated embodiment, the flyback converter 200 includes: a transformer T that receives an input voltage V in , the transformer includes a primary winding T1 and a secondary winding T2, and the transformer T also has a leakage inductance Lk, which can be equivalent to being connected in series with the primary winding T1; a main power switch 101, coupled between the primary winding T1 and the primary reference ground, wherein the common connection point of the main power switch 101 and the primary winding T1 is the switching node SW; an auxiliary switch 102 and an auxiliary capacitor 103, coupled between the switching node SW and the input voltage V in ; a feedback voltage generation circuit 104 that generates a feedback voltage V pri according to a current signal I in representing the current flowing through the primary winding, the input voltage V SW , the switching node voltage V 102 , and an auxiliary control signal G fb for controlling the auxiliary switch 102; a control circuit 105 that generates a main control signal G fb and the auxiliary control signal G 101 according to the feedback voltage V 102 to control the main power switch 101 and the auxiliary switch 102 respectively.

[0018] In one embodiment of the present invention, the main control signal G 101 and the auxiliary control signal G 102 are complementary. In other embodiments of the present invention, there is a certain dead time between the main control signal G 101 and the auxiliary control signal G 102 .

[0019] In Figure 2 the illustrated embodiment, the active clamp flyback converter 200 further includes a secondary diode, coupled to the secondary winding T 2 to provide an output voltage V O . In other embodiments, the secondary diode can be replaced with a controllable power switch (such as a so-called synchronous rectifier tube) to improve efficiency.

[0020] Figure 3 Schematic diagram of the circuit structure of the feedback voltage generation circuit 104 in the active clamp flyback converter 200 according to an embodiment of the present invention. In Figure 2 the illustrated embodiment, the feedback voltage generation circuit 104 includes: a first delay circuit 41 that performs a first duration t Figure 3 on the auxiliary control signal G 102 for a first time period t 1The delay generates a first delay signal TD 1 ; a second delay circuit 42 delays the auxiliary control signal G 102 for a second duration t 2 to generate a second delay signal TD 2 ; a first sample-and-hold circuit 43 responds to the first delay signal TD 1 to sample and hold the current signal I pri to obtain a first hold signal SH 1 ; a second sample-and-hold circuit 44 responds to the second delay signal TD 2 to sample and hold the current signal I pri to obtain a second hold signal SH 2 ; an arithmetic circuit 45 multiplies the difference between the first hold signal SH 1 and the second hold signal SH 2 , the time difference between the first duration t 1 and the second duration t 2 , and the inductance value L of the leakage inductance Lk lk , adds the product of the three to the switch node voltage V SW , and subtracts it from the input voltage V in to obtain a feedback voltage V fb . That is, the relationship between the feedback voltage V fb and the input signals of the arithmetic circuit 45 is:

[0021] V fb =(SH 2 -SH 1 )×L lk / (t 2 -t 1 )+V SW -V in

[0022] In an embodiment of the present invention, the first duration t 1 is less than the second duration t 2 .

[0023] In an embodiment of the present invention, the arithmetic circuit 45 can be implemented by a digital circuit or an analog circuit.

[0024] Figure 4 is a schematic circuit diagram of the arithmetic circuit 405 according to an embodiment of the present invention. In Figure 4 the shown embodiment, the arithmetic circuit 405 includes: a first arithmetic unit 51 that performs a subtraction operation on the first hold signal SH 1 and the second hold signal SH 2 ; a second arithmetic unit 52 that performs an operation on the first hold signal SH1 and a second hold signal SH 2 The difference between and the first duration t 1 and the second duration t 2 The time difference of, the inductance value L of the leakage inductance Lk lk The three are multiplied to obtain a product signal (SH 2 -SH 1 ) × L lk / (t 2 -t 1 ); The third operation unit 53 subtracts the switch node voltage V SW and the input voltage V in ; The third sample and hold circuit 54 responds to the second delay signal TD 2 , samples and holds the difference between the switch node voltage V SW and the input voltage Vin to obtain a difference hold signal (V SW -V in ); The fourth operation unit 55 adds the difference hold signal (V SW -V in ) and the product signal (SH 2 -SH 1 ) × L lk / (t 2 -t 1 ) to obtain the feedback voltage V fb .

[0025] Figure 5 is a schematic circuit diagram of the feedback voltage generation circuit 104 according to an embodiment of the present invention. In Figure 5 In the illustrated embodiment, the first delay circuit 41 and the second delay circuit 42 are both implemented by RC circuits; the first sample and hold circuit 43 and the second sample and hold circuit 44 are both implemented by a sample switch and a sample capacitor. The RC delay circuit and the sample and hold circuit are both technical means familiar to those skilled in the art and will not be described in detail here for the sake of brevity.

[0026] When the active clamp flyback converter is operating, when the main power switch 101 is turned off and the auxiliary switch 102 is turned on, the voltage V 1 across the primary winding T T1 is the sum of the voltage V 103 across the auxiliary capacitor 103 and the leakage inductance voltage V Lk , that is, V T1 = V 103 +V Lk .

[0027] And when the auxiliary switch 102 is turned on, the voltage V 103 across the auxiliary capacitor 103 is the switch node voltage VSW The difference from the input voltage V in is, i.e., V 103 = V SW - V in .

[0028] The relationship between the output voltage V O and the voltage V 1 across the primary winding T T1 is as follows:

[0029] V O = V T1 × N S / N P

[0030] where N S / N P is the turns ratio of the primary winding T 1 and the secondary winding T 2 .

[0031] Therefore, as long as the leakage inductance voltage V Lk is obtained, the information of the output voltage V O can be obtained.

[0032] At the feedback voltage generation circuit 104, the first delay circuit 41 and the first sample and hold circuit 43 fix the current signal I 1 representing the primary current as the first hold signal SH pri at the time point t 1 after the auxiliary switch 102 is turned on; the second delay circuit 42 and the second sample and hold circuit 44 fix the current signal I 2 representing the primary current as the second hold signal SH pri at the time point t 2 after the auxiliary switch 102 is turned on; the calculation circuit 45 multiplies the difference between the first hold signal SH 1 and the second hold signal SH 2 by the time difference between the first duration t 1 and the second duration t 2 and the inductance value L lk of the leakage inductance Lk. According to the relationship between the voltage across the inductor and the rate of change of the current flowing through it, it can be known that the product of the difference between the first hold signal SH 1 and the second hold signal SH 2 by the time difference between the first duration t 1 and the second duration t 2 and the inductance value L lk of the leakage inductance Lk is the leakage inductance voltage V Lk .

[0033] Therefore,Figure 3 The feedback voltage generation circuit 104 shown realizes accurate sampling of the output voltage information. After the sampled output voltage information passes through the control circuit 105, a main control signal G is generated. 101 and an auxiliary control signal G 102 , which are used to control the main power switch 101 and the auxiliary switch 102 respectively.

[0034] Figure 6 FIG. is a schematic circuit diagram of the control circuit 105 according to an embodiment of the present invention. In Figure 6 the embodiment shown, the control circuit 105 includes: a proportional-integral circuit (such as an error amplifier EA) 51 that amplifies and integrates the difference between the feedback voltage V fb and the reference voltage V ref to obtain a compensation signal CMP; a comparator 52 that compares the compensation signal CMP with a current signal I pri representing the primary current to generate a reset signal R; and a logic circuit 53 that responds to the reset signal R to generate a main control signal G 101 for controlling the main power switch 101. After the main power switch 101 is turned off, after a dead time, the auxiliary switch 102 is turned on.

[0035] In an embodiment of the present invention, the main control signal G 101 is converted into an auxiliary control signal G 102 via an inverter for controlling the auxiliary switch 102.

[0036] Figure 6 The control circuit 105 shown is an implementation of closed-loop control. However, those skilled in the art should be aware that according to other embodiments of the present invention, the control circuit 105 can also be implemented in other forms, such as a single voltage loop, peak current control, average current control, etc. How to implement the control circuit 105 is not a technical point discussed in the present invention. For the sake of simplicity of description, it will not be elaborated here.

[0037] Figure 7 FIG. schematically shows a method flowchart 700 for an active clamp flyback converter according to an embodiment of the present invention. The active clamp flyback converter includes: a transformer having a primary winding, a secondary winding, and a leakage inductance, a main power switch coupled between the primary winding and the primary reference ground, an auxiliary switch and an auxiliary capacitor coupled between the common connection point of the main power switch and the primary winding and the input voltage. The method includes:

[0038] Step 701, the primary winding receives the input voltage, and by periodically turning on and off the main power switch, the input voltage is transferred to the secondary side to obtain an output voltage.

[0039] Step 702: Turn on the auxiliary switch during the period when the main power switch is off.

[0040] Step 703: Sample the current flowing through the primary winding, the voltage at the common coupling point of the main power switch and the primary winding, and the input voltage.

[0041] Step 704: Generate a feedback voltage in response to the current flowing through the primary winding, the input voltage, the voltage at the common coupling point of the main power switch and the primary winding, and an auxiliary control signal for controlling the auxiliary switch.

[0042] Step 705: Generate a main control signal and the auxiliary control signal in response to the feedback voltage to control the main power switch and the auxiliary switch respectively.

[0043] In an embodiment of the present invention, the method further includes: delaying the auxiliary control signal for a first duration to generate a first delay signal, sampling and holding the current flowing through the primary winding in response to the first delay signal to obtain a first hold signal; delaying the auxiliary control signal for a second duration to generate a second delay signal, sampling and holding the current flowing through the primary winding in response to the second delay signal to obtain a second hold signal; multiplying the difference between the first hold signal and the second hold signal by the time difference between the first duration and the second duration and the inductance value of the leakage inductance to obtain a product signal; adding the product signal to the voltage at the common coupling point of the main power switch and the primary winding and subtracting the input voltage to obtain the feedback voltage.

[0044] In an embodiment of the present invention, the step of "generating a main control signal and the auxiliary control signal in response to the feedback voltage" includes: amplifying and integrating the difference between the feedback voltage and a reference voltage to obtain a compensation signal; comparing the compensation signal with a current signal representing the primary current to generate a set signal; generating the main control signal and the auxiliary control signal in response to the set signal.

[0045] Although the present invention has been described with reference to several exemplary embodiments, it should be understood that the terms used are illustrative and exemplary, rather than restrictive. Since the present invention can be embodied in many forms without departing from the spirit or essence of the invention, it should be understood that the above embodiments are not limited to any of the foregoing details, but should be broadly construed within the spirit and scope defined by the appended claims. Therefore, all changes and modifications falling within the scope of the claims or their equivalents should be covered by the appended claims.

Claims

1. An active clamp flyback converter, comprising: A transformer that receives an input voltage, the transformer including a primary winding and a secondary winding, and the transformer further having a leakage inductance that is equivalent to being connected in series with the primary winding; A main power switch coupled between the primary winding and a primary reference ground, wherein a common connection point of the main power switch and the primary winding is a switching node; An auxiliary switch and an auxiliary capacitor coupled between the switching node and the input voltage; A feedback voltage generation circuit that generates a feedback voltage based on a current signal representing the current flowing through the primary winding, the input voltage, the switching node voltage, and an auxiliary control signal for controlling the auxiliary switch; A control circuit that generates a main control signal and the auxiliary control signal based on the feedback voltage for respectively controlling the main power switch and the auxiliary switch.

2. The active clamp flyback converter according to claim 1, wherein the feedback voltage generation circuit comprises: A first delay circuit that delays the auxiliary control signal for a first duration to generate a first delayed signal; A second delay circuit that delays the auxiliary control signal for a second duration to generate a second delayed signal; A first sample and hold circuit that samples and holds the current signal in response to the first delayed signal to obtain a first held signal; A second sample and hold circuit that samples and holds the current signal in response to the second delayed signal to obtain a second held signal; An arithmetic circuit that performs a multiplication operation on the difference between the first held signal and the second held signal, the time difference between the first duration and the second duration, and the inductance value of the leakage inductance, and adds the product of the three to the switching node voltage and subtracts the product of the three from the input voltage to obtain the feedback voltage.

3. The active clamp flyback converter according to claim 2, wherein the arithmetic circuit comprises: A first arithmetic unit that performs a subtraction operation on the first held signal and the second held signal; A second arithmetic unit that performs a multiplication operation on the difference between the first held signal and the second held signal, the time difference between the first duration and the second duration, and the inductance value of the leakage inductance to obtain a product signal; A third arithmetic unit that performs a subtraction operation on the switching node voltage and the input voltage; A third sample and hold circuit that samples and holds the difference between the switching node voltage and the input voltage in response to the second delayed signal to obtain a difference held signal; A fourth arithmetic unit that performs an addition operation on the difference held signal and the product signal to obtain the feedback voltage.

4. The active clamp flyback converter according to claim 1, wherein the control circuit comprises: A proportional-integral circuit that amplifies and integrates the difference between the feedback voltage and a reference voltage to obtain a compensation signal; A comparator that compares the compensation signal and the current signal to generate a reset signal; A logic circuit that generates a main control signal in response to the reset signal for controlling the main power switch.

5. The active clamp flyback converter according to claim 1, wherein after the main power switch is turned off, after a dead time, the auxiliary switch is turned on.

6. A feedback voltage generation circuit for an active clamp flyback converter, the active clamp flyback converter comprising: A transformer having a primary winding, a secondary winding and leakage inductance, a main power switch coupled between the primary winding and a primary reference ground, an auxiliary switch and an auxiliary capacitor coupled between a common connection point of the main power switch and the primary winding and an input voltage, the feedback voltage generating circuit comprising: A first delay circuit that delays an auxiliary control signal for controlling the auxiliary switch for a first duration to generate a first delayed signal; A second delay circuit that delays the auxiliary control signal for a second duration to generate a second delayed signal; A first sample and hold circuit that samples and holds a current signal representing the current flowing through the primary winding in response to the first delayed signal to obtain a first held signal; A second sample and hold circuit that samples and holds the current signal in response to the second delayed signal to obtain a second held signal; An arithmetic circuit that performs a multiplication operation on the difference between the first held signal and the second held signal, the time difference between the first duration and the second duration, and the inductance value of the leakage inductance, and performs an addition operation on the product of the three and the voltage at the common connection point of the main power switch and the primary winding, and a subtraction operation from the input voltage to obtain a feedback voltage.

7. The feedback voltage generating circuit according to claim 6, wherein the arithmetic circuit comprises: A first arithmetic unit that performs a subtraction operation on the first held signal and the second held signal; A second arithmetic unit that performs a multiplication operation on the difference between the first held signal and the second held signal, the time difference between the first duration and the second duration, and the inductance value of the leakage inductance to obtain a product signal; A third arithmetic unit that performs a subtraction operation on the voltage at the common connection point of the main power switch and the primary winding and the input voltage; A third sample and hold circuit that samples and holds the difference between the switch node voltage and the input voltage in response to the second delayed signal to obtain a difference held signal; A fourth arithmetic unit that performs an addition operation on the difference held signal and the product signal to obtain the feedback voltage.

8. A method for an active clamp flyback converter, the active clamp flyback converter comprising: A transformer having a primary winding, a secondary winding and leakage inductance, a main power switch coupled between the primary winding and a primary reference ground, an auxiliary switch and an auxiliary capacitor coupled between a common connection point of the main power switch and the primary winding and an input voltage; the method comprising: The primary winding receives an input voltage, and the input voltage is transferred to the secondary side by periodically turning on and off the main power switch to obtain an output voltage; During the period when the main power switch is turned off, the auxiliary switch is turned on; Sample the current flowing through the primary winding, the voltage at the common connection point of the main power switch and the primary winding, and the input voltage; In response to the current flowing through the primary winding, the input voltage, the voltage at the common connection point of the main power switch and the primary winding, and an auxiliary control signal for controlling the auxiliary switch, generate a feedback voltage; In response to the feedback voltage, generate a main control signal and the auxiliary control signal to respectively control the main power switch and the auxiliary switch.

9. The method according to claim 8, further comprising: Delay the auxiliary control signal for a first duration to generate a first delayed signal, and in response to the first delayed signal, sample and hold the current flowing through the primary winding to obtain a first held signal; Delay the auxiliary control signal for a second duration to generate a second delayed signal, and in response to the second delayed signal, sample and hold the current flowing through the primary winding to obtain a second hold signal; Perform a multiplication operation on the difference between the first hold signal and the second hold signal, the time difference between the first duration and the second duration, and the inductance value of the leakage inductance to obtain a product signal; Perform an addition operation on the product signal and the voltage at the common coupling point of the main power switch and the primary winding, and a subtraction operation with the input voltage to obtain the feedback voltage.

10. The method according to claim 8, wherein the step "in response to the feedback voltage, generate the main control signal and the auxiliary control signal" comprises: Amplify and integrate the difference between the feedback voltage and the reference voltage to obtain a compensation signal; Compare the compensation signal with the current signal representing the primary current to generate a reset signal; In response to the reset signal, generate the main control signal and the auxiliary control signal.

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