Circuit and method for predicting a valley timing for a voltage across a switching device

• US 8,279,634 B2
• Filed: 01/07/2011
• Issued: 10/02/2012
• Est. Priority Date: 11/24/2006
• Status: Active Grant

• Current Assignee: Richtek Technology Corporation (MediaTek, Inc.)
• Original Assignee: Richtek Technology Corporation (MediaTek, Inc.)
• Inventors: Lin, Tzu-Chen
• Primary Examiner(s): Tran, Nguyen
• Application Number:
  US12/986,648
• Publication Number:
  US 20110095735A1
• Time in Patent Office: 634 Days
• Field of Search: 363 17- 20, 363/21.04, 363/21.05, 363/21.07, 363/21.08, 363 2111- 21, 363/21.16, 363/49, 363/78
• US Class Current: 363/21.05
• CPC Class Codes: H02M 3/01 Resonant DC/DC converters
• CPC Class Codes: H02M 3/33592 having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
• 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

Abstract

Disclosed are circuits and methods for use in a control circuit of a switching mode power supply for turning on a switching device in the switching mode power supply when the voltage across the switching device is at a minimum. A voltage detector is provided for detecting the voltage across the switching device to produce a detection voltage which is a function of the voltage across the switching device, and circuit arrangement is used to predict a valley timing for the voltage across the switching device by evaluating the time period that the detection voltage falls down from a first threshold to a second threshold.

Claims

1. A circuit for predicting a valley timing for a voltage across a switching device, comprising:

a voltage detector for detecting the voltage across the switching device to produce a detection voltage as a function of the voltage detected across the switching device;

a first comparator for comparing the detection voltage with a first threshold to produce a first comparison signal;

a second comparator for comparing the detection voltage with a second threshold to produce a second comparison signal;

a logic circuit for producing a first logic signal and a second logic signal according to the first and second comparison signals; and

,

a timer for predicting the timing of a valley for the detection voltage according to the first and second logic signals, the timer thereby determining a valley signal corresponding to the detection voltage valley used to trigger turn on of the switching device, the valley signal being thereby generated responsive to the timing of detection voltage change between the first and second thresholds, said timer including a capacitor for providing a timer voltage, a first current source controlled by the first logic signal for determining a first current to charge the capacitor, a second current source controlled by the second logic signal for determining a second current to discharge the capacitor and a third comparator for comparing the timer voltage with a third threshold to produce the valley signal.

2. The circuit of claim 1, wherein the logic circuit comprises:

a first negative-edge triggering circuit for triggering a setting signal in response to the first comparison signal;

a second negative-edge triggering circuit for triggering a resetting signal in response to the second comparison signal; and

a flip-flop for determining the first and second logic signals according to the setting and resetting signals.

3. The circuit of claim 1, further comprising a blanking signal for selectively blanking the valley signal.

4. The circuit of claim 3, wherein the logic circuit further provides a third logic signal and a fourth logic signal according to the first and second comparison signals.

5. The circuit of claim 1, further comprising a sample and hold circuit for sampling the detection voltage to produce the first threshold.

6. A method for predicting valley timing for a switching device, comprising the steps of:

(a) detecting the voltage across the switching device for producing a detection voltage as a function of the voltage detected across the switching device; and

,

(b) predicting the timing of a valley for the detection voltage by evaluating a first time period wherein the detection voltage goes from a first threshold to a second threshold, to generate a valley signal corresponding to the detection voltage valley used to trigger turn on of the switching device, the valley signal being thereby generated responsive to the timing of detection voltage change between the first and second thresholds where the step of predicting the timing of a valley further includes establishing a capacitor for providing a timer voltage, controlling a first current source by a first logic signal for determining a first current to charge a capacitor, controlling a second current source by a second logic signal for determining a second current to discharge the capacitor, and providing a comparator for comparing the timer voltage with a third threshold to produce the valley signal.

7. The method of claim 6, wherein the step (b) comprises the steps of:

determining the first current and the second current according to the first time period and a second time period that the detection voltage goes from the second threshold to a valley;

charging a capacitor by the first current when the detection voltage is lower than the first threshold; and

stopping charging the capacitor by the first current and discharging the capacitor by the second current when the detection voltage is lower than the second threshold, and triggering the valley signal when the capacitor is discharged completely.

8. The method of claim 7, further comprising the step of selectively blanking the valley signal.