DC-DC converter with resonant gate drive

[0002] The present patent application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 60/337,837 entitled “Novel DC-DC Step-Down Converter with Resonant Gate Drive”; filed on Nov. 5, 2001, the full disclosure of which is incorporated herein by reference.

[0003] The following references to non-provisional patent applications are also incorporated by reference herein:

[0004] “Monolithic Battery Charging Device” to Shenai et al., Ser. No. 10/288,177, filed concurrently herewith;

[0005] “DC—DC Converter with Current Control” to Shenai et al., Ser. No. 10/289,115, filed concurrently herewith; and

[0006] “Synchronous Switched Boost and Buck Converter” to Shenai et al., Ser. No. 10/288,987, filed concurrently herewith.

[0007] The present invention relates to power converters and, more specifically, to direct current to direct current step-down voltage converters (buck converters), and to direct current to direct current step-up voltage converters (boost converters).

[0008] Direct-current to direct current voltage converters (DC—DC converters) are used frequently in electrical and electronic systems to convert one voltage potential to another voltage potential. Such DC—DC converters typically have some form of regulation that controls an output voltage for the DC—DC converter as the electrical power consumed by an electrical load connected with the DC—DC converter changes. Such loads may include microprocessors, wireless communication devices, or any other electronic system or component that uses a DC voltage. One common type of DC—DC converter may be referred to as a buck converter. Buck converters step down an input voltage to provide a lower voltage potential output voltage. Another common type of DC—DC converter may be referred to as a boost converter. Boost converters step up an input voltage to provide a higher voltage potential output voltage.

[0009] One challenge that is faced when designing DC—DC converters, such as buck and boost converters, is the efficiency of such converters. Efficiency may be measured by the ratio of output power to input power. Therefore, efficiency for a given DC—DC converter indicates the amount of power consumed, or lost, as a result of the conversion from the input voltage potential to the output voltage potential. Current approaches for implementing DC—DC buck converters may have efficiencies on the order of 50% percent. As electrical and electronic systems continue to increase in complexity, such power losses due to voltage conversion may present more significant design challenges. Therefore, alternative approaches for buck converters may be desirable.

[0010] A direct current to direct current boost or buck voltage converter in accordance with the invention includes a plurality of switching devices that effect voltage conversion and control current flow direction in the converter. The converter also includes a control circuit for comparing an output voltage of the converter with a reference voltage, where the control circuit produces a comparison signal based on that comparison. A resonant gate-drive circuit, also included in the converter and coupled with the control circuit and the plurality of switching devices, opens and closes the plurality of switches in response to the comparison signal to effect voltage conversion and control current flow direction.

[0011] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, as to both organization and method of operation, together with features and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

[0012] FIG. 1 is a schematic diagram illustrating a prior art direct current to direct current voltage step-down converter (buck converter);

[0013] FIG. 2 is a schematic drawing illustrating an embodiment of a buck converter with resonant gate drive in accordance with the invention;

[0014] FIG. 3 is a schematic diagram illustrating an embodiment of a control circuit that may be used in the buck converter illustrated in FIG. 2;

[0015] FIG. 4 is a schematic diagram illustrating an embodiment of a resonant gate-drive circuit in accordance a preferred embodiment;

[0016] FIG. 5 is a schematic diagram of a timer circuit that may be included in the resonant gate-drive circuit depicted in FIG. 4;

[0017] FIG. 6 is a schematic diagram illustrating an alternative embodiment of a buck converter;

[0018] FIG. 7 is a schematic diagram illustrating an alternative embodiment of a resonant gate-drive circuit;

[0019] FIG. 8 is a schematic diagram illustrating another alternative embodiment of a resonant gate-drive circuit;

[0020] FIG. 9 is schematic diagram illustrating another alternative embodiment of a buck converter, and

[0021] FIG. 10 is a schematic diagram illustrating another embodiment of a direct current voltage step-up converter.

[0022] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail, so as not to obscure the present invention.

[0023] As was previously indicated, current approaches for implementing a buck or boost converter may have efficiencies in the range of 50% percent. Such efficiencies may create significant design challenges in certain applications, such as, for example, monolithic direct current to direct-current step-down voltage converters (buck converters) or step-up voltage converters (boost converters) integrated on a semiconductor device with other circuitry. Such challenges may include power consumption, circuit element sizes for such DC—DC converters, among other issues.

[0024] FIG. 1 is a schematic diagram illustrating such a prior-art buck converter 100. Buck converter 100 comprises a direct current voltage supply 110. Supply 110 is coupled with a switch 120, which takes the form of a bipolar transistor for this embodiment. Buck converter 100 also includes inductor 130, diode 140, control circuit 150 and capacitor 160. Such a configuration is well-known and will not be discussed in detail here. Briefly, however, control circuit 150 typically includes a startup oscillator and either a pulse-width-modulated (PWM) circuit or a pulse-frequency-modulated (PFM) circuit. In such configurations, the PWM or PFM circuit is coupled directly with switch 120. Such a configuration typically results in power loss due the power consumed to turn switch 120 on and off during operation of buck converter 120. These switching losses adversely impact the efficiency of buck converter 100 and similar converter topologies. Therefore, alternative techniques for implementing a buck (or boost) converter may be desirable.

[0025] Various exemplary embodiments of the invention will now be described. Some of these embodiments include analogous elements that are referenced with like reference numbers. It will be appreciated, however, that the invention is not limited in scope in this respect, as many alternatives for such elements may exist. Also, specific implementations of such buck and boost converters may vary. For example, converters in accordance with the invention may be implemented using discrete components or, alternatively, may be implement monolithically on a single integrated circuit device.

[0026] FIG. 2 illustrates an embodiment of a buck converter 200 in accordance with the invention that may, at least in part, overcome some of the issues discussed with respect to current boost and buck converter configurations. Buck converter 200 includes a substantially static direct current (DC) voltage power supply 210. Power supply 210 is coupled with a series synchronous rectifier that includes a p-type field effect transistor (FET) 220 and an n-type FET 240. Buck converter 200 further includes an inductive-capacitive output voltage filter that includes inductor 230 and capacitor 260. Buck converter 200 may supply voltage to a load resistance 270. It will be appreciated that load resistance 270 typically is a time varying load impedance. In this regard, buck converter 200 is typically regulated based, at least in part, on the power consumed by load resistance 270.

[0027] Buck converter 200 also includes a voltage reference source 290, which may generate a reference voltage (e.g. a desired output voltage) for buck converter 200. The reference voltage produced by voltage reference 290 may compared with an output voltage of buck converter 200 by control circuit 250. In this regard, the reference voltage may be communicated to control circuit 250 via a signal line 295 while the output voltage of buck converter 200 may be communicated to control circuit 250 via a signal line 275. Control circuit 250 then generates a signal based on that comparison. Such voltage comparison is discussed in further detail hereinafter with respect to FIG. 3.

[0028] The signal generated by control circuit 250 may be communicated, via a signal line 255, to a resonant gate-drive circuit 280. Resonant gate-drive circuit 280 may, in response to this signal, operate, via signal line 285, p-type FET 220 and n-type FET 240 (the series synchronous rectifier) so as to regulate the output voltage of buck converter 200 so that it approximates the reference voltage generated by reference voltage source 290. Such a configuration may be advantageous as the resonant gate-drive circuit 280 may reduce power losses that result from switching without such drive circuits, such as in previous buck converter configurations. Each of the components of buck converter 200 will now be described in further detail.

[0029] FIG. 3 is a schematic diagram illustrating an embodiment of control circuit 250 in accordance with the invention that may be used in buck converter 200, as shown in FIG. 2. Control circuit 250 includes a voltage amplifier 300. The input terminals of voltage amplifier 300 are coupled with signal lines 275 and 295, respectively the output voltage of buck converter 200 and the reference voltage produced by voltage reference source 290. Voltage amplifier 300, in operation, compares these voltages and produces a signal that indicates whether the output voltage of buck converter 200 is above or below the reference voltage, where the reference voltage is typically the desired operating voltage of buck converter 200.

[0030] This indication signal is then communicated to a PWM circuit 320 where it is compared with a pulse train (typically a series of triangular pulses or voltage ramps) that is produced by a signal source 310. PWM circuit 320 may then produce the comparison signal that is used by resonant gate-drive circuit 280 to regulate the output voltage of buck converter 200 using the series synchronous rectifier.

[0031] FIG. 4 is a schematic diagram illustrating an embodiment of resonant gate-drive circuit 280 that may be used in buck converter 200, as shown in FIG. 2. Resonant gate-drive circuit 280, for this embodiment, includes a buffer 400 and a timer 410 coupled with buffer 400. Timer 410 is further coupled, via signal lines 415 and 417, with an inverter circuit that includes p-type FET 420 and n-type FET 430. An inductive-capacitive resonant circuit is coupled between electrical ground and an output terminal of the inverter circuit (signal line 285), as is shown in FIG. 4. The inductive-capacitive circuit includes inductor 440 and capacitor 450. The output terminal of the inverter circuit (and the inductive-capacitive resonant circuit) is coupled with the gates of p-type FET 420 and n-type FET 430 of the series synchronous rectifier.

[0032] Timer circuit 410 operates, based on signals communicated from control circuit 250 (via buffer 400), such that p-type FET 420 and n-type FET 430 have overlapping “off” times without a corresponding overlap in “on” times. The overlap in “off” times is desirable as it allows for electrical energy to be transferred between inductor 440 and capacitor 450, and the gates of p-type FET 220 and n-type FET 240. Such a configuration may also reduce losses due to switching because electrical energy for charging the gate capacitance (or electrical energy discharged from the gates) of these FETs is stored using resonant gate-drive circuit 280, as opposed to being supplied from (or communicated to) a power supply rail via a control circuit, such as is shown in FIG. 1.

[0033] Resonant gate-drive circuit 280 operates by providing a gate current sink or source by building a drive current in the inductor 440. Specifically, when capacitor 450 is fully charged, and FET 430 is in a conductive state (the signal on line 417 is high), capacitor 450 discharges through inductor 440 and FET 430 to ground (or a reference voltage). When FET 430 is turned off (and before FET 420 is turned on), the current through inductor 440 is provided at node 285 and is used to provide gate current to FETs 220 and 240, thereby turning on n-type FET 240, and turning off p-type FET 220. Timer circuit 410 then provides a logic low on line 415, thereby turning on p-type FET 420, which provides a current path for the inductor current from inductor 440 through p-type FET 420 to the positive supply. Eventually capacitor 450 discharges and the inductor current decreases, and then builds in the opposite direction. Current begins flowing from the positive supply through p-type FET 420, through the inductor 440, and into capacitor 450, thereby recharging capacitor 450. At the appropriate time, timer circuit 410 turns off p-type FET 420, so that both FETs 420 and 430 are not conducting. At this instant, the inductor current is near its negative peak, and as an inductor's current must be continuous in time, it begins drawing current from the gates of FETs 220 and 240 through node 285, thereby turning off n-type FET 240, and turning on p-type FET 220. In this manner, the inductor 440 of the gate drive circuit is used as a current source or sink to provide a gate drive current. By exchanging energy between the capacitor 450 and inductor 440 in a resonant fashion, the switching losses associated with charging and discharging the converter's synchronous FET switches (220 and 240) is reduced. That is, by sequentially increasing the inductor current to a positive value (current provided from the inductor to the gates of the FET devices 420 and 430), and then momentarily diverting sourcing the inductor current to the FET gates, followed by decreasing the inductor current to a negative value and then momentarily sinking the inductor current from the FET gates, the FET gates may be charged and then discharged, respectively.

[0034] FIG. 5 is a schematic diagram illustrating an embodiment of timer circuit 410 that may be used in resonant gate-drive circuit 280, as is shown in FIG. 4. As was indicated above, timer circuit 410 may operate so as to effect overlapping “off” times for p-type FET 420 and n-type FET 430 so as to provide inductor 440 to operate as a current source or sink for the gates of FETs 420 and 430. In this regard, timer circuit 410 operates such that the FETs are both in a non-conduction state (“off”) for a brief period of time.

[0035] In this respect, timer 410 includes multiple circuit paths, 505 and 570, that each includes delay elements. Single delay elements may be used or, as in FIG. 5, multiple delay elements may be used in each path. Such a configuration results in a signal that is communicated to an input terminal of the timer (from buffer 400 on signal line 405) reaching the end of each of the multiple circuit paths (505 and 570) at different times. Also, circuit path 505 has a first delay time for a low state to high state transition and a second delay time for a high state to low state transition, where the second delay is longer than the first delay. While the specific operation of timer 410 is now described, it will be appreciated that many alternative timers may be used and the invention is not limited in scope to the use of this, or any particular timer circuit.

[0036] In operation, timer 410 receives a signal on signal line 405 from buffer 400. This signal is then communicated to both signal paths 505 and 570. Due to propagation delay for each of the inverters and the NOR gate included in timer 410, the signal received on signal line 405 results in p-type FET 420 and n-type FET 430 being controlled in the fashion described above with respect to “on” and “off” times.

[0037] In this regard, looking first at a transition from a low state to a high state on signal line 405, a NOR gate 560 of circuit path 505 produces (or transitions from a high state to) a low state, regardless of the initial state of its other input. This low state is communicated to a fifth inverter 550, which then inverts it to a high state. The high state is then communicated to p-type FET 420, turning it off.

[0038] The transition from low state to high state of the signal on line 405 is also communicated to a second circuit path 570. Circuit path 570 includes a sixth inverter 580 that inverts the high state to produce a low state. This low state is communicated to a seventh inverter 590, which inverts its incoming signal to produce a high state. The high state is then communicated to n-type FET 450, which switches it to its ON state.

[0039] Like NOR gate 560 and fifth inverter 550, sixth 580 and seventh inverter 590 are in series. As such, the high state that is fed to n-type FET 450 lags behind the high state of the signal communicated from buffer 400 on signal line 405 by the combined propagation delay of sixth inverter 580 and seventh inverter 590, which may have the same duration or different duration.

[0040] The individual propagation delays of sixth inverter 580 and seventh inverter 590 may, respectively, have the same duration as NOR gate 560 and fifth inverter 550. Assuming no propagation delay difference for the pinch-off of p-type FET 420 and n-type FET 430, preferably and in practice, the combined propagation delay of sixth inverter 580 and seventh inverter 590 would be longer than the combined propagation delay of the NOR gate 560 and the fifth inverter 550. This situation ensures that when n-type FET 430 switches to its on state, p-type FET 420 is already in its off state.

[0041] Looking now at a transition from high state to low state of a signal communicated to timer 410 from buffer 400, the signal is communicated to circuit path 570 and sixth inverter 580 inverts the low state to produce a high state. This high state is communicated to seventh inverter 590, which inverts its incoming signal to produce a low state. The low state is then communicated to n-type FET 430, which switches off.

[0042] For at least the combined propagation delay of a first, second, third and fourth inverter (510, 520, 530 and 540) the output of circuit path 505 remains unchanged after receiving the high-to-low state signal transition. Although the low state is communicated directly to the first input of NOR gate 560, the output of NOR gate 560 continues to provide a low state until fourth inverter 540 provides a low state signal. This period of time is at least the combined propagation delay of first, second, third and fourth inverters 510, 520, 530, 540.

[0043] When the fourth inverter provides the low state to the second input of NOR gate 560, the output of NOR gate 560 produces a high state signal that is fed to fifth inverter 570. Fifth inverter 570 then supplies a low state to p-type FET 420, which turns on. It is noted that, when used in conjunction with timer 410, p-type FET 420 and n-type FET 430 have over-lapping “off” times, non-overlapping “on” times, and operate substantially out of phase with each other.

[0044] FIG. 6 illustrates an alternative embodiment of a buck converter 600 in accordance with the invention. Buck converter 600 is similar to buck converter 200 in a number of respects. In this regard, similar elements of buck converter 600 and buck converter 200 are indicated with the same reference numerals. It will be appreciated that the invention is not so limited, and alternatives for these elements of buck converters 200 and 600 exist.

[0045] Buck converter 600 differs from buck converter 200 in at least the following respects. Resonant gate-drive circuit 680 includes two output terminals (which are coupled with signal lines 685 and 687). This allows for individual control of the FETs included in the series synchronous rectifier (e.g. p-type FET 220 and n-type FET 240). Such a configuration may further reduce switching losses for buck converter 600 as opposed to other approaches. In this respect, such a configuration may reduce shoot-through current in the series synchronous rectifier as a result of resonant gate-drive circuit 680 individually controlling the FETs.

[0046] FIGS. 7 and 8 illustrate two alternative embodiments of a resonant gate-drive circuit, respectively 680 and 680′, that may be used with buck converter 600, as shown in FIG. 6. These circuits are similar in certain respects to the gate-drive circuit shown in 4 and like reference numbers have been used in such cases. Again, it will be appreciated that alternatives exist and the invention is not limited to the particular exemplary embodiments described herein.

[0047] Resonant gate-drive circuit 680 differs from circuit 280 shown in FIG. 4 in that it includes multiple cascaded timers 710, 712 and 714 forming a first and second timing path. Individual timers 710, 712 and 714 may be of similar configuration as timer 410 shown in FIG. 5, or may have completely different configurations, depending on the particular embodiment. The additional timer path shown in FIG. 7 may be used to control a second inverter circuit (p-type FET 435 and n-type FET 440) that includes a corresponding inductive-capacitive resonant circuit (inductor 445 and capacitor 450). In this respect, an output terminal of the first inverter circuit is coupled with the gate of p-type FET 220 via signal line 685 and an output terminal of the second inverter is coupled with the gate of n-type FET 240 via signal line 687.

[0048] FIG. 8 is a schematic diagram that illustrates another alternative resonant gate-drive circuit 680′ that may be used with buck converter 600 shown in FIG. 6. Resonant gate-drive circuit is similar in a number of respects to resonant gate-drive circuit 680 shown in FIG. 7 and those aspects are indicated with like reference numerals. Resonant gate-drive circuit 680′ varies from the circuit shown in FIG. 7 in that resonant gate-drive circuit 680′ includes only a single timer 810 with four output terminals that control the p-type and n-type FETs included in the two inverter circuits. Timer 810 may include multiple circuit paths, as was discussed with respect to timer 410 shown in FIG. 4. Such circuit paths may be configured in a similar fashion as with timer 410, though the invention is not so limited.

[0049] FIG. 9 is a schematic diagram that illustrates yet another alternative embodiment of a buck converter 900 in accordance with the invention. Buck converter 900 comprises a static direct current voltage source 905, which provides an input voltage for buck converter 900. Buck converter 900 also comprises an output voltage filter that includes an inductor 950 and a capacitor 955, which store electrical energy and reduce variation of the regulated, stepped-down output voltage produced by buck converter 900.

[0050] Voltage source 905 is coupled, in series, with two switching devices, which, for this embodiment, take the form of n-type field effect transistor (FET) 910 and p-type FET 945. In this configuration, n-type FET 910 and p-type FET 945 may function as a series synchronous rectifier. In this respect, when p-type FET 945 is on and n-type FET 910 is off, electrical energy from voltage source 905 may be supplied to inductor 950 and capacitor 955. Conversely, when n-type FET 110 is on and p-type FET 945 is off, electrical energy from inductor 950 and capacitor 955 may be diverted to ground. Control circuit 925, in combination with resonant gate-drive circuits, as will be described in further detail below, would determine, at least in part, the timing of opening and closing n-type FET 910 and p-type FET 945. Such timing control regulates the voltage potential across capacitor 955, which may be supplied to a load impedance 960. It will be appreciated that load impedance 960, as has been previously described with respect to FIGS. 2 and 6, may vary over time, and the output voltage of buck converter 900 may be regulated in response to that variation.

[0051] For buck converter 900, n-type FET 910 and p-type FET 945 would operate out of phase with each other. That is, n-type FET 910 will be open (not conducting current) when p-type FET 945 is closed (conducting current), and vice-versa. Opening and closing of n-type FET 910 and p-type FET 945 is accomplished, at least in part, using respective resonant circuits, as was indicated above. For n-type FET 910, the resonant gate-drive circuit comprises inductor 915 and the capacitance of the gate of n-type FET 910, forming an inductive-capacitive resonant circuit. Likewise, the resonant gate-drive circuit for p-type FET 945 comprises inductor 940 and the capacitance of the gate of p-type FET 945.

[0052] Buck converter 900 further comprises a control circuit 925, an inverter 930 and n-type FETs 920 and 935. Control circuit 925 senses an output voltage of buck converter 900 via feedback signal line 965. Based on that output voltage, control circuit 925 activates one of the two resonant gate-drive circuits. This is done via a single control signal line 927, which is coupled with a gate of n-type FET 920 and further coupled with a gate of n-type FET 935 via inverter 930. The sensed voltage may be compared with a reference voltage (e.g. the desired output voltage) by control circuit 925. If the sensed voltage is above the reference voltage, n-type FET 910 may be turned on and p-type FET 945 may be turned off to divert current from inductor 950 to electrical ground. Conversely, if the sensed voltage is below the reference voltage, p-type FET 945 may be turned on and n-type FET 910 may be turned off, so as to supply electrical energy from voltage source 905 to inductor 950 and capacitor 955.

[0053] This configuration provides for out of phase operation of the two resonant gate-drive circuits and their associated FETs. Such a configuration may reduce the power consumption of a buck converter as a result of a reduction in power consumption due to switching the series synchronous rectifier switching devices, n-type FET 910 and p-type FET 945 for buck converter 900.

[0054] Regulation of voltage conversion using buck converter 900 depends, at least in part, on the desired switching frequency of such a converter. In this regard, switching in such a buck converter would be done at approximately zero current crossings of the resonant gate-drive circuits. Such an approach may reduce the occurrence of current spikes from inductors 915 and 940, which may damage the gates of n-type FETs 920 and 935 if switching is not done in this manner. In this regard, the values for inductors 915 and 940 depend, at least in part, on a desired switching frequency for a particular embodiment of buck converter 900. While the invention is not limited to any particular embodiment, the following table illustrates inductor values for inductors 915 and 940 for embodiments of buck converter 900 with a one hundred megahertz switching frequency at various control signal duty cycles (produced by control 925) and various resonant frequencies for the gate-drive circuits.

[0055] FIG. 10 is a schematic diagram illustrating an embodiment of a direct current voltage step-up (boost) converter 1000 in accordance with the invention. Boost converter 1000 includes a direct current voltage input supply 1010, which is coupled with an inductor 1020. Inductor 1020 stores electrical energy from supply 1010 that is used by converter 1000 to boost that voltage. Converter 1000 also includes n-type FET 1030 and p-type FET 1040, which act, respectively as a step up switch and a synchronous switch for converter 1000. Voltage converted by converter 1000 may be stored and filtered by a capacitor 1060 and supplied to a load resistance 1070, which may be a time varying impedance, as has been previously described with respect to FIG. 2.

[0056] Boost converter 1000 also includes a control circuit 1050 and a voltage reference 1090. Control circuit 1050 may compare an output voltage of converter 1000 with a reference voltage generated by voltage reference 1090 using signal lines 1075 and 1095. A signal based on that comparison may then be sent to a resonant gate-drive circuit 1080. Resonant gate-drive circuit 1080 may be of a similar configuration as resonant gate drive 280, shown in FIG. 4, though the invention is not so limited. For this embodiment, resonant gate drive 1080 would switch n-type FET 1030 and p-type FET 1040 in a similar fashion as was discussed with respect to buck converter 200 and FIGS. 2 and 4. Additionally, resonant gate-drive circuit 1080 may be replaced with a resonant gate-drive circuit such as resonant gate-drive circuits 680 and 680′, shown respectively in FIG. 7 and FIG. 8.

[0057] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.