Resonant converter and voltage stabilizing method thereof

A resonant converter and voltage stabilizing method thereof are provided. The resonant converter includes a converting stage circuit, a diode-rectifying stage circuit, a filter and load stage circuit, a logic circuit, a driving circuit, and an energy-recycling circuit. The method includes steps of recycling an energy from the filter and load stage circuit to the converting stage circuit when the resonant converter is light- or zero-loaded.

FIELD OF THE INVENTION

The present invention relates to a resonant converter and a voltage stabilizing method for the resonant converter operating at light load or no load condition, and more particularly to a resonant dc/dc converter for a power supply.

BACKGROUND OF THE INVENTION

Recently, more and more strict demands are asked on the DC/DC converter such as high efficiency, high power density, high reliability and low cost. Please refer toFIG. 1, which is a block diagram showing a dc/dc converter according to the prior art. InFIG. 1, the dc/dc converter10includes a converting stage circuit11, a diode-rectifying stage circuit12, and a filter and load stage circuit13. The operation principle of the dc/dc converter10includes the steps of: a dc voltage Vin is firstly modulated by the converting stage circuit11, then rectified by the diode-rectifying stage circuit12, and finally filtered by the filter and load stage circuit13to be sent to a load (not shown).

In the dc/dc converter10, the energy is delivered from the converting stage circuit11to the filter and load stage circuit13, which is a uni-direction path. And sometimes this “uni-directional path” energy transferring method will cause the output voltage of the dc/dc converter10unstable when operating at light or no load condition, as shown inFIG. 2(a).

Please refer toFIG. 2(a), which is a circuit diagram showing a full-bridge LLC converter according to the prior art. The full-bridge LLC converter is generally operated using Pulse Frequency Modulation (PFM) technique. InFIG. 2(a), the full-bridge LLC converter20includes a converting stage circuit, a diode-rectifying stage circuit, and a filter and load stage circuit. The converting stage circuit includes four switches Q1˜Q4, a resonant capacitor C1, a resonant inductor L1, a magnetizing inductor L2, and a transformer T1. The diode-rectifying stage circuit includes two diodes D1˜D2. The filter and load stage circuit includes a filter capacitor Cout. The switches Q1and Q2constitute one bridge arm and the switches Q3and Q4constitute the other. The respective driving signal of the switches Q1and Q4and the switches Q2and Q3drives the switches at nearly 50% duty cycle. Between the midpoint of the two bridge arms are the resonant capacitor C1, the resonant inductor L1, and the primary side of the transformer T1, which are connected in series. The secondary side of the transformer T1, which is a center-tap structure, includes two diodes D1and D2to form a full-wave rectifier. The output side of the full-bridge LLC converter20includes a capacitor Cout for filtering and stabilizing the output voltage.

For a resonant converter with the diode rectifying technique, there exists a minimum voltage gain in the range of the operation frequency thereof, for example, the minimum voltage gain obtained when the above full-bridge LLC converter20is operated at the highest operation frequency. Generally, a converter is designed to have its gain more than the above minimum voltage gain when operating in the range of the operation frequency thereof, and the converter is theoretically able to be operated stably with complete zero load. In practice, due to the parasitic oscillation generated by the parasitic parameters of the elements, e.g. the parasitic capacitor at the primary or secondary side of the transformer, an excess of energy will be injected into the output terminal so as to cause the output voltage to rise when using the diode-rectification at the secondary side, as shown inFIG. 2(a). Thus, the converter will be unstable when operating at light or no load condition.

To solve the aforementioned problems, there are at least four technical schemes in the prior art, which are provided as follows.

The first is to consume the excess energy injected into the output terminal. The practical method is to install an adequate dummy load. However, the dummy load will cause the converter to be operated in a lower efficiency and consume more power when operating at no load condition. Besides, the size and the cost are also increased, too.

The second is to install an independent auxiliary circuit. When operating at light or zero load condition, the main circuit is switched off and the auxiliary circuit is operated to maintain the output voltage. In this regard, there will be no additional loss at normal load. However, it needs load judgment additionally and switching between the auxiliary circuit and the main circuit, which increases the control complexity and adds the additional requirements on the dynamic performance of the converter.

The third is to adopt burst mode control technique to reduce the energy transferred from the input terminal to the output terminal when operating at light load or no load condition.

The fourth is to prevent the excess energy from being injected into the output terminal when operating at light load or no load condition, which is realized by changing the resonant parameters or the resonant impedance. There are at least three methods as follows:

Please refer toFIG. 2(b), which is a circuit diagram showing the full-bridge LLC converter provided in U.S. Pat. No. 5,388,040. In the full-bridge LLC converter21, the elements which are the same as those inFIG. 2(a) are marked with the same numerical symbols.

The technical scheme adopted in U.S. Pat. No. 5,388,040 is to change the resonant parameters according to the load conditions. AsFIG. 2(b) shows, a switch Sa is introduced into the main circuit to be connected with the magnetizing inductor L2in series. The equivalent magnetizing inductance is able to be adjusted by controlling the switch Sa. When operating at light or no load condition, the equivalent magnetizing inductance of the main circuit will be decreased after the switch Sa is turned on. Therefore, the minimum voltage gain of the main circuit will also be decreased in a specific range of operation frequency. Thus the main circuit will be operated stably.

Please refer toFIG. 2(c), which is a circuit diagram showing the full-bridge LLC converter provided in JP Patent No. 8,033,329. In the full-bridge LLC converter22, the elements, which are the same as those inFIG. 2(a) are marked with the same numerical symbols.

The technical scheme adopted in JP Patent No. 8,033,329 is to change the resonant impedance at different load conditions. AsFIG. 2(c) shows, a parallel resonant unit composed of an inductor L2and a capacitor C2is in the resonant loop constituted by the resonant capacitor C1, the resonant inductor L1, and the primary side of the transformer T1, so as to increase the impedance of the resonant loop when the converter22is light- or zero-loaded. Thus the whole system will be operated stably accordingly. However, the drawback is that the parallel resonant unit will bear a large voltage current stress when the main circuit is light- or zero-loaded.

Please refer toFIG. 2(d), which shows the full-bridge LLC converter provided in JP Patent No. 2,106,164. In the full-bridge LLC converter23, the elements, which are the same as those inFIG. 2(a) are marked with the same numerical symbols.

AsFIG. 2(d) shows, a series circuit composed of an auxiliary switch S and a resistor R is connected to the resonant capacitor C1in parallel. With the series circuit, the energy at the resonant capacitor C1will be consumed when operating at zero load condition, in order to prevent the excess of energy from being injected into the output terminal.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a resonant converter and a voltage stabilizing method thereof, so that the drawback that the converter being operated unstably when operating at light or zero load condition resulting from the parasitic parameters, can be eliminated.

According to the foregoing object of the present invention, a resonant converter is provided. The resonant converter includes additionally an energy-recycling circuit. The energy-recycling circuit is able to recycle the excess of energy, from the output terminal of the converter, to the input terminal of the converter.

According to the foregoing object of the present invention, a voltage stabilizing method for a resonant converter with light or open load is provided. With the method, the energy will be recycled from the output terminal of the converter to the input terminal of the converter through a substantial circuit or an inducting circuit, so that the stable operation of the converter when operating at light load or zero load condition is achieved.

The foregoing and other features and advantages of the present invention will be more clearly understood through the following descriptions with reference to the drawings, wherein:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Please refer toFIG. 3, which is a block diagram showing a converter according to one preferred embodiment of the present invention. In the converter30, the blocks, which are the same as those inFIG. 1are marked with the same numerical symbols. The converter30includes a converting stage circuit11, a diode-rectifying stage circuit12, a filter and load stage circuit13, a logic circuit31, a driving circuit32, and an energy-recycling circuit33.

The diode-rectifying stage circuit12is connected with the converting stage circuit11in series to rectify the output thereof. The filter and load stage circuit13is connected with the diode-rectifying stage circuit12in series to filter the output thereof. The logic circuit31is coupled with the converting stage circuit11to generate a logic signal in response thereto. The driving circuit32is coupled with the logic circuit31in series to generate a driving signal in response thereto. The energy-recycling circuit33is coupled with the converting stage circuit11, the filter and load stage circuit13, and the driving circuit32. When the converter30is operating at light load or no load condition, the energy-recycling circuit33will recycle the energy from the filter and load stage circuit13back to the converting stage circuit11in response to the driving signal.

Please refer toFIG. 4, which is a circuit diagram showing a converter according to one preferred embodiment of the present invention. In the converter40, the blocks, which are the same as those inFIG. 3are marked with the same numerical symbols. The resonant converter40is a series resonant converter, which includes a dc/dc converter10, a logic circuit31, a driving circuit32, and an energy-recycling circuit33.

InFIG. 4, the converting stage circuit includes an input-voltage generating circuit composed of four switches Q1˜Q4, a resonant capacitor C1, resonant inductor L1, a magnetizing inductor L2, and a transformer T1. The magnetizing inductor L2is firstly connected with the primary side of the transformer T1in parallel and then connected with the resonant circuit in series. Although the four switches Q1˜Q4are adopted to constitute a full-bridge circuit as the input-voltage generating circuit in the preferred embodiment, two switches are also able to be adopted to constitute a half-bridge circuit as the input-voltage generating circuit alternatively.

InFIG. 4, the diode-rectifying stage circuit and the filter and load stage circuit are sequentially coupled to the secondary side of the transformer T1. Although the two diodes D1˜D2are adopted to constitute a diode fall-wave rectifying circuit as the diode-rectifying stage circuit, the diode full-wave rectifying circuit is also able to be replaced with a diode half-wave rectifying circuit or a diode full-bridge rectifying circuit. The filter and load stage circuit includes a capacitor Cout while the load is not shown inFIG. 4.

Although the logic circuit31inFIG. 4is composed of a resistor Ra, a diode Da1, a capacitor Ca, and an AND logic gate, the practical circuit is not limited thereto. One skilled in the art is able to figure out other types of circuit as the logic circuit31providing the similar circuit functions.

Although the energy-recycling circuit33inFIG. 4is composed of a switch Sa and a diode Da, the practical circuit is not limited thereto. One skilled in the art is able to figure out other types of circuit as the energy-recycling circuit33providing the similar circuit functions. In the present invention, the energy-recycling circuit33includes at least a switch. In this regard, the energy-recycling circuit33could be composed of the single switch shown inFIG. 5(a), the single transistor switch Sa shown inFIG. 5(b), the series connection of the single switch Sa and the diode Da shown inFIG. 5(c), the series connection of the single switch Sa and the resistor Ra shown inFIG. 5(d), the series connection of the single switch Sa, the resistor Ra and the diode Da shown inFIG. 5(e), or the series connection of the two switches Sa shown inFIG. 5(f). Provided that the energy is able to be recycled from the output terminal to the input terminal, the energy-recycling circuit33could be composed of one or more switches. Each of the switches could be a uni-directional switch or a bi-directional switch. The switches could be connected with an outer resistor in series, as long as the switches are able to provide a controllable energy channel from point B to point A, as shown inFIG. 5.

As mentioned above, the diode rectifying stage circuit could be a diode half-wave rectifying circuit, a diode fall-wave rectifying circuit or a diode full-bridge rectifing circuit.FIG. 6(a)˜(c) are circuit diagrams showing the connection of the energy-recycling circuit ofFIG. 5(a) and the respective three diode rectifying stage circuits. AsFIG. 6(a)˜(c) show, the energy-recycling circuit is a single substantial circuit. The energy-recycling frequency of is at most equal to the switch frequency of the converting stage circuit, which means that the recycle of the energy from the output terminal to the input terminal can be achieved at most once during a switch cycle. Certainly, the recycle of the energy could also be achieved once during several switch cycles. In the respectiveFIG. 6(d) andFIG. 6(e), two substantial circuits are provided and each diode of the diode rectifying stage circuit is connected with a switch in parallel. With the additional energy-recycling unit (the switch), the energy recycle could also be achieved at most twice during a switch cycle.

The operation principles shown inFIG. 4will be described below with the waveform shown inFIG. 7.

The input signal of the logic circuit31includes the driving signal gQ1of the switches Q1& Q4and the driving signal gQ2of the switches Q3& Q2. After passing through the resistor Ra, the diode Da1and the capacitor Ca, the falling edge of the driving signal gQ2is delayed, and then becomes a signal u1. The signal u1undergoes a logic AND operation with the driving signal gQ1and then becomes a logic signal u2, which has a rising edge synchronized with the driving signal gQ1and has a pulse width no more than that of the driving signal gQ1. After being amplified by the driving circuit32, the logic signal u2drives the switch Sa of the energy-recycling circuit33. The switch Sa is firstly connected to the diode Da in series and then connected to the main power diode D1in parallel for providing a uni-directional energy transmission which has a direction opposite to that of the diode D1. Therefore, the unit composed of the switch Sa, the diode Da and the diode D1is able to achieve the bi-directional energy transmission. When the voltage u3at the secondary side of the transformer T1rises to be positive, the switch Sa is turned on. If there is an excess of energy transmitted to the output terminal when the converter is zero-loaded, the voltage of the output terminal will be higher than the voltage u3. A current will flow back from the output terminal to the primary side of the transformer T1and the recycle of the energy will thus be achieved. When the load increases, the voltage u3is higher than the voltage of the output terminal and a current flows from the diode D1to the output terminal. Because of the diode Da, there is no current flowing through the switch Sa. Therefore, the energy-recycling circuit33has no influence on the operation of the main circuit with an increased load. The transistor Sa and the diode Da can also able to be adopted with the components of lower rating.

Please refer toFIG. 8, which is a circuit diagram showing another variation of the energy-recycling circuit according to the present invention. Differing from the substantial circuits shown inFIG. 5, an inducting circuit is adopted inFIG. 8. That is, an auxiliary secondary winding and a switch S constitute the energy-recycling circuit81for controlling the energy to be recycled to the primary side. The energy-recycling circuit81is suitable for the converter with variable output-filtering circuit. If a capacitor is adopted to filter directly in the output, the circuit topology is equal to the circuit shown inFIG. 6(b). Besides, the switch S could be replaced with the respective circuits shown inFIG. 5(a)˜(f), as long as the controllable energy channel from the output terminal to the input terminal can be formed. For the diode full-bridge rectifying circuit shown inFIG. 6(c), one switch element can be saved if the above circuit topology is adopted.

Please refer toFIG. 9, which is a circuit diagram showing a converter with the energy-recycling circuit ofFIG. 8according to another preferred embodiment of the present invention, and the waveform thereof is shown inFIG. 7.

In the converter90, the blocks which are the same as those inFIGS. 3,4and8are marked with the same numerical symbols. The resonant converter90is a series resonant converter, which includes a dc/dc converter10, a logic circuit31, a driving circuit32, and an energy-recycling circuit81. Especially, the input-voltage generating circuit is a half-bridge circuit composed of two switches Q1& Q2. The diode-rectifying stage circuit is a diode full-wave rectifying circuit. The energy-recycling circuit81is composed of an auxiliary secondary winding, a switch Sa and a diode Da.

The logic circuit31is completely the same with the logic circuit shown inFIG. 4. The logic circuit31finally generates a logic signal u2, which has a rising edge synchronized with the driving signal gQ1and has a pulse width no more than that of the driving signal gQ1. Certainly, the control signal could also be generated by using normal synchronous rectification control method of flyback converter. When the voltage u3at the secondary side of the transformer T1rises to be positive, the switch Sa is turned on. If there is an excess of energy transmitted to the output terminal when the converter is zero-loaded, the voltage of the output terminal will be higher than the voltage u3. A current will flow back from the output terminal to the primary side of the transformer T1, thus the recycle of the energy and the stable operation is achieved. Similarly, when the load increases, the voltage u3is higher than the voltage of the output terminal. Because of the diode Da, there is no current flowing through the switch Sa. Therefore, the energy-recycling circuit81has no influence on the operation of the main circuit with an increased load. The switch Sa and the diode Da can also able to be chosen with the components of low rating.

In conclusion, a resonant converter and a voltage stabilizing method thereof are provided in the present invention. To eliminate the drawback that the converter being operated unstably when being at light or zero load condition resulting from the parasitic parameters, an energy-recycling circuit is introduced to achieve the bi-directional transmission of energy. With this energy-recycling circuit, the excess of energy transmitted through the rectifying diode to the output terminal can be recycled to the input terminal. The stable operation of the system is thus achieved. The additional energy-recycling circuit has no influence on the operation of the main circuit. The switch and the diode can also able to be used with the components of low rating.