Power converter with a snubber circuit

A power converter circuit includes a switching circuit with at least one electronic switch, a capacitor configured to provide or receive a voltage with a predefined voltage level, at least one first inductor, and a snubber circuit. The snubber circuit includes at least one second inductor inductively coupled to the at least one first inductor and electrically coupled to the capacitor.

TECHNICAL FIELD

This disclosure in general relates to a power converter, in particular a switched mode power converter.

BACKGROUND

Switched-mode power converters are widely used in many different electronic applications such as automotive, industrial, household or consumer electronic applications, to name only a few. A power converter is configured to convert an input power received at an input into an output power available at an output. The input power is defined by an input current and an input voltage received at the input, and the output power is defined by an output current and an output voltage available at an output, wherein at least one parameter of at least one of the input current and the input voltage is different from the corresponding parameter of the output current and the output voltage. A DC/DC converter, for example, may receive a DC input voltage with a first voltage level and supply a DC output voltage with a second level higher or lower than the first level. An AC/DC converter, for example, may receive an AC input voltage and supply a DC output voltage.

A power converter includes a plurality of electronic devices. These devices may include parasitic inductances and/or parasitic capacitances. For example, a transformer may include a parasitic inductance (often referred to as leakage inductance) and a power transistor such as a power MOSFET may include a parasitic capacitance (often referred to as output capacitance). Such parasitic devices may form a parasitic resonant circuit, whereas the parasitic resonant circuit can be excited during operation of the power converter. Exciting a parasitic resonant circuit can cause voltage oscillations with voltage amplitudes that exceed the voltage rating of the devices employed in the power converter. There is therefore a need to limit the amplitude of parasitic oscillations in a power converter.

SUMMARY

One example relates to a power converter circuit. The power converter circuit includes a switching circuit with at least one electronic switch, a capacitor configured to provide or receive a voltage with a predefined voltage level, at least one first inductor, and a snubber circuit. The snubber circuit includes at least one second inductor inductively coupled to the at least one first inductor and electrically coupled to the capacitor.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings. The drawings form a part of the description and by way of illustration show specific embodiments in which the invention may be practiced. It is to be understood that the features of the various embodiments described herein may be combined with each other, unless specifically noted otherwise.

FIG. 1shows a power converter circuit according to one example. The power converter circuit includes a first inductor2, a capacitor3, a switching circuit4, and a snubber circuit5. The snubber circuit5includes a second inductor51inductively coupled to the first inductor2and electrically coupled to the capacitor3. In the example shown inFIG. 1, the second inductor51is electrically coupled to the capacitor3via a rectifier element52. For example, the rectifier element52is a bipolar diode (as shown), a Schottky diode, or the like.

Referring toFIG. 1, the power converter circuit includes a first port at circuit nodes11,12and a second port at circuit nodes13,14. The power converter circuit is configured to receive an input power at one of the first and second ports and to provide an output power at the other one of the first and second ports. Just for the purpose of explanation it is assumed that the input power P1is received at the first port11,12and that the output power P3is provided at the second port13,14. In this case, the first port11,12may be referred to as input and the second port13,14may be referred to as output of the power converter. The input power P1is defined by a voltage V1between the circuit nodes11,12of the first port and a current I1at the first port, and the output power P3is defined by a voltage V3between the circuit nodes13,14of the second port and a voltage13at the second port. The voltage V1and the current I1at the first port11,12will be referred to as input voltage V1and input current I1in the following, and the voltage V3and the current I3at the second port will be referred to as output voltage V3and output current I3in the following.

According to one example, the power converter circuit is configured to convert the input power P1into the output power P3such that at least one of the signal waveform and the voltage level of the input voltage V1is different from the signal waveform and the voltage level, respectively, of the output voltage V3. According to one example, the power converter is a DC/DC power converter so that both the input voltage V1and the output voltage V3are DC voltages, but have different voltage levels. According to one example, the power converter circuit is configured to generate the output voltage V3with a lower voltage level than the input voltage V1, and according to another example, the power converter circuit is configured to generate the output voltage V3with a higher voltage level than the input voltage V1.

According to one example, the power converter circuit is configured to regulate the voltage level of the output voltage V3such that a voltage level of the output voltage V3has a predefined voltage level. To regulate the output voltage V3the switching circuit4generates a pulse-width voltage VPWMbased on a feedback signal SFB. In the power converter circuit shown inFIG. 1, the inductor2and the capacitor3are connected in series and the series circuit with the inductor2and the capacitor3receives the pulse-width modulated voltage VPWMgenerated by the switching circuit4.

For example, the feedback signal SFBis generated by a feedback circuit7(illustrated in dashed lines inFIG. 1) that receives the output signal that is to be regulated. For example, if the output voltage V3is to be regulated the feedback circuit7receives the output voltage V3(as shown) or a signal representing the output voltage V3. If the output current I3is to be regulated the feedback circuit7receives the output voltage or a signal representing the output current I3. According to one example, the feedback circuit7is configured to generate an error signal based on the output signal and a reference signal and to filter the error signal in order to obtain the feedback signal. Filtering the error signal may include using a filter with one of a proportional (P) characteristic, a proportional-integral (PI) characteristic, a proportional-integral-derivative (PID) characteristic, or the like.

A signal diagram of a pulse-width modulated voltage VPWMaccording to one example is shown inFIG. 2. Referring toFIG. 2, the pulse-width modulated voltage VPWMincludes a sequence of voltage pulses which are timely spaced apart from each other by pause periods. According to one example, the switching circuit4is configured to regulate the output voltage V3by varying, based on the feedback signal SFB) at least one of a duration of the voltage pulses and the pause periods between the voltage pulses. This is commonly known as varying a duty-cycle of the PWM voltage VPWM. Varying the duty cycle of a PWM voltage such as the PWM voltage shown inFIG. 1based on a feedback signal such as the feedback signal SFBshown inFIG. 1is commonly known in power converter circuits so that no further explanation is required in this regard.

The switching circuit4may include parasitic capacitances, examples of which are explained in greater detail herein below. These parasitic capacitances together with the first inductor2and/or parasitic inductances in the switching circuit4may form a resonant circuit which may be excited by the switched-mode operation of the switching circuit4. Excitation of this resonant circuit may result in high voltage peaks of the voltage VPWMat the output of the switching circuit and, therefore, across the first inductor2and the capacitor. The snubber circuit5serves to limit these voltage peaks in the way explained in the following.

A voltage V51across the second inductor51is given by
V51=V3+VF52  (1),
where VF52is the forward voltage of the rectifier element52. If the output voltage V3is significantly higher than the forward voltage VF52, the forward voltage can be neglected, so that the voltage V51across the second inductor51is substantially given by the output voltage V3. By virtue of the voltage V3having a predefined (regulated) voltage level, the voltage V51across the second inductor51is substantially clamped to the regulated voltage level of the voltage V3, as explained in the following. The voltage across the series circuit with the first inductor2and the capacitor3is given by
VPWM=V2+V3  (2).

By virtue of the first inductor2being inductively coupled with the second inductor51, the voltage across the first inductor2is given by (clamped to)

V⁢⁢2=N2N51·V⁢⁢51=N2N51·V⁢⁢3(3)
here N51denotes the number of turns of the second inductor51, and N2denotes the number of turns of the first inductor2. Using equations (2) and (3), a maximum voltage level of the output voltage of the switching circuit4is given by

Thus, the snubber circuit5limits (clamps) the output voltage VPWMof the switching circuit4to a voltage level defined by the voltage level of the regulated voltage V3and a winding ratio N51/N2of the second inductor51and the first inductor2. The first inductor2and the capacitor3are circuit elements required to ensure proper operation of the power converter circuit so that only two additional circuit elements, namely the second inductor51and the rectifier element52, are additionally required to provide for the voltage clamping (snubber) capability.

The switching circuit4can be implemented in many different ways. Some examples of how the switching circuit4can be implemented are explained below.FIG. 3shows one example of a power converter circuit with a full-bridge phase-shift topology. In this power converter circuit, the switching circuit4includes a full-bridge coupled to the input11,12, a rectifier circuit coupled to the series circuit with a first inductor2and a capacitor3, and a transformer42coupled between the full-bridge and the rectifier circuit. The full-bridge includes a first half-bridge and a second half-bridge. The first half-bridge includes a first electronic switch411and a second electronic switch412connected in series between the circuit nodes11,12of the input, and the second half-bridge includes a third electronic switch413and a fourth electronic switch414connected in series between the circuit nodes11,12of the input. A circuit node common to the first switch411and the second switch412forms an output of the first half-bridge, and a circuit node common to the third switch413and the fourth switch414forms an output of the second half-bridge. A primary winding421of the transformer42is connected between the output of the first half-bridge and the output of the second half-bridge. The rectifier circuit can be implemented as a full-bridge rectifier, as shown inFIG. 3. In this case, the rectifier circuit includes a first series circuit with a first rectifier element431and a second rectifier element432and a second series circuit with a third rectifier element433and a fourth rectifier element434. Each of these series circuits is connected in parallel with the series circuit including the first inductor2and the capacitor3. A secondary winding422of the transformer42is connected between a tap of the first series circuit and a tap of the second series circuit. The tap of the first series circuit is a circuit node common to the first rectifier element431and the second rectifier element432, and the tap of the second series circuit is a circuit node common to the third rectifier element433and the fourth rectifier element434.

A control circuit44modulates a voltage V421across the primary winding421of the transformer by controlling operation of the full-bridge. The primary winding421and the secondary winding422have the same winding sense so that a voltage V422across the secondary winding422is substantially proportional to the voltage V421across a primary winding421. In particular, V422=V421/n, wherein n is the transformer winding ratio, which is given by the number N421of turns of the primary winding421divided by the number N422of turns of the secondary winding.

One way of operation of the power converter shown inFIG. 3is explained with reference toFIG. 4.FIG. 4shows timing diagrams of an output voltage V41aof the first half-bridge, an output voltage V41bof the second half-bridge, the voltage V421across the primary winding421, and the pulse-width modulated voltage VPWMat an output of the switching circuit4. Referring toFIG. 3, the output voltage V41aof the first half-bridge is a voltage between the output of the first half-bridge and the second circuit node12, the output voltage V41bof the second half-bridge is the voltage between the output of the second half-bridge and the second input node12, and the voltage V421across the primary winding421is given by the output voltage V41aof the first half-bridge minus the output voltage V41bof the second half-bridge. According to one example, the control circuit4is configured to control operation of the first half-bridge and the second half-bridge such, that each of the output voltages V41a, V41bis a pulse-width modulated voltage with a duty-cycle of substantially 50%. There is a phase-shift φ between these output voltages V41a, V41bso that the voltage V421across the primary winding421includes positive voltage pulses, negative voltage pulses and pause periods between the individual voltage pulses. That is, the voltage V421across the primary winding421has three different voltage levels, a positive level, zero, and a negative level. The absolute value of each of the positive level and the negative level is substantially given by the voltage level of the input voltage V1(if voltage drops across the individual electronic switches411-414are neglected).

Referring to the above, and as shown inFIG. 4, the voltage V422across the secondary winding422has the same signal waveform as the voltage V421across the primary winding421but a different absolute value of the positive level and the negative level. The rectifier circuit431-434rectifies the voltage V422across the secondary winding to generate the pulse-width modulated output voltage VPWMof the switching circuit4, so that the pulse-width modulated output voltage VPWMis a rectified version of the voltage V422across the secondary winding422. The pulse-width modulated output voltage VPWMhas a duty-cycle D, which is given by
D=Th/(Th+Tl)  (5),
where This the duration of a voltage pulse, and Tlis the duration of a pause period between two voltage pulses. By varying this duty-cycle D, the electric power provided to the series circuit with the first inductor2and the capacitor3and, therefore, the output voltage V3can be regulated.

In the power converter circuit shown inFIG. 3, the duty-cycle of the pulse-width modulated output voltage VPWMof the switching circuit4can be adjusted by adjusting the phase-shift φ between the output voltages V41aand V41bof the half-bridges. According to one example, the control circuit44, which controls operation of the switches411-414via drive signals S411, S412, S413, S414, is configured to vary the phase-shift φ of these voltages V41a, V41bbased on the feedback signal SFBin order to regulate the output voltage V3.

The rectifier elements431-434of the rectifier circuit may include parasitic capacitances. These parasitic capacitances are illustrated as capacitors in dashed lines inFIG. 3. The rectifier elements431-434can be passive rectifier elements, such as bipolar diodes (as shown), Schottky diodes, or the like. In case of bipolar diodes, the parasitic capacitances can include junction capacitances of the bipolar diodes.

In the power converter circuit shown inFIG. 3, the electronic switches411-414of the full-bridge are schematically illustrated as switches controlled by the control circuit44. For example, these electronic switches411-414are transistors such as MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors). IGBTs (Insulated Gate Bipolar Transistors), BJTs (Bipolar Junction Transistors), HEMTs (High Electron Mobility Transistors), or the like. Optionally, a freewheeling element such as a diode (illustrated in dashed lines inFIG. 2) is connected in parallel with at least one or each of the electronic switches411-414. These freewheeling elements allow a current to flow in one direction given by the forward direction of the respective freewheeling element when the corresponding switch has been switched off. The freewheeling elements may be used to clamp the voltage across the corresponding electronic switch411-414to zero before the electronic switch411-414switches on. This is known as zero voltage switching (ZVS).

According to one example, shown inFIG. 5, at least one or each of the electronic switches411-414is implemented as a MOSFET. The MOSFET41shown inFIG. 5represents one of these four electronic switches of the full-bridge. A MOSFET includes an internal diode (often referred to as body diode) between a drain node and a source node. This body diode can be used as the respective freewheeling element described above so that no additional freewheeling element is required when implementing the electronic switch as a MOSFET. The body diode is explicitly shown inFIG. 5.

FIG. 6shows a modification of the power converter circuit shown inFIG. 3. In the power converter circuit shown inFIG. 6, the rectifier circuit includes active rectifier elements (which are drawn as electronic switches inFIG. 6) instead of passive rectifier elements shown inFIG. 3. Referring toFIG. 7, each of these active rectifier elements can be implemented as a transistor, such as a MOSFET. According to one example, these MOSFETs are connected such that their internal body diodes are interconnected in the same way as the passive rectifier elements shown inFIG. 3. The active rectifier elements431-434are controlled by a control circuit45that generates drive signals S431-S434for these active rectifier elements431-434. According to one example, the control circuit45is configured to sense a voltage across the individual active rectifier elements431-434and is configured to switch on the respective active rectifier element when the sensed voltage indicates that the corresponding body diode is forward biased. Using a rectifier circuit with active rectifier elements instead of passive rectifier elements, as shown inFIG. 3, can help to reduce losses occurring in the power converter circuit.

FIG. 8shows a power converter circuit according to another example. The power converter circuit shown inFIG. 8is based on the power converter circuit shown inFIG. 3and includes a full-bridge411-414controlled by a control circuit44and connected between an input11,12and a primary winding421of a transformer42. Coupled to a secondary winding422of the transformer421the switching circuit4includes two rectifier elements, a first rectifier element461connected between a first circuit node of the secondary winding422and a first node13of the output13,14, and a second rectifier element462connected between a second circuit node of the secondary winding422and the first node13of the output13,14. Furthermore, the power converter circuit includes two first inductors, one inductor21connected between the second circuit node of the secondary winding422and a second node14of the output13,14, and another inductor22connected between the first circuit node of the secondary winding422and the second node14of the output13,14. This secondary side topology with the secondary winding422, the two rectifier elements461,462and the two first inductors21,22is usually referred to as current doubler topology.

In this topology, voltage peaks of the output voltage VPWMof the switching circuit4may affect both first inductors21,22. Thus, the power converter circuit shown inFIG. 8includes two snubber circuits, a first snubber circuit51inductively coupled with the first inductor21, and a second snubber circuit52inductively coupled with the other first inductor22. Each of these first and second snubber circuits51,52is implemented like the snubber circuit5explained with reference toFIG. 1. That is, each of these snubber circuits51,52includes a second inductor511,512inductively coupled with the respective first inductor21,22, and a rectifier element521,522connected in series with the respective second inductor511,512. Each of the series circuits with one second inductor511,512and one rectifier element521,522is connected in parallel with the capacitor3.

The operating principle of the power converter circuit shown inFIG. 8is similar to the operating principle of the power converter circuit shown inFIG. 3. The control circuit44receives the feedback signal SFBand generates a 3-level signal V421across the primary winding421of the transformer42in the way explained with reference toFIG. 4. In the power converter circuit shown inFIG. 8, however, the output signal VPWMof the switching circuit4is a 3-level signal which is in phase with the voltage V422across the secondary winding. A voltage level of the output voltage VPWMsubstantially equals the voltage level of the secondary side voltage V422(to be more precisely, the voltage level of the output voltage VPWMequals the voltage level of the secondary side voltage V422minus the forward voltage of the rectifier element461,462). When the output voltage VPWMhas a positive voltage pulse, the secondary side current flows through the rectifier element461, the capacitor3and the first inductor21, and when the output voltage VPWMhas a negative voltage pulse, the secondary side current flows through the rectifier element462, the output capacitor3and the first inductor22.

FIG. 9shows a power converter circuit according to another example. This power converter circuit is based on the power converter circuit shown inFIG. 3and is different from the power converter circuit shown inFIG. 3in that the switching circuit4on the secondary side includes only two rectifier elements431.433instead of a full-bridge rectifier. The rectifier circuit with the two rectifier elements431,433shown inFIG. 9can be obtained from the rectifier circuit with the four rectifier elements431-434shown inFIG. 3by omitting rectifier element432and replacing rectifier element434with a short-circuit. Thus, a first circuit node of the secondary winding422is connected to the first inductor2via rectifier element431, and a second circuit node of the secondary winding422is connected to the second node14of the output13,14.

On the primary side, the power converter circuit shown inFIG. 9includes a full-bridge connected between the input11,12and the primary winding421. However, the full-bridge shown inFIG. 9is different from the one shown inFIG. 3in that each of the first and second half-bridges includes a series circuit with one electronic switch411,414, respectively, and a rectifier element471,474, respectively. The operating principle of the primary side circuit is similar to the operating principle of the primary side circuit shown inFIG. 3. The electronic switches411and414are switched on and off simultaneously (synchronously) by the control circuit44. In the on-state of the electronic switches411,414the voltage V421across the primary winding substantially equals the input voltage V1; on the secondary side the current then flows through the rectifier element431connected between the second inductor2and the secondary winding422, the second inductor2, the output capacitor3and the output13,14, respectively. When the electronic switches411,414switch off the voltage across the primary winding421changes its polarity and has a voltage level that is given by the input voltage V1plus forward voltages of the rectifier elements472,474on the primary side. Usually these forward voltages are much smaller than the input voltage so that voltage V421across the primary winding421is substantially given by −V1(the inverted input voltage) after the electronic switches411.414have been switched off and until the transformer42has been demagnetized. After the electronic switches411,414have switched off the voltage V422across the secondary winding422also changes its polarity so that the rectifier element431prevents a current flow through the secondary winding422; a freewheeling current driven by energy magnetically stored in the first inductor2flows through the first inductor2, the capacitor3and the output13,14, respectively, and the further rectifier element432on the secondary side. Of course, in each of the power converter circuits shown inFIGS. 8 and 9, the rectifier element2461,462,431and433can be replaced by active rectifier elements as explained with reference toFIGS. 6 and 7herein before.

The power converter circuits explained before each have an isolated topology. That is, the input11,12and the output12,14are galvanically isolated by a transformer42. The use of the snubber circuit5explained before, however, is not restricted to power converters with an isolated topology, but may be used in power converter circuits with a non-isolated topology as well.FIG. 10shows one example of a power converter circuit with a non-isolated topology. This power converter circuit is implemented as buck converter. In this case, the switching circuit4includes a half-bridge with a first electronic switch411and a rectifier element471connected between the first circuit node11and the second circuit node12of the input. The series circuit with the first inductor2and the capacitor3is connected in parallel with the rectifier element472. In the example shown inFIG. 10, the rectifier element472is drawn as a bipolar diode. This, however, is only an example. The rectifier element471may be implemented as another type of passive rectifier element, such as a Schottky diode, or an active rectifier element as well. A control circuit44receives the feedback signal SFBand controls operation of the first electronic switch411. In particular, the control circuit44controls a duty-cycle of the pulse-width modulated output voltage VPWMof the switching circuit4.

In the power converter circuits explained herein before, the second inductor51of the snubber circuit5are coupled to a capacitor3connected between the circuit nodes13,14of the output. This, however, is only an example. According to another example, shown inFIG. 11, the power converter circuit includes a capacitor6connected between the first circuit node11and the second circuit node12of the input, and the second inductor51of the snubber circuit5is connected to the capacitor6, which will be referred to as input capacitor in the following. In particular, a series circuit with a second inductor51and the rectifier element52is connected in parallel with the input capacitor6. The switching circuit4, which is only schematically shown inFIG. 11, can be implemented in accordance with any of the switching circuits4explained herein before.

In the power converter circuits explained above, the second inductor2is connected between the switching circuit4and the output13,14. This, however, is only an example. According to another example, shown inFIG. 12, the second inductor2is connected between the input11,12and the switching circuit4. In the example shown inFIG. 12, the second inductor51of the snubber circuit5is connected to the input capacitor6. This, however, is only an example. According to another example (not shown), the second inductor51is connected to the output capacitor3.

FIG. 13shows one example of the switching circuit4shown inFIG. 12. In this example, the power converter circuit has a boost converter topology. In this case, the switching circuit4includes a switch connected in parallel with a series circuit that includes the input capacitor6and the first inductor2. A rectifier element47is connected between a circuit node common to the switch41and the first inductor2and the first circuit node13of the output. An output capacitor3is connected in parallel with the series circuit including the switch41and the rectifier element47. A control circuit44controls operation of the switch41such that one of the output voltage V3and the output current I3has a predefined signal level.

In the power converter circuits explained above, the second inductor51of the snubber circuit5is connected to one of the output capacitor3and the input capacitor6of the power converter circuit. This, however, is only an example. Basically, the second inductor51can be coupled to each capacitor across which a voltage with a predefined or regulated voltage level is available.FIG. 14shows one example of a power converter circuit, in which the second inductor51is coupled to a capacitor82of an auxiliary voltage source8. The power converter circuit shown inFIG. 14is based on the power converter circuit shown inFIG. 3. This, however, is only an example. Connecting the second inductor51to an auxiliary voltage source8is not restricted to the specific topology shown inFIG. 14but may be used in other topologies as well.

Referring toFIG. 14, the auxiliary voltage source includes an auxiliary winding423of the transformer42. This auxiliary winding423is inductively coupled with the primary winding421and the secondary winding422. A series circuit with the capacitor82and a rectifier element83is connected in parallel with the auxiliary winding423. In this circuit, the capacitor82is charged from the auxiliary winding61via the rectifier element83, so that a supply voltage VSUPis available across the capacitor82. For example, the control circuit44receives this supply voltage VSUP. A voltage limiting element84, such as, for example, a Zener diode is connected in parallel with the capacitor82, so as to limit the supply voltage VSUPto a predefined voltage level.