Soft start scheme for resonant converters having variable frequency control

• US 6,154,375 A
• Filed: 10/08/1999
• Issued: 11/28/2000
• Est. Priority Date: 10/08/1999
• Status: Expired due to Fees

• Current Assignee: Philips Electronics North America Corporation (Koninklijke Philips N.V.)
• Original Assignee: Philips Electronics North America Corporation (Koninklijke Philips N.V.)
• Inventors: Antheunes, Fernand Raphael Cornelis, Janczak, Jerzy, Majid, Naveed
• Primary Examiner(s): BERHANE, ADOLF D
• Application Number:
  US09/415,160
• Time in Patent Office: 417 Days
• Field of Search: 363/16, 363/17, 363/20, 363/21, 363/56, 363/49, 363/58, 363/95, 363/97, 363/98, 363/131, 323/901, 323/902
• US Class Current: 363/16
• CPC Class Codes: H02M 1/36 Means for starting or stopping converters
• CPC Class Codes: H02M 3/3376 with automatic control of output voltage or current
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes
• CPC Class Codes: Y10S 323/901 Starting circuits

Abstract

A resonant mode power supply includes a soft-start circuit for, at start-up, for sweeping the frequency of the power supply from a maximum value to a minimum value. The soft-start circuit performs this frequency sweep using the function

F(x)=1-e.sup.-t

Using this function, change in the frequency, and correspondingly the increase of the output voltage decreases over time, thereby allowing a feedback circuit of the resonant mode power supply to respond to the level of the output voltage and to begin regulation when the output voltage achieves the desired regulation level. As such, overshoot of the output voltage with respect to the regulation level is lessened, if not eliminated.

Claims

1. A resonant mode power supply comprising:

means for generating a d.c. supply voltage having a first terminal and a second terminal;

a first switching element having a first terminal coupled to said first terminal of said generating means, and a second terminal connected to a supply node;

a second switching element having a first terminal coupled to said supply node and a second terminal coupled to said second terminal of said generating means;

a transforming comprising a primary winding having a first end, and a second end connected to the second terminal of said second switching element, and a secondary winding having a first end, a second end and a central tap connected to ground;

a capacitor for coupling the first end of the primary winding to the supply node;

a sensing resistor for connecting the second end of the primary winding to said second terminal of said generating means;

a first diode for connecting the first end of said secondary winding to an output node;

a second diode for connecting the second end of said secondary winding to said output node; and

a controller connected to control inputs of said first and second switching elements for alternatively turning on and off said first and second switching elements for inducing an oscillation in an oscillating circuit including at least said capacitor and said primary winding of said transformer, wherein a frequency of said oscillation is inversely proportional to an amount of power being delivered by said resonant mode power supply, characterized in that said resonant mode power supply further comprises a soft-start circuit having means for sweeping the frequency of oscillation from a maximum value to a minimum value, said sweeping means using the function

space="preserve" listing-type="equation">f(x)=1-e.sup.-t

whereby the rate of decrease of the frequency reduces as the amount of power delivered to the output increases and as the output voltage approaches a regulation value.

2. The resonant mode power supply as claimed in claim 1, wherein said soft-start circuit compensates for variations in a value of said d.c. supply voltage, said sweeping means using the function

3. The resonant mode power supply as claimed in claim 2, wherein said controller comprises a voltage-controlled oscillator for controlling the switching of said first and second switching elements, and said sweeping means comprises:

an input for receiving a voltage Vsup in accordance with the following formula

space="preserve" listing-type="equation">Vsup=1/Vin,

Said input being coupled to an output node; and

a parallel combination of a resistor and a capacitor coupling said output node to ground, whereby said output node provides a signal having the function

space="preserve" listing-type="equation">f(x)=1-e.sup.-1/Vin,

to an input of said voltage-controlled oscillator.

4. The resonant mode power supply as claimed in claim 1, wherein said soft-start circuit monitors the output voltage and regulates the sweeping means based on the output voltage.

5. The resonant mode power supply as claimed in claim 4, wherein said transformer further comprises an auxiliary winding on which the voltage on said secondary winding is impressed, and said controller comprises an input coupled to said auxiliary winding and means for regulating the oscillation frequency based on a voltage on said auxiliary winding, and wherein said sweeping means comprises:

a series arrangement of a capacitor and a resistor coupling the auxiliary voltage to ground; and

a comparator having a first input coupled to a junction between said capacitor and said resistor, a second input coupled to receive a threshold voltage, and an output coupled to an input of said regulating means.