Switching Power Supply

Smoothing circuit for switching power supply

Switching Power Supply Abstract
A smoothing circuit for a switching power supply is formed of a combination of a reactor and a capacitor having a small equivalent internal resistance, and a MOSFET connected in series to the capacitor. The MOSFET is turned on by an output voltage of the smoothing circuit and has an ON-state resistance compensating for an equivalent internal resistance of the capacitor.

Switching Power Supply Claims
What is claimed is:

1. A smoothing circuit for a switching power supply, comprising:

a combination of a reactor, and a capacitor connected to the reactor and having an equivalent internal resistance, and

a MOSFET connected in series to the capacitor and having a gate connected to a connecting point between the reactor and the capacitor, said MOSFET being turned on by an output voltage of the smoothing circuit and having an ON-state resistance compensating for the equivalent internal resistance of the capacitor.

2. A smoothing circuit for a switching power supply according to claim 1, wherein said equivalent internal resistance of the capacitor decreases as an ambient temperature increases, and the ON-state resistance of the MOSFET increases as the ambient temperature increases to compensate for the equivalent internal resistance of the capacitor.

3. A smoothing circuit for a switching power supply, comprising:

a reactor,

a first capacitor connected to the reactor and having an equivalent internal resistance which decreases as an ambient temperature increases,

a second capacitor connected to the reactor and having an equivalent internal resistance,

a thermistor and a resistor connected in series to divide an output voltage of the smoothing circuit, and

a MOSFET connected in series to the second capacitor and being turned on by an output voltage divided by the thermistor and resistor, said MOSFET having an ON-state resistance and operating such that as the ambient temperature increases, the output voltage divided by the thermistor and resistor is reduced to thereby increase the ON-state resistance so that a combined equivalent internal resistance of the first and second capacitors is compensated.

4. A smoothing circuit for a switching power supply according to claim 1, further comprising a first relay formed of a series connection of a diode and a relay coil and situated between output terminals of the smoothing circuit for blocking an exciting current when a battery is connected in reverse polarity between the output terminals; a first relay contact situated between the capacitor and one of the output terminals and being actuated by the relay coil, said first relay contact connecting the capacitor and the one of the output terminals when the relay coil is excited, a first resistor situated between a drain and a source of the MOSFET; and a second resistor connected to the gate of the MOSFET.

5. A smoothing circuit for a switching power supply according to claim 4, wherein said second resistor is connected at one end to the gate of the MOSFET and at the other end to a portion between the first relay contact and the capacitor.

6. A smoothing circuit for a switching power supply according to claim 4, further comprising a second relay excited by a switching power-supply activation signal, and a second relay contact connected in series to the first relay, said second relay being turned on when the second relay is excited.

7. A smoothing circuit for a switching power supply according to claim 6, further comprising a delay circuit connected to the second relay, a voltage control circuit, and an AND circuit connected to the delay circuit and the voltage control circuit for activating a switching power supply.

8. A smoothing circuit for a switching power supply according to claim 1, further comprising means for detecting an overvoltage applied between output terminals of the smoothing circuit and connected to the gate of the MOSFET, said detecting means, when an overvoltage is detected, outputting a signal to turn the gate of the MOSFET off to thereby separate the capacitor from the output terminals.

9. A smoothing circuit for a switching power supply according to claim 8, wherein said detecting means includes a diode situated between the output terminals for allowing an electric signal to pass only when the overvoltage is applied between the output terminals, and a transistor connected to the diode and the gate of the MOSFET to provide the signal for turning the MOSFET off when the overvoltage passes through the diode.

10. A smoothing circuit for a switching power supply according to claim 1, wherein said capacitor becomes conductive when the output voltage of the smoothing circuit increases to exceed an on-threshold value of the MOSFET to thereby turn on the MOSFET.

11. A smoothing circuit for a switching power supply according to claim 10, wherein said capacitor and MOSFET are arranged in series between output terminals of the smoothing circuit.

Switching Power Supply Description
A switching power supply has first and second switching elements that are alternately turned on to cause an alternating current to flow through a primary winding of a transformer. The alternating current flowing through the primary winding induces voltages respectively across first and second secondary windings of the transformer. Synchronous rectifying transistors are turned on by the voltages induced across the first and second secondary windings, causing a current to flow alternately through the first and second secondary windings. Either one of the first and second switching elements is turned on while the other is being turned off, except for short off-times in which both the first and second switching elements are prevented from being turned on. As a result, when currents flow through the first and second secondary windings, the synchronous rectifying transistors are turned on. Therefore, no current flows through internal parasitic diodes of the synchronous rectifying transistors, and hence any loss caused thereby is very small.