WO2019224165A1

WO2019224165A1 - Multiphase dc-dc converter and method of controlling such

Info

Publication number: WO2019224165A1
Application number: PCT/EP2019/063011
Authority: WO
Inventors: Simon RAVYTS; Johan Driesen
Original assignee: Katholieke Universiteit Leuven
Priority date: 2018-05-22
Filing date: 2019-05-21
Publication date: 2019-11-28
Also published as: GB201808364D0

Abstract

A multi-phase DC-DC converter (1) is disclosed. The converter (1) comprises at least two phases (P1, P2, P3); at least one temperature sensor (2) configured to sense ambient temperature (T_a) of the converter (1); and a control module (3) electrically coupled to the plurality of phases (P1, P2, P3) and to the temperature sensor (2). The control module (3) is configured to determine a component temperature of at least one component of at least one of the phases (P1, P2, P3) in dependence upon the ambient temperature (T_a) and to determine an activation state of each phase (P1, P2, P3) in dependence upon the component temperature.

Description

MULTIPHASE DC-DC CONVERTER AND METHOD OF CONTROLLING SUCH

Field of the invention

The present invention relates to the field of electronics. More particularly, the present invention relates to a multiphase DC-DC converter and methods of controlling a multiphase DC-DC converter.

Background of the invention

Reliability of electronic components is an issue in many fields of electronics. For example, in building- integrated photovoltaics (BIPV) applications, electronic components may not be easily accessible for replacement in the event of failure, and so components which maintain their function over a duration of years are required.

Parameters which can affect reliability and lifetime of a component may be referred to as stressors. One such stressor is the temperature experienced by the component. One example is the junction temperature of a MOSFET. The functionality of a MOSFET can be permanently damaged if the junction temperature rises above a predetermined value. A BIPV module can include such temperature- sensitive components, for example in a multiphase DC-DC converter.

US2006/061339 describes a multiphase voltage regulator which automatically senses the temperature of components from each phase and lowers the current through hot phases while raising the current through cool phases.

However, in US2006/061339, the temperature of each phase is measured by a sensor located in close proximity to the bank of FETs for that phase. Thus, a separate temperature sensor is required for each phase. Furthermore, the regulator is a low-power device.

Therefore there is a need for devices having an improved reliability under temperature variations and methods of operating such devices.

Summary of the invention

It is an object of the present invention to provide a DC-DC converter having a good reliability under temperature variations and methods of operating such a device.

According to a first aspect of the present invention there is provided a multi-phase DC-DC converter comprising at least two phases; at least one temperature sensor configured to sense ambient temperature of the converter; and a control module electrically coupled to the plurality of phases and to the temperature sensor. The control module is configured to determine a component temperature of at least one component of at least one of the phases in dependence upon the ambient temperature and to determine an activation state of each phase in dependence upon the component temperature. Determining an activation state of each phase may comprise comparing the component temperature with a predetermined threshold value.

It is an advantage of embodiments of the present invention that the internal temperature of the component is estimated. The internal temperature of the component refers to the temperature increase being a consequence of the losses caused by the operation of the component, e.g. by the thermal losses created in the component.

The control module thus estimates the component temperature of at least one component. Where in embodiments of the present invention reference is made to determining an activation state of each phase, reference is made to selecting an activation state of each phase. Selecting an activation state of a phase may comprise maintaining the current activation state of a phase or switching the current activation state of a phase.

Determining an activation state of each phase may comprise, when the component temperature is greater than the predetermined threshold value, changing an activation state of at least one phase from an inactive state to an active state.

Each phase in an active state may receive substantially the same portion of a total current wherein the total current is a sum of current amounts received by each phase in an active state

The at least one component of at least one of the phases may comprise a switch.

A temperature of at least one component may be a junction temperature of the switch.

The component temperature may be determined in dependence upon an input voltage V_m of the converter.

The component temperature may be determined in dependence upon an input current of the converter.

The at least one component of at least one of the phases may comprise a diode.

The at least one component of at least one of the phases may comprise an inductor.

The at least one component of at least one of the phases may comprise a capacitor.

According to a second aspect of the present invention there is provided a photovoltaic device comprising a converter according to the first aspect.

According to a third aspect of the present invention there is provided a method of operating a converter according to the first aspect or a device according to the second aspect, the method comprising determining a component temperature of at least one component of at least one of the phases in dependence upon the ambient temperature; and determining an activation state of each phase in dependence upon the component temperature.

Determining an activation state of each phase may comprise, if the component temperature is greater than a predetermined threshold value, changing an activation state of at least one phase from an inactive state to an active state.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

Brief description of the drawings

Further features of the present invention will become apparent from the examples and figures, wherein:

Figure 1 is a schematic circuit diagram of a three-phase interleaved Boost converter according to embodiments of the present invention;

Figure 2 is a schematic view of a control module which may be comprised in a converter according to embodiments of the present invention;

Figure 3 is a flow diagram of a method according to embodiments of the present invention;

Figure 4a is a plot of the number of active phases required to keep junction temperature below a threshold temperature value, for a range of voltage and current values, at a first ambient temperature; Figure 4b is a plot of the number of active phases required to keep junction temperature below a threshold temperature value, for a range of voltage and current values, at a second ambient temperature value;

Figure 5a is a top view of Figure 4a;

Figure 5b is a top view of Figure 4b;

Figure 6 shows a three-phase boost converter according to embodiments of the present invention, comprising two temperature sensors;

Figure 7 illustrates a converter according to embodiments of the present invention comprising two temperature sensors. Detailed description of preferred embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated. The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein. In the drawings, like reference numerals indicate like features; and, a reference numeral appearing in more than one figure refers to the same element.

Referring to Figure 1, a multiphase, or multi-leg, DC-DC converter 1 according to embodiments of the present invention is shown. The converter 1 is configured to convert an input voltage V_m to an output voltage V_out which is greater than V_m. The output voltage is the voltage across an output resistor R. The converter 1 includes a first phase PI, a second phase P2, and a third phase P3. However, the present invention is not limited to embodiments having precisely three phases. Embodiments of the present invention may comprise two, three, four, five or more phases.

The converter 1 has an input current which is supplied by the input voltage V_m. Each phase advantageously includes a capacitor C connected across the output of the phase which can function to filter out voltage ripple. The first phase PI includes a first inductor LI, a first diode D1 and a first switch SI. The second phase PI includes a second inductor L2, a second diode D2 and a second switch S2. The third phase includes a third inductor L3, a third diode D3 and a third switch S3. The use of each phase can be enabled or disabled by operation of its respective switch.

Herein, a phase being "in use" or "activated" means that the phase is used for the voltage conversion of Vi_n to V_out. The corresponding switch comprised in a phase which is "in use" is activated and deactivated (or opened and closed) with an on-time which is dependent on the desired output voltage. Thus the switch comprised in a phase which is in use may not be in the closed configuration for the duration of the period that the phase is in use. Herein, a switch may also be described as "in use", which means that the switch is activated and deactivated so as to allow the phase comprising the switch to be used for the voltage conversion. When the switch comprised in a phase is "in use", the corresponding phase is also in use. Herein, where a switch is described as "not in use", this means that the switch is open (i.e., does not allow current to pass) for the entire period of time that the switch is not in use, and the corresponding phase is not activated and does not contribute to the voltage conversion.

When switch SI is in use and switches S2 and S3 are not in use, phase 1 is activated and phases 2 and 3 are not activated. The converter 1 operates as a standard DC-DC boost converter with the first inductor LI providing energy storage. When switches SI and S3 are not in use and switch S2 is in use, phase 2 is activated and phases 1 and 3 are not activated. The converter 1 operates as a standard DC- DC boost converter with the second inductor L2 providing energy storage. When switches SI and S2 are not in use and switch S3 is in use, phase 3 is activated and phases 1 and 2 are not activated. The converter 1 operates as a standard DC-DC boost converter with the third inductor L3 providing energy storage.

When more than one switch is in use, the corresponding phases are activated. For example, when switches SI and S2 are in use and switch S3 is not in use, the first phase PI and the second phase P2 are both activated and the third phase P3 is not activated. The current load of the converter 1 is shared between the first phase PI and the second phase P2, wherein the current load may be divided equally or in unequal proportions. Thus a current flowing through each switch SI, S2 is less than a current which would flow through a switch in use in the case that the other switches are not in use (through SI when SI is in use and S2 and S3 are not in use, or through S2 when S2 is in use and SI and S3 are not in use). The switches SI, S2, S3 are preferably transistor-based switches, for example a MOSFET, a SiC MOSFET, an IGBT, a BJT, a GaN FET. In such switches, when in use, the junction can experience thermal stress. Such thermal stress may arise as a consequence of current flowing through the switch when the switch is closed and switching actions. Losses inside the switch will lead to a temperature increase of the junction. This can cause the functionality of the switch to be impaired. For example, the temperature increase can lead to wear-out of the switch and if the temperature exceeds a certain limit, such as a maximum temperature value which may be set by the manufacturer, the switch may be almost immediately be destroyed and/or permanently damaged. The wear-out can be bond-wire lift off or a steady increase in the on-resistance. The switch or other component will then need to be replaced. Thermal stress can result in shortening of the lifetime of a component, such as a switch, diode, or inductor, as one or more bond wires configured to provide an electrical connection between the component and a support for the component such as a circuit board or printed circuit board can break or lift off, and the component is then required to be replaced. In some embodiments, the wire may be re-connected; however, in some other embodiments, a wire that lifts off may be located inside a component and may not be accessible for reconnection. The thermal stress can also be caused by the component being subject to thermal cycling. Thermal stress resulting from a junction temperature greater than a maximum junction temperature of the component (for example that given by the manufacturer) can cause immediate failure of the component, for example by explosion.

The present invention provides for additional phase(s) to be activated if the junction temperature of a component comprised in an activated phase is greater than a threshold value. The junction temperature may be calculated or estimated according to embodiments of the present invention as described herein. This can provide increased reliability of the converter by providing a redistribution of current within the converter, which results in a decrease of the current flowing through said component and thus the junction temperature of the component.

If all phases are permanently activated, the efficiency of the converter may be lower as compared to a case wherein one or more phases are inactive. Therefore some embodiments of the present invention provide a balance between efficiency and reliability of the converter by only activating an additional phase in the case that the junction temperature of a phase is greater than a threshold value, as opposed to all phases being permanently simultaneously activated.

The converter 1 includes a temperature sensor 2. The temperature sensor 2 is configured to sense an ambient temperature of the converter. For example, the converter may be contained within a casing (not shown) and the ambient temperature may be the temperature inside the casing as sensed by the temperature sensor 2. The temperature sensor 2 may be arranged, or mounted, adjacent to or nearest to one of the switches SI, S2, S3. The temperature sensor 2 may be arranged so as to be equidistant from each of the switches. The converter may be supported by a printed circuit board (PCB) and the temperature sensor may also be supported by the PCB.

Although in Figure 1 only one temperature sensor 2 is shown, in some embodiments of the present invention more than one temperature sensor may be provided. In these embodiments the temperature values measured by the sensors may be averaged and the resulting average value used to calculate the junction temperature. Alternatively, the highest measured temperature value may be used to calculate the junction temperature. The at least one temperature sensor may be located at a position so as to not be significantly affected by the component of interest (the component for which the junction temperature is calculated).

The ambient temperature can be affected by several factors. For example, if the converter 1 is comprised in a building-integrated photovoltaic module, the ambient temperature may be greater when receiving direct sunlight than during cloudy periods. The ambient temperature may be affected by heat produced by any component of the converter, for example through thermal losses of any component of the converter.

In some embodiments it is assumed for the purposes of determining the junction temperature that the ambient temperature does not vary across the converter. That is, for example, the ambient temperature at a point for example between switch S2 and switch S3 and closer to switch S3 than switch S2 is the same as the ambient temperature at a point for example between switch S2 and SI and closer to switch SI than switch S2. This assumption provides for simple calculation of junction temperatures based on one ambient temperature for all components and is easily applicable to any circuit topology.

The converter 1 includes a control module 3 configured to receive signals from the temperature sensor 2 and to provide control signals to the switches SI, S2, S3. The control module 3 may for example receive a voltage measurement from the temperature sensor 2 and use an internal lookup table or conversion factor, formula, or other suitable means to convert the voltage measurement to a temperature measurement representing the ambient temperature of the converter 1.

The control module 3 is also configured to receive measurements of the input voltage V_m and the input current , for example from a first voltage sensor VI in parallel with the input voltage and a first current sensor II in series with the input voltage, respectively. The control module 3 is configured to receive measurements of the output voltage V_out, for example from a second voltage sensor V 2 in parallel with the output resistor R. The control module 3 is also configured to control an activation state of each phase (that is, whether the switch comprised in each phase is in use). Thus the control module is configured to control the number of phases which are active, that is, contributing to the power conversion.

In some embodiments, each switch comprises a MOSFET which can be activated and deactivated by applying a gate voltage to a gate of the MOSFET. The control module may be configured to provide such a gate voltage or series of gate voltages in order to activate a switch.

The control module is configured to determine a junction temperature of each of the switches SI, S2, and S3 in dependence upon at least the ambient temperature as measured by the temperature sensor, the values of the input voltage V_m, and the input current . The calculation may also depend on the thermal resistance of a component (e.g. switch), the losses generated within the component (e.g. switch) and/or other parameters of the component which may be found in the component manufacturer's specifications or datasheet, for example for a switch, the drain-source resistance R₀s, one or more switching times. The calculation of junction temperature is dependent upon the topology of the converter (for example, the number of phases, whether the converter is a buck converter, a boost converter, a buck-boost converter).

For example, for the three-phase interleaved boost converter according to embodiments of the present invention, as shown in Figure 1, the estimated temperature (T) of the component can be calculated according to equation 1:

T— T_a + Pi_oss R_t (1)

where T_a is the ambient temperature (measured by the temperature sensor), P_|0Ss are the losses that are generated within the component and R_th is the thermal resistance of the component. The thermal resistance is a component parameter that can be found in the datasheet of the manufacturer and the losses are calculated according to the component of interest. For example, when the switch is considered, the junction temperature is the most relevant and the thermal resistance is the resistance between the junction (j) and the ambient (a). The formula then becomes according to equation 2:

For the calculation of the losses, one possible method is to consider the type of switch (MOSFET, IGBT, GaN FIEMT, BJT,..) and a specific loss model for the switch, which can depend on the desired accuracy of the designer to include or exclude certain loss mechanisms. For example, when a MOSFET is used, three major types of losses can be distinguished: on-state losses, off-state losses and switching losses. In general, the off-state losses are not taken into account as they are negligibly small compared to the other two. The off-state losses are excluded in this example of loss analysis. However, it will be understood that choice of loss model is not restricted to the described example and the skilled person will select any suitable loss model.

For the switching P_sw and on-state losses P_¥nd (also generally referred to as conduction losses) in case of a MOSFET in a Boost converter, one can find:

Ros_,on , t_on and t_off are parameters that can be found in the datasheet of the MOSFET. The switching frequency f_sw is a design parameter of the circuit and is usually constant. The currents ls,rms , I_on and l_0ff are measured by the current sensor (which may be a current sensor II in series with the input voltage or may be an individual current sensor for each component which is located in series with the concerned component) and the output voltage V_out is sensed by the voltage sensor V 2.

The control module is configured to compare the junction temperature of each switch in use with a temperature threshold value T_th. The temperature threshold value is chosen as a value which is less than a failure temperature of the switch, the failure temperature being for example a maximum junction temperature as provided by the manufacturer. For example, the failure temperature of the switch may be 150°C and the temperature threshold value may be chosen to be 125°C. In some embodiments, the temperature threshold value is no more than 80% of the failure temperature of the switch. However, in some embodiments the temperature threshold may be no more than 90% of the failure temperature of the switch, or no more than 70%, 60%, or 50% of the failure temperature of the switch.

In some embodiments, the control module 3 comprises a microcontroller, for example a C2000 series microcontroller (Texas Instruments, Dallas, USA), a PIC microcontroller (Microchip Technology, Arizona, USA), or other suitable microcontroller.

Referring to Figure 2, the control module 3 may comprise a voltage controller module 10, a component temperature estimator module 11, a current controller module 12, and a PWM modulator and gate driver module 13.

The voltage controller module 10 of the present example is configured to receive measurements of the output voltage V_out, for example from the second voltage sensor V 2 if included. The voltage controller module 10 of the present example is configured to provide an input current reference value li_{n, ref} to the current controller module 12. The input current reference value of the present example is the desired reference value of the input current. The component temperature estimator module 11 of the present example is configured to receive measurements of the input current , for example from the first current sensor II if included, and to receive measurements of the input voltage V_m, for example from the first voltage sensor VI if included. The component temperature estimator module 11 of the present example is configured to receive temperature measurements from the temperature sensor 2. The component temperature estimator module 11 of the present example is configured to determine a number N of phases to be activated, in dependence upon the input voltage, input current, and ambient temperature, for example using a method according to embodiments of the present invention as described herein, and to provide the number N of phases to be activated to the current controller module 12.

The current controller module 12 of the present example is configured to calculate the required duty cycle d (or the equivalent control voltage U_c) from the measured phase currents (II, 12, 13), the reference current ( lin.ref), the maximum reference current ( L, ref, max) and the amount of active phases (N). This can be implemented via a simple PI, PID or more advanced control algorithm. The maximum reference current is a stored value and depends on the particular type of component. For example, this may be taken from the datasheet of the component and generally depends on the power or current level for which the component is designed.

The PWM modulator and gate driver module 13 of the present example is configured to provide an interface between the switch and the current controller. It comprises a PWM modulator which is configured to receive value of the control voltage U_c or the duty cycle d and to pass this signal to a gate driver associated with each switch, which are connected physically to the switch and are configured to turn the respective switch on or off by applying a voltage or a current signal to the gate of the switch.

The temperature threshold value for a particular component may not be a fixed value for the lifetime of the converter. For example, losses can increase over the lifetime of the component, which can lead to a junction temperature that is higher than the value calculated according to embodiments of the present invention. In some embodiments, a margin may be applied to the threshold temperature so as to decrease the threshold temperature to account for increased losses. This margin may be a constant value for the lifetime of the component. In some embodiments, the margin may increase over the lifetime of the component.

In the event that the junction temperature of a switch in use is greater than the temperature threshold value T_th, the control module is configured to activate at least one phase which is currently in an inactive state. This can allow the input current to be divided between a greater number of phases than before the activation of the additional one or more phases. This can allow the junction temperature of the switch in use, which was determined to be greater than the threshold temperature value T_th, to decrease. Preferably, the input current is shared substantially equally between activated phases, as this can lead to optimized efficiency of the converter, for example as discussed in 'Extreme efficiency power electronics', J. W. Kolar et al., 7th International Conference on Integrated Power Electronics Systems (CIPS), 2012. In some embodiments, the division of input current may be unequal between activated phases. For example, in preferred embodiments a current sensor is provided for each phase in series with the inductor of that phase and a PI control loop may be used to regulate the current of each phase. The current sensor for each phase may provide the measured current to the controller 3 for calculation of the junction temperature of one or more components in that phase. In some embodiments, the division of input current between activated phases is achieved by varying the duty cycle of the each phase, which determines the amount of current in each phase.

If no phase is in an inactive state, that is, all phases are activated or in use, the control module may shutdown the converter, for example by setting the duty cycle of each phase to zero. Alternatively, if all phases are in use, the controller may decrease the input power to a value that leads to low enough losses, for example by decreasing the value of the maximum reference current _ef_max·

The control module may then repeat the steps of comparing junction temperature, of the same switch and/or of one or more other switches comprised in phases which are activated, with the threshold value and activating one or more additional phases in dependence upon the comparison.

Referring to Figure 3, a flow chart of an exemplary method of operating a controller according to embodiments of the present invention is shown. The number of active phases is denoted by N and the total number of phases is denoted by N_max. Thus, T(N) denotes the temperature of a component when N phases are active.

In the control module receives the ambient temperature measurement and the input voltage and input current measurements (step SI).

The control module calculates, in the example, the junction temperature T_j(N) of a switch comprised in an activated phase (step S2). In the case that the input current is equally divided between the active phases (and so the current in one phase is calculated by dividing the input current by the number of active phases), and each phase has the same model of active component (diode, capacitor, etc), the junction temperatures are assumed to be equal for each phase. The control module of the present example compares the calculated junction temperature with the threshold temperature value T_th (step S3).

If the calculated junction temperature is less than the threshold value, the process proceeds to step S4, where it is determined whether the number of activated phases N is greater than 1. If the number of activated phases is not greater than 1, the process returns to step SI. If the number of activated phases is greater than 1, the junction temperature is calculated for N-l, that is, assuming that the current per phase is equal to the input current divided by (N-l), and this is step S5. The new junction temperature T_j(N-l) is compared with the threshold temperature (step S6). If the new junction temperature T_j(N-l) is greater than the threshold temperature, the process returns to step SI and the number of activated phases N is not changed. If the new junction temperature T_j(N-l) is less than the threshold temperature, the number of activated phases is decreased by 1 (step S7), that is, one phase which was active is deactivated. The process then returns to step SI.

If the calculated junction temperature is greater than the threshold value, the process proceeds to step S8, where the number of active phases N is compared with the number of available phases (N_max). If the number of active phases N is less than the number of available phases N_max, the number of active phases is increased by 1 (step S9), and the process returns to step SI. If the number of active phases N is greater than the number of available phases N_max, the converter is shut down or the input current is decreased (step S10), for example by decreasing the maximum input current reference value.

In some embodiments, if a phase is activated or deactivated (i.e. if N changes, e.g. in step S7 or S9), a wait time may be provided before the number of phases is allowed to be changed subsequently. This can provide time for the ambient temperature to equalize following a change in the number of active phases.

Preferably, the input current is equally divided between the active phases. However, in some embodiments, the input current may not be equally divided between the active phases. In these embodiments, the method of Figure 3 is followed and the operations of steps S2, S3, S5, S6, S10 are performed for all of the active phases.

For example, in step SI, the currents of each phase are received and may be arranged as a vector of individual phase currents. Referring again to Figure 2, the maximum reference input current value

_ref _max may be replaced by a vector of maximum reference current values, each corresponding to a respective phase.

In step S2, a vector of temperatures may be calculated, the vector including respective elements for each active phase; in step S3, the vector of calculated temperatures may be compared with a vector of threshold temperature values. A 'yes' decision in step S3, resulting in proceeding to step S8, would require at least one of the calculated temperature vector elements to be greater than its corresponding threshold temperature vector element. If all of the calculated temperature vector elements are less than the corresponding threshold temperature vector elements, the method proceeds to step S4 instead.

In step S5, the phase which is not included in the calculation of the junction temperature for N-l phases may be chosen randomly. In some embodiments, the method may 'cycle through' the phase to be excluded from this calculation, choosing a different phase for each iteration.

In step S10, the maximum reference current value I i_{n, re}f _max may be replaced by a vector of maximum reference current values, each corresponding to a respective phase and one or more of the elements of the vector may be decreased, in dependence upon the comparison of calculated junction temperatures in step S3.

In the present example, the junction temperature of each phase is calculated using parameters of each phase (for example, the current of the phase as provided to the controller 3 by a current sensor in each phase, the maximum current of the phase).

Methods according to embodiments of the present invention have the advantage that a high junction temperature in a first switch of a first phase, which could lead to damage to the junction and impair the operation of the switch, can be detected and subsequently decreased by activating a previously inactive phase, thus decreasing the portion of the input current received by the first switch of the first phase and reducing the possibility of damage to the switch. As a consequence, the possibility of damage to the switch, which may require the switch to be replaced, is decreased, which provides a corresponding improvement in the reliability of the converter. This can have advantages in many applications, for example in applications wherein the converter is not easily accessible and it is preferred that no replacement of components is needed. Such applications can include, for example, building-integrated photovoltaics modules, or mission-critical systems where maintenance is undesired or impossible, for example in satellite or space applications. A converter according to embodiments of the present invention may be comprised in a photovoltaic module, such as a building- integrated photovoltaic module.

It is a further advantage of embodiments of the present invention that, by only activating a previously inactive phase under the condition that a junction temperature of a switch comprised in an active phase is greater than a threshold temperature value, the efficiency of the converter can be preserved by minimizing the number of phases which are required to be activated. Referring to Figure 4a, a plot is shown of the number of phases required to be activated to keep the junction temperature of all phases below 125°C, as a function of the input voltage and input current, for an ambient temperature of 25°C (which is assumed to be in a steady state during calculation of the number required number of active phases). Referring to figure 4b, a plot is shown of the number of phases required to be activated to keep the junction temperature below 125°C, as a function of the input voltage and input current, for an ambient temperature of 70°C.

It can be seen from these figures that, for example, at an input current of 6A and an input voltage of 20V, 2 phases are required to be activated in the case that the ambient temperature is 25°C and 3 phases are required to be activated in the case that the ambient temperature is 70°C. Such simulations can be used as a design guideline to choose the number of required phases for a desired range of input voltages, input currents and ambient temperatures.

In the simulations of Figures 4a and 4b, the number of phases available was not limited. This can allow to determine the number of phases that are required to span a certain region of voltage and current, in combination with a specified maximum ambient temperature.

Figures 5a and 5b are the top views of Figures 4a and 4b respectively. The areas bounded by lines in Figures 5a and 5b correspond to current-voltage regions for which a particular number of active phases is required to keep the junction temperature below the threshold temperature.

Referring to Figure 6, a flow chart of a modified method according to embodiments of the present invention is shown. The method has steps S1-S8 and S10 in common with the flow chart of Figure 3 and their description will not be repeated here.

In step S9' of the modified method, if the number of active phases is less than the total number of phases, the junction temperature is calculated for N+l. In step Sll, the junction temperature for N+l is compared with the threshold temperature. If the junction temperature for N+l is less than the threshold temperature, then N+l phases are activated (step lib) and the process returns to step SI. If the junction temperature for N+l is greater than the threshold temperature, then N is assumed to be equal to N+l (step Slla) and the process returns to step S8. Flowever, in step Slla, no additional phases are activated. This means that an optimal number of active phases can be calculated, without restricting the number of additional phases to be activated to only 1. For example, if the required number of additional active phases is 3, then according to the modified method all three of the additional phases can be activated, instead of only one phase being activated and the process then returning to step SI. Referring to Figure 7, one embodiment of a converter according to the present invention is shown. The converter 20 comprises two temperature sensors T1 and T2.

It will be appreciated that many modifications may be made to the embodiments hereinbefore described.

For example, one or more additional temperature sensors may be provided which are spaced apart from each other. The control module may then be configured to receive a measurement from each of the temperature sensors and to determine a mean value of the ambient temperature based on these measurements.

The present invention is not limited to determining the junction temperature of switches. The methods and devices according to embodiments of the present invention may be used to determine the temperature of any component of the converter which is capable of being isolated (that is, can be controllably placed in an activated state and an inactive state), for example, one or more of diodes, inductors, capacitors. Activation of one or more additional phases may then be based upon a comparison of the component temperature with a component temperature threshold which is specific to the type of component under consideration.

For example, the control module may determine the temperature of a diode comprised in an active phase and may activate one or more additional phases if the diode temperature is determined to be greater than a diode temperature threshold value. The control module may additionally or alternatively determine the temperature of one or more other components in an active phase and may activate one or more additional phases if the temperature of the other (i.e. non-diode) component is determined to be greater than a threshold value for that component.

In some embodiments, not just components but entire phases may be isolated by including two extra relays at the input and output of each phase. This can allow to isolate, for example, the output capacitor of a phase as well as the diode, which can allow to prolong the lifetime of the capacitor as well as the diode.

The present invention is not limited to a boost converter. For example, the converter according to embodiments of the present invention may be comprised in a buck converter, a buck-boost converter, a series resonant converter, a Cuk converter. For example, a series resonant converter, a Cuk converter may comprise a capacitor in each leg, or phase, and the capacitor may be a component of the converter to be monitored as described herein.

Claims

1. A multi-phase DC-DC converter (1) comprising:

at least two phases (PI, P2, P3);

at least one temperature sensor (2) configured to sense ambient temperature of the converter (1);

a control module (3) electrically coupled to the plurality of phases and to the temperature sensor (2);

wherein the control module (3) is configured to determine a component temperature of at least one component of at least one of the phases (PI, P2, P3) in dependence upon the ambient temperature (T_a ) and to determine an activation state of each phase (PI, P2, P3) in dependence upon the component temperature.

2. A converter (1) according to any preceding claim, wherein determining an activation state of each phase (PI, P2, P3) comprises comparing the component temperature with a predetermined threshold value (T_th).

3. A converter (1) according to claim 2, wherein determining an activation state of each phase (PI, P2, P3) comprises, when the component temperature is greater than the predetermined threshold value (T_th), changing an activation state of at least one phase (PI, P2, P3) from an inactive state to an active state.

4. A converter (1) according to any preceding claim, wherein a total current is a sum of current amounts received by each phase (PI, P2, P3) in an active state, and wherein each phase (PI, P2, P3) in an active state receives substantially the same portion of the total current.

5. A converter (1) according to any preceding claim, wherein the at least one component of at least one of the phases (PI, P2, P3) comprises a switch (SI, S2, S3).

6. A converter (1) according to claim 5, wherein a temperature of at least one component is a junction temperature of the switch (SI, S2, S3).

7. A converter (1) according to any preceding claim, wherein the component temperature is determined in dependence upon an input voltage V_m of the converter (1).

8. A converter (1) according to any preceding claim, wherein the component temperature is determined in dependence upon an input current of the converter (1).

9. A converter (1) according to any preceding claim, wherein the at least one component of at least one of the phases comprises a diode.

10. A converter (1) according to any preceding claim, wherein the at least one component of at least one of the phases comprises an inductor.

11. A converter (1) according to any preceding claim, wherein the at least one component of at least one of the phases comprises a capacitor.

12. A photovoltaic device comprising a converter (1) according to any preceding claim.

13. A method of operating a converter (1) according to any one of claims 1 to 11 or a device according to claim 12, the method comprising:

determining a component temperature of at least one component of at least one of the phases in dependence upon the ambient temperature (T_a ); and

determining an activation state of each phase in dependence upon the component temperature.

14. A method according to claim 13, wherein determining an activation state of each phase comprises, if the component temperature is greater than a predetermined threshold value (T_th), changing an activation state of at least one phase from an inactive state to an active state.