CN106549563A - A kind of flyback exports high-voltage diode replacement circuit - Google Patents

Abstract

The present invention includes a transformer, a filter unit, and a load. The transformer includes a primary winding, an iron core, and a secondary winding; it also includes a first diode, a second diode, and a voltage clamping unit; the secondary winding The first diode, the second diode and the load are connected in series; the secondary winding includes a first terminal and a second terminal, and the first terminal is connected to the anode of the first diode; the The first diode and the second diode are connected in series in the same direction; one end of the voltage clamping unit is connected to the cathode of the first diode, and the voltage clamping unit The other end is connected to the second terminal. Ming proposed to reduce the cost of components and parts, improve the electromagnetic interference characteristics, and replace the circuit with a flyback output high-voltage diode. When outputting high voltage, due to the problem of diode withstand voltage, it may not be possible to use the flyback topology. This solution continues to use the flyback topology. topology is possible.

Description

A Flyback Output High Voltage Diode Replacement Circuit

technical field

The invention relates to the field of electricity, in particular to a flyback output high voltage diode replacement circuit.

Background technique

When the LED drive power supply is implemented with a flyback topology, when the output voltage is high, the reverse withstand voltage requirements of the output rectifier diode are correspondingly increased. The higher the output voltage, the worse the EMI, but now the widely used high-voltage diode reverse withstand voltage The voltage is only 1200V, and the reverse recovery is slow, the price is high, and the reliability is not as high as that of common fast recovery diodes.

Figure 1 is a common flyback topology. The voltage stress of the output diode of the flyback power supply increases with the increase of the output voltage, and there are voltage spikes. The specific expression is: Vdiode=Vin/N+Vo+Vspike, where Vdiode represents The peak voltage of the output diode, Vin represents the input voltage of the flyback converter, N represents the turn ratio of the transformer, turn ratio = Np/Ns, Vo represents the output voltage, and Vspike represents the peak voltage part. When the output voltage is relatively high, such as above 300V, since the maximum specification of the conventional diode is generally 1200V, and the voltage peak of the flyback will reach 300V or even 500V during the start-up and lightning surge test, it can be seen that in this case it needs to be adjusted The transformer turns ratio ensures that the stress on the diode does not exceed its rating. Due to this limitation, it is difficult to achieve an optimal design for the flyback converter.

The general solution is to add another diode in series, as shown in Figure 2. The two diodes shown in Figure 2 may have the problem of not being able to equalize the voltage in actual use, which is a common sense problem in the industry. The problem of uneven voltage division is related to the batch of diodes, heat dissipation, stray capacitance, etc. In other words, if the stress of a single diode needs to be 1000V, when replacing two diodes in series, it cannot be replaced with two 600V or 500V diodes. Integrating two 800V diodes will cause the stress of a single diode to exceed the standard during a lightning surge, and the cost will increase a lot. It is meaningless to replace it with two 1000V diodes. In the same way, if a single 1200V diode is used, the problem of excessive stress will occur. Since there is no higher specification diode to choose from, it is necessary to replace it with two 1200V diodes in series to solve the problem. In fact, due to the batch problem of diodes , the form of two series cannot completely guarantee to solve the problem of excessive stress, not to mention the increase of cost and the decrease of efficiency.

In order to solve the problem that the stress of the diode is too large and there are no optional components, the present invention proposes a method of adding an auxiliary diode.

Contents of the invention

In order to solve the problems existing in the prior art, the present invention proposes to reduce the cost of components and parts, improve the electromagnetic interference characteristics, and replace the high-voltage diode circuit with the flyback output. Excitation topology, this solution makes it possible to continue to use the flyback topology.

The present invention adopts following technical scheme:

The present invention includes a transformer, a filter unit, and a load. The transformer includes a primary winding, an iron core, and a secondary winding; it also includes a first diode, a second diode, and a voltage clamping unit;

The secondary winding is connected in series with the first diode, the second diode and the load; the secondary winding includes a first terminal and a second terminal, and the first terminal is connected to the first diode the anode of the tube; the first diode and the second diode are connected in series in the same direction; one end of the voltage clamping unit is connected to the cathode of the first diode, and The other end of the voltage clamping unit is connected to the second terminal.

Preferably, the voltage clamping unit includes a third diode, the cathode of the third diode is connected to the cathode of the first diode, and the anode of the third diode is connected to the second terminal.

Preferably, the voltage clamping unit further includes a triode, and the triode and the third diode are connected in parallel; the collector of the triode is connected to the cathode of the first diode, so The emitter of the triode mentioned above is connected to the second terminal.

Preferably, the voltage clamping unit includes a MOSFET, the drain of the MOSFET is connected to the cathode of the first diode, and the source of the MOSFET is connected to the second terminal.

Preferably, the voltage clamping unit includes a resistor.

Preferably, the anode of the second diode is connected to the voltage clamping unit, and the cathode of the second diode is connected to the load.

Preferably, the anode of the second diode is connected to the load, and the cathode of the second diode is connected to the voltage clamping unit.

Preferably, the filter unit is connected in parallel with the load.

Preferably, the filtering unit is a capacitor.

The beneficial effects of the present invention are: the present invention arranges the first diode VD1 and the second diode VD2 on the secondary winding Ns side of the flyback power supply, and between the first diode VD1 and the second diode VD2 Add a voltage clamp unit. In the prior art, before the second diode VD2 and the voltage clamping unit are added, when the voltage across the secondary winding is high at the lower end and low at the upper end, the voltage at both ends of the first diode VD1 is relative to the output ground They are Vo and -(Vin/N+Vspike); Vin represents the input voltage of the flyback converter, N represents the turn ratio of the transformer, N=Np/Ns, Vo represents the output voltage, and Vspike represents the peak voltage part. As shown in Figure 5-6, after adding the second diode VD2 and the voltage clamping unit, the anode voltage of the first diode VD1 is -(Vin/N+Vspike) relative to the output ground, and the negative voltage is relative to the output ground is Vf. Among them, Vf is the forward conduction voltage drop of the voltage clamping unit, which is generally less than 2V. Compared with the higher output voltage, it can be considered as 0 in theoretical analysis. At this time, the lower end of the voltage clamping unit is at high level, the upper end is at low level, and is forward-conducting, while the cathode of the first diode VD1 is at high level, and the anode is at low level, and is reversely cut off. After the second diode VD2 and the third diode VD3 are added, both the first diode and the second diode are cut off to form a voltage divider network, and the voltage at both ends of the flyback transformer winding is generated relative to the output ground. The large difference is equivalent to splitting a higher voltage into the sum of two voltages, dV/dt becomes smaller, and its EMI characteristics have changed greatly. In practical applications, due to the electromagnetic flux generated by the transformer output winding and diode The interference part is reduced, which improves the EMI performance. It can be seen that after adding the second diode VD2 and the voltage clamping unit, since the second diode VD2 can choose a diode with better reverse recovery characteristics, the EMI characteristics are only different from the output of the first diode VD1 Larger, the radiation in some frequency bands has been significantly improved. The present invention reduces the stress of the first diode VD1 and at the same time reduces the diode dv/dt (reduces the voltage peak value and slows down the voltage rise) and improves the electromagnetic compatibility performance of the power supply. After changing the conventional output of flyback topology to this technical solution, diodes with lower reverse withstand voltage and faster reverse recovery time can be used. Since the amplitude of the transient change point of the transformer is greatly reduced, the overall electromagnetic compatibility of the power supply can be improved. performance. When outputting high voltage, due to the problem of diode withstand voltage, it may not be possible to use the flyback topology. This solution makes it possible to continue to use the flyback topology.

Description of drawings

FIG. 1 is a flyback topology diagram in the prior art.

Figure 2 is a circuit diagram using two diodes connected in series.

Fig. 3 is a measured waveform diagram of voltage stress when two diodes of the same type are connected in series in Fig. 2 .

FIG. 4 is a schematic diagram of the first circuit of the voltage clamping unit of the present invention.

FIG. 5 is a schematic diagram of the second circuit of the voltage clamping unit of the present invention.

FIG. 6 is a circuit schematic diagram of the first type of diode used in the voltage clamping unit of the present invention.

FIG. 7 is a schematic diagram of a second circuit using diodes in the voltage clamping unit of the present invention.

FIG. 8 is a schematic circuit diagram of a resistor used in the voltage clamping unit of the present invention.

FIG. 9 is a schematic circuit diagram of a voltage clamping unit using a MOSFET in the present invention.

FIG. 10 is a schematic circuit diagram of a voltage clamping unit using a triode in the present invention.

detailed description

The present invention will be further described below in conjunction with the accompanying drawings, but the present invention is not limited to the following examples.

As shown in Figures 4-10, the present invention includes a transformer, a filter unit, and a load. The transformer includes a primary winding, an iron core, and a secondary winding; it also includes a first diode, a second diode, and a voltage clamp unit;

The secondary winding is connected in series with the first diode VD1, the second diode VD2 and the load; the secondary winding NS includes a first terminal and a second terminal, and the first terminal is connected to the first An anode of a diode VD1; the first diode VD1 and the second diode VD2 are connected in series in the same direction; one end of the voltage clamping unit is connected to the first two The cathode of the pole tube VD1, the other end of the voltage clamping unit is connected to the second terminal. The voltage clamping unit includes a third diode VD3, the cathode of the third diode VD3 is connected to the cathode of the first diode VD1, and the anode of the third diode VD3 Connect the second terminal as described. The voltage clamping unit further includes a triode, and the triode and the third diode VD3 are connected in parallel; the collector of the triode is connected to the cathode of the first diode VD1, and the triode is connected to the cathode of the first diode VD1. The emitter of the triode is connected to the second terminal. The voltage clamping unit includes a MOSFET, the drain of the MOSFET is connected to the cathode of the first diode VD1, and the source of the MOSFET is connected to the second terminal. The voltage clamping unit includes a resistor. The anode of the second diode VD2 is connected to the voltage clamping unit, and the cathode of the second diode VD2 is connected to the load. The anode of the second diode VD2 is connected to the load, and the cathode of the second diode VD2 is connected to the voltage clamping unit. The filtering unit C2 is connected in parallel with the load. The filtering unit C2 is a capacitor.

Embodiment one

6-7 are schematic circuit diagrams of the voltage clamping unit of the present invention using the second diode VD2 connected in series with the third diode VD3. Its forms include the schematic circuit diagrams of the two series connection modes of the second diode VD2 respectively shown in FIGS. 4-5 .

The present invention adds an auxiliary diode, the third diode VD3, on the basis of adding FIG. 2 . The anode of the second diode VD2 is connected to the cathode of the third diode VD3, and the cathode of the second diode VD2 is connected to the load. In order to illustrate the working principle, the selection of the device after the third diode VD3 is explicitly added here.

Assuming that the output voltage is 300V, since the withstand voltage of the second diode VD2 plus the third diode VD3 will not exceed the output voltage and it only plays a conduction role and is not responsible for switching, the withstand voltage of the second diode VD2 can be selected as 400V The diode of the 1A level and the current level is slightly lower, and its cost is much lower than that of the 1200V diode; the third diode VD3 can choose a fast diode of the 1A400V level, and the cost of the diode with a small withstand voltage is very low, only a few cents. Below, the advantages of this embodiment are specifically described through Table 1 through the comparison of three solutions including the prior art,

Table 1 Comparison table of three schemes

Scheme 1, that is, Figure 1 is a common flyback topology diagram. The voltage stress of the output diode of the flyback power supply increases with the increase of the output voltage, and there are voltage spikes. The specific expression is: Vdiode=Vin/N+Vo+Vspike , where Vdiode represents the peak voltage of the output diode, Vin represents the input voltage of the flyback converter, N represents the turn ratio of the transformer, N=Np/Ns, Vo represents the output voltage, and Vspike represents the peak voltage part. When the output voltage is relatively high, such as above 300V, since the maximum specification of the conventional diode is generally 1200V, and the voltage peak of the flyback will reach 300V or even 500V during the start-up and lightning surge test, it can be seen that in this case it needs to be adjusted The transformer turns ratio ensures that the stress on the diode does not exceed its rating. Due to this limitation, it is difficult to achieve an optimal design for the flyback converter.

Solution 2, as shown in Figure 2, is to connect another diode in series. The two diodes shown in Figure 2 have the problem of not being able to equalize the voltage in actual use, which is a common-sense problem in the industry. As shown in Figure 3, the measured waveform of the voltage stress when two diodes of the same type are connected in series in Figure 2, it can be seen that the voltage of the two diodes is uneven (615V for a single diode). Figure 3 only quantitatively illustrates the problem of uneven voltage division. As shown in Fig. 1, when the first diode VD1 is single, the stress is 770V. The problem of uneven voltage division is related to the batch of diodes, heat dissipation, stray capacitance, etc. In other words, if the stress of a single diode needs to be 1000V, when replacing two diodes in series, it cannot be replaced with two 600V or 500V diodes. Integrating two 800V diodes will cause the stress of a single diode to exceed the standard during a lightning surge, and the cost will increase a lot. It is meaningless to replace it with two 1000V diodes. In the same way, if a single 1200V diode is used, the problem of excessive stress will occur. Since there is no higher specification diode to choose from, it is necessary to replace it with two 1200V diodes in series to solve the problem. In fact, due to the batch problem of diodes , the form of two series cannot completely guarantee to solve the problem of excessive stress, not to mention the increase of cost and the decrease of efficiency.

Scheme 3, that is, the scheme of the first implementation, the working principle is as follows: when the winding voltage of the transformer is positive and negative, the first diode VD1 and the second diode VD2 are turned on, and the two diodes work like one diode. And the third diode VD3 is cut off and does not work; when the winding voltage of the transformer is lower than positive and upper negative, the third diode VD3 is turned on, the first diode VD1 is cut off, and the second diode VD2 is also cut off; The voltage stress of the second diode VD2 , that is, the back voltage is equal to the output voltage Vo, and the voltage stress of the first diode VD1 is equal to Vin/N+Vspike. Therefore, when the output voltage is relatively high (greater than 300V), 1200V or 1000V diodes can be used. More importantly, at this time, the flyback topology cannot be used because the diode stress problem cannot be solved.

After adding the second diode VD2 and the third diode VD3, since the second diode VD2 can choose a diode with better reverse recovery characteristics, the EMI characteristics are only different from the output of the first diode VD1 Larger, the radiation in some frequency bands has been significantly improved.

It should be noted that the third diode VD3 of the added auxiliary diode can use a very cheap and small-sized device because it hardly flows current; the second diode VD2 can also use a device with a small rated voltage and current stress, and the cost lower;

Embodiment two

The voltage clamping unit described in 9 in the figure may be a MOSFET. Its forms include the schematic circuit diagrams of the two series connection modes of the second diode VD2 respectively shown in FIGS. 4-5 . The voltage clamping unit includes a MOSFET, the drain of the MOSFET is connected to the cathode of the first diode VD1, and the source of the MOSFET is connected to the second terminal. When the voltage clamping unit is a MOSFET, due to the body diode, no additional parallel diode is required to perform the clamping function. The principle is the same as that in Embodiment 1, and will not be repeated here.

Embodiment Three

The voltage clamping unit described in 10 in the figure may be a triode. Its forms include the schematic circuit diagrams of the two series connection modes of the second diode VD2 respectively shown in FIGS. 4-5 . When the voltage clamping unit is a triode, two external parallel diodes are required. The principle is the same as that in Embodiment 1, and will not be repeated here.

Embodiment Four

The voltage clamping unit described in 8 in the figure may be a resistor. Resistors are another, less expensive alternative. Its forms include the schematic circuit diagrams of the two series connection modes of the second diode VD2 respectively shown in FIGS. 4-5 . The working principle is as follows: when the upper end of the winding voltage of the transformer is at a high level and the lower end is at a low level, the first diode VD1 and the second diode VD2 are turned on, and the two diodes work as one diode. At the same time, the voltage There is current passing through the resistance of the clamping unit; when the lower end of the winding voltage of the transformer is at a high level and the upper end is at a low level, the first diode VD1 and the second diode VD2 are cut off, at this time the first diode VD1 1. There is no current passing through the second diode VD2 and the resistor, which can be regarded as infinite resistance. The first diode VD1 and the second diode VD2 form a voltage divider network. The resistance value of the resistor is small, and the voltage divider can be ignored. Excluding. In this way, the purpose of voltage division by the first diode VD1 and the second diode VD2 is achieved.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which the present invention belongs can make various modifications or supplements to the described specific embodiments or adopt similar methods to replace them, but they will not deviate from the spirit of the present invention or go beyond the definition of the appended claims These changes should also be regarded as the protection scope of the present invention.

Claims (9)

1. a kind of flyback exports high-voltage diode replacement circuit, including transformator, filter unit, load, described transformator bag Include armature winding, iron core and secondary windings；It is characterized in that：Also include the first diode, the second diode, voltage clamp list Unit；

Described secondary windings are connected the first diode, the second diode and load；Described secondary windings include the first terminal And Second terminal, the anode of the first described diode of described the first terminal connection；The first described diode and described Second diode series aiding connection；The negative electrode of the first diode described in one end connection of described voltage clamp unit, it is described Second terminal described in the other end connection of voltage clamp unit.

2. a kind of flyback according to claim 1 exports high-voltage diode replacement circuit, it is characterised in that：Described voltage Clamp units include the 3rd diode, and the negative electrode of the 3rd described diode connects the negative electrode of the first described diode, described The 3rd diode the described Second terminal of anode connection.

3. a kind of flyback according to claim 2 exports high-voltage diode replacement circuit, it is characterised in that：Described voltage Clamp units also include audion, and described audion is parallel with one another with the 3rd described diode；The collection of described audion The negative electrode of the first described diode of electrode connection, the emitter stage of described audion connect described Second terminal.

4. a kind of flyback according to claim 1 exports high-voltage diode replacement circuit, it is characterised in that：Described voltage Clamp units include MOSFET, and the drain electrode of described MOSFET connects the negative electrode of the first described diode, described MOSFET The described Second terminal of source electrode connection.

5. a kind of flyback according to claim 1 exports high-voltage diode replacement circuit, it is characterised in that：Described voltage Clamp units include resistance.

6. a kind of flyback according to any one of claim 1-5 exports high-voltage diode replacement circuit, it is characterised in that： Voltage clamp unit described in the anode connection of the second described diode, the negative electrode connection of the second described diode are loaded.

7. a kind of flyback according to any one of claim 1-5 exports high-voltage diode replacement circuit, it is characterised in that： The anode connection load of the second described diode, the negative electrode of the second described diode connect described voltage clamp unit.

8. a kind of flyback according to claim 1 exports high-voltage diode replacement circuit, it is characterised in that：Described filtering Unit is in parallel with described load.

9. a kind of flyback according to claim 1 exports high-voltage diode replacement circuit, it is characterised in that：Described filtering Unit is electric capacity.