Regulated power supply voltage and triac holding current for a switching power converter

Abstract

Procedure that includes: while a phase-cut dimmer switch (320) is operating to dim a light from an LED (LED1) which is supplied by a switching power converter (300); Supplying a current through a power source to a first node (310); In response to an internal power supply voltage (Vcc) falling below a maximum value, a first switch (S4) coupled between the first node (310) and a storage capacitor (C1) is switched on to maintain a leakage current for the phase-cut dimmer switch (320) until the storage capacitor (C1) is charged to a desired level for the internal power supply voltage (Vcc); In response to the internal power supply voltage (Vcc) being at or above its maximum value, a second switch (S3) coupled between the first node and ground is switched on to maintain the leakage current for the phase-cut dimmer switch (320); and Regulation of an output voltage for the switching power converter (300) by cyclically switching a power switch (S2) coupled to a rectified input voltage, wherein the supply of the current by the current source includes a supply of the current by a current mirror (125).

Description

TECHNICAL FIELD

[0001] This application relates to switching power converters and, in particular, to regulating a power supply voltage for a switching power converter while maintaining a sufficient triac holding current. BACKGROUND

[0002] Due to their advantageous low energy consumption and the absence of toxic materials, solid-state light-emitting diode (LED) lighting applications are rapidly replacing conventional incandescent and fluorescent lighting systems. However, an LED cannot be exposed to the AC mains like an incandescent lamp. Solid-state lighting applications therefore contain a switching power converter to convert the input AC voltage into a rectified output voltage that can power the LED. It is also common to power incandescent light bulbs via a dimmer switch that includes a triac. The triac requires a minimum current, known as the holding current, for its operation. In an incandescent lamp application, the incandescent lamp is connected directly to the dimmer, so there is no need to support the triac holding current.

[0003] Since users expect an LED to mimic the behavior of incandescent lamps, the corresponding switching power converter must support the required holding current. It is therefore common for solid-state lighting (SSL) switching power supplies to include a bypass circuit that is active during periods of dimming applied by the dimmer switch. This bypass circuit generates sufficient current despite the application of dimming so that the triac in the dimmer for the SSL switching power converter can dim the required holding current.

[0004] To supply the internal power supply voltage, conventional SSL switching power converters may include a switch that connects a storage capacitor to a charged node via a resistor. Since the internal power supply voltage is commonly referred to as VCC, the corresponding switch is referred to herein as the VCC switch to distinguish it from the power switch that the switching power converter cycles to regulate the output voltage. The VCC voltage drops when the VCC switch is turned off by a product of the charging current drawn from the storage capacitor and an equivalent series resistance (ESR) for the storage capacitor. If the product of the capacitor ESR and the charging current is large, the VCC voltage may drop below a minimum required value, causing the power converter to perform a power-on reset (POR).The storage capacitor can then be recharged, causing the VCC switch to turn off and the VCC voltage to fall below the minimum required value. The result is that the LED lamp powered by the corresponding SSL switching power converter cannot start properly.

[0005] DE 10 2015 215 658 A1 relates to controlling LED lights and generating a voltage for supplying internal components of LED lights with energy or current.

[0006] US 2014 / 0 346 874 A1 concerns driver circuits for solid-state lighting devices.

[0007] US 8 937 435 B1 provides a circuit comprising a first circuit, a second circuit, and a bridge circuit. The first circuit is coupled to a magnetic component to receive electrical energy transmitted via the magnetic component and is configured to store the electrical energy and generate a supply voltage. The second circuit is also coupled to the magnetic component and is configured to consume a portion of the electrical energy when the second circuit is switched on. Accordingly, there is a need in the art for a switching power converter with improved regulation of the internal power supply voltage. SUMMARY

[0008] This problem is solved by the subject matter of the independent claims. Advantageous embodiments and further developments are the subject matter of the dependent claims.

[0009] A switching power converter is provided, comprising a power switch controlled by a controller to regulate an output voltage. The controller is powered by an internal power supply voltage. To ensure stable startup for the internal power supply voltage, the switching power converter includes a current mirror that controls a current that charges a storage capacitor. The internal power supply voltage is provided by the charged storage capacitor.

[0010] These advantageous features can be better appreciated by considering the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram of an exemplary switching power converter. Fig. Figure 2 shows a startup waveform for the internal power supply voltage for the switching power converter of Fig. 1. Fig. 3 is a circuit diagram of an exemplary switching power converter according to one aspect of the disclosure.

[0011] Embodiments of the present disclosure and their advantages are best understood by reference to the following detailed description. It should be understood that like reference numerals are used to identify like elements illustrated in one or more of the figures. DETAILED DESCRIPTION

[0012] To meet the need for improved regulation of the internal power supply voltage, a switching power converter is provided with a controlled current source configured to charge a storage capacitor to provide the internal power supply voltage. The storage capacitor is thus charged at a rate substantially independent of the voltage applied to the controlled current source. In this way, a stable internal power supply voltage is provided to the switching power converter to ensure that startup and subsequent operation can be achieved without triggering power-on resets when the internal power supply voltage is maintained above a power-on reset trigger level.

[0013] Referring to the drawings, an exemplary buck-boost converter 100 is illustrated. However, it should be understood that the circuits and methods discussed herein may be applied to a diverse range of alternative switching converter topologies, such as a flyback converter or a buck converter. A rectified input voltage V_IN is received at a first terminal of an inductor L1 and a resistor R1. For example, a bridge rectifier (not shown) or other suitable rectifier may rectify the AC input voltage from an AC mains to provide a rectified input voltage, which is processed via a phase-cut dimmer switch (not shown) to generate a rectified input voltage V_IN.

[0014] The power converter 100 includes a cascode transistor S1 (e.g., an NMOS transistor) whose gate is coupled to a second terminal of the resistor R1 at a node 105. The source of the cascode transistor S3 is coupled to a node 130, which in turn is coupled to a drain of a power switch transistor S2 (e.g., another NMOS transistor) having a gate voltage controlled by a controller U1. The source for the power switch transistor S2 is coupled to ground. The controller U1 is configured to control a cycle of the power switch S2 to regulate the amount of current pulsed into at least one LED (LED1). The output filtering is provided by an output capacitor C2 connected in parallel with the LED1 and also by a load diode coupled between a drain for the cascode transistor S1 and a second terminal for the inductor L1.Node 105 is also coupled to a Zener diode D2, which in turn is coupled to ground. When the rectified input voltage V_IN is initially applied during power-up of the power converter 100, the voltage of node 105 is equal to the Zener breakdown voltage for the Zener diode D2 minus a gate-source voltage for the power switch S1.

[0015] The cascode transistor disconnects the power switch S2 from the relatively high voltage for the rectified input voltage V_IN. If a user applies dimming via a dimmer with a triac, the triac requires a sufficient holding current. To maintain this minimum holding current while dimming is applied, the power converter 100 includes a shunt circuit 110. For example, the shunt circuit 110 may include a current source 115 controlled by a control signal CNTL. In this regard, it is common for a switching power converter such as the power converter 100 to include a dimming detection circuit 120 that detects whether phase-cut dimming has been applied to the AC input voltage from which the rectified input voltage V_IN is derived.The dimming detection circuit may be configured to establish the control signal CNTL for triggering the diverter circuit 110 by turning on the current source 115. For example, the diverter circuit 110 may comprise a current mirror formed by transistors 125, each having a drain coupled to the node 130.

[0016] An internal power supply 135 regulates an internal power supply voltage VCC via a current source 140 coupled between node 130 and an output node 145 carrying the internal power supply voltage VCC. A storage capacitor C1 with an equivalent series resistance (ESR) has a first terminal coupled to the output node 145 and a second terminal coupled to ground. A feedback circuit 150 is configured to turn off the current source 140 once the internal supply voltage VCC has reached the desired operating level. It should be understood that the current source 140 may instead be coupled to the storage capacitor C1 via a switch regulated by the feedback circuit 150.

[0017] Advantageously, the charging rate of storage capacitor C1 is substantially independent of the voltage for node 130. Depending on the breakdown voltage for Zener diode D2, the voltage of node 130 during startup is generally approximately 12 to 18 volts. Current source 140 charges storage capacitor C1 at substantially the same rate regardless of the voltage for node 130. For example, in Fig. 2 illustrates a power-on / reset cycle for the internal power supply voltage. At time t0, power is applied, after which the internal power supply voltage VCC is charged at a rate determined by the current supplied by current source 140 and the capacitance of storage capacitor C1. When the desired (POR) level for the VCC voltage is reached, the current source 140 is turned off or otherwise prevented, for example, by opening a switch to stop charging the storage capacitor C1. The power converter 100 continues to draw operating current, so the VCC voltage then decreases. However, since the VCC voltage was not increased at a relatively high rate, its decrease also occurs gradually, so that the VCC voltage does not fall below the required minimum value.This way, the boot process proceeds normally and is not disrupted by repeated resets that can occur with conventional architectures.

[0018] An embodiment of the invention for a switching power converter 300 is shown in Fig.3. Inductor L1, output diode D1 and capacitor C2, LED1, dimming detection circuit 120, resistor R1, Zener diode D2, controller U1, power switch S2, and cascode transistor S1 are all arranged as discussed with respect to power converter 100. However, power converter 300 includes a combined shunt and internal voltage regulator circuit 305. Combined circuit 300 includes current source 140 and the current mirror formed by transistors 125. Current mirror current I_Source is coupled to ground via a shunt switch transistor S3, which is controlled by controller U2. In particular, a source of one of current mirror transistors 125 is coupled to a node 310, which in turn is coupled to a drain for shunt switch transistor S3.A regulation switch transistor S4 is also coupled between node 310 and an output node 315, which carries the internal power supply voltage VCC. The storage capacitor C1 with an equivalent series resistance ESR is coupled between the output node 315 and ground. The power converter 300 thus advantageously saves component costs by using the current source 140 for both sinking and voltage regulation.

[0019] The controller U2 regulates a switching rate for the regulation switch transistor S4 to regulate the internal power supply voltage VCC to the desired value. In particular, the regulation switch transistor S4 conducts a current I_VCC that is greater than a leakage current I_Drain conducted by the leakage switch transistor S3. A dimmer switch 320 includes a triac 325 controlled by a user dimming input 330. If a user applies dimming via the dimming input 330, the controller U2 responds to dimming detection by the dimming circuit 120 to ensure that a sufficient holding current flows through the triac. For example, the controller U2 may be configured to respond to dimming detection by only cycling the regulation switch transistor S4 while the internal power supply voltage VCC is less than a maximum amount.This reduces power dissipation and improves operating efficiency. If no dimming is detected, the controller U2 leaves the bypass switch transistor S3 off. On the other hand, if dimming is detected but the power supply voltage VCC is already at or near the maximum level, the controller U2 can cycle the bypass switch S3 while leaving the regulation switch transistor S4 off. If no dimming is detected but the internal power supply voltage VCC requires an increase, the controller U2 can cycle the regulator S4 again while leaving the bypass transistor off.

[0020] As those skilled in the art will now appreciate, depending on the particular application at hand, many modifications, substitutions, and variations may be made in and to the materials, devices, configurations, and methods of using the devices of the present disclosure without departing from the spirit and scope thereof. In view of this, the scope of the present disclosure is not intended to be limited to the specific embodiments illustrated and described herein, which are merely examples thereof, but rather to be entirely consistent with that of the appended claims and their functional equivalents.

Claims

[1] Procedure which includes: while a phase-cut dimmer switch (320) operates to dim a lighting from an LED (LED1) powered by a switching power converter (300); supplying a current through a current source into a first node (310); in response to an internal power supply voltage (Vcc) being below a maximum value, turning on a first switch (S4) coupled between the first node (310) and a storage capacitor (C1) to maintain a leakage current for the phase-cut dimmer switch (320) until the storage capacitor (C1) is charged to a desired level for the internal power supply voltage (Vcc); in response to the internal power supply voltage (Vcc) being at or above the maximum value, turning on a second switch (S3) coupled between the first node and ground to maintain the leakage current for the phase-cut dimmer switch (320); and Regulating an output voltage for the switching power converter (300) by cyclically switching a power switch (S2) coupled to a rectified input voltage, wherein supplying the current through the current source comprises supplying the current through a current mirror (125). [2] Switching power converter (300) containing: a current mirror (125) configured to drive a current into a first node (310); a storage capacitor (C1) for storing a power supply voltage (Vcc); a first switch (S4) connected between the first node (310) and the storage capacitor (C1); a second switch (S3) connected between the first node (310) and ground; a dimming detector configured to detect dimming from a phase-cut dimmer switch; and a controller supplied by the power supply voltage (Vcc), the controller being configured to turn on the first switch (S4) and turn off the second switch (S3) in response to the power supply voltage (Vcc) being below a maximum value and to dimming detection by the dimming detector, and to turn off the first switch (S4) and turn on the second switch (S3) in response to the power supply voltage (Vcc) being at or above the maximum value and to dimming detection by the dimming detector. [3] Switching power converter (300) according to claim 2, further comprising: a circuit breaker (S2); an inductor (L1) having a first terminal coupled to an input node; a cascode transistor (S1) coupled between the inductor (L1) and the power switch (S2); a resistor (R1) coupled between the input node and a second node; and a Zener diode (D2) coupled between the second node and ground, wherein the second node is coupled to a gate of the cascode transistor (S1). [4] The switching power converter (300) of claim 3, further including an LED coupled to the input node. [5] The switching power converter (300) of claim 4, further including an output diode (D1) coupled between the LED and a second terminal for the inductor (L1). [6] Switching power converter (300) according to claim 3, wherein the power switch (S2) comprises an NMOS transistor. [7] Switching power converter (300) according to claim 3, wherein the power switch (S2) comprises a bipolar transistor.

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