Current limiting circuit and method for LED driver

Aspects of the disclosure provide a circuit that includes a driver circuit and a current limiter circuit. The driver circuit is configured to drive a load with an output current when the load is coupled with the driver circuit. The current limiter circuit is configured to turn on a path to deplete a portion of the output current from the driver circuit in order to prevent a load current flowing through the load from exceeding a current limit.

BACKGROUND

Light emitting diode (LED) lighting devices provide the advantages of low power consumption and long service life. Thus, LED lighting devices may be used as general lighting equipment to replace, for example, fluorescent lamps, bulbs, halogen lamps, and the like.

SUMMARY

Aspects of the disclosure provide a circuit that includes a driver circuit and a current limiter circuit. The driver circuit is configured to drive a load with an output current when the load is coupled with the driver circuit. The current limiter circuit is configured to turn on a path to deplete a portion of the output current from the driver circuit in order to prevent a load current flowing through the load from exceeding a current limit.

In an embodiment, the current limiter circuit includes a current sensor and a switch. The current sensor is configured to sense the load current flowing through the load. The switch is configured to turn on the path when the load current flowing through the load reaches the current limit. Further, the switch includes a transistor that is controlled by a voltage generated based on the sensed load current. In an example, the current sensor is configured to have a resistance determined based on the current limit.

According to an aspect of the disclosure, the driver circuit includes a capacitor that is charged to a voltage limit before a light emitting diode (LED) string is coupled with the driver circuit. The voltage limit is larger than a forward voltage of the LED string. In an embodiment, the driver circuit is configured to output an initial current to discharge the capacitor when the LED string is initially coupled with the driver circuit and is configured to output a controlled current to the LED string after the initial discharging. The current limiter circuit is configured to turn on the path when the LED string is initially coupled with the driver circuit to deplete a portion of the initial current, and is configured to turn off the path after the initial discharging.

Aspects of the disclosure provide a current limiting method. The method includes driving a load by a driver circuit with an output current when the load is coupled with the driver circuit, and turning on a path to deplete a portion of the output current from the driver circuit to prevent a load current flowing through the load from exceeding a current limit.

Aspects of the disclosure also provide a light emitting diode (LED) driving system. The LED driver includes a driver circuit configured to drive an LED string with an output current when the LED string is coupled with the driver circuit, and a current limiter circuit configured to turn on a path to deplete a portion of the output current from the driver circuit to prevent an LED current flowing through the LED string from exceeding a current limit.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1shows a diagram of an electronic system100according to an embodiment of the disclosure. The electronic system100includes a driver circuit160, a current limiter circuit170and a load169. These elements are coupled together as shown inFIG. 1. The driver circuit160is coupled to a power source101and is configured to receive electrical power from the power source101, regulate a power supply to have a desired power characteristic for the load169, and provide regulated power supply to the load169. In an example, during a normal operation, the driver circuit160provides a power supply having a controlled current to the load169that has at least an operational parameter depending on the current. Thus, the driver circuit160controls the current provided to the load169in order to control the operation of the load169. Generally, the current output from the driver circuit160is within a specific range, such as a range defined by a minimum current and a maximum current. However, under certain circumstances, the current output from the driver circuit160may be out of the specific range. The current limiter circuit170is configured to limit a current flowing through the load169, and thus to protect the load169from damage.

In an embodiment, the load169is swappable. In an example, the electronic system100includes an interface component having two connectors OUT1and OUT2. The load169includes two nodes that can be connected to the OUT1and OUT2, and thus the load169is electrically coupled with the rest of the electronic system100. The load169can be pulled out to be disconnected from the two connectors OUT1and OUT2, and thus to be decoupled from the rest of the electronic system100, and a new load169can be plugged in to be connected with the connectors OUT1and OUT2and thus the new load169is electrically coupled with the rest of the electronic system100. Further, the load169can be hot-swappable which means the load169is swapped without powering down the electronic system100.

According to an aspect of the disclosure, at the time of a hot-swap, when the new load169is plugged in to be connected with the connectors OUT1and OUT2, an instantaneous current outrushes from the driver circuit160. The instantaneous current can be larger than the maximum current for example, and then cause the load169to be damaged. The current limiter circuit170is configured to limit the current flowing through the load169below the maximum current for example, and thus to protect the load169. In an example, when the current flowing through the load169is larger than a current limit, such as the maximum current, the current limiter circuit170switches on a bleeder path in parallel with the load169to allow the outrush current to flow through the bleeder path, and thus limit the current flowing through the load169and protect the load169from damage.

During a normal operation, in an embodiment, the controlled current is generally smaller than the current limit, and the current limiter circuit170switches off the bleeder path to save power.

The electronic system100can be any suitable system, such as a lighting system, a fan system, and the like. The power source101can be any suitable power source, such as an alternating current (AC) power source, a direct current (DC) power source, and the like.

In theFIG. 1example, the electronic system100is a lighting system in which the load169includes a string of light emitting diodes (LEDs). Further, the driver circuit160is coupled to an AC power source101, converts the AC power to a DC power supply, and provides a controlled DC current to the string of LEDs in the load169during a normal operation. The string of LEDs emits light having a light intensity, and the light intensity depends on the controlled DC current. The driver circuit160controls the DC current in order to control the light intensity emitted by the string of LEDs.

Specifically, inFIG. 1example, the driver circuit160includes a rectifier103, a control circuit110, and an energy transfer module120. These elements are coupled together as shown inFIG. 1. The rectifier103rectifies an AC voltage to a fixed polarity, such as to be positive. In theFIG. 1example, the rectifier103is a bridge rectifier. The bridge rectifier103receives the AC voltage, and rectifies the received voltage to a fixed polarity, such as to be positive. The rectified voltage VRECTis provided to the following circuits, such as the control circuit110, the energy transfer module120, and the like, in the electronic system100.

The energy transfer module120transfers electric energy provided by the rectified voltage VRECTto the load169under the control of the control circuit110. In an embodiment, the energy transfer module120is configured to use a magnetic component, such as a transformer, an inductor, and the like to transfer the electric energy. The energy transfer module120can have any suitable topology, such as a fly-back topology, a buck-boost topology, a buck topology, a boost topology, and the like. In theFIG. 1example, the energy transfer module120uses a fly-back topology that includes a transformer122, a switch SW, a diode D and a capacitor C coupled together as shown inFIG. 1. The transformer122includes a primary winding (P) coupled with the switch SW to receive the rectified voltage VRECT, and a secondary winding (S) coupled with the diode D and the capacitor C to drive the load169.

The control circuit110includes any suitable circuits to detect circuit parameters, such as current and voltage in the electronic system100, and generate one or more control signals accordingly to control the operation of the electronic system100. In an example, the control circuit110includes a detecting circuit (not shown) configured to detect a current IPflowing through the primary winding P and the switch SW. Further, the control circuit110includes a PWM generation circuit (not shown) configured to generate a pulse width modulation (PWM) signal with pulses having a relatively high frequency, such as in the order of 100 KHz, and the like. The control circuit110uses the PWM signal to control the switch SW to transfer the electric energy from the primary winding P to the secondary winding S in the transformer122.

Specifically, in an example, when the switch SW is switched on, a current IPflows through the primary winding of the transformer122, and the switch SW. The polarity of the transformer122and the direction of the diode D can be arranged such that there is no current in the secondary winding S when the switch SW is switched on. Thus, the received electric energy is stored in the transformer122.

When the switch SW is switched off, the current IPbecomes zero. The polarity of the transformer122and the direction of the diode D can enable the secondary winding S to deliver the stored electric energy to the capacitor C, and the energy stored in the capacitor C can be used to drive the load169.

According to an aspect of the disclosure, the LED string in the load169is swappable. In an example, the load169and the other components of the electronic system100, such as the driver circuit160, the current limiter circuit170and the like, are assembled into a package to form an LED lighting device to replace, for example, a fluorescent lamp, a halogen lamp, and the like. The LED string can be pulled out of the LED lighting device and replaced with another LED string. Further, in an embodiment, the LED string is hot-swappable. For example, the present LED string can be pulled out and a new LED string can be plugged in while the electronic system100remains powered on. The new LED string plugged in can be the same type as the LED string that has been pulled out or can be a different type from the LED string that has been pulled out. In an example, the new LED string has the same number of LEDs in the string as the LED string that has been pulled out. In another example, the new LED string has a different number of LEDs from the LED string that has been pulled out.

In an embodiment, at the time of a hot-swap, when the new LED string is plugged in, an instantaneous current output from the driver circuit160is relatively large. Specifically, when the previous LED string is pulled out, because the electronic system100is still powered on, the control circuit110keeps generating pulses in the PWM signal to repetitively switch on/off the switch SW. Thus, the capacitor C keeps charging and the voltage on the capacitor C raises. In an embodiment, the control circuit110includes an over voltage protection module111configured to monitor the voltage on the capacitor C, and disable the pulse generation for the PWM signal when the voltage on the capacitor C is too large, such as larger than a voltage limit.

Generally, the voltage limit is higher than a forward voltage of a specific LED string. When the new LED string has fewer number of LEDs than the specific LED string, the voltage on the capacitor C is much higher than the forward voltage of the new LED string, and then the instantaneous current can be larger than a current limit, such as a maximum current, and the like, for the new LED string and can cause damage to the new LED string.

The current limiter circuit170senses a current flowing through the load169, and switches on a bleeder path when the current flowing through the load169is larger than a current limit. Specifically, in theFIG. 1example, the current limiter circuit170includes a switch171, a first resistor172and a second resistor173. These elements are coupled together as shown inFIG. 1. The first resistor172is coupled in series with the connector OUT1/OUT2. When a load169is plugged in, a current flows through the load169. The current also flows through the first resistor172and a voltage drop on the first resistor172is indicative of the current flowing through the load169.

When the voltage drop on the first resistor172is larger than a threshold, the switch171switches on a bleeder path. In theFIG. 1example, the switch171is an NPN bipolar junction transistor, and the voltage drop on the first resistor172is applied on the base-emitter of the NPN bipolar junction transistor171. When the voltage drop on the first resistor172is larger than a cut-in voltage of the NPN bipolar junction transistor171(e.g., P-N junction forward voltage, about 650 mV for silicon bipolar junction transistor at room temperature), the NPN bipolar junction transistor171is in active mode. This applied voltage causes the P-N junction between the base and emitter to be forward biased, allowing a current flowing through the NPN bipolar junction transistor171from the collector to the emitter. The current flowing through the NPN bipolar junction transistor171from the collector to the emitter relates to the voltage on the first resistor172exponentially. In an example, an increase of the voltage drop on the first resistor172by approximately 60 mV increases the emitter current by a factor of 10.

According to an aspect of the disclosure, the resistance (Rsns) of the first resistor172is suitably designed to protect the LED string. In an example, during a normal operation, the electronic system100is specified to provide a normal LED current (ILED) to the LED string. Further, during the normal operation, the NPN bipolar junction transistor171is turned off due to relatively small voltage drop on the first resistor172. Thus, the output current from the driver circuit160(I1) is the same as the current flowing through the LED string (I2), and the current flowing through the NPN bipolar junction transistor (I3) is zero.

Further, in an example, the LED string is specified with a maximum LED current (Ilimit) that is N times of the normal LED current. N is a multiple factor of the maximum LED current to the normal LED current. In an example, N is a value in the range of [2.5, 3]. Then, in an example, the resistance of the first resistor172is calculated using Eq. 1:

Rsns=VbeIlimit=VbeN×ILEDEq.⁢1
Where Vbeis the cut-in voltage of the NPN bipolar junction transistor, and is about 650 mV for silicon bipolar junction transistor at room temperature. For example, when the normal LED current is 1 A and the multiple factor N is 3, the first resistor172is designed to have a resistance of about 217 mΩ.

During a normal operation, the driver circuit160outputs the normal LED current to the load169, and the voltage on the capacitor C is about the forward voltage of the LED string in the load169. Further, the voltage drop on the first resistor172is smaller than the cut-in voltage, and thus the NPN bipolar junction transistor171is turned off.

When the load169is pulled out, charges are accumulated on the capacitor C and thus raise the voltage on the capacitor C. When the voltage on the capacitor C is large enough to activate the over voltage protection, the pulse generation for the PWM signal is disabled. When a new load169is plugged in, the voltage on the capacitor C may be much larger than the forward voltage of the new load169, for example the new LED string has fewer number of LEDs than the previous LED string. The voltage on the capacitor C causes an initial current outrushing to the load169for discharging. When the discharging current via the load169is about the maximum LED current, the voltage drop on the first resistor172is about the cut-in voltage of the NPN bipolar junction transistor171and thus the NPN bipolar junction transistor171turns on, and the charges on the capacitor can be discharged via the NPN bipolar junction transistor171.

Due to the discharging, the voltage on the capacitor C drops and the current flowing through the load169decreases. When the voltage drop on the first resistor172is below the cut-in voltage, the NPN bipolar junction transistor171turns off. The control circuit110resumes the pulse generation for the PWM signal, and the electronic system100enters the normal operation.

According to an aspect of the disclosure, the NPN bipolar junction transistor171provides low cost, high robustness, and fast switching performance. Further, in an example, the second resistor173is configured to protect the NPN bipolar junction transistor171. The resistance of the second resistor173is suitably designed to allow the NPN bipolar junction transistor171to be the major discharging path.

It is noted that the electronic system100can be suitably modified. In an example, the second resistor173is not needed. In another example, other suitable switch device, such as a PNP bipolar junction transistor, a metal-oxide-semiconductor field effect transistor (MOSFET), and the like, can be used to replace the NPN bipolar junction transistor171.

FIG. 2shows another diagram of an electronic system200according to an embodiment of the disclosure. The electronic system200operates similarly to the electronic system100described above. The electronic system200also utilizes certain components that are identical or equivalent to those used in electronic system100; the description of these components has been provided above and will be omitted here for clarity purposes. In this embodiment, the current limiter circuit270uses a PNP bipolar junction transistor271that operates similarly to the NPN bipolar junction transistor171.

FIG. 3shows a flowchart outlining a process example300according to an embodiment of the disclosure. In an example, the process300is executed by the electronic system100during a hot-swap that replaces an LED string without shutting down the power source101. The process starts at S301and proceeds to S310.

At S310, an LED string is pulled out of a lighting system without shutting down a power source that provides power to the lighting system. In theFIG. 1example, the LED string in the load169is pulled out to be disconnected from the connectors OUT1and OUT2while the driver circuit160remains in the power-up state. Thus, the driver circuit160keeps driving a controlled LED current. Because the LED string has been removed, charges are accumulated on the capacitor C and raise the voltage on the capacitor C.

At S320, an over voltage protection is activated. In theFIG. 1example, when the voltage on the capacitor C reaches a voltage limit, the over voltage protection module111activates over voltage protection and disables the pulse generation for the PWM signal. Then, charges stop building up on the capacitor C, and the voltage on the capacitor C remains about the level of the voltage limit.

At S330, a new LED string is installed under hot-swap condition. In theFIG. 1example, a new LED string is plugged in to be connected with the connectors OUT1and OUT2. When the voltage on the capacitor C is larger than the forward voltage of the new LED string, the charges on the capacitor C start discharging through the new LED string.

At S340, a current bleeding path to limit the current flowing through the new LED string is switched on. In theFIG. 1example, the voltage on the capacitor C is larger than the forward voltage of the new LED string, and causes an initial current outrushing to the new LED string. When the current flowing through the load169is about the maximum LED current, the voltage drop on the first resistor172is about the cut-in voltage for the NPN bipolar junction transistor171and thus the NPN bipolar junction transistor171turns on. In an example, because of the current amplifying operation of the NPN bipolar junction transistor171, a majority of the charges on the capacitor C can discharge via the NPN bipolar junction transistor171, and the current flowing through the new LED string is limited not to exceed the maximum LED current.

At S350, the electronic system enters a normal operation mode after the initial current outrushing. In theFIG. 1example, after the initial current outrushing, the voltage on the capacitor C reduces to about the forward voltage of the new LED string, then the control circuit110resumes the pulse generation for the PWM signal, and the driver circuit160resumes providing the controlled current to the new LED string. The controlled current is smaller than the maximum LED current (e.g., about one third of the maximum LED current), thus the voltage drop on the first resistor172is smaller than the cut-in voltage for the NPN bipolar junction transistor171, and the NPN bipolar junction transistor171turns off during the normal operation. The process then proceeds to S399and terminates.