US-12627237-B2 - Bootstrap recharge system in dual-switch flyback converters

Abstract

According to an embodiment, a converter includes a bootstrap capacitor, a high-side switch, a low-side switch, an auxiliary switch, and a controller. The bootstrap capacitor has a first terminal coupled to a floating ground node. The high-side switch has a source terminal coupled to the bootstrap capacitor through the floating ground node. The auxiliary switch has a drain terminal coupled to the bootstrap capacitor through the floating ground node. The controller provides a first control signal to a control terminal of the high-side switch, provides a second control signal to a control terminal of the low-side switch, and provides a third control signal to a control terminal of the auxiliary switch. The third control signal is based on a condition associated with the converter after the first control signal and the second control signal deactivate the high-side switch and the low-side switch respectively.

Inventors

• Claudio Adragna

Assignees

• STMICROELECTRONICS INTERNATIONAL N.V.

Dates

Publication Date

20260512

Application Date

20231103

Claims (20)

1. 1 . A converter, comprising: a transformer having a primary side and a secondary side; a bootstrap capacitor having a first terminal coupled to a floating ground node; a high-side switch having a source terminal coupled to the bootstrap capacitor through the floating ground node and a drain terminal coupled to the primary side of the transformer; a low-side switch having a source terminal coupled to ground and a drain terminal coupled to the primary side of the transformer; an auxiliary switch having a drain terminal coupled to the bootstrap capacitor through the floating ground node; and a controller configured to: provide a first control signal to a control terminal of the high-side switch, provide a second control signal to a control terminal of the low-side switch, and provide a third control signal to a control terminal of the auxiliary switch, wherein the third control signal activates the auxiliary switch for a current cycle based on a condition associated with the converter after the first control signal and the second control signal deactivate the high-side switch and the low-side switch, respectively.
2. 2 . The converter of claim 1 , further comprising a control logic circuit configured to monitor a voltage across the bootstrap capacitor.
3. 3 . The converter of claim 2 , wherein the third control signal is set to activate the auxiliary switch for the current cycle in response to detecting, by the control logic circuit, that the voltage across the bootstrap capacitor is less than a threshold.
4. 4 . The converter of claim 2 , wherein the third control signal is set to keep the auxiliary switch deactivated for the current cycle in response to detecting, by the control logic circuit, that the voltage across the bootstrap capacitor is greater than a threshold.
5. 5 . The converter of claim 1 , further comprising a control logic circuit configured to monitor a voltage at the floating ground node for a duration within a period immediately after the high-side switch is turned OFF.
6. 6 . The converter of claim 5 , wherein the third control signal is set to activate the auxiliary switch for the current cycle in response to detecting, by the control logic circuit, that the voltage at the floating ground node for the duration within the period immediately after the high-side switch is turned OFF is greater than a threshold.
7. 7 . The converter of claim 5 , wherein the third control signal is set to keep the auxiliary switch deactivated for the current cycle in response to detecting, by the control logic circuit, that the voltage at the floating ground node for the duration within the period immediately after the high-side switch is turned OFF is less than a threshold.
8. 8 . A method of operating a converter, the method comprising: generating, by a controller, a first control signal to a control terminal of a high-side switch of the converter, the high-side switch having a source terminal coupled to a bootstrap capacitor through a floating ground node and a drain terminal coupled to a primary side of a transformer; generating, by the controller, a second control signal to a control terminal of a low-side switch of the converter, the low-side switch having a source terminal coupled to ground and a drain terminal coupled to the primary side of the transformer; and determining, by the controller, whether to activate an auxiliary switch for a current cycle using a third control signal, the auxiliary switch having a drain terminal coupled to the bootstrap capacitor through the floating ground node, wherein the determining is based on a condition associated with the converter after the first control signal and the second control signal deactivate the high-side switch and the low-side switch, respectively.
9. 9 . The method of claim 8 , further comprising monitoring, by a control logic circuit, a voltage across the bootstrap capacitor.
10. 10 . The method of claim 9 , wherein the third control signal is set to activate the auxiliary switch for the current cycle in response to detecting, by the control logic circuit, that the voltage across the bootstrap capacitor is less than a threshold.
11. 11 . The method of claim 10 , wherein the third control signal is set to keep the auxiliary switch deactivated for the current cycle in response to detecting, by the control logic circuit, that the voltage across the bootstrap capacitor is greater than the threshold.
12. 12 . The method of claim 8 , further comprising monitoring, by a control logic circuit, a voltage at the floating ground node for a duration within a period immediately after the high-side switch is turned OFF.
13. 13 . The method of claim 12 , wherein the third control signal is set to activate the auxiliary switch for the current cycle in response to detecting, by the control logic circuit, that the voltage at the floating ground node for the duration within the period immediately after the high-side switch is turned OFF is greater than a threshold.
14. 14 . The method of claim 12 , wherein the third control signal is set to keep the auxiliary switch deactivated for the current cycle in response to detecting, by the control logic circuit, that the voltage at the floating ground node for the duration within the period immediately after the high-side switch is turned OFF is less than a threshold.
15. 15 . A device comprising a converter, the converter comprising: a transformer having a primary side and a secondary side; a bootstrap capacitor having a first terminal coupled to a floating ground node; a high-side switch having a source terminal coupled to the bootstrap capacitor through the floating ground node and a drain terminal coupled to the primary side of the transformer; a low-side switch having a source terminal coupled to ground and a drain terminal coupled to the primary side of the transformer; an auxiliary switch having a drain terminal coupled to the bootstrap capacitor through the floating ground node; and a controller configured to: provide a first control signal to a control terminal of the high-side switch, provide a second control signal to a control terminal of the low-side switch, and provide a third control signal to a control terminal of the auxiliary switch, wherein the third control signal activates the auxiliary switch for a current cycle based on a condition associated with the converter after the first control signal and the second control signal deactivate the high-side switch and the low-side switch, respectively.
16. 16 . The device of claim 15 , wherein the converter further comprises a control logic circuit configured to monitor a voltage across the bootstrap capacitor.
17. 17 . The device of claim 16 , wherein the third control signal is set to activate the auxiliary switch for the current cycle in response to detecting, by the control logic circuit, that the voltage across the bootstrap capacitor is less than a threshold, and wherein the third control signal is set to keep the auxiliary switch deactivated for the current cycle in response to detecting, by the control logic circuit, that the voltage across the bootstrap capacitor is greater than the threshold.
18. 18 . The device of claim 15 , wherein the converter further comprises a control logic circuit configured to monitor a voltage at the floating ground node for a duration within a period immediately after the high-side switch is turned OFF.
19. 19 . The device of claim 18 , wherein the third control signal is set to activate the auxiliary switch for the current cycle in response to detecting, by the control logic circuit, that the voltage at the floating ground node for the duration within the period immediately after the high-side switch is turned OFF is greater than a threshold.
20. 20 . The device of claim 19 , wherein the third control signal is set to keep the auxiliary switch deactivated for the current cycle in response to detecting, by the control logic circuit, that the voltage at the floating ground node for the duration within the period immediately after the high-side switch is turned OFF is less than the threshold.

Description

TECHNICAL FIELD The present disclosure generally relates to electric circuits and, in particular embodiments, to a bootstrap recharge system in dual-switch flyback converters. BACKGROUND The dual-switch flyback converter is a design topology used in power electronics. However, just like any other electronic system, it grapples with specific technical challenges that need addressing for optimal performance. One of the prominent issues at the forefront is ensuring that the bootstrap capacitor maintains an adequate charge under all operating conditions, particularly to drive the high-side switch of the converter. To initiate the charge of the bootstrap capacitor, the low-side switch can be turned ON for a specific duration—an approach mirroring techniques used in half-bridge structures. By activating low-side switch, the potential at the floating ground (FGND) node is essentially zero. Facilitating the charging of the bootstrap capacitor through the low-side switch and the primary winding of the converter's transformer. As the converter operates and switches, the recharging of the bootstrap capacitor comes into play. The presence of substantial energy from the leakage inductance of the transformer can draw the potential at the floating ground (FGND) close to zero resulting in the recharging of the bootstrap capacitor. This is further amplified when the recirculation diodes of the converter are activated during the OFF states of the high-side and low-side switches. The voltage shift across the primary winding, upon the deactivation of high-side and low-side switches, is equivalent to the sum of the reflected voltage (VR) and the voltage spike due to leakage inductance (Vx). However, a potential challenge arises if the combined value of the reflected voltage (VR) and the voltage spike due to leakage inductance (Vx) falls short of the input voltage (VIN). Under such circumstances, the potential at the floating ground (FGND) node doesn't reach zero. The integrity of the converter's operation hinges on the adequate recharging of bootstrap capacitor. When the energy from the leakage inductance is insufficient to drive the potential at the floating ground (FGND) node close to zero, the bootstrap capacitor cannot recharge. Over time, this culminates in an inability to activate the high-side switch, derailing the operation of the converter. SUMMARY Technical advantages are generally achieved by embodiments of this disclosure, which describe a bootstrap recharge system in dual-switch flyback converters. A first aspect relates to a converter that includes a bootstrap capacitor, a high-side switch, a low-side switch, an auxiliary switch, and a controller. The bootstrap capacitor has a first terminal coupled to a floating ground node. The high-side switch has a source terminal coupled to the bootstrap capacitor through the floating ground node. The auxiliary switch has a drain terminal coupled to the bootstrap capacitor through the floating ground node. The controller is configured to provide a first control signal to a control terminal of the high-side switch, provide a second control signal to a control terminal of the low-side switch, and provide a third control signal to a control terminal of the auxiliary switch. The third control signal to activate the auxiliary switch for a current cycle is based on a condition associated with the converter after the first control signal and the second control signal deactivate the high-side switch and the low-side switch respectively. A second aspect relates to a method of operating a converter. The method includes generating, by a controller, a first control signal to a control terminal of a high-side switch of the converter, the high-side switch having a source terminal coupled to a bootstrap capacitor through a floating ground node; generating, by the controller, a second control signal to a control terminal of a low-side switch of the converter; and determining, by the controller, whether to activate an auxiliary switch for a current cycle using a third control signal, the auxiliary switch having a drain terminal coupled to the bootstrap capacitor through the floating ground node. The determining is based on a condition associated with the converter after the first control signal and the second control signal deactivate the high-side switch and the low-side switch respectively. A third aspect relates to a device with a converter. The converter includes a bootstrap capacitor, a high-side switch, a low-side switch, an auxiliary switch, and a controller. The bootstrap capacitor has a first terminal coupled to a floating ground node. The high-side switch has a source terminal coupled to the bootstrap capacitor through the floating ground node. The auxiliary switch has a drain terminal coupled to the bootstrap capacitor through the floating ground node. The controller is configured to provide a first control signal to a control terminal of the high-side switch, provide a second cont