Switched-mode power converter with sampled feedback signal

In accordance with an embodiment, a method of operating a switched-mode power converter includes driving a switching element in successive drive cycles, in which the switching element is driven to switch on for an on-period and subsequently driven to switch off for an off-period; sampling a feedback signal two or more times during the drive cycles, where the feedback signal includes a signal representative of an operation parameter of the switched-mode power converter and noise. The method further includes filtering the sampled feedback signal to extract the signal representative of the operation parameter from the sampled feedback signal and controlling the switching element according to the filtered feedback signal.

TECHNICAL FIELD

The disclosure relates to a switched-mode power converter, a method configured to operate a switched-mode power converter, and a computer-readable storage device storing computer-executable instructions that, in response to execution, cause a processing component to perform operations.

BACKGROUND

Each time a switch, such as a transistor, is turned on or off in switched-mode power converter, energy is dissipated in proportion to the current and voltage being switched. The power losses associated with switch operation, referred to as switching losses, represent a significant source of power dissipation and therefore a significant source of inefficiency within conventional switch mode power converters. In addition to increasing switching loss, large rates of change for voltages and/or currents (i.e., dv/dt and/or di/dt) at the time of switch transition increase stress upon the switch and the amount of electromagnetic interference (EMI) produced by the switch. Some switching schemes have been developed that take advantage of resonance within switch-mode power converters to turn on switches at times when the voltages applied to the switches are at a local minimum, referred to as a valley. Other switching schemes use a constant switching frequency or a frequency of certain bursts.

SUMMARY

A method configured to operate a switched-mode power converter, in which the power converter is operable to convert an input voltage into an output voltage at a variable switching frequency of a switching element in the power converter comprises: a method of operating a switched-mode power converter, in which the power converter is operable to convert an input voltage into an output voltage at a switching frequency of a switching element in the power converter; the method comprising driving the switching element in successive drive cycles, in which the switching element is driven to switch on for an on-period and subsequently driven to switch off for an off-period; sampling a feedback signal two or more times during the drive cycles; the feedback signal comprising a signal representative of an operation parameter of the switched-mode power converter and noise; filtering the sampled feedback signal to extract the signal representative of the operation parameter from the sampled feedback signal; and controlling the switching element according to the filtered feedback signal.

A switched-mode power converter, which is operable to convert an input voltage into an output voltage, comprises a control unit configured to drive the switching element in successive drive cycles, in which the switching element is driven to switch on for an on-period and subsequently driven to switch off for an off-period; wherein the control unit is further configured to sample a feedback signal two or more times during the drive cycles; the feedback signal comprising a signal representative of an operation parameter of the switched-mode power converter and noise; filter the sampled feedback signal to extract the signal representative of the operation parameter from the sampled feedback signal; and control the switching element according to the filtered feedback signal.

A computer-readable storage device stores computer-executable instructions that, in response to execution, cause a processing component to drive a switching element of a switched-mode power converter in successive drive cycles, in which the switching element is driven to switch on for an on-period and subsequently driven to switch off for an off-period; sample a feedback signal two or more times during the drive cycles; the feedback signal comprising a signal representative of an operation parameter of the switched-mode power converter and noise; filter the sampled feedback signal to extract the signal representative of the operation parameter from the sampled feedback signal; and control the switching element according to the filtered feedback signal.

Other converters, methods, software, features and advantages will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional converters, methods, programs, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Referring toFIG. 1, an exemplary switched-mode power converter may include a power conversion unit100, e.g., a DC-DC flyback converter with a switching element101, a magnetic element such as a transformer102, and other parts (not shown) such as rectifiers, capacitors etc. The power conversion unit100has an input which is supplied with a DC voltage, bulk voltage VBULK, and an output which supplies an output voltage VOUTto a load103. Switching element101may be a semiconductor switch, such as metal oxide semiconductor field-effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT) or the like, which is configured to connect the transformer to voltage VBULKin accordance with a respective control signal. The switched-mode power converter may further include a current sense module104that provides a current sense signal, e.g., a (primary) current sense voltage VCSrepresenting the current flowing through a primary winding of transformer102. A switch control unit105is configured to generate a control signal VGDsupplied to the switching element103for switching the switching element101on (conducting) and off (non-conducting) according to the control signal VGD. If the switching element101is a MOSFET or an IGBT, the control signal VGDmay be applied to the gate thereof.

Switch control unit105is configured to control the switching operation of the power conversion unit100. In the following examples, the switch control unit105is configured to control the power conversion unit100to operate in at least one of a multiplicity of operation modes such as a quasi-resonant mode, i.e. self-oscillating mode. The control unit10may further be configured to compare the current sense voltage VCSwith a reference voltage. The control signal VGDis set to switch off a primary current flowing into transformer102when the current sense signal VCSequals or exceeds the reference voltage. In quasi-resonant mode, switching element101may be switched on when the voltage across the switching element101is at a (local) minimum, also known as voltage valley or valley. To achieve this, the switched-mode power converter may comprise a voltage sense module106for directly or indirectly monitoring the voltage drop VDacross the switching element101during the off-time of switching element101in order to allow for detecting the time instant when the voltage is at the minimum (valley). A feedback path107from the switched-mode power converter output (i.e., from the output of a power conversion unit100) to switch control unit105may provide an additional input signal, a feedback signal FB, to the switch control unit105. The feedback path107may include basic signal processing (e.g., PI or PID regulation) and galvanic isolation (e.g., by way of opto-electronic couplers etc.).

Switching at the valleys minimizes switching losses and electromagnetic emissions (EMI). Commonly, a fixed valley of a number of valleys in a row, e.g., the first, second or any other valley, is used as a trigger for controlling the switching element101to switch on. The switching frequency of the switched-mode power converter, when in quasi-resonant mode, corresponds to the load103and, thus, may widely vary. Other modes of operation may be, for example, continuous current mode, discontinuous current mode and burst mode.

Referring toFIG. 2, an exemplary method of operating a switched-mode power converter may include the following procedures, which may be implemented in hardware, software or any combination thereof. The switching element is driven in successive drive cycles, in which the switching element is driven to switch on for an on-period and subsequently driven to switch off for an off-period (200). Rising and fallen edges of the on-period may be detected and sampling may be blocked for a first time period (see time period TR inFIG. 7) before rising edges and a second time period after falling edges (see time period TF inFIG. 7) (201). A feedback signal is sampled two or more times during the on-periods or off-periods (or both, on-periods and off-periods) of the drive cycles (202), wherein the feedback signal comprises a signal representative of an operation parameter of the switched-mode power converter and noise. Then the sampled feedback signal is filtered to extract the signal representative of the operation parameter from the sampled feedback signal (203). The switching element is controlled according to the filtered feedback signal (204). The process starts again with the next drive cycle (205).

FIG. 3depicts another exemplary switched-mode power converter (e.g., a pulse width modulated flyback converter) in which a switching device304is operable to connect a transformer300to a DC input voltage, bulk voltage VBULK, and to control the power delivered from a primary winding301to the secondary winding302of transformer300. Energy is stored into the transformer300when the switching device304is turned on. As the switching device304is switched off, the energy of the transformer300is discharged to a capacitor306and to a load307at the output of the power converter through a rectifier305so that an output voltage VOUTappears at the load307. As shown inFIG. 4, a reflected voltage VRis generated at this time in the primary winding301of transformer300in accordance with the output voltage VOUTand the turn-ratio of the transformer300. Therefore, a voltage VDacross the switching device304is equal to the bulk voltage VBULKplus the reflected voltage VRonce the switching device304is turned off. The energy corresponding to the voltage VDis stored in a parasitic capacitor308of the switching device304. After a discharge period TDS, the energy of the transformer300is fully discharged and the energy stored in the parasitic capacitor308flows back to the bulk voltage VBULKthrough the primary winding301of the transformer300. Capacitance Cj, which represents all applicable parasitic capacitance like drain-source capacitance308of switching device304or winding capacitance of transformer300, and the inductance LPof the primary winding301develop a resonant tank, whose resonant frequency fRcan be described as equation (1a):

During the resonant period, the energy of the parasitic capacitor308is delivered to the inductance provided by primary winding301back and forth. Discharging of the parasitic capacitor308to a (first) valley voltage401(followed by subsequent valleys402etc.) on voltage VDtakes places during a delay time Tq. The delay time Tqis one half of the period of the quasi-resonant and can be expressed as equation (lb):

Tq=12·fR.(1⁢b)
If the switching device304is turned on during the valley voltage401across the switching device308, switching loss and EMI can be decreased.

A switching control circuit309receives a feedback signal FB, a current sense signal VCS(e.g., a voltage representing the current) and a zero-crossing detection signal ZCD, and provides an output signal VGD(e.g., a voltage). The output signal VGDis to drive the switching device304such as a MOSFET. The switching device304is further coupled to the transformer300to generate a switching current signal on a current-sense resistor310that transforms a primary current IPflowing through primary winding301into the voltage VCSthat forms current sense signal. The transformer300includes, besides primary winding301and secondary winding302, an auxiliary winding303with an inductance LA. The primary winding301is connected to the bulk voltage through switching device302and the secondary winding302provides the output voltage of the power converter through rectifier305. The auxiliary winding303provides the zero-crossing detection signal ZCD to the switching control circuit309and may provide, through a rectifier arrangement311, a supply voltage to the switching control circuit309. An output voltage sense unit312is equipped (e.g., with an optical coupler and PI or PID regulator) to generate the galvanically isolated feedback signal FB. Thus, the input of the output voltage sense unit312is coupled to the output of the power converter and its output is coupled to the respective input of switching control circuit309to generate the switch control signal VGDfor regulating the output voltage VOUTof the power converter.

As further shown inFIG. 3, the DC bulk voltage VBULKmay be derived from an AC input voltage VINby way of a bridge rectifier313and a smoothing capacitor314. Input of switching control circuit309may include a control law unit such as a frequency law module315that generates from the feedback signal FB a peak current threshold IPKand a switching time signal representing the switching time period TSWor the switching time period thresholds TSWmin and TSWmax:

As can be seen fromFIG. 4, the current sense signal VCS, which is representative of the primary current IP, increases during the time period TONin which the switching element304is switched on until the peak current threshold IPKis reached and then switching element304is switched off until the next switch on. The time period between rising edges of successive on-time periods TONis referred to as switching time period TSW. Frequency law module315may include a look-up table or a calculation module to generate the peak current threshold IPKand switching time period TSWfrom feedback signal FB. Although not shown inFIG. 4, switching-on may be performed not only when a certain valley occurs but can alternatively be performed at other times as well, such as at zero crossing.

FIG. 5is a timing diagram showing the characteristics of a voltage VAUXacross the auxiliary winding303in the power converter shown inFIG. 3in connection with transformer current IL(i.e., primary current IPand secondary current IS) and the output signal VGDof switch control unit304. The signal curves between a time instant t1and a time instant t5when the switching element304is switched on again after it has been switched off at a time instant t2, are continuously repeated during operation. At time instant t1switching element304is switched on and the primary current IPstarts ramping up until a maximum current (peak current threshold IPK) is reached at time instant t2, when switching element304is switched off again. As a result, primary current IPquickly drops to zero, while a secondary current ISflowing through the secondary winding302steeply rises to its maximum value and then ramps down until it reaches zero at a time instant t4. While switching element304is switched on between time instants t1and t2the voltage VAUXacross the auxiliary winding303is almost zero. When switching element304is switched off at time instant t2, the voltage VAUXsteeply rises to a maximum voltage. Some ringing of the voltage VAUXmay be observed between time instants t2and t3(i.e. during a settling time), and between time instants t3and t4, when the secondary current IShas dropped to zero, the voltage VAUXdrops to a value VAUX(t4) that can be described with equation 3:

VAUX⁡(t4)=VOUT·NAUXNSEC,(3)
in which

VOUT=LS·d⁢⁢ISd⁢⁢t,(4)
NAUXis the number of turns of the auxiliary winding303, NSECis the number of turns of the secondary winding302and LSis the inductance of the secondary winding302.

Accordingly, the drop of the voltage VAUXto a value VAUX(t2) at time instant t2can be described with equation 5:

NPRIis the number of turns of the primary winding301, LP is the inductance of the primary winding301and the maximum primary current IPmaxis equal to the maximum transformer current ILPK.

In the time interval between time instants t4and t5, voltage VAUXrings again. When operating in a quasi-resonant mode, the switching element304is switched on again when the voltage VAUXreaches a certain one of a number of (local) minimums, valleys501-504, which in the present example is at time instant t5. At time instant t5the cycle starts over again. In the following, the time interval between t1and t2is referred to as the on-time TON, the time interval between t2and t4is referred to as off-time TOFFand the time interval between t4and t5is referred to as wait time TW. On-time TON, off-time TOFFand wait time TWcan be described with equations 7-9:

TON=LP·ILPKVBULK,(7)TOFF=LP·ILPKVR,(8)TW=(n-12)·1fR⁢⁢with⁢⁢n=1,2,3⁢…(9)
in which fRis the resonant frequency in the quasi-resonant mode (see equation 1) and n is the number of the valley in a row starting with 1, and

TSW=TON+TOFF+TW=1fSW,(10)
in which fSWis the switch frequency of switching element304. The resulting primary power PPis according to equation 11:

The zero crossing signal ZCD may not only provide the zero crossings of the auxiliary voltage VAUXbut also can form the basis for the calculation of the bulk voltage VBULKand the output voltage VOUT. However, other ways to determine those parameters are applicable as well.

Instead of the quasi-resonant mode illustrated inFIGS. 4 and 5, alternative modes of operation may be employed such as the two modes that are illustrated in the timing diagrams ofFIG. 6. The diagrams in the left column refer to the discontinuous current mode (DCM) whereas the diagrams in the right column refer to the continuous current mode (CCM). Selection of a certain mode of operation may depend on the level of the bulk voltage VBULK. In DCM and CCM the time instants t6and t10, at which the semiconductor switch is switched on, and the time instants t7and t11, at which the semiconductor switch is switched off, may be time-triggered or event-triggered. In a switching power converter that operates with a forced frequency, e.g., a fixed frequency, and variable on-time (i.e. a variable duty-cycle) these time instants are time-triggered. In a switching power converter that performs a quasi-resonant oscillation, the switch-on time as well as the switch-off time may be event-triggered. The “event”, which triggers a switch-off of the semiconductor switch may be, for example, the primary current IPreaching a threshold value, whereas the event which triggers a switch-on of the semiconductor switch may be the voltage across the switching element being at a (local) minimum. For the further discussion, a flyback converter operated at a forced frequency, e.g., a fixed frequency fSW(fSW=TSW−1), and having a variable duty cycle D (D=TON/TS) is considered as an illustrative example.

The following considerations assume stationary operation in DCM, as illustrated in the diagrams in the left column ofFIG. 6. A switching cycle starts at time instant t1and ends at time instant t9when the subsequent switching cycle begins. The period TSof one cycle can be calculated as:
TSW=(t10−t6)=fSW−1.  (12)

The drive signal (e.g. the gate voltage VGA), which controls the switching state of switching element304, is switched on at time instant t6and switched off at time instant t7. The on-time TONcan be calculated as
TON=(t7−t6)=D·TSW=D·(t9−t6),  (13)
wherein D denotes the duty cycle (Dε[0, 1]). The remaining time of one cycle is the off-time TOFF, which can be calculated as
TOFF=(t9−t7)=(1−D)·TSW.  (14)

During the on-time TONthe primary current IPrises from zero to its peak value IPP. The gradient of the current ramp is VBULK/LP(LPrepresenting the inductance of the primary winding) which is almost constant if the input voltage VBULKdoes not significantly change during one cycle. At time instant t2the energy EDCMstored in the primary winding can be calculated as
EDCM(t7)=LP·IPP2/2.  (15)

As the switching element304is switched off at time instant t7the energy EDCMis “transferred” from the primary side to the secondary side of the transformer300due to inductive coupling. The secondary current ISis zero during the on-time TON(as the rectifier diode305is reversed biased) and falls from its initial peak value ISP, which occurs at time instant t7, down to zero, which is reached at time instant t8. The secondary current ISremains zero between time instants t8and t9. It is noted that delay times (rise and fall times) are neglected in the present discussion in order focus attention on the basic function of the circuit. The secondary peak current ISPfulfils the equation EDCM(t7)=LS·ISP2/2=LP·IPp2/2, wherein LSagain represents the inductance of the secondary winding. The gradient of the secondary current ramp during the off-time TOFFis (VOUT+V305)/LSwherein the V305is the forward voltage of the rectifier diode305. In contrast to CCM, in DCM the secondary current ISdrops (beginning at ISP) to zero during a switching cycle whereas this is not the case during CCM—in the present example, time t7.

Continuous current mode (CCM) is illustrated in the diagrams in the right column inFIG. 6. A switching cycle starts at time instant t5and ends at time instant t7when the subsequent switching cycle begins. In stationary operation, the initial primary current IP0(IP0=IP(t10)), which would be zero in DCM, is greater than zero in CCM as the energy ECCM“stored” in the transformer windings LPand LSnever falls to zero during the off time TOFF. At time instant t5the semiconductor switch T6is closed (drive signal VGDis at a high level) and the primary current IPstarts to ramp up from the initial value IP0to the peak level IPP, wherein IPp=IP0+ΔIP. When the primary current reaches its peak value IPP(defined by a current threshold) at time instant t11switching element304is switched off and the secondary current ramps down from its peak value ISPto its final value IS0, wherein ISP=IS0=ΔIS. The energy “stored” in the transformer varies from ECCMmin=LS·IS02/2=LP·IP02/2 to ECCMmax=LS·ISP2/2=LP·IPP2/2. At the time instant t12the cycle starts over again, the secondary current ISis blocked by the rectifier diode305, and the primary current “jumps” to its initial value IP0and ramps up as in the previous cycle.

Referring toFIG. 7, the power converter shown inFIG. 3is, for example, operated in DCM, so that during on-time period TONof switching element304the primary current ramps up to its peak value IPpand where switching element304is switched off for the time period TOFFand secondary current ISramps down to zero from peak value ISP. One on-time period TONand one off-time period TOFFform one drive cycle. In order to keep the output voltage (power) at a desired value, parameters such as the output voltage etc. are monitored by way of output voltage sense unit312which provides the feedback signal FB to the full or partly digital switching control circuit309. Output voltage sense unit312may provide basic signal processing (e.g., PI or PID regulation) and galvanic isolation (e.g., by way of opto-electronic couplers etc.). In particular, PI or PID regulation may amplify noise, such as voltage ripples, that may interfere with the output voltage VOUTso that the feedback signal FB may contain a relatively high amount of noise. InFIG. 7, the feedback signal FB is assumed to be almost constant but superimposed by noise in the form of sinusoidal ripples. When sampling only one time during period TOFF, the noise may lead to an incorrect assessment of the output voltage VOUT, as only this particular sample, which may include a considerable amount of noise, serves as a basis for the subsequent control of switching element304. However, sampling the feedback signal FB two or more times (i.e., at s times TSAMx, x=1, 2, . . . s, s≧2) during the on-periods and/or off-periods of the drive cycles gives a much clearer picture of the feedback signal FB, including a signal representative of an operation parameter (e.g., output voltage VOUT) of the switched-mode power converter and noise. Although more samples are more precise, noise (e.g., voltage ripples) is still present in the feedback signal FB. In order to eliminate the noise, filters, e.g., dedicated digital filters as shown inFIGS. 8-10, may be employed. The power converter may produce additional noise in the rising and falling edges RE, FE of the on-periods of switching element304. To avoid that, switching noise is sampled. Sampling may be blocked for a certain time period TR shortly before or after the rising edges RE occur and a certain time-period TF shortly before or after the falling edges FE occur.

As can be seen fromFIG. 7, higher sampling rates may be employed during turn-off periods than during turn-on phases in a “normal operation mode”, i.e., in CCM, DCM or QRn mode. During normal operation mode dedicated sampling is synchronized to the switching frequency to determine the next pulse width modulation (PWM) cycle. This may be performed either at the very beginning of the turn-on phase, to consider the sampled value for the already ongoing turn-on phase, or at the end of the turn-off phase in view of the next PWM cycle. The other samples taken during the turn-off phase can be used for a dv/dt detection on the feedback signal in order to indentify in advance a load jump at the output. Based on this there can be, e.g., an immediate change to the lowest QR-n mode or a positive offset can support the compensation network in reacting to the load jump.

In burst mode operation, the sampling rate may be increased compared to normal mode and all or at least the majority of sampled values may be evaluated (e.g., for load detection), as it is desired to immediately detect whether the feedback signal has exceeded a burst mode exit threshold in order to exit the burst mode at once. In burst mode, there is no correlation between sampled feedback values and defining the PWM cycle. The feedback signal and, thus, the samples are only used for two-point regulation in which the feedback signal toggles between two thresholds.

FIG. 8shows a sampling module801, which receives the feedback signal FB and which samples with a sample frequency fSAM, and a linear predictor module802, which comes after the sampling module801. The output signal of linear predictor module802, which is a signal representing the predicted noise, is subtracted from the output signal of sampling module by a subtractor803whose output signal is supplied to a PWM modulator. The PWM modulator may include the frequency law unit315shown inFIG. 3. Linear prediction is a mathematical operation with which future values of a discrete-time signal are estimated as a linear function of previous samples. It is often used in linear predictive coding (LPC) and can be viewed as a subset of filter theory.

FIG. 9shows a sampling module901, which receives the feedback signal FB and which samples with the sample frequency fSAM, and a notch filter module902, which comes after the sampling module901. The output signal of sampling module901is supplied to the PWM modulator, which may include the frequency law unit315shown inFIG. 3. The pass frequency of notch filter module902may be controlled with a control signal CF to let only wanted signals pass and to keep unwanted noise signals from passing.

FIG. 10shows a sampling module1001, which receives the feedback signal FB, and a notch filter module902, which comes after sampling module901.

FIG. 10shows a sampling module1001, which receives the feedback signal FB and which samples with the sample frequency fSAM, and an inverse multiplexer module1002, which comes after the sampling module1001and which provides a multiplicity of output signals. The output signals may be stored in a buffer memory module1003. One or more of the available output signals are selected for the PWM modulation by a selection unit1004. The PWM modulator may include the frequency law unit315shown inFIG. 3. Selection criteria can be that the latest available sample prior to the PWM modulation update has been taken or that those samples where the least distortion from switching is expected have been taken.

In one or more examples, the functions described herein may be implemented at least partially in hardware, such as specific hardware components or a processor. More generally, the techniques may be implemented in hardware, processors, software, firmware, or any combination thereof. If implemented in software, the functions, as one or more instructions or code, may be stored on or transmitted via a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media may generally include (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium.