Secondary rectification circuit suitable for high output voltage flyback converter and flyback converter

Abstract

The invention discloses a secondary side rectification circuit suitable for a high output voltage flyback converter and the flyback converter, the secondary side rectification circuit is connected in parallel through secondary side capacitors and resistors for voltage sharing, and the central point is connected with the secondary side center tap of a transformer, so that the whole secondary side circuit is divided into a high side circuit and a low side circuit; and meanwhile, synchronous rectifier tubes are respectively arranged at a high side position and a low side position. The two rectifier tubes operate in series in a period and are switched on and off simultaneously, so that the reverse voltage of a secondary rectifier tube is reduced to half of the reverse voltage of a secondary rectifier tube of a traditional flyback power supply module, the loss of the secondary synchronous rectifier tube is reduced, the requirements of high stability and high efficiency are met, and the flyback power supply module is simple in topology and easy to implement. By adopting the secondary side rectifying circuit designed by the invention, the limitation on reverse withstand voltage can be relaxed when the synchronous rectifying tube is selected, and the synchronous rectifying tube with excellent reverse recovery characteristic and higher reliability can be selected under the condition of meeting the basic requirements.

Description

Technical Field

[0001] The invention relates to a secondary side rectifier circuit and a flyback converter suitable for a high output voltage flyback converter, belonging to the technical field of power electronic devices. Background Art

[0002] In low-power module power supplies, flyback power supply modules are widely used. Their advantages are simple circuit topology, electrical isolation of input and output, wide voltage rise and fall range, and easy multi-channel output. However, the secondary rectifier of the traditional flyback converter uses diode rectification, and has high requirements for its reverse withstand voltage. Therefore, the circuit will produce large conduction loss and turn-off loss, and the circuit reliability is not high. In order to solve the problem of large loss of the secondary rectifier, the Virginia Power Electronics Technology Center in the United States has proposed the use of synchronous rectification technology to improve the efficiency of the converter, using MOS tubes instead of diode rectification. Although this method reduces the conduction loss of the rectifier, it does not solve the problem of high reverse voltage of the rectifier.

[0003] In high output voltage applications, the reverse voltage of the rectifier tube is extremely high when the secondary side is cut off. Therefore, in order to meet the circuit requirements, a synchronous rectifier tube with high reverse withstand voltage must be selected. However, synchronous rectifier tubes with high reverse withstand voltage often have poor reverse recovery characteristics and large on-resistance, which will produce huge conduction loss and turn-off loss. In addition, the high withstand voltage device package is larger, which will affect the overall power density of the circuit.

[0004] In summary, in the use of converters, the conversion efficiency of the circuit topology and the safety of the working environment are factors that must be considered. Although the synchronous rectification method proposed in the prior art can improve the overall efficiency of the converter by utilizing the device advantages of the MOS tube, the secondary rectifier tube still needs to withstand high reverse voltage during the shutdown period. The reliability problem of the circuit still needs to be solved urgently. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a secondary side rectifier circuit and a flyback converter suitable for a high output voltage flyback converter, and reduce the high reverse voltage stress during the shutdown period of the secondary side rectifier tube on the basis of a flyback power supply module.

[0006] The present invention adopts the following technical solutions to solve the above technical problems:

[0007] A secondary side rectifier circuit suitable for a high output voltage flyback converter, comprising: a secondary side transformer module, a synchronous rectifier module, a voltage balancing module and a load; the secondary side transformer module comprises first and second secondary side windings; the synchronous rectifier module comprises first and second synchronous rectifier tubes; the voltage balancing module comprises first and second voltage balancing capacitors and first and second voltage balancing resistors;

[0008] The first and second secondary windings are connected in series, and the first secondary winding is at the secondary high side, and the second secondary winding is at the secondary low side; the source of the first synchronous rectifier is connected to the opposite end of the first secondary winding, and the drain is connected to one end of the load; the drain of the second synchronous rectifier is connected to the same end of the second secondary winding, and the source is connected to the other end of the load;

[0009] One end of the first capacitor is connected to the drain of the first synchronous rectifier, one end of the second capacitor is connected to the source of the second synchronous rectifier, the other end of the first capacitor is connected to the other end of the second capacitor, the center point of the first capacitor and the second capacitor is connected to the center tap of the secondary side of the transformer, the center tap of the secondary side of the transformer is located at the midpoint of the first secondary winding and the second secondary winding, and the same-name end of the first secondary winding is connected to the opposite-name end of the second secondary winding;

[0010] The first balancing resistor is connected in parallel with the first balancing capacitor, and the second balancing resistor is connected in parallel with the second balancing capacitor.

[0011] As a preferred solution of the present invention, the first secondary winding and the second secondary winding have the same number of turns and the same winding direction, and the first and second secondary windings evenly divide the voltage transferred from the primary side of the transformer to the secondary side.

[0012] As a preferred solution of the present invention, the first synchronous rectifier and the second synchronous rectifier are in the same state within one cycle, and are turned on or off at the same time.

[0013] As a preferred solution of the present invention, when the polarities of the first and second secondary windings are both negative at the opposite end and positive at the same end, the first and second synchronous rectifiers are both turned off, and the reverse voltages borne by the first and second synchronous rectifiers are both Among them, V in is the input voltage, U o is the output voltage, N is the total turns ratio of the primary and secondary sides of the transformer; when the polarities of the first and second secondary windings are both positive at the opposite ends and negative at the same end, the first and second synchronous rectifiers are both turned on.

[0014] A secondary side rectifier circuit suitable for a high output voltage flyback converter, comprising: a secondary side transformer module, a synchronous rectifier module, a voltage balancing module and a load; the secondary side transformer module comprises first and second secondary side windings; the synchronous rectifier module comprises a PMOS tube and an NMOS tube; the voltage balancing module comprises first and second voltage balancing capacitors and first and second voltage balancing resistors;

[0015] The first and second secondary windings are connected in series, and the first secondary winding is at the secondary high side, and the second secondary winding is at the secondary low side; a PMOS tube is used at the secondary high side, and an NMOS tube is used at the secondary low side, the drain of the PMOS tube is connected to the opposite end of the first secondary winding, and the source is connected to one end of the load; the drain of the NMOS tube is connected to the same end of the second secondary winding, and the source is connected to the other end of the load;

[0016] One end of the first capacitor is connected to the source of the PMOS tube, one end of the second capacitor is connected to the source of the NMOS tube, the other end of the first capacitor is connected to the other end of the second capacitor, the center point of the first capacitor and the second capacitor is connected to the center tap of the secondary side of the transformer, the center tap of the secondary side of the transformer is located at the midpoint of the first secondary winding and the second secondary winding, and the same-name end of the first secondary winding is connected to the opposite-name end of the second secondary winding;

[0017] The first balancing resistor is connected in parallel with the first balancing capacitor, and the second balancing resistor is connected in parallel with the second balancing capacitor.

[0018] A high output voltage flyback converter, comprising: a flyback primary circuit and a secondary rectifier circuit suitable for the high output voltage flyback converter, wherein the flyback primary circuit comprises a transformer primary winding, a main switch tube, a clamping resistor, a clamping capacitor, a diode, and first to second inductors;

[0019] The drain of the main switch tube is connected to the opposite end of the primary winding of the transformer and the positive pole of the diode, and the source is connected to the negative pole of the input voltage; the negative pole of the diode is connected to one end of the clamping capacitor, the other end of the clamping capacitor is connected to the positive pole of the input voltage, and the clamping resistor is connected in parallel with the clamping capacitor; one end of the first inductor is connected to the positive pole of the input voltage, and the other end is connected to the same end of the primary winding of the transformer; one end of the second inductor is connected to the drain of the main switch tube, and the other end is connected to the same end of the primary winding of the transformer.

[0020] Compared with the prior art, the present invention adopts the above technical solution and has the following technical effects:

[0021] 1. The present invention divides the secondary circuit into two paths, high side and low side, by connecting the secondary capacitors and resistors in parallel to equalize the voltage and connecting the center point to the center tap of the secondary side of the transformer. At the same time, synchronous rectifiers are added to the high side and the low side respectively. The two rectifiers are connected in series within the cycle and are turned on and off at the same time, thereby achieving the purpose of reducing the reverse voltage of the secondary rectifier to half of the reverse voltage of the secondary rectifier of the traditional flyback power module.

[0022] 2. The present invention reduces the reverse voltage of the synchronous rectifier, so that a synchronous rectifier with superior reverse recovery characteristics and higher reliability can be selected, reducing the loss of the secondary synchronous rectifier, improving circuit reliability, and achieving high stability and high efficiency requirements.

[0023] 3. The secondary rectifier circuit designed in the present invention has a simple topology and is easy to implement; the volume of the secondary rectifier tube is reduced, the package is smaller, and the overall power density of the circuit is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a circuit topology diagram of a high output voltage flyback converter of the present invention;

[0025] Figure 2 It is a comparison diagram between the conventional flyback secondary circuit and the secondary circuit of the present invention;

[0026] Figure 3 It is a comparison diagram of the reverse voltage curve of the secondary rectifier tube of the conventional flyback and the reverse voltage curve of the secondary rectifier tube of the present invention;

[0027] Figure 4 It is a comparison diagram of the conduction loss and the reverse recovery loss of the body diode of the secondary rectifier tube of the traditional flyback circuit and the secondary rectifier tube of the present invention;

[0028] Figure 5 It is a same type of deformed circuit topology suitable for a secondary side rectifier circuit of a high output voltage flyback converter of the present invention, wherein a PMOS tube is used at the high side and an NMOS tube is used at the low side. DETAILED DESCRIPTION

[0029] The embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be interpreted as limiting the present invention.

[0030] like Figure 1 As shown, the present invention proposes a high output voltage flyback converter, including a flyback primary circuit and a secondary rectifier circuit. The flyback primary circuit is the same as the primary part of any flyback converter, including a main switch tube Q1, an RCD clamp circuit R c , C c , D c And the primary winding of the transformer T; the present invention focuses on the secondary winding part.

[0031] In view of the fact that the reverse voltage of the rectifier tube of the traditional flyback secondary circuit is relatively high when the secondary side is cut off, the requirement for reverse withstand voltage of the synchronous rectifier tube is relatively high. The synchronous rectifier tube with high reverse withstand voltage often has poor reverse recovery characteristics and large on-resistance, which will produce great conduction loss and turn-off loss. Therefore, the present invention designs a secondary rectifier circuit suitable for a high output voltage flyback converter, including: a secondary transformer module, a synchronous rectifier module and a voltage equalization module.

[0032] The secondary transformer module includes transformer secondary windings LS1 and LS2. LS1 is on the secondary high side and LS2 is on the secondary low side. The midpoint of LS1 and LS2, i.e. the center tap of the secondary transformer, is recorded as point A. The opposite-name terminal of LS1 is connected to the drain of synchronous rectifier SR1, and the same-name terminal is connected to point A. The opposite-name terminal of LS2 is connected to point A, and the same-name terminal is connected to the drain of synchronous rectifier SR2. Point A is connected to the midpoint of equalizing capacitors C1 and C2, which improves the stability of the circuit.

[0033] The synchronous rectification module includes the secondary synchronous rectification tubes SR1 and SR2. When the polarity of the secondary side LS1 and LS2 of the transformer T is negative at the top and positive at the bottom, SR1 and SR2 are turned off and withstand The reverse voltage (N is the total turns ratio of the primary and secondary sides of the transformer) is connected to the reverse voltage (N is the total turns ratio of the primary and secondary sides of the transformer). When the polarity of LS1 and LS2 is positive at the top and negative at the bottom, SR1 and SR2 are turned on to transfer energy to the load; the source of SR1 is connected to the opposite end of the secondary winding LS1 of the transformer T, and the drain is connected to the positive output terminal. The source of SR2 is connected to the negative output terminal, and the drain is connected to the same end of the secondary winding LS2 of the transformer T. The source voltage of SR2 is a constant voltage within a cycle, which is easy to drive.

[0034] The voltage balancing module includes voltage balancing capacitors C1 and C2 and voltage balancing resistors R1 and R2. C1 is connected in parallel with R1, and C2 is connected in parallel with R2. C1 and C2 are used to increase the output voltage U o R1 and R2 are used to ensure that the voltage-equalizing capacitors C1 and C2 can still evenly distribute the output voltage within the tolerance range; the center point of C1 and C2 is connected to the center tap point A of the secondary side of the transformer. The voltage across C1 and C2 is constant in one cycle, and C1 and C2 do not cause loss.

[0035] Figure 2 For the traditional flyback secondary circuit and Figure 1 Comparison diagram of the secondary side circuit. Figure 3 The figure is a comparison diagram of the reverse voltage curves of the secondary rectifier tube of the traditional flyback and the secondary rectifier tube of the present invention.

[0036] In a traditional flyback converter, when the main switch is turned on and the secondary rectifier is not turned on, the reverse voltage of the secondary rectifier is as follows:

[0037]

[0038] Among them, V SR Represents the reverse voltage of the secondary rectifier of the traditional flyback, V in represents the input voltage, N represents the primary-to-secondary turns ratio of the transformer, U o Indicates the output voltage.

[0039] The present invention realizes voltage balancing of the output capacitor by connecting voltage balancing capacitors C1 and C2 in parallel with voltage balancing resistors R1 and R2, and connecting the center point of the capacitors to the center tap of the secondary side of the transformer, and divides the secondary side circuit into two paths, the high side and the low side. On this basis, synchronous rectifiers SR1 and SR2 are added to the high side and the low side of the secondary side respectively. Compared with the traditional flyback circuit, when the total turns ratio of the original secondary side remains unchanged, the reverse withstand voltage of SR1 and SR2 is as follows:

[0040]

[0041] Among them, V SR1 、V SR2Indicates the reverse withstand voltage of the secondary rectifier tubes SR1 and SR2.

[0042] From the above formula, we can see that the reverse withstand voltage of SR1 and SR2 is reduced by half. Therefore, when selecting synchronous rectifiers, the restrictions on reverse withstand voltage can be relaxed. Under the condition of meeting the basic requirements, synchronous rectifiers with superior reverse recovery characteristics and higher reliability can be selected.

[0043] The losses of the secondary rectifier tubes of the traditional flyback circuit and the present invention are compared using 40V input voltage, 36V output voltage, 40W power, 300K frequency, and a transformer primary-secondary turns ratio of 1:3 as the main technical indicators. Figure 4 shown.

[0044] According to formula (1), the reverse withstand voltage of the secondary rectifier tube of the traditional flyback circuit is 156V, so the NMOS tube ISZ520N20NM6 with a reverse withstand voltage of 200V is selected.

[0045] According to formula (2), the reverse withstand voltage of the secondary rectifier tube of the present invention is 78V, so the NMOS tube ISZ034N06LM5 with a reverse withstand voltage of 100V is selected.

[0046] Table 1 shows the on-resistance R of the selected conventional flyback secondary side NMOS tube and the secondary side synchronous rectifier tube of the present invention. ds 、Package ackage , body diode reverse voltage V DR , body diode reverse recovery charge Q rr contrast.

[0047] Table 1

[0048] ISZ520N20NM6 ISZ034N06LM5 <![CDATA[P ackage ]]> PG-TSDSON-8FL PG-TSDSON-8FL <![CDATA[R ds ]]> 52mΩ 3.4mΩ <![CDATA[V DR ]]> 156V 78V <![CDATA[Q rr ]]> 143nC 50nC

[0049] Calculation of main circuit parameters:

[0050] The main switch duty cycle D is obtained by equation (3):

[0051]

[0052] The duty cycle D is obtained to be 0.24.

[0053] Secondary side average current I S From formula (4), we can get:

[0054]

[0055] Where P is the rated output power, and the average secondary current I S It is 1.46A.

[0056] (1) Conduction loss comparison analysis

[0057] The conduction loss P1 is obtained from equation (5):

[0058] P1=I S 2 R ds (5)

[0059] Among them, R ds is the on-resistance of the synchronous rectifier.

[0060] The conduction loss of ISZ520N20NM6, i.e. the conduction loss of the secondary MOS tube of the traditional flyback circuit, is 0.111 W. The present invention adopts dual-tube rectification, so the conduction loss of two synchronous rectifier tubes must be calculated. From the above formula, the conduction loss of two ISZ034N06LM5, i.e. the total conduction loss of the secondary synchronous rectifier tube of the present invention, is 0.0145 W.

[0061] (2) Comparative analysis of reverse recovery loss

[0062] The reverse recovery loss P2 is obtained from equation (6):

[0063]

[0064] Among them, V DR is the reverse voltage drop of the diode, Q rr is the body diode reverse recovery charge.

[0065] The body diode reverse recovery loss of ISZ520N20NM6, i.e., the body diode reverse recovery loss of the secondary MOS tube of the traditional flyback circuit, is 1.673 W. The present invention adopts dual-tube rectification, so the body diode reverse recovery loss of two synchronous rectifiers must be calculated. From the above formula, the body diode reverse recovery loss of two ISZ034N06LM5, i.e., the overall diode reverse recovery loss of the secondary synchronous rectifier of the present invention, is 0.585 W.

[0066] From the comparative analysis of the above two parts, it can be concluded that the secondary side topology adopted by the present invention can reduce the conduction loss by 87% and the body diode reverse recovery loss by 65% ​​while meeting the above technical indicators.

[0067] Figure 5 This is a modified circuit topology of the secondary rectifier circuit of the high output voltage flyback converter of the present invention. Under the condition that the other circuits remain unchanged, SR1 is replaced by a PMOS tube and SR2 is replaced by an NMOS tube. The source voltages of SR1 and SR2 are both constant voltages within one cycle, which is easy to control.

[0068] The above embodiments are only for illustrating the technical idea of ​​the present invention, and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the present invention.

Claims

1. A secondary side rectifier circuit suitable for a high output voltage flyback converter, characterized in that: include: A secondary transformer module, a synchronous rectification module, a voltage balancing module and a load; the secondary transformer module includes first to second secondary windings; the synchronous rectification module includes first to second synchronous rectification tubes; the voltage balancing module includes first to second voltage balancing capacitors and first to second voltage balancing resistors; The first and second secondary windings are connected in series, and the first secondary winding is at the secondary high side, and the second secondary winding is at the secondary low side; the source of the first synchronous rectifier is connected to the opposite end of the first secondary winding, and the drain is connected to one end of the load; the drain of the second synchronous rectifier is connected to the same end of the second secondary winding, and the source is connected to the other end of the load; One end of the first capacitor is connected to the drain of the first synchronous rectifier, one end of the second capacitor is connected to the source of the second synchronous rectifier, the other end of the first capacitor is connected to the other end of the second capacitor, the center point of the first capacitor and the second capacitor is connected to the center tap of the secondary side of the transformer, the center tap of the secondary side of the transformer is located at the midpoint of the first secondary winding and the second secondary winding, and the same-name end of the first secondary winding is connected to the opposite-name end of the second secondary winding; The first balancing resistor is connected in parallel with the first balancing capacitor, and the second balancing resistor is connected in parallel with the second balancing capacitor.

2. The secondary side rectifier circuit suitable for a high output voltage flyback converter according to claim 1, characterized in that: The first secondary winding and the second secondary winding have the same number of turns and the same winding direction, and the first and second secondary windings evenly divide the voltage transferred from the primary side of the transformer to the secondary side.

3. The secondary side rectifier circuit suitable for a high output voltage flyback converter according to claim 1, characterized in that: The first synchronous rectifier and the second synchronous rectifier are in the same state within one cycle, and are turned on or off at the same time.

4. The secondary side rectifier circuit suitable for a high output voltage flyback converter according to claim 1, characterized in that: When the polarity of the first and second secondary windings is negative at the opposite end and positive at the same end, the first and second synchronous rectifiers are both turned off, and the reverse voltages borne by the first and second synchronous rectifiers are both Among them, V in is the input voltage, U o is the output voltage, N is the total turns ratio of the primary and secondary sides of the transformer; when the polarities of the first and second secondary windings are both positive at the opposite ends and negative at the same end, the first and second synchronous rectifiers are both turned on.

5. A secondary side rectifier circuit suitable for a high output voltage flyback converter, characterized in that: include: A secondary transformer module, a synchronous rectifier module, a voltage balancing module and a load; the secondary transformer module includes first and second secondary windings; the synchronous rectifier module includes a PMOS tube and an NMOS tube; the voltage balancing module includes first and second voltage balancing capacitors and first and second voltage balancing resistors; The first and second secondary windings are connected in series, and the first secondary winding is at the secondary high side, and the second secondary winding is at the secondary low side; a PMOS tube is used at the secondary high side, and an NMOS tube is used at the secondary low side, the drain of the PMOS tube is connected to the opposite end of the first secondary winding, and the source is connected to one end of the load; the drain of the NMOS tube is connected to the same end of the second secondary winding, and the source is connected to the other end of the load; One end of the first capacitor is connected to the source of the PMOS tube, one end of the second capacitor is connected to the source of the NMOS tube, the other end of the first capacitor is connected to the other end of the second capacitor, the center point of the first capacitor and the second capacitor is connected to the center tap of the secondary side of the transformer, the center tap of the secondary side of the transformer is located at the midpoint of the first secondary winding and the second secondary winding, and the same-name end of the first secondary winding is connected to the opposite-name end of the second secondary winding; The first balancing resistor is connected in parallel with the first balancing capacitor, and the second balancing resistor is connected in parallel with the second balancing capacitor.

6. A high output voltage flyback converter, characterized in that: include: A flyback primary circuit and a secondary rectifier circuit suitable for a high output voltage flyback converter as claimed in any one of claims 1 to 4, wherein the flyback primary circuit comprises a transformer primary winding, a main switch tube, a clamping resistor, a clamping capacitor, a diode and first to second inductors; The drain of the main switch tube is connected to the opposite end of the primary winding of the transformer and the positive pole of the diode, and the source is connected to the negative pole of the input voltage; the negative pole of the diode is connected to one end of the clamping capacitor, the other end of the clamping capacitor is connected to the positive pole of the input voltage, and the clamping resistor is connected in parallel with the clamping capacitor; one end of the first inductor is connected to the positive pole of the input voltage, and the other end is connected to the same end of the primary winding of the transformer; one end of the second inductor is connected to the drain of the main switch tube, and the other end is connected to the same end of the primary winding of the transformer.

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