JPH08336279A - Chopper type switching power supply - Google Patents

Abstract

PURPOSE: To obtain a small-sized maintenance free switching power supply by connecting a temperature-sensitive element, exhibiting high resistance when a predetermined temperature is reached due to current supply, in series between the drain of a power MOSFET and a choke coil. CONSTITUTION: Upon short circuit of a load 3 when a desired output voltage is appearing between second and fourth terminals, a short circuit current flows through a circuit of a DC power supply 1 →the first terminal → a power MOSFET 5 → a PTC element 13 → a choke coil 7 → the second terminal →the load 3 → the GND. Since the temperature of PTC element 13 increases when an overcurrent flows through the element 13, the resistance thereof increases abruptly. Since the PTC element 13 blocks a short circuit current flowing through the chopper circuit, the power MOSFET 5 is protected against breakdown. Even if the DC power supply 1 is connected reversibly with a step- down chopper type switching power supply, the PTC element 13 blocks the reverse current thus protecting the power MOSFET 5 and a flywheel diode D2 against breakdown.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chopper type switching power supply, and more particularly to a chopper type switching power supply which requires no maintenance.

[0002]

2. Description of the Related Art Generally, automobiles are provided with various electric systems. And this electric system requires a plurality of power sources of different voltages according to the load, and using the voltage of the DC power source,
A chopping method is often used to obtain a desired output voltage according to the load. And, such a switching power supply mounted on a vehicle is
Generally, the switching power supply is unitized in order to facilitate maintenance and protect it from a bad environment.

For this reason, for example, when the switching power supply fails, if the units are replaced for maintenance, the DC power supply unit and the switching power supply unit may be connected in reverse. When connected in reverse, the unit's terminal, which should be connected to the positive side of the DC power supply, is connected to the negative side of the DC power supply, and the terminal to be connected to the negative side is connected to the positive side of the DC power supply. Connected (hereinafter referred to as reverse power supply).

As described above, when the power source is reversely connected, the element inside the switching power source is destroyed due to dead short circuit, so that it may be protected by a fuse or the like as shown in FIG. 3, for example. Such a switching power supply will be described below.

FIG. 3 is a schematic configuration diagram of a conventional step-down chopper type switching power supply using a fuse. As shown in FIG. 3, the power MOSFET 1, the choke coil 5, the smoothing capacitor 7 are connected to the positive terminal of the DC power source, the first terminal connected to the positive terminal of the DC power supply, and the second terminal to which one end of the load 3 is connected. A chopper circuit including a flywheel diode D2 and a control unit 9 for turning on / off the power MOSFET 1 at a duty ratio according to the voltage difference across the smoothing capacitor 7 are provided.

A fuse 11 is provided between the connection point where the choke coil 5 of the chopper circuit and the cathode of the flywheel diode D2 are connected and the source S of the power MOSFET 1.

Also, the anode of the flywheel diode D2 and the other of the smoothing capacitor 7 are both connected to a third terminal connected to the negative side of the DC power supply (hereinafter referred to as the GND side) and the other end of the load 3. It is connected to the fourth terminal.

In the step-down chopper type switching power supply shown in FIG. 3, the control circuit 9 uses a pulse signal to generate a power MOS.
The FET1 is turned on / off, and the rectangular wave obtained by this on / off is flattened by the choke coil 5, the smoothing capacitor 7 and the flywheel diode D2, and an output voltage Vo lower than the DC voltage VB is obtained across the smoothing capacitor 7. .

When the step-down chopper type switching power supply is reversely connected to the power supply, the current of the DC power supply is changed to the third
Terminal → Flywheel diode D2 → Fuse 11
-> Parasitic diode D1 of power MOSFET 1-> 1st terminal-> It was made to flow in the circuit route of a DC power supply, and fuse 11 was destroyed, and the destruction of power MOSFET 1 and flywheel diode D2 was protected.

However, until the fuse 11 is blown, the flywheel diode D2 and the power MOSF
Since overcurrent flows through ET1, it causes considerable damage.

Further, when a step-down chopper type switching power supply provided with a fuse is used in an automobile, when the step-down chopper type switching power supply is mounted inside the vehicle, the fuse is installed in a place where it can be easily replaced. Must be done. For example, the fuse must be designed so that it can be attached to a panel portion of an automobile.

Further, when it is not possible to secure a place where this fuse can be attached, a step-down chopper type switching power supply is provided and maintenance work is performed to replace this fuse each time the fuse is blown, but replacement is easy. If the step-down chopper type switching power supply is not installed in a different place, replacement is not easy.

From the above, as shown in FIG.
Stop the dead short circuit due to the reverse connection of the power supply with the fuse, and insert the backflow prevention diode D3 in series between the source of the power MOSFET 1 and the cathode of the flywheel diode D2 to prevent the dead short circuit due to the reverse connection of the power supply. I was trying to eliminate the need for maintenance.

However, since a diode is inserted in the current path, in order to prevent a dead short by the diode D3 for backflow prevention, the diode D3 for backflow prevention is inserted in series in the current path when the power MOSFET 1 is turned on. Therefore, the backflow prevention diode D3 must be a large one having a large capacity so that the amount of flowing current is not attenuated, resulting in a large device.

Therefore, for example, Japanese Patent Laid-Open No. 63-1676.
The chopper type switching power supply of Japanese Patent No. 66 monitors the output voltage, and when there is a voltage drop that causes a load short circuit abnormality, turns on the thyristor and causes the current flowing through the current path to the ground to flow the current flowing through the load at this time. Protects the elements on the current path.

[0016]

However, the chopper type switching power supply disclosed in Japanese Patent Laid-Open No. 167666/1988 requires a control circuit and a thyristor for driving the thyristor, which makes the device large and There was a problem that it became expensive.

Further, the thyristor has a characteristic that a current easily flows when the junction temperature rises. Therefore, even if the thyristor is turned on to interrupt the current flowing to the load side in the current path during a load short-circuit abnormality, a large current will flow through the thyristor even for a moment, so in the worst case, the thermal There was a problem that it could cause a runaway and burn.

Further, in the method of preventing the destruction of the element due to the short circuit or the reverse connection of the power source by the fuse, the fuse must be replaced every time the fuse is melted, so that the number of maintenance times for the fuse replacement increases. There was a problem.

The present invention has been made to solve the above problems, and an object of the present invention is to obtain a chopper type switching power supply which is compact and requires no maintenance.

[0020]

According to a first aspect of the present invention, a power MOSFE is provided between a positive terminal of a DC power supply and a first terminal connected to the second terminal to which one end of a load is connected.
T, a choke coil, a smoothing capacitor, a flywheel diode, a chopper circuit, a control circuit for turning on / off the power MOSFET with a pulse signal having a duty ratio according to a voltage difference across the smoothing capacitor, and a power MOS.
A temperature-sensitive element, which is connected in series between the drain of the FET and the choke coil and has a high resistance to a predetermined temperature when a current flows, is provided.

According to a second aspect of the invention, the power MOSFET and the temperature sensitive element are superposed and integrated.

According to a third aspect of the present invention, the control circuit maintains the power MOSFET in the off state when the voltage across the both ends becomes lower than a predetermined value.

[0023]

In the first aspect of the present invention, when a large current flows due to a load short circuit or a power source reverse connection, the power MOSFE
A large current flows through T, the temperature sensitive element, the choke coil, the smoothing capacitor, and the flywheel diode. At this time, the temperature sensitive element becomes high in temperature and has high resistance, and a large current is blocked.

In the second aspect of the invention, the power MOSFET and the temperature sensitive element are superposed and integrally inserted in the chopper circuit, so that the device is downsized and the temperature of the temperature sensitive element rises quickly.

According to the third invention, when a large current flows due to a short circuit of the load or a reverse connection of the power source, the power MOSFE
A large current flows through T, the temperature sensitive element, the choke coil, the smoothing capacitor, and the flywheel diode, and the temperature sensitive element becomes a high temperature and becomes a high resistance.
The control circuit maintains the power MOSFET in the off state when the output voltage falls below a predetermined level.

[0026]

【Example】

Example 1 FIG. 1 is a schematic configuration diagram of an example. As shown in Figure 1,
This step-down chopper type switching power supply is unitized, and a power MOS is connected to a first terminal connected to the positive terminal of the DC power supply 1 and a second terminal connected to one end of the load 3.
A chopper circuit including an FET 5, a choke coil 7, a smoothing capacitor 9, and a flywheel diode D2,
A control circuit 11 for turning on / off the power MOSFET 5 at a duty ratio according to the voltage difference across the smoothing capacitor 9 is provided. In addition, a third terminal in which the anode of the flywheel diode D2 and the other of the smoothing capacitor 9 are both connected to the ground side of the DC power supply (hereinafter referred to as the GND side) and the other end of the load 3 are connected to the fourth terminal. It is connected to the terminal.

Further, a PTC element 13 which is a kind of temperature sensitive element is connected to the source S of the power MOSFET 5, the cathode of the flywheel diode D2 and one of the yoke coils 7.

The PTC element 13 is an element which has an extremely low resistance at a normal temperature and rapidly has a high resistance when the temperature rises, and for example, a PTC thermistor or a polymer PTC is used.

The operation of the step-down chopper type switching power supply of the first embodiment constructed as described above will be described below. For example, if the ± of the DC power supply and the step-down chopper type switching power supply are normally connected and the control circuit 11 turns on and off the power MOSFET 11, the current at the time of turning on is
Positive side of DC power supply → first terminal → power MOSFET
5 → PTC element 13 → choke coil 7 → second terminal →
Return to the GND side of the DC power supply 1 through the load 3 → GND.

When the power MOSFET 5 is off and the smoothing capacitor 9 is fully charged, the load 3
→ Return to the choke coil 7 through the circuit route of the flywheel diode D2 and the smoothing capacitor 9
Discharge current flows from the same circuit route.

When the desired output voltage is obtained at the second terminal and the fourth terminal in such a state, and the load 5 is short-circuited for some reason, the power MOSFET 5 is turned on, as shown in FIG. As indicated by A, the short circuit current flows through the circuit route of DC power supply 1 → first terminal → power MOSFET 5 → PTC element 13 → choke coil 7 → second terminal → load 3 → GND. When the overcurrent flows, the temperature rises, so that the resistance suddenly becomes high.

Therefore, even if the load 3 is short-circuited, PT
Since the short-circuit current flowing through the chop circuit is blocked by the C element 13, the power MOSFET 5 can be prevented from being destroyed.

Next, when the DC power source 1 and the step-down chopper type switching power source are mistakenly connected in reverse, the DC power source 1 → the third terminal → the flywheel diode D2 →
A large reverse current flows through the circuit route of the PTC element 13 → the parasitic diode D1 of the power MOSFET 5 → the first terminal → the DC power supply 1, but the PTC element 13 suddenly becomes a high resistance because the temperature rises when an overcurrent flows. .

Therefore, even if the power source is reversely connected, the PTC element 13 blocks the reverse current, so that the power MOSF
It is possible to prevent the ET5 and the flywheel diode D2 from being destroyed.

That is, when a fuse is used in the current path, it is necessary to replace the fuse each time the fuse is blown due to reverse connection of the power source or short circuit of the load 3. However, the PTC element has a large current. When the current flows, the resistance of the power MOS suddenly becomes high and the current is blocked.
Since the FET 5 and the flywheel diode D2 are prevented from being destroyed, maintenance is unnecessary.

Second Embodiment In the first embodiment, the PTC element is used to prevent the overcurrent that flows when the power source is reversely connected or the load 3 is short-circuited. ~ 180
Since the radiant heat may adversely affect the surrounding circuit elements, it is desirable to suppress the spread of the radiant heat as much as possible.

For this reason, not only a protection circuit for overcurrent and overheat protection but also an intelligent power IC (hereinafter referred to as IPS) which informs the computer of abnormality of the load 3 or abnormality of elements is used. But I
PS is expensive.

On the other hand, the power MOSFET 5 is
Since a large current flows through the drain D, the drain D has a heat sink structure.

Therefore, in the second embodiment, PT
The C element 13 and the power MOSFET 5 are combined.

FIG. 2 is a schematic configuration diagram of the second embodiment. In the figure, 1 to 11, D1 and D2 are the same as above. However, the power MOSFET 5 and the PTC element 13 are
As shown in FIG. 2, it is screwed and integrated with the pattern of the substrate. Of course, when screwing cannot be performed, the power MOSFET 5 and the PTC element 13 may be surface-mounted.

The operation of the step-down chopper type switching power supply of the second embodiment having the above-mentioned structure will be described below. For example, when the load 5 is short-circuited for some reason, when the power MOSFET 5 is turned on,
As shown in, DC power supply 1 → first terminal → power MOS
Since a short-circuit current flows through the circuit route of FET 5 → PTC element 13 → choke coil 7 → second terminal → load 3 → GND, the temperature of both the power MOSFET 5 and the PTC element 13 rises. Therefore, the PTC element 13 is the same as that of the first embodiment.
Since the temperature rises more quickly, the short-circuit current can be elementized quickly.

Further, the drain D of the power MOSFET 5
Has a heat sink structure and a large pattern area. Therefore, even if the temperature of the PTC element 13 rises, heat is radiated, and it is possible to prevent the surrounding elements from being affected.

Further, since the power MOSFET 5 and the PTC element 13 are integrated, the unit can be downsized.

Furthermore, both ends of the smoothing capacitor 9 are monitored, and when the output voltage reaches the GND level, the control circuit 1
1 judges that the power supply is reversely connected or the load 3 is short-circuited, and the power MO
The SFET 5 may be turned off to prevent a short-circuit current flowing in the current path and prevent the power MOSFET 5 and the PTC element 13 from generating heat.

[0045]

As described above, according to the first aspect of the present invention, the power source is connected between the positive terminal of the DC power source and the second terminal to which one end of the load is connected. MOSF
If a large current flows due to a short circuit of the load or reverse connection of the power supply by forming a chopper circuit consisting of an ET, a semiconductor temperature sensitive element, a choke coil, a smoothing capacitor, and a flywheel diode, the temperature sensitive element becomes a high resistance and a large current. Since it is not necessary to replace it like a fuse by blocking the above, the effect that maintenance is unnecessary is obtained.

According to the second aspect of the present invention, the power MOSFET and the temperature sensitive element are superposed and integrally inserted in the chopper circuit, so that the device is downsized and the temperature of the temperature sensitive element rises quickly. When a large current flows due to a short circuit or reverse connection of the power source, it is possible to instantaneously prevent the large current from flowing.

According to the third aspect of the invention, the power MOSFET and the temperature sensitive element are superposed and integrated on the chopper circuit, and when the output voltage becomes lower than a predetermined level, the power MOSFET is integrated.
Power MOSFET by keeping the off state
Since an electric current does not flow through the temperature sensitive element thereafter, it is possible to prevent the influence of radiation heat on surrounding elements.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a first embodiment.

FIG. 2 is a schematic configuration diagram of a second embodiment.

FIG. 3 is a schematic configuration diagram of a step-down chopper type switching power supply in which a fuse is inserted.

FIG. 4 is a schematic configuration diagram in which a diode for preventing backflow is inserted.

[Explanation of symbols]

 1 DC power supply 3 Load 5 Power MOSFET 7 Choke coil 9 Smoothing capacitor 11 Control circuit 13 PTC element D1 Parasitic diode D2 Flywheel diode

Claims (3)

[Claims] 1. A power MOSFET, a choke coil, a smoothing capacitor, and a flywheel diode are provided between a first terminal connected to a positive terminal of a DC power source and a second terminal connected to one end of a load. A chopper circuit, a control circuit for turning on / off the power MOSFET with a pulse signal having a duty ratio according to a voltage difference across the smoothing capacitor, a drain connected to the power MOSFET, and a choke coil connected in series, and a current flows. A chopper-type switching power supply, which has a temperature sensitive element that becomes high in resistance at a predetermined temperature. 2. The chopper-type switching power supply according to claim 1, wherein the power MOSFET and the temperature-sensitive element are overlapped and integrated. 3. The chopper type switching power supply according to claim 1, wherein the control circuit keeps the power MOSFET in an off state when the voltage across the both ends becomes lower than a predetermined value.