Self-discharge control for an EMI filter capacitor

An independent bleeding integrated circuit device is provided to replace the bleeding resistor for an EMI filter capacitor, to establish a discharge path between the two terminals of the EMI filter capacitor when the EMI filter capacitor is disconnected from an AC power source, for discharging the EMI filter capacitor. When the EMI filter capacitor is connected with an AC power source, the discharge path is cut off to avoid power loss.

FIELD OF THE INVENTION

The present invention is related generally to an alternating current/direct current (AC/DC) power interface and, more particularly, to a bleeding circuit for an electro-magnetic interference (EMI) filter capacitor.

BACKGROUND OF THE INVENTION

An AC/DC power converter is typically used to convert a commercial AC power source into a DC power supply of a specific voltage. Referring toFIG. 1, at the AC power input terminals12and14of an AC/DC power converter for connecting to an AC power source10, there is an AC/DC power interface16which includes a bridge rectifier18to rectify the AC voltage VAC supplied by the AC power source10into a DC input voltage Vin for the AC/DC power converter to further converter into a DC output voltage. The AC/DC power interface16also includes an EMI filter capacitor X-CAP connected between the AC power input terminals12and14. However, the presence of the EMI filter capacitor X-CAP brings a risk of electric shock because once the AC power source10is removed, the EMI filter capacitor X-CAP will sustain the voltage of the AC power source10that occurs at the instant moment when the AC power source10is removed, and may be up to hundreds of volts. For this issue, conventional solutions to comply with safety specification NE60950 or IEC950 are to connect a bleeding resistor Rb parallel to the EMI filter capacitor X-CAP, such that upon removal of the AC power source10, the bleeding resistor Rb and the EMI filter capacitor X-CAP establish a circuit loop for discharging the EMI filter capacitor X-CAP. Nevertheless, the presence of the bleeding resistor Rb connected between the AC power input terminals12and14establishes a normally conductive current path between the AC power input terminals12and14, resulting in power loss Ploss=VAC2/Rb as long as the AC power source10is connected with the AC power input terminals12and14, and thereby reducing the efficiency of the AC/DC power converter. In addition, IEC950 requires the discharge time of the EMI filter capacitor X-CAP be shorter than the time constant of one second. Therefore, to conform to this safety specification, the larger the EMI filter capacitor X-CAP is, the smaller the bleeding resistor Rb must be, and yet the smaller this resistance, the greater the power loss caused by the bleeding resistor Rb. For example, under VAC=230V, if X-CAP=5 μF and Rb=150KΩ, then Ploss=353 mW. In other words, power loss of 353 mW occurs even when the AC/DC power converter is at no loading state or in a standby mode. While specifications on power loss were relatively loose in the past, the rising awareness of environmental protection has led to more tight requirements on power loss; however, power loss resulting from the bleeding resistor Rb has hindered compliance of the AC/DC power converter with today's much stricter environmental protection specifications.

U.S. Pat. No. 7,046,529 uses the circuit of the AC/DC power converter to generate a control signal for switching a plurality of resistors in the AC/DC power interface16between a plurality of configurations. Once the AC power source10is removed, the resistors are reconfigured to establish a smaller equivalent resistance as the bleeding resistor Rb to discharge the EMI filter capacitor X-CAP within a required time period. When the AC power source10is connected, the resistors are reconfigured to establish a larger equivalent resistance as the bleeding resistor Rb to reduce power loss. However, this approach does not eliminate the use of the bleeding resistor Rb, and thus power loss still occurs. Moreover, this approach requires modification of the circuit of the AC/DC power converter and is thus not applicable to existing AC/DC power converters unless the circuits of existing AC/DC power converters are redesigned.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a self-discharge bleeding circuit for an EMI filter capacitor.

Another objective of the present invention is to provide an independent bleeding integrated circuit device for an EMI filter capacitor.

Yet another objective of the present invention is to provide a bleeding method for an EMI filter capacitor.

Still another objective of the present invention is to provide an AC/DC power interface.

According to the present invention, a self-discharge bleeding circuit for an EMI filter capacitor includes a bleeding switch and a controller. The bleeding switch is connected between the two terminals of the EMI filter capacitor, and the controller turns on the bleeding switch when the voltage of the EMI filter capacitor has continuously exceeded a threshold value for a threshold time, such that a discharge path is established between the two terminals of the EMI filter capacitor for the EMI filter capacitor to discharge through the discharge path.

The self-discharge bleeding circuit does not need an external control signal and therefore can be produced as an independent bleeding integrated circuit device directly applicable to any AC/DC power converters.

According to the present invention, a bleeding method for an EMI filter capacitor includes connecting a bleeding switch between two AC power input terminals that are connected by the EMI filter capacitor, wherein the bleeding switch is open circuit when the two AC power input terminals are connected to an AC power source, and turning on the bleeding switch when the voltage of the EMI filter capacitor has continuously exceeded a threshold value for a threshold time, such that a discharge path is established between the two terminals of the EMI filter capacitor for the EMI filter capacitor to discharge through the discharge path.

According to the present invention, an AC/DC power interface connected between two AC power input terminals includes an external bridge rectifier connected to the two AC power input terminals, an EMI filter capacitor connected between the two AC power input terminals, a bleeding switch connected between the two AC power input terminals, and a controller connected to the bleeding switch. The controller turns on the bleeding switch when the voltage of the EMI filter capacitor has continuously exceeded a threshold value for a threshold time, such that a discharge path is established between the two terminals of the EMI filter capacitor for the EMI filter capacitor to discharge through the discharge path.

Since the bleeding switch is open circuit when the EMI filter capacitor is connected to an AC power source, additional power loss can be avoided, and this helps increase the efficiency of an AC/DC power converter.

DETAILED DESCRIPTION OF THE INVENTION

The bleeding resistor Rb shown inFIG. 1is only for discharging the EMI filter capacitor X-CAP after the AC power source10is removed to conform to safety specifications such as NE60950 and IEC950. In other words, the bleeding resistor Rb is totally unnecessary while the AC power source10supplies power to the AC/DC power converter. Based on this characteristic, a self-discharge bleeding circuit is designed as shown inFIG. 2, which includes a controller22and a bleeding switch24both connected between the AC power input terminals12and14. The bleeding switch24is open circuit when the AC power source10supplies power to the AC/DC power converter, and thus power loss attributable to the bleeding switch24can be avoided regardless of whether the AC/DC power converter is in normal operation or in a standby mode. However, once the AC power source10is removed, the bleeding switch24becomes a closed circuit such that the EMI filter capacitor X-CAP can discharge. Specifically, this self-discharge bleeding circuit can work independently, i.e., without cooperation of or control signals provided by other circuits, and thus it can be made into an independent bleeding integrated circuit (IC) device20as shown inFIG. 2, which can be directly applied to an AC/DC power converter without having to modify or redesign the circuit of the AC/DC power converter. The terminals V1and V2of the bleeding switch24are connected to the pins26and28of the independent bleeding IC device20, respectively, and the pins26and28are connected to the AC power input terminals12and14, respectively. The controller22is connected to the bleeding switch24and can turn on the bleeding switch24upon detecting that the voltage across the pins26and28has continuously exceeded a threshold value TH for a threshold time Tpre, so as to establish a discharge path between the pins26and28through which the EMI filter capacitor X-CAP can discharge. When the bleeding switch24is on, its equivalent resistance is very small, and this allows an AC/DC power converter to comply with NE60950 and IEC950.

Alternatively, as shown inFIG. 3, high voltage blocking elements Q1and Q2are additionally inserted between the self-discharge bleeding circuit and the pin26, and between the self-discharge bleeding circuit and the pin28, respectively, for blocking high voltages to the self-discharge bleeding circuit, such that the controller22and the bleeding switch24can be made by low-voltage manufacturing processes. In this embodiment, the high voltage blocking elements Q1and Q2use junction field effect transistors (JFETs), and according to the JFET's characteristics, the maximum value of each of the voltages V1and V2will be clamped under VB-Vp, where VB and Vp are the gate bias and the pinch-off voltage of the JFET, respectively. The bleeding switch24may include two face-to-face bleeding switches QA and QB, which are in an opposite series connection. In this embodiment, both the bleeding switches QA and QB are PMOS transistors and are controlled by a control signal Vg provided by the controller22, and the interconnection node N1between the bleeding switches QA and QB is also connected to the controller22. The bleeding switch24is on when the controller22turns on either the discharge switch QA or the discharge switch QB, and the bleeding switch24is off when the controller22turns off both the bleeding switches QA and QB. When the first discharge switch QA is on and the second discharge switch QB is off, a bleeding current Iqa can flow from the first AC power input terminal12to the second AC power input terminal14. When the first discharge switch QA is off and the second discharge switch QB is on, a bleeding current Iqb can flow from the second AC power input terminal14to the first AC power input terminal12.

FIG. 4shows an embodiment of the controller22inFIG. 3, in which the input terminals of the bridge rectifier30are connected to the terminals V1and V2of the bleeding switch24, respectively, the output terminals of the bridge rectifier30are connected to the node N1and the positive input terminal VS of a comparator32, respectively, and a voltage source Vref1is connected between the negative input terminal of the comparator32and the node N1. For the sake of distinction, the bridge rectifier18shown inFIG. 3is hereinafter called the external bridge rectifier, and the bridge rectifier30in the self-discharge bleeding circuit is called the internal bridge rectifier. In this embodiment, the internal bridge rectifier30is a full-wave bridge rectifier for rectifying the voltage across the terminals V1and V2of the bleeding switch24and to generate an under detection voltage VS, the voltage source Vref1provides a reference voltage Vref1, the comparator32compares the under detection voltage VS with the reference voltage Vref1and, if the under detection voltage VS is higher than the reference voltage Vref1, triggers a comparison signal V3, a timer34triggers a control signal V6when the comparison signal V3has lasted for the threshold time Tpre, and an inverting driver36generates the control signal Vg from the control signal V6so as to turn on one of the bleeding switches QA and QB. The timer34includes an inverter38for inverting the comparison signal V3into a reset signal V4, which will reset a switch SWc connected in parallel to a capacitor C1, the capacitor C1and a current source40jointly establish an integration circuit, the current source40provides a charging current I1for charging the capacitor C1to thereby generate a voltage V5, and a comparator42compares the voltage V5with a time-setting voltage Vpre to generate the control signal V6. In this embodiment, the reference voltage Vref1and the time-setting voltage Vpre determine the aforesaid threshold value TH and threshold time Tpre, respectively.

FIG. 5is a waveform diagram produced when the circuit shown inFIG. 4is applied to the controller22shown inFIG. 3, in which waveform50represents the voltage Vcap across the terminals of the EMI filter capacitor X-CAP, waveform52represents the voltage V1at the first terminal of the bleeding switch24, waveform54represents the voltage V2at the second terminal of the bleeding switch24, waveform56represents the under detection voltage VS, waveform58represents the reference voltage Vref1, waveform60represents the comparison signal V3, waveform62represents the voltage V5of the capacitor C1, waveform64represents the time-setting voltage Vpre, and waveform66represent the control signal Vg. Referring toFIGS. 3,4and5, when the AC power source10is in supplying power, the voltage Vcap across the terminals of the EMI filter capacitor X-CAP has a sine wave as shown by the waveform50, the maximum voltages applied to the terminals V1and V2of the bleeding switch24are clamped by the high voltage blocking elements Q1and Q2, respectively, such that the voltages V1and V2are as shown by the waveforms52and54, respectively, and the internal bridge rectifier30rectifies the voltages V1and V2to generate the under detection voltage VS as shown by the waveform56. When the under detection voltage VS rises above the reference voltage Vref1, as indicated at time t1inFIG. 5, the comparator32triggers the comparison signal V3, and until time t2, the under detection voltage VS falls below the reference voltage Vref1, the comparison signal V3terminates. During the period Ts in which the comparison signal V3is asserted, the switch SWc is open circuit and thus the charging current I1continues charging the capacitor C1, causing the voltage V5increasing. When the AC power source10is in supplying power, the time Ts that the comparison signal V3lasts is shorter than the threshold time Tpre determined by the time-setting voltage Vpre, thus the voltage V5will not reach the time-setting voltage Vpre, and the control signal Vg stays high, thereby keeping both the bleeding switches QA and QB being open circuits. After the AC power source10is removed at time t3shown inFIG. 5, the voltage Vcap remains as high as at the instant moment of removal of the AC power source10, i.e. time t3. If this voltage Vcap makes the under detection voltage VS higher than the reference voltage Vref1, the comparator32will continue the comparison signal V3, and when the time Ts that the comparison signal V3has lasted reaches the threshold time Tpre as shown at time t4inFIG. 5, the voltage V5of the capacitor C1reaches the time-setting voltage Vpre; as a result, the control signal Vg terminates, and the first discharge switch QA becomes closed circuit and thereby establishes a discharge path. Since the on resistance of the first discharge switch QA is very low, the bleeding current Iqa can be rather large, thereby allowing the EMI filter capacitor X-CAP to discharge rapidly. If, however, the voltage Vcap occurring when the AC power source10is removed is in a negative half cycle of the AC voltage VAC, termination of the control signal Vg will turn the second discharge switch QB into closed circuit so as to establish the bleeding current Iqb.

In more details, referring toFIGS. 3 and 4, if the AC power source10is removed in a positive half cycle of the AC voltage VAC, the gate-source voltage Vg-V2of the second discharge switch QB will be greater than the threshold voltage Vt of the second discharge switch QB, and thus the second discharge switch QB will not be conductive, and the discharge current Iqa of the EMI filter capacitor X-CAP will flow from the high voltage blocking element Q1to the high voltage blocking element Q2through the first discharge switch QA and the internal bridge rectifier30. In other words, the discharge path in this case consists of the high voltage blocking elements Q1and Q2, the first discharge switch QA, and the internal bridge rectifier30. If the AC power source10is removed in a negative half cycle of the AC voltage VAC, the gate-source voltage Vg-V1of the first discharge switch QA will be greater than the threshold voltage Vt of the first discharge switch QA, and thus the first discharge switch QA will not be conductive, and the discharge current Iqb of the EMI filter capacitor X-CAP will flow from the high voltage blocking element Q2to the high voltage blocking element Q1through the second discharge switch QB and the internal bridge rectifier30. In other words, the discharge path in this case consists of the high voltage blocking elements Q1and Q2, the second discharge switch QB, and the internal bridge rectifier30.

FIG. 6shows a simulation result obtained when the controller22ofFIG. 3uses the circuit ofFIG. 4. Regardless of whether the AC power source10is removed at a peak or valley of the voltage Vcap, the independent bleeding IC device20can relax the voltage Vcap to the range specified in the aforementioned safety specifications within one second.

In an embodiment as shown inFIG. 7, the bleeding switch24may include two bleeding switches QA and QB in an opposite parallel connection, where both the bleeding switches QA and QB are PMOS transistors and are controlled by a controller22.

FIG. 8shows a first embodiment of the controller22inFIG. 7, which includes comparators32and44both connected to the first terminal V1of the bleeding switch24for comparing the voltage V1with reference voltages Vref1and Vref2, respectively, where Vref1>Vref2. The comparator32triggers a first comparison signal V3when the voltage V1becomes higher than the first reference voltage Vref1, and the comparator44triggers a second comparison signal V7when the voltage V1becomes lower than the second reference voltage Vref2. A timer34counts the time Ts1that the comparison signal V3lasts or the time Ts2that the comparison signal V7lasts. After removal of the AC power source10, when either the time Ts1or the time Ts2reaches a threshold time Tpre, the timer34will trigger a control signal V6, from which an inverting driver36generates the control signal Vg. The control signal Vg turns on the discharge switch QA or QB, thereby establishing a discharge path for discharging the EMI filter capacitor X-CAP. In the timer34ofFIG. 8, the inverter38, the reset switch SWc, the capacitor C1, the current source40and the comparator42are identical to their counterparts inFIG. 4, and an OR gate46is additionally provided for generating a signal V8as the input of the inverter38according to the comparison signals V3and V7. The signal V8will be triggered regardless of whether the comparison signal V3or V7is triggered. In other embodiments, it may configure the comparators32and44to compare the voltage V2, instead of the voltage V1, at the second terminal of the bleeding switch24with the reference voltages Vref1and Vref2, respectively, thereby identifying whether the AC power source10has been removed.

FIG. 9is a waveform diagram of the circuit shown inFIG. 8when the AC power source10is removed in a positive half cycle of the AC voltage VAC, in which waveform70represents the half-wave rectified voltage of the voltage Vcap across the terminals of the EMI filter capacitor X-CAP, waveform52represents the voltage V1, waveform58represents the first reference voltage Vref1, and waveform72represents the second reference voltage Vref2. Referring toFIGS. 7,8and9, after the AC power source10is removed at time t1, the voltage Vcap stays as high as at the instant moment when the AC power source10is removed as shown by the waveform70. If the voltage V1is higher than the first reference voltage Vref1, the comparator32will keep the comparison signal V3asserted. Once the time Ts1that the comparison signal V3has lasted reaches the threshold time Tpre at time t2, the voltage V5reaches the time-setting voltage Vpre as shown by waveforms62and64ofFIG. 9, respectively; in consequence, the control signal Vg is turned to low. With the first discharge switch QA having a gate-source voltage Vgsa less than its threshold voltage Vt, and the second discharge switch QB having a gate-source voltage Vgsb greater than its threshold voltage Vt, the first discharge switch QA is on while the second discharge switch QB is off. Thus, the high voltage blocking elements Q1and Q2and the first discharge switch QA jointly establish a discharge path for discharging the EMI filter capacitor X-CAP.

FIG. 10is a waveform diagram of the circuit shown inFIG. 8when the AC power source10is removed in a negative half cycle of the AC voltage VAC. At time t1, the voltage V1drops below the second reference voltage Vref2as shown by waveforms52and72, respectively, and because of that, the comparator44triggers the second comparison signal V7. At time t2, as shown by waveforms62and64, respectively, the time Ts2that the second comparison signal V7has lasted reaches the threshold time Tpre, and the voltage V5reaches the time-setting voltage Vpre. The controller22thus identifies the AC power source10as having been removed, and the control signal Vg is turned to low; as a result, the first discharge switch QA has a gate-source voltage Vgsa greater than the threshold voltage Vt, and the second discharge switch QB has a gate-source voltage Vgsb less than the threshold voltage Vt. In other words, the first discharge switch QA is off and the second discharge switch QB is on, such that the second discharge switch QB and the high voltage blocking elements Q1and Q2jointly establish a discharge path for discharging the EMI filter capacitor X-CAP.

FIG. 11shows a simulation result obtained when the controller22ofFIG. 7uses the circuit ofFIG. 8. Regardless of whether the AC power source10is removed at a peak or valley of the voltage Vcap, the independent bleeding IC device20can relax the voltage Vcap to the range specified in the aforementioned safety specifications within one second.

FIG. 12shows a second embodiment of the controller22inFIG. 7, in which a first comparator32is connected to the first terminal V1of the bleeding switch24for comparing the voltage V1with a reference voltage Vref1to trigger a first comparison signal V3_1when the voltage V1becomes higher than the reference voltage Vref1, a first timer34will trigger a first control signal V6_1once the time that the first comparison signal V3_1has lasted reaches the threshold time Tpre, a first inverting driver36generates a control signal Vga according to the first control signal V6_1to turn on the first discharge switch QA, a second comparator80is connected to the second terminal V2of the bleeding switch24for comparing the voltage V2with a reference voltage Vref1to trigger a second comparison signal V3_2when the voltage V2becomes higher than the reference voltage Vref1, a second timer82will trigger a second control signal V6_2once the time, that the second comparison signal V3_2has lasted reaches the threshold time Tpre, and a second inverting driver84generates a control signal Vgb according to the second control signal V6_2to turn on the second discharge switch QB. The timers34and82shown inFIG. 12may have the same circuit as detailed inFIG. 4.

In a different embodiment, it may use only one of the two groups of circuits shown inFIG. 12to detect one of the voltages V1and V2of the bleeding switch24for identifying whether the AC power source10has been removed.

FIG. 13shows a third embodiment of the self-discharge bleeding circuit shown inFIG. 2, in which the input terminals of an internal bridge rectifier30are connected to the pins26and28, respectively, so as for the internal bridge rectifier30to rectify the voltage Vcap of the EMI filter capacitor X-CAP to generate an under detection voltage Vh, and a high voltage blocking element Q1is connected between the first output terminal Vh of the internal bridge rectifier30and the bleeding switch24for clamping the under detection voltage Vh to generate an under detection voltage VS. As the maximum value of the under detection voltage VS is clamped by the high voltage blocking element Q1, both the controller22and the bleeding switch24can be made by low-voltage manufacturing processes. The bleeding switch24includes a discharge switch QA connected between the high voltage blocking element Q1and the second output terminal of the bridge rectifier30. The controller22detects the under detection voltage VS to identify whether the AC power source10has been removed. Once the AC power source10is removed, the controller22turns on the discharge switch QA such that the discharge switch QA, the high voltage blocking element Q1, and the bridge rectifier30jointly establish a discharge path for discharging the EMI filter capacitor X-CAP.

FIG. 14shows an embodiment of the controller22inFIG. 13. Referring toFIG. 14in conjunction withFIG. 5, a comparator32compares the under detection voltage VS with a reference voltage Vref1to trigger a comparison signal V3, where the under detection voltage VS has a waveform as shown by the waveform56inFIG. 5. As soon as the under detection voltage VS rises above the reference voltage Vref1, the comparison signal V3is asserted as shown by the waveform60, and the timer34counts the time Ts that the comparison signal V3lasts. Upon removal of the AC power source10, if the under detection voltage VS is higher than the reference voltage Vref1, as shown by waveforms56and58at time t3inFIG. 5, respectively, the comparator32will continue the comparison signal V3, and until time t4, the time Ts that the comparison signal V3has lasted reaches the threshold time Tpre, the voltage V5rises to the time-setting voltage Vpre, as shown by the waveforms62and64, respectively, the timer34triggers the control signal V6, and the control signal Vg generated by the inverting driver36according to the control signal V6is turned to low, thereby turning on the discharge switch QA for discharging the EMI filter capacitor X-CAP.

FIG. 15shows a simulation result obtained when the controller22ofFIG. 13uses the circuit ofFIG. 14. Regardless of whether the AC power source10is removed at a peak or valley of the voltage Vcap, the independent bleeding IC device20can relax the voltage Vcap to the range specified in the aforementioned safety specifications within one second.