# Patent

## Title
Power supply circuit for charging a bootstrap capacitor

## Abstract

A switching power supply circuit is provided for capacitor charging, wherein a power device is coupled to a terminal node of a capacitor to be charged and has a control terminal coupled to the output of an associated drive circuit. This circuit includes a second power element being associated with the first power device, coupled to said terminal node, and provided with a control terminal which is connected directly to the output of respective drive logic. The second power element is driven to turn on when a voltage below a predetermined minimum is present at the capacitor, thereby pulling the voltage at the terminal node to ground and further charging the capacitor.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
The present application claims priority from Italian patent application No. MI2001A002605, filed Dec. 11, 2001, which is incorporated by reference.
TECHNICAL FIELD
The present invention relates generally to a power supply circuit for optimum bootstrap capacitor charging. In particular, the invention relates to a switching power supply circuit for the optimum charging of a bootstrap capacitor, wherein a power device is connected to a terminal node of the capacitor to be charged and has a control terminal connected to the output of an associated drive circuit.
BACKGROUND OF THE INVENTION
In a switching power supply of the step-down type employing an N-channel transistor for a power device, the power transistor is driven by a bootstrapping or charge-pump voltage boosting technique. FIG. 1 herewith is a basic diagram of a bootstrapped power supply 1.
The above technique comprises charging a capacitor, called the bootstrap capacitor, which is placed between the output of the power device and the supply VDrive to the drive circuit or driver of the device. In FIG. 1, this bootstrap capacitor is referenced Cb. The supply voltage to the drive circuit is approximately the combined values of the supply voltage VPow to the power device T1 and the voltage at the capacitor Cb, i.e.:

VDrive=VPow+VCb−VD0.

Thus, the voltage drop Vgs across the gate and source terminals of the power device T1 is always near-constant under varying conditions of operation, i.e., on/off switching of the power device always provides good overdrive.
Assuming the capacitor Cb to be in a charged initial state, when the power device T1 is turned on, the node Vpow is at Vcc, but the power device will stay on, since voltage VDrive is equal to Vcc+Vcb−VD0.
Also, with the power device T1 on, a current will be circulated such that, as the power device is switched off, the ratio dIl/dt makes the diode D1 conductive and VPow≈0V. Accordingly, the capacitor Cb will be charged by the voltage generator connected to it through the link 2 comprised of components VCb, D0, Cb, and D1. Of course, this operation would be feasible only when the coil contains sufficient energy to pull the cathode of the diode D1 below ground.
The fundamental law for inductors, ΔVL=−L·dIL/dt, indicates that, in the above instance, with the output current IOut being small, ΔIL will be low, and Δt finite, so that, when the power device changes over, the voltage variation across the coil will be insufficient to pull the cathode of diode D1 below ground due to parasitic capacitances. Therefore, the capacitor Cb cannot be charged within time Toff, and will keep being discharged due to a continual current draw from the drive circuit.
Thus a condition is ultimately reached of the supply voltage to the drive circuit being unable to drive the power device T1 as expected.
An attempt at overcoming this problem is represented by European Patent Application No. EP 0 822 475, which is herein incorporated by reference.
However, the proposal of that patent application cannot overcome the problem at 100% duty cycle.
FIG. 2A is a plot with respect to time for a number of signals that are present in the power supply described in the above patent application. These signal plots clearly show that the above patent application will only drive the duty cycle of the PWM signal to 100% for one period, this PWM signal being the control signal to the power device T1. Basically, the coil current is raised to a sufficient ΔV for the cathode of diode D1 to be pulled to ground and the capacitor Cb charged.
At the following cycle, when the duty cycle is driven to 0%, the increase in the coil current ΔIL is large enough at low duty cycles to produce a voltage differential ΔVL that can place the diode D1 in forward conduction (proper operation).
FIG. 2B shows that at high duty cycles, the relative increase in the current ΔIL is so small that the voltage differential ΔVL is inadequate to place the diode D1 in the forward mode. Thus, the aforementioned patent application cannot overcome the problem when conditions are as outlined above.
Consequently, there is a need for a power supply that can keep the bootstrap capacitor charged under conditions of a small current IL and a very high duty-cycle value (close to 100%). As discussed above, with prior art circuits the current will be so small that in this case it cannot pull the cathode of the loop diode below ground, and thus will inhibit the charge current to the bootstrap capacitor.
SUMMARY OF THE INVENTION
An embodiment of the present invention is a circuit having a second power device associated with the power device, wherein the second power device is managed by a logic drive circuit for pulling the potential at the drain terminal of the power device to ground and enabling the charge current to the bootstrap capacitor only when the loop diode cannot be turned conductive.
Advantageously, the second power device may be associated with any type of supply circuit that employs the bootstrapping technique. Based on this idea, the technical problem is solved by a power supply circuit, as previously indicated, being characterized in that it comprises a second power element, said second power element being associated with said power device, connected to said terminal node, and connected with a control terminal directly to the output of respective drive logic. Advantageously, the second power element is driven to turn on when a lower voltage than a predetermined minimum is present at the capacitor, thereby pulling the voltage at said terminal node to ground and further charging the capacitor.



BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the power supply circuit according to this invention will become understood from the following description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings.
FIG. 1 shows schematically a power supply circuit according to the prior art.
FIG. 2A shows plots against time for several signals, such as would be present in the power supply circuit of FIG. 1 during low duty-cycle operation according to the prior art.
FIG. 2B shows plots against time for several signals, such as would be present in the power supply circuit of FIG. 1 during high duty-cycle operation according to the prior art.
FIG. 3 shows schematically a power supply circuit according to an embodiment of the invention.
FIG. 3A shows schematically a power supply circuit according to another embodiment of the invention.
FIG. 4 is a more detailed view of the power supply circuit according an embodiment of to the invention.
FIG. 5 shows plots against time for several signals, such as would be present in the power supply circuit of FIG. 4 during high duty-cycle operation according to an embodiment of the invention.
FIG. 6 shows a second embodiment of a portion of the power supply circuit of FIG. 3.
FIG. 7 shows plots against time for several signals, such as would be present in the power supply circuit of FIG. 6 during high duty-cycle operation according to an embodiment of the invention.



DETAILED DESCRIPTION
The following discussion is presented to enable a person skilled in the art to make and use the invention. The general principles described herein may be applied to embodiments and applications other than those detailed below without departing from the spirit and scope of the present invention. The present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed or suggested herein.
With reference to the drawings, in particular to the example of FIG. 3, a power supply circuit for optimum bootstrap capacitor charging, according to an embodiment of this invention, is shown generally at 10 in schematic form.
Preferably, the circuit 10 would be used as a step-down switching power supply. However, there is no reason why the circuit 10 could not be used in other applications where a capacitor of a given capacitance was to be charged in an optimum manner.
As shown best in FIG. 3, the differences of the inventive circuit 10 from a conventional design like that shown in FIG. 1 are apparent. A power device T1 is connected to a terminal node A of a capacitor Cb, the latter also having an output circuit LC connected to it. A second power element, in particular a transistor T2, is associated with the power device T1 of FIG. 1 and connected to the node A, it being driven directly from dedicated drive logic 11.
Generally, the second power element T2 is controlled from a logic circuit 11 to pull to ground the potential at its drain terminal, thereby allowing the charge current to the capacitor Cb to go through the link 13 that includes elements such as VCb, D0, Cb, and T2. The second power element T2 would take over whenever the diode D1 is prevented from turning conductive. Thus, when the coil lacks sufficient energy to allow the capacitor Cb to be charged, i.e. when the voltage VCb at the bootstrap capacitor is below an admissible minimum for the system, the power element T2 will be turned on and pull the voltage VPow at node A to ground, so that the capacitor Cb can be charged. This removes typical malfunctions of such circuit designs.
FIG. 4 shows the construction of the logic circuit 11 in greater detail. The construction of the logic circuit 11 can be said to include the three parts specified here below:
an output buffer stage 16 connected directly to a control terminal of the second power element T2;
a level-shift circuit 14 operative to raise the voltage to the buffer stage 16, the circuit 14 shifting the voltage level to 12V from 5V;
a logic network 15 driving the level-shift circuit 14 by appropriate outputs from the power supply.
FIG. 4 also shows that the logic circuit 11 is input four signals, namely:
Referring to the circuit 10 of FIG. 3A, which is similar to the circuit 10 of FIG. 3 but for the addition of a comparator COMP. OUT_COMP is the output from the comparator COMP, which is arranged to compare the value of the voltage VCb across the bootstrap capacitor to a predetermined threshold THR; upon this voltage falling below THR, the comparator output is changed over, and is again changed over upon the voltage VCb exceeding a second set value “Hist”;
PWM: this is the signal that drives the power device T1 in the control loop;
VREF_OK: this is a signal from a band-gap voltage regulator which will take a high value as the system supply voltage attains steady state; broadly in the circuit of this embodiment, this is an enable signal to the logic network 15 for a proper start-up of the logic circuit 11;
MASK: this signal monitors the voltage at the terminal “Pow”; in practice, it detects the moment that the voltage Vpow goes below a given value, i.e. that the bias of diode D1 changes from reverse to forward.
Let us see now the construction and operation of the logic circuit 11 in greater detail. As the signal OUT_COMP takes a high value, corresponding to the bootstrap voltage being below its designed minimum, the negation of PWM will generate a pulse through a delay block Delay1 causing the negated output Q of a flip-flop FF to go low.
This takes place over a sufficiently long time to ensure that the power device T1 is off. Since the buffer stage 16 is to invert the output signal from the flip-flop FF, the power element T2 will be conducting (ON). A reset signal is generated to the flip-flop FF through a second delay block Delay2, causing the power device T2 to stop conducting.
If before the resetting pulse comes in from the second delay block Delay2 the potential VPow already is above the sensitivity threshold of the signal MASK, this signal is propagated to the flip-flop FF through a series of logic gates, 18 and 19, and resets the logic network 15 such that the power element T2 can be opened.
It should be noted that this system maximizes the efficiency of charging the capacitor Cb, because the time for closing the power element T2 is not made longer than is strictly required for charging the capacitor by the provision of the two delay blocks, the network of logic gates connected to the blocks to generate the pulses, and the signal MASK.
Should the power element T2 be held on for a longer time than is required for the supply circuit to operate properly, i.e. until the voltage VPow is approximately 0V, then the node at potential “Pow” would see its voltage raised by the power element T2 becoming current-saturated, IT2=ICb+IL. Under this condition, the capacitor Cb is no longer charged, and merely the output voltage from the supply circuit is discharged.
FIG. 5 shows, by way of example only, waveforms plotted against time for a number of signals present in the logic network 15. How the voltage Vgate at the gate terminal of the power element T2 varies with the other signals is readily evinced from these plots.
The first pulse of voltage Vgate goes high with a time delay over PWM, as due to the block Delay1. This voltage signal goes low again because it is assumed that the diode D1 will change from a reverse bias to a direct bias. The second, shorter pulse of Vgate changes to low from high upon the voltage VCb at the bootstrap capacitor exceeding a threshold VHist. Accordingly, OUT_COMP goes low again and resets the flip-flop FF.
Briefly, the power supply circuit of this embodiment solves the technical problem and affords several advantages, foremost among which is the fact that the problems connected with charging the bootstrap capacitor Cb at any duty-cycle values of the system are now overcome.
In addition, the system performance is improved by that the energy expended to charge the capacitor Cb is now minimized.
A second embodiment of the logic circuit 11 associated with the second power element will now be described with reference to the example of FIG. 6. Compared to the previously described embodiment, this embodiment is no optimum as far as optimizing the system efficiency is concerned, yet does overcome the problem of discharging the bootstrap capacitor.
Thus, the second embodiment further overcomes the problem of charging the capacitor Cb at values of the system duty cycle close to 100%.
Compared to the previously described embodiment, the bootstrap capacitor Cb is not charged in an optimum manner because neither the signal MASK nor the delay blocks Delay1 and Delay2 for controlling the power element T2 are utilized. This embodiment only uses two signals: OUT_COMP and OSC_scarica.
These signals are each applied to a respective input of a logic gate 20, preferably a NAND gate. The output of the logic gate 20 is connected directly to the input of the level shifter 14.
The signal OSC_scarica controls discharge of the sawtooth that, in switching power supplies of this type, is necessary in order to produce fixed-frequency PWM.
As the signal OUT_COMP goes high, this corresponding to the bootstrap voltage being below its designed level, each pulse OSC_scarica will close the power element T2 and hold it closed through the duration of the latter. Thus, the capacitor Cb is charged, and upon attaining steady state, OUT_COMP will go to a logic low. As a result, the following pulses OSC_scarica are disabled, thereby enabling the power element T2 by short pulses only when required. Although efficiency is not optimized in this way, a significant step toward maximization is made.

## Claims

1. A switching power supply circuit for optimum capacitor charging, wherein a power device is coupled to a first terminal node of a capacitor to be charged and has a control terminal coupled to the output of an associated drive circuit having a supply node coupled to a second terminal node of the capacitor, wherein the switching power supply circuit comprises a second power element being associated with said power device, coupled to said first terminal node, and provided with a control terminal which is connected directly to an output of a respective drive logic circuit, and wherein the drive logic circuit turns on said second power element in response to a voltage below a predetermined minimum being present at the second terminal node of the capacitor, thereby pulling the voltage at said first terminal node to ground and further charging the capacitor.

2. A power supply circuit according to claim 1, wherein said second power element is a MOS power transistor having a drain terminal coupled to said first terminal node.

3. A power supply circuit according to claim 1, wherein said drive logic comprises:
an output buffer stage connected directly to the control terminal of the second power element; 
a level-shift circuit arranged to raise the voltage to the buffer stage; and 
a logic network driving the level-shift circuit.

4. A power supply circuit according to claim 3, wherein said logic network comprises:
a set of logic gates; 
at least one pair of delay blocks; and 
a storage element.

5. A power supply circuit according to claim 3, wherein said logic network has four signal inputs.

6. A power supply circuit according to claim 3, wherein said logic network comprises a two-input NAND gate.

7. A power supply circuit according to claim 1, wherein said capacitor to be charged is a bootstrap capacitor of a voltage booster.

8. A power supply circuit comprising:
a bootstrap capacitor coupled between a load terminal and a voltage supply terminal; 
a first power circuit coupled to the load terminal and operable to supply power to the load terminal; and 
a second power circuit coupled to the load terminal and operable to charge the bootstrap capacitor in response to the voltage across the capacitor falling below a predetermined threshold voltage.

9. The power supply circuit of claim 8 wherein the first power circuit comprises a MOS transistor having a source terminal coupled to the load terminal and a drive circuit coupled to the voltage-supply terminal and operable to drive the MOS transistor.

10. The power supply circuit of claim 8 wherein the second power circuit comprises a MOS transistor having a drain terminal coupled to the load terminal.

11. The power supply circuit of claim 8 wherein the second power circuit comprises:
a transistor having a drive terminal coupled to the load terminal and having a control terminal; 
an output buffer stage coupled to the control terminal of the transistor; 
a level-shift circuit operable to raise the voltage to the buffer stage; and 
a logic network operable to drive the level-shift circuit.

12. The power supply circuit of claim 11 wherein the logic network further comprises:
a set of logic gates; 
at least one pair of delay circuits coupled to the set of logic gates; and 
a storage element coupled to the at least one pair of delay circuits.

13. The power supply circuit of claim 12 wherein the set of logic gates comprises four signal input nodes.

14. The power supply circuit of claim 12 wherein the set of logic gates comprises two-input NAND gates.

15. The power supply circuit of claim 8 further comprising a comparator operable to compare the voltage across the bootstrap capacitor to the threshold voltage, the comparator generating an output signal for turning on the second power circuit when the voltage across the bootstrap capacitor falls below the threshold voltage.

16. A method comprising:
driving a load with a power circuit; 
driving the power circuit with a boosted voltage from a bootstrap capacitor; 
monitoring a voltage across the bootstrap capacitor; and 
charging the bootstrap capacitor in response to the voltage across the bootstrap capacitor being lower than a first predetermined threshold voltage.

17. The method of claim 16 wherein charging the bootstrap capacitor comprises charging the bootstrap capacitor when the power circuit is inactive.

18. The method of claim 16 wherein charging the bootstrap capacitor comprises charging the bootstrap capacitor when a voltage at a load-driving node is greater than a second predetermined threshold.

19. The method of claim 16 wherein charging the bootstrap capacitor comprises charging the bootstrap capacitor when a voltage at a load-driving node is less than a second predetermined threshold.

20. The method of claim 16, further comprising halting the charging of the bootstrap capacitor in response to the voltage across the bootstrap capacitor exceeding a second predetermined threshold.

21. The method of claim 16, further comprising halting the charging of the bootstrap capacitor after a predetermined time.

22. The method of claim 16 wherein charging the bootstrap capacitor comprises charging the bootstrap capacitor a predetermined time after the power circuit becomes inactive.