Boot strap circuit for power up control of power supplies

• US 5,459,652 A
• Filed: 01/28/1994
• Issued: 10/17/1995
• Est. Priority Date: 01/28/1994
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Development Company, L.P. (HP Inc.)
• Original Assignee: Compaq Computer Corporation (HP Inc.)
• Inventors: Faulk, Richard A.
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): Riley, Shawn
• Application Number:
  US08/188,108
• Time in Patent Office: 627 Days
• Field of Search: 363/20, 363/21, 363/41, 363/49, 363/131, 363/141, 363/95, 363/97, 323/907, 323/901
• US Class Current: 363/49
• CPC Class Codes: H02M 1/36 Means for starting or stopping converters
• CPC Class Codes: Y10S 323/901 Starting circuits

Abstract

A boot strap control circuit including a depletion mode NMOS transistor to effectively switch a boot strap bleed resistor out of the circuit after power up is achieved. The NMOS transistor is initially turned fully on to allow current to flow through the bleed resistor to a pulse width modulation circuit (PWM) upon power up and to allow early control by the PWM. When the PWM reaches operating power, it asserts its reference voltage output high, activating a transistor switch to turn the NMOS transistor fully off. A capacitor maintains power to the PWM until an auxiliary winding of the power transformer develops sufficient voltage to operate the PWM. In the event of failure of the auxiliary winding, the NMOS transistor operates discretely, turning on and off at a low duty cycle, rather than operating in the linear mode. In this manner, the NMOS transistor and bleed resistor assure low power during normal operation and safe operation at all times.

Claims

1. A boot strap control circuit for providing initial power from a DC input signal to the power input of a pulse width modulation circuit of a switching power supply, wherein the pulse width modulation circuit includes an output for asserting a reference voltage after receiving sufficient operating voltage, said boot strap control circuit comprising:

a bleed resistor for coupling in series between the DC input signal and the power input of the pulse width modulation circuit, said bleed resistor having a resistance to allow sufficient current from the DC input signal to power the pulse width modulation circuit;

a current pass device having a control terminal and having a current path for coupling in series between said bleed resistor and the power input of the pulse width modulation circuit, wherein said current pass device allows current to flow through said bleed resistor to the pulse width modulation circuit when the voltage of said control terminal is greater than or equal to the voltage of said current path, and does not allow such current flow when the voltage of said control terminal is less than the voltage of said current path;

a capacitor for coupling between the power input of the pulse width modulation circuit and ground; and

a switch coupled between said control terminal of said current pass device and ground and for receiving the reference voltage from the pulse width modulation circuit, wherein said switch is off, thereby turning said current pass device on, while the reference voltage is not asserted, and wherein said switch is turned on, thereby turning off said current pass device, when the reference voltage is asserted.

2. The boot strap control circuit of claim 1, wherein said current pass device comprises an N-type depletion-mode MOSFET.

3. The boot strap control circuit of claim 2, further comprising:

a resistor coupled between the gate and source of said N-type depletion-mode MOSFET.

4. The boot strap control circuit of claim 1, wherein said switch comprises a MOSFET transistor.

5. The boot strap control circuit of claim 1, wherein the power supply includes an auxiliary circuit coupled to the power input of the pulse width modulation circuit for supplying power to the pulse width modulation circuit after it begins operating.

6. The boot strap control circuit of claim 5, wherein the switching power supply includes a power transformer and wherein said auxiliary circuit includes an auxiliary winding provided on said power transformer.

7. The boot strap control circuit of claim 6, wherein said bleed resistor has a resistance of approximately 100 Kohms.