JP2009077554A - Dc-dc converter and its reverse-current suppression method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DC-DC converter that suppresses output fluctuations due to switching operation of switching elements while suppressing a reverse current due to a relationship with a load current, and also to provide its reverse-current suppression method. <P>SOLUTION: Driving switching elements for synchronous rectification is set as an operation such that pulse signals (PWM_A, PWM_B) for switching operation and a load-current detection value (CURR_OUTPUT) are inputted and a duty ratio is increased/decreased according to the load-current detection value CURR_OUTPUT. An ON-duty is increased in accordance with an increase in load current regarding drive signals (synchronous rectification A, synchronous rectification B) for synchronous rectification so as to make adjustment that finally achieves the maximum ON-duty. The optimal ON/OFF switching is made according to a load current. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a DC-DC converter and a reverse current suppressing method thereof, and more specifically, includes an inductance element, a switching element for rectification and / or reflux, and synchronizes the rectification operation and / or the reflux operation. The present invention relates to improvement of switching control for synchronous rectification of a synchronous rectification type DC-DC converter that performs switching control.

  A DC-DC converter adopts a synchronous rectification type configuration in order to increase a low voltage and a large current. As seen in, for example, Patent Documents 1 and 2 and the like, there is a synchronous rectification type DC-DC converter that is replaced with a switching element instead of a diode and turned on / off at an appropriate timing. With this configuration, since a MOSFET or the like is used as the switching element, there is an advantage that a low on-resistance can be obtained and high efficiency can be obtained.

  The DC-DC converter will be described with reference to FIG. 1 according to the present invention. The DC-DC converter includes an inductance element L1 and switching elements Q5 and Q6 for rectification and / or recirculation, and a DC input voltage is switched by a switching operation. And the switching elements Q5 and Q6 are supplied with alternate driving signals (synchronous rectification A, synchronous rectification B) to perform the rectification operation and / or the reflux operation. ing.

In the on / off switching of the switching elements Q5 and Q6, there is a problem that a reverse current flows from the output side to the switching element side due to the relationship with the load current. Therefore, in the above-mentioned Patent Documents 1 and 2 and the like, there is a proposal of a technique in which a reverse current is suppressed by detecting a situation in which a reverse current flows and appropriately restricting on / off switching control.
JP 2004-201436 A JP 2007-1224749 A

  However, when the switching elements Q5 and Q6 are controlled to be switched on / off, the change in the output voltage at the time of switching on / off is essentially a problem. That is, when switching from off to on in MOSFETs Q5 and Q6, the forward voltage Vf due to the parasitic diode is switched to the voltage drop at the on-resistance, so the voltage drop at the switching element instantaneously decreases, and as a result, The output Vout is increased. Conversely, in switching from on to off in MOSFETs Q5 and Q6, the voltage drop at the on-resistance switches from the voltage drop at the on-resistance to the forward voltage Vf due to the parasitic diode, so the voltage drop at the switching element rises momentarily. This causes a fluctuation that the output Vout drops.

  The present invention solves the above-described problems, and an object of the present invention is to provide a DC-DC converter capable of suppressing a reverse current resulting from a relationship with a load current and capable of suppressing an output fluctuation accompanying a switching operation, and the reverse current thereof. It is to provide a suppression method.

  In order to achieve the above-described object, a DC-DC converter according to the present invention includes (1) an inductance element and a switching element for rectification and / or reflux, and converts a DC input voltage into high-frequency power by a switching operation. In the synchronous rectification type DC-DC converter in which the rectifying operation and / or the recirculating operation are performed by supplying synchronous alternating drive signals to the inductance elements, The detected value is input and the duty ratio is increased / decreased according to the detected load current, and the on-duty is increased as the load current increases for the synchronous rectification drive signal. A configuration is provided with duty ratio adjusting means for adjusting the duty.

  (2) A transformer may be arranged to electrically insulate the input side and the output side.

  (3) A MOSFET can be used as the switching element.

  The reverse current suppression method according to the present invention includes an inductance element and a switching element for rectification and / or return, converts a DC input voltage into high-frequency power by switching operation, and supplies the high-frequency power to the inductance element. In a synchronous rectification type DC-DC converter that applies alternating drive signals in synchronization with each other to perform rectification operation and / or return operation, a pulse signal of switching operation and a detection value of load current are input, and the load current detection value The duty ratio is increased / decreased in accordance with the method, and the on-duty of the synchronous rectification drive signal is increased in accordance with the increase of the load current, and finally the maximum on-duty is adjusted.

  In the present invention, whether the load current increases or decreases, the on / off switching is appropriately performed by the control operation for increasing or decreasing the duty ratio. In the switching operation of the switching element, control is performed to increase or decrease the duty ratio according to the detected value of the load current, and the on-duty is increased as the load current increases. Therefore, optimal on / off switching corresponding to the load current can be performed.

  In the DC-DC converter and the reverse current suppressing method thereof according to the present invention, in any situation where the load current increases or decreases, the on / off switching is appropriately performed by the control operation for increasing or decreasing the duty ratio. The reverse current can be suppressed without causing a situation in which the reverse current flows.

  In the switching operation of the switching element, control is performed to increase or decrease the duty ratio according to the detected value of the load current, the on-duty is increased as the load current increases, and finally the maximum on-duty is adjusted Therefore, optimal on / off switching corresponding to the load current can be performed. Therefore, the output fluctuation accompanying the switching operation can be suppressed.

  FIG. 1 shows a preferred embodiment of the present invention. In the present embodiment, the DC-DC converter includes a main transformer T1, four switching elements Q1 to Q4 are disposed on the primary side, and two switching elements Q5 and Q6 are disposed on the secondary side, so-called full bridge. The structure is a synchronous rectification system in which the primary side and the secondary side are insulated by a mold.

  The switching elements Q1 to Q4 use MOSFETs, are bridge-connected on the primary side of the main transformer T1, and have two gates crossed and connected to the primary-side control circuit 1 to connect two control signals ( The switching operation is alternately performed by PWM_A and PWM_B). A capacitor C1 is connected to the input side of the primary side of the main transformer T1 to stabilize the voltage on the primary side.

  Switching elements Q5 and Q6 use MOSFETs and are connected to the secondary side of the main transformer T1, and each gate is connected to the synchronous rectification control circuit 2 and is switched alternately by two control signals (synchronous rectification A and synchronous rectification B). It is supposed to work. These two MOSFETs Q5 and Q6 perform rectification operations alternately.

  On the secondary side of the main transformer T1, a smoothing circuit is formed by connecting a choke coil L1 and a capacitor C2 in series and in parallel to the center tap side. On the output side of the smoothing circuit, a resistor Rs is connected in series to the ground side of the output line, and both ends of the resistor Rs are connected to the current detection circuit 3 to detect the load current. The current detection circuit 3 is connected to the synchronous rectification control circuit 2 and outputs a detection value (CURR_OUTPUT) of the load current.

  The primary side control circuit 1 is connected to the synchronous rectification control circuit 2 via the sub-transformer T2, and the primary side control signal is guided to the secondary side in an electrically insulated state. Diodes D1 and D2 are connected to the secondary side of the sub-transformer T2 from the output side of the smoothing circuit, and clamping that applies a zero potential to the secondary side is performed.

  The primary side control circuit 1 performs pulse width control to increase or decrease the pulse widths of the two control signals (PWM_A, PWM_B) so that the output Vout of the secondary side smoothing circuit becomes a target value.

  As shown in FIG. 2, the synchronous rectification control circuit 2 includes a triangular wave generation circuit 21, a comparator 22, an OR circuit 23, and a duty ratio adjustment circuit 24, and a control signal (synchronous rectification A, For the synchronous rectification B), the duty ratio is adjusted to optimize the switching operation.

  Two control signals (PWM_A, PWM_B) guided to the secondary side via the sub-transformer T2 are sent to the triangular wave generation circuit 21, and the triangular wave generation circuit 21 generates a triangular wave corresponding to the time difference between the two signals. This triangular wave is taken into IN + of the comparator 22, and the load current detection value (CURR_OUTPUT) is taken into IN− of the comparator 22.

  The two control signals PWM_A and PWM_B are also sent to the OR circuit 23 and the output of the OR circuit 23 is sent to the duty ratio adjustment circuit 24. The output of the comparator 22 is sent to the duty ratio adjustment circuit 24, and two control signals (synchronous rectification A and synchronous rectification B) synchronized with the signal from the OR circuit 23 as a trigger are output.

  FIG. 3 is a time chart for explaining the operation of the synchronous rectification control circuit, and shows a situation where the load current changes from zero to a light load. The synchronous rectification control circuit 2 performs the adjustment operation in three stages according to the voltage comparison by the comparator 22, that is, the detection value CURR_OUTPUT of the load current, and the switching operation is optimized.

  First, (1) in the voltage comparison of the comparator 22, in a situation where the detected value CURR_OUTPUT of the load current is lower than the DC offset of the triangular wave signal, the comparator 22 outputs an “H” level, and the control signal (synchronous rectification) of the switching operation Both A and synchronous rectification B) are controlled to output an “L” level. Therefore, the MOSFETs Q5 and Q6 are driven off, and the parasitic diode performs a rectifying operation.

(2) In the voltage comparison of the comparator 22, when the load current detection value CURR_OUTPUT is not less than the DC offset of the triangular wave signal and not more than the peak value of the triangular wave signal, the comparator 22 outputs a pulse in synchronization with the coincidence point of the voltage comparison. The synchronous rectification A and the synchronous rectification B are pulse outputs synchronized with the control signals PWM_A and PWM_B. This pulse output is a pulse width control operation in which the pulse width is increased on the rear side as the load current increases. Become. Accordingly, the MOSFETs Q5 and Q6 are driven to be turned on / off in synchronization with the primary side control signals PWM_A and PWM_B, and the on-time is increased in accordance with the increase of the load current, so that the load increases. An operation in a direction to compensate for the voltage drop is performed.

(3) In the voltage comparison of the comparator 22, in a situation where the detected value CURR_OUTPUT of the load current is higher than the peak value of the triangular wave signal, the comparator 22 outputs “L” level, and the synchronous rectification A and the synchronous rectification B are control signals. Although the pulse output is synchronized with PWM_A and PWM_B, this pulse output is an operation of pulse width control with the maximum on-duty in which the pulse width corresponding to the dead time is increased before and after. Therefore, the MOSFETs Q5 and Q6 are driven to be turned on / off in synchronization with the primary side control signals PWM_A and PWM_B. At this time, the driving is performed with the maximum on-duty. An operation in a direction to compensate for the voltage drop is performed.

  In a situation where the load current changes from light load to zero, the control operation is the reverse of the example shown in FIG. That is, the synchronous rectification A and the synchronous rectification B are first performed from (3) the pulse width control operation of the maximum on-duty to (2) the operation of synchronous rectification in which the on-time is increased in accordance with the increase of the load current. (1) The MOSFETs Q5 and Q6 are rectified by the parasitic diodes in the drive-off state.

  As described above, in this embodiment, on / off switching is appropriately performed by the control operation for increasing / decreasing the duty ratio regardless of whether the load current increases or decreases. For this reason, the reverse current can be suppressed without causing a situation in which the reverse current flows.

  In the switching operation of MOSFETs Q5 and Q6, control is performed to increase or decrease the duty ratio according to the detected value of the load current, the on-duty is increased in accordance with the increase of the load current, and finally the maximum on-duty is set. Since adjustment is performed, optimal on / off switching corresponding to the load current can be performed. Therefore, output fluctuations associated with the switching operation can be suppressed.

1 is a circuit diagram showing a preferred embodiment of a DC-DC converter according to the present invention. It is a circuit diagram explaining the structure of a synchronous rectification control circuit. It is a time chart explaining operation | movement of a synchronous rectification control circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Primary side control circuit 2 Synchronous rectification control circuit 3 Current detection circuit 21 Triangular wave generation circuit 22 Comparator 23 OR circuit 24 Duty ratio adjustment circuit Rs Load current detection resistor C1 Primary side stabilization capacitor C2 Secondary side smoothing Capacitors Q1, Q2, Q3, Q4 for primary side full bridge MOSFET
Q5, Q6 MOSFET for synchronous rectification (switching element)
T1 main transformer T2 control signal primary side secondary side separation sub transformer L1 smoothing choke coil (inductance element)
D1, D2 Secondary zero-potential clamping diodes

Claims (4)

Including an inductance element and a switching element for rectification and / or reflux,
Synchronous rectification DC that converts a DC input voltage into high-frequency power by a switching operation and supplies the high-frequency power to the inductance elements, and applies a synchronous alternating drive signal to the switching elements to perform a rectifying operation and / or a reflux operation. -In a DC converter,
The switching operation pulse signal and the load current detection value are input, and the duty ratio is increased or decreased according to the load current detection value. The synchronous rectification drive signal is turned on / duty as the load current increases. A DC-DC converter comprising a duty ratio adjusting means for adjusting the output to a maximum on-duty.   The DC-DC converter according to claim 1, wherein a transformer is disposed to electrically insulate the input side and the output side.   The DC-DC converter according to claim 1, wherein the switching element is a MOSFET. An inductance element and a switching element for rectification and / or reflux are provided, and a DC input voltage is converted into high frequency power by a switching operation and supplied to the inductance element, and alternate drive signals synchronized with the switching element In a synchronous rectification type DC-DC converter that performs rectification operation and / or reflux operation by providing
The switching operation pulse signal and the load current detection value are input, and the duty ratio is increased or decreased according to the load current detection value. The synchronous rectification drive signal is turned on / duty as the load current increases. The reverse current suppression method for a DC-DC converter is characterized in that adjustment is performed so that the maximum on-duty is finally obtained.

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