Patent Application
20070216396
This application claims priority under 35 U.S.C. 119 from British Application No. 0605065.2 filed 14 Mar. 2006, which application is incorporated herein by reference.
This invention generally relates to apparatus and methods for Switch Mode Power Supply (SMPS) Sensing Systems, more particularly to methods and apparatus for sensing the output current of a Switch Mode Power Supply using primary side sensing.
We will describe improved techniques which enable the design of a Switch Mode Power Supply with a relatively accurately controlled output current limit which, in embodiments, do not need current sensing components on the secondary side of the power supply.
Many SMPS applications require the output current to be either limited to, or maintained at a particular value. One way of achieving this is by including some form of output current sensing, located on the secondary side of the converter, communicating this information back to the power converter controller located on the primary side. This provides an accurate method of current sensing but incurs the cost of additional secondary side components.
A relatively crude current limiting may be implemented by monitoring and limiting the primary side switch current to a particular value. The accuracy may be improved by sensing and integrating the current through the primary switch, correlating the time constant of the integrator to the switching period, in this way estimating the output current. However, the accuracy of output current sensing in this way is dependent, among other things, upon the efficiency of power conversion, the switching time of the switch and the like.
Background prior art relating to SMPS output current control can be found in: U.S. Pat. No. 6,833,692: Method and apparatus for maintaining an approximate constant current output characteristic in a switched mode power supply; U.S. Pat. No. 6,781,357: Method and apparatus for maintaining a constant load current with line voltage in a Switch Mode Power Supply; U.S. Pat. No. 6,977,824: Full-Text Control circuit for controlling output current at the primary side of a power converter; U.S. Pat. No. 6,977,824: Control circuit for controlling output current at the primary side of a power converter; U.S. Pat. No. 6,862,194: Flyback power converter having a constant voltage and a constant current output under primary-side PWM control; U.S. Pat. No. 6,853,563: Primary-side controlled flyback power converter; and U.S. Pat. No. 6,625,042: Power supply arrangement comprising a DC/DC converter with primary-side control loop.
We will describe improved techniques for sensing the output current of an SMPS, and for measuring the output current by means of primary side sensing.
According to the present invention there is therefore provided a current sensing system for sensing an output current of a Switch Mode Power Supply (SMPS), the SMPS including a magnetic energy storage device for transferring power from an input side to an output side of the SMPS, the current sensing system comprising: a flux model system to generate a waveform representing a magnetic flux in said magnetic energy storage device; and an output current model system to generate an output current sensing signal responsive to said magnetic flux waveform.
In preferred embodiments the energy storage device has a primary side coupled to the input side of the SMPS and a secondary side coupled to the output side of the SMPS, and the SMPS includes a power switch for switching power to the primary side of the energy storage device (for transferring power from the input to the output side in the usual manner of an SMPS), and a controller for controlling the power switch. In the context of such an arrangement in preferred embodiments the flux model system then further comprises a current sense input to receive a current sense signal responsive to current flowing in the primary side of the energy storage device, and a power switch timing input to receive a power switch timing signal, for example a signal which substantially corresponds to a drive signal for the power switch. The flux model system may then further comprise a flux waveform generator to generate the magnetic flux waveform, more particularly to generate a first (for example rising) part of the flux waveform when the power switch is on and a second (for example falling) part of the flux waveform when the power switch is off, the rates of change or slopes of the first and second (for example rising and falling) parts of the flux waveform being responsive to the current sense signal. Thus typically the magnetic flux waveform is generally triangular with substantially linear rising and falling portions, this modelling of flux in the energy storage device. Thus the flux waveform represents that the flux in the energy storage device gradually builds up whilst the power switch is on and current is supplied to the primary side of the energy storage device, and then gradually decays when the power switch is off and power is drawn from the secondary side of the energy storage device to contribute to the output current from the SMPS.
The drive control signal for the power switch may be used to control when the magnetic flux waveform ramps up and down, so that it ramps up when the power switch is on. Whilst the power switch is on the current through the primary side of the energy storage device is ramping up, sensed by the current sense signal. Preferably a first portion of the ramp is used to control a rate at which the flux waveform falls (modelling the secondary side output current). Preferably a second later portion of the current sense signal is then used to control the rising portion of the flux waveform (modelling the build-up of flux in response to the primary side input current. Counter-intuitively controlling the falling part of the flux model waveform using the initial rising part of the current sense signal provides a form of negative feedback which helps to stabilise the flux model system. This technique enables both rising and falling parts of the flux model waveform generated from a current sense signal which, in effect, has only a rising part. Moreover the technique pulls the magnetic flux waveform into amplitude (more particularly, voltage level) lock with the current sense signal.
In some preferred embodiments a signal-level-locked loop, more particularly a voltage-locked loop is implemented using a (triangle) waveform generator which has controllable up and down ramp rates. The up ramp rate is controlled by integrating an error signal dependent upon a difference between the magnetic flux waveform and current sense signal, and the down ramp rate is similarly controlled, the integration is being performed over a second part and a first part of the current sense signal respectively. Preferably these two portions of the current sense signal are substantially equal in duration; they may be derived, for example, by comparing the current sense signal with a reference which is midway between the start and end points of its ramp. As previously mentioned, whether the waveform generator ramps up or down may be controlled according to whether the power switch is on or off. Optionally a reset input may be provided to the waveform generator to reset the flux waveform, for example to zero, at a point in the SMPS cycle at which the flux is known to be zero. Such a point may correspond, for example, to a point when the secondary side current is known to be zero.
In preferred embodiments the output current model system comprises an averager to average the magnetic flux waveform over a period when this waveform represents decreasing magnetic flux in the energy storage device, that is when a current is flowing in (out of) the secondary side of the energy storage device). This period may be determined from the flux waveform itself or, for example, may be approximated by the timing of the power switch (at least in a continuous conduction mode). Alternatively a period when an output current is flowing in the secondary side of the energy storage device may be determined by monitoring the energy storage device using an auxiliary winding. In some preferred embodiments the current model system has an input to receive a signal indicating when such a secondary side current is flowing (which is different to the substantially continuous output current of the SMPS itself), this signal being used to gate the magnetic flux waveform into a low pass filter with a relatively long time constant so that the output of the filter represents a time-averaged output current from the SMPS.
The invention also provides an SMPS including a current sensing system as described above. Preferably the energy storage device has an auxiliary winding, as mentioned above, to generate a voltage signal which can be used to determine when a secondary side current is flowing in the energy storage device. The voltage waveform from such an auxiliary winding falls gradually whilst secondary side current is flowing until a knee is reached at which point the voltage drops rapidly to zero. The current timing signal may be derived by identifying this knee, either directly or, for example, by counting backwards from when this auxiliary voltage reaches zero by a quarter of a cycle of the ringing which then follows. This signal, which indicates when the secondary side current falls to zero, may optionally be used to reset the waveform generator generating the magnetic flux waveform or, alternatively a separate reset signal may be derived.
In another aspect the invention provides a system to generate a waveform representing a level of magnetic flux in an magnetic energy storage device, the system comprising: an input to receive a signal sensing a current flowing through a winding of said magnetic energy storage device; a system output to output said magnetic flux level waveform; a first error detector having a first enable input to, when enabled, determine a first control signal responsive to a difference between said magnetic flux level waveform and said current sensing signal; and a second error detector having a second enable input to, when enabled, determine a second control signal responsive to a difference between said magnetic flux level waveform and said current sensing signal; a magnetic flux waveform generator configured to generate a generally triangular waveform, said waveform generator having: a rising ramp control input coupled to said first error detector to control a rate of a rising ramp part of said generally triangular waveform responsive to said first control signal, and a falling ramp control input coupled to said second error detector to control a rate of a falling ramp part of said generally triangular waveform responsive to said second control signal, a third timing control input to control a timing of said rising and falling ramp parts of said generally triangular waveform, and an output for said generally triangular waveform, coupled to said system output.
In a related aspect the invention provides a method of sensing the output current of a Switch Mode Power Supply (SMPS) by sensing on the primary side of a magnetic energy storage device of said SMPS, the method comprising: generating a waveform representing a level of magnetic flux in said energy storage device by said primary side sensing; and generating a signal representing an output current of said SMPS from said magnetic flux waveform.
Preferably the method includes controlling rates of rising and falling parts of the flux waveform using the primary side sensing, more particularly using an initial rate of rise of primary side current to determine a rate of fall of the flux waveform. As mentioned above, preferably the output signal current is generated by averaging the magnetic flux waveform over a period when flux in the energy storage device is decreasing (secondary side current is flowing); this may be performed by switching or gating the magnetic flux waveform into a low pass filter.
The skilled person will understand that the above described systems and methods may be implemented using processor control code. Thus the invention further provides such processor control code, in particular on a carrier medium such as a disk, programmed memory, or on a data carrier such as an optical or electrical signal carrier. The code may comprise conventional computer program code (either source, object or executable, high or low level) and/or code for setting up or controlling an ASIC or FPGA, or code for a hardware description language such as RTL (Register Transfer Level) code, VeriLog™, VHDL, SystemC or similar.
In a further aspect the invention provides a system for sensing the output current of a Switch Mode Power Supply (SMPS) by sensing on the primary side of a magnetic energy storage device of said SMPS, the system comprising: means for generating a waveform representing a level of magnetic flux in said energy storage device by said primary side sensing; and means for generating a signal representing an output current of said SMPS from said magnetic flux waveform.
The skilled person will further understand that the above described systems and methods may be employed in a wide variety of SMPS architectures including (but not limited to) a flyback converter, and a direct-coupled boost converter. The SMPS may operate in either a Discontinuous Conduction Mode (DCM) or in a Continuous Conduction Mode (CCM) or at the boundaries of the two in a Critical Conduction Mode.
These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:
a to 1d show, respectively, a block diagram of a Switch Mode Power Supply incorporating a current sensing system according to an embodiment of the invention, an example mains power supply input, an example volt output voltage sense circuit, and an outline block diagram of an example SMPS controller;
Referring to
b and 1c illustrate, by way of example only, a DC source 100 and a voltage sensing circuit 111 respectively. In the example DC source a (domestic grid) mains power supply input is rectified to provide the DC power. In the example voltage sensing circuit the DC output voltage of the SMPS drives a current through a resistor 118 and opto-isolator 117 to a reference voltage generated by a linear shunt regulator. The transistor of the opto isolator passes a current which depends upon the sensed output voltage of the SMPS.
d shows an example oscillator and timing block, more particularly an internal block diagram of an integrated circuit of the applicant. For the purposes of describing the present invention the details of this block are not important (generation of the control signals T0-T4 is described later), except that the feedback FB generates a demand signal (DEMAND) which controls an oscillator which provides a DRIVE signal output to a power switch, as illustrated comprising an IGBT (Insulated Gate Bipolar Transistor). The DEMAND signal may control either or both of a pulse width and pulse frequency of the DRIVE signal. For further details reference may be made to the applicant's co-pending applications PCT/GB2005/050244, PCT/GB2005/050242, GB 0513772.4, and GB 0526118.5 (all of which applications are hereby incorporated by reference in their entirety).
To aid in understanding embodiments of the invention and the context in which they operate a generalised SMPS will be described. Broadly speaking, a SMPS comprises an energy transfer device for transferring energy cyclically from an input to an output of the power supply (in a flyback regulator design), a power switching device coupled to the input of the power supply and to the energy transfer device, and a control system for controlling the power switching device. The power switching device has a first state in which energy is stored in the energy transfer device and a second state for transferring the stored energy to the power supply output. Typically the energy transfer device comprises an inductor or transformer and the power switching device is controlled by a series of pulses, the transfer of power between the input and the output of the power supply being regulated by either pulse width modulation and/or pulse frequency modulation.
The control system controls the power switching device in response to a feedback signal to regulate the output voltage of the power supply by regulating the energy transferred per cycle. There are many ways of deriving a feedback signal for the control system to regulate the power supply. Direct feedback from the power supply output may be employed, generally with some form of isolation between the output and input such as an opto-isolator or pulse transformer. Alternatively, if a transformer is used as the energy transfer device, an additional or auxiliary winding on the transformer can be used to sense the reflected secondary voltage, which approximates to the power supply output voltage.
In a discontinuous conduction (DCM) mode of operation the energy stored in the energy transfer device falls to substantially zero between power switching cycles (and where the energy transfer device comprises a transformer then the secondary current goes to approximately zero between each cycle). In a continuous conduction (CCM) mode of operation the energy transferred in one cycle depends upon that transferred in previous cycles (and where the energy transfer device comprises a transformer the secondary current does not fall to zero). Embodiments of the techniques we describe may be used in both these modes, and in a critical conduction mode in which the power switch is closed just as the secondary current (stored energy) falls to zero.
Referring again to
Referring now to
The waveform generator 115 has up and down-slope control inputs receiving respective signals CTLA, CTLB and generates up and down-slopes proportional to the analogue voltages on these respective inputs. The triangular output waveform FLUX is subtracted from CS and the difference integrated to provide the CTLA and CTLB signals. In this way the (voltage) amplitude of the FLUX waveform is locked to the (voltage) amplitude of the CS waveform. The waveform generator 115 also has a RESET input driven by signal T3 which, when active resets the triangle waveform (down-slope) to zero. A further input, UP/DN is provided by signal T0 and controls whether the waveform generator 115 generates a rising or falling ramp.
In more detail, the summing junction 112 subtracts the FLUX value from the CS signal value, generating a small error value. This error value is integrated by two error integrators, 113 and 114, which generate CTLA and CTLB values (shown greatly expanded in the waveforms of
It is helpful at this stage to refer to the timing diagram of
Referring next to the FLUX waveform, this rises linearly together with CS and then falls linearly when the power switch is open (T0 is low), as secondary side current is drawn reducing the energy stored in transformer 104. As previously described, in its rising portion (more specifically, in the T1 part of its rising portion) the FLUX waveform is amplitude (voltage) locked to CS. In the falling part of the FLUX waveform, in DCM mode (
Referring again to
In DCM mode an optional RESET signal (T3) may also be generated. This can be used to reset the triangle waveform generator 115 of
Referring next to timing signals T1 and T2, it can be seen that in both
Continuing to refer to
Referring to
The skilled person will recognise that this IOUT signal may be used in a variety of different ways. One example application shown in
T0 functionally corresponds to the primary switch state, being active when the primary switch 106 is closed.
T1 goes active high at a point during the on-time of the primary switch, preferably when the FLUX value reaches the mean value of the peak and trough values CS(PK) and CS(TR), and goes inactive at the same time as T0 goes inactive.
T2 is a logical function of signals T0 and T1, such that:
T2=T0&!T1
T3 goes active at the end of the transformer discharge period (preferentially on the next transition of the VAUX through zero) and remains high until the primary switch closes at the start of the next cycle.
T4 goes active high at the start of the transformer discharge time (preferentially when the VAUX signal first passes through zero after the on-time of the primary switch), and goes inactive at the end of the transformer discharge period (preferentially on the next transition of VAUX through zero).
In some preferred embodiments the majority of the SMPS and current sensing system, in particular blocks 105, 108, 109, 110, is implemented on a single integrated circuit, preferably together with power switching device 106; the hardware circuitry itself may be generated, for example, from an RTL-level functional description as indicated above.
Broadly speaking we have described a method and system of generating a model waveform of the FLUX of a transformer (or other magnetic energy storage device) in an SMPS. The method/system uses a triangular ramp generator with independent control for the rising and falling ramp waveform portions, which is preferably amplitude (voltage)-locked to the primary current waveform. Thus an amplitude (voltage)-locked loop generates a model FLUX waveform representing the total FLUX in the transformer. An oscillator generates a switching signal for switching the power converter, and a zero-current detector detects a discharge time of a secondary-side switching current by means of an auxiliary winding of the transformer. An averaging block averages the FLUX model waveform during the transformer discharge time. The integrated value is proportional to the output current of the power converter. Embodiments of this system and method provide a relatively low cost method of accurately estimating the output current of an SMPS. Embodiments work in both DCM and CCM modes and have the potential for improved accuracy. This is because embodiments are substantially independent of the effects of variations in the characteristics of the power switch and system efficiency.
No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0605065.2 | Mar 2006 | GB | national |
