Capacitor charging methods and apparatus

• US 7,787,262 B2
• Filed: 12/19/2006
• Issued: 08/31/2010
• Est. Priority Date: 05/09/2005
• Status: Active Grant

• Current Assignee: Allegro Microsystems Incorporated (Sanken Electric Company Limited)
• Original Assignee: Allegro Microsystems Incorporated (Sanken Electric Company Limited)
• Inventors: Humenick, John, Tsai, Tseng-Nan, Lee, Nai-Chi, Mangtani, Vijay
• Primary Examiner(s): Vu; Bao Q
• Assistant Examiner(s): Tran; Nguyen
• Application Number:
  US11/612,564
• Publication Number:
  US 20070103943A1
• Time in Patent Office: 1,351 Days
• Field of Search: 363/18, 363/19, 363/20, 363/21.12, 363/21.13, 363/56.09
• US Class Current: 363/18
• CPC Class Codes: H02M 3/33507 with automatic control of the output voltage or current, e.g. flyback converters H02M3/33561, H02M3/33569 take precedence

Abstract

A capacitor charging circuit is provided with a primary side output voltage sensing circuit including an RC network having an RC time constant with a predetermined relationship to the RC time constant of the output capacitor. Once the capacitor voltage reaches a fully charged level, the charging mode is terminated. The output voltage is continuously detected by measuring the voltage across the primary side RC network that decays at a predetermined rate with respect to the output capacitor and the charging mode is commenced once the RC voltage falls to a predetermined level. According to a further aspect of the invention, a switch control circuit in a flyback converter controls the switch off time in response to detection of a change in the slope polarity of the voltage at a terminal of the switch.

Claims

1. A flyback converter comprising:

a transformer having a primary winding and a secondary winding;

a switch having a terminal coupled to the primary winding; and

a switch control circuit having a first circuit portion for controlling the on time of the switch and a second circuit portion for controlling the off time of the switch, wherein the second circuit portion comprises a slope detection circuit to detect a change of the slope of the voltage at the switch terminal from a negative slope, wherein the slope detection circuit comprises;

a capacitor having a first terminal coupled to the switch terminal and a second terminal;

a resistor coupled to the second terminal of the capacitor; and

a comparator coupled to the second terminal of the capacitor and providing a comparator output signal that transitions from a first logic level to a second logic level when the slope of the voltage at the switch terminal changes from a negative slope.

2. The converter of claim 1 wherein the slope detection circuit further comprises:

a sensing switch coupled to the switch terminal and having a control terminal coupled to the second terminal of the capacitor; and

a clamp coupled to the control terminal.

3. The converter of claim 2 wherein the switch is comprised of a plurality of switches and wherein the sensing switch is one of the plurality of switches.

4. The converter of claim 1 further comprising a clamp coupled between the second terminal of the capacitor and ground.

5. The converter of claim 1 further comprising a flip-flop having a set input responsive to the comparator output signal and an output at which a switch control signal is provided, wherein the second circuit portion of the switch control circuit comprises:

a second resistor coupled to the switch; and

a second comparator having a first input coupled to the second resistor, a second input responsive to a reference voltage, and providing an output signal to a reset input of the flip-flop.

6. In a flyback converter having a transformer with a primary winding coupled to a terminal of a switch and a secondary winding, a method comprising:

detecting of a change in the slope of a voltage at the terminal of the switch from a negative slope; and

turning the switch on in response to the detection, wherein detecting comprises;

providing a capacitor having a first terminal coupled to the switch terminal and having a second terminal;

providing a resistor coupled to the second terminal of the capacitor; and

comparing the voltage at the second terminal of the capacitor to a reference voltage to detect a change in the slope of the voltage at the switch terminal from a negative slope.

7. The method of claim 6 wherein detecting further comprises:

providing a sense switch having a first terminal coupled to the switch terminal and having a second, control terminal, coupled to the second terminal of the capacitor; and

clamping the voltage at the control terminal.

8. The method of claim 6 further comprising clamping the voltage at the second terminal of the capacitor.

9. The method of claim 6 further comprising:

measuring a current in the primary winding to determine when the primary winding current exceeds a predetermined level; and

turning the switch off in response to the determination.

10. A flyback converter comprising:

a transformer having a primary winding and a secondary winding;

a switch having a terminal coupled to the primary winding; and

a switch control circuit having a first circuit portion for controlling the on time of the switch and a second circuit portion for controlling the off time of the switch, wherein the second circuit portion comprises a slope detection circuit to detect a change of the slope of the voltage at the switch terminal from a negative slope, wherein the slope detection circuit comprises;

a sensing switch coupled to the switch terminal and having a control terminal;

a clamp coupled to the control terminal;

a resistor coupled to the control terminal; and

a comparator coupled to the control terminal and providing a comparator output signal that transitions from a first logic level to a second logic level when the slope of the voltage at the switch terminal changes from a negative slope.

11. The converter of claim 10 wherein the switch is comprised of a plurality of switches and wherein the sensing switch is one of the plurality of switches.