Abstract:
A power limiter for a switched mode power supply includes an operational amplifier and a comparator circuit. The operational amplifier is configured to receive an input voltage supplied to the SMPS as a first input and a reference voltage as a second input. The comparator circuit is configured to receive an output of the operational amplifier, receive a current sense signal, and generate an output signal configured to control a power generator. The output signal is based on a comparison between the output of the operational amplifier and the current sense signal.

Description:
PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/364,131 filed on Jul. 19, 2016, which is hereby incorporated by reference herein for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to power limiters and, in particular, switched mode power supplies (SMPS). 
       BACKGROUND 
       [0003]    An SMPS actively regulates its output voltage by switching the output on and off in a duty cycle. The output of the SMPS may be given by the relative duration of the on and off portions of the duty cycle. This may be in contrast to linear power supplies, wherein power is dissipated in, for example, a transistor. SMPSs may be implemented as, for example, buck converters, boost converters, or buck-boost converters. An SMPS may regulate output voltage or current by switching ideal storage elements such as inductors and capacitors into and out of different electrical configurations. If a power source, an inductor, a switch, and the corresponding electrical ground are placed in series and the switch is driven by a square wave, the peak-to-peak voltage of the waveform measured across the switch can exceed the input voltage from the DC source. This is because the inductor responds to changes in current by inducing its own voltage to counter the change in current, and this voltage adds to the source voltage while the switch is open. If a diode-and-capacitor combination is placed in parallel to the switch, the peak voltage can be stored in the capacitor, and the capacitor can be used as a DC source with an output voltage greater than the DC voltage driving the circuit. This boost converter acts like a step-up transformer for DC signals. A buck-boost converter works in a similar manner, but yields an output voltage which is opposite in polarity to the input voltage. Other buck circuits exist to boost the average output current with a reduction of voltage. In an SMPS, the output current flow depends on the input power signal, the storage elements and circuit topologies used, and also on the modulation and duty cycle to drive the switching elements. The spectral density of these switching waveforms has energy concentrated at relatively high frequencies. As such, switching transients and ripple introduced onto the output waveforms can be filtered with a small LC filter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is an illustration of an example SMPS using an adaptive input power limiter, according to embodiments of the present disclosure. 
           [0005]      FIG. 2  is an illustration of an example adaptive input power limiter within the context of an SMPS, according to embodiments of the present disclosure. 
           [0006]      FIG. 3  illustrates a problem of potential overloads or short circuits addressed by embodiments of the present disclosure. 
           [0007]      FIG. 4  illustrates a problem of potential overloads or short circuits when current limiters are used and addressed by embodiments of the present disclosure. 
           [0008]      FIG. 5  illustrates example performance of an SMPS using an adaptive input power limiter, according to embodiments of the present disclosure. 
           [0009]      FIG. 6  illustrates further example performance of SMPS  100  using an adaptive input power limited, according to embodiments of the present disclosure. 
       
    
    
     SUMMARY 
       [0010]    Embodiments of the present disclosure include a power limiter. The power limiter may be for an SMPS. The power limiter may include an operational amplifier and a comparator circuit. The operational amplifier may be configured to receive an input voltage supplied to the SMPS as a first input and a reference voltage as a second input. The comparator circuit may be configured to receive an output of the operational amplifier, receive a current sense signal, and generate an output signal configured to control a power generator. In combination with any of the above embodiments, the output signal may be based on a comparison between the output of the operational amplifier and the current sense signal. In combination with any of the above embodiments, the output of the operational amplifier may be a current limit. In combination with any of the above embodiments, the comparator circuit may be further configured to generate the output signal based on a comparison between a set current limit and a measured current by comparing the output of the operational amplifier and the current sense signal. In combination with any of the above embodiments, the output signal may be configured to overwrite a feedback signal from output of the SMPS. In combination with any of the above embodiments, the output signal may be configured to overwrite a feedback signal from output of the SMPS. In combination with any of the above embodiments, the comparator circuit may be further configured to generate the output signal before reception of the feedback signal. In combination with any of the above embodiments, the operational amplifier and the comparator unit may be incorporated into a microcontroller. In combination with any of the above embodiments, the operational amplifier may be further configured to lower a current input limit as input voltage increases. In combination with any of the above embodiments, the operational amplifier may be further configured to lower a current input limit as input voltage increases according to a slope defined by one or more resistors connected between an input voltage source and the operational amplifier. In combination with any of the above embodiments, the operational amplifier may be further configured to maintain a current limit above zero based upon the reference voltage. 
         [0011]    Also, embodiments of the present disclosure include an SMPS, including any of the power limiters of the above embodiments. The SMPS may include a pulse-width modulation power generator circuit configured to produce an output voltage based on a duty cycle of a pulse-width modulation output. The comparator circuit may be configured to receive a current sense signal based on output of the pulse-width modulation power generator circuit. 
         [0012]    Furthermore, embodiments of the present disclosure include a microcontroller including any of the power limiters or SMPSs of the above embodiments. 
         [0013]    In addition, embodiments of the present disclosure include methods performed by any of the power limiters, SMPSs, or microcontrollers of the above embodiments. 
       DETAILED DESCRIPTION 
       [0014]      FIG. 1  is an illustration of an example SMPS  100  using an adaptive input power limiter, according to embodiments of the present disclosure. 
         [0015]    SMPS  100  may include a variable input power source  102 , a power conversion stage  104 , SMPS control  110 , and an adaptive power limiting function  108 . Each of such elements may be implemented in any suitable combination of analog or digital circuitry, including instructions for execution by a processor. Power source  102  may include a voltage or current source. Power conversion stage  104  may switch power on and off according to a duty cycle to generate output power  106 . Output power  106  may be fed back to SMPS control  110  in order to determine whether to adjust the power conversion stage  104  so as to maintain an expected level of output power  106 . SMPS control  110  may specify the duty cycle and other operational parameters of SMPS  100 . 
         [0016]    SMPS  100  may prevent power conversion that exceeds the specified design by measuring output of the converter where the power is delivered, such as output power  106 . In a fixed output voltage conversion design, the limit is placed on the delivered output current, while for a fixed output current conversion design, the limit is placed on the measured output voltage. Thus, while SMPS  100  is illustrated in  FIG. 1  and  FIG. 2  as measuring and evaluating current with respect to output voltage of SMPS  100 , SMPS  100  might be implemented instead as measuring and evaluating voltage with respect to output current of SMPS  100 . 
         [0017]    Limiting techniques may be used to prevent damage or unwanted operation, even when the converted power is within specifications. Such a case can happen when the input voltage is smaller than the rated value. An undervoltage lockout will stop the conversion until the input voltage is within specified levels. Another case where the power conversion can be within specifications is an output overvoltage. In such a case, power conversion enters a shutdown state to protect the load from irreversible damage. 
         [0018]    In one embodiment, adaptive power limiting function  108  may implement adaptive input power limits for SMPS  100 . Embodiments of adaptive power limiting function  108  are shown in more detail in  FIG. 2 . 
         [0019]    As discussed above, in SMPS  100  a variable input power source  102  may provide variable input voltage to SMPS  100 . When there is variable input voltage, an overload or short circuit condition may arise when output power  106  rises too high. 
         [0020]      FIG. 3  illustrates a problem of possible overloads or short circuits with other SMPSs. For example, an offline 20 W flyback SMPS was tested with different input voltages and the output current was varied to simulate an overload. Current and power values where the output voltage dropped by 0.5V with respect to the desired value are plotted in  FIG. 3 . 90, 100, and 110 volts AC inputs are shown as reference. Response with respect to current and power output are shown in  FIG. 3 . Another input value, 170 volts AC, not shown in  FIG. 3 , produced an 8 A output and 93 W before the output voltage decreased 0.5V. Other solutions may prevent such overloading by adding an input current limit, using a digital-to-analog converter (DAC). However, a further problem arises because such solutions are based upon a calculated input voltage. The input voltage may change when, for example, a variable input voltage or power source is used such as an SMPS  100  or when a voltage or power source, otherwise assumed to be constant, degrades, changes, or is affected by noise or other ambient conditions and does not provide the assumed voltage source.  FIG. 4  illustrates such a problem of possible overloads or short circuits with other SMPSs that use current limiting. A similar test as was performed for  FIG. 3  to determine at what output current and power that the output voltage would drop by 0.5 volts while a current limit is calculated to limit output power to 31 W at 90 volts AC. As shown in  FIG. 4 , even while the intended power limit was approximately functional at the desired set point of 30 W at 90 volts AC, as the input voltage increased the output current and power continued to rise significantly, which would still cause overload and short circuit problems. 
         [0021]    These other SMPS systems often determine a fixed threshold reference on inductor current that will take over the control loop and will end the pulse earlier. The fixed threshold level is often implemented with a comparator with a fixed limit level or an error signal. A comparator may stop the PWM pulse earlier if the threshold level is passed, or another comparator with a higher, fixed threshold level may stop the IC for a limited time to prevent component damage. 
         [0022]    These other SMPS systems may utilize a fixed duty-cycle limit to prevent damage. However, this works only for a fixed input voltage and fixed output voltage converter. Furthermore, the converter may have to be calculated exactly to limit the desired power limit to the duty cycle and still work properly in normal operation conditions. Once designed, the limit is not configurable. In such cases, output power rises with the increase of the input voltage. In such a solution using only a maximum duty cycle limit, there is no current limit from the primary cycle. At 160 VAC input, the load can take 93 W of power from the SMPS without losing output regulation; thus, the SMPS components will fail. 
         [0023]    These other SMPS systems may utilize a fixed primary-peak current limit. However, in these systems when the input voltage of the converter is variable for the same power transfer, the current signal will become smaller. This may result in a higher power transfer before reaching the set limit. Because the limit is fixed, configuration is not possible. 
         [0024]    These other SMPS systems may utilize an output current limit. However, this technique becomes very expensive in isolated designs as having multiple signals pass through the isolation barrier can become very costly. As the limit is fixed, configuration is not possible. These results prove that the classic limit approach works only if there is direct access to the output current signal. It does not work with a system with large input variations and isolation needs. 
         [0025]    Returning to  FIG. 1 , SMPS  100  may be configured to address aspects of the shortcomings of systems whose performance is shown in  FIGS. 3 and 4 . SMPS  100  may be configured to limit the total power conversion produced as an output. Furthermore, SMPS  100  may be configured to prevent damage to a load connected to output of SMPS  100 . In addition, SMPS  100  may be configured to prevent high short-circuit impulses. Also, SMPS  100  may be configured to prevent transformer core saturation. In one embodiment, SMPS  100  may be configured to provide user control over power limits. 
         [0026]    In one embodiment, SMPS  100  can be configured to adapt the limit of the output power of SMPS  100  while still giving the user of SMPS  100  the ability to change the limit. In another embodiment, the limit, after being set, may apply to variable input voltages. SMPS  100  may be configured to adapt to a wide variety of input voltages from variable input power source  102 . The variations in power may be by design, or may arise from degradation, malfunction, or noise affecting the source. The output power may be maintained below a set limit. The adaptable limit of the output power of SMPS  100  may be implemented with circuitry, including hardware or software executed by a microcontroller. 
         [0027]    Adaptive power limiting function  108  may be communicatively coupled to variable input power source  102  and to SMPS control  110 . Thus, adaptive power limiting function  108  may be placed on the primary side of SMPS  100  and of the converter (power conversion stage  104 ) thereof. Accordingly, adaptive power limiting function  108  may be able to set, change, or affect the duty-cycle values controlled by SMPS control  110 . This may include the maximum duty-cycle limit set therein. In one embodiment, this limit may adapt to the input voltage variation. In a further embodiment, this may be implemented by an inverse proportional function implemented in adaptive power limiting function  108 . Consequently, adaptive power limiting function  108  may limit the maximum output power delivery in output power  106  from the primary of the power conversion stage  104  by limiting the amount of primary current into power conversion stage  104  based on the voltage from variable input power source  102 . When output power  106  reaches sufficient limits, further, higher output voltage feedback will be ignored. 
         [0028]    An adaptable software limit may be implemented by an ADC and a DAC to change the set peak current limit. An internal set of limits is defined and attributed to a certain range of input voltages. The ADC measures a proportion of the input voltage and stores it in the memory. After each acquisition, the value is compared to the input voltage attributed in the limit array and the peak current limit is updated in the DAC. This solution is feasible when the input voltage changes are slow. 
         [0029]    A hardware-based implementation may free a processor core from other SMPS-related tasks. The implementation may change the current limit inversely proportional to the input voltage change using one internal op-amp and one DAC. A resistor divider scales down the rectified input voltage and the op-amp is used to invert the signal, so when the input voltage rises, the current limit will adapt and fall. The DAC may be set as an op-amp positive input to raise the inverted signal over the 0V line. This signal may be compared with the current from the transformer primary and trigger a falling event on the pulse-width modulation generation circuit when the current reaches the set limit. The limit trigger may also be used to implement a shutdown or other functions in the circuit. 
         [0030]      FIG. 2  is an illustration of an example adaptive input power limiter within the context of an SMPS, according to embodiments of the present disclosure. Furthermore, the adaptive input power limiter may be implemented in a microcontroller  204 , such as a PIC microcontroller unit. In various embodiments, the adaptive input power limiter may be implemented in any other suitable electronic device. 
         [0031]    Operation of adaptive power limiting function  108  may be performed in part by an op-amp  214 . Op-amp  214  may receive as one input a voltage reference values, VREF. The value of VREF may be provided by a DAC  212  and may be output from elsewhere in SMPS  100 , such as another component of microcontroller  204 . Op-amp  214  may receive as another input a proportion of the input voltage  202  applied to SMPS  100 . The input voltage, VIN  202 , may be provided by variable input power source  102 . In one embodiment, VIN  202  may be passed to op-amp  214  through a resistor network. The resistor network may include resistors  206 ,  208 ,  210  denoted R 1 , R 2 , R 3 . VIN  202  may be connected to resistor  206 , which may be connected to resistor  208  and op-amp  214 . Resistor  208  may be connected to ground. Resistor  210  may be connected between the output of op-amp  214  and the input of op-amp  214  as well as resistors  206 ,  208 . 
         [0032]    In one embodiment, op-amp  214  may be implemented within microcontroller  204 . In another embodiment, op-amp  214  may be implemented separately from microcontroller  204 . In yet another embodiment, resistors  206 ,  208 ,  210  may be implemented outside microcontroller  204 . 
         [0033]    Output of op-amp  214  may be shown as OPAMP OUT  216  in  FIG. 2 . OPAMP OUT  216  may be also denoted as VINV. Op-amp  214  may amplify and invert VIN  202  to product VINV. VINV may be passed as an input to a comparator  218 . Comparator  218  may be implemented in any suitable combination of analog or digital circuitry, such as with an op-amp. Comparator  218  may accept as another input output of a current sensor, ISENSE  220 . The current sensor may be implemented in any suitable combination of analog or digital circuitry. ISENSE  220  may represent the current produced as part of feedback received through output power  106 . 
         [0034]    The output of comparator  218  may be passed to a complementary output generator (COG)  222 . COG  222  may be implemented in any suitable combination of analog or digital circuitry. COG  222  may be configured to, based upon voltage signal input, issue a drive signal representing the duty cycle signal that will drive the switched-mode operation of SMPS  100 . COG  222  may include inputs for rising event (RE) and falling event (FE) signals. If input is connected to RE input, COG  222  may issue the duty cycle signal upon rising of its input. If input is connected to FE input, COG  222  may issue the duty cycle signal upon falling of its input. COG  222  may implement a driver of the duty-cycle signal. 
         [0035]    As shown, ISENSE  220  illustrates current as issued with respect to output power  106 . ISENSE illustrates that the current is zero during the off portion of the duty cycle, rising quickly to a base level when the on portion of the duty cycle is initiated, and rising thereafter until the off portion of the duty cycle. 
         [0036]    Resistor  206  may limit the current sampled from the input. Op-amp  214  and resistor  210  may combine to implement an inverting function to VIN  202 . Resistor  210  may dictate the intervening proportion. VREF  212  rises the output function of op-amp  214  above the 0V with the set value of VREF  212  so that the resulting function  216  can be used by comparator  218 . Comparator  218  may provide a falling event to COG  222  when the output of op-amp  214  is compared with ISENSE  220 . This may happen before the output feedback regulation signal from  106  provides a falling event on COG  222  and overwrite the feedback falling event with an early falling event from comparator  218 . 
         [0037]    When op-amp  214  takes a portion of VIN  202  and inverses it, as VIN increases, OPAMP OUT  216  decreases. Accordingly, when voltage rises to otherwise dangerous levels, such as higher than 90V, op-amp  214  would otherwise go negative except for the shift provided by VREF  212 . VREF  212  may thus cause OPAMP OUT  216  to stay above zero volts. Furthermore, a user or designer of SMPS  100  may set VREF according to desirable outputs. As shown in  FIG. 2 , at a 90V level of VIN  202 , OPAMP OUT  216  may be at 1.9V. After rising to, for example, 360V of VIN  202 , OPAMP OUT  216  may be at 1.5V. OPAMP OUT  216  may represent or be proportional to an effective current limit for SMPS  100 . Thus, as VIN  202  rises, the current limit of SMPS  100  may decrease. The slopes and values of OPAMP OUT  216  may be dependent upon resistor and VREF values, discussed further below. 
         [0038]    Accordingly, when the decreased current limit is reached, as ascertained by comparator  218 , the output drive signal is stopped and thus the output power is lessened. COG  222  may drive a pulse width modulated square wave signal, the width of which powers output of SMPS  100 . 
         [0039]    As a result, SMPS  100  may implement a direct duty-cycle limit on the driver (COG  222 ). This limit, to the outside of SMPS  100 , may be seen as a maximum power limitation. Moreover, this power limitation may be independent to adapt with the input voltage variation. 
         [0040]    A user or designer of SMPS  100  may allow users to change the output power limit. As op-amp  214  generates an inverse-proportional waveform to the input voltage, the proportionality may be given by the relationship between resistors  206 ,  208 ,  210 . This relationship may be given by: 
         [0000]    
       
         
           
             
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         [0041]    This relationship expresses how resistors  206 ,  208 ,  210  and op-amp  204  are used to generate a waveform inversely-proportional to VIN  202 , so when the output voltage rises the limit will drop and adjust. In one example, resistor  206  may be selected as 1 MOhm to limit the current that goes to the rest of the adaptive limit circuit, resistor  208  may be selected as 51 KOhm, resistor  210  may be selected as 1.5 KOhm, and VREF  212  can be set to 2V. Thus, the set limit of VINV will be 1.9V when the rectified input voltage is 90 VDC, and 1.5V when the rectified input voltage is 360 VDC. 
         [0042]    The limits used in SMPS  100  may be automatically changed or utilized in various situations. For example, a designer or user of SMPS  100  may control the maximum power delivery at any moment during runtime. A user may be notified or the power limit further reduced after multiple instances of limiting operation. A time limit may be applied to the maximum allowed power conversion. For example, an 100 W converter can be set to deliver a maximum 80 W after five years of operation to extend the lifetime of the product and prevent wear and tear. Furthermore, a user may use the same instance of SMPS  100  for multiple applications, wherein the maximum allowed power is adjusted on the basis of the application. For example, the user may use a 100 W/12V converter to power a 100 W load, a 60 W load and a 20 W load separately without the need of having three separate converters. Furthermore, the necessary protections and functions will work correctly in all instances. SMPS Control  110  and Adaptive Power Limiting Function  108 , though implementable in a microcontroller, need not use the microcontroller core. Accordingly, logic may be installed to detect triggering of the limiting operation to identify when the power delivery is at limit, without compromising the safety of the conversion. 
         [0043]      FIG. 5  illustrates example performance of SMPS  100  using an adaptive input power limiter, according to embodiments of the present disclosure. A set limit for 30 W may be set. The minimum and maximum values of testing are 30 W and 31.5 W, respectively. As the input voltage rises, the output power is nonetheless controlled. 
         [0044]      FIG. 6  illustrates further example performance of SMPS  100  using an adaptive input power limiter, according to embodiments of the present disclosure. In particular,  FIG. 6  illustrates current waveforms such as ISENSE  220  under different input voltage VIN  202 . In (a), the input voltage is 90V, and in (b) the input voltage is 130V. As the input voltage rises from (a) to (b), the current limit adapts and falls from 2 A to 1.6 A based upon the operation of SMPS  100 . 
         [0045]    Although embodiments have been described in detail with particular reference to figures and examples, one of skill in the art would recognize that variations, additions, and modifications can be effected within the spirit and scope of the present disclosure.