DE102021208153A1 - Electrical circuit for limiting an inrush current and power pack and charger with such an electrical circuit - Google Patents

Abstract

The present invention relates to an electrical circuit (10) for limiting an inrush current, comprising a voltage input (1), a thermistor (NTC), an energy store (CZK), a switch-off device (4) and a voltage output (2), the thermistor (NTC ) is set up to limit an inrush current (I) from the voltage input (1) into the energy storage device (CZK) to a greater extent in the cold state than in the hot state, and the switch-off device (4) is set up to limit the voltage output (2) depending on a voltage input or at the energy storage device (CZK) to separate or connect the electrical voltage falling to the energy storage device (CZK).

Description

State of the art

The present invention relates to an electrical circuit for limiting an inrush current and a power pack and a charging device with such a circuit. In particular, the present invention relates to reducing an inrush current when using a thermistor (NTC).

Switching converters (AC/DC converters) are known in the prior art, which are part of a charging device or a power pack, for example. In other words, in the prior art, a charger or power pack or the like consists of an AC/DC converter and also a DC/DC converter. Such an AC/DC converter usually contains a rectifier, which converts the AC voltage into a pulsating DC voltage. An intermediate circuit capacitor is charged via this rectifier. This capacitor supports the input voltage of the DC/DC converter that usually follows. When connecting the power pack to the AC voltage network, a high inrush current (also: pulsed current) usually occurs, which charges the intermediate circuit capacitor. This pulsed current depends on the state of charge of the intermediate circuit capacitor and on the series impedance. In the prior art, the series impedance is usually designed as a thermistor (NTC). This is mainly used to protect the contacts on the AC plug and to protect the rectifier, which provides a high resistance when cold and thus limits the inrush current. As the voltage of the intermediate circuit capacitor increases and the series impedance is adequately dimensioned, the inrush current falls into value ranges that are thermally and electrically unproblematic. When the converter is in operation, the NTC becomes hot due to its power loss and reduces its resistance in order to keep the power loss low during operation. The NTC represents a cost-effective way of limiting the inrush current. Problems can arise if the converter is disconnected from the mains while warm (after it has been in operation for a while) and is then reconnected to the mains shortly thereafter. The hot NTC now still has a low resistance. The intermediate circuit capacitor, which is now uncharged, is charged with a very high (maximum) input current (pulse current or "inrush current"), with the rectifier diodes requiring a very high pulse current strength.

In order to counteract this problem, a rectifier with a sufficiently high pulse current capability is often used in the prior art. However, this requires higher component costs. In addition, measures are proposed on the website "https://www.ametherm.com/blog/inrush-current/cool-down-time-inrush-currentlimiter" to reduce the cooling time of the NTCs. There are also other costly methods of avoiding the use of an NTC entirely (Application note AN4606, "Inrush-current limiter circuits (ICL) with Triacs and Thyristors (SCR) and controlled bridge design tips", ST Microelectronics, 2016, https ://www.st.com/resource/en/application_note/dm00140117-inrushcurrentlimiter-circuits-icl-with-triacs-and-thyristors-scr-and-controlled-bridge-design-tipsstmicroelectronics.pdf).

Accordingly, there is a need for an inexpensive solution to the problem of high inrush currents.

Disclosure of Invention

According to the invention, an electrical circuit for limiting an inrush current with the features of claim 1 is proposed. The dependent claims show preferred developments of the invention. Accordingly, an electrical circuit for limiting an inrush current is proposed, which has a voltage input, a thermistor (also known as a negative temperature coefficient (NTC) resistor) and an energy store (e.g. designed as an intermediate circuit capacitance). According to the invention, a switch-off device and a voltage output are also provided. The voltage output can operate a load/consumer or a switching converter or be electrically connected to one. The thermistor is set up to limit an inrush current from the voltage input into the energy storage device more when it is cold than when it is hot. For this purpose, the thermistor can be electrically connected in series with a positive pole of the voltage input or with a negative pole of the voltage input. In other words, the (switch-on) current that feeds/charges the energy store on the input side is applied to the thermistor. The switch-off device is set up to ensure that the energy store does not discharge (for example via the primary-side voltage output) when the voltage at the energy store falls below a predefined value and/or when a predefined negative gradient of the voltage at the voltage input or at the energy store is exceeded. If the voltage input delivers supply voltage again within a short period of time after falling below the predefined voltage or the predefined negative gradient (the predefined voltage value or the predefined negative gradient is exceeded again), the energy store is still charged sufficiently to limit a current conducted through the still warm thermistor. In other words, a potential difference between a potential at the voltage input and a potential across the energy store is kept sufficiently low at least for so long that supplying the voltage input with electrical voltage again does not lead to an inrush current, which is problematic for the selected components. It is thus ensured according to the invention that the current-limiting capability of the circuit based on a (residual) voltage present across the energy store always compensates for a reduced current-limiting capability of the thermistor at least to such an extent that damage to the circuit according to the invention due to an input current is effectively prevented.

The voltage input can be supplied with an AC voltage, among other things. This is generally the case with conventional chargers. They convert an incoming AC voltage into a DC voltage. Such electrical circuits are included in chargers or other power supplies. Alternatively or additionally, the voltage input can be set up to accept a DC voltage. This is also usually the case for chargers and power packs supplying direct current. Alternatively, the voltage output can be set up to output an AC voltage. In this case, a different frequency of the AC voltage can be generated than that present at the voltage input. According to the invention, there are therefore no limits to the freedom to design an electrical output voltage.

For rectification, the electrical circuit can have a bridge rectifier, which is supplied with electrical energy via the thermistor with the voltage input. On the other hand, the bridge rectifier is coupled to the energy store, which it charges or keeps at a high voltage level during operation. For this purpose, the bridge rectifier can be arranged in parallel with the energy store.

The switch-off device is preferably set up to disconnect the voltage output from the energy store or to reconnect it to it depending on the time elapsed since a voltage at the voltage input or the voltage at the energy store dropped. Correspondingly, the switch-off device can alternatively or additionally be set up, depending on a temperature of the thermistor, to disconnect the voltage output from the energy store or to connect it (again) to it. In other words, the shutdown device can monitor the suitability of the thermistor for limiting any input current. In the case of time-controlled monitoring, an empirically or arithmetically determined relationship between the temperature level of the thermistor during operation and the cooling characteristics of the thermistor after the input voltage/mains voltage has been switched off can be taken into account. The temperature of the thermistor can be determined even more precisely, but with greater effort, by means of a temperature sensor and from this the current electrical resistance of the thermistor and thus also its current-limiting characteristics over time can be inferred. As soon as the current-limiting capability of the thermistor has been adequately restored by the cooling process, the switch-off device can reconnect the voltage output to the energy store and thus prepare the circuit for renewed operation.

The electrical circuit can preferably have a switching converter, which is arranged between the switch-off device and the voltage output. Alternatively, another switching converter can be connected to the voltage output. These converters can be designed as buck converters or boost converters, for example. The design of a switching converter is known to those skilled in the art and is not explained in more detail here.

The switch-off device can preferably have a voltage divider which is arranged in parallel with the energy store. In other words, a ratio of the electrical voltage present across the energy store is made available via the voltage divider, which can be used as an input variable (command variable) for the switch-off device. The dimensioning of the voltage divider can be dimensioned depending on a difference between the (maximum) voltage across the energy store and the (maximum permissible) voltage at the input of the switch-off device.

The switch-off device can have an integrated circuit, which interrupts an electrical connection between the energy store and the voltage output when the voltage at its input falls below that level. The circuit can, for example, have a so-called error input, which is electrically connected to the tap of the above-mentioned voltage divider. The maximum operating voltage of the energy store can be adjusted to the maximum permissible input voltage at the fault input via the voltage divider.

A transistor can preferably be provided in the electrical circuit, which acts as a component of the switch-off device. The transistor can, for example, interrupt a ground-side or a supply-side electrical connection to the switching converter if the voltage at the voltage input or at the energy store falls below a predefined value or falls below a predefined negative gradient. Combinations of the two aforementioned conditions are also possible according to the invention.

The energy store can have a capacitance, in particular a capacitor, which is preferably designed as an electrolytic capacitor or as a film capacitor. In particular, the energy storage device can be understood as an intermediate circuit capacitor, which buffers a (pulsating) DC voltage produced on the input side before this is made available on the output side as a DC voltage (e.g. with a changed voltage level) or is converted into an AC voltage (in particular with a changed amplitude and/or a changed frequency). .

According to a second aspect of the present invention, a power pack and/or a charging device is/are proposed, which includes/have an electrical circuit in accordance with the above statements. In particular, the power pack can be understood for operation on a standard household 230 V AC voltage or any other household socket. However, this does not rule out the possibility of the power pack being provided in a wall box or some other charging device for electrically powered means of transport. Appropriate dimensions are not a hurdle for a person skilled in the relevant field. Of course, the applications for the subject matter of the present disclosure are not limited to the aforementioned examples.

In other words, a core of the invention, without any restrictive character, consists in avoiding the premature and in particular complete discharge of the intermediate circuit capacitor after the disconnection of a converter from the network. It is detected that the intermediate circuit capacitor is discharged below a certain voltage potential and/or with a predefined (in particular negative) gradient in order to stop the converter in its energy conversion. The electrical energy no longer drawn from the intermediate circuit capacitor as a result remains, apart from leakage currents or low currents for supplying auxiliary circuits, on the intermediate circuit capacitor, which thus holds a residual voltage. If the converter is now reconnected to the same or a different power supply shortly after it was disconnected from the mains (on the input side), the intermediate circuit capacitor only has to be partially charged. In other words, the voltage difference between the mains and the intermediate circuit capacitor is significantly lower than if the intermediate circuit capacitor is not disconnected from the mains in the manner according to the invention with a load on the output side (such as a switching converter). If the intermediate circuit capacitor is discharged by the above-mentioned leakage currents after a longer period of inactivity, the thermistor has had enough time to cool down. Due to the restored resistance value of the NTC (after cooling down), the inrush current limitation based solely on the NTC thermistor is sufficient again. Theoretically, at this point in time, the shutdown device can close the electrical connection that has been separated for protection, in order to prepare the circuit for a new operating cycle.

Accordingly, according to a core of the invention, an electrical circuit can be provided which has an inrush current limitation by means of a thermistor (NTC), a bridge rectifier with a downstream intermediate circuit capacitor for providing a DC voltage, a control device for a switching converter (e.g. an integrated circuit, an integrated control component or similar). Ä.) And has a switch-off device, which can limit or interrupt a current consumption/energy consumption of the control device from the intermediate circuit capacitor, so that the switching converter is deactivated. The energy and voltage across the intermediate circuit capacitor therefore remain at a high level for significantly longer than in the prior art.

character list

• 1 eine schematische Ausführung einer elektrischen Schaltung zur Begrenzung eines Einschaltstromes gemäß einem ersten Ausführungsbeispiel der vorliegenden Erfindung;
• 2 eine schematische Ausführung einer elektrischen Schaltung zur Begrenzung eines Einschaltstromes gemäß einem zweiten Ausführungsbeispiel der vorliegenden Erfindung;
• 3 eine schematische Ausführung einer elektrischen Schaltung zur Begrenzung eines Einschaltstromes gemäß einem dritten Ausführungsbeispiel der vorliegenden Erfindung;
• 4 eine schematische Ausführung einer elektrischen Schaltung zur Begrenzung eines Einschaltstromes gemäß einem vierten Ausführungsbeispiel der vorliegenden Erfindung;
• 5 ein Zeitdiagramm einer Zwischenkreiskondensatorspannung über der Zeit ohne eine erfindungsgemäße elektrische Schaltung;
• 6 ein Zeitdiagramm einer Zwischenkreiskondensatorspannung über der Zeit mit einer erfindungsgemäßen elektrischen Schaltung;
• 7 eine schematische Darstellung elektrischer Kenngrößen in einer herkömmlichen elektrischen Schaltung, umfassend einen Einschaltstrom;
• 8 eine schematische Darstellung elektrischer Kenngrößen in einer erfindungsgemäßen elektrischen Schaltung, umfassend einen Einschaltstrom.

Exemplary embodiments of the invention are described in detail below with reference to the accompanying drawings. In the drawing is:

• 1 a schematic embodiment of an electrical circuit for limiting an inrush current according to a first embodiment of the present invention;
• 2 a schematic embodiment of an electrical circuit for limiting an inrush current according to a second embodiment of the present invention;
• 3 a schematic embodiment of an electrical circuit for limiting an inrush current according to a third embodiment of the present invention;
• 4 a schematic embodiment of an electrical circuit for limiting a Inrush current according to a fourth embodiment of the present invention;
• 5 a timing diagram of an intermediate circuit capacitor voltage over time without an electrical circuit according to the invention;
• 6 a timing diagram of an intermediate circuit capacitor voltage over time with an electrical circuit according to the invention;
• 7 a schematic representation of electrical parameters in a conventional electrical circuit, comprising an inrush current;
• 8th a schematic representation of electrical parameters in an electrical circuit according to the invention, comprising an inrush current.

Embodiments of the invention

1 shows an electrical circuit 10 for limiting an inrush current, in which a voltage input 1 supplies electrical voltage to a bridge rectifier 3 comprising four diodes D ₁ -D ₄ via an NTC thermistor. In parallel with the bridge rectifier 3, an energy store is provided in the form of an intermediate circuit capacitance C _ZK , the voltage V _ZK of which is also present across a switch-off device 4 according to the invention. The switch-off device 4 is connected in parallel after the energy store C _ZK . The switching converter 5 is connected in parallel with the switch-off device 4 . The switch-off device 4 either supplies the switching converter 5 or interrupts the flow of energy to it. On the output side, the switching converter 5 is connected to a voltage output 2, which is optionally buffered via an output _capacitor CA. The switch-off device 4 is set up to disconnect the switching converter 5 from the intermediate circuit capacitance C _ZK when the intermediate circuit voltage V _ZK falls below a predefined level. In this way, such a high voltage remains on the intermediate circuit capacitance C _ZK for a sufficiently long period of time that a renewed supply of electrical energy to the electrical circuit 10 via the voltage input 1 does not result in high switch-on currents, which pose problems for the components used. In other words, the supply at voltage input 1 is interrupted, as a result of which voltage input 1 no longer receives any energy. The voltage value at the intermediate circuit capacitance C _ZK reaches a critical voltage (threshold). The switch-off device 4 then disconnects the energy supply to the switching converter 5. As a result, the voltage level at the capacitor remains almost the same or drops only very slowly. The NTC thermistor now also cools down. Even with an immediate resupply of the electrical circuit 10 at the voltage input 1 no longer causes problematic pulse currents.

2 shows a second exemplary embodiment of an electrical circuit 10 according to the invention, in which the switch-off device 4 is represented by a voltage divider R ₁ , R ₂ and a bipolar transistor as transistor T ₁ . The switching converter 5 is supplied with voltage via the intermediate circuit capacitance and itself generates a supply voltage VCC from the voltage across the intermediate circuit capacitance C _ZK for the primary side. In the switching converter 5 there is generally a galvanic isolation required by standards. The galvanic isolation is generated/provided almost exclusively by a transformer (coupled inductance). This is referred to as the primary and secondary side. The switching converter 5 in turn generates the output voltage U _A . The switching converter is supplied with V _ZK and itself generates VCC from V _ZK for the primary side (example in this figure) and furthermore the switching converter generates U _A . If a threshold voltage drops across the resistor R ₂ , which is less than the voltage required to turn on the transistor T ₁ , the transistor T ₁ blocks and thus switches the switching converter 5 off.

The intermediate circuit capacitor C _ZK is mainly discharged by the switching converter 5 . The voltage divider is dimensioned with such a high resistance that after switching off the switching converter 5 the potential at V _ZK remains for a sufficiently long time.

3 shows a third exemplary embodiment of an electrical circuit 10 according to the invention, in which the switch-off device 4 has a second transistor T ₂ and a third resistor R ₃ . In this case, the switch-off device 4 is fitted between the supply voltage V _cc and the primary-side voltage V _cc generated by the switching converter 5 itself. In other words, placed in the supply path of the IC. The transistor T ₁ regulates the input voltage or supply voltage V _CC of the IC from the self-generated supply voltage V _cc . The voltage at the base of T ₁ represents a reference value for the transistor T _1. The reference value is set as a function of the voltage V _ZK across the intermediate circuit capacitance C _ZK . The intermediate circuit capacitance voltage V _ZK is determined using a voltage divider consisting of R ₁ and R ₂ . The voltage U _Div tapped off at the voltage divider R ₁ , R ₂ sets the threshold voltage via the transistor T ₂ . When the intermediate circuit capacitance voltage V _ZK falls, the regulated supply voltage V _IC of the switching converter IC also falls. The voltage divider R ₁ , R ₂ is set so that when the intermediate circuit capacitance voltage V _ZK drops below a predetermined value ned voltage threshold, the undervoltage detection at the supply pin of the switching converter ICs responds and switching converter 5 is deactivated.

4 shows a fourth exemplary embodiment of an electrical circuit 10 according to the invention, in which the voltage divider R ₁ , R ₂ acts with an error input of the switching converter IC (number still assigned) as part of the switch-off device 4 . The voltage V _F present at the error input is set as a function of the intermediate circuit capacitance voltage V _ZK . The intermediate circuit capacitance voltage V _ZK is reduced and forwarded to the fault input with the aid of the voltage divider R ₁ , R ₂ . The voltage divider R ₁ , R ₂ is set in such a way that when the intermediate circuit capacitance voltage V _ZK falls below a specific voltage, the error pin of the switching converter IC is triggered and the switching converter 5 is deactivated.

5 shows a time chart. This illustrates the intermediate circuit capacitance voltage V _ZK over time t. This applies to intermediate circuit capacitances known from the prior art, which are initially (time range -80 ms - -20 ms) held on the input side with a pulsating DC voltage at a comparatively high voltage level (298 V - 330 V). At around -20 ms, the voltage supply for the intermediate circuit capacitor is disconnected from the mains supply, whereupon its voltage drops to 0 V. When the supply voltage is switched on again, the entire input voltage (approx. 325 V in the example) would drop immediately across the input-side circuit part and ensure an extremely high input pulse current. This can lead to problems with component strength.

6 shows a result of a modification of the invention 5 underlying electrical circuit. After switching off the supply voltage at approximately -19 ms, the intermediate circuit capacitance voltage V _ZK drops to a voltage level of approximately 175 V. Due to a shutdown of loads on the output side according to the invention, the intermediate circuit capacitance voltage V _ZK does not decrease any further from 0 ms. If the supply voltage is switched on again at a later point in time (in the range of 20 ms - 160 ms), the voltage difference between the supply network and the intermediate circuit capacitor is lower (in this example approx. 150V), which is why the input pulse current is lower than according to 5 fails. The component strength is not endangered.

7 shows a timing diagram of a current I in an electrical circuit according to the prior art. In 7 and 8th the resistance value of the NTC is lower, since the case is simulated when the charger is unplugged and immediately plugged in again after it is warm. This creates the high input pulse currents that 8th be avoided with the circuit according to the invention. The negatively plotted inrush current without the switch-off device according to the invention and despite the insufficient protection due to the heated thermistor (the resistance value is then much lower) protection via a thermistor is -80 A, which means that the electrical components can be significantly stressed or endangered/destroyed.

8th shows the inrush current I of an electrical circuit 10 modified according to the invention for limiting an inrush current. Due to a compared to about 150 V 7 higher voltage levels of the intermediate circuit capacitance voltage V _ZK at the time when the input current I rises, the peak current value falls by comparison 7 with a maximum of approx. -35 A, the amount is significantly lower, which protects the electrical components.

Claims (10)

Electrical circuit (10) for limiting an inrush current, comprising - a voltage input (1) - a thermistor (NTC), - an energy store (C _ZK ), - a switch-off device (4) and - a voltage output (2), the thermistor (NTC ) is set up to limit an inrush current (I) from the voltage input (1) into the energy store (C _ZK ) more than in the hot state in the cold state, and the switch-off device (4) is set up to limit the voltage output (2) as a function of an am To separate voltage input and / or at the energy storage (C _ZK ) falling electrical voltage of the energy storage (C _ZK ) or to connect with it. Electrical circuit after claim 1 , wherein - the voltage input (1) is set up to accept an AC voltage and/or a DC voltage and/or - the voltage output (2) is set up to output a DC voltage and/or an AC voltage. Electrical circuit after claim 1 or 2 further comprising - a rectifier (3) which is coupled via the thermistor (NTC) to the voltage input and which on the other hand to the energy store (C _ZK ). Electrical circuit according to one of the preceding claims, wherein the switch-off device is set up, depending on - a lapse of time since a drop in a voltage at the voltage input or the voltage at the energy store (C _ZK ) and / or - a temperature of the thermistor (NTC) the voltage output from the To separate energy storage or to connect it with this. Electrical circuit according to one of the preceding claims, further comprising a switching converter (5) which is arranged between the switch-off device (4) and the voltage output (2). Electrical circuit according to one of the preceding claims, in which the switch-off device has a voltage divider (R ₁ , R ₂ ) which is arranged in parallel with the energy store (C _ZK ). Electrical circuit according to one of the preceding claims, wherein the switch-off device has an integrated circuit, which in particular with a tap of the voltage divider according to claim 6 is electrically connected. Electrical circuit according to one of the preceding claims further comprising a transistor as part of the switch-off device, which in particular with a tap of the voltage divider according to claim 6 or 7 is electrically connected. Electrical circuit according to one of the preceding claims, wherein the energy store (C _ZK ) comprises a capacitance, in particular a capacitor. A power pack or charger comprising an electrical circuit according to any one of the preceding claims.

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