Constant-voltage switching power supply provided with overvoltage output protecting circuit, and electronic apparatus provided with overvoltage protecting circuit

• US 7,215,517 B2
• Filed: 05/08/2003
• Issued: 05/08/2007
• Est. Priority Date: 05/08/2002
• Status: Active Grant

• Current Assignee: Seiko Epson Corporation (Seiko Group)
• Original Assignee: Seiko Epson Corporation (Seiko Group)
• Inventors: Takamatsu, Seiji
• Primary Examiner(s): Sircus; Brian
• Assistant Examiner(s): Benenson; Boris
• Application Number:
  US10/431,571
• Publication Number:
  US 20040021992A1
• Time in Patent Office: 1,461 Days
• Field of Search: 361/18, 363/21.1, 363/21.15
• US Class Current: 361/18
• CPC Class Codes: H02M 1/32 Means for protecting converters other than automatic disconnection
• CPC Class Codes: H02M 3/3385 with automatic control of output voltage or current H02M3/33561 takes precedence

Abstract

An overvoltage output protector is electrically connected to a constant-voltage switching power supply which includes a switching transistor converting a DC voltage obtained by smoothing an AC voltage supplied from an AC power source into a cyclic pulse signal. In the overvoltage output protector, an overvoltage monitor whether a potential of the cyclic pulse signal is a predetermined value or more. A deactivator turns off the switching transistor in a case where the overvoltage monitor judges that the potential of the cyclic pulse signal is the predetermined value or more.

Claims

1. A constant-voltage switching power supply, comprising:

a smoother, which converts an AC voltage supplied from an AC power source into a first DC voltage;

a switching transistor, which converts the first DC voltage into a first cyclic pulse signal;

a transformer, which voltage-transforms the first cyclic pulse signal to obtain a second cyclic pulse signal, the transformer comprising a first primary coil to which the first cyclic pulse signal is applied, a second primary coil at which a third cyclic pulse signal corresponding to the first cyclic pulse signal is induced;

an overvoltage monitor, which monitors a potential of the third cyclic pulse signal to judge whether a potential of the first cyclic pulse signal is a predetermined value or more, the overvoltage monitor comprising a Zener diode through which a backward current flows in a case where the potential of the third cyclic pulse signal reaches a Zener voltage of the Zener diode; and

a deactivator, which turns off the switching transistor when the backward current flows through the Zener diode;

wherein;

the overvoltage monitor comprises a transistor having a grounded base, an emitter to which the Zener diode is electrically connected, and a collector which is electrically connected to the deactivator; and

the deactivator turns off the switching transistor when the backward current turns on the transistor.

2. An electronic apparatus, connected to a constant-voltage DC power supply provided with a first controller which is externally controlled to vary a DC output voltage to be supplied, the electronic apparatus comprising:

an overvoltage detector, which detects whether a power voltage of the electronic apparatus which is generated from a rated output voltage in the constant-voltage DC power supply is a predetermined value or more, the overvoltage detector comprising a Zener diode; and

a second controller, which controls the first controller to vary the DC output voltage of the constant-voltage DC power supply in a case where the overvoltage detector detects that the power voltage is the predetermined value or more,

wherein the overvoltage detector deactivates the second controller when the power voltage reaches a Zener voltage of the Zener diode and maintains a deactivated state of the second controller even when the power voltage decreases lower than the Zener voltage of the Zener diode afler the second controller is once deactivated.

3. The electronic apparatus as set forth in claim 2, wherein:

the first controller comprises a control terminal which sets the output DC voltage at a first voltage in a case where the control terminal is short-circuited to a predetermined potential, and which sets the output DC voltage at a second voltage lower than the first voltage in a case where the control terminal is brought into a high-impedance condition;

the second controller comprises a first transistor having a collector electrically connected to the control terminal;

the first transistor is turned on in a case where the overvoltage detector detects that the internal potential is less than the predetermined value, so that the control terminal is short-circuited to the predetermined potential; and

the first transistor is turned off in a case where the overvoltage detector detects that the internal potential is the predetermined value or more, so that the control terminal is brought into the high-impedance condition.

4. The electronic apparatus as set forth in claim 3, further comprising an activator/deactivator, which turns on or off the first transistor to activate or deactivate the electronic apparatus, in a case where the overvoltage detector detects that the internal potential is less than the predetermined value.

5. The electronic apparatus as set forth in claim 4, wherein the activator/deactivator comprises a switch operable to cut off a base current of the first transistor.

6. The electronic apparatus as set forth in claim 5, wherein:

the activator/deactivator comprises a second transistor electrically connected to the first transistor and a third controller which turns on or off the second transistor; and

the base current of the first transistor is cut off in a case where the third controller turns off the second transistor.

7. The electronic apparatus as set forth in claim 6, wherein:

the activator/deactivator allows to flow the base current in a case where the switch is turned on, and cut off the base current in a case where the switch is turned off;

the third controller detects that an ON/OFF state of the switch; and

the third controller turns on the second transistor when an ON state of the switch is detected on under a condition that the base current is cut off, maintains an ON state of the second transistor even if an OFF state of the switch is detected, and turns off the second transistor in a case where the OFF state of the switch is again detected afler the ON state is again detected.

8. The electronic apparatus as set forth in claim 2, wherein the constant-voltage DC power supply is provided as an AC-DC converter which converts an AC voltage supplied from a domestic plug socket into the output DC voltage having a predetermined potential.

9. An overvoltage protector, provided within an electronic apparatus and connected to a constant-voltage DC power supply provided with a first controller which is externally controlled to vary a DC output voltage to be supplied, the overvoltage output protector comprising:

an overvoltage detector, which detects whether a power voltage of the electronic apparatus which is generated from a rated output voltage in the constant-voltage DC power supply is a predetermined value or more, the overvoltage detector comprising a Zener diode; and

a second controller, which controls the first controller to vary the DC output voltage of the constant-voltage DC power supply in a case where the overvoltage detector detects that the power voltage is the predetermined value or more,

wherein the overvoltage detector deactivates the second controller when the power voltage reaches a Zener voltage of the Zener diode and maintains a deactivated state of the second controller even when the power voltage decreases lower than the Zener voltage of the Zener diode afler the second controller is once deactivated.

10. The overvoltage protector as set forth in claim 9, wherein the constant-voltage DC power supply is provided within the electronic apparatus.