JP2009065741A - Dc-dc converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DC-DC converter which can bring the recovery loss to substantially zero. <P>SOLUTION: In a DC-DC converter 100 satisfying a basic equation of Vout=D×(1-D)×Vin, where Vin is the input voltage, Vout is the output voltage and D is the time ratio, zero voltage switching (ZVS) is achieved while suppressing the surge voltage of a switch element by connecting an active clamp circuit 20 consisting of a capacitor 201 and a switch element 202, and a capacitor 21 for resonance in parallel with the switch element 18 on the secondary of a transformer 15 having the function of an inductor out of the switch elements 17 and 18, i.e. the rectifier cells, on the secondary of the transformer 15 thus bringing the recovery loss to substantially zero. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a DC-DC converter having a large step-down ratio for converting a commercial power source into DC (direct current) and converting it to a low voltage, and a switch control method thereof.

  When the input voltage is Vin, the output voltage is Vout, and the duty ratio is D, in the DC-DC converter in which the basic expression Vout = D × (1−D) × Vin is established, the turn-off flowing to the secondary side of the transformer As for the slope of the current, a relatively gentle current waveform is obtained due to the leakage inductance component. For this reason, the recovery current of the switch element at the time of turn-off is smaller than that of a normal converter, and the recovery loss is also small (see, for example, Patent Document 1).

However, when a switching element such as an FET (field effect transistor) is used for the purpose of reducing conduction loss, the recovery time of the body diode of the FET is long, causing problems such as an increase in recovery loss and an increase in surge voltage. In particular, the voltage applied to the switch element connected to the secondary side of the transformer having the inductance function is higher than that of the normal converter system, and thus the problem is serious. As means for suppressing the surge voltage, there are a CR snubber circuit, a CRD snubber circuit, etc., but they are generated as a recovery loss and the efficiency is deteriorated. Further, even if the surge voltage of the switch element used in the converter is suppressed, a recovery loss of the diode occurs, so that a reduction in efficiency is inevitable.
JP 2007-074830 A

  As described above, in the conventional DC-DC converter, the recovery loss of the switch element and the surge voltage have a great influence on the efficiency.

  In order to solve the above-described problems, the present invention realizes zero voltage switching (ZVS) while suppressing a surge voltage of a switch element, and a DC-DC converter capable of almost zero recovery loss and a switch control method thereof The purpose is to provide.

  In order to solve the above-described problem, a DC-DC converter according to the present invention includes first and second switch elements that are serially connected between a positive electrode end and a negative electrode end of an input DC power supply, and the first and second switch elements. A first capacitor connected to a midpoint of a switch element; a first transformer having one end of a primary winding connected to the first capacitor and having an inductance function; and One end of the primary side winding is connected to the other end of the primary side winding, the other end of the primary side winding is connected to the negative end of the input DC power source, and the secondary side winding is the first side winding. A second transformer that is serially connected to the secondary winding of the first transformer, and serially connected between both ends of the serially connected secondary windings of the first and second transformers; The third switch connected to the secondary winding of the transformer A fourth switch element connected to the secondary winding of the element and the second transformer, a midpoint of the serially connected secondary winding of each of the first and second transformers, and the third switching element. And a second capacitor that is connected between the middle point of the fourth switch element and whose both ends serve as output terminals, and a third capacitor and a fifth switch element between both ends of the third switch element. And an active clamp circuit connected in serial.

  In addition, the switch control method is to operate the first and second switch elements by always turning on one of the switches except for the dead time period for preventing conduction and realizing zero voltage switching. The third and fourth switch elements are turned on during the conduction period to operate, and the fifth switch element of the active clamp circuit is connected to the secondary winding of the first transformer. The fourth switch element is turned on and operated during a period when the recovery current is finished.

  That is, in the DC-DC converter having the above configuration, when the input voltage is Vin, the output voltage is Vout, and the time ratio is D, a DC-DC that satisfies the basic equation Vout = D × (1−D) × Vin holds. In the converter, third and fourth switch elements as rectifier elements are provided at both ends of the serially connected secondary windings of the first and second transformers, and in parallel with the fourth switch element on the first transformer side. By connecting an active clamp circuit composed of a capacitor and a fifth switch element to the capacitor, zero voltage switching (ZVS) is realized while suppressing the surge voltage of the switch element, thereby making the recovery loss almost zero.

  ADVANTAGE OF THE INVENTION According to this invention, zero voltage switching (ZVS) is implement | achieved, suppressing a surge voltage, The DC-DC converter which can make recovery loss substantially zero, and its switch control method can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a circuit diagram showing an embodiment of a DC-DC converter according to the present invention. The DC-DC converter 100 shown in FIG. 1 is supplied with a DC voltage from an input voltage source (Vin) 10. The positive side of the input voltage source 10 is connected to one end of a first switch element (for example, FET) 11, and the other end of the first switch element 11 is one of the second switch elements (for example, FET) 12. The other end of the second switch element 12 is connected to the negative side of the input voltage source 10.

  The midpoint of connection between the first switch element 11 and the second switch element 12 that are serially connected as described above is the primary side winding of the first capacitor 13, the inductor (coil) 14, and the first transformer 15. The wire 151 is connected to the other end of the second switch element 12 via the primary winding 161 of the second transformer 16. Note that the inductor 14 is included in the function of the primary winding of the first transformer 15.

  The secondary winding 152 facing the primary winding 151 of the first transformer 15 is serially connected to the secondary winding 162 of the second transformer 16. A fourth switch connected to the secondary winding 152 of the first transformer 15 is provided between both ends of the serially connected secondary windings 152 and 162 of the first and second transformers 15 and 16. The element (for example, FET) 18 and the third switch element (for example, FET) 17 connected to the secondary winding 162 of the second transformer 16 are serially connected.

  A second capacitor 19 is connected between the connection midpoint of the secondary windings 152 and 162 and the connection midpoint of the third and fourth switch elements 17 and 18, and both ends thereof are output terminals (DC Output voltage Vout). An active clamp circuit 20 formed by serially connecting a third capacitor 201 and a fifth switch element (eg, FET) 202 is connected between both ends of the fourth switch element 18. In addition, a resonance fourth capacitor 21 is connected between both ends of the fourth switch element 18 together with the active clamp circuit 20.

  Here, the control input terminals of the first to fifth switch elements 11, 12, 17, 18, 202 are connected to the control circuit 22, and the control circuit 22 respectively determines predetermined values in accordance with changes in the output voltage Vout. ON / OFF control is performed at the timing. ON / OFF of the first switch element 11 is controlled by the drive signal DrvA, ON / OFF of the second switch element 12 is controlled by the drive signal DrvB, and ON / OFF of the third switch element 17 is controlled by the drive signal DrvC. The fourth switch element 18 is turned on / off by the drive signal DrvD, and the fifth switch element 202 is turned on / off by the drive signal DrvE.

  The operation of the above configuration will be described below with reference to FIG. For the sake of explanation, the voltage drop of the switch element, transformer, inductor, and recovery current other than the switch element (202) are ignored.

  The drive signals DrvA and DrvB are turned off as shown in FIGS. 2A and 2B in order to prevent simultaneous conduction of the first and second switch elements 11 and 12 and to realize zero voltage switching. Except for the period (dead time period), one of the switch elements 11 and 12 is controlled to be turned on. As shown in FIGS. 2C and 2D, the drive signal DrvC and the drive signal DrvD are turned on during the period in which the forward current flows through the third switch element 17 and the fourth switch element 18, respectively. Let me control. As shown in FIG. 2E, the drive signal DrvE is controlled so that the drive signal DrvD is turned on while the recovery current flows through the fifth switch element 202.

  Under the switch control by the drive signal, the current Ip flowing through the capacitor 13, the inductor 14, and the primary windings 151 and 161 is shown in FIG. 2F, and the current Irec1 flowing through the third switch element 17 is shown in FIG. ), The current Irec2 flowing through the fourth switch element 18 is shown in FIG. 2 (h), the current Is1 flowing through the capacitor 21 is shown in FIG. 2 (i), and the current Is2 flowing through the active clamp circuit 20 is shown in FIG. The voltage Vrec2 relating to the fourth switch element 18 shown in j) is shown in FIG.

At this time, the duty ratio D is defined by equation (1).
D = Ton / Ts (1)
However, Ton is the ON time of the drive signal DrvA, and Ts is a switching period.

Further, when the number of turns of the primary side winding 151 of the first transformer 15 is Nl1, and the number of turns of the secondary side winding 152 of the second transformer 16 is Nl2, the turn ratio nl1 is expressed by the following equation (2). Define
nl1 = Nl1 / Nl2 (3)
When the number of turns of the primary side winding 161 of the second transformer 16 is Nt1, and the number of turns of the secondary side winding 162 is Nt2, the turn ratio nt1 is defined as in equation (4).
nt1 = Nt1 / Nt2 (4)
In order to minimize the output ripple voltage, the turn ratios nl1 and nt1 of the first and second transformers 15 and 16 may be made equal, and newly defined as n as shown in equation (5).

n = nl1 = nt1 (5)
At this time, the output voltage is expressed by equation (6).
Vout = D × (1-D) × Vin / n (6)
Therefore, when setting to an arbitrary output voltage, it becomes possible by changing the duty ratio, that is, the ON time of the drive signal DrvA. The drive signal DrvB is a signal that is turned on other than the period when the drive signal DrvA is on and the dead time for zero voltage switching of the primary side switch elements 11 and 12. The drive signal DrvC is a signal that is turned on in a period in which the third switch element 17 flows in the forward direction, that is, a period in which both conduction periods with the fourth switch element 18 are added to the on time of the drive signal DrvA. . The drive signal DrvD is a signal that is turned on during a period in which the fourth switch element 18 flows in the forward direction, that is, a period in which both conduction periods with the third switch element 17 are added to the on time of the drive signal DrvB. . The drive signal DrvE is a signal that is turned on after the fourth switch element 18 is turned off and the current flows to the negative side due to the recovery time, and is turned on until the drive signal DrvA is turned off.

When operated by the above signal, the capacitor 201 of the active clamp circuit 20 has a voltage obtained by dividing the difference between the input voltage Vin and the voltage Vin × D applied to the capacitor 13 by the number of turns n.
(Vin−Vin × D) / n = Vin (1−D) / n (7)
Is applied. This voltage is clamped when the capacitor capacity is sufficiently larger than the recovery current of the fourth switch element 18 × the recovery time.

  When the drive signal DrvB is turned off, the current Irec2 (FIG. 2 (h)) flowing through the fourth switch element 18 begins to gradually decrease. Even when the switch element 18 is turned off, the current is gradually increased by the rectifying action of the switch element 18. When the current approaches 0 and further begins to flow in the negative direction due to the recovery current and reaches the peak point depending on the recovery time and the current gradient inherent in the switch element 18, the current Is1 in the negative direction (FIG. 2 (i)) is used for resonance. It starts to flow from the capacitor 21. When the resonance peak current of the inductor 14 and the resonance capacitor 21 is reached, the current Is2 (FIG. 2 (j)) starts to flow from the active clamp circuit 20. During this period, the voltage Vrec2 applied to the fourth switch element 18 gradually reaches the voltage value of the expression (7) from zero (so-called zero voltage switching (ZVS)), so that the recovery current and the switch element 18 are applied. Switching is possible with almost no recovery loss caused by the voltage Vrec2. Since the same phenomenon occurs during the switching period of the switch element 18 from OFF to ON, almost no switching loss occurs.

  As described above, according to the configuration of the above embodiment, when the input voltage is Vin, the output voltage is Vout, and the time ratio is D, the basic equation Vout = D × (1−D) × Vin is obtained. In the formed DC-DC converter 100, the rectifier elements on the secondary side of the transformer 15 are the switch elements 17 and 18, and the capacitor 201 and the switch element 202 are connected in parallel with the switch element 18 on the secondary side of the transformer 15 having the function of an inductor. By connecting the active clamp circuit 20 and the resonance capacitor 21 to each other, zero voltage switching (ZVS) can be realized while suppressing the surge voltage of the switching element. Voltage suppression and efficiency can be improved.

  In the embodiment of FIG. 1, the fifth switch element 202 is connected to the negative voltage (−V0) side of the output end, but the same is true even if it is connected to the positive voltage (+ V0) side of the output end. An effect is obtained. Further, the fifth switch element 202 can obtain the same effect even if it is a P-channel FET. In the active clamp circuit 20, the same effect can be obtained even if the capacitor 201 and the switch element 202 are replaced. The control of DrvC, DrvD, and DrvE may be a fixed value if the efficiency is not particularly problematic. Further, the fourth capacitor 21 used for resonance may not be particularly problematic in efficiency and the like.

  In addition, the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

1 is a circuit diagram showing an embodiment of a DC-DC converter according to the present invention. The wave form diagram which shows the output state of each part in order to demonstrate the operation | movement principle of the DC-DC converter shown in FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100 ... DC-DC converter, 11 ... 1st switch element, 12 ... 2nd switch element, 13 ... 1st capacitor | condenser, 14 ... Inductor, 15 ... 1st transformer, 151 ... Primary side winding, 152 ... Secondary winding, 16 ... Second transformer, 161 ... Primary winding, 162 ... Secondary winding, 17 ... Third switch element, 18 ... Fourth switch element, 19 ... Second 20 ... active clamp circuit, 201 ... third capacitor, 202 ... fifth switch element, 21 ... fourth capacitor, 22 ... control circuit, DrvA, DrvB, DrvC, DrvD, DrvE.

Claims (5)

First and second switch elements serially connected between a positive terminal and a negative terminal of an input DC power supply;
A first capacitor connected to a midpoint of the first and second switch elements;
A first transformer having one end of a primary winding connected to the first capacitor and having an inductance function;
One end of the primary side winding is connected to the other end of the primary side winding of the first transformer, and the other end of the primary side winding is connected to the negative end of the input DC power source. A second transformer whose side winding is serially connected to the secondary side winding of the first transformer;
A third switch element serially connected between both ends of a serially connected secondary winding of each of the first and second transformers, and connected to a secondary winding of the second transformer; A fourth switch element connected to the secondary winding of one transformer;
Each of the first and second transformers is connected between the midpoint of the serially connected secondary windings and the midpoint of the third and fourth switch elements, and both ends thereof are output ends. Two capacitors,
An active clamp circuit formed by serially connecting a third capacitor and a fifth switch element between both ends of the fourth switch element;
The DC-DC converter characterized by comprising.   The DC-DC converter according to claim 1, further comprising a fourth capacitor for resonance connected between both ends of the fourth switch element together with the active clamp circuit.   When the power supply voltage of the input DC power supply is Vin, the output voltage generated at both ends of the second capacitor is Vout, and the time ratio is D, the basic expression Vout = D × (1−D) × Vin holds. The DC-DC converter according to claim 1, wherein:   For the first and second switch elements, one of the switches is always turned on and operated except for a dead time period for preventing conduction and realizing zero voltage switching. The switch element is turned on during the conduction period to operate, and for the fifth switch element of the active clamp circuit, the fourth switch element connected to the secondary winding of the first transformer is turned off. 2. The DC-DC converter according to claim 1, further comprising a control unit that is turned on and operated during a period when the recovery current is completed. First and second switch elements serially connected between a positive terminal and a negative terminal of an input DC power supply;
A first capacitor connected to a midpoint of the first and second switch elements;
A first transformer having one end of a primary winding connected to the first capacitor and having an inductance function;
One end of the primary side winding is connected to the other end of the primary side winding of the first transformer, and the other end of the primary side winding is connected to the negative end of the input DC power source. A second transformer whose side winding is serially connected to the secondary side winding of the first transformer;
A third switch element serially connected between both ends of a serially connected secondary winding of each of the first and second transformers, and connected to a secondary winding of the second transformer; A fourth switch element connected to the secondary winding of one transformer;
Each of the first and second transformers is connected between the midpoint of the serially connected secondary windings and the midpoint of the third and fourth switch elements, and both ends thereof are output ends. Two capacitors,
An active clamp circuit formed by serially connecting a third capacitor and a fifth switch element between both ends of the fourth switch element;
Is used for a DC-DC converter characterized by comprising:
For the first and second switch elements, one of the switches must be turned on and operated except for a dead time period for preventing conduction and realizing zero voltage switching,
The third and fourth switch elements are turned on during the conduction period to operate.
The fifth switch element of the active clamp circuit is turned on while the fourth switch element connected to the secondary winding of the first transformer is turned off and the recovery current is finished. A switch control method for a DC-DC converter, characterized by comprising:

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