High efficiency cascade MOSFET flyback converter for generating supply voltage for power converter integrated circuit

Abstract

A system is provided that includes a diode bridge for rectifying an AC input voltage. A main switching power converter converts a rectified input voltage of the diode bridge to an output voltage. The flyback converter is coupled to the pre-bridge node or the post-bridge node to receive an input voltage that is converted to a supply voltage for the controller integrated circuit. The controller integrated circuit regulates both the output voltage and the supply voltage.

Description

Technical Field

[0001] This application relates to switching power converters, and more specifically, to a cascaded MOSFET flyback converter for generating the supply voltage of a power converter integrated circuit. Background Technology

[0002] In a switching power converter, one or more controller integrated circuits control the cycling of power switching transistors to regulate the output voltage. These controller integrated circuits require their own supply voltage to control the power switching transistors. Therefore, a power generation circuit is needed to generate the supply voltage in a switching power converter. Designing a suitable power generation circuit presents several challenges. For example, the power generation circuit must regulate the supply voltage and keep it within an operating range to ensure proper functioning of the integrated circuits. Furthermore, the power generation circuit should be highly efficient, especially in applications requiring minimal losses under extremely low load conditions. The power generation circuit should meet these requirements at low cost and low specifications. Additionally, the power generation circuit should operate noiselessly throughout its entire operating range and maintain high reliability under customer-specified ambient temperature conditions. Summary of the Invention

[0003] According to one aspect of this disclosure, a flyback converter is provided for generating a supply voltage for a controller integrated circuit for a main switching power converter. The flyback converter includes: a transformer including a primary winding and a secondary winding; a first transistor having a drain coupled to the primary winding; a supply voltage capacitor having a positive terminal coupled to the secondary winding and a negative terminal coupled to ground; and a second transistor having a drain coupled to the source of the first transistor and a drain coupled to ground, wherein the controller integrated circuit includes a supply voltage terminal coupled to the drain of the first transistor and the positive terminal of the supply voltage capacitor.

[0004] According to another aspect of this disclosure, a system is provided comprising: a diode bridge including a pair of input terminals and a pair of output terminals; a main switching power converter coupled to the pair of output terminals and configured to convert a rectified input voltage into an output voltage; a controller integrated circuit configured to provide a modulation signal to the main switching power converter to regulate the output voltage; a first diode having an anode coupled to a positive input terminal of the pair of input terminals; a second diode having an anode coupled to a negative input terminal of the pair of input terminals; and a flyback converter including a transformer having a primary winding and a secondary winding, the primary winding being coupled to the cathodes of the first diode and the second diode, and the secondary winding being coupled to a supply voltage capacitor configured to store a supply voltage for the controller integrated circuit.

[0005] According to another aspect of this disclosure, a system is provided comprising: a diode bridge including a pair of input terminals and a pair of output terminals; an input voltage capacitor coupled between the pair of output terminals; a main switching power converter coupled to the pair of output terminals and configured to convert a rectified input voltage stored by the input voltage capacitor into an output voltage; a controller integrated circuit configured to provide a modulation signal to the main switching power converter to regulate the output voltage; and a flyback converter including a transformer having a primary winding and a secondary winding, the input terminal of the primary winding coupled to a positive output terminal of the pair of output terminals, the secondary winding coupled to a supply voltage capacitor configured to store a supply voltage for the controller integrated circuit.

[0006] These and other aspects of the invention will be more fully understood after reading the following detailed description. Other aspects, features, and embodiments will become apparent to those skilled in the art upon reading the following description of specific exemplary embodiments in conjunction with the accompanying drawings. While features relating to certain embodiments and figures may be discussed below, all embodiments may include one or more advantageous features discussed herein. In other words, while one or more embodiments may be discussed as having certain advantageous features, one or more such features may also be used according to the various embodiments discussed herein. Similarly, while exemplary embodiments may be discussed below as embodiments of devices, systems, or methods, it should be understood that these exemplary embodiments can be implemented in various devices, systems, and methods. Attached Figure Description

[0007] Figure 1A system according to one aspect of this disclosure is shown, the system including a main switching power converter controlled by a controller, the supply voltage of which is generated by a VCC charging circuit having bridge post-coupling.

[0008] Figure 2 A system according to one aspect of the present disclosure is shown, the system including a main switching power converter controlled by a controller, the supply voltage of which is generated by a VCC charging circuit having bridge pre-coupling.

[0009] Figure 3 One aspect of this disclosure is shown in more detail. Figure 1 Post-bridge coupling of the system.

[0010] Figure 4 One aspect of this disclosure is shown in more detail. Figure 2 Pre-bridge coupling of the system.

[0011] Figure 5A The starting current flowing through a system with pre-bridge coupling according to one aspect of this disclosure is shown.

[0012] Figure 5B The primary winding current flowing during normal operation of a system having pre-bridge coupling, according to one aspect of this disclosure, is shown.

[0013] Figure 5C The secondary winding current flowing during normal operation of a system with pre-bridge coupling, according to one aspect of this disclosure, is shown.

[0014] Figure 6 It shows the use of Figure 5B and Figure 5C Some waveforms of the system.

[0015] Figure 7 The discharge current of an X capacitor flowing through a system having pre-bridge coupling is shown according to one aspect of this disclosure.

[0016] Figure 8 A system with post-bridge coupling according to one aspect of this disclosure is shown.

[0017] Figure 9 The rectified AC line voltage and PWM_Vcc modulation signal for a system including valley stop mode and peak stop mode are shown according to one aspect of this disclosure.

[0018] The embodiments and advantages of this disclosure can be best understood by referring to the following detailed description. It should be understood that the same reference numerals are used to identify the same elements shown in one or more figures. Detailed Implementation

[0019] Given the stringent requirements for a suitable IC supply voltage (VCC) generation circuit, it is not surprising that conventional VCC charging circuits suffer from numerous drawbacks. For example, the VCC charging circuit in a flyback converter typically includes a startup metal-oxide-semiconductor field-effect transistor (MOSFET) coupled to the rectified input voltage rail. During power-up of the flyback converter, the startup MOSFET turns on to generate the supply voltage for the flyback converter integrated circuit (IC) controller. As the flyback converter transitions to normal operation, an auxiliary winding begins to generate voltage. Once this voltage is sufficient, it bypasses the startup MOSFET, and the auxiliary winding functions as part of the VCC charging circuit to generate the IC supply voltage. However, the voltage generated by the auxiliary winding is proportional to the output voltage of the flyback converter, as determined by the transformer turns ratio. If the flyback converter has a wide output voltage range, the IC supply voltage may be forced out of its operating range. To keep the IC supply voltage within its operating range, the VCC charging circuit can include a low-dropout regulator (LDO) that steps down the auxiliary winding voltage to generate the IC supply voltage. In this way, despite the higher output voltage, the IC supply voltage can still be maintained within the desired operating range. However, adding an LDO to the VCC charging circuit increases cost and circuit size, while also reducing efficiency.

[0020] The auxiliary winding also affects the coupling between the primary and secondary windings of the flyback transformer and increases leakage inductance. Therefore, the addition of the auxiliary winding reduces the efficiency of the flyback converter and increases oscillations and voltage stress on its components. Furthermore, since the IC supply voltage depends on the auxiliary winding voltage, which in turn depends on the output voltage, the operation of the VCC charging circuit depends on the main circuit output voltage. This dependence on the output voltage significantly limits the potential for efficiency optimization in the generation of the IC supply voltage. Finally, the auxiliary winding increases cost and adds complexity to the transformer design.

[0021] To address these issues, a VCC charging circuit is provided that generates the IC supply voltage without the need for an auxiliary winding. The VCC charging circuit generates the supply voltage for the controller IC. The controller IC can then control the cycling of one or more power switching transistors in a switching power converter stage. The switching power converter stage (also referred to herein as the main circuit) can employ any suitable topology, including flyback, buck, boost, buck / boost, and other switching power converter configurations. The switching power converter stage converts the rectified input voltage (or rectified input current) from the diode bridge, which rectifies the AC input voltage. The controller IC modulates the cycling of one or more power switches to produce the desired output voltage (or output current). In the following discussion, it will be assumed that the controller IC uses pulse width modulation (PWM) to modulate the power switch cycling; however, it is understood that pulse frequency modulation (PFM) or pulse train modulation may also be used in alternative embodiments.

[0022] To generate the IC supply voltage, the VCC charging circuit disclosed herein uses a flyback topology, which is separate from the topology used in the main circuit. Therefore, the VCC charging circuit includes a transformer with primary and secondary windings. The primary winding is coupled to the input voltage node. The input voltage node can be located before or after the diode bridge, which also supplies power to the main circuit. The before-and-after connection points for the input voltage node each have their advantages and disadvantages, which will be discussed further herein. Figure 1 An example system 100 with a VCC charging circuit 125 is shown, where the VCC charging input voltage is taken from the bridge of a diode bridge 110. The VCC charging circuit 125 provides a supply voltage (VCC) to a controller IC 130, which controls the cycling of one or more power switches in a main switching power converter 135 via a pulse width modulation control signal (PWM_main) to regulate the output voltage (Vout). To provide the VCC charging input voltage to the VCC charging circuit 125, the diode bridge 110 rectifies the AC input voltage (VAC), which is filtered by an electromagnetic interference filter (EMI) 105 to prevent switching noise from the main circuit 135 from affecting the AC input voltage. The diode bridge 110 can be charged to a relatively high voltage using the AC input voltage. If the main switching power converter 135 is disconnected from the AC input voltage, or if an AC input voltage failure occurs, the charge stored on the X capacitor Cx will dissipate as further described herein.

[0023] As previously described, the main switching power converter 135 can be a flyback converter, a buck converter, a buck / boost converter, or some other suitable switching power converter. The main switching power converter 135 receives a rectified input voltage Vin on voltage rail 115, which is charged by diode bridge 110. The main switching power converter 135 also receives ground (Vss) from ground rail 140, which is coupled between diode bridge 110 and main circuit 135. VCC charging circuit 125 is also coupled to voltage rail 115 to receive the VCC charging input voltage (the ground connection between VCC charging circuit 125 and ground rail 140 is not shown for clarity). Therefore, it can be understood that the input voltage node for VCC charging circuit 125 is voltage rail 115, which is a post-bridge coupled voltage. VCC charging circuit 125 is also coupled to voltage rail 115 to sense the AC input voltage VAC via VAC sensing circuit 120. Compared to front-bridge coupling, this back-bridge topology or coupling, which both receives the input voltage and senses the AC input voltage, offers several advantages. For example, the VCC charging circuit 125 can be directly coupled to the voltage rail 115 without the need for additional high-voltage diodes, reducing manufacturing costs and system size. Furthermore, sampling on the DC side (back-bridge side) of the diode bridge 110 facilitates power factor correction (PFC) in the VCC charging circuit 125, improving the overall efficiency and stability of the system 100. Effective PFC reduces harmonic components in the input current conducted from the diode bridge 110 to the VCC charging circuit 125, thereby minimizing contamination of the AC power supply trunk (not shown) providing the AC input voltage. Additionally, back-bridge coupling allows the VCC charging circuit 125 to bypass switching frequency noise, which will be further explained herein. The front-bridge implementation will now be discussed.

[0024] Figure 2 An example system 200 with a VCC charging circuit 225 is shown, where the VCC charging input voltage is taken before the diode bridge 210. As discussed similarly for system 100, the diode bridge 210 in system 200 rectifies the AC input voltage (VAC), which is filtered by an electromagnetic interference (EMI) filter 205 to prevent switching noise from the main switching power converter 235 from affecting the AC input voltage. The X capacitor Cx in system 100 functions the same as in system 100 in reducing EMI pollution of the AC power supply trunk from system 200. Because the coupling with the VCC charging circuit 225 is pre-bridge coupling, the coupling is directly exposed to the relatively high fluctuations of the AC input voltage. Therefore, the coupling is achieved via a high-voltage diode, which, for simplicity, is shown in... Figure 2The high-voltage diode is shown as coupling circuit 215. Coupling circuit 215 provides the VCC charging input voltage to VCC charging circuit 225. Furthermore, coupling circuit 215 is coupled to VCC charging circuit 225 via VAC sensing circuit 220, allowing VCC charging circuit 225 to sense the AC input voltage VAC. VCC charging circuit 225 uses the supply voltage (VCC) to power controller IC 230. After being powered, controller IC 230 can control the cycling of one or more power switches in main switching power converter 235 via pulse width modulation control signal (PWM_main) to regulate the output voltage (Vout).

[0025] Pre-bridge coupling via coupling circuit 215 offers several advantages to system 200. For example, system 200 can discharge capacitor Cx of the X capacitor more quickly compared to post-bridge coupling in system 100. Furthermore, pre-bridge sampling of the AC input voltage allows VCC charging circuit 225 to measure the AC input voltage more accurately because this measurement is direct, whereas system 100 must estimate the AC input voltage after rectification by diode bridge 110. After rectification of the AC input voltage, post-bridge measurements in system 100 lose some information about the original AC frequency, amplitude, and phase. In contrast, pre-bridge sensing of the AC input voltage in system 200 remains unchanged, allowing VCC charging circuit 225 to accurately and directly measure the amplitude, frequency, and phase of the AC input voltage. Moreover, pre-bridge sampling of the AC input voltage enables VCC charging circuit 225 to detect transient changes in the AC input voltage, which is crucial for functions such as power factor correction. Therefore, pre-bridge coupling with VCC charging circuit 225 provides better protection for system 200. Furthermore, because the VCC charging circuit 225 can respond to changes in the AC input voltage more quickly and accurately, this more accurate sensing of the AC input voltage, despite the presence of such changes, also ensures better regulation of the supply voltage VCC, thereby allowing for earlier fault detection. Some examples of pre-bridge and post-bridge coupling will now be discussed in more detail.

[0026] Figure 3An example post-bridge system 300 is shown in part. For simplicity, the EMI filter and main circuitry are not shown. The diode bridge is implemented using diodes D1, D2, D3, and D4, where the AC input voltage is received via an X capacitor Cx coupled between a pair of input terminals of the diode bridge. A post-bridge capacitor C1, connected across a pair of output terminals of the diode bridge, helps smooth the rectified input voltage Vin, which serves as the rectified input voltage of a transformer (not shown). The rectified input voltage Vin is also the input voltage of the main switching power converter (not shown). To receive the rectified input voltage Vin, a VCC charging circuit (not shown) is coupled to the positive terminal of the post-bridge capacitor C1. The VAC sensing circuit is implemented using a resistive voltage divider, such as a pair of series resistors R1 and R2 coupled between the positive terminal of the post-bridge capacitor C1 and ground. To sense the AC input voltage (VAC sensing), the VCC charging circuit (not shown) is coupled to the node between resistors R1 and R2.

[0027] Figure 4 An example pre-bridge system 400 is partially shown. For simplicity, the EMI filter and main switching power converter are not shown. As described with respect to system 300, the diode bridge in system 400 is implemented by diodes D1, D2, D3, and D4, where the AC input voltage is received via an X capacitor Cx. A post-bridge capacitor C1 helps smooth the rectified input voltage Vin, which drives the main switching power converter. Since the coupling with the VCC charging circuit (not shown) is pre-bridge coupled, the coupling is achieved via a pair of high-voltage diodes D5 and D6, each having an anode coupled to a corresponding terminal of the X capacitor Cx. The cathodes of diodes D5 and D6 form the input node of the VCC charging circuit to provide the VCC charging input voltage. Furthermore, the input node formed by the cathodes of diodes D5 and D6 is coupled via a VCC sensing circuit implemented by a resistive voltage divider, such as formed by a pair of resistors R1 and R2 coupled in series between the input node and ground. To sense the AC input voltage (VAC sensing), the VCC charging circuit is coupled to the node between resistors R1 and R2.

[0028] The operations implemented before and after the bridge will be discussed in more detail below. For example, Figure 5AThe startup behavior of the pre-bridge system 500 is illustrated. The arrangement of the X capacitor Cx, high-voltage diodes D5 and D6, resistors R1 and R2, diode bridge diodes D1, D2, D3, and D4, and post-bridge capacitor C1 is the same as that discussed for the pre-bridge system 400. The positive terminal of the post-bridge capacitor C1 provides a rectified input voltage to the main switching power converter (not shown). The cathodes of diodes D5 and D6 are coupled to the input terminals of the primary winding of transformer T. The output terminals of the primary winding are coupled to ground via a pair of MOSFETs, including a depletion-mode n-type metal-oxide-semiconductor (NMOS) transistor Q1 with a drain coupled to the output terminals of the primary winding. The pair of MOSFETs also includes an enhancement-mode NMOS transistor Q2 with a drain coupled to the source of transistor Q1. Since the output source current of transistor Q1 forms the input current of transistor Q2, transistors Q1 and Q2 can be considered cascaded.

[0029] The controller IC 505 controls the cycling of one or more power switches in the main switching power converter to regulate the output voltage generated by the main switching power converter. At startup, the controller IC 505 has no power and cannot turn on transistor Q2. However, since transistor Q1 is a depletion-mode transistor, it will turn on at startup even though the controller IC 505 initially has no power. Therefore, at startup, charging current flows from the cathodes of diodes D5 and D6 through the primary winding and through transistor Q1. The source of transistor Q1 is coupled to the VCC terminal of the controller IC 505 through resistor R4 and diode D7. Specifically, the anode of diode D7 is coupled to resistor R4, while its cathode is coupled to the VCC terminal. Furthermore, the cathode of diode D7 is coupled to the positive terminal of the VCC capacitor (CVcc), the negative terminal of which is coupled to ground. The secondary winding of the transformer is coupled to the positive terminal of capacitor CVcc through diode D8. The positive terminal of the VCC capacitor CVcc is also coupled to ground through the bleeding resistor Rbleeding and the bleeding transistor Q3, the operation of which will be discussed later.

[0030] Since transistor Q1 is turned on during startup, the startup charging current flows from the source of transistor Q1 through resistor R4 and diode D7 to begin charging the VCC capacitor using the supply voltage Vcc. To control the operation of transistor Q1, the active startup (ASU) terminal of controller IC 505 is coupled to the gate of transistor Q1. To prevent the gate of transistor Q1 from floating during startup, resistor R5 is coupled from the anode of diode D7 to the ASU terminal and then to the gate of transistor Q1. Resistor R5 can have a relatively high resistance so that it does not conduct a large current, but rather serves to prevent an undefined voltage from appearing on the gate of transistor Q1 during startup. As the charging current charges capacitor CVcc, the supply voltage Vcc will begin to rise. The startup charging current is determined by the threshold voltage of transistor Q1 and the resistance of resistor R4. Once the supply voltage VCC stored on the VCC capacitor reaches the operating level that powers controller IC 505, the startup phase ends, and controller IC 505 can begin regulating the cycle of transistor Q2 by driving the gate of transistor Q2 using the pulse width modulation signal PWM_Vcc. Therefore, in system 500, the VCC charging circuit will be formed by transformer T, transistors Q1, Q2 and Q3, resistors R3, R4, R5 and Rbleding, diodes D7 and D8, and capacitors C2 and CVcc.

[0031] Figure 5B The diagram illustrates the charging current when the controller IC 505 issues a PWM_Vcc signal to turn on transistor Q2. This charging current is now conducted through the primary winding and the channel of transistor Q1 to the drain of transistor Q2, and from the source of transistor Q2 to ground. To enable the controller IC 505 to monitor the charging current, the source of transistor Q2 can be coupled to ground via a sensing resistor (such as resistor R3). Therefore, the controller IC 505 may include a terminal (not shown) coupled to the node between the sensing resistor R3 and the source of transistor Q2. By monitoring the voltage across the sensing resistor, the controller IC 505 can turn off the cycle of transistor Q2 once the desired peak charging current has been reached, thereby regulating the supply voltage Vcc.

[0032] Figure 5CThe diagram illustrates the charging current flowing when the controller IC 505 releases the PWM_Vcc signal to turn off transistor Q2. Before transistor Q1 is turned off, transformer T stores magnetic energy conducted from the charging current through the primary winding. With transistor Q1 off, the stored magnetic energy drives the secondary winding current. To prevent the secondary winding current from conducting when transistor Q1 is on, the secondary winding current is rectified through diode D8, which has an anode coupled to the secondary winding and a cathode coupled to the positive terminal of capacitor CVcc (VCC). Alternatively, the secondary winding current can also be rectified through a synchronous rectifier (SR) switching transistor (not shown), which will be appropriately controlled by controller IC 505. With transistor Q2 off, the secondary winding current is conducted as an output current through diode D8 to charge capacitor CVcc using the supply voltage Vcc. Controller IC 505 can use any suitable control method to regulate the cycling of transistor Q2, including peak current control or constant on-time control of transistor Q2.

[0033] Figure 6 The following waveforms illustrate some of the operating waveforms used in System 500. At time t0, the controller IC sends a PWM_Vcc modulation signal to turn on transistor Q2. The primary winding current (Ip) begins to rise until it reaches its peak current at time t2. While the primary winding current conducts, the secondary winding current (Isec) does not conduct due to rectification by the secondary-side diode D8. At time t1, the controller IC de-asserts the PWM_Vcc modulation signal to turn off transistor Q2. Therefore, the primary winding current drops to zero, while the secondary winding current rises to a high level and begins to decline. At time t2, the transformer reset cycle ends, and the secondary winding current ramps to zero. The drain-to-source voltage Vds1 of transistor Q1 and the drain-to-source voltage of transistor Q2 both begin to resonate until another turn-on of transistor Q1 begins at time t3. During the cycle of transistor Q2, the ASU signal remains deactivated, so that transistor Q1 will conduct when transistor Q2 is turned on. When transistor Q2 is turned off, the source voltage of transistor Q1 increases, causing the gate-to-source voltage of transistor Q1 to fall below its threshold voltage, thus also turning off transistor Q1. With both transistors Q1 and Q2 off, they act as a capacitive voltage divider, blocking relatively high input voltages. In some implementations, the output capacitance of transistor Q2 may be much higher than that of transistor Q1, allowing transistor Q1 to withstand most of the input voltage when both transistors Q1 and Q2 are off. See again... Figure 5ACapacitor C2 can be coupled between the drain of transistor Q2 and ground to ensure that the drain-to-source voltage of transistor Q2 remains relatively low. In this way, transistor Q2 can be integrated into the controller IC to reduce manufacturing costs and simplify transistor control, since only transistor Q1 needs to withstand a high drain-to-source voltage. Therefore, it will be understood that transistor Q1 is shown separately from the controller IC simply for clarity. If the output capacitance of transistor Q2 is much higher than that of transistor Q1, capacitor C2 can be omitted.

[0034] Now refer to Figure 7 The discharge of capacitor X in system 500 is discussed. This discharge involves a bleeder transistor having a source coupled to ground and a drain coupled to the positive terminal of capacitor VCC via a bleeder resistor Rbleeding. Note that the discharge current path from capacitor X Cx in system 500 is substantially similar to the current path during startup. This discharge current is generated if the system is disconnected from the AC power supply line, or in response to a fault condition such as an AC power supply line failure (e.g., a power outage). To control the discharge, controller IC 505 activates bleeder transistor Q3 by asserting a discharge enable signal (Bleeding_EN), which charges the gate of bleeder transistor Q3, allowing capacitor X Cx to discharge through diode D5 (or diode D6), and then through the primary winding, transistor Q1, resistor R4, diode D7, bleeder resistor Rbleeding, and bleeder transistor Q3 to ground. This discharge proceeds faster via pre-bridge coupling compared to post-bridge coupling. The magnitude of the discharge current is determined by the threshold voltage of transistor Q1 and the resistance of resistor R4.

[0035] Figure 8 The bridge-back system 800 is shown in more detail. The arrangement of the VAC sensor, diode bridge, and X capacitor is related to... Figure 4The system 800 is the same as that discussed for system 500. The input voltage Vin at the positive plate of capacitor C1 after the bridge is coupled to the primary winding of transformer T. The remaining parts of system 800, including transistors Q1 and Q2, resistors R3, R4 and R5, capacitor C2, diodes D7 and D8, secondary winding, controller IC 505, capacitor CVccc, bleeder resistor Rbleeding, and bleeder transistor Q3, are arranged the same as those discussed for system 500. Therefore, except that the current in the primary winding must flow through the diode bridge to the node of input voltage Vin, the start-up current path, the charging current path during the cycle of turning on and off transistor Q2, and the discharge path of capacitor X will all be similar. Therefore, in system 800, the VCC charging circuit will be formed by transformer T, transistors Q1, Q2 and Q3, resistors R3, R4, R5 and Rbleeding, diodes D7 and D8, and capacitors C2 and CVcc.

[0036] Regardless of whether post-bridge coupling or pre-bridge coupling is used, the knowledge of the resulting AC input voltage waveform can be used to achieve, for example... Figure 9 The diagram illustrates valley-stop and peak-stop modes. In valley-stop mode, the controller IC stops issuing the PWM_Vcc signal before the AC input voltage is considered to have dropped to the threshold voltage Vlow. Note that at the valley (zero point) of the rectified AC input voltage waveform, the error signal between the supply voltage Vcc and its desired level (Vref) is at its maximum. This error signal is used by the controller IC, such as through constant on-time or peak current control methods, to determine the PWM_Vcc modulation signal. However, since the error signal is proportional to the switching frequency of transistor Q2, the switching frequency is also at its maximum when the AC line voltage approaches its zero-crossing point. This increase in the switching frequency of transistor Q2 causes the system to transition from discontinuous conduction mode to continuous conduction mode. Figure 6 The waveform illustrates the discontinuous conduction mode because the secondary current drops to zero (the transformer reset cycle ends) before transistor Q2 cycles back on. However, if the secondary current has not yet reached zero and transistor Q2 cycles on, the system has transitioned to continuous conduction mode. Operation in continuous conduction mode can lead to undesirable high current and subharmonic problems. However, the controller IC disclosed herein can prevent transistor Q2 from cycling by canceling the PWM_Vcc modulation signal in response to a drop in the AC line voltage below the threshold voltage Vlow, thus avoiding the danger of transitioning to continuous conduction mode.

[0037] The controller IC disclosed herein can also respond to an AC line voltage rise above a threshold voltage Vhigh by canceling the PWM_Vcc modulation signal again, thus achieving a peak-stop mode. The threshold voltage Vhigh is lower than the peak AC line voltage and sufficiently higher than the threshold voltage Vlow. When the AC line voltage rises above the threshold voltage Vhigh, the system avoids operation when the AC line voltage is too high by stopping the cycling of transistor Q2. If transistor Q2 cycles when the AC line voltage is close to its peak, switching losses can be significant. Therefore, peak-stop mode improves efficiency, especially during light load conditions. Furthermore, when transistor Q1 is turned off from cycling when the AC line voltage is close to its peak, the drain-to-source voltage oscillation on that transistor introduces additional stress, especially during heavy load conditions. While a snubber circuit could be used to suppress this voltage oscillation, it reduces efficiency. Therefore, peak-stop mode reduces voltage stress on transistor Q1 without the need for a snubber circuit.

[0038] Those skilled in the art will now understand that many modifications, substitutions, and variations can be made to the materials, apparatus, configuration, and methods of use of the devices disclosed herein without departing from the scope of this disclosure. Therefore, the scope of this disclosure should not be limited to the specific embodiments shown and described herein, as they are merely examples, but rather should be fully consistent with the scope of the appended claims and their functional equivalents.

Claims

1. A flyback converter for generating a supply voltage for a controller integrated circuit of a main switching power converter, the flyback converter comprising: A transformer, comprising a primary winding and a secondary winding; A first transistor having a drain coupled to the primary winding; A power supply voltage capacitor having a positive terminal coupled to the secondary winding and a negative terminal coupled to ground; as well as A second transistor having a drain coupled to the source of the first transistor and a drain coupled to ground, wherein the controller integrated circuit includes a power supply voltage terminal coupled to the drain of the first transistor and the positive terminal of the power supply voltage capacitor.

2. The flyback converter according to claim 1, characterized in that, The flyback converter also includes: The first resistor; and A first diode, wherein the power supply voltage terminal is coupled to the drain of the first transistor via a series combination of the first resistor and the first diode, wherein the first resistor is connected between the drain of the first transistor and the anode of the first diode, and wherein the cathode of the first diode is connected to the power supply voltage terminal.

3. The flyback converter according to claim 2, characterized in that, The flyback converter also includes: A sensing resistor connected between the source of the second transistor and ground; and A second resistor is coupled between the gate of the first transistor and the anode of the first diode, wherein the active startup terminal of the controller integrated circuit is coupled to the gate of the first transistor.

4. The flyback converter according to claim 1, characterized in that, The first transistor is an n-type depletion mode metal-oxide-semiconductor field-effect transistor.

5. The flyback converter according to claim 4, characterized in that, The second transistor is an n-type enhancement mode metal-oxide-semiconductor field-effect transistor.

6. The flyback converter according to claim 5, characterized in that, The second transistor is integrated within the controller integrated circuit, and the first transistor is located outside the controller integrated circuit.

7. The flyback converter according to claim 3, characterized in that, The controller integrated circuit includes a pulse width modulation signal terminal coupled to the gate of the second transistor.

8. The flyback converter according to claim 3, characterized in that, The flyback converter also includes: A first capacitor is coupled between the drain of the second transistor and ground.

9. The flyback converter according to claim 1, characterized in that, The flyback converter also includes: A diode bridge is used to rectify an AC input voltage into a rectified input voltage, wherein the main switching power converter is configured to convert the rectified input voltage into an output voltage in response to a modulation signal from the controller integrated circuit.

10. The flyback converter according to claim 9, characterized in that, The flyback converter also includes: A voltage divider is coupled between a node of the rectified input voltage and ground, wherein the controller integrated circuit is further configured to sense the AC input voltage through the voltage divider.

11. A system comprising: A diode bridge, which includes a pair of input terminals and a pair of output terminals; A main switching power converter coupled to the pair of output terminals and configured to convert a rectified input voltage into an output voltage; A controller integrated circuit configured to provide a modulation signal to the main switching power converter to regulate the output voltage; A first diode having an anode coupled to the positive input terminal of the pair of input terminals; A second diode having an anode coupled to the negative input terminal of the pair of input terminals; as well as A flyback converter includes a transformer having a primary winding and a secondary winding, the primary winding being coupled to the cathodes of a first diode and a second diode, and the secondary winding being coupled to a supply voltage capacitor configured to store a supply voltage for the controller integrated circuit.

12. The system according to claim 11, characterized in that, The controller integrated circuit is further configured to modulate the loop of a first transistor coupled between the primary winding and ground to regulate the supply voltage.

13. The system according to claim 12, characterized in that, The system also includes: A second transistor having a drain coupled to the primary winding and a source coupled to the drain of the first transistor, wherein the controller integrated circuit includes an active startup terminal coupled to the gate of the second transistor.

14. The system according to claim 13, characterized in that, The first transistor is an enhancement-mode transistor, and the second transistor is a depletion-mode transistor.

15. The system according to claim 11, characterized in that, The system also includes: A resistor divider coupled between ground and a node, the node being coupled to the cathode of the first diode and the cathode of the second diode, wherein the controller integrated circuit is further configured to sense the AC input voltage via the resistor divider.

16. A system comprising: A diode bridge, which includes a pair of input terminals and a pair of output terminals; An input voltage capacitor is coupled between the pair of output terminals; A main switching power converter coupled to the pair of output terminals and configured to convert the rectified input voltage stored by the input voltage capacitor into an output voltage; A controller integrated circuit configured to provide a modulation signal to the main switching power converter to regulate the output voltage; as well as A flyback converter includes a transformer having a primary winding and a secondary winding, the input terminal of the primary winding being coupled to a positive output terminal of a pair of output terminals, and the secondary winding being coupled to a supply voltage capacitor configured to store the supply voltage for the controller integrated circuit.

17. The system according to claim 16, characterized in that, The system also includes: A voltage divider coupled between the pair of output terminals, wherein the controller integrated circuit is further configured to sense the AC input voltage of the diode bridge via the voltage divider.

18. The system according to claim 17, characterized in that, The system also includes: An X capacitor, which is coupled between the pair of input terminals; and A bleed switch transistor coupled between the secondary winding and ground, wherein the controller integrated circuit is further configured to turn on the bleed switch transistor to discharge the X capacitor in response to sensing that the AC input voltage meets a threshold condition.

19. The system according to claim 16, characterized in that, The system also includes: A first transistor having a drain coupled to the primary winding; The second transistor has a drain coupled to the source of the first transistor and a source coupled to ground; A first diode having a cathode coupled to a power supply voltage terminal of the controller integrated circuit and a power supply voltage capacitor; A first resistor is coupled between the drain of the first transistor and the anode of the first diode; and A second resistor is coupled between the anode of the first diode and the gate of the first transistor, wherein the controller integrated circuit has an active start-up terminal coupled to the gate of the first transistor and a pulse width modulation terminal coupled to the gate of the second transistor.

20. The system according to claim 19, characterized in that, The first transistor is a depletion-mode transistor, and the second transistor is a enhancement-mode transistor.

Images