TECHNICAL FIELD

[0001] The present invention relates to discharging circuit. The present invention more particularly relates to a discharging circuit to discharge a DC-link capacitor connected across DC links of a power electronic system.
BACKGROUND
[0002] In electronic power systems typically used in applications such as variable speed AC drives, uninterruptible power supplies (UPS), stand-alone and grid-connected systems, etc., AC power supply is rectified and converted to DC which is then routed through an inverter circuit to obtain the final output. There are two stages i.e. AC-DC converter stage and DC-AC inverter stage which are linked together with an intermediate filter capacitor known as DC-link capacitor to minimize the effects of voltage variations due to changes in load and to provide a low impedance path for high frequency switching currents and also to provide energy storage.
[0003] In conventional power electronic converters, an active discharge device/circuit is connected across the DC-link capacitor to reduce the idle time of the converter. Such active discharge device includes a discharging switch which helps in improving the safety of the converter by dissipating energy stored in different components that could cause potential damage to the operators of a power system where the converter is used, if left unnoticed. In case of short circuit defect of the discharging switch in the discharge circuit, the converter has to load more in normal operating condition which leads to heavier losses and affects its operating performance; while in case of open circuit defect of the discharging switch, irrespective of its gate signal, the discharging switch is in OFF mode which can put human safety at risk.

[0004] Thus, there is a need of an invention which overcomes the problems associated with the discharging device/circuit connected across the DC-link capacitor in conventional power electronic converters.
SUMMARY
[0005] This summary is provided to introduce concepts of the present invention. This summary is neither intended to identify essential features of the present invention nor is it intended for use in determining or limiting the scope of the present invention.
[0006] In accordance with an embodiment of the present invention, there is provided a DC-link discharge circuit for discharging a DC-link capacitor connected across DC bus links. The DC-link discharge circuit includes: a charging detection unit configured to detect a mode of operation of the DC-link capacitor and generate a charging detection signal based on the mode of operation; a voltage transducing unit capable of generating a DC-link voltage signal from a voltage across the DC bus links; a discharge signal generation unit configured to receive the charging detection signal and the DC-link voltage signal, and to generate a discharge signal based on at least one of the charging detection signal and the DC-link voltage signal; a discharging unit configured to receive the discharge signal and discharge the DC-link capacitor to a predefined value and to generate a discharge feedback signal; a fault monitoring unit configured to receive the discharge signal and the discharge feedback signal, and to generate a fault signal based on at least one of the discharge signal and the discharge feedback signal; and a signal isolation unit coupled to the fault monitoring unit and configured to transmit the fault signal to an external control system for fault detection.
[0007] Typically, the DC-link capacitor, the charging detection unit, the voltage transducing unit and the discharging unit are each connected across a positive bus and a negative bus of the DC bus links.

[0008] In an aspect, the voltage transducing unit includes a set of serially connected resistors and an output terminal, wherein a first end of the serially connected resistors is connected to the positive bus of the DC bus links, a second end of the serially connected resistors is connected to the negative bus of the DC bus links, and the output terminal providing the DC-link voltage signal is taken from a node of the serially connected resistors based on a scaling factor.
[0009] Typically, the charging detection unit is configured to monitor the voltage across the DC bus links to detect the mode of operation of the DC-link capacitor, and wherein the mode of operation is selected from a normal mode, a charging mode, and a discharging mode.
[0010] Typically, the charging detection signal is logic low when the DC-link capacitor is discharging in the discharging mode, and the charging detection signal is logic high when the DC-link capacitor is charging in the charging mode or when the DC-link capacitor is at a constant voltage in the normal mode.
[0011] In an aspect, the charging detection unit includes serially connected bleeder resistors, a differentiator circuit and a control power supply circuit including serially connected zener diodes, wherein the bleeder resistors, the differentiator circuit and the control power supply circuit are interconnected.
[0012] In an aspect, the DC-link voltage signal includes a scaled down DC-link voltage signal.
[0013] Typically, the discharge signal generation unit is configured to generate the discharge signal by comparing the DC-link voltage signal with a reference voltage.

[0014] In an aspect, the discharge signal generation unit includes a gate signal generation circuit and a gate pull down switch, wherein the charging detection signal clips a gate signal of the gate pull down switch to zero during charging of the DC-link capacitor whereby the gate pull down switch clips the discharge signal to zero during charging of the DC-link capacitor.
[0015] In an aspect, the discharging unit includes a discharging resistor, a discharge switch, and a feedback resistor, wherein: the discharging resistor is configured to dissipate energy of the DC-link capacitor while discharging, the discharge switch is configured to receive the discharge signal at a gate thereof for turning on the switch and dissipating the energy of the DC-link capacitor through the discharging resistor while discharging, and the feedback resistor includes an emitter follower resistor configured to generate the discharge feedback signal indicative of operating status of the discharge switch.
[0016] In an aspect, the fault monitoring unit includes at least one of an open circuit fault detection circuit and a short circuit fault detection circuit, wherein: the open circuit fault detection circuit is configured to generate an open circuit fault signal indicative of open circuit fault of the discharge switch when the discharge signal is high and the discharge feedback signal is low, and the short circuit fault detection circuit is configured to generate a short circuit fault signal indicative of short circuit fault of the discharge switch when the discharge signal is low and the discharge feedback signal is high.'
[0017] In an aspect, the signal isolation unit includes a pulse transformer having a primary winding coupled at an output of the fault monitoring unit and the negative bus of the DC bus links and a secondary winding connected to the external control system for fault detection, wherein the generated fault signals from the fault monitoring unit are transmitted to the external control system through magnetic isolation by the pulse transformer.

[0018] In accordance with another embodiment of the present invention, there is provided a method of discharging a DC-link capacitor connected across DC bus links. The method includes: detecting a mode of operation of the DC-link capacitor and generating a charging detection signal based on the mode of operation; generating a DC-link voltage from a voltage across the DC bus links; generating a discharge signal based on at least one of the charging detection signal and the DC-link voltage; discharging, based on the discharge signal, the DC-link capacitor to a predefined value and generating a discharge feedback signal; generating a fault signal based on at least one of the discharge signal and the discharge feedback signal; and transmitting the fault signal to an external control system for fault detection.
[0019] In an aspect, the step of detecting the mode of operation includes monitoring the voltage across the DC bus links for detecting the mode of operation of the DC-link capacitor.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
[0020] The detailed description is described with reference to the accompanying figures. In the figures, same reference numbers are used throughout the drawings to reference like features and modules.
[0021] Figure 1 illustrates a block diagram of a DC-link discharge device according to an exemplary embodiment of the present invention.
[0022] Figure 2 illustrates a detailed block diagram of the DC-link discharge device illustrated in Figure 1.
[0023] Figures 3 illustrates a circuit diagram of a charging detection unit of the DC-link discharge device illustrated in Figure 2.

[0024] Figures 4 illustrates a circuit diagram of a discharge signal generation unit of the DC-link discharge device illustrated in Figure 2.
[0025] Figures 5 illustrates a circuit diagram of a discharging unit of the DC-link discharge device illustrated in Figure 2.
[0026] Figures 6 illustrates a circuit diagram of a fault monitoring unit of the DC-link discharge device illustrated in Figure 2.
[0027] Figures 7 illustrates a circuit diagram of a signal isolation unit of the
DC-link discharge device illustrated in Figure 2.
[0028] Figures 8 illustrates a flow chart depicting the steps involved in a method of discharging a DC-link capacitor according to another exemplary embodiment of the present invention.
DETAILED DESCRIPTION
[0029] The various embodiments of the present invention describe about a DC-link discharge device.
[0030] In the following description, for purpose of explanation, specific details are set forth in order to provide an understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these details. One skilled in the art will recognize that embodiments of the present invention, some of which are described below, may be incorporated into a number of systems.
[0031] However, the systems and methods are not limited to the specific embodiments described herein. Further, structures and devices shown in the figures

are illustrative of exemplary embodiments of the present invention and are meant to avoid obscuring of the present invention.
[0032] It should be noted that the description merely illustrates the principles of the present invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present invention. Furthermore, all examples recited herein are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.
[0033] A DC-link discharge device in accordance with an embodiment of the present invention is an analog design circuit configured to discharge a DC-link capacitor deployed in a power electronic converter / power system, and to monitor the faults of a discharging switch in the discharge device. The design also includes a charging detector to avoid discharge of the DC-link capacitor while the capacitor is charging. The design focuses on avoiding inappropriate operating conditions of the discharging switch by suitable fault monitoring so as to protect the power electronic converter / power system as well as provide human safety.
[0034] A simplified block diagram and a detailed block diagram of a DC-link discharge device/circuit (100) in accordance with an exemplary embodiment of the present invention, are illustrated in Figures 1 and 2. The discharge device (100) is a low loss DC-link discharge device with fault feedback to discharge a bulky DC-link capacitor (200) connected across DC bus links (50) of a power converter in an electronic power system. The discharge device (100) includes a charging detection unit (110), a voltage transducing unit (120), a discharge signal generation unit

(130), a discharging unit (140), a fault monitoring unit (150) and a signal isolation unit (160).
[0035] The charging detection unit (110) is connected across a positive and a negative bus of the DC bus links (50) and is configured to generate a charging detection signal based on a mode of operation of the DC-link capacitor (200) connected across the DC bus links (50) by monitoring the voltage across the DC bus links (50). The voltage transducing unit (120) includes a set of serially connected resistors (not particularly shown) and an output terminal, wherein a first end of the serially connected resistors is connected to the positive bus of the DC bus links (50), a second end of the serially connected resistors is connected to the negative bus of the DC bus links (50), and the output terminal of the voltage transducing unit is taken from a node of the serially connected resistors depending upon required scaling factor and is capable of generating a DC-link voltage signal (Vm). In an embodiment, the DC-link voltage signal (Vm) may include a scaled down DC-link voltage signal. The discharge signal generation unit (130) is configured to receive the charging detection signal from the charging detection unit (110) and the DC-link voltage signal (Vm) from the voltage transducing unit (120) and is further configured to generate a discharge signal based on the charging detection signal of the charging detection unit (110) and/or the DC-link voltage signal (Vm) of the voltage transducing unit (120). The discharging unit (140) is connected across the positive and negative bus of the DC links (50) and is configured to discharge the DC links (50) from a predefined value, for example 900 V for a power system with nominal rating of 1000V, and up to a required value, for example 75V, while the discharging unit (140) receives the discharge signal from the discharge signal generation unit (130). It may be appreciated that the aforesaid value of 900V is mentioned only for the sake of understanding, and that the discharging unit (140) is configurable to discharge DC links (50) of different voltages based on application. The fault monitoring unit (150) includes a short circuit fault detection unit (152) and an open circuit fault detection unit (151) and is configured to receive the discharge signal from the discharge signal generation

unit (130) and a feedback signal from the discharging unit (140) and generate fault signals based upon the discharge signal from the discharge signal generation unit (130) and the feedback signal from the discharging unit (140). The signal isolation unit (160) includes a pulse transformer having a primary winding coupled at an output of the fault monitoring unit (150) and the negative bus of the DC links, and a secondary winding connected to an external control system/unit (300) for fault detection. In an operative condition of the DC-link discharge device (100), the charging detection signal is logic low when the DC-link capacitor (200) is discharging, and the charging detection signal is logic high when the DC-link capacitor (200) is charging or when the DC-link capacitor (200) is at a constant voltage.
[0036] A circuit diagram of the charging detection unit (110) is illustrated in
Figure 3. The charging detection unit (110) includes bleeder resistors (111), a differentiator unit/circuit (112) and a control power supply unit/circuit (113) which are also illustrated in Figure 2. The bleeder resistors (111) including serially connected resistors, the differentiator circuit (112) and the control power supply circuit (113) including serially connected zener diodes, are all interconnected. The charging detection unit (110) is configured to detect a mode of operation of the DC-link capacitor (200) and further configured to generate a gate pull down signal i.e. the charging detection signal based on the mode of operation of the DC-link capacitor (200). The charging detection unit (110) is configured to monitor the voltage across the DC bus links (50) which indicates the mode of operation of the DC-link capacitor (200) connected across the DC bus links (50). The mode of operation is either a normal mode or a charging mode or a discharging mode. In an operative condition of the DC-link discharge device (100), the gate pull down signal i.e. the charging detection signal is logic low when the DC-link capacitor (200) is discharging i.e. mode of operation is discharging mode, and the gate pull down signal i.e. the charging detection signal is logic high when the DC-link capacitor is charging i.e. mode of operation is charging mode or when the DC-link capacitor is at a constant voltage i.e. mode of operation is normal mode.

[0037] A circuit diagram of the discharge signal generation unit (130) is illustrated in Figure 4. The discharge signal generation unit (130) includes a gate signal generation unit/circuit (131) and a gate pull down switch (132) which are also illustrated in Figure 2. The discharge signal generation unit (130) is configured to generate a discharge gate signal i.e. the discharge signal by comparing the DC-link voltage signal (Vm) with a reference voltage typically of 15 V, wherein the gate pull down switch (132) clips the discharge gate signal i.e. the discharge signal to zero during the charging time of the DC-link capacitor (200). Typically, the gate pull down signal i.e. the charging detection signal clips the gate signal of the gate pull down switch (132) to zero during the charging time of DC-link capacitor (200), and the gate pull down switch (132) clips the discharge signal to zero during charging of the DC-link capacitor (200).
[0038] A circuit diagram of the discharging unit (140) is illustrated in Figure 5. The discharging unit (140) includes a discharging resistor (141), a discharge switch (142) and feedback resistor (143) which are also illustrated in Figure 2. The discharging resistor (141) is used for dissipating energy of the DC-link capacitor (200) while discharging. The discharge signal from the discharge signal generation unit (130) is provided to a gate of the discharge switch (142) for turning on the switch and thereby the discharging unit (140) to dissipate the energy of the DC-link capacitor (200) through the discharging resistor (141) while discharging. The feedback resistor (143) which is an emitter follower resistor provides operating status of the discharge switch (142) in the form of a discharge feedback signal to the fault monitoring unit (150). In an exemplary embodiment the discharge switch (142) is an insulated-gate bipolar transistor (IGBT).
[0039] A circuit diagram of the fault monitoring unit (150) is illustrated in Figure 6. The fault monitoring unit (150) includes an open circuit fault detection unit/circuit (151) and a short circuit fault detection unit/circuit (152) which are also illustrated in Figure 2. The discharge signal from the discharge signal generation

unit (130) and the discharge feedback signal from the discharging unit (140) are provided to the fault monitoring unit (150) to generate fault signals, wherein a short circuit fault signal indicating short circuit fault of discharge switch (142) is generated by the short circuit fault detection circuit (152) when the discharge gate signal is low and discharge feedback signal is high, and an open circuit fault signal indicating open circuit fault of discharge switch (142) is generated by the open circuit fault detection circuit (151) when discharge gate signal is high and discharge feedback is low.
[0040] A circuit diagram of the signal isolation unit (160) is illustrated in Figure 7. In an embodiment, the signal isolation unit (160) includes a pulse transformer having a primary winding coupled at an output of the fault monitoring unit and the negative bus of the DC links (50), and a secondary winding connected to an external control system/unit (300) for fault detection. The generated fault signals from the fault monitoring unit (150) are transmitted to the external control system (300) through magnetic isolation by the pulse transformer. In other embodiments, the signal isolation unit (160) may include different isolation mediums such as capacitive, optical etc., which can be decided based on the application.
[0041] The DC-link discharge device (100) in accordance with an embodiment of the present invention provides a method of discharging a DC-link capacitor (200), as illustrated in the flowchart shown in Figure 8. The method is performed by the DC-link discharge device (100) connected across DC bus links (50) of a power converter. The method includes the following steps. At step 801 - detecting, by a charging detection unit (110), a mode of operation of the DC-link capacitor (200) and generating a charging detection signal based on the mode of operation, wherein the mode of operation is either normal mode or charging mode or discharging mode, and wherein the charging detection signal is logic low when the DC-link capacitor (200) is discharging i.e. mode of operation is discharging mode, and the charging detection signal is logic high when the DC-link capacitor (200) is

charging i.e. mode of operation is charging mode or when the DC-link capacitor (200) is at a constant voltage i.e. mode of operation is normal mode. The step of detecting the mode of operation involves monitoring, by the charging detection unit (110), the voltage across the DC bus links (50) which indicates the mode of operation of the DC-link capacitor (200) connected across the DC bus links (50). At step 802 - generating, by a voltage transducing unit (120), a DC-link voltage signal (Vm) from a voltage across the DC bus links (50). In an embodiment, the DC-link voltage signal (Vm) may include a scaled down DC-link voltage signal. At step 803 - generating, by a discharging signal generation unit (130), a discharge signal based on at least one of the charging detection signal and the DC-link voltage signal (Vm). At step 804 - discharging, by a discharging unit (140), based on the discharge signal, the DC-link capacitor (200) to a predefined value and generating a discharge feedback signal. At step 805 - generating, by a fault monitoring unit (150), a fault signal based on at least one of the discharge signal and the discharge feedback signal. At step 806 -transmitting, by a signal isolation unit (160), the fault signal to an external control system (300) for fault detection.
[0042] The low loss DC-link discharge device with fault feedback as disclosed herein is capable of achieving lesser restart/idle time and is realized typically by a plurality of analog ICs. The discharge device may discharge the DC-link only when it is desired, thus avoiding continuous power loss in normal operation of the power system.
[0043] The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the invention.

We Claim:

1. A DC-link discharge circuit (100) for discharging a DC-link capacitor (200)
connected across DC bus links (50), the DC-link discharge circuit (100)
comprising:
a charging detection unit (110) configured to detect a mode of operation of the DC-link capacitor (200) and generate a charging detection signal based on the mode of operation;
a voltage transducing unit (120) capable of generating a DC-link voltage signal (Vm) from a voltage across the DC bus links (50);
a discharge signal generation unit (130) configured to receive the charging detection signal and the DC-link voltage signal (Vm), and to generate a discharge signal based on at least one of the charging detection signal and the DC-link voltage signal (Vm);
a discharging unit (140) configured to receive the discharge signal and discharge the DC-link capacitor (200) to a predefined value and to generate a discharge feedback signal;
a fault monitoring unit (150) configured to receive the discharge signal and the discharge feedback signal, and to generate a fault signal based on at least one of the discharge signal and the discharge feedback signal; and
a signal isolation unit (160) coupled to the fault monitoring unit (150) and configured to transmit the fault signal to an external control system (300) for fault detection.
2. The DC-link discharge circuit (100) as claimed in claim 1, wherein the DC-link
capacitor (200), the charging detection unit (110), the voltage transducing unit (120)
and the discharging unit (140) are each connected across a positive bus and a
negative bus of the DC bus links (50).

3. The DC-link discharge circuit (100) as claimed in claim 2, wherein the voltage transducing unit (120) comprises a set of serially connected resistors and an output terminal, wherein a first end of the serially connected resistors is connected to the positive bus of the DC bus links (50), a second end of the serially connected resistors is connected to the negative bus of the DC bus links (50), and the output terminal providing the DC-link voltage signal (Vm) is taken from a node of the serially connected resistors based on a scaling factor.
4. The DC-link discharge circuit (100) as claimed in claim 1, wherein the charging detection unit (110) is configured to monitor the voltage across the DC bus links (50) to detect the mode of operation of the DC-link capacitor (200), and wherein the mode of operation is selected from a normal mode, a charging mode, and a discharging mode.
5. The DC-link discharge circuit (100) as claimed in claim 4, wherein the charging detection signal is logic low when the DC-link capacitor (200) is discharging in the discharging mode, and the charging detection signal is logic high when the DC-link capacitor (200) is charging in the charging mode or when the DC-link capacitor (200) is at a constant voltage in the normal mode.
6. The DC-link discharge circuit (100) as claimed in claim 1, wherein the charging detection unit (110) comprises serially connected bleeder resistors (111), a differentiator circuit (112) and a control power supply circuit (113) comprising serially connected zener diodes, wherein the bleeder resistors (111), the differentiator circuit (112) and the control power supply circuit (113) are interconnected.
7. The DC-link discharge circuit (100) as claimed in claim 1, wherein the DC-link voltage signal (Vm) comprises a scaled down DC-link voltage signal.

8. The DC-link discharge circuit (100) as claimed in claim 7, wherein the discharge signal generation unit (130) is configured to generate the discharge signal by comparing the DC-link voltage signal (Vm) with a reference voltage.
9. The DC-link discharge circuit (100) as claimed in claim 1, wherein the discharge signal generation unit (130) comprises a gate signal generation circuit (131) and a gate pull down switch (132), wherein the charging detection signal clips a gate signal of the gate pull down switch (132) to zero during charging of the DC-link capacitor, and the gate pull down switch (132) clips the discharge signal to zero during charging of the DC-link capacitor (200).
10. The DC-link discharge circuit (100) as claimed in claim 1, wherein the
discharging unit (140) comprises a discharging resistor (141), a discharge switch
(142), and a feedback resistor (143), wherein:
the discharging resistor (141) is configured to dissipate energy of the DC-link capacitor (200) while discharging,
the discharge switch (142) is configured to receive the discharge signal at a gate thereof for turning on the switch (142) and dissipating the energy of the DC-link capacitor (200) through the discharging resistor (141) while discharging, and
the feedback resistor (143) comprises an emitter follower resistor configured to generate the discharge feedback signal indicative of operating status of the discharge switch (142).
11. The DC-link discharge circuit (100) as claimed in claim 1, wherein the fault
monitoring unit (150) comprises at least one of an open circuit fault detection circuit
(151) and a short circuit fault detection circuit (152), wherein:
the open circuit fault detection circuit (151) is configured to generate an open circuit fault signal indicative of open circuit fault of the discharge switch (142) when the discharge signal is high and the discharge feedback signal is low, and

the short circuit fault detection circuit (152) is configured to generate a short circuit fault signal indicative of short circuit fault of the discharge switch (142) when the discharge signal is low and the discharge feedback signal is high.
12. The DC-link discharge circuit (100) as claimed in claim 1, wherein the signal isolation unit (160) comprises a pulse transformer having a primary winding coupled at an output of the fault monitoring unit (150) and the negative bus of the DC bus links (50) and a secondary winding connected to the external control system (300) for fault detection, wherein the generated fault signals from the fault monitoring unit (150) are transmitted to the external control system (300) through magnetic isolation by the pulse transformer.
13. A method of discharging a DC-link capacitor (200) connected across DC bus links (50), the method comprising:
detecting a mode of operation of the DC-link capacitor (200) and generating a charging detection signal based on the mode of operation;
generating a DC-link voltage (Vm) from a voltage across the DC bus links (50);
generating a discharge signal based on at least one of the charging detection signal and the DC-link voltage (Vm);
discharging, based on the discharge signal, the DC-link capacitor (200) to a predefined value and generating a discharge feedback signal;
generating a fault signal based on at least one of the discharge signal and the discharge feedback signal; and
transmitting the fault signal to an external control system (300) for fault detection.
14. The method as claimed in claim 13, wherein the step of detecting the mode of
operation comprises monitoring the voltage across the DC bus links (50) for
detecting the mode of operation of the DC-link capacitor (200).

15. The method as claimed in claim 13, wherein the DC-link voltage (Vm) comprises a scaled down DC-link voltage signal.