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
First Claim
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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.

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