JPH07222438A - Synchronous-rectification dc-dc converter - Google Patents

Abstract

PURPOSE:To obtain the synchronous-rectification DC-DC converter in which, when a load is short-circuited in a heavy load mode, a current for a light load rises immediately with the restoration of the load by a method wherein, when a synchronous rectifying element is set to continuity in the heavy load mode, a main switching element for a light load is turned off and a light-load control circuit having an overcurrent drooping characteristic is operated irrespective of the heavy load mode or a light load mode. CONSTITUTION:In a heavy load mode, a light-load control circuit 122 is utilized, a FET 5 is operated, a heavy-load control circuit 121 controls FETs 3, 4 so as to be turned on and off in such a way that a load voltage detected by resistances 9, 10 is kept at a prescribed value. At this time, the FET 5 is turned off during the ON- period of the FET 4 by an interlock circuit 123. When a load 11 is short-circuited in the heavy load mode, an overcurrent protective circuit 124 cuts off a power supply for the heavy-load control circuit 121 due to the voltage rise of a load-current detection resistance 13, and it turns off the FETs 3, 4. At this time, an overcurrent drooping circuit inside the light-load control circuit 122 functions, and a load current is limited to a prescribed current. As soon as the load 11 is restored, the light-load control circuit 122 is returned to an operation in a normal state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter called a DC chopper, which produces a constant voltage DC power supply from a DC power supply whose voltage fluctuates.
Two DC choppers with a common output smoothing reactor and capacitor, namely a light load DC chopper with a commutation diode and a main switch element, and a commutation switch element (synchronous rectification element) in parallel with the commutation diode. And a main switch element different from the above main switch element, and the synchronous rectification is provided with a DC hopper for heavy load of a synchronous rectification, which turns on / off the two new switch elements in synchronization with each other. The present invention relates to a DC-DC converter.

In the following drawings, the same reference numerals indicate the same or corresponding parts.

[0003]

2. Description of the Related Art FIG. 2 shows a conventional synchronous rectification DC-D of this type.
The structural example of a C converter (DC chopper) is shown. In the figure, 1 is a DC power supply whose voltage fluctuates, 2 is an electrolytic capacitor that suppresses the pulsation of the power supply voltage, 3 is an N-channel FET as a main switching element in the heavy load mode, 4 is a commutation diode in the heavy load mode. N-channel FET as an alternative switch element (referred to as a synchronous rectifying element), 5 is a P-channel FET as a main switch element in the light load mode, 6
Is a commutation diode (also called a flywheel diode) that operates in the light load mode, 7 is a reactor for smoothing the output current, 8 is an electrolytic capacitor for smoothing the output voltage, and 9, 1
Reference numeral 0 is a voltage dividing resistor for detecting the output voltage, and 11 is a load. Further, 12L detects the output voltage through the resistors 9 and 10, and the FE is fixed at a relatively long fixed period so as to keep the output voltage constant.
A light load control unit for turning on / off T5, and a heavy load control unit 12H for detecting the output voltage and turning on / off the FETs 3 and 4 in a relatively short fixed period so as to keep the output voltage constant.

In this example, the heavy load mode refers to the operation mode of portable equipment and the like, and the light load mode refers to the standby mode. In this standby mode, the DC output is supplied only to a minute load such as display on the liquid crystal panel or memory retention. Then, the heavy load mode and the light load mode are discriminated externally, and in FIG.
And the operation of the heavy load control unit 12H is switched.

In the light load mode, the light load control unit 12H is powered off while only the light load control unit 12L is used. Therefore, FET3 and FET4 are always off, and FET5
Is turned on / off at a predetermined cycle by the light load control unit 12L. When the FET 5 is off, the current in the reactor 7 is maintained by the commutation diode 6. As is well known, the output voltage of the FET 5 is represented by the following equation (1) or (1a).

[0006]

## EQU1 ## Output voltage (average value) = (input voltage) × (on time) / (cycle) ... (1) = (input voltage) × (on ratio) ... (1a) In this example, The input voltage is the voltage of the power source 1, and the on-time is the on-time in one cycle of turning on / off the FET 5. The light load control unit 12L detects the voltage of the load 11 via the resistors 9 and 10 and FET so as to keep this value at a predetermined value.
Control the ON ratio of 5. In this example, the open / close cycle of the FET 5 is relatively long and the ON ratio is relatively small.

On the other hand, in the heavy load mode, the light load control unit 12
The power of L is turned off, and only the heavy load control unit 12H is used. Therefore, FET5 is always off, and FET3 and FET
4 are alternately turned on in a predetermined cycle (that is, the FET 4 is turned off when the FET 3 is turned on, and the FET 4 is turned on when the FET 3 is turned off). That is, the FET 4 serves as a commutation diode in parallel with the commutation diode 6. The reason for this is that the forward voltage drop VF of the commutation diode 6 is large and this loss cannot be ignored under heavy load, so FE with a small voltage drop is used.
This is because a commutation current is passed through T4 to improve the efficiency of the DC chopper. In this way, the FET 4 is turned on / off in synchronism with the turning on / off of the FET 3, and this DC chopper is called a synchronous rectification type DC chopper.

Even in this heavy load mode, the output voltage of the FET 3 is expressed by the above equation (1). And the heavy load control unit 12H
Detects the voltage of the load 11 via the resistors 9 and 10, controls the ON ratio of the FET 3 to keep this value at a predetermined value, and turns the FET 4 ON / OFF in synchronization with the FET 3. When the FET 3 is turned on and the FET 4 is turned off here, a current flows to the output through the reactor 7. However, the current gradually increases due to the reactor 7. Next, FET3 is off, FET
When 4 is turned on, the energy stored in the reactor 7 flows through the load 11 and the FET 4, and the current gradually decreases. In this example, the open / close cycle of the FETs 3 and 4 is relatively short in the heavy load mode, and the ON ratio of the FET 3 is relatively large.

[0009]

By the way, in the circuit of FIG. 2, since the control sections 12L and 12H are completely switched between the light load mode and the heavy load mode by the signal input from the outside, in the heavy load mode. When the short circuit protection is activated due to an instantaneous load short circuit, the heavy load control unit 12H is powered off, and then the light load control unit 12L is powered off even if the light load DC value is used for backup. Therefore, even if the load short circuit is released, it takes time to recover because the output is not output unless the power supply is restarted.
The method of FIG. 2 may not be used depending on the application of the converter. Therefore, an object of the present invention is to provide a synchronous rectification DC-DC converter that can solve this problem.

[0010]

In order to solve the above-mentioned problems, the synchronous rectification DC-DC converter according to claim 1 is connected in series with a series circuit of a smoothing reactor (7 etc.) and a load (11 etc.). A DC power source (1 or the like) is connected through a first switch element (such as a light load main switch element 5) provided in a polarity for supplying power to a load, and a smoothing capacitor (8 or the like) is connected in parallel with the load, A diode (commutation diode 6 or the like) is connected in parallel to the series circuit of the smoothing reactor and the load so as to maintain the current of the smoothing reactor, and
The second switch element (main switch element 3 for heavy load, etc.) is connected in parallel with the same switch polarity as the above switch element, and the third switch element (for heavy load) is connected in parallel with the diode and has the same switch polarity. Synchronous rectifier 4 etc.) is connected,
A means for detecting a load voltage (resistors 9, 10, etc.), and an ON / OFF drive control for the first switch element at a first predetermined cycle so as to keep the load voltage detected by the voltage detecting means at a predetermined value. First control means (light load control circuit 1
22), so that the load voltage detected by the voltage detecting means is kept at a predetermined value, the second and third switch elements are turned on in the second predetermined cycle, the latter is turned off when the former is turned on, and the former is turned on. In the synchronous rectification DC-DC converter including the second control means (the heavy load control circuit 121 and the like) for synchronously performing ON / OFF drive control when turning off the latter, the light load mode and the heavy load mode Based on an external signal that specifies whether the first control means is activated in the light load mode and the first and second control means are activated in the heavy load mode (control section 120 or the like), and the second control means. It is assumed that the first control means invalidates a signal for turning on the first switch element while the third control element turns on the third switch element (interlock circuit 123 or the like).

A synchronous rectification DC-DC converter according to a second aspect of the present invention is the synchronous rectification DC-DC converter according to the first aspect, wherein a means for detecting a load current (load current detection resistor 13, etc.) and a heavy load. In the mode, when the load current detected by the current detection means exceeds a predetermined value, at least means for invalidating the ON signal given to the second and third switch elements from the second control means (overcurrent protection circuit And the like) and the like.

Further, in the synchronous rectification DC-DC converter according to claim 3, in the synchronous rectification DC-DC converter according to claim 2, the invalidating means of the ON signal cuts off the power supply of the second control means. To do so. Further, in the synchronous rectification DC-DC converter according to claim 4, in the synchronous rectification DC-DC converter according to claim 2 or 3, the first control means controls the first control circuit when the load current exceeds a predetermined value. A means for lowering the ON ratio of the switch element to droop the load current (such as an overcurrent drooping circuit) is provided.

[0013]

[Function] While the short circuit protection of the heavy load switching circuit turns off the power supply of the heavy load control circuit by the latch operation, the short circuit protection of the light load switching circuit does not turn off the power supply of the light load control circuit. The current is limited by the overcurrent droop circuit in the control circuit. Even when the heavy load switching circuit is operating, the light load switching circuit is always operated, but the heavy load synchronous rectifying element (that is, the FET 4) is forcibly lightened at the timing of the ON period. The main switch element for load (that is, FET 5) is interlocked so as to be turned off.

As a result, even if the load causes a momentary short circuit in the heavy load mode and the short circuit protection works to stop the heavy load switching circuit, the light load circuit immediately starts switching. , The power required for the backup can be supplied. Further, when the synchronous rectifying element FET4 is turned on, the light load main switching element FET5 does not turn on, so that a short-circuit current between the input and the ground does not flow.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a circuit configuration as an embodiment of the present invention, and this figure corresponds to FIG. In FIG. 1, 13 is a load current detection resistor, and 120 is a new control unit. In the control unit 120, a FET 121 turns on FETs 3 and 4 /
A heavy load control circuit for controlling OFF, a light load control circuit for controlling ON / OFF of the FET 5, a reference numeral 123 for an interlock circuit described later, and a reference numeral 124 for an overcurrent protection circuit also described later. The detection voltage of the voltage dividing resistors 9 and 10 for detecting the output voltage is the heavy load control circuit 121 and the light load control circuit 122.
Entered in. The detection voltage of the load current detection resistor 13 is input to the overcurrent protection circuit 124 and the light load control circuit.

In the light load mode, the power source of the heavy load control circuit 121 is turned off by an external signal, and the FETs 3 and 4 are kept off because there is no signal to turn them on. On the other hand, the light load control circuit 122 is utilized to control the FET 5 on / off at a predetermined frequency so as to keep the voltage of the load 11 detected by the resistors 9 and 10 at a predetermined value. Next, in the heavy load mode, the light load control circuit 122 is kept alive by the external signal, so that the FET 5 is kept operating and the power source of the heavy load control circuit 121 is kept alive. Therefore, the heavy load control circuit 121 uses a synchronous rectification method to keep the load voltage detected by the resistors 9 and 10 at a predetermined value, and in this example, turns on / off the FETs 3 and 4 at a predetermined frequency different from that of the FET 5. Here, the interlock circuit 123 is the F by the heavy load control circuit 121.
The timing of turning on the ET4 is detected, and during the ON period of the FET4, the ON signal of the FET5 from the light load control circuit 122 is invalidated and interlocked so as to forcibly turn off the FET5, thereby preventing a power supply short circuit.

In this heavy load mode. N channel F
There is a timing at which the P-channel FET 5 is turned off and a timing at which it is turned on when the ET3 is turned on. In the latter timing, that is, when the FETs 3 and 5 are both turned on, the on-resistance of the N-channel FET 3 is lower than the on-resistance of the P-channel FET 5, and the current flows through the N-channel FET 3 so that F
The on-effect of ET5 does not appear.

When the load 11 is short-circuited in this heavy load mode, the overcurrent protection circuit 124 in the control unit 120 knows this short circuit because the voltage of the load current detection resistor 13 exceeds a predetermined value, and the heavy load control circuit Turn off the power of 121 and turn off the FET
Turn off 3 and 4. However, the light load control circuit 122 is used as it is. However, at this time, since the voltage of the load current detection resistor 13 is also input to the light load control circuit 122,
An overcurrent drooping circuit (not shown) in the light load control circuit 122 operates to lower the ON ratio of the FET 5 and lower the output voltage (droop).
Then, the load current is limited to a predetermined value. However, as soon as the load is restored, the light load control circuit 122 returns to the normal operation and restores the output voltage.

[0019]

According to the present invention, in the heavy load mode, only when the synchronous rectifying element FET4 is conductive, the light load main switch element FET5 is forcibly turned off through the interlock circuit 123. Since the light load control circuit 122 having the overcurrent drooping circuit is utilized regardless of the light load mode, the power supply of the heavy load control circuit 121 is turned off by the overcurrent protection circuit 124 when the load is short-circuited in the heavy load mode. Even if the load is restored, the output power supply for the light load immediately rises, and the power required for backup can be supplied.

[Brief description of drawings]

FIG. 1 is a circuit diagram as an embodiment of the present invention.

FIG. 2 is a conventional circuit diagram corresponding to FIG.

[Explanation of symbols]

 1 DC power supply 2 Electrolytic capacitor 3 Main switch element for heavy load (N channel FET) 4 Synchronous rectifying element for heavy load (n channel FET) 5 Main switch element for light load (P channel FET) 6 Commutation diode 7 Smoothing reactor 8 Smoothing capacitor 9,10 Output voltage detection voltage dividing resistor 11 Load 13 Load current detection resistor 120 Control unit 121 Heavy load control circuit 122 Light load control circuit 123 Interlock circuit 124 Overcurrent protection circuit

Claims (4)

[Claims] 1. A direct current power supply is connected in series to a series circuit of a smoothing reactor and a load via a first switch element provided in a polarity for supplying power to the load, and a smoothing capacitor is connected in parallel with the load, A diode is connected in parallel to the series circuit of the smoothing reactor and the load in a polarity that maintains the current of the smoothing reactor, a second switching element is connected in parallel with the first switching element and in the same conduction polarity, and A third switch element is connected in parallel with the diode with the same conduction polarity, a means for detecting a load voltage, and a first predetermined value so as to keep the load voltage detected by the voltage detection means at a predetermined value. First control means for ON / OFF driving control of the first switch element in a cycle, second and third in a second predetermined cycle so as to keep the load voltage detected by the voltage detection means at a predetermined value The switching element, turns off the latter when turning on the former, to turn on the latter when turning off the former, synchronous rectification DC-DC having a second control means for controlling synchronization with ON / OFF driving
In the converter, means for activating the first control means in the light load mode and activating the first and second control means in the heavy load mode based on an external signal designating the light load mode and the heavy load mode. And a period in which the second control means turns on the third switch element,
Synchronous rectification D, characterized in that the first control means comprises means for invalidating a signal for turning on the first switch element.
C-DC converter. 2. The synchronous rectification DC-DC converter according to claim 1, wherein: means for detecting a load current; and a load current detected by the current detecting means in the heavy load mode exceeds a predetermined value. A synchronous rectification DC-DC converter comprising at least means for invalidating an ON signal given to the second and third switch elements from the second control means. 3. The synchronous rectification DC-DC converter according to claim 2, wherein the means for invalidating the ON signal shuts off the power supply of the second control means.
C converter. 4. The synchronous rectification DC- according to claim 2 or 3.
In the DC converter, the first control means includes means for lowering the ON ratio of the first switch element to droop the load current when the load current exceeds a predetermined value.
DC converter.