Preceding-stage conversion circuit for DC-DC load

Abstract

The utility model relates to the technical field of power supply conversion, and discloses a preceding stage conversion circuit for a DC-DC load, which comprises an equivalent load, a transformer, a first rectification circuit, a second rectification circuit and a switch switching circuit. A primary coil of the transformer is connected with the alternating-current power supply end; the first rectifying circuit is connected with a first secondary coil of the transformer, and the first rectifying circuit is connected with the equivalent load; the second rectifying circuit is connected with a second secondary coil of the transformer, and the second rectifying circuit is connected with the equivalent load; the switch switching circuit is respectively connected with the first rectifying circuit and the second rectifying circuit, the switch switching circuit is connected with an equivalent load, the switch switching circuit is communicated with the first rectifying circuit and the second rectifying circuit along with the increase of the input alternating current voltage, and the two rectifying output circuits are connected in series; a boost or buck type inflection point voltage can be well avoided, voltage clamping is prevented, and the conversion power of an alternating current power supply is improved.

Description

Technical Field

[0001] The utility model relates to the technical field of power conversion, in particular to a pre-stage conversion circuit for a DC-DC load. Background Art

[0002] In a converter or power supply system, during the startup process, the relationship between the current and output voltage of the limit output may be approximately a straight line, which makes the converter look like a source that provides a constant transient current. When the equivalent load with constant power demand characteristics formed by the constant current output DC-DC backstage and the LED-like load it drives is powered on and started, as the output voltage of the converter or power supply system gradually increases and gradually reaches the startup voltage of the equivalent load, the constant power demand characteristics of the equivalent load will soon generate a large current demand. If the converter or power supply system cannot meet this current, its output voltage will be clamped at this point, resulting in a "voltage dead" phenomenon, causing the converter or power supply system to be in a high current and low voltage output state. In this state, the conversion efficiency of the converter or power supply system is extremely low, the electrical stress and thermal stress of the internal devices are extremely large, and it works in an extremely unsafe state. This phenomenon greatly limits the application scope of the converter or power supply system.

[0003] In the prior art, when the power converter supplied to the DC-DC load is in output condition, during the startup process of the converter being powered, it will be "pulled to death" at a certain output voltage. This startup process is an instantaneous process. In this process, the secondary of the transformer coil is not processed. If the primary coil is used to pull up the voltage for power supply, the primary coil will tend to be saturated, and the current and voltage capacity passing through may not be able to supply the required voltage. The entire front-stage circuit is difficult to bear, and the progress of this process will result in a steady state. When the output end of the constant current output type for the DC-DC load is connected to a constant voltage load (similar to LEDs), the circuit for the DC-DC load and the corresponding connected load are considered as a whole. Then this whole will have the characteristics of a power demand after startup, and most of the conversion circuits for DC loads have the above-mentioned problems. Utility Model Content

[0004] The purpose of the utility model is to provide a pre-stage conversion circuit for a DC-DC load to solve one or more technical problems existing in the prior art and at least provide a beneficial choice or create conditions.

[0005] The utility model solves the technical problem by providing a pre-stage conversion circuit for a DC-DC load, comprising:

[0006] Equivalent load;

[0007] An AC power supply terminal, the AC power supply terminal is used to output an AC voltage;

[0008] A transformer, wherein the transformer is provided with a primary coil, a first secondary coil and a second secondary coil, the primary coil is connected to an AC power supply end, and the transformer is used to transform the AC voltage;

[0009] a first rectifier circuit, wherein an input end of the first rectifier circuit is connected to the first secondary coil, and an output end of the first rectifier circuit is connected to an equivalent load;

[0010] a second rectifier circuit, wherein an input end of the second rectifier circuit is connected to the second secondary coil, and an output end of the second rectifier circuit is connected to an equivalent load;

[0011] A switch switching circuit, wherein the input end of the switch switching circuit is respectively connected to the output end of the first rectifier circuit and the output end of the second rectifier circuit, the output end of the switch switching circuit is connected to the equivalent load, and the switch switching circuit is used to connect the first rectifier circuit and the second rectifier circuit to switch to a series output.

[0012] Furthermore, the switch switching circuit includes:

[0013] A switch tube, wherein the drain of the switch tube is connected to the output end of the first rectifier circuit, and the source of the switch tube is connected to the output end of the second rectifier circuit;

[0014] A first filter capacitor, wherein the first filter capacitor is connected to the gate and source of the switch tube respectively;

[0015] A first resistor, wherein the first resistor is respectively connected to the gate and the source of the switch tube;

[0016] A second resistor is connected to the equivalent load, the first resistor and the second rectifier circuit respectively.

[0017] Further, the switch switching circuit includes: a relay;

[0018] The switch of the relay is connected to the output end of the first rectifier circuit and the output end of the second rectifier circuit respectively, and the driving end of the relay is connected to the equivalent load.

[0019] Furthermore, a front-stage conversion circuit for a DC-DC load further includes:

[0020] A control pulse input terminal, wherein the control pulse input terminal is used to output a control pulse;

[0021] A control circuit, wherein the input end of the control circuit is connected to the control pulse input end, the output end of the control circuit is connected to the primary coil, and the control circuit is used to receive the control pulse to control the transformer.

[0022] Furthermore, the first rectifier circuit comprises:

[0023] a first rectifier diode, wherein a cathode of the first rectifier diode is connected to the first secondary coil;

[0024] a second rectifying diode, wherein a cathode of the second rectifying diode is respectively connected to an anode of the first rectifying diode and an input end of the switch switching circuit, and an anode of the second rectifying diode is grounded;

[0025] A second filter capacitor, one end of which is respectively connected to the anode of the first rectifier diode and the cathode of the second rectifier diode, and the other end of which is respectively connected to the first secondary coil and the equivalent load.

[0026] Further, the second rectifier circuit includes:

[0027] a third rectifier diode, wherein an anode of the third rectifier diode is connected to the second secondary coil;

[0028] a fourth rectifier diode, wherein an anode of the fourth rectifier diode is respectively connected to a cathode of the third rectifier diode and an input end of the switch switching circuit, and a cathode of the fourth rectifier diode is respectively connected to the other end of the second filter capacitor and an equivalent load;

[0029] A third filter capacitor, one end of the third filter capacitor is connected to the cathode of the third rectifier diode, and the other end of the third filter capacitor is grounded.

[0030] Furthermore, a front-stage conversion circuit for a DC-DC load further includes: a rectifier and filter circuit;

[0031] The input end of the rectifying and filtering circuit is connected to the AC power supply end, and the output end of the rectifying and filtering circuit is connected to the primary coil.

[0032] Furthermore, the rectification and filtering circuit comprises:

[0033] A rectifier bridge, wherein an input end of the rectifier bridge is connected to an AC power supply end;

[0034] A fourth filter capacitor is connected to the rectifier bridge and the primary coil of the transformer respectively.

[0035] Furthermore, the control circuit includes: a field effect tube; the gate of the field effect tube is connected to the control pulse input terminal, the source of the field effect tube is grounded, and the drain of the field effect tube is connected to the primary coil.

[0036] The beneficial effect of the utility model is as follows: corresponding rectifier output circuits are arranged on the two secondary coils of the transformer, the two rectifier output circuits are connected in parallel, and the two transformed voltages are correspondingly output to the equivalent load, the switch switching circuit is respectively connected to the two rectifier output circuits, and as the input AC voltage increases, the switch switching circuit is turned on, thereby connecting the two rectifier output circuits, and connecting the two rectifier output circuits in series, which can better avoid the inflection point voltage of the boost or buck type, prevent voltage clamping, so as to achieve the maximum power of the circuit and improve the conversion power of the AC power supply. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a partial circuit diagram of a pre-stage conversion circuit for a DC-DC load provided by an embodiment of the utility model;

[0038] Figure 2 This is a partial circuit diagram of a front-stage conversion circuit for a DC-DC load provided by another embodiment of the utility model;

[0039] Figure 3 This is an example curve diagram of different converters and load matching in a front-stage conversion circuit for a DC-DC load provided by the utility model. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and cannot be understood as limiting the present utility model.

[0041] It should be noted that although the functional module division is performed in the schematic diagram, in some cases, the module division can be different from that in the system.

[0042] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, the features defined as "first" and "second" may explicitly or implicitly include one or more features.

[0043] In the description of the present invention, it should be noted that, unless otherwise clearly defined, terms such as setting, installing, connecting, etc. should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in the present invention in combination with the specific content of the technical solution.

[0044] In the prior art, the power conversion circuit supplied to the DC-DC load will enter a constant power mode after long-term operation. In this mode, the secondary of the transformer coil is not processed. If the primary coil is used to pull up the voltage for power supply, the primary coil will tend to be saturated, and the current and voltage capacity of the corresponding switch tube may not be able to supply the required voltage, and the entire front-stage circuit is difficult to bear. If applied to the product (LED as the back stage), it will become a constant power source, and the "power dead" situation will occur. The conversion efficiency will also decrease accordingly, and the circuit dedicated to the DC-DC load and the corresponding connected load will be considered as a whole. Then this whole will have the characteristics of showing a power demand after startup. Most of the conversion circuits used for DC loads have the above problems.

[0045] Reference Figures 1 to 3 In some embodiments of the present invention, a pre-stage conversion circuit for a DC-DC load includes: an equivalent load RS, an AC power supply end 400, a transformer T1, a first rectifier circuit 100, a switch switching circuit 300 and a second rectifier circuit 200.

[0046] The transformer T1 includes a primary coil, a first secondary coil and a second secondary coil.

[0047] The AC power supply terminal 400 is electrically connected to the primary coil, and the AC power supply terminal 400 outputs an AC voltage to the transformer T1. The transformer T1 receives the AC voltage, transforms the AC voltage, and outputs the transformed output voltage. The AC power supply terminal 400 can output an AC voltage of 220V and 50Hz.

[0048] The first secondary coil is electrically connected to the input end of the first rectifier circuit 100, and the output end of the first rectifier circuit 100 is electrically connected to the equivalent load RS. The first rectifier circuit 100 is used to receive the output voltage, rectify the output voltage, and output the rectified output voltage to the equivalent load RS.

[0049] The second secondary coil is electrically connected to the input end of the second rectifier circuit 200, and the output end of the second rectifier circuit 200 is electrically connected to the equivalent load RS. The second rectifier circuit 200 is used to receive the output voltage, rectify the output voltage, and output the rectified output voltage to the equivalent load RS.

[0050] The input end of the switch switching circuit 300 is electrically connected to the output end of the first rectifier circuit 100, and the input end of the switch switching circuit 300 is also electrically connected to the output end of the second rectifier circuit 200. The output end of the switch switching circuit 300 is electrically connected to the equivalent load RS, and the switch switching circuit 300 can connect the first rectifier circuit 100 and the second rectifier circuit 200 to switch the two parallel rectifier outputs to series outputs.

[0051] It should be noted that the equivalent load RS is an equivalent load with constant power demand characteristics jointly constituted by the constant current output type DC-DC post-stage circuit and the load with LED-like characteristics driven by it.

[0052] In one embodiment, referring to Figure 3 , Figure 3 This is an example diagram of the matching of the conversion circuit and the load. In the figure, the starting voltage of the connected load is 5V, the corresponding required power is 400V and the corresponding required current is 80A.

[0053] According to the VA curve of the DC-DC rear stage, during the startup of the front-stage conversion circuit, a current greater than or equal to 80A needs to be provided when passing through 5V in order to gradually increase the output voltage to exceed the 5V voltage point. Otherwise, the output voltage of the front-stage conversion circuit will be clamped at 5V and cannot rise. As the voltage increases, the current continues to decrease, resulting in a "voltage pull-down" phenomenon, resulting in a steady-state phenomenon in the prior art.

[0054] In the prior art, the traditional pre-stage conversion circuit considers the balance between cost and applicability when designing. Figure 3 The circuit configuration corresponding to the V-Apeak curve of the traditional converter in FIG. That is, the overcurrent control point of the primary circuit of the conversion circuit limits the output current Apeak value of the conversion circuit output to 40.74A, which is 2.44 times the normal stable output current of 16.6667A. However, when the load with constant power demand (starting voltage of 5V, corresponding power demand of 400V and corresponding current demand of 80A) in the traditional front-stage conversion circuit connection diagram is started, it still cannot exceed the current demand of 80A at 5V. Therefore, the output voltage of the traditional front-stage conversion circuit will be clamped at 5V for a long time, and the corresponding current will be maintained at 40A for a long time.

[0055] In the front-stage conversion circuit of the present application, the switch switching circuit 300 is connected to the two rectifier output circuits respectively. As the input AC voltage increases, the two rectifier outputs are connected, and the two parallel rectifier outputs are switched to series outputs. Figure 3 From the V-Apeak curve of the front-stage conversion circuit of the present application, it can be seen that the front-stage conversion circuit of the present application can not only smoothly reach the current demand point of 80A at 5V, but also after the front-stage conversion circuit is connected in series, the current can still exceed and stabilize at the current demand point corresponding to 10V, so that when the output voltage climbs to 24V, the output current can still meet the 16.667A required by the load (the starting voltage is 5V, the corresponding required power is 400V and the corresponding required current is 80A).

[0056] In the above embodiment, corresponding rectifier output circuits are arranged on the two secondary coils of the transformer T1, and the two rectifier output circuits are connected in parallel, and the two transformed voltages are correspondingly output to the equivalent load RS. The switch switching circuit 300 is connected to the two rectifier output circuits respectively. As the input AC voltage increases, the switch switching circuit 300 is turned on to connect the two rectifier output circuits. The two rectifier output circuits are connected in series, which can better avoid the inflection point voltage of the boost or buck type and prevent voltage clamping, so as to achieve the maximum power of the circuit and improve the conversion power of the AC power supply.

[0057] Reference Figure 1 In some embodiments of the present invention, the switch switching circuit 300 includes: a switch tube Q2, a first filter capacitor C3, a first resistor R1 and a second resistor R2.

[0058] The drain of the switch tube Q2 is electrically connected to the output end of the first rectifier circuit 100, and the source of the switch tube Q2 is electrically connected to the output end of the second rectifier circuit 200. As the voltage increases, it reaches the turn-on voltage of the switch tube Q2, and the switch tube Q2 is turned on, connecting the first rectifier circuit 100 and the second rectifier circuit 200 to switch the two parallel rectifier outputs to series outputs.

[0059] One end of the first filter capacitor C3 is electrically connected to the gate of the switch tube Q2, the other end of the first filter capacitor C3 is electrically connected to the source of the switch tube Q2, one end of the first resistor R1 is electrically connected to the gate of the switch tube Q2, and the other end of the first resistor R1 is electrically connected to the source of the switch tube Q2, so as to realize the first filter capacitor C3 and the first resistor R1 being connected in parallel.

[0060] One end of the second resistor R2 is electrically connected to the equivalent load RS, one end of the second resistor R2 is electrically connected to the second rectifier circuit 200, and the other end of the second resistor R2 is electrically connected to one end of the first resistor R1, that is, the other end of the second resistor R2 is electrically connected to the gate of the switch tube Q2.

[0061] In this embodiment, the switch tube Q2 is used as the main component of the switch switching circuit 300. As the voltage increases, the turn-on voltage of the switch tube Q2 is reached, and the switch tube Q2 is turned on, connecting the first rectifier circuit 100 and the second rectifier circuit 200 to switch the two parallel rectifier outputs to series outputs, which can better avoid the inflection point voltage of the boost or buck type and prevent voltage clamping, so as to achieve the maximum power of the circuit and improve the conversion power of the AC power supply.

[0062] Reference Figure 2 In some embodiments of the present invention, the switch circuit 300 includes: a relay K1.

[0063] One end of the switch of the relay K1 is electrically connected to the output end of the first rectifier circuit 100, and the other end of the switch of the relay K1 is electrically connected to the output end of the second rectifier circuit 200. Both ends of the driving end of the relay K1 are electrically connected to the equivalent load RS, that is, the driving end of the relay K1 is connected in parallel with the equivalent load RS, so that the output voltages output by the two rectifier output circuits will flow through the driving end of the relay K1 and then flow into the equivalent load RS.

[0064] The switch switching circuit 300 may also include other circuit components to improve the stability and applicability of the switch switching circuit 300 , and is not limited to only setting the relay K1 alone.

[0065] In this embodiment, the relay K1 is used as the main component of the switch switching circuit 300. As the voltage increases, the conduction voltage of the driving end of the relay K1 is reached, and the switch of the relay K1 is closed, connecting the first rectifier circuit 100 and the second rectifier circuit 200, so as to switch the two parallel rectifier outputs to series outputs, which can better avoid the inflection point voltage of the boost or buck type and prevent voltage clamping, so as to achieve the maximum power of the circuit and improve the conversion power of the AC power supply.

[0066] Reference Figure 1 or Figure 2 In some embodiments of the present invention, the first rectifier circuit 100 includes: a first rectifier diode D1, a second filter capacitor C1 and a second rectifier diode D2.

[0067] The cathode of the first rectifier diode D1 is electrically connected to one end of the first secondary coil, and the anode of the first rectifier diode D1 is electrically connected to one end of the second filter capacitor C1, the cathode of the second rectifier diode D2 and the input end of the switch circuit 300 respectively.

[0068] The anode of the second rectifying diode D2 is electrically connected to the ground terminal, the anode of the second rectifying diode D2 is electrically connected to the second rectifying circuit 200 , and the anode of the second rectifying diode D2 is electrically connected to the other end of the second secondary coil.

[0069] The cathode of the second rectifier diode D2 is electrically connected to the input end of the switch circuit 300, the cathode of the second rectifier diode D2 is also electrically connected to one end of the second filter capacitor C1, and the cathode of the second rectifier diode D2 is also electrically connected to the anode of the first rectifier diode D1.

[0070] One end of the second filter capacitor C1 is electrically connected to the anode of the first rectifier diode D1, the cathode of the second rectifier diode D2 and the input end of the switch circuit 300. The other end of the second filter capacitor C1 is electrically connected to the other end of the first secondary coil, the output end of the switch circuit 300 and the equivalent load RS.

[0071] Reference Figure 1 In one embodiment, when the switch tube Q2 is used as the main component of the switch switching circuit 300, the source of the switch tube Q2 is electrically connected to the anode of the first rectifier diode D1 and the cathode of the second rectifier diode D2 respectively.

[0072] Reference Figure 2 In one embodiment, when the relay K1 is used as a main component of the switch switching circuit 300, one end of the switch of the relay K1 is electrically connected to the anode of the first rectifier diode D1 and the cathode of the second rectifier diode D2 respectively.

[0073] Reference Figure 1 or Figure 2 In some embodiments of the present invention, the second rectifier circuit 200 includes: a third rectifier diode D3, a third filter capacitor C2 and a fourth rectifier diode D4.

[0074] The anode of the third rectifier diode D3 is electrically connected to one end of the second secondary coil, and the cathode of the third rectifier diode D3 is electrically connected to one end of the third filter capacitor C2, the anode of the fourth rectifier diode D4 and the output end of the switch circuit 300 respectively.

[0075] The anode of the fourth rectifier diode D4 is electrically connected to one end of the third filter capacitor C2 , the cathode of the third rectifier diode D3 , and the output end of the switch circuit 300 , respectively.

[0076] The cathode of the fourth rectifier diode D4 is electrically connected to the other end of the first secondary coil, the output end of the switch circuit 300 and the equivalent load RS respectively.

[0077] Reference Figure 1 In one embodiment, when the switch tube Q2 is used as the main component of the switch switching circuit 300, the drain of the switch tube Q2 is electrically connected to the cathode of the third rectifier diode D3 and the cathode of the fourth rectifier diode D4 respectively.

[0078] Reference Figure 2 In one embodiment, when the relay K1 is used as the main component of the switch switching circuit 300, the other end of the switch of the relay K1 is electrically connected to the cathode of the third rectifier diode D3 and the cathode of the fourth rectifier diode D4 respectively.

[0079] Reference Figure 1 or Figure 2 In some embodiments of the present invention, the front-stage conversion circuit further includes: a control circuit 500 , a rectifier and filter circuit 600 , and a control pulse input terminal 510 .

[0080] The input end of the control circuit 500 is electrically connected to the control pulse input end 510 , and the output end of the control circuit 500 is connected to the primary coil in the transformer T1 .

[0081] The control pulse input terminal 510 can supply a control pulse to the control circuit 500 . The control circuit 500 receives the control pulse and controls the transformer T1 to control the conversion power of the circuit.

[0082] The control circuit 500 includes a field effect transistor Q1.

[0083] The gate of the field effect transistor Q1 is electrically connected to the control pulse input terminal 510, the source of the field effect transistor Q1 is electrically connected to the ground terminal, and the drain of the field effect transistor Q1 is electrically connected to the other end of the primary coil in the transformer T1.

[0084] The control circuit 500 may also include other circuit components to improve the stability and applicability of the control circuit 500 , and is not limited to only setting the field effect transistor Q1 alone.

[0085] An input end of the rectifying and filtering circuit 600 is electrically connected to the AC power supply end 400 , and an output end of the rectifying and filtering circuit 600 is electrically connected to the primary coil in the transformer T1 .

[0086] The rectifying and filtering circuit 600 includes: a rectifying bridge BD1 and a fourth filtering capacitor.

[0087] The input end of the rectifier bridge BD1 is electrically connected to the AC power supply end 400, the output end of the rectifier bridge BD1 is connected to the fourth filter capacitor, one end of the fourth filter capacitor is electrically connected to the output end of the rectifier bridge BD1 and one end of the primary coil in the transformer T1 respectively, the other end of the fourth filter capacitor is electrically connected to the ground end, and the other end of the fourth filter capacitor is also connected to the output end of the rectifier bridge BD1.

[0088] Among them, refer to Figure 1 , when the switch tube Q2 is used as the main component of the switch switching circuit 300, the fourth filter capacitor is the capacitor C4. Figure 2 When the relay K1 is used as the main component of the switch circuit 300, the fourth filter capacitor is the capacitor C3.

[0089] Through the scheme of the above embodiment, the utility model sets corresponding rectifier output circuits on the two secondary coils of the transformer T1, and the two rectifier output circuits are connected in parallel, and the two transformed voltages are correspondingly output to the equivalent load RS. The switch switching circuit 300 is connected to the two rectifier output circuits respectively. As the input AC voltage increases, the switch switching circuit 300 is turned on to connect the two rectifier output circuits. The two rectifier output circuits are connected in series, which can better avoid the inflection point voltage of the boost or buck type and prevent voltage clamping, so as to achieve the maximum power of the circuit and improve the conversion power of the AC power supply.

[0090] The preferred implementation methods of the utility model are specifically described above, but the invention of the utility model is not limited to the described embodiments. Those skilled in the art can make various equivalent modifications or substitutions without violating the spirit of the utility model. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A pre-stage conversion circuit for a DC-DC load, characterized in that: include: Equivalent load; An AC power supply terminal, the AC power supply terminal is used to output an AC voltage; A transformer, wherein the transformer is provided with a primary coil, a first secondary coil and a second secondary coil, the primary coil is connected to an AC power supply end, and the transformer is used to transform the AC voltage; a first rectifier circuit, wherein an input end of the first rectifier circuit is connected to the first secondary coil, and an output end of the first rectifier circuit is connected to an equivalent load; a second rectifier circuit, wherein an input end of the second rectifier circuit is connected to the second secondary coil, and an output end of the second rectifier circuit is connected to an equivalent load; A switch switching circuit, wherein the input end of the switch switching circuit is respectively connected to the output end of the first rectifier circuit and the output end of the second rectifier circuit, the output end of the switch switching circuit is connected to the equivalent load, and the switch switching circuit is used to connect the first rectifier circuit and the second rectifier circuit to switch to a series output.

2. A pre-stage conversion circuit for a DC-DC load according to claim 1, characterized in that: The switch switching circuit comprises: A switch tube, wherein the drain of the switch tube is connected to the output end of the first rectifier circuit, and the source of the switch tube is connected to the output end of the second rectifier circuit; A first filter capacitor, wherein the first filter capacitor is connected to the gate and source of the switch tube respectively; A first resistor, wherein the first resistor is connected to the gate and the source of the switch tube respectively; A second resistor is connected to the equivalent load, the first resistor and the second rectifier circuit respectively.

3. A pre-stage conversion circuit for a DC-DC load according to claim 1, characterized in that: The switch switching circuit comprises: a relay; The switch of the relay is connected to the output end of the first rectifier circuit and the output end of the second rectifier circuit respectively, and the driving end of the relay is connected to the equivalent load.

4. A pre-stage conversion circuit for a DC-DC load according to claim 1, characterized in that: Also includes: A control pulse input terminal, wherein the control pulse input terminal is used to output a control pulse; A control circuit, wherein the input end of the control circuit is connected to the control pulse input end, the output end of the control circuit is connected to the primary coil, and the control circuit is used to receive the control pulse to control the transformer.

5. The pre-stage conversion circuit for a DC-DC load according to claim 1, characterized in that: The first rectifier circuit comprises: a first rectifier diode, wherein a cathode of the first rectifier diode is connected to the first secondary coil; a second rectifying diode, wherein a cathode of the second rectifying diode is respectively connected to an anode of the first rectifying diode and an input end of the switch switching circuit, and an anode of the second rectifying diode is grounded; A second filter capacitor, one end of which is respectively connected to the anode of the first rectifier diode and the cathode of the second rectifier diode, and the other end of which is respectively connected to the first secondary coil and the equivalent load.

6. A pre-stage conversion circuit for a DC-DC load according to claim 5, characterized in that: The second rectifier circuit comprises: a third rectifier diode, wherein an anode of the third rectifier diode is connected to the second secondary coil; a fourth rectifier diode, wherein an anode of the fourth rectifier diode is respectively connected to a cathode of the third rectifier diode and an input end of the switch switching circuit, and a cathode of the fourth rectifier diode is respectively connected to the other end of the second filter capacitor and an equivalent load; A third filter capacitor, one end of the third filter capacitor is connected to the cathode of the third rectifier diode, and the other end of the third filter capacitor is grounded.

7. A pre-stage conversion circuit for a DC-DC load according to claim 1, characterized in that: Also includes: Rectification and filtering circuit; The input end of the rectifying and filtering circuit is connected to the AC power supply end, and the output end of the rectifying and filtering circuit is connected to the primary coil.

8. A pre-stage conversion circuit for a DC-DC load according to claim 7, characterized in that: The rectifier and filter circuit comprises: A rectifier bridge, wherein an input end of the rectifier bridge is connected to an AC power supply end; A fourth filter capacitor is connected to the rectifier bridge and the primary coil of the transformer respectively.

9. A pre-stage conversion circuit for a DC-DC load according to claim 4, characterized in that: The control circuit comprises: a field effect tube; the gate of the field effect tube is connected to the control pulse input terminal, the source of the field effect tube is grounded, and the drain of the field effect tube is connected to the primary coil.

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