Current control circuit with in parallel transistor modules

Consistent with an example embodiment, there is a current control circuit for controlling current flow between a first terminal and second terminal. The current control circuit comprises a current-sensing power MOSFET (metal-oxide semiconductor field effect transistor). Current control is useful for limiting current flow during linear mode operations such as “hotswap”, “soft start” and “eFuse” operations, in particular, the reducing of or the preventing of high current surges due to discharged capacitive loads suddenly being switched into circuit. Such current surges can cause supply interference or cause malfunction of sensitive circuits due to the effects of the noise pulse. In extreme cases, fuses may blow or circuit breakers may trip due to the high current surges, therefore taking one or more systems offline.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority under 35 U.S.C. §119 of European patent application no. 13182494.8, filed on Aug. 30, 2013, the contents of which are incorporated by reference herein.

This disclosure relates to a current control circuit for controlling current flow between a first terminal and second terminal. In particular, although not exclusively, this disclosure relates to a current control circuit comprising a current-sensing power MOSFET (metal-oxide semiconductor field effect transistor).

Current control is useful for limiting current flow during linear mode operations such as “hotswap”, “soft start” and “eFuse” operations. in particular, to reduce or prevent high current surges due to discharged capacitive loads suddenly being switched into circuit.

Such current surges can cause supply interference or cause malfunction of sensitive circuits due to the effects of the noise pulse. in extreme cases fuses may blow or circuit breakers may trip due to the high current surges, therefore taking one or more systems offline.

According to a first aspect of the invention there is provided a current control circuit, comprising:a first terminal;a second terminal;a plurality of transistor modules in parallel with each other between the first terminal and the second terminal, each transistor module comprising:a main transistor having a main drain, a main source and a main gate, wherein the main source and the main drain define a main source-drain path, the main drain is coupled to the first terminal, the main source is coupled to the second terminal and the main gate is coupled to a local control terminal;a sense transistor having a sense drain, a sense source and a sense gate, wherein the sense source and the sense drain define a sense source-drain path, the sense drain is coupled to the first terminal, the sense gate is coupled to the local control terminal, and the sense source is configured to provide a local feedback signal;a local controller configured to:receive the local feedback signal and a main control signal; andprovide a local control signal to the local control terminal in accordance with the local feedback signal and the main control signal in order to control the current through the main source-drain path and the sense source-drain path.

The current control circuit can advantageously reduce the amount of power that is consumed in measuring the current through the circuit, Also, the likelihood that any of the transistors is operated outside of its safe operating (SOA) is reduced because of the improved current sharing due to the local control of each transistor module.

The local controller may be configured to determine the local control signal in accordance with a difference between the local feedback signal and the main control signal.

The local feedback signal may be representative of the current flowing through the sense transistor, which may also be representative of the current flowing through the main transistor.

The main control signal may be representative of a desired current level for the sense transistor or the transistor module. The local controller of each of the plurality of transistor modules may he configured to receive the same main control signal.

The local control signal may be configured to control the current such that the current through the sense source-drain path tends towards a desired current level represented by the main control signal.

The main transistor and the sense transistor may be provided on a common die. The main transistor and the sense transistor may comprise a current sensing transistor such as a current sensing power MOSFET.

An area of the main transistor may be at least an order of magnitude greater than an area of the sense transistor, and optionally a plurality of orders of magnitude greater, An area of the drain of the main transistor may be at least an order of magnitude greater than the area of the drain of the sense transistor, and optionally a plurality of orders of magnitude greater.

The local controller may include a difference amplifier comprising two inputs that respectively receive signals from a Kelvin source contact of the main transistor and the sense source of the sense transistor; and an output that provides the local feedback signal

The current control circuit may further comprise a main controller. The local may be configured to provide a main feedback signal to the main controller. The main controller may be configured to provide the main control signal in accordance with the main feedback signals received from the plurality of transistor modules. The main feedback signal may be representative of the current through the sense transistor.

The main controller may be configured to:determine a maximum power that can be applied to the main transistor and sense transistor such that they stay within their safe operating areas in accordance with the main feedback signal; andset the main control signal in accordance with the determined maximum power.

There may be provided a module configured for plugging into a backplane power supply, the module comprising any current control circuit disclosed herein.

There may be provided a soft start circuit for controlling a current flow to a capacitive load, the soft start circuit comprising any current control circuit disclosed herein.

Current-sensing power metal-oxide semiconductor field effect transistors (MOSFETs) provide a low loss method of measuring load current, in some examples eliminating the need for a current shunt resistor.

A sensing power MOSFET can include several thousand transistor cells on a common substrate arranged in parallel and sharing common drain and gate electrodes. Each transistor cell or element within the sensing power MOSFET is identical such that current applied at the drain terminal of the sensing power MOSFET is shared equally between them. In such designs, the source electrodes of several of the transistors are separated from the remaining source electrodes and connected to a separate source terminal. Accordingly, the resulting current-sensing MOSFETs can be thought of as equivalent to two transistors in parallel, these having common gate and drain terminals, but separate source terminals. The two transistors may he referred to as a main FET, which comprises the majority of the transistor cells, and a sense FET that comprises much fewer transistor cells. A sense ratio, n, is defined by the area of the main FET divided by the area of the sense FET. In use, the sense FET conducts only a small fraction of current applied to the common drain terminal, the fraction being inversely proportional to the sense ratio, n. More details on sensing power transistors are provided in the NXP application note AN10322 entitled “Current-sensing power MOSFETs”, rev.02, 24 Jun. 2009, available on the NXP website at http://www.nxp.com/documents/application_note/AN10322.pdf.

FIG. 1illustrates a current control circuit100for controlling current flow between a first terminal102and a second terminal104, The circuit100includes a plurality of transistor modules106in parallel with each other between the first terminal102and the second terminal104, In this example there are two transistor modules106, although it will be appreciated that additional transistor modules106could be easily added to the circuit ofFIG. 1. Also shown inFIG. 1is a main controller122.

Each transistor module106includes a sensing power MOSFET108and a local controller120. The sensing power MOSFET108, as described above, includes a main transistor and a sense transistor on a common substrate/die. The main transistor will be referred to as a main FET and the sense transistor will be referred to as a sense FET.

The main FET has a main drain, a main source112and a main gate. The sense FET has a sense drain, a sense source116and a sense gate, The main drain and the sense drain are provided as a common drain, which is shown with reference110inFIG. 1. The main gate and the sense gate are both connected to a local control terminal114.

The main source112and the common drain110define a main source-drain path. The common drain110is connected to the first terminal102. The main source112is connected to the second terminal104. The sense source116and the common drain110define a sense source-drain path, The sense source116is configured to provide a local feedback signal118to the local controller120.

The local controller120receives the local feedback signal118and also a main control signal124from the main controller122. The main control signal122is representative of a desired current level for the transistor module106. As described in more detail below, the main control signal124may be a fixed value that is set in accordance with the expected operating conditions of the current control circuit100or it may be dynamically set in accordance with measured parameters of the current control circuit100.

The local controller120provides a local control signal126to the local control terminal114of the sensing power MOSFETs108in accordance with the local feedback signal118and the main control signal124. The local control signal126controls the current through the main source-drain path and the sense source-drain path. For example, the voltage level at the gates of the main FET and sense FET can be set such that the current through the sensing power MOSFET108(as determined by the local feedback signal118) tends towards a value that is represented by the main control signal124.

Use of both the main control signal124and the local control signal118can enable the sensing power MOSFETs103to be controlled independently of the sensing power MOSFETs in the other transistor modules106such that they efficiently and effectively share the current serving requirements of the circuit100. This may be in contrast to a circuit that does not have such control, in which case any variations between the sensing power MOSFETs108(for example due to tolerance variations between components) can lead to an imbalance in the current sharing between the sensing power MOSFETs108, particularly at startup. For example, one of the sensing power MOSFETs108may start conducting before the others, which could lead to that sensing power MOSFET108passing more than its share of the total current if local control is not provided. This imbalance may lead to one or more of the sensing power MOSFETs108passing too much current such that its power exceeds its safe operating area (SOA), which could cause the sensing power MOSFET108to fail.

The current control circuit100can advantageously reduce the amount of power that is consumed in measuring the current through the circuit because high power sense resistors may not be used. Also, the likelihood that any of the sensing power MOSFETs108is operated outside of its safe operating (SOA) is reduced because of the improved current sharing that is enabled by the local control of each transistor module106. Further advantages that follow from the reduced power consumption and improved current sharing include:Improved efficiency due to reduced power consumption.Better thermal management of the printed circuit board (PCB) as less power is being used for measuring the current, and therefore less heat is being generated.Increased reliability of the current control circuit100, and possibly associated circuits, as less heat is being generated in measuring the current.Space saving of the PCB area because sense resistors may not be required and/or thermal management components may not be required.Cost savings in running the current control circuit100due to the decreased power consumption of the circuit100.

FIG. 2illustrates another current control circuit200for controlling current flow between a first terminal202and a second terminal204. Components inFIG. 2that have corresponding components inFIG. 1have been given similar reference numbers in the200series, and will not necessarily be described in detail again.

The plurality of transistor modules206are each provided in series with a common load234between the first terminal202and the second terminal204, The local controller220ofFIG. 2includes a current to voltage converter230and an error amplifier and compensation block232. The current to voltage converter230and the error amplifier and compensation block232may be provided on a single integrated circuit (IC), An example implementation of the current to voltage converter230is disclosed in US 2008/0191779 A1 , the content of which is incorporated herein by reference,

The main FET of the sensing power MOSFET208in this example includes two main source contacts: a high current source contact212a, and a Kelvin source contact212b.

The high current source contact212ais provided by an impedance such as a bond wire such that it can handle relatively high currents. The Kelvin source contact212bis connected to the main source of the main transistor in order to provide an accurate determination of the main transistor source potential,

The Kelvin source contact212bof the main FET is connected to a first input of the current to voltage converter230, The sense source216of the sense FET is connected to a second input of the current to voltage converter230. A purpose of the current to voltage converter230is to reliably and conveniently present the signal at the sense source216of the sense FET for subsequent processing. In this example, the current to voltage converter230includes a difference amplifier (not shown) with two inputs that respectively receive signals from the Kelvin source contact212bof the main FET and the sense source207of the sense FET. The difference amplifier equalises the potential at the Kelvin source contact212bof the main FET and the sense source207of the sense FET. In this way, a constant ratio between the amount of current that flows through the drain-source path of the sense FET and the amount of current that flows through the drain-source path of the main FET can be maintained and so the sense ratio, n, can be used to accurately calculate the total current flow through the sensing power MOSFET208using the determined current through the sense FET,

The current to voltage converter230has an output that provides a signal representative of the current through the sense FET, The output of the current to voltage converter230is connected to an inverting input of an operational amplifier228in the error amplifier and compensation block232. The main control signal224from the main controller222is provided as an input to a non-inverting input of the operational amplifier228. The main control signal224is representative of a desired current level for the transistor module206. The output of the operational amplifier228is representative of the difference between the main control signal224and the current through the sense FET.

It will be appreciated that the implementation of the error amplifier and compensation block232that is shown inFIG. 2is exemplary, and that any other implementation can be used that provides the required functionality.

The main control signal224may be a fixed value that is set in accordance with the expected operating conditions of the current control circuit100. The fixed value may be set by a user based on design parameters. For example, if a total current of20A is desired through the load234then the main control signal224may be set such that each of the two sensing power MOSFETs208will contribute10A.

Alternatively, the main control signal224may be dynamically set in accordance with measured parameters of the current control circuit200. As shown inFIG. 2, a main feedback signal236is provided by the transistor module206to the main controller222.

In this embodiment, the main feedback signal236is provided by the output of the current to voltage converter230and is representative of the current through the sensing power MOSFET208. Each of the transistor modules206provides a similar main feedback signal236. The main feedback signals236are combined together in this example, thereby adding together the individual current values, before being provided to a feedback terminal of the main controller222. The feedback terminal of the main controller222inFIG. 2is labeled as “ISENSE”. The main controller222can then use the signal received at ISENSE to determine how much current should be allowed to flow through the sensing power MOSFETs208without exceeding their safe operating areas. The power that the MOSFET can handle without exceeding its safe operating area (SOA) is a function of the voltage dropped across the drain-source path of the sensing power MOSFET208(Vds) multiplied by the current at the drain of the sensing power MOSFET208(Id), along with thermal arrangement, time and secondary breakdown mechanisms. The SOA is typically represented by a graph. The main controller222can process the signal received at ISENSE in order to determine the power that is being applied to the sensing power MOSFETs208and then set the main control signal224such that current flow through the sensing power MOSFETs208is controlled such that the MOSFETs208operate within the SOA. In some examples the main controller222can process the signal received at SENSE along with the known resistance of the drain-source path of the sensing power MOSFETs208and/or a measured value of the voltage dropped across the sensing power MOSFETs208in order to determine the power that is being applied to the sensing power MOSFETs208. In this way, the current flow through the sensing power MOSFETs208can be advantageously controlled such that the likelihood of component failure is reduced.

The main controller222may be provided as a simple adaption of existing controllers such as the LT4256 component from Linear Technology Corporation, or other controllers, such as those provided by Maxim Integrated, Texas Instruments, or Analog Devices, for example.

One or more of the examples disclosed herein can provide soft start/hot swap control, for applications where power-up current surges should be controlled. Such applications include examples where modules/boards can be plugged into a backplane in order to be connected to a power supply, such as in server systems and telecoms equipment. Examples can also include high power load switching applications where discharged capacitive loads are switched onto power rails. Also, examples disclosed herein can be used in soft start circuits for controlling a voltage supply to a capacitive load. MOSFET safe operating area can be a key parameter in these applications, and low RDSONthe resistance between the common drain and the high current source contact when the main FET is on) can be important since it can helps to reduce power dissipation when the MOSFET is fully switched on.

It will be appreciated that any of the controllers disclosed herein can be analogue or digital controllers,

The sensing power MOSFETs disclosed herein may be power FETs such as those described in the NXP application note AN10322. Also, the field effect transistors disclosed in NXP B.V.'s U.S. Pat. No. 7,737,507 B2 can have safe operating areas (SOAs) that are particularly suitable for one or more of the circuits disclosed herein,

Any components that are described herein as being “coupled” or “connected” could be directly or indirectly coupled or connected. That is, one or more components could be located between two components that are said to be coupled or connected whilst still enabling the required functionality to be achieved.