Abstract:
A DC-DC converter includes: a high-side MOSFET as a main switching element which is driven by using a bootstrap capacitor; a low-side MOSFET as a synchronous rectifier, wherein a series circuit of the high-side MOSFET and the low-side MOSFET is connected to a DC power supply; and a coil and a smoothing capacitor, which are serially connected between the drain and the source of the low-side MOSFET; an overvoltage protection unit, which clamps an overvoltage; an overcurrent interrupting unit, which interrupts an overcurrent that flows when the overvoltage protection unit clamps the overvoltage; and a protection circuit, wherein the protection circuit includes: a differential-voltage detecting unit detecting the voltage of both ends of the bootstrap capacitor; and a control unit that, when the voltage detected by the differential-voltage detecting unit exceeds a predetermined value, turns OFF the low-side MOSFET and turns ON the high-side MOSFET.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2011-013466 filed on Jan. 25, 2011, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FILED 
     This disclosure relates to a DC-DC converter. 
     BACKGROUND 
       FIG. 2  is a circuit diagram illustrating a part of a general configuration of a step-down DC-DC converter. The DC-DC converter shown in  FIG. 2  is a step-down DC-DC converter, which uses an N-channel Metal Oxide Semiconductor Field Effect Transistor (MOSFET) at a high-side, and a N-channel MOSFET for synchronous rectification is connected at a low-side to reduce a loss of a flywheel diode D 12  and to increase the power conversion efficiency. An input voltage Vin is applied to a drain terminal of the high-side MOSFET Q 1 , and the drain terminal of the low-side MOSFET Q 2  is connected to a source terminal of the high-side MOSFET Q 1 . One end of a coil L 11  is connected to a connection point where the high-side MOSFET Q 1  and the low-side MOSFET Q 2  are connected, and a smoothing capacitor C 12  and a load RL are connected between the other end of the coil L 11  and the GND. The coil L 11  and the smoothing capacitor C 12  configure a direct current smoothing circuit at an output part of the DC-DC converter. The drive circuit of the high-side MOSFET Q 1  comprises a bootstrap capacitor C 11 , which supplies an approximately constant voltage to a buffer circuit BF 1  based on the electric potential of the source terminal of the high-side MOSFET Q 1  (the electric potential of the terminal SW shown in  FIG. 2 ), in order to provide a voltage higher than the input voltage Vin to the buffer circuit BF 1  and the gate terminal of the high-side MOSFET Q 1 . 
     In the DC-DC converter, the high-side MOSFET Q 1  and the low-side MOSFET Q 2  are controlled to be ON/OFF, in response to inputting a high-side drive signal into the gate terminal of the high-side MOSFET Q 1  through the buffer circuit BF 1  and inputting a low-side drive signal into the gate terminal of the low-side MOSFET Q 2  through a buffer circuit BF 2 . 
     In the above-described DC-DC converter, due to use the N-channel MOSFET as a switching element at the high-side of the power supply device, it is necessary to raise the voltage of the gate electrode of the MOSFET to be higher than the input voltage Vin. A bootstrap circuit, which uses the bootstrap capacitor C 11 , is used for this purpose. However, when the terminal to which the input voltage Vin is applied and the terminal to which the bootstrap voltage is applied (the terminal BS shown in  FIG. 2 ) are shorted by adhesion of dust or the like, since the input voltage Vin is applied between the terminal BS and the terminal SW, each of a withstand voltage of the buffer circuit BF 1  between the terminal BS and the terminal SW, a withstand voltage between the gate and the source of the high-side MOSFET Q 1  and a withstand voltage between the gate and the drain of the high-side MOSFET Q 1  are exceeded, and thus the buffer circuit and MOSFET Q 1  may be broken down and shorted. 
     Such short between the drain and the source of the high-side MOSFET Q 1  due to the breakdown of withstand may be caused when the low-side MOSFET Q 2  is turned on, in which the voltage of the terminal SW becomes Low. At this time, because a through current flows through the high-side MOSFET Q 1  and the low-side MOSFET Q 2  from the Vin to the GND, the Area of Safe Operation (ASO) breakdown of the low-side MOSFET Q 2  is also caused by the overcurrent. Further, when the low-side MOSFET Q 2  is broken, excessive current flows between the Vin and the GND, and the product may be broken due to intense damages. 
     In view of this, a protection circuit is proposed. When the voltage of the bootstrap circuit increases due to some abnormality as above and thus the switching elements cannot be normally turned on and off, the protection circuit interrupts the power supply to the switching elements, and thus safety is improved by suppressing damages of the switching elements (for example, refer to JP-A-2006-280014). 
       FIG. 1  is the circuit diagram of an example of power supply devices including a protection circuit and a bootstrap circuit. In  FIG. 1 , a bootstrap circuit  106  includes capacitors C 5  and C 6 , and a diode D 3 . Further, the protection circuit includes a diode D 5  as a voltage detecting part, a transistor Tr 1  and a fuse F 2 . 
     Behaviors of the bootstrap circuit  106  will be described below. The GND of the bootstrap circuit  106  is a floating GND, which is connected to the source terminal of a switching element S 2 . Thus, the terminal Vb of the bootstrap circuit  106  is always kept higher than the source terminal voltage of the switching element S 2  as the terminal voltage Vboot, namely the charging voltage of the capacitor C 6 , regardless of ON/OFF of the switching element S 2 . This is because the capacitor C 5  is charged by the capacitor C 6  through the diode D 3  when the switching element S 2  is turned on. 
     When the bootstrap circuit  106  works normally, the bootstrap circuit  106  control the switching element S 2  to turn ON/OFF by repeating the charging and discharging of the capacitors C 5  and C 6 . On the other hand, when the diode D 3  in the bootstrap circuit  106  is shorted due to some abnormality and the voltage of the terminal Vboot of the control IC increases, the diode D 5  is turned on and thus the transistor Tr 1  is turned on. Therefore, the input and the GND are shorted, and the fuse F 2  is meltdown. The power supply to the switching element S 2  is interrupted by such behaviors, and operations of the switching element S 2  is stopped. Therefore, damages of the switching element S 2  is to be suppressed and safety is to be improved. 
     SUMMARY 
     However, in the protection circuit used for the above-described bootstrap circuit  106 , because the transistor Tr 1 , which makes the input and the GND short, is required to discharge a combined current of the interrupting current of the fuse F 2  and the charging current of the capacitor C 7 , it is necessary to use a big transistor which has a current specification or an area of safe operation, meeting this requirement. 
     In view of the above, this disclosure to provide a DC-DC converter whose safety is improved. 
     A DC-DC converter of one aspect of this closure, which is a synchronous step-down DC-DC converter, the DC-DC converter comprises: a high-side MOSFET as a main switching element which is driven by using a bootstrap capacitor; a low-side MOSFET as a synchronous rectifier, wherein a series circuit of the high-side MOSFET and the low-side MOSFET is connected to a DC power supply; and a coil and a smoothing capacitor, which are serially connected between the drain and the source of the low-side MOSFET, wherein a direct current is output from the terminals of the smoothing capacitor by turning ON/OFF the high-side MOSFET and the low-side MOSFET complementarily; an overvoltage protection unit, which clamps an overvoltage when the direct current output voltage of the smoothing capacitor exceeds a predetermined value; an overcurrent interrupting unit, which interrupts an overcurrent that flows when the overvoltage protection unit clamps the overvoltage; and a protection circuit, wherein the protection circuit comprises: a differential-voltage detecting unit detecting the voltage of both ends of the bootstrap capacitor; and a control unit that, when the voltage detected by the differential-voltage detecting unit exceeds a predetermined value, turns OFF the low-side MOSFET and turns ON the high-side MOSFET. 
     A DC-DC converter of another aspect of this disclosure, which is a synchronous step-down DC-DC converter, the DC-DC converter comprises: a high-side MOSFET as a main switching element which is driven by using a bootstrap capacitor; a low-side MOSFET as a synchronous rectifier, wherein a series circuit of the high-side MOSFET and the low-side MOSFET is connected to a DC power supply; and a coil and a smoothing capacitor, which are serially connected between the drain and the source of the low-side MOSFET, wherein a direct current is output from the terminals of the smoothing capacitor by turning ON/OFF the high-side MOSFET and the low-side MOSFET complementarily; and a protection circuit, wherein the protection circuit comprises: a differential-voltage detecting unit detecting the voltage of both ends of the bootstrap capacitor; a control unit that, when the voltage detected by the differential-voltage detecting means exceeds a predetermined value, turns OFF the low-side MOSFET, a switching element which is connected in parallel with the bootstrap capacitor; a voltage detecting unit, when the voltage of both ends of the bootstrap capacitor exceeds a predetermined value, turns ON the switching element; and an element, which is provided between the terminal of the bootstrap capacitor at the high voltage side and the drain terminal of the switching element, and which is meltdown due to a current that flows when the switching element is turned on. 
     This disclosure is to provide the DC-DC converter in which safety is improved when the terminal, to which the input voltage is applied, and the terminal, to which the bootstrap voltage is applied, are shorted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed descriptions considered with the reference to the accompanying drawings, wherein: 
         FIG. 1  is a circuit diagram illustrating a switching control circuit including a protection circuit according to background art; 
         FIG. 2  is a circuit diagram illustrating a synchronous step-down DC-DC converter according to background art; 
         FIG. 3  is a circuit diagram illustrating a DC-DC converter of an illustrative embodiment 1 of this disclosure; and 
         FIG. 4  is a circuit diagram illustrating a DC-DC converter of an illustrative embodiment 2 of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Next, the illustrative embodiments of this disclosure will be explained in detail with reference to the drawings. 
     (Illustrative embodiment 1) The illustrative embodiment 1 of this disclosure is shown in  FIG. 3  in which a DC-DC converter includes a bootstrap circuit. 
     In  FIG. 3 , a power MOSFET used as the high-side switching element is shown by a symbol Q 1 , and a power MOSFET used as the low-side switching element is shown by a symbol Q 2 . An input voltage Vin is applied to the drain terminal of the high-side MOSFET Q 1  through a fuse F 11 , and the source terminal of the high-side MOSFET Q 1  is connected to the drain terminal of the low-side MOSFET Q 2 . One end of a coil L 11  is connected to the connection point where the high-side MOSFET Q 1  and the low-side MOSFET Q 2  are connected, and a smoothing capacitor C 12 , a load RL and a clamp diode D 13  for load protection are connected between the other end of the coil L 11  and the GND. The coil L 11  and the smoothing capacitor C 12  configure a direct current smoothing circuit at an output part of the DC-DC converter. One end of a bootstrap capacitor C 11  is connected to a terminal SW, and the other end of the bootstrap capacitor C 11  is connected to a terminal BS. Thus, the bootstrap capacitor C 11  is charged by applied a predetermined voltage generated by a regulator (not shown) through a back-flow preventing diode (not shown) and the terminal BS, and thus a bootstrap voltage is generated. A potential difference between a voltage of the output signal of the buffer circuit BF 1  and a voltage of the terminal SW is generated by the bootstrap voltage, and the potential difference is to be higher than a gate threshold voltage of the high-side MOSFET Q 1 . Therefore, the high-side MOSFET Q 1  is turned on, and current is flown to the terminal SW through the drain and the source of the high-side MOSFET Q 1 . 
     Complementary drive signals are input into the gate terminals of the high-side MOSFET Q 1  and the low-side MOSFET Q 2 , respectively. In other words, drive signal input terminals whose signals are input from a drive signal generating circuit (not shown) are connected to one of the input terminals of an OR-circuit OR and an AND-circuit AND, respectively. Further, an output signal from the overvoltage protection circuit  1  is connected through the latch circuit LC 1  to the other input terminal of the OR-circuit OR, and to the other of the input terminals of the AND-circuit AND through an inverter circuit INV, respectively. The output of the OR-circuit OR is connected to the gate terminal of the high-side MOSFET Q 1  through a buffer circuit BF 1 . The output of the AND-circuit AND is connected to the gate terminal of the low-side MOSFET Q 2  through a buffer circuit BF 2 . 
     The overvoltage protection circuit  1  detects an overvoltage by comparing the potential difference between the electric potentials of both ends of the bootstrap capacitor C 11 , namely the electric potentials between the terminal BS and the terminal SW shown in  FIG. 3 , with a reference voltage Vref. The potential difference between the terminal BS and the terminal SW is detected by a differential amplifier, which is configured by an operational amplifier OP 1  and resistors R 1 -R 4 , and the output from the differential amplifier is connected to the non-inverting input terminal of a comparator CP 1 . On the other hand, Vref being the reference voltage is supplied to the inverting input terminal of the comparator CP 1 . Through a latch circuit LC 1 , the output of the comparator CP 1  is connected to the input terminal of the OR-circuit OR side to which the drive signal input terminal is not connected, as stated above, and is connected to the input terminal of the AND-circuit AND to which the drive signal input terminal is not connected further through an inverter circuit INV. 
     When the bootstrap circuit works normally, the bootstrap capacitor C 11  stably controls the ON/OFF of the high-side MOSFET Q 1  by repeating the charging and discharging of energy. The overvoltage protection circuit  1  compares the potential difference between the terminal SW and the terminal BS with the reference voltage Vref, and the output of the comparator CP 1  usually becomes a signal of Low. 
     However, when the terminal, to which the input voltage is applied, and the terminal BS, to which the bootstrap voltage is applied, are shorted due to adhesion of dust or the like and when the low-side MOSFET Q 2  is turned on, since the potential difference between the terminal BS and the terminal SW, at first, becomes higher than the normal state, the potential difference is converted and then output by the differential amplifier configured by the operational amplifier OP 1  and the resistors R 1 -R 4 . Then, when the potential difference signal converted by the differential amplifier is compared with the reference voltage Vref by the comparator CP 1  and exceed the threshold, a signal of High is output from the comparator CP 1 . The signal output from the comparator CP 1  is connected through the latch circuit LC 1  to the input terminal of the OR-circuit OR, to which the drive signal input terminal is not connected, and the input terminal of the AND-circuit AND, to which the drive signal input terminal is not connected, through an inverter circuit INV, respectively. 
     Since a signal of High output by the latch circuit LC 1  is input to the input terminal of the OR-circuit OR, to which the drive signal input terminal of is not connected, a signal of High is always output from the OR-circuit OR, regardless of the input drive signal input to the other input terminal of the OR-circuit OR. This output signal is input to the gate terminal of the high-side MOSFET Q 1  through the buffer circuit BF 1 , and then the high-side MOSFET Q 1  is kept being ON, namely, the drain and the source are conducted. 
     On the other hand, since the output of the latch circuit LC 1  is input through the inverter circuit INV into the input terminal of the AND-circuit AND, to which the drive signal input terminal is not connected, a signal of Low is output from the inverter circuit INV and input into one of the input terminals of the AND-circuit AND. Therefore, regardless of the drive signal input into the other input terminal of the AND-circuit AND, a signal of Low is always output from the AND-circuit AND and is input into the gate terminal of the low-side MOSFET Q 2  through a buffer circuit BF 2 . Thus, the drain and the source of the low-side MOSFET Q 2  are kept in an interrupted condition. 
     In this case, the current flows through a path of the fuse F 11  connected to the input power supply line, the high-side MOSFET Q 1  and coil L 11  which is connected to the source terminal of the high-side MOSFET Q 1 , and flows to the clamp diode D 13  for load protection (described by the dotted arrow line). The fuse F 11  connected to the input power supply line is meltdown, and thus the voltage from the power supply line is interrupted. 
     According to this configuration, since the through current from the power supply line to the GND is suppressed and the voltage from the power supply line is to be interrupted, fuming, burning and exploding sound, etc., of the high-side MOSFET Q 1  and the low-side MOSFET Q 2  is suppressed and safety is to be improved. 
     (Illustrative embodiment 2) The illustrative embodiment 2 of this disclosure is shown in  FIG. 4  in which a DC-DC converter includes a bootstrap circuit. 
     The illustrative embodiment 2 includes an overvoltage protection circuit  2  between the terminal BS and the terminal SW instead of the OR-circuit OR and the latch circuit LC 1  in the DC-DC converter of illustrative embodiment 1. The overvoltage protection circuit  2  includes a resistor R 5 , which is connected between the terminal BS and the buffer circuit BF 1 , a N-channel MOSFET Q 3 , of which drain electrode is connected to the terminal BS and of which source electrode is connected to the terminal SW, and a zener diode D 14  and a resistor R 6  which are connected between the terminal BS and the terminal SW. 
     In the above overvoltage protection circuit  2 , when the terminal, to which an input voltage is applied, and the terminal BS, to which the bootstrap voltage is applied, are shorted due to some abnormality, the zener diode D 14  is turned on by a potential difference produced between the terminal BS and the terminal SW, and the N-channel MOSFET Q 3  is turned on. Accordingly, the drain and the source of the N-channel MOSFET Q 3  are conducted, and an excessive current flow through the resistor R 5  that is connected between the terminal BS and the buffer circuit BF 1 , and thus the resistor R 5  is meltdown due to the excessive current. Since the bootstrap capacitor C 11  and the buffer circuit BF 1  are cut off after the resistor R 5  is meltdown, an overvoltage will not be applied to the buffer circuit BF 1  and the gate of the high-side MOSFET Q 1 . Thus, the buffer circuit BF 1  and the high-side MOSFET Q 1  are protected. In addition, since the bootstrap capacitor C 11  and the buffer circuit BF 1  are cut off, the high-side MOSFET Q 1  cannot be turned on. Even in this case, the overvoltage protection circuit  1  functions as the same as that of the illustrative embodiment 1 described above. Therefore, even if the terminal, to which the input voltage is applied, and the terminal BS, to which the bootstrap voltage is applied, are shorted in the timing that the low-side MOSFET Q 2  is on, since the drain and the source of the low-side MOSFET Q 2  are cut off instantly, the through current between the Vin and the GND does not flow. Accordingly, it is possible to protect the high-side MOSFET Q 1  and the low-side MOSFET Q 2  without damages. Further, by using a thin film resistor as the resistor connected between the terminal BS and the buffer circuit BF 1 , the fuming when melting due to the overcurrent is suppressed, and the power supply line is to be safely cut off. 
     Thus, since the Vin is cut off by the melting of the resistor R 5 , it is not necessary to provide usually the externally attached fuse F 11  and the clamp diode D 13  for load protection, and it is possible to improving safety and to reduce the number of externally attached elements.