Technical field

The present invention relates to functional safety in a power electronic device.

Background of the invention

The objective of functional safety is freedom from unacceptable risk of physical injury or of damage to the health of people either directly or indirectly. The principles on how to carry out safety functions are covered by international standards, e.g. IEC61508 gives the requirements for electric systems.

An example of functional safety is the so-called safe turn-off (STO) function, which in connection to power electronic devices means that it must be possible to switch the output power off in a reliable way. The reliability requirement typically means that the design of the electric circuitry between the operator (i.e. a person who presses the on/off button) and the actuator (i.e. the power electronic component which connects the output power on/off) must be reliable and its functionality needs to be testable.

In modern power electronic devices, controllable power semiconductor switches, e.g. insulated gate bipolar transistors (IGBT), are normally used as power switches in main circuits. In a STO situation it should be ensured that all power switches stay in off-state. One safe way to ensure this is to switch off the auxiliary voltage, which supplies energy to the gate drivers for generating control pulses for the controllable power semiconductor switches. A problem when using this method is that the functional test of the safety function interrupts the operation of the device, which is not desirable in many processes operating continuously.

Summary of the invention

The objective of the present invention is to provide a novel arrangement and a novel method for ensuring that the functionality of the output power turn off circuitry can be tested. According to the invention, the operating condition of critical safety related components in a DC auxiliary voltage delivery system can be tested regularly during the operation of a power electronic device, without interrupting its operation. The following is a brief summary in order to provide basic understanding of some aspects of various embodiments of the invention, a more detailed description of exemplifying embodiments are given later. The objective of the invention is achieved by what is stated in the independent claims, other preferred embodiments are disclosed in the dependent claims.

The basic characteristic feature of the safe turn-off arrangement according to the present invention is that a DC auxiliary voltage of a functional unit, which as unpowered ensures the safety of the output connection of a power electronic device, is supplied via a safety circuit comprising the following features:

- Each pole of the DC auxiliary voltage can be disconnected separately,

- When only one DC pole is disconnected, an energy storage device maintains the output voltage of the safety circuitry above a limit value for at least a predefined time period,

- When both DC poles are disconnected, the output voltage drops below a limit value without delay, and

- The safe turn-off circuit generates component-specific feedback signals indicating the operating states of those critical components which perform the DC pole disconnection.

In a safe turn-off arrangement, the comparison of operating instructions and corresponding feedback signals provide a reliable indication of the functionality of the safety-critical DC circuit disconnecting components, which is a condition to meet the requirements of safety standards, e.g. IEC61508. The compari- son can be performed in any control block supervising the functional safety of a power electronics device, e.g. in a control unit of a frequency converter.

The component used to disconnect a DC auxiliary voltage pole may be e.g. a MOSFET transistor, a bipolar transistor, a mechanical switch etc. The feedback signal for monitoring the functionality of the disconnecting component may be formed e.g. by using an optocoupler.

In a method according to the present invention, the functionality of the safety circuitry can be tested regularly by short disconnecting periods of one DC auxiliary voltage pole at a time. The testing time period can be selected to be sufficiently short that during the test pulse the output voltage of the safety circuitry stays above a minimum operation level of the load circuit. In a safe turn off situation both poles of the DC auxiliary voltage are disconnected simultaneously.

The arrangement and method according to the present invention makes it possible to test the functionality of a safe turn off circuit during the operation of the power electronics device without interrupting the normal operation of the power electronics device. In a safe turn off situation, the circuit disconnects the auxiliary voltage, thereby ensuring that energizing the output terminals of the power electronic device is prevented quickly.

Description of drawings

Below the invention is explained more detailed by using examples with references to the enclosed figures, wherein

Fig.1 presents a main circuit of a frequency converter drive,

Fig.2 presents an auxiliary voltage arrangement,

Fig.3 presents a safety circuit,

Fig.4 illustrates operation of the safety circuit,

Fig. 5 presents a flow chart showing an exemplary testing algorithm for the safety circuit, and

Fig. 6 presents an auxiliary voltage arrangement including a safety circuit.

Detailed description of the invention Fig. 1 presents a simplified main circuit diagram of a variable speed motor drive as an example of a power electronic device wherein a functional safety circuitry according to the present invention is applicable. In the figure, a frequency converter FC is used to control the shaft rotating speed of an AC motor M. The frequency converter FC in this example comprises a rectifier REC, rectifying the three-phase supply voltage UL into a constant DC-link voltage filtered by a capacitor CDC, and a three-phase inverter unit INU, creating a three-phase adjustable output voltage UM for supplying the motor M. INU consists of controllable power semiconductor switches, normally IGBTs (not presented), and free-wheeling diodes (not presented). The frequency converter FC comprises also a control unit CU and an auxiliary voltage power supply POW, which converts an input voltage from the DC-link into several lower level output DC-voltages for e.g. the control unit CU and the gate driver unit of INU (not presented). In a STO situation the rotation of a motor shaft, induced by the output voltage of the frequency converter, should be prevented. This target can be met by ensuring that all controllable power semiconductor switches of INU stay in an off- state.

Fig. 2 presents a simplified example of a control and auxiliary voltage supply system in a frequency converter. The first auxiliary power supply POWi converts the DC-link voltage UDC into a first lower voltage Ucu for the control unit CU and into a second lower voltage UGD for the gate driver unit GD. Inside the gate driver unit GD a second auxiliary voltage power supply POW2 converts the UGD voltage into isolated auxiliary voltages for each gate driver GDi, GD2,... which form the control signals of the inverter controllable power semiconductor switches (only Vi presented) according to the control signals V_G received from the control unit CU.

Fig. 3 presents an example of a safety circuit SC according to the present invention. The circuit SC is located between a first auxiliary voltage power supply POW1 , having an output DC voltage UGDI with a positive pole UGDU- and a negative pole UGDI-, and a second auxiliary power supply POW2, having input DC voltage UGD2 with a positive pole UGD2+ and a negative pole UGD2-. The safety circuit comprises the following:

- A series connection of a first diode D-i , a first switch Si and a second diode D2 between UGDU- and UGD2+ such that the forward direction of both diodes are connected towards UGD2+,

- A series connection of a third diode D3, a second switch S2 and a fourth diode D ₄ between UGD-I- and UGD2- such that the forward direction of both diodes are connected towards UGDI -,

- A first energy storage device, advantageously a capacitor Ci , connected between UGD2+ and the anode terminal of D ₄ ,

- A second energy storage device, advantageously a capacitor C2, connected between the cathode terminal of Di and UGD2-,

- A first feedback circuit comprising a series connection of a resistor Ri and a light emitting photodiode of an optocoupler H-i , connected between the anode terminals of D2 and D ₄ such that the forward direction of the optocoupler light emitting diode is connected towards S2, and

- A second feedback circuit comprising a series connection of a resistor P»2 and a light emitting photodiode of an optocoupler H2, connected between the cathode terminals of Di and D3 such that the forward direction of the optocoupler light emitting diode is connected towards S2. Fig. 4 illustrates operation of the safety circuit presented in Fig. 3. The purpose of the curves is just to illustrate operating principles, they are not drawn to scale relative to each other. Signals S and H indicate the operation of switches Si, S2 and operating states of the feedback signals H-i , H2, such that a high state means a close-state of a switch and an active state of the feedback signal.

In normal operating situation, before time instant t-i , both switches S1 , S2 are in close-state which means that UGDU is connected to UGD2+ via diodes D-i, D2, and UGDI- is connected to UGD2- via diodes D3, D ₄ . Thus the input voltage UGD2 of POW2 is close to the output voltage UGDI of POW-i . Further, with both switches S1 , S2 in the close-state, current flows through both the optocouplers Hi, H2, thereby indicating the normal functionality of the safety circuit SC.

At time instant ti the switch Si turns to open-state. Since current cannot flow to the optocoupler H 1 from either the first auxiliary power supply POW1 (due to the open switch S1 ) or the second auxiliary power supply POW2 (due to the blocking diode D2), the feedback signal of H-i , indicating the operating state of Si, turns into a non-active state. In Si open state, UGDU is not any more connected to UGD2+, but due to the energy charged in Ci before ti its voltage uci and also the voltage UGD2 decreases at a limited rate. Si is turned back to close-state at time instant .2 before UGD2 has reached the minimum operating voltage limit ULIM of POW2. Thus POW2 can continue its normal operation also during time period ti - .2, and at the same time the feedback signal Hi indicates that the switch Si is operative.

A similar operating condition test described above with respect to Si is made for S2 during the time period .3 - 14. Similar to Ci above, during the test the energy of capacitor C2 prevents the voltage UGD2 from falling below the limit ULIM. As shown in Figure 4, at time instant t3 the switch S2 turns to open-state. Since current cannot flow from the optocoupler H2 to either the first auxiliary power supply POW1 (due to the open switch S2) or the second auxiliary power supply POW2 (due to the blocking diode D3), the feedback signal of H2, indicating the operating state of S2, turns into a non-active state. In S2 open state UGDU is not any more connected to UGD2+, but due to the energy charged in C2 before .3, its voltage uc2 and also the voltage UGD2 decreases at a limited rate. S2 is turned back to close-state at time instant t4 before UGD2 has reached the minimum operating voltage limit ULIM of POW2. Thus POW2 can continue its normal operation also during time period .3 - 14, and at the same time the feedback signal H2 indicates that the switch Si is operative.

At time instant ts both switches Si , S2 are turned to open-state, as a consequence of a STO command. Now the open switches prevent direct connections between UGDI and UGD2 via diodes Di - D ₄ , and the open switches prevent also the full charged capacitors Ci , C2 to supply energy to POW2. Thus the voltage UGD2 falls immediately to 0, which means that if POW2 is not any more capa- ble to supply auxiliary voltages for the gate drivers in an arrangement like presented in Fig. 2. Missing auxiliary voltage prevents the gate drivers to generate turn-on pulses for the main circuit controllable power semiconductor switches, which is the target in a safe turn-off situation. Noteworthy is also that in case of a switch component failure, i.e. Si or S2 stuck in close-state, the condition of a safe turn-off will be met, but with a delay due to the stored energy of Ci or C2.

Fig. 5 is a flow chart showing an exemplary testing algorithm for the safety circuit. The flow chart starts in a normal operation mode of the safety circuit (in which the switches Si, S2 are closed and the optocouplers Hi , H2 are in an active state. Next, the first switch Si is opened and it is determined whether the opto- coupler Hi changes to an inactive state. If so, the algorithm proceeds to the next step, otherwise the algorithm terminates with a fault being reported. At the next step, the first switch Si is closed and the second switch S2 is opened and it is determined whether the optocoupler H2 changes state. If so, the algorithm ter- minates with an indicates that the circuit is working normally; if not, the algorithm terminates with a fault being reported.

Fig. 6 presents an otherwise similar simplified example of a control and auxiliary voltage supply system in a frequency converter as presented in Fig. 2, but with an added safety circuit SC. In this example the first auxiliary voltage UGDI is wired via a safety circuit SC, corresponding to the circuit of Fig. 3, in order to form the final auxiliary voltage UGD2 for the gate driver unit GD. The control unit CU forms the control signals S_c for the switches (Si, S2 in Fig. 3), receives the feedback signals S_F, indicating the operating states of the switches, and per- forms the logical operations needed to meet the functional safety requirements. The feedback signals S_F may include the output of optocoupler phototransistors.

The phototransistor parts of the above-mentioned optocouplers Hi and H2 are shown only in highly schematic form in Figure 6. Advantageously the photo- transistors are connected to a safety logic (which may form part of the control unit CU and may be separate to the control unit CU) belonging to a safety arrangement according to the present invention, in which arrangement the functionality of the safety-critical components are monitored continuously by comparing the operating instructions of switches S-i , S2, and corresponding feedback signals from optocouplers H-i , H2. Note that above an optocoupler is used just as an example of an advantageous component, also other commercial signal transmitting devices with isolation between the sender (corresponding a photodiode above) and the receiver (corresponding a phototransistor above) exist.

The specific examples provided in the description above are not exhaustive unless otherwise explicitly stated, nor should they be construed as limiting the scope and/or the applicability of the accompanied claims. The features re- cited in the accompanied dependent claims are mutually freely combinable unless otherwise explicitly stated. The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.