Power supplies for computers, personal digital assistants, cellular phones and other hand held mobile electronic devices and systems have exacting demands. Two types of converters are used to meet these demands. One type is a pulse width modulated (PWM) converter and the other is a hysteretic (ripple) converter. A typical single mode DC-to-DC converter 8 with either a PWM or hysteretic controller is shown in FIG. 1 a. The converter 8 has a PWM controller 12 or a hysteretic controller 14. The output of the controller drives the gates of output switches, typically upper and lower mosfet power transistors 16, 18. The mosfets are connected together at a switching node 20. At high output current levels, the PWM controller with a synchronous rectifier provides efficient and controllable output regulation. For low output currents, the efficiency of the DC-to-DC converter operating in a fixed frequency PWM mode gets lower because PWM switching losses become dominant. One the other hand, the hysteretic converter is efficient for low output currents but is not efficient for high output currents.
Demands on a system can change from tens of amps to milliamps in as short a time as a few microseconds. In order to address the variable and frequently inconsistent current requirements, many DC-to-DC converters, especially those used in mobile systems, include both a pulse width modulation (PWM) converter and a hysteretic converter. Such dual mode controllers provide a high efficiency over a wide range of load current levels.
A typical dual mode converter 10 is shown in FIG. 1b. The converter 10 has a PWM controller 12 and a hysteretic controller 14. The output of the controller drives the gates of upper and lower mosfet power transistors 16, 18. The mosfets are connected together at a switching node 20. The switching node 20 is connected to an inductor 22 that is connected to a parallel network comprising an output capacitor 23 and a load as represented by resistor 24. A sense resistor 26 is connected in series with the inductor 22. The voltage across the sense resistor is coupled to the controller 10 to provide data on the load current. A comparator 13 in the controller receives the voltage signal from the sense resistor 26, compares it to a reference value indicative of a critical current, and operates a switch to switch the controller between the PWM modulator 12 and the hysteretic controller 14 when the sensed current falls below the threshold value of the reference input to the comparator.
At high output current levels, the PWM controller with a synchronous rectifier provides efficient and controllable output regulation. As the load current gets lower, the efficiency of the DC-to-DC converter operating in a fixed frequency PWM mode gets lower because PWM switching losses become dominant. A simple hysteretic (ripple) controller improves the converter efficiency for light loads. The integrated circuit senses the load current and, when load current falls below a minimum threshold, it invokes the hysteretic (ripple) controller and disables the PWM controller. When the load current increases above the minimum threshold, the PWM controller resumes control. In this way high efficiency is maintained over a wide range of load currents.
The optimal transition point for the threshold current usually lies at the current level where the inductor current becomes “critical.” Critical current is a value of load current for which the total energy stored in the inductor 22 is delivered to the load each cycle. At load currents below the critical value, the inductor current must go through zero and reverse direction at some point in the cycle. When the inductor current changes direction, energy is taken from the output filter capacitor 23 due to the bidirectional conductivity of the synchronous rectifier, lower fet 18. To maintain the output in regulation more energy will be delivered to the filter capacitor 23 at the next operating cycle. Unless the controller is switched to the hysteretic controller 14, the converter efficiency dramatically degrades. Power and energy are wasted. In mobile systems that rely on battery power, the overall life of the system is likewise reduced.
To prevent the energy losses when operating at sub-critical currents, diode-like conduction is required of the lower mosfet 18. This assures discontinuous inductor current operation. Operating the converter 10 in the discontinuous conduction mode under fixed PWM mode control creates its own challenges because the small-signal loop becomes broken, closed-loop gain increases and the converter easily becomes unstable. This leads to the conclusion that hysteretic mode is preferred for safe, stable and efficient operation at sub-critical current.
In order to select the PWM or the hysteretic mode of operation, the controller 10 senses the load current or any current in the circuit proportional to the load current and compares the sensed load current to a reference. If the load current is higher than the reference, the PWM mode of operation is activated. Otherwise, the converter 10 operates in the hysteretic mode. This widely used approach depends on the tolerance of the currently sensing circuitry. As the output voltages of DC-to-DC converters for modem computer applications are getting lower and the output currents are getting higher and vary widely over short periods of time, it is becoming very difficult to measure the current precisely and efficiently. This leads to increased uncertainty of the switch over point and, therefore, to unpredictable operation of the whole converter.
The invention is a new DC-to-DC converter circuit and a method for DC to DC conversion. The circuit includes a pulse width modulator controller and a hysteretic controller. Both controllers convert an input first voltage into an output second voltage during a series of repeated switching cycles. The circuit has a mode selection circuit for selecting one of the two controllers in accordance with the current demand of a load coupled through an inductor to the second voltage. The mode selection switch has a comparator for comparing the second voltage to a reference voltage (ground) for sensing the polarity of the second voltage at the end of each switching cycle. The polarity of the output voltage at the end of the switching cycle is a measure of the state of the inductor. If the inductor is in continuous operation and the load current is above the critical current, then the polarity of the switch node will be positive. If the inductor is in discontinuous operation, then the polarity of the switch node will be negative.
One or more counters are coupled to the comparator and to the mode selection switch. The counters record the polarity of the second voltage at the end of each switching cycle and keep that data for a given number of cycles. If the polarity of the switch node does not change, then the controller remains in whichever mode (PWM or hysteretic) that it has been operating in. By using counters, the invention avoids premature switching for a single change in polarity. Such changes may occur for spurious reasons that are not related to enduring load conditions. As such, the counters maintain the mode selection switch in its current mode so long as the polarity of the second voltage at the end of each cycle does not change. However, when the polarity changes and the changes endure for more than n switching cycles, then the counters operate the mode selection switch to change the mode of operation to the other mode. If the converter was operating in the PWM mode, then it is switched to the hysteretic mode and vice versa.
The DC-to-DC converter circuit operates to switch the mode selection switch to the hysteretic mode controller when the polarity of the second voltage for n number of cycles is positive and to switch the mode selection switch to the pulse width modulator controller when the polarity of the second voltage for n number of cycles is negative. There are separate counters for counting the positive and negative cycles. The number n may be the same or different for both counters.
DESCRIPTION OF THE DRAWING
FIG. 1a is a schematic of a single mode DC-to-DC converter.
FIG. 1b is a schematic of a dual mod DC-to-DC converter.
FIGS. 2a-2 c are graphs of the converter performance during continuous conduction.
FIGS. 2d-2 f are graphs of the converter performance during discontinuous conduction.
FIGS. 3a-3 d are graphs of the invention operation during PWM and hysteretic operations.
FIG. 4 is a schematic of a mode-controlled DC-to-DC controller.
FIG. 5 is a detailed schematic of a mode-controlled DC-to-DC controller that includes the invention.
FIG. 5 shows a detailed schematic of a possible embodiment of the mode control switch with counters for and polarity sensors for distinguishing continuous from discontinuous currents in the inductor. The mode switch 50 includes a comparator 52 that compares the switching node to ground. The output of comparator 52 is clocked to the input of D-flip-flop 54 by the PWM signal. Two counters 56, 57 count to eight cycles, one counting positive switch node voltage, the other, negative. The counters 56, 57 are reset when the clocked switch node polarity (Q output of 54) changes. In the preferred embodiment each counter uses a count of eight to invoke the appropriate operating mode. However, the number of counts is arbitrary. The mode switch circuit 50 also provides a means to override the HYSTERETIC-to-PWM delay or counter in order to permit fast response to load current transients. The embodiment of this means in the mode switch control 50 uses a comparator 58 for monitoring the output feedback voltage and causing an immediate resetting of the MODE latch out of the HYSTERETIC mode when the feedback voltage is below the reference by an amount set to be greater than the normal ripple voltage expected. In other words, the delay of eight cycles in going from HYSTERETIC to PWM mode can be entirely eliminated via this means. A PWM reset comparator 58 compares the output voltage on the load VFB to a reference voltage. If VFB is greater than the reference, the circuit 50 immediately switches to the PWM mode. The reference is set to detect large output voltages. If the triggering voltage is spurious, the circuit 50 will reset to the HYSTERETIC mode after n cycles.