HK1165107B - Adaptive current limiter and dimmer system including the same - Google Patents

Description

Adaptive current limiter and dimmer system including the same

Technical Field

The present disclosure relates generally to current limiter circuits and, more particularly, to an adaptive current limiter suitable for phase cut dimmer circuits having electromagnetic interference (EMI) filter circuits.

Background

An incandescent lamp is dimmed when it is operated at a voltage lower than the nominal voltage with which it is designed. When the applied voltage is reduced, the power and intensity of the lamp are correspondingly reduced. Conventionally, dimmers are solid state switching devices that turn a light on and off at a rate that is related to the frequency of the applied power. In the united states, the standard Alternating Current (AC) supply voltage oscillates at a rate of 60 times per second (or 60 hertz), so the dimmer circuit is designed to turn the lamp on and off at a rate of twice the supply frequency (i.e., a frequency of 120 hertz). The combined thermal mass and resistance of the incandescent filament smoothes the pulses. Because the brightness at which the filament emits light depends on the amount of current flowing through the filament, the human eye sees brighter or darker light, depending on the ratio of the on-time to the off-time. In other words, the input voltage is switched on at a phase point that reduces the average voltage delivered to the bulb, which is referred to as phase cut dimming.

There are two types of phase-cut dimming: forward phase-cut dimming and reverse phase-cut dimming. When dimming phase-cut forward, the circuit controls the power applied to the lamp so that the lamp is energized during the last portion of each power line half cycle. Such a phase-cut forward dimming circuit, which includes a standard incandescent lamp and a magnetic transformer, as well as neon, cold cathode, and some types of fluorescent dimming ballasts, is inexpensive, relatively robust, and suitable for most load circuits. During reverse phase-cut dimming, the circuit controls power to the lamp such that the lamp is energized during the beginning portion of each power line half cycle. Reverse phase-cut dimming is generally more expensive because it uses more complex electronics. However, some loads, such as electronic transformers, work better and produce less audible noise (such as buzzing or ringing) when using a reverse phase-cut dimmer circuit.

In both cases, the actual lighting load operates at the power line frequency. The switching frequency can cause the load circuit to emit audible noise. Sometimes, noise reduction devices, such as de-buzzing coils, are used to reduce or eliminate the noise generated by these systems.

One type of phase cut dimmer circuit is known as a TRIAC (TRIAC) circuit or TRIAC dimmer. TRIAC dimmers can be incorporated into light switches to provide dimming functionality. A TRIAC dimmer controls the power to a load, such as a light bulb, by turning on (gate) an applied sinusoidal AC waveform from a current source based on either of forward or reverse type dimming. In particular, TRIAC dimmers switch on over a half-cycle of the AC supply and switch off at the end of each half-cycle, modulating the power delivered to the load. The on-off cycle of the TRIAC dimmer repeats during operation to drive the load, and the timing of the switching on of the TRIAC dimmer controls the amount of energy provided. Generally, when a TRIAC dimmer is configured to turn on at the peak of the AC supply waveform, the TRIAC dimmer delivers the peak supply to the load.

Such TRIAC dimmers are used in many residential, commercial, and industrial environments to provide adjustable lighting. Conventional TRIAC dimmers are designed to control the power provided to a resistive load, such as a standard incandescent or halogen lamp. Recent developments in light bulbs such as compact fluorescent lamps and light emitting diode lamps provide enhanced efficiency and lifetime; however, from the perspective of a TRIAC dimmer, such a device does not necessarily work as a resistive load. Accordingly, in such newer lighting solutions, conventional TRIAC dimmer circuits may not function properly and/or may not function at all. Instead of dimming the lamp, such a TRIAC dimmer circuit may cause the lamp to flicker or may not work at all.

Drawings

Fig. 1 is a timing diagram of current versus time produced by a conventional TRIAC dimmer connected to the circuit in fig. 2.

Fig. 2 is a block diagram of a representative example of a system including a conventional electromagnetic interference (EMI) filter circuit for a conventional TRIAC dimmer.

Fig. 3 is a block diagram of an embodiment of a phase cut dimmer system with an adaptive current limiter.

Fig. 4 is a circuit diagram of the phase cut dimmer system of fig. 3 showing circuit details.

Fig. 5 is a timing diagram of current versus time for the phase cut dimmer system of fig. 4.

In the following description, the use of the same reference numbers in different figures indicates similar or identical items.

Detailed Description

Embodiments of a system are disclosed below that include an adaptive current limiter configured to limit a burst power step from a TRIAC dimmer switch, prevent ringing in an associated EMI filter, and increase a current limit as a function of time and based on a feedback current from the EMI filter. By limiting the burst power step to prevent the passage of high rate of change signals, the adaptive current limiter provides a more consistent load that maintains proper TRIAC operation even when used with LEDs or other types of lights that do not present a resistive load to the system. The current signal generated by a conventional TRIAC dimmer system is described below with respect to fig. 1.

Fig. 1 is a timing diagram 100 of current versus time produced by a conventional phase cut dimmer system, such as a TRIAC dimmer circuit. Within timing diagram 100, the vertical axis represents current in amperes and the horizontal axis represents time in milliseconds. At 102, the phase cut dimmer circuit is turned on over a half cycle in the negative portion of an Alternating Current (AC) supply from the power source. Assuming a particular load, at 102, the negative current spike has a peak current of approximately minus 0.95 amps. After the peak current spike, the current level is reduced to a level related to the effective current consumption of the load circuit (generally indicated at 104). The active current consumption level 104 has a current level of approximately negative 0.2A. The effective current consumption is reduced by the remaining part of the quarter cycle. At 106, the TRIAC dimmer circuit is turned off during the next quarter cycle.

At 108, the positive current spike also has a peak of about 0.95 amps, which is generally indicated by horizontal line 114. The positive current spike 108 is related to the turn-on of the phase cut dimmer circuit during the half cycle of the positive portion of the AC supply. After the positive current spike 108, the current level is reduced at 110 to an operating current level, which represents a peak current of approximately 0.2A, and which discharges for the remainder of the half cycle. At 112, the phase cut dimmer circuit remains off until the next half-cycle. This signal pattern is repeated indefinitely until the phase cut dimmer circuit is switched off. Phase cut dimmer circuits typically include an adjustment mechanism (such as a slider or other user accessible element) that can be used to dim the relevant light by changing the amount of time the phase cut dimmer circuit is on or off, thereby changing the timing and duration of the output pulses relative to the AC supply phase.

The timing diagram 100 is illustrative of peak current, but does not represent the worst case, which occurs at higher voltages, such as at 230 volts AC and 277 volts AC, where the voltage of the peak step can exceed 400 volts, where the current will be significantly higher. These high peak voltages and associated peak current spikes increase losses and place excessive stress on circuit components. In addition, these peak currents can resonate among components of an electromagnetic interference (EMI) circuit, such as an EMI circuit, which will be discussed with respect to the system 200 designated in fig. 2, which produces oscillating currents that can interfere with the operation of the phase cut dimmer circuit. When these oscillating currents occur, insufficient current can cause the phase cut dimmer circuit to inadvertently turn off when the phase cut dimmer circuit is on or at any time during a cycle.

For example, at the transition between the positive current spike 108 and the operating current level 110, the current can exceed or not exceed the operating current level 110 (as indicated generally at 116), which can cause the current level to fall below the holding current (i.e., the operating current level 110), causing the switch to unexpectedly turn off. A certain amount of holding current is required to maintain the phase cut dimmer circuit in the "on" state to provide proper operation. The oscillating current can weaken the current level, causing the current level to fall below the holding current level, accidentally turning off the phase cut dimmer circuit. Such accidental turn-off of the phase cut dimmer circuit can produce flicker in an electronic lamp (LED or compact fluorescent lamp). One technique for preventing such flicker involves drawing additional current to achieve a holding current level sufficient to prevent the phase cut dimmer circuit from turning off. An embodiment of a conventional circuit configured to maintain a TRIAC dimmer holding current is described below with respect to fig. 2.

The magnitude of the current spike generated by the TRIAC turn-on is a function of the instantaneous voltage difference between the power supply and the capacitor of the system. If the TRIAC is turned on near the beginning or end of an input half cycle, the instantaneous voltage difference will be small and thus the current spike will be low. The delay period of activating the TRIAC close to the peak of the input power waveform introduces a high instantaneous voltage difference, producing a high amplitude current spike.

Fig. 2 is a block diagram of a representative example of a system 200, the system 200 including an input filter circuit for providing electromagnetic interference (EMI) filtering between a phase cut dimmer circuit, such as a TRIAC dimmer circuit, and a resistive load. The system 200 includes inputs 202 and 208 for receiving ac power from a TRIAC dimmer circuit (not shown). A fuse 204 and a resistor 210 connect the inputs 202 and 208, respectively, to the common mode inductor circuit 206.

Common mode inductor circuit 206 includes a first terminal connected to a first terminal of inductor 212 and a first terminal of resistor 214 arranged in parallel. Inductor 212 includes a second terminal connected to a first electrode of capacitor 216. Resistor 214 also includes a second terminal connected to a first electrode of capacitor 216. Common mode inductor circuit 206 also includes a second terminal connected to a second electrode of capacitor 216. The second terminal of common mode inductor circuit 206 is also connected to a first terminal of resistor 218 and a first terminal of inductor 220, which are connected in parallel. The resistor 218 includes a second terminal connected to a full-wave rectifier bridge (diode bridge). Inductor 220 also includes a second terminal connected to a full wave rectifier bridge.

The full wave rectifier bridge includes diodes 222, 224, 226, and 228. Diode 222 includes an anode connected to the second terminal of inductor 212 and resistor 214, and includes a cathode connected to positive power supply terminal 223. Diode 226 includes an anode connected to negative supply terminal 225, and a cathode connected to the anode of diode 222. Diode 228 includes an anode connected to negative supply terminal 225, and a cathode connected to resistor 218 and to the second terminal of inductor 220. Diode 224 includes an anode connected to the cathode of diode 228, and a cathode connected to positive power supply terminal 223. Further, the diode bridge includes a transient voltage surge protector 236 having a first terminal connected to the cathode of diode 226 and the anode of diode 222, and having a second terminal connected to the cathode of diode 228 and the anode of diode 224.

The system 200 further comprises a resistor 230 having a first terminal connected to the positive supply terminal 223, and a second terminal connected to one electrode of a capacitor 232, said capacitor 232 comprising a second electrode connected to the negative supply terminal 225. The capacitor 234 includes a first electrode connected to the positive power supply terminal 223, and a second electrode connected to the negative power supply terminal 225. The voltage potential across capacitor 234 is provided as an output voltage to the load circuit.

In operation, common mode inductor circuit 206, inductors 212 and 220, resistors 214, 218 and 230, and capacitors 216, 232 and 234 cooperate to provide EMI noise reduction for nearby circuits. Resistor 210 is selected to limit peak current and common mode inductor circuit 206 uses two coupled inductors for EMI filtering and to help reduce inrush current. In addition, the EMI filter includes a relatively large inductive-resistive-capacitive (LRC) filter that includes resistors 214, 218, and 230, and capacitors 216, 232, and 234, providing additional holding current to compensate for current ringing and drawing additional holding current to prevent inadvertent disconnection of the phase cut dimmer circuit connected to inputs 202 and 208.

The input current from the phase cut dimmer circuit can include large current spikes at the "on" transition as previously described in fig. 1. In some examples, surges from these current spikes produce audible noise (sometimes referred to as "buzzing" or "ringing") from the circuit elements. Such noise is undesirable, particularly in quiet residential settings. In addition, the higher supply voltage exacerbates current spikes and related problems of oscillating currents in EMI circuits, particularly for environments with 230 volts AC and industrial 277 volts AC.

In addition, excessive current peaks can stress the phase cut dimmer circuit itself. These phase cut dimmer circuits typically have a specific current rating, and non-productive current spikes limit the useful load rating of the phase cut dimmer circuit. The lower peak current allows more fixtures to be connected to the phase cut dimmer circuit. Therefore, the high peak currents that produce the oscillations need to be managed to ensure correct performance to allow for more fixtures and to avoid collapse of the phase cut dimmer operation. Reducing the current can also reduce component stress and acoustic noise. An example of a circuit that reduces peak current spikes from such a phase cut dimmer circuit is described below with respect to fig. 3.

Fig. 3 is a block diagram of an embodiment of a phase cut dimmer system 300, the phase cut dimmer system 300 including a phase cut or TRIAC dimmer circuit 302 and an adaptive current limiter 308 configured to provide consistent TRIAC operation for various load circuits. The phase cut dimmer system 300 includes an AC supply 304 having a first terminal connected to a TRIAC dimmer circuit 302 having a first input connected to an EMI filter and an output of a full wave rectifier 305. AC power supply 304 includes a second terminal connected to a second input of EMI filter and full wave rectifier 305. The EMI filter and full wave rectifier 305 includes a first terminal connected to a first terminal of a power converter 306 (such as a transformer or a Direct Current (DC) to DC converter, and associated power control circuitry) and includes a second terminal connected to an adaptive current limiter 308. The adaptive current limiter includes a terminal coupled to a second terminal of the power converter 306. The power converter 306 includes output terminals that are connected to an LED or other load circuit 310, such as an LED lamp, compact fluorescent lamp, motor, or other load circuit.

In this particular example, as indicated in fig. 2, the EMI filter and full wave rectifier 305 comprises a diode rectifier bridge comprising diodes 222, 224, 226, and 228 and a transient surge protector 236. Also indicated in fig. 2, EMI filter and full wave rectifier 305 includes common mode inductor circuit 206, resistors 210 and 218, inductor 220, and capacitors 232 and 234. In alternative embodiments, different types of EMI filter circuits can be used.

Adaptive current limiter 308 includes a current pass element 318 that includes a first current electrode (currenteleclectrode) connected to a second terminal of power converter 306; a second current electrode connected to a first terminal of sensing element 312; and a control electrode. Sensing element 312 includes a second terminal connected to the second terminal of EMI filter and full wave rectifier 305. The sense element 312 provides a sense voltage (senseovoltage) proportional to the current flowing through the sense element 312 to the negative input of the differential amplifier 314. The differential amplifier 314 includes a positive input connected to a reference generator 316, and an output connected to a control electrode of a current passing element 318. The current pass element 318 may be a power Field Effect Transistor (FET) or other device configured to control current in response to a control signal.

The reference generator 316 provides a time-varying signal to the positive input of the differential amplifier 314. The differential amplifier 314 generates a control signal, which is itself time-varying, in response to receipt of the sense voltage and the time-varying signal. The time-varying control signal controls the current flowing through element 318, allowing the current to increase over time in response to the time-varying control signal.

In one example, the reference generator 316 can include a capacitor and/or other circuitry configured to automatically generate a time-varying signal over a predetermined time. In some embodiments, the reference generator 316 is programmable, such as by a host system (not shown). In a particular example, the reference generator 316 can be implemented by a system that generates a time-varying signal that can be provided to the positive input of the amplifier 314 through a pin connection.

The time-varying signal may be a ramp signal having a substantially linear portion with respect to the predetermined time period, may be a signal that varies exponentially over the predetermined time period, or may be some other signal that can be used to increase the current limit over the predetermined time. The predetermined time can be configured to correspond to a time of a current spike related to an on-time of an associated phase cut dimmer circuit. The current pass element 318 is capable of controlling the current to increase substantially linearly, substantially exponentially, or in other controlled manner over a predetermined time period in response to a time-varying control signal generated by the amplifier 314. The amplifier 314 generates a time-varying control signal in response to a time-varying signal from the reference generator 316 and in response to a sense voltage from the sense element 312.

In operation, adaptive current limiter 308 limits the power surge generated by the switching of TRIAC dimmer circuit 302 to a rate of change suitable for energizing the EMI filter and the EMI filter portion of full-wave rectifier 305 with reduced peak current. In particular, by controlling or limiting the peak current, the adaptive current limiter 308 allows current to be increased at a controlled rate after the TRIAC dimmer circuit 302 is turned on, provides time to charge the EMI filter capacitor, avoids oscillations from excessively stored energy, and prevents interference with proper TRIAC operation. Adaptive current limiter 308 gradually increases the current limit as a function of the on-time of the sensed feedback current through sensing element 312.

Adaptive current limiter 308 controls the current flowing through element 318, initially limiting the current and gradually increasing the allowed current over a predetermined period of time, avoiding the initial surge, and then allowing full operating current to flow. During the initial surge, adaptive current limiter 308 dissipates the surge energy. The magnitude of the feedback current through the sensing element 312 allows the EMI filter circuit to be charged as quickly as possible to minimize power loss. Thus, the adaptive current limiter 308 limits the current as a function of the time that the feedback current flows through the sensing element 312 (i.e., the amount of time the TRIAC dimmer circuit 302 is on), and as a function of the feedback current flowing through the sensing element 312. Thus, adaptive current limiter 308 allows the magnitude of the current to vary as a function of time and as a function of the feedback current, substantially independent of the magnitude of the input current. In this manner, adaptive current limiter 308 generates a variable current limit, starting at a lower limit and gradually increasing, allowing the capacitive elements in the EMI filter circuit of the EMI filter and full-wave rectifier 305 to charge at a controlled rate with minimal overcharging.

The time-varying feature allows the current to build up over time corresponding to a variable instantaneous voltage difference introduced by the dimmer setting. The current limiter adapts to the magnitude of the voltage difference, which provides a controlled current to charge the capacitor without overcharging, which could cause oscillations and disturb the correct system operation. The time-varying characteristics match or adapt to prevailing operating conditions, compensating for system loads and dimmer settings.

The current limit set by adaptive current limiter 308 will continue to increase after the EMI filter circuit is charged. Additional current is not forced into the power converter 306 because the adaptive current limiter 308 controls the current limit as a function of the feedback current.

The adaptive current limiter 308 limits the peak current during the time that the TRIAC dimmer circuit 302 is turned on, reducing the peak current. To this end, the turn-on event of the TRIAC dimmer circuit 302 has passed each half-cycle, and the adaptive current limiter 308 has increased its limit to a level at or above the active current level of the load circuit 310, thereby reducing losses. Further, the reference generator 316 and the sensing element 312 cooperate to control the rate at which the current changes as a function of the on-time of the TRIAC dimmer circuit 302, as well as a function of the sensed voltage across the sensing element 312. The reference generator 316 is reset during the period after the TRIAC dimmer circuit 302 turns on in preparation for controlling the rate of current change. Thus, the phase cut dimmer system 300 has current feedback and maintains effective control of the current flowing through the element 318 regardless of the magnitude of the AC input voltage.

The adaptive current limiter 308 allows a wide range of load power to be used for a given set of circuit components, making the phase cut dimmer system 300 tolerant of supply and load range variations while limiting the current provided to the power converter 306, which would result in oscillation and incorrect TRIAC operation. This tolerance allows the adaptive current limiter 308 to be used with standard TRIAC dimmer circuits to control LED lamps, compact fluorescent lamps, and other loads that are non-linear or not purely resistive.

The adaptive current limiter 308 limits the peak current flowing into the power converter 306 in response to the TRIAC dimmer circuit 302 turning on and in response to the feedback current measured by the sensing element 312. Adaptive current limiter 308 allows the current to increase as a function of the instantaneous voltage between the input source and the capacitor voltage, and as a function of the feedback current. An example of a circuit that implements adaptive current limiter 308 is discussed below with respect to fig. 4.

Fig. 4 is a circuit diagram of an embodiment implementing the system in fig. 3. As indicated in fig. 2, system 400 includes fuse 204, resistors 210 and 218, common mode inductor circuit 206, inductor 220, transient surge protector 236, diodes 222, 224, 226, and 228, positive and negative supply terminals 223 and 225, and capacitors 232 and 234. Unlike system 200, which is illustrated in fig. 2, system 400 includes the details of additional circuit implementations of adaptive current limiter 308. In comparison to system 200 in fig. 2, in system 400, inductor 212, resistors 214 and 230, and capacitor 232 are omitted.

In the illustrated embodiment, diodes 222, 224, 226, and 228 cooperate to provide rectified power to positive and negative power supply terminals 223 and 225. Further, adaptive current limiter 308 includes resistors 410 and 412 connected in series between positive supply terminal 223 and the collector of a double junction transistor (BJT) 414. Resistors 410 and 412 form a bias circuit for connecting the gate of a Field Effect Transistor (FET)428 to the positive supply terminal 223. Further, the zener diode 426 includes an anode connected to the source of the FET428, and a cathode connected to the gate of the FET 428. In some embodiments, an avalanche diode or a breakdown diode may be used in place of zener diode 426. FET428 also includes a drain connected to resistor 218 and inductor 220, and a source connected to a first terminal of sense resistor 420, sense resistor 420 having a second terminal connected to negative supply terminal 225. Resistors 418 and 416 form a voltage divider circuit across sense resistor 420 and provide a divided sense voltage to the base of BJT414, BJT414 having an emitter connected to negative supply terminal 225.

It should be appreciated that in the illustrated embodiment, FET428 is an N-channel MOSFET. However, in other embodiments, another type of transistor (such as a P-channel MOSFET or BJT) may be used in place of FET428 or in place of BJT 414. The term "current electrode" is used to refer to a current conducting electrode of a transistor, such as a collector or emitter of a BJT, or a source or drain of a FET. The gate of a FET or the base of a BJT, respectively, which can conduct a small amount of current is referred to as the "control electrode".

Positive power supply terminal 223 provides power to load circuit 310 through an isolation circuit, such as transformer circuit 430. The capacitor 432 includes a first electrode connected to the positive power supply terminal 223, and a second electrode connected to the power control circuit 436. Power FET438 includes a drain connected to the primary winding of isolation transformer circuit 430, a gate connected to power control circuit 436, and a source connected to sense resistor 440, sense resistor 440 being connected to inductor 220. The power control circuit 436 is also connected to a sense resistor 440 to detect a sense voltage with respect to the primary winding flowing through the isolation transformer circuit 430 and to control the current flowing through the primary winding by controlling the voltage applied to the gate of the power FET 438. The primary winding of isolation transformer circuit 430 is inductively coupled to the secondary winding, which includes output terminals connected to load circuit 310.

In the illustrated embodiment, the FET428 controls the feedback current flowing through the adaptive current limiter 308 to the negative supply terminal 225. The FET428 provides secondary control of the current flowing into the capacitors 216 and 234, which provides EMI filtering to limit the current surge generated by the TRIAC dimmer circuit 302.

In operation, resistors 410 and 412 draw a small amount of current from positive supply terminal 223 to drive the gate of FET428, allowing current to flow through FET428 when a phase cut dimmer circuit (such as TRIAC dimmer circuit 302) is in an off state. This small current can be used by the phase cut dimmer circuit to manage its on/off timing. When current increases (such as when the phase cut dimmer circuit is on), current flows through resistor 424 and charges capacitor 422, causing BJT414 to turn on, pulling the voltage on the gate of FET428 low, reducing the current flowing through the drain to the source of FET 428. As the capacitor charges, the current flowing to the capacitor 422 decreases and the base voltage of the BJT414 drops, allowing the voltage on the gate of the FET428 to increase over time, thus increasing the current limit of the FET 428. In one example, when the voltage at the base of BJT414 reaches approximately 0.65 volts, BJT414 turns on and conducts, reducing the voltage at the gate of FET 428. The reduced voltage on the gate of FET428 reduces the current flowing from the drain to the source through FET428 to a controlled current level.

Resistor 424 and capacitor 422 form a time-dependent network that bypasses resistor 418 for a predetermined period of time. When the current limiter is first activated and capacitor 422 discharges, resistor 424 appears directly in parallel with resistor 418, increasing the current flowing through resistor 416 and thereby turning on BJT414 and lowering the effective current limit point. As the capacitor 422 charges, the current level decreases, reducing the current flowing through the BJT414, and the current limit allowed through the FET428 increases over time. During operation, the voltage at the base of BJT414 can reach an equilibrium point at which the current limit of FET428 is set until the phase cut dimmer is turned off. During the off phase, the capacitor 422 discharges, resetting the timing. In one embodiment, the values of resistor 424 and capacitor 422 are selected to achieve a time constant that allows the current limit to increase at a controlled rate over time so as not to overdrive the input filter. Zener diode 426 acts as a gate-to-source voltage limiter in FET428, clamping the gate voltage to a voltage level that corresponds to the breakdown voltage of zener diode 426. The maximum voltage (i.e., the breakdown voltage of zener diode 426) is set high enough to allow FET428 to turn on fully, but not so high as to take too much time to gain control BJT414 after the TRIAC dimmer turns off, and then turns on again in the next input half-cycle.

In a particular embodiment, the resistor 210 has a resistance of approximately 2.2 Ω, and the fuse 204 has a current limit of 1 ampere. In this embodiment, resistors 410 and 412 have a resistance of approximately 62k Ω, resistor 218 has a resistance of approximately 5.6k Ω, and capacitors 216 and 234 have a capacitance of approximately 100 nF. Further, in this embodiment, zener diode 426 has a breakdown voltage of approximately 9.1 volts, resistor 418 has a resistance of 10k Ω and sense resistor 420 has a resistance of 10 Ω, and resistor 416 has a resistance of approximately 2.7k Ω. Further, resistor 424 has a resistance of 270 Ω, and capacitor 422 has a capacitance of approximately 47 nF.

In this particular configuration, BJT414 will be biased in the active region when the line current reaches approximately 0.3 amps. Resistor 424 and capacitor 422 form a time-dependent network bypass resistor 418. When the current limiter is first activated and the capacitor 422 discharges, the resistor 424 appears in parallel with the resistor 418, reducing the current limit to approximately 0.066 amps, so that a low current of approximately 0.066 amps is sufficient to activate the BJT414, controlling the voltage on the gate of the FET 428. As the capacitor 422 charges, the current limit level shifts up to approximately 0.3 amps over time. In particular, as capacitor 422 charges, the voltage at the base of BJT414 increases across resistors 418 and 416, turning on the collector-to-emitter current. Thus, capacitor 422 charges according to a time constant, increasing the impedance associated with the time-dependent network over time, and varying the current across resistor 416 over time, thereby varying the current in the base of BJT 414. The resistance and capacitance values of resistor 424 and capacitor 422 can be selected to produce a time constant configured to slowly increase the current to allow time for charging the EMI filter circuit (i.e., common mode inductor circuit 206, capacitors 216 and 234, resistor 218, and inductor 220) and to avoid overdriving the EMI filter circuit.

Thus, the system 400 limits the current, reducing the peak current magnitude during the on portion of the TRIAC dimmer cycle. Further, as described below with respect to fig. 5, the system 400 maintains the operating current draw to a level sufficient to drive the associated load circuit for the remaining half-cycle, and resets to operate in the next half-cycle.

Fig. 5 is a timing diagram 500 of current versus time for the system 400 indicated in fig. 4. The scale of the vertical axis is changed to provide greater resolution than the timing diagram 100 in fig. 1. At 502, when the phase cut dimmer is turned on, the current rapidly decreases to a negative peak current of approximately minus 0.20A, which is less than one-fourth of the negative current spike 102 (minus 0.95 amps) of the timing diagram 100 indicated by fig. 1. The current then slowly increases (as indicated generally at 504) according to the operating current consumed by the associated load circuit. During the period indicated at 506, the phase cut dimmer is turned off and the adaptive current limiter 308 is reset. At 508, the phase cut dimmer turns on again, and the current level increases to a peak of approximately 0.20A (as indicated by line 514), followed by a discharge cycle at 510, and then a phase cut dimmer off cycle at 512. The cycle is then repeated.

As discussed previously, the negative current spike 102 and the positive current spike 108 in fig. 1 represent non-productive portions of the phase cut dimmer power cycle. As indicated in fig. 5, the system 400 in fig. 4 substantially eliminates current spikes by ramping the current limit over a predetermined time period corresponding to the on-phase in the phase cut dimmer circuit. By adaptively varying the current limit as a function of time and as a function of the feedback current across the sense resistor 420, the peak current is limited to a level approximately equal to the operating current level (i.e., approximately 0.20A). Thus, adaptive current limiter 308 substantially eliminates non-productive portions of the phase cut dimmer power cycle, reduces power losses and forces experienced on the phase cut dimmer circuit and other associated circuitry, and substantially reduces current oscillations that can cause the current level to fall below the holding current of the phase cut dimmer circuit, thereby causing the phase cut dimmer circuit to inadvertently open.

Adaptive current limiter 308 can be used to dim the LED lamp. In particular, because of the reduced size, the adaptive current limiter 308 can be useful in applications where space is at a premium for LEDs instead of light bulbs. In addition, adaptive current limiter 308 can be used with other circuits, such as snubber circuits and over-voltage and over-current protection circuits, to suppress transient power events while protecting the associated circuitry.

Although adaptive current limiter 308 is described using discrete components, it should be understood that adaptive current limiter 308 can be applied on a single or multiple die. Further, in some embodiments, adaptive current limiter 308 includes analog control circuitry, such as current mirrors, voltage compensation, or other circuitry, configured to provide the same functionality at lower sense voltages (such as between 100 and 300mV instead of 650 mV). At lower sensing voltages, circuitry can be introduced to further reduce the power consumption of the switching inductor.

In certain embodiments, FET428 and sense resistor 420 are external to adaptive current limiter 308, making it possible to adjust performance for a particular application. In a specific example, an adaptive current limiter solution using a control circuit to simulate BJT414, capacitor 422, resistor 424, zener diode 426, resistors 410, 412, 416, and 418, sense resistor 420, and FET428 can be produced in the same package. In some examples, the sense resistor 420 is external and one or more sense resistors 420 having different resistance values are utilized with the adaptive current limiter 308. In such an example, the value of the sense resistor 420 may be selected to control a particular current level. In a particular example, the FET428 and the sense resistor 420 can be designed for a particular power range.

In another embodiment, FET428 and sense resistor 420 can be implemented integrally using a sense FET (sensefet) to monitor current. Furthermore, a higher degree of integration is possible from a product perspective. The four-diode rectifier bridge formed by diodes 222, 224, 226 and 228 can be included in the same package as adaptive current limiter 308, providing a four-lead, small, single-component solution that is advantageous for space-limited applications.

While the above discussion has focused on the adaptive current limiter 308 for limiting the magnitude of the on-current peak in dimming solutions for compact fluorescent lamps and LEDs and the like or solid state lighting solutions, it should be noted that the adaptive current limiter 308 can be used for other functions. Furthermore, when the TRIAC dimmer is not connected to the input, the adaptive current limiter 308 does not unduly interfere with the operation of the system operation, making it possible to incorporate the adaptive current limiter 308 into an EMI filter, for example, for use within a light socket. When such a device is used without a dimmer, the adaptive current limiter 308 consumes a small amount of power, primarily due to the sense resistor 420, and does not interfere with system functions. In addition, the adaptive current limiter 308 can be used with leading edge or trailing edge type dimmers. Thus, the adaptive current limiter 308 can be used with or without an external dimmer circuit.

Along with the systems and techniques described above with respect to fig. 3-5, the adaptive current limiter controllably adjusts the current limit as a function of time and as a function of the feedback current, limits the peak current based on the controlled current and allows the EMI circuit to charge. By limiting the peak on-current spike, the oscillating current within the EMI filter is substantially eliminated and power loss due to current surges is reduced. Furthermore, because the adaptive current limiter is configured to limit peak current independent of input current, providing time for charging the EMI filter components, the adaptive current limiter can be used to adapt to different types of phase cut dimmers to facilitate various types of lamps, including compact fluorescent lamps, LED bulbs, and other solid state lamps, to work together, reducing ringing and flicker. In particular, by using the feedback current, the adaptive current limiter circuit adjusts the current limit as a function of the current drawn by the load and is not based on the input voltage.

In one aspect, an adaptive current limiter includes a sensing element having a first sensing terminal coupled to a supply terminal, and a second sensing terminal. The sense element generates a sense voltage in response to the feedback current. The adaptive current limiter also includes a current pass element including a first terminal for receiving a feedback current, a second terminal coupled to the second sense terminal, and a control terminal. Further, the adaptive current limiter includes a controller coupled to the sensing element and to the control terminal, the controller adjusting a feedback current conducted by the current passing element based on the sensed voltage and the time-varying voltage signal.

In one particular aspect, the controller includes a reference generator to generate a time-varying voltage signal. In another aspect, the time-varying voltage signal comprises a ramp signal that increases substantially linearly over a predetermined time. In yet another aspect, the time-varying voltage signal comprises a ramp signal that increases substantially exponentially over a predetermined time period. In yet another particular aspect, the controller further includes an amplifier including a first input coupled to the sensing element to receive the sensed voltage, a second input coupled to the reference generator to receive the time-varying voltage signal, and an output coupled to the control terminal of the current passing element.

In another aspect, a current limiter circuit includes a current pass element including a first current electrode adapted to be coupled to a load circuit to receive a feedback current, a second current electrode, and a control electrode. The current limiter circuit further comprises a sense resistor comprising a first sense terminal adapted to be coupled to the supply terminal, and a second sense terminal coupled to the second current electrode of the current passing element. The sense resistor generates a sense voltage proportional to the feedback current. The current limiter circuit also includes a controller coupled to the sense resistor and to the control electrode. The controller generates a control signal based on the sense voltage and the time-varying signal to controllably increase the current conducted by the current-passing element over a predetermined time.

In a particular aspect, the controller includes a reference generator for providing a time-varying signal and an amplifier including a first input for receiving the time-varying signal, a second input for receiving a sense voltage, and an output coupled to a control electrode of the current passing element. In another particular aspect, the current limiter circuit includes a resistor including a first terminal coupled to the second sense terminal and including a second terminal, and a capacitor including a first electrode coupled to the second terminal of the resistor, and a second electrode. The current limiter circuit also includes a transistor including a first terminal coupled to the control electrode of the current pass element, a control terminal coupled to the second electrode of the capacitor, and a second terminal coupled to the first sense terminal of the sense resistor. The resistor and the capacitor define a time constant for generating a time varying signal applied to the control terminal of the transistor.

In yet another particular aspect, the controller includes a second resistor including a first terminal coupled to the second sense terminal of the sense resistor and including a second terminal coupled to the control terminal of the transistor. The controller also includes a third resistor including a first terminal coupled to the control terminal of the transistor and including a first sense terminal coupled to the sense resistor and a second terminal coupled to the second terminal of the transistor. In yet another aspect, the current limiter circuit includes a breakdown diode including an anode coupled to the second sense terminal of the sense resistor and a cathode coupled to the control electrode of the current passing element. The breakdown diode is adapted to conduct current from the cathode to the anode when a voltage potential between the control electrode and the second current electrode of the current passing element exceeds a breakdown voltage of the breakdown diode.

In yet another aspect, a phase cut dimmer system includes first and second power supply terminals and a phase cut dimmer circuit adapted to be coupled to an Alternating Current (AC) power supply for generating a phase cut signal. The phase cut dimmer system further includes a full wave rectifier coupled to the phase cut dimmer circuit for generating a rectified version of the phase cut signal. A full wave rectifier is coupled to the first and second supply terminals for receiving a rectified supply. The phase cut dimmer system also includes an electromagnetic interference (EMI) filter coupled to the first and second power supply terminals, and includes a current limiter circuit coupled to the second power supply terminal and to the EMI filter. The current limiter circuit is responsive to the feedback current and the time varying signal to regulate the feedback current over a predetermined time.

In one particular aspect, the current limiter circuit increases the feedback current substantially linearly over a predetermined time. In another particular aspect, a current limiter circuit includes a first transistor including a first current electrode for receiving a feedback current, a second current electrode, and a control electrode. The current limiter circuit also includes a bias circuit including a first terminal coupled to the first power supply terminal and a second terminal coupled to the control electrode of the transistor. The current limiter circuit also includes a second transistor including a first current electrode coupled to the control electrode of the first transistor, a control electrode, and a second current electrode coupled to the second power supply terminal. Further, the current limiter circuit includes a sense resistor including a first sense terminal coupled to the second current electrode of the first transistor and a second sense terminal coupled to the second power supply terminal, and includes a first resistor including a first terminal coupled to the second sense terminal and a second terminal coupled to the control electrode of the second transistor. The current limiter circuit further includes a second resistor including a first terminal coupled to the control electrode of the second transistor and a second terminal coupled to the second current electrode of the second transistor, and a signal generator coupled between the second sense terminal of the sense resistor and the control electrode of the second transistor for providing a time-varying signal.

In yet another particular aspect, the time-varying signal generator includes a third resistor having a first terminal coupled to the second sense terminal of the sense resistor and a second terminal, and includes a capacitor including a first electrode coupled to the second terminal of the third resistor and a second electrode coupled to the control electrode of the second transistor. In yet another particular aspect, the phase cut dimmer system further comprises first and second output terminals coupled to the EMI filter and an isolation circuit comprising a first terminal coupled to the first output terminal and a second terminal coupled to the second output terminal. The isolation circuit is adapted to be coupled to a load circuit that includes one of a Light Emitting Diode (LED) light, a compact fluorescent light, and a motor.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the scope of the invention.

Claims (10)

1. An adaptive current limiter comprising:

a sense transistor including a first sense terminal coupled to a supply terminal, and a second sense terminal, the sense transistor generating a sense voltage in response to a feedback current;

a current pass transistor comprising a first terminal for receiving an input current, a second terminal coupled to the second sense terminal, and a control terminal; and

a controller coupled to the sense transistor and to the control terminal, the controller to sense a start of an on phase of a phase cut signal and limit the feedback current conducted by the current pass transistor during the start of the on phase of the phase cut signal.

2. The adaptive current limiter of claim 1, wherein the controller increases the feedback current conducted by the current pass transistor from a first current level to a second current level over a predetermined period of time.

3. The adaptive current limiter of claim 1, wherein the current pass transistor comprises:

a field effect transistor including a first current electrode for receiving the feedback current, a control electrode coupled to the controller, and a second current electrode coupled to the sense transistor.

4. The adaptive current limiter of claim 3, wherein the controller comprises:

a second transistor comprising a first current electrode coupled to the control electrode of the field effect transistor, a control electrode, and a second current electrode coupled to the power supply terminal;

a first resistor comprising a first terminal coupled to the second sense terminal, and a second terminal coupled to a control electrode of the second transistor;

a second resistor comprising a first terminal coupled to a control electrode of the second transistor, and a second terminal coupled to the supply terminal; and

a third resistor comprising a first terminal coupled to the second sense terminal, and a second terminal; and

a capacitor including a first electrode coupled to the second terminal of the third resistor and a second electrode coupled to the control electrode of the second transistor.

5. The adaptive current limiter of claim 4, wherein the third resistor and the capacitor determine a time-varying current limit.

6. A current limiter circuit comprising:

a current pass transistor comprising a first current electrode adapted to be coupled to a load circuit to receive a feedback current, a second current electrode, and a control electrode;

a sense resistor including a first sense terminal adapted to be coupled to a supply terminal, and a second sense terminal coupled to a second current electrode of the current pass transistor, the sense resistor to generate a sense voltage proportional to the feedback current; and

a controller coupled to the sense resistor and to the control electrode, the controller to sense a start of an on phase of a phase cut signal to controllably increase a current conducted by the current pass transistor at the start of the on phase of the phase cut signal.

7. The current limiter circuit of claim 6, wherein the controller comprises:

a reference generator responsive to the sense voltage to generate a time-varying voltage signal during a start of an on phase of the phase-cut signal; and

an amplifier including a first input for receiving the time-varying voltage signal, a second input for receiving the sense voltage, and an output coupled to a control electrode of the current-passing transistor.

8. The current limiter circuit of claim 6, further comprising:

a first resistor comprising a first terminal coupled to the second sense terminal and comprising a second terminal;

a capacitor comprising a first electrode coupled to a second terminal of the first resistor, and a second electrode; and

a second transistor comprising a first terminal coupled to a control electrode of the current pass transistor, a control terminal coupled to a second electrode of the capacitor, and a second terminal coupled to a first sense terminal of the sense resistor; and

thereby, the first resistor and the capacitor define a time constant for limiting the current conducted by the current pass transistor.

9. A phase cut dimmer system comprising:

a first power supply terminal and a second power supply terminal;

a phase cut dimmer circuit adapted to be coupled to an alternating current, AC, supply for generating a phase cut signal;

a full wave rectifier coupled to the phase cut dimmer circuit for generating the phase cut signal in a rectified form, the full wave rectifier coupled to the first and second power supply terminals for providing a rectified power supply;

an electromagnetic interference filter coupled to the first and second power supply terminals; and

a current limiter circuit coupled to the second supply terminal and to the electromagnetic interference filter, the current limiter circuit responsive to a feedback current sensing a start of an on phase of a phase cut signal and adjusting the feedback current at the start of the on phase of the phase cut signal.

10. The phase cut dimmer system of claim 9, wherein the current limiter circuit comprises:

a reference generator for generating a time-varying voltage signal during the start of an on-phase of the phase-cut signal in response to a sense voltage;

a sense resistor for converting the feedback current to the sense voltage;

an amplifier responsive to the time-varying voltage signal and the sense voltage to generate a time-varying control signal; and

a current pass transistor including a first terminal for receiving the feedback current, a second terminal coupled to the sense resistor, and a control terminal coupled to the amplifier and responsive to the time-varying control signal for limiting the feedback current.