JP5326634B2 - Synchronous rectification type DC-DC converter - Google Patents

Description

  The present invention relates to a synchronous rectification type DC-DC converter that connects a power source and a coil at a predetermined period and supplies power to the load by connecting the coil and the load at a timing when the power source and the coil are cut off.

  FIG. 6 is a diagram showing a general configuration of a conventional synchronous rectification type DC-DC converter (or switching regulator). In such a conventional configuration, the energy supplied from the power source B is accumulated in the coil by turning on the switching element M1, and the load is obtained by turning off the switching element M1 and turning on the switching element M2. Is supplying power.

  At this time, the ratio of the output voltage Vout to the input voltage Vin can be adjusted by adjusting the duty ratio in PWM (Pulse Width Modulation) control to turn on the switching element M1.

  The switching element M2 can be replaced by a diode. However, as described above, by controlling the switching element M2 in the opposite phase to the switching element M1 (synchronous rectification), it is possible to reduce a loss during rectification. have.

  An invention relating to a switching regulator that supplies power according to such a principle is disclosed (for example, see Patent Document 1). This switching regulator is a synchronous rectification switching regulator, and is between a first power source that supplies a first potential and a second power source that supplies a second potential lower than the first potential. A first switch and a second switch arranged in series with each other; a switch control unit that controls on / off of the first and second switches according to a control signal; the first switch; And a smoothing circuit that smoothes the potential of the output node between the output switch and the switch control unit when the second switch is turned on in accordance with an instruction of the control signal. When the potential of the second switch exceeds the first predetermined potential, the second switch is turned off.

  By the way, in the synchronous rectification type DC-DC converter (or switching regulator) having such a configuration, when the current supplied to the load is small (low load), the output side to the input side (in the configuration of FIG. There is a problem that current may flow backward from the load side to the power source B side. FIG. 7 is a diagram showing temporal changes in the gate voltage and coil current of the switching elements M1 and M2 in a conventional synchronous rectification type DC-DC converter (or switching regulator) at high load and low load, respectively. . In the figure, the hatched area is the portion where the backflow current is generated.

  When a backflow current is generated, heat is generated due to the winding resistance of the coil L and the on-resistance of the switching element M2, which leads to a decrease in conversion efficiency. Therefore, it is preferable to perform some processing by detecting the backflow current. In the switching regulator described in Patent Document 1, the arrangement of the switching elements is different from the configuration shown in FIG. 6, but the backflow current is detected by measuring the voltage across the switching element M2.

JP 2000-92824 A

  However, since the on-resistance of the switching element used in such a device is small and the backflow current is not so large, the voltage across the switching element has a small fluctuation. Therefore, in order to measure the voltage across the switching element, a high-precision comparator or the like is required. However, if a high-precision comparator is provided, the circuit scale is large and the structure is complicated for canceling the offset voltage. The problem of not getting. Such a problem similarly occurs even when a shunt resistor is used instead of the comparator.

  The present invention is intended to solve such problems, and a main object of the present invention is to provide a synchronous rectification type DC-DC converter that can more easily detect a reverse current.

In order to achieve the above object, one embodiment of the present invention provides:
A first switching element for storing energy in the coil by power supplied from a power source;
A second switching element for connecting or disconnecting the coil and a load;
Control means for controlling the switching of the first and second switching elements;
A synchronous rectification type DC-DC converter comprising:
Comprising a potential comparison means for comparing the load side potential of the coil with a reference potential;
The control means includes
Controlling the first and second switching elements to be in an off state at a predetermined period,
Said first and second switching elements are have you to the period of time that the OFF state together, the comparison result of the first of the potential comparison means switching element is outputted at the timing relative to the timing which is turned on On the basis of, the backflow current in the coil flowing from the load side to the power supply side is detected,
It is a synchronous rectification type DC-DC converter.

  According to this aspect of the present invention, it is provided with potential comparison means for comparing the load side potential of the coil with the reference potential, and the control means is configured so that both the first and second switching elements are turned off in a predetermined cycle. Since the reverse current flowing from the load side to the power source side is detected based on the comparison result of the potential comparison means during the period, the reverse current can be detected more easily.

in this case,
The timing based on the timing at which the first switching element is turned on is, for example, a timing before a predetermined time before the timing at which the first switching element is turned on.

In one embodiment of the present invention,
The control unit is preferably a unit that suppresses driving of the second switching element when detecting a backflow current in the coil.

  By so doing, it is possible to prevent a decrease in conversion efficiency caused by the backflow current.

  ADVANTAGE OF THE INVENTION According to this invention, the synchronous rectification type DC-DC converter which can detect a backflow current more easily can be provided.

It is a circuit structural example of the synchronous rectification type DC-DC converter 1 which concerns on one Example of invention. It is a timing chart which shows the time change of the state of a standard clock used as a standard of a PWM signal, a PWM signal, a synchronous rectification OFF signal, and switching elements M1 and M2. 5 is a timing chart showing temporal changes in the states of switching elements M1 and M2, the current IL flowing through the coil L, and the output-side potential VX of the coil L when the synchronous rectification is off, when the load is high and when the load is low. 7 is a flowchart for executing control to turn off synchronous rectification for a predetermined period including the time when the control circuit Ctrl determines that a backflow current is generated. Reference clock, PWM signal, synchronous rectification off signal, and switching, which are the reference of PWM signal, reflecting the control to turn off synchronous rectification for a predetermined period including the time when it is determined that a reverse current has occurred It is a timing chart which shows the time change of the state of elements M1 and M2. It is a figure which shows the general structure of the conventional synchronous rectification type DC-DC converter (or switching regulator). It is a figure which shows the time change of the gate voltage of each of the switching elements M1 and M2, and the coil current at the time of high load and low load in the conventional synchronous rectification type DC-DC converter (or switching regulator).

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings.

  Hereinafter, a synchronous rectification type DC-DC converter 1 according to an embodiment of the present invention will be described. FIG. 1 is a circuit configuration example of a synchronous rectification type DC-DC converter 1 according to an embodiment of the present invention. The synchronous rectification type DC-DC converter 1 is a device for stepping up and down the electric power supplied from the power source B and supplying it to a load. The main components are switching elements M1, M2, a coil L, a comparator CP, And a control circuit Ctrl. The power source B is a secondary battery such as a lead storage battery.

  The switching elements M1 and M2 are N-channel MOS (Metal Oxide Semiconductor) transistors, respectively, and parasitic diodes are connected in parallel. The switching elements M1 and M2 are basically subjected to switching control in an opposite phase by a control circuit Ctrl to which a PWM signal is input.

  The switching element M1 is attached in the middle of the power line connecting the power supply B and the ground terminal via the coil L. Therefore, while the switching element M1 is in the on state, a current flows from the power source B through the coil L to the ground terminal, and energy is stored in the coil L.

  The switching element M2 is attached in the middle of the power line that connects the output side of the coil L (load side; the side opposite to the power source B) and the load. When the switching element M2 is turned on (the switching element M1 is turned off), the energy stored in the coil L is released and power is supplied to the load.

  The comparator CP compares the output side potential (load side potential) VX of the coil L with the reference potential, and if the difference between the output side potential VX of the coil L and the reference potential is equal to or greater than the threshold value Vth, the comparator CP controls the control signal Ctrl. Output to. The reference potential is set by connecting a battery or the like to the ground terminal. Therefore, the comparator CP according to the present embodiment has a configuration different from that for measuring the voltage across the switching element M1 or M2.

  The control circuit Ctrl may employ any configuration such as a microcomputer, IC, analog or digital circuit.

  A PWM signal is input to the control circuit Ctrl. This PWM signal, for example, alternately shows a Hi signal that indicates the timing when the switching element M1 is turned on and a Lo signal that indicates the timing when the switching element M1 is turned off, and changes in the output voltage Vout. Thus, feedback control is performed so that the ratio (duty ratio) of the Hi signal is changed.

  As the duty ratio increases, the amount of energy stored in the coil L increases, and the output voltage Vout increases. On the other hand, when the duty ratio decreases, the amount of energy stored in the coil L decreases, and the output voltage Vout decreases. Utilizing this, a constant voltage can be supplied by decreasing the duty ratio when the output voltage Vout increases and increasing the duty ratio when the output voltage Vout decreases.

  The control circuit Ctrl basically turns on the switching element M1 while the Hi signal is input, turns off the switching element M2, and turns off the switching element M1 during the period when the Lo signal is input. At the same time, the switching element M2 is turned on. In order to prevent a through current generated when the switching elements M1 and M2 are simultaneously turned on, the timing when the switching element M2 is turned on is slightly later than the timing when the switching element M1 is turned off. The timing at which the switching element M2 is turned off is controlled to be slightly earlier than the timing at which the switching element M1 is turned on. This can prevent damage to the apparatus.

  Further, the control circuit Ctrl controls the switching elements M1 and M2 to be turned off at every predetermined period. FIG. 2 is a timing chart showing temporal changes in the state of the reference clock, the PWM signal, the synchronous rectification off signal, and the switching elements M1 and M2 that are the reference of the PWM signal. In the drawing, the drive suppression control of the switching element M2 at the time of detecting the backflow current described later is not reflected.

  The synchronous rectification off signal is an instruction signal for turning off the switching element M2 regardless of the PWM signal (turning off the synchronous rectification), and is generated by a counter circuit or the like inside the control circuit Ctrl. Hereinafter, it is assumed that the synchronous rectification OFF signal is generated every four cycles (the period from when the PWM signal changes from the Hi state to the Lo state and then to the Hi state is referred to as one cycle). The four periods are merely examples, and the present invention is not limited to these. The switching element M2 is originally controlled to be turned on when the PWM signal is in the Lo state, but is maintained in the off state regardless of the PWM signal during the period in which the synchronous rectification off signal is in the Hi state.

  Then, the control circuit Ctrl detects a reverse current flowing from the load side to the power source B side based on the output of the comparator CP during the period when both the switching elements M1 and M2 are in the off state. FIG. 3 shows the states of the switching elements M1 and M2, the current IL flowing through the coil L, and the output-side potential of the coil L at the time of high load and low load (a reverse current can be generated) when the synchronous rectification is off. It is a timing chart which shows the time change of VX. As shown in the figure, when the synchronous rectification is turned off at the time of low load, first, the backflow current in such a period is prevented.

  Further, when the synchronous rectification is turned off, the fall of the output side potential VX of the coil L becomes earlier than that at the time of high load, and at a certain timing, the output of the comparator CP becomes an on signal at the time of high load. It is an off signal at low load. As shown in FIG. 3, the timing is slightly before the switching element M1 is turned on.

  Using this phenomenon, the control circuit Ctrl detects whether or not a backflow current is generated with reference to the timing when the switching element M1 is turned on. More specifically, a predetermined time before the timing when the switching element M1 is turned on (for example, about 1/10 period) is set in advance as a detection timing, and the output of the comparator CP is turned on at this detection timing. If no, no reverse current is generated, and if it is an off signal, it is determined that a reverse current is generated.

  In addition, when the control circuit Ctrl determines that a backflow current is generated, the control circuit Ctrl is in a predetermined cycle including the time (which may be synchronized with the output cycle of the synchronous rectification off signal or may be different). Controls the switching element M2 to turn off synchronous rectification. Assuming that the predetermined period is four periods, the switching element M2 is maintained in the OFF state for four periods including the period in which it is determined that the backflow current is generated.

  FIG. 4 is a flowchart for executing such control. This flow is repeatedly executed while the synchronous rectification type DC-DC converter 1 is operating.

  First, the counter value n is set to the initial value 1 and the synchronous rectification flag is set to 0 (S100). The synchronous rectification flag is a control flag set in a register or the like in the control circuit Ctrl.

  Next, it waits until the switching element M1 is turned on (rising timing of M1) (S102).

  When the timing at which the switching element M1 is turned on is reached, it is determined whether or not the counter value n is 1 (S104).

  When the counter value is 1, the synchronous rectification of the cycle is turned off (S106). That is, the period maintains the switching element M2 in the off state. Here, the aforementioned synchronous rectification off signal is generated by a counter circuit or the like.

  Then, at the detection timing described above, it is determined whether or not the output of the comparator CP is an off signal (S108).

  When the output of the comparator CP is an OFF signal at the detection timing, the synchronous rectification flag is set to a value 1 (S110), and the counter value is increased by a value 1 (S112). On the other hand, if the output of the comparator CP is an ON signal at the detection timing, the counter value is simply increased by 1 (S112).

  If the process of S104-S112 is performed, it will return to S102. Next, when the timing at which the switching element M1 is turned on is reached, it is determined in S104 that the counter value is not 1;

  In S114, it is determined whether the synchronous rectification off flag is a value of 1. When it is determined that the synchronous rectification off flag has the value 1, the synchronous rectification in the period is turned off and the counter value is incremented by 1 (S116). That is, the period maintains the switching element M2 in the off state.

  On the other hand, when it is determined that the synchronous rectification off flag is not the value 1, the synchronous rectification of the period is turned on and the counter value is increased by the value 1 (S118). That is, the switching element M2 and the switching element M1 are included in the period. Turn on with opposite phase.

  When the process of S116 or S118 is executed, it is determined whether or not the counter value is a value 4 (S120). If it is determined that the counter value is not 4, the process returns to S102.

  If it is determined that the counter value is 4, one routine of this flow is terminated. Thereafter, the processing is executed again in order from S100.

  Accordingly, when the four cycles from the first cycle in which the synchronous rectification is always turned off are completed, the synchronous rectification flag is set to the value zero (S100). In the next four cycles, synchronous rectification is turned on if no reverse current is detected in the first cycle.

  FIG. 5 is a timing chart showing a time change in the state of the reference clock, the PWM signal, the synchronous rectification OFF signal, and the switching elements M1 and M2 that are the reference of the PWM signal, reflecting such control. As shown in the figure, when it is determined that the backflow current is generated in the period when the synchronous rectification off signal is output, the synchronous rectification is turned off for the subsequent three periods, and the driving of the switching element M2 is suppressed. As a result, it is possible to prevent a decrease in conversion efficiency caused by the backflow current. Then, the output of the synchronous rectification type DC-DC converter 1 is changed to a low output suitable for a low load.

  On the other hand, when it is determined that no reverse current is generated in the period in which the synchronous rectification OFF signal is output, the synchronous rectification is turned on for the subsequent three periods, and the switching element M2 is driven in the opposite phase to the switching element M1. Is done.

  Further, in the synchronous rectification type DC-DC converter 1 of the present embodiment, the reverse current is not detected by measuring the voltage across the switching element M1 or M2, but the output side potential VX of the coil L is used as the reference potential. Since the reverse current is detected by comparison, the necessity for making the comparator CP highly accurate is small. This is because the difference between the output-side potential VX of the coil L and the reference potential varies more than the voltage across the switching element M1 or M2.

  Accordingly, the backflow current can be detected more easily, and the necessity of making the comparator CP highly accurate is small, so that an increase in circuit scale and weight can be suppressed.

  And the change of the output side electric potential VX of the coil L by a backflow current appears notably when synchronous rectification is OFF. Therefore, such an effect is realized by a configuration in which both the switching elements M1 and M2 are periodically turned off as in the synchronous rectification type DC-DC converter 1 of the present embodiment.

  According to the synchronous rectification type DC-DC converter 1 of the present embodiment, the backflow current can be detected more easily. And when a backflow current is detected, the drive of the switching element M2 can be suppressed, and the fall of the conversion efficiency which arises by a backflow current can be prevented.

  The best mode for carrying out the present invention has been described above with reference to the embodiments. However, the present invention is not limited to these embodiments, and various modifications can be made without departing from the scope of the present invention. And substitutions can be added.

  The present invention can be used in the automobile manufacturing industry, the automobile parts manufacturing industry, and the like.

1 Synchronous rectification type DC-DC converter
B Power supply CP Comparator Ctrl Control circuit
L Coil M1, M2 Switching element

Claims (3)

A first switching element for storing energy in the coil by power supplied from a power source;
A second switching element for connecting or disconnecting the coil and a load;
Control means for controlling the switching of the first and second switching elements;
A synchronous rectification type DC-DC converter comprising:
Comprising a potential comparison means for comparing the load side potential of the coil with a reference potential;
The control means includes
Controlling the first and second switching elements to be in an off state at a predetermined period,
Said first and second switching elements are have you to the period of time that the OFF state together, the comparison result of the first of the potential comparison means switching element is outputted at the timing relative to the timing which is turned on On the basis of, the backflow current in the coil flowing from the load side to the power supply side is detected,
Synchronous rectification type DC-DC converter. The timing based on the timing when the first switching element is turned on is a timing before a predetermined time before the timing when the first switching element is turned on.
The synchronous rectification type DC-DC converter according to claim 1 . The control means is means for suppressing driving of the second switching element when detecting a backflow current in the coil.
The synchronous rectification type DC-DC converter according to claim 1 or 2 .

Images