Title of Invention: Programmable power supply with dual mode cross over clamps
Abstract: A power supply programmable for a constant current mode and a constant voltage mode and selectively operable in either mode. Respective circuits associated with each of a constant voltage loop and a constant current loop track a control voltage of the other loop when the other loop is operating closed loop and maintains a control voltage of the open loop at a tracking offset value from the control voltage of the closed loop, allowing the open loop to rapidly assume control of the power supply during a mode transition. The tracking offset value is briefly increased during a mode transition to interrupt oscillation tendencies, resulting in faster mode cross-over and reduced overshoots, while maintaining stability.
Detailed Description: Part general_description_paragraphs: Part CROSS-REFERENCE TO RELATED APPLICATIONS: Not Applicable
Part BACKGROUND OF THE INVENTION: In a programmable DC power supply having a CV mode and a CC mode, modes of operation are implemented with a voltage feedback loop for the CV mode and a current feedback loop for the CC mode, both attempting to regulate their respective parameter to user-specified settings. The DC operating point is determined by a combination of a load line, the large signal voltage-current (V-I) relationship of the load, and either a CV setting or a CC setting, whichever is lower, i.e., whichever setting results in a lower output power level. In the CC and CV modes of operation, the loop that is not regulating opens and relinquishes control to the other loop. Changes in the load may cause the operating point to vary such that the power supply switches modes (CV mode to CC mode or CC mode to CV mode), which is termed mode cross-over.
During a mode cross-over, the loop which was not in control (i.e., open) when the mode cross-over began, seizes control of the supply (i.e., closes) and stabilizes. In an ideal case, the mode cross-over transition is seamless and the supply immediately begins regulating the new parameter. In practice, however, the transition takes a finite amount of time, referred to as mode cross-over latency, during which the operating point may exceed the bounds set by the CV and CC settings. An amount by which the operating point exceeds the set bounds is referred to as mode cross-over overshoot. An amplitude and duration of the mode cross-over overshoots are directly related to a magnitude of the mode cross-over latency.
Conventionally, tracking clamps have been used in an effort to reduce mode cross-over over shoot, however such clamps have a tendency to cause non-linear mode cross-over oscillations (rapid, perpetual switching between CV and CC), making them unusable. These problems have led to the general practice of using a tracking clamp on only one control loop, optimizing mode cross-over overshoots on either the CV to CC, or the CC to CV transition, but not both.
Part SUMMARY OF THE INVENTION: In the present invention, a pair of clamps, one for the CV loop and one for the CC loop, continually regulate (or clamp) the control voltage of the open loop to be a small, fixed voltage, referred to as a tracking offset, above a voltage of the closed loop, allowing the open loop to rapidly assume control of the power supply should the programmed limit of the open loop be exceeded. The rapid assumption of control dramatically reduces the mode cross-over latency and consequently, the severity of the associated overshoots.
The present invention addresses the tendency of the tracking clamps to cause the power supply to oscillate. The tendency to oscillate is suppressed by incorporating a circuit that briefly increases the tracking offset of the CC clamp in response to a CV to CC mode transition, temporarily reducing sensitivity of the CC clamp, thereby interrupting any impending oscillation. Alternatively, comparable results are achievable by incorporating a similar circuit in the CV clamp instead of the CC clamp. Briefly increasing the tracking offset allows the use of dual clamps, which results in a programmable power supply with much faster mode cross-over transitions, overshoots that are either substantially reduced or completely eliminated, without generating secondary adverse effects on other aspects of power supply performance, such as for example, stability.
Part DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain the present invention by referring to the figures.
Referring now to FIG. 1, a power supply 100 according to the present invention comprises a power mesh 110 which drives a user's load 112 in response to a voltage Vctrl to produce a current Imon through the load 112 and a voltage Vmon across the load 112. The voltage Vctrl is selectively controlled by a CV loop 114 and a CC loop 134. The CV loop 114 and the CC loop 134 demand power necessary to regulate voltage and current, respectively, by amplifying an error signal, i.e., differences between user settings, Vprog and Iprog, and actual operating points Vmon and Imon, respectively. An adder 116 provides a difference signal Verr which is amplified by a CV error amplifier 118 to provide a control voltage CVctrl for the CV loop 114. An adder 136 provides a difference signal Ierr which is amplified by a CC error amplifier 138 to provide a control voltage CCctrl. The respective transfer functions (k/s) of the error amplifiers 118 and 138 are application specific and preferably linear.
The voltage Vctrl which drives the power mesh 110 is derived from the voltages CVctrl and CCctrl. In response to the voltage Vctrl, the power mesh 110 drives the load in accordance with Vctrl. A relationship between the voltage Vctrl and the power supplied by the power mesh 110 is monotonic, but not necessarily linear. Vmon and Imon are then fed-back, which represent the voltage across the load 112 and the current through the load 112, respectively. The voltage Vmon may be provided by directly monitoring a voltage at the load 112 and Imon may be provided by a current monitor 113. Suitable current monitors are well known in the art of power supplies and will not be further described.
The voltage Vctrl is maintained at a diode drop above the lower of the voltages CVctrl and CCctrl through diodes 119 and 139, respectively, allowing the loop 114 or 134 demanding the least power to forward bias the respective diode, 119 or 139, to control the power mesh, leaving the other diode, 119 or 139, reverse-biased. This arrangement inherently means that the loop not in control (i.e., open), if un-bounded, will force the respective control voltage, Ierr or Verr, up until the output, CCctrl or CVctrl, of the respective error amp, 138 or 118, saturates.
CVctrl and CCctrl are generally subject to slew limits, either as a direct result of the transfer functions of error amps 118 and 138, respectively, or due to an amplifier parasitic parameter. The slew limits have a side-effect of causing the control voltage of the open loop (which is saturated high) to be slow to slew down past that of the closed loop and assume control when necessary due to a magnitude of a difference between the voltage of the open or non controlling loop and the voltage of the closed or controlling loop.
The power supply 100 further comprises a CV clamp 120, a CC clamp 140, a dynamic offset injector 160 and an adder 180 which are adapted to control differences between the loop voltage of the open loop and the loop voltage of the closed loop, regulating the control voltage of the open loop to be very near that of the closed loop.
The CV clamp 120 comprises an adder 122, a CV clamp error amplifier 124 and a clipper 126. The CC clamp 140 comprises an adder 142, a CC clamp error amplifier 144 and a clipper 146. The dynamic offset injector 160 comprises a comparator 162, a high pass filter 164 and a clipper 166.
The adder 122 provides a control signal CV clamp error CVce by subtracting the voltage CVctrl from the voltage CCctrl and adding a static CV tracking offset voltage 128. The adder 142 provides a control signal CC clamp error CCce by subtracting the voltage CCctrl from the voltage CVctrl and adding a sum of a static CC tracking offset 148 and a pulse 168 output by the dynamic tracking offset injector 160. The pulse 168 occurs at a time of mode transition as further explained below.
The CV clamp error signal CVce is provided to the CV clamp error amplifier 124. An output of the CV clamp error amplifier 124 is clipped by the clipper 126 so that only negative voltages pass through the clipper 126. The clipped clamp error signal is fed into the adder 116 and summed with Vprog and Vmon to regulate the open loop control voltage. The clipper 126 allows the CV clamp 120 to reduce the CV loop control voltage CVctrl but not increase the CV loop control voltage CVctrl.
In an analogous manner, the CC clamp error signal CCce is provided to the CC clamp error amplifier 144. An output of the CC clamp error amplifier 144 is clipped by the clipper 146 so that only negative voltages pass through the clipper 146. The clipped clamp error signal is provided to the adder 136 and summed with Iprog and Imon to regulate the open loop control voltage. The clipper 146 allows the CC clamp 140 to reduce the CC loop control voltage CCctrl but not increase the CC loop control voltage CCctrl.
To enhance stability, the dynamic offset injector 160 adds the pulse 168 to the CC tracking offset 148 when comparator 162 detects that the CC loop 134 assumes control. The addition of the pulse 168 temporarily increases the tracking offset of the CC clamp, thereby reducing a likelihood that a rapid subsequent transition into the CV mode, accompanied by a small perturbation on the CV control voltage, will cause an undesired transition back into CC mode, thus initiating an oscillation.
The dynamic offset injector 160 further comprises a high pass filter 164 and a clipper 166. Operation of the high pass filter 164 and the clipper 166 will be explained using the waveforms shown in FIG. 2. The output of the comparator 162 changes from a low value to a high value 204 during a transition from a first mode of operation (CC) to a second mode of operation (CV) of the power supply and from the high value 204 to the low value 202 during a transition from the second mode of operation (CV) to the first mode of operation (CC). During the transition of the comparator 162 from the low value 202 to the high value 204, the high pass filter 164 outputs a voltage having a waveform 205 with an initial step value 206 and a decay time 207. During the transition of the comparator 162 from the high value 204 to the low value 202, the high pass filter 164 outputs a voltage having a waveform 211 with an initial step value 212 and a decay time 213. The clipper 166 passes only the waveform 205 and rejects the waveform 211 so that only the waveform 205 is applied to the adder 180 as the injected offset 168. A time constant of the high pass filter 166 determines the decay time 207 and thus a duration of the injected offset 168. The time constant is made long enough to ensure that cross-over transients abate and that the operating point stabilizes. The initial step value 206 of the waveform 205 is made large enough to ensure that the clamp 140 avoids mode changes due to the cross-over transients.
Comparative results for several examples of responses to operating mode transitions are shown in FIGS. 3A through 5B. In FIGS. 3A, 4A and 5A, the mode transitions in response to load transients are shown with the clamps 120 and 140 functionally disabled, such as, for example, by temporarily removing the outputs of clamps 120 and 140 from the adders 116 and 136, respectively. In FIGS. 3B, 4B and 5B, the mode transitions in response to load transients are shown with the clamps 120 and 140 enabled as shown in FIG. 1.
Referring now to FIG. 3A, during a first example of mode transition, at time (t=0), the power supply is operating in the CC mode, supplying 20 amps (A) to the load at 5 V, and the load switches to an open-circuit, causing the power supply to switch to the CV timothy34@example.com V. In the first example, the clamps 120 and 140 are disabled as described above. At the load transition (t=0), the output voltage initially overshoots to about 7.5 V, decaying to about 5 V after about 1 ms, undershooting to about 4 V for about 0.7 ms and then settling to about 5.1 V at about 2 ms without overshooting.
Referring now to FIG. 3B, during a second example of mode transition, at t=0, the power supply is operating in the CC mode, supplying 20 A to the load at 5 V, and the load switches to an open circuit, causing the power supply to switch to CV timothy34@example.com V. During the second example, the clamps 120 and 140 are enabled. At the load transition (t=0), the output voltage initially overshoots to only about 5.5 V and then settles to about 5.1 V at about 0.5 ms without undershooting.
Referring now to FIG. 4A, during a third example of mode transition, at t=0, the power supply is operating in the CC mode, supplying 20 A into a short circuit at about 0.8 V, and the load switches to an open-circuit, causing the power supply to switch to the CV mode, at 5 V. During the third example, the clamps 120 and 140 are disabled as described above. At the load transition (t=0), the output voltage initially overshoots to about 8 V, decaying to about 5 V after about 1 ms, undershooting to about 3.8 V for about 0.6 ms, and then settling to about 5 V at about 2.3 ms after t=0.
Referring now to FIG. 4B, during a fourth example of mode transition, at t=0, the power supply is operating in the CC mode, supplying 20 A into a short circuit at about 0.8 V, and the load switches to an open-circuit, causing the power supply to switch to the CV mode, at 5 V. During the fourth example, the clamps 120 and 140 are enabled. At the load transition (t=0), the output voltage rises and settles to about 5 V within about 0.5 ms without overshooting.
Referring now to FIG. 5A, during a fifth example of mode transition, at t=0, the power supply is operating in the CV mode supplying about 5 V into an open circuit and the load switches to a short circuit, causing the power supply to switch to the CC mode, supplying 20 A. During the fifth example, the clamps 120 and 140 are disabled as described above. The load current initially overshoots to about 63 A and begins to approach the 20 A value only after about 50 ms.
Referring now to FIG. 5B, during a sixth example of mode transition, at t=0, the power supply is operating in the CV mode supplying about 5 V into an open circuit and the load switches to a short circuit, causing the power supply to switch to the CC mode, supplying 20 A. During the sixth example of mode transition, the clamps 120 and 140 are enabled. The load current initially overshoots to about 50 A, rapidly returns to a level near the programmed value within about 2 ms and reaches the programmed value of about 20 A within less the 10 ms.
As shown by the above examples, the present invention achieves significant reductions in overshoot during mode transitions and significant improvement in settling time after the mode transitions.
Referring now to FIG. 6, a power supply 200 according to a second embodiment of the present invention is illustrated. The power supply 200 operates in a similar manner as the power supply 100. The difference between power supply 100 and the power supply 200 is the point of injection of the output pulse 168 of the dynamic offset injector 160 and the polarity of the inputs of the comparator 162. As shown in FIG. 6, the pulse 168 is provided to an adder 182 and summed with the static CV tracking offset 128 and the sum of the pulse 168 and the static CV tracking offset 128 is provided to the CV clamp 120 so that clamp 120 avoids unintended mode changes due to cross-over transients.
Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.


{'bibliographic_information': "{'country': 'US', 'doc-number': '10973297', 'kind': 'B2', 'date': '20041027', 'extension': '150', 'invention_title': 'Programmable power supply with dual mode cross over clamps'}", 'source_file': './grant/2006/ipg060829.xml', 'source_data': 'USPTO-Grant'}