# Patent

## Title
High-efficiency, low-power power supply circuit

## Abstract

A power supply circuit includes a rectifier, a charging circuit, and a storage capacitor. An AC signal is rectified by the rectifier thereby generating a rectified signal VR between a VR node and a GND node. The capacitor is coupled between an output voltage VO node and the GND node. If VR is greater than a first predetermined voltage VP then the VO node is decoupled from the VR node. If VR is below VP then the charging circuit supplies a substantially constant charging current from the VR node, through the charging circuit, to the VO node, and to the capacitor, provided that VO on the capacitor is below a second predetermined voltage VO(MAX) and provided that VR is adequately high with respect to VO. Due to the charging current, the voltage VO on the storage capacitor is restored to the desired second predetermined voltage.

## Background
TECHNICAL FIELD
The described embodiments relate to power supply circuits suitable for supplying small amounts of power in an efficient manner.
BACKGROUND INFORMATION
Multiple types of power supplies exist that can be used to supply a small amount of supply power to an individual integrated circuit. One type of integrated circuit that sometimes needs to be powered is a microcontroller integrated circuit. A typical microcontroller integrated circuit might require only about ten milliamperes of supply current at a low supply voltage of 3.3 volts. A way to provide this small amount of power to the microcontroller integrated circuit at a low cost without wasting a lot of power is desired.
FIG. 1 (Prior Art) is a diagram of a first type power supply circuit 1 usable for this purpose. Power supply circuit 1 is called a linear regulator. A bridge rectifier 2 converts an AC input voltage from a source 15 into a rectified signal VR on node 3. If, for example, the AC input voltage is 120VAC 60 Hz ordinary wall power, then the peak of the rectified signal VR across capacitor 4 may be about +169 volts. If, for example, the AC input voltage is 240VAC 50 Hz wall power, then the peak of the rectified signal VR across capacitor 4 is about +338 volts. If only ten milliamperes at 3.3 volts is needed to supply power to the microcontroller 5, then the linear regulator is quite inefficient. Considering the 240VAC situation, the voltage drop across the linear regulator 6 is approximately 338 volts minus 3.3 volts. At an average current flow of ten milliamperes, approximately 3.3 watts of power is dissipated by linear regulator 6. The power needed is only 33 mW (10 mA multiplied by 3.3 volts). Efficiency of the power supply is therefore about 33 mW/3.3 W, or 0.1 percent efficiency.
FIG. 2 (Prior Art) is a simplified diagram of a second type of power supply circuit 8 usable for the purpose of supplying power to microcontroller 5. The power supply circuit 8 is a flyback converter. The flyback converter 8 is about 80 to 85 percent efficient when in normal steady state operation, but the flyback converter 8 has several problems. First, the flyback converter 8 has relatively expensive and large components such as transformer 9 and output voltage sense circuitry 10-14. Second, the control circuitry 16 and the output voltage sense circuitry 10-14 of the flyback converter 8 must be powered. During initial start up of the flyback converter 8, before the auxiliary winding 17 can supply power to the control circuitry, power is supplied to the control circuitry 16 via resistor 18. Supplying power from the high rectified voltage VR on node 19 through this resistor 18 to a lower supply voltage used to power the control circuitry 16 is inefficient. Efficiency of the switching power supply during initial startup is similar to that of the linear regulator described above.
In some applications, the overall circuit being powered is powered up, and powered down, frequently. When the circuit is in a powered down condition, the flyback converter is not powered and is not switching. Next, the overall circuit is to power up and operate for a short period in a normal operating mode. The switching power supply therefore is powered up, and operates in an initial start up mode until it is operating in regulation, at which point the switching power supply starts operating in its normal operating mode. After only a brief amount of time in normal operating mode operation, the overall circuit is powered down again. The switching power supply is therefore powered down along with the other parts of the overall circuit. Because the period of normal mode operation is so short as compared to the amount of time the switching power supply spends in its initial start up mode, the efficiency of the switching power supply is undesirably low.
SUMMARY
A power supply circuit includes a bridge rectifier, a charging circuit, and a storage capacitor. An AC signal (for example, 120VAC 60 Hz) is received onto two nodes of the bridge rectifier. The bridge rectifier outputs a rectified signal VR onto a VR node and a GND node. In the case of the AC signal being a sinusoidal 120VAC 60 Hz voltage signal, VR may be a full wave rectified signal having a maximum voltage of about +169 volts. The storage capacitor is coupled between an output voltage VO node and the GND node. The charging circuit keeps the VR node decoupled from the VO node (so that no charging current flows to the VO node) if VR is higher than a predetermined first voltage VP. If VR is below the first predetermined voltage, then the charging circuit supplies a substantially constant charging current ICHARGE from the VR node, through the charging circuit, to the VO node, and to the storage capacitor, provided that: 1) the voltage VO on the storage capacitor is below a second predetermined voltage VO(MAX), and 2) VR is adequately higher than VO. In one specific example, VR is adequately higher than VO if VR is 1.5 volts higher than VO. Due to the flow of the charging current, the voltage VO on the storage capacitor increases up to the desired output voltage VO(MAX). The voltage VO on the storage capacitor decreases between times of charging, but the amount of decrease in VO is predetermined to be an acceptable amount for the circuit being driven. In one specific example, VO ripples from a maximum value VO(MAX) of 3.3 volts to a minimum value VO(MIN) of 3.0 volts.
Efficiency of the power supply circuit is high because the storage capacitor is only charged at times when VR is only slightly higher than the voltage VO on the storage capacitor. At other times when VR is much higher than VO there is no charging current flowing.
In one example, the charging circuit includes a switch (for example, a depletion mode n-channel field effect transistor) and a voltage detector circuit. When the depletion mode n-channel field effect transistor (dep-FET) is conductive, a charging current may flow from the VR node, through the dep-FET, to the VO node, and into the storage capacitor. The voltage detector circuit monitors VR on the VR node. If VR is detected to be greater than the first predetermined voltage VP, then the voltage detector circuit increases the VGS of the dep-FET so that the dep-FET is off and remains off. In this way, the voltage detector keeps the charging circuit from conducting a charging current at all times when VR is large.
If, however, VR is determined to be less than the first predetermined voltage VP, then the voltage detector circuit does not disable the dep-FET. Provided that the output voltage VO on the storage capacitor is below VO(MAX), and provided that VR is adequately higher than VO, a substantially constant charging current ICHARGE is made to flow from the VR node, through the dep-FET, to the VO node, and to the storage capacitor. Such pulses of the substantially constant charging current ICHARGE serve to restore the voltage VO on the storage capacitor to its desired VO(MAX) value.
The power supply circuit can be realized in discrete form, or may be realized in integrated circuit form. In one example, the bridge rectifier and the charging circuit are realized in integrated circuit form. The resulting integrated circuit is packaged along with another integrated circuit and a storage capacitor in an integrated circuit package. The power supply circuit supplies power to the other integrated circuit (for example, a microcontroller integrated circuit) and also provides an amount of supply current for use by external off-chip circuits. The package may have a VO terminal and a GND terminal to which an external storage capacitor is coupled. The amount of supply current for use by external off-chip circuits can be taken from the VO terminal or from a lead of the external storage capacitor.
Further details and embodiments and techniques are described in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, where like numerals indicate like components, illustrate embodiments of the invention.
FIG. 1 (Prior Art) is a diagram of a linear regulator.
FIG. 2 (Prior Art) is a simplified diagram of a flyback switching power supply.
FIG. 3 is a circuit diagram of a high-efficiency, low-power, power supply circuit 20 in accordance with one novel aspect.
FIG. 4 is a simplified waveform diagram that illustrates operation of the power supply circuit 20 of FIG. 3.
FIG. 5 is a simplified waveform diagram that shows, in further detail, two times when the charging current ICHARGE flows in the waveforms of FIG. 4. It is to be understood that the waveforms of FIGS. 4 and 5 are idealized waveforms presented for illustrative purposes. The idealized waveforms are inaccurate in certain minor respects. More accurate waveforms can be determined by using the well known circuit simulator SPICE (using an accurate depletion mode FET model) to simulate the actual power supply circuit of interest and/or by fabricating the actual circuit and then testing it.
FIG. 6 is a table that sets forth operational characteristics of the power supply circuit 20 of FIG. 3.
FIG. 7 is a diagram of a packaged microcontroller integrated circuit that includes the power supply circuit 20 of FIG. 3.
FIG. 8 is a flowchart of a method in accordance with one novel aspect.



DETAILED DESCRIPTION
Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings.
FIG. 3 is a circuit diagram of a high-efficiency, low-power, power supply circuit 20 in accordance with one novel aspect. Power supply circuit 20 includes a power supply integrated circuit 21 and a storage capacitor 22. The power supply circuit 20 receives sinusoidal AC wall power from an AC power source 23 and supplies twenty milliamperes of 3.3 volt DC supply current IO to a load, such as for example a microcontroller integrated circuit 24. In one example, AC power source 23 is 120VAC 60 Hz power such as is typically available from a wall socket in a home in the United States. In another example, the AC power source 23 is 240VAC 50 Hz power such as is typically available from a wall socket in European countries.
Power supply integrated circuit 21 includes a bridge rectifier 25 and a charging circuit 26. Charging circuit 26 in turn includes a voltage detector circuit 27, a depletion mode n-channel field effect transistor (dep-FET) 28, three resistors 29-31, and a zener diode 32. Voltage detector circuit 27 includes a comparator 33, a reference voltage generating circuit 34, a voltage divider involving resistors 35 and 36, an N-channel field effect transistor (NFET) 37, and an optional resistor 38. The bridge rectifier 25 and the charging circuit 26 are packaged together in a semiconductor package such as an ordinary injection molded package. The package has terminals 39-42. AC power is received onto integrated circuit 21 from AC power source 23 via terminals 39 and 40. Terminal 39 is coupled to lead or node 53 of bridge rectifier 25. Terminal 40 is coupled to lead or node 54 of bridge rectifier 25. Charging current is supplied from integrated circuit 21 to capacitor 22 via terminal 41 to capacitor lead 51, and via terminal 42 to capacitor lead 52.
In the present example, power supply circuit 20 is to supply an average of twenty milliamperes of supply output current IO at 3.3 volts, where the supply voltage VO between terminals 41 and 42 is to vary by not more than ten percent. The output voltage VO output by the power supply circuit 20 therefore has a maximum value VO(MAX) of 3.3 volts and has a minimum value VO(MIN) of 3.0 volts. In the present example, AC power source 23 supplies a sinusoidal 120VAC RMS voltage signal at 60 Hz. Bridge rectifier 25 receives the 120VAC RMS sinusoidal 60 Hz voltage signal via terminals 39 and 40 and outputs a full wave rectified signal VR onto node VR 43. Node 44 is a ground node. Lead 55 of bridge rectifier 25 is coupled to and is a part of VR node 43. Lead 56 of bridge rectifier 25 is coupled to, and is a part of, GND node 44. The full wave rectified signal VR has a minimum value VR(MIN) of approximately 3.15 volts (as is explained in further detail below) and has a maximum value VR(MAX) of about +169 volts.
Voltage detector circuit 27 detects when VR is above a first predetermined voltage VP, and when VR is detected to be above VP then the voltage detector circuit 27 disables charging circuit 26 so that the VO node 45 is decoupled from the VR node 43. When charging circuit 26 is disabled in this way, charging current does not flow from the VR node, through the charging circuit 26, through the VO node, and to the storage capacitor 22. The magnitude of the first predetermined voltage VP is determined by the ratio of the resistances R1/R2 of the voltage divider 35 and 36.
In the specific example of FIG. 3, the first predetermined voltage VP is 4.8 volts. The non-inverting input lead 46 of comparator 33 is coupled to node 47 whereas the inverting input lead 48 of comparator 33 is coupled to receive a 1.25 volt reference voltage from reference voltage generator 34. The supply voltage lead 49 of comparator 33 is coupled to node 47 whereas the ground lead 50 of comparator 33 is coupled to ground node 44. If voltage VR on VR node 43 is higher than the first predetermined voltage VP, then the voltage on node 47 is greater than 1.25 volts, and comparator 33 drives the voltage on the gate of NFET 37 high. NFET 37 is turned on. NFET 37 becomes conductive and pulls the voltage on the gate of dep-FET 28 down. As a result, the gate-to-source voltage VGS of dep-FET 28 exceeds the 2.5 volt VGS(OFF) of dep-FET 28, and dep-FET 28 is turned off. No charging current therefore can flow from VR node 43, through dep-FET 28, to VO node 45, and into capacitor 22. Dep-FET 28 remains off as long as VR is above the first predetermined voltage VP of 4.8 volts.
If, however, VR drops below the first predetermined voltage VP, then NFET 37 is turned off. Provided that VO on node 45 is lower than a second predetermined voltage VO(MAX) and provided that VR on node 43 is adequately higher than VO on node 45, charging circuit 26 supplies a substantially constant charging current (ICHARGE) from the VR node 43 and onto VO node 45 and to storage capacitor 22. In the present example, storage capacitor 22 has a capacitance of sixty microfarads. If storage capacitor 22 is not fully charged and the voltage VO on storage capacitor 22 is less than VO(MAX), then the charging current ICHARGE will flow to the storage capacitor to increase the voltage on the storage capacitor. If storage capacitor 22 is fully charged to the desired VO(MAX) voltage (in the present example, 3.3 volts), then the VO voltage on VO node 45 is not lower than the second predetermined voltage VO(MAX) and no charging current flows. The magnitude of the second predetermined voltage VO(MAX) is determined by the zener voltage of zener diode 32.
In the specific example of FIG. 3, the second predetermined voltage VO(MAX) is 3.3 volts and VGS(OFF) of dep-FET 28 is 2.5 volts. In the present example, dep-FET 28 is an IXTA6N50D2 depletion-mode field effect transistor available from IXYS Corporation of 1590 Buckeye Drive, Milpitas, Calif. 95035. The zener voltage VZ of zener diode 32 is therefore 0.8 volts. With NFET 37 being off, the voltage on the gate of dep-FET 28 is fixed at 0.8 volts due to the zener diode. As current flows through the dep-FET, the magnitude of the voltage drop VS across RS resistor 31 increases. (VS here is the voltage drop across RS resistor 31, not the absolute voltage on the source of dep-FET 28). The increase in voltage VS serves to increase VGS of the dep-FET 28. Increasing VGS causes the internal resistance of the dep-FET to increase, thereby decreasing current flow through dep-FET. Due to the voltage drop VS across RS resistor 31, and due to the gate voltage on dep-FET 28 being fixed by zener diode 32, current flow through dep-FET 28 remains substantially constant. This substantially constant charging current ICHARGE charges storage capacitor 22.
If after a period of charging the voltage VO reaches 3.3 volts, then the source voltage on dep-FET 28 is large enough with respect to the gate voltage as set by zener diode 32 that dep-FET 28 is turned off. The second predetermined voltage VO(MAX) is equal to the sum of VZ and VGS(OFF).
FIG. 4 is a waveform diagram that illustrates operation of power supply circuit 20 of FIG. 3. In the upper part of the diagram, the thin dashed line 60 represents the incoming 120VAC sinusoidal 60 Hz voltage received from AC source 23. The heavy dashed line 61 represents rectified periodic voltage signal VR on node 43. The maximum value VR(MAX) of the VR signal is +169 volts. The signal VR, at its lowest point, does not drop below 3.2 volts because there is no way for VR node 43 to discharge due to the blocking action of bridge rectifier 25. At most, VR is 0.7 volts lower than VO due to the body diode 62 of dep-FET 28. The anode of body diode 62 is coupled to VO node 45 and the cathode of body diode 62 is coupled to VR node 43.
Accordingly, in steady state operation of the power supply circuit 20, signal VR is always of adequate magnitude to supply power to comparator 33 via supply lead 49. In other embodiments, the supply lead 49 of comparator 33 is connected to VO node 45 rather than to node 47. A signal diode is then provided between the non-inverting input lead 46 of comparator 33 and supply voltage lead 49 of comparator to protect the comparator 33 from overvoltage. The anode of this diode is coupled to the non-inverting input lead 467 and the cathode of this diode is coupled to the supply voltage lead 49 so that the diode can source current from node 47 into capacitor 22.
As indicated in the bottom portion of FIG. 4, NFET 37 is turned on whenever VR is detected to be higher than the first predetermined voltage VP of 4.8 volts. Line 63 represents the output voltage VO. During the times when NFET 37 is on, VO slowly decreases because capacitor 22 is being discharged due to its having to supply microcontroller 24 with supply current. The magnitude of the decrease in VO is determined by the frequency of the AC input voltage waveform, the amount of supply current IO to be supplied to the load, and by the capacitance of storage capacitor 22. In the present example, VO(MIN) is 3.0 volts. During the times when NFET 37 is off, the charging circuit 26 may supply the substantially constant charging current ICHARGE to storage capacitor 22 so that the voltage VO on the storage capacitor will increase up to its maximum value of VO(MAX). The substantially constant charging current ICHARGE in the present example is one ampere.
FIG. 5 is a waveform diagram that shows two times when the charging current ICHARGE flows in further detail. During each low point in VR, there are two intervals of time when the substantially constant charging current ICHARGE flows. At time T1, VR is greater than VP, so NFET 37 is turned on and dep-FET 28 is turned off and VO node 45 is decoupled from the VR node 43. No charging current flows. At time T2, when VR drops to be less than VP, NFET 37 is turned off. Because VO is below the second predetermined voltage VO(MAX) and because VR is adequately higher than VO, dep-FET 28 starts to turn on. By time T3, the charging current has reached its substantially constant value of one ampere. The charging current ICHARGE flows from VR node 43 to VO node 45 and to storage capacitor 22. As explained above, the magnitude of the charging current ICHARGE is substantially constant due to increased charging current flow serving to decrease the on resistance of dep-FET 28. All this time, VR is decreasing. At time T4, VR is so low with respect to VO that the charging circuit can no longer maintain constant current charging. For constant current charging at one ampere, VR must be 1.5 volts higher than approximately VO. The 1.5 volts is the sum of the RDS(ON) of the dep-FET and the resistance of RS, multiplied by one ampere. At time T5, VR is so low with respect to VO that the charging current flow stops entirely. From time T5, to T6, to T7, no charging current flows. The voltage VO on storage capacitor 22 decreases slightly due to capacitor 22 being discharged by the load. At time T7, when VR is again higher than VO, charging resumes. When VR is adequately higher than VO at time T8 (adequately higher considering the specific RDS(ON) of dep-FET 28 at this time), charging continues with the substantially constant charging current ICHARGE. The voltage VO on storage capacitor 22 increases back up to its VO(MAX) value of 3.3 volts. At time T9 the voltage VO on the storage capacitor 22 reaches 3.3 volts. The source-to-gate voltage across the dep-FET is therefore 2.5 volts due to the gate voltage of the dep-FET being fixed by the zener diode at 0.8 volts. Because VGS(OFF) of the dep-FET is 2.5 volts, the dep-FET is turned off at time T9 when VO reaches 3.3 volts. Regardless of whether the storage capacitor 22 is fully charged or not, when VR then increases further and exceeds the first predetermined voltage VP, NFET 37 is turned on again, and the charging circuit 26 is disabled. This occurs at time T10 in the illustrative waveform diagram.
Because storage capacitor 22 is only charged when the magnitude of VR is close to VO, there is not a large voltage drop across the charging circuit 26 during the times when the charging current ICHARGE flows. As a result, power dissipation in power supply circuit 20 is reduced as compared to the power dissipation in a linear regulator of the type set forth in FIG. 1. Power supply circuit 20 includes no inductive components or opto-isolators as compared to the flyback converter of FIG. 2. Such inductive components and opto-isolators are expensive. Moreover, there is no expensive and failure-prone large capacitor attached to the VR node of the bridge rectifier. In one specific example, there is no such capacitor coupled to VR node 43 and there is less than 0.5 microfarads of total capacitance on VR node 43. In a power on reset condition starting with a fully discharged storage capacitor 22, the voltage VO on storage capacitor 22 reaches VO(MAX) relatively quickly in about 90 microseconds. This is acceptable for many microcontroller applications where execution of microcontroller self-initialization routines after power up take about 100 microseconds to complete. In applications where the overall system is being frequently power switched to spend a substantial proportion of its operating time in a start up condition, the power supply circuit 20 is efficient as compared to a switching power supply that exhibits linear low efficiency performance during start up operation.
FIG. 6 is a table that sets forth operational characteristics of power supply circuit 20 of FIG. 3.
FIG. 7 is a diagram of a packaged microcontroller integrated circuit 70. Packaged microcontroller integrated circuit 70 includes a microcontroller integrated circuit die 71, an on-chip capacitor 72, and a power supply integrated circuit die 84. Power supply integrated circuit die 84 is an unpackaged die version of the power supply integrated circuit 21 illustrated in FIG. 3. All three components 71, 72 and 84 are packaged together in a package 83. Package 83 may be any type of package in which a microcontroller is packaged such as an injection molded plastic quad flat pack or DIP package. Terminals 73-77 are terminals of package 83. Terminals 39-42 and 78-82 are integrated circuit terminals such as bond pads. Wire bonds and lead frame details are not shown in the simplified illustration of FIG. 7. Microcontroller integrated circuit 71 consumes about ten milliamperes of the twenty milliamperes of supply current provided by power supply circuit 84, 72 and 22. This ten milliamperes of supply current flows into microcontroller integrated circuit 71 via supply input voltage terminal 81. The remaining ten milliamperes of supply current is available for use by other external circuits.
Although a microcontroller die is set forth as an example of a die that can be powered by the power supply circuit, the packaged device 70 of FIG. 7 may involve another type of circuit in the place of die 71. For example, the packaged device may involve a memory integrated circuit or linear analog circuitry in the place of die 71. In one example, a whole family of AC-powered parts is realized, where each part of the family includes an instance of integrated circuit 84 and on-chip capacitor 72, but where each part includes a different type of integrated circuit in place of the microcontroller integrated circuit 71 of FIG. 7. Parts of the family are provided with intercommunication capabilities such that parts of the family provide add on auxiliary capabilities to the microcontroller member of the family. In addition, power supply integrated circuit 21 is made available separately in its own package so that a printed board level designer can provide integrated circuit 21 and capacitor 22 on the printed circuit board to supply a small amount of supply power to other circuitry on the printed circuit board.
FIG. 8 is a flowchart of a method 100 in accordance with one novel aspect. A full wave bridge rectifier is used to rectify (step 101) an incoming AC voltage signal (for example, a sinusoidal 120VAC 60 Hz voltage) thereby generating a full wave rectified signal VR. The full wave rectified signal VR is present between a VR node and a ground potential GND node. A storage capacitor is provided (step 102) between an output voltage VO node and the ground potential node GND. If VR is greater than VP then the VR node is decoupled (step 103) from the VO node such that no current can flow from the VR node to the VO node. If VR is less than VP then a substantially constant charging current ICHARGE is supplied (step 104) from the VR node and to the VO node provided that VO is below a second predetermined voltage VO(MAX) and provided that VR is adequately larger than VO. The exposition of method 100 in the form of blocks in a flowchart does not indicate that the operations set forth in the various blocks must necessarily be performed in some order. The operations of blocks 101 and 102 are generally being performed continuously. At a given time, VP can only be either larger or smaller than 4.8 volts. Accordingly, although the determining of whether VR is above or below VP may occur continuously, at most one of the operations set forth in blocks 103 and 104 is being performed at a given time.
Although certain specific embodiments are described above for instructional purposes, the teachings of this patent document have general applicability and are not limited to the specific embodiments described above. In some examples, the reference voltage generator for the comparator of the voltage detector circuit is not a bandgap voltage reference but rather is another type of reference such as a zener diode. Although an example is set forth above where the storage capacitor is charged with pulses of a substantially constant charging current, in other examples the pulses of charging current are not pulses having a substantially constant magnitude. The second predetermined voltage VO(MAX) need not be set by a zener diode but rather may be set in other ways. The power supply circuit can be adapted to use a switch other than a depletion mode n-channel field effect transistor. The rectifier need not be a full wave rectifier, but in some embodiments is a half wave rectifier. In some embodiments, the first predetermined voltage VP and/or the magnitude of the substantially constant charging current ICHARGE are software programmable due to resistances R2 and/or RS being digitally adjustable under software control. The software (processor-executable instructions) for adjusting VP and ICHARGE in one example is stored in memory (a processor-readable medium) on microcontroller die 71, and a digital interface is provided on die 84 via which the microcontroller writes control information into die 84. Accordingly, various modifications, adaptations, and combinations of various features of the described embodiments can be practiced without departing from the scope of the invention as set forth in the claims.

## Claims

1. A circuit for charging a capacitor, wherein the capacitor has a first lead coupled to an output voltage VO node, and wherein the capacitor has a second lead coupled to a GND node, the circuit comprising:
a full wave bridge rectifier that supplies a rectified voltage signal (VR) onto a VR node, and that supplies a ground potential GND onto the GND node; and
a charging circuit that: 1) decouples the VO node from the VR node when VR is greater than a first predetermined voltage VP and, 2) supplies a charging current (ICHARGE) from the VR node and onto the VO node when VR is less than VP provided that VO is less than a second predetermined voltage VO(MAX) and provided that VR is greater than VO, wherein the second predetermined voltage VO(MAX) is determined by a Zener voltage of a Zener diode and a source-to-gate voltage of a depletion mode field effect transistor (dep-FET).

2. The circuit of claim 1, wherein the charging circuit comprises:
a zener diode, and wherein the zener diode determines the second predetermined voltage VO(MAX) of the output voltage signal VO.

3. The circuit of claim 1, wherein the charging circuit comprises:
a switch through which ICHARGE flows from the VR node to the VO node; and
a zener diode that determines the second predetermined voltage VO(MAX) of the output voltage signal VO.

4. The circuit of claim 3, wherein the switch is a field effect transistor, and wherein the zener diode is coupled via a resistor to a gate of the switch.

5. The circuit of claim 3, wherein the switch is a depletion mode field effect transistor (dep-FET).

6. The circuit of claim 1, wherein the VR signal has a maximum value VR(MAX) of more than approximately +150 volts, and wherein VR has a minimum value VR(MIN) that is greater than zero volts.

7. The circuit of claim 1, wherein there is less than approximately 0.5 microfarads of capacitance on the VR node.

8. The circuit of claim 1, wherein the charging circuit supplies ICHARGE onto the VO node through a depletion mode field effect transistor (dep-FET), wherein the dep-FET has a body diode, and wherein a cathode of the body diode is coupled to the VR node.

9. The circuit of claim 1, wherein the charging circuit comprises:
a depletion mode field effect transistor (dep-FET) through which the ICHARGE flows from the VR node to the VO node; and
a voltage detector circuit that detects when VR is greater than the first predetermined voltage VP and that causes the dep-FET to be turned off when VR is greater than the first predetermined voltage VP.

10. The circuit of claim 9, wherein the voltage detector circuit comprises:
a reference voltage generator;
a comparator having a first input lead, a second input lead, and an output lead, wherein the first input lead of the comparator is coupled via a voltage divider to the VR node, and wherein the second input lead of the comparator is coupled to receive a reference voltage from the reference voltage generator; and
a switch that receives a control signal from the output lead of the comparator, wherein when the switch is on a current can flow from the gate of the dep-FET to the GND node through the switch.

11. The circuit of claim 1, wherein the charging circuit comprises a comparator, wherein the comparator has a supply voltage input lead, and wherein the supply voltage input lead is coupled to the VR node so that the comparator is powered by the VR node.

12. The circuit of claim 1, wherein the circuit is a power supply integrated circuit, the circuit further comprising:
a first AC input terminal that is coupled to a first lead of the full wave bridge rectifier;
a second AC input terminal that is coupled to a second lead of the full wave bridge rectifier;
a third terminal that is coupled to and is a part of the GND node; and
a fourth terminal that is coupled to and is a part of the VO node, wherein the first, second, third and fourth terminals are terminals of the power supply integrated circuit.

13. The circuit of claim 1, wherein the circuit is a packaged integrated circuit device, wherein the capacitor is a part of the packaged integrated circuit device, the circuit further comprising:
a first AC input terminal that is coupled to a first lead of the full wave bridge rectifier;
a second AC input terminal that is coupled to a second lead of the full wave bridge rectifier;
a third terminal that is coupled to and is a part of the GND node; and
a fourth terminal that is coupled to and is a part of the VO node, wherein the first, second, third and fourth terminals are terminals of the packaged integrated circuit device.

14. The circuit of claim 13, wherein the full wave bridge rectifier and the charging circuit are parts of a first integrated circuit, the circuit further comprising:
a second integrated circuit that is a part of the packaged integrated circuit device, wherein a supply voltage input terminal of the second integrated circuit is coupled to the fourth terminal, and wherein a ground terminal of the second integrated circuit is coupled to the third terminal.

15. The circuit of claim 1, wherein the charging current supplied from the VR node and onto the VO node is a substantially constant charging current.

16. A method comprising:
(a) using a full wave bridge rectifier to rectify AC voltage signals and thereby generating a full wave rectified signal VR, wherein VR is present on a VR node, and wherein VR is a periodic signal and has a cycle, and wherein VR is at times during the cycle greater than a predetermined voltage VP and at other times during the cycle is less than the predetermined voltage VP;
(b) providing a capacitor between an output voltage VO node and a ground potential GND node;
(c) if VR has a voltage greater than VP then decoupling the VR node from the output voltage VO node; and
(d) if VR has a voltage less than the VP then supplying a substantially constant charging current from the VR node and onto the VO node provided that a magnitude of a voltage signal VO on the VO node is below a second predetermined voltage and provided that a magnitude of VR is greater than the magnitude of VO on the VO node, wherein the second predetermined voltage is determined by a Zener voltage of a Zener diode and a source-to-gate voltage of a depletion mode field effect transistor (dep-FET).

17. The method of claim 16, wherein in (d) the substantially constant charging current flows from the VR node, through a depletion mode field effect transistor, and to the VO node.

18. A circuit for charging a capacitor, wherein the capacitor has a first lead coupled to an output voltage VO node, and wherein the capacitor has a second lead coupled to a GND node, the circuit comprising:
a rectifier that supplies a rectified voltage signal (VR) onto a VR node; and
means for decoupling the VO node from the VR node when VR is greater than a first predetermined voltage VP such that no current flows from the VR node to the VO node, and wherein the means is also for supplying a charging current (ICHARGE) from the VR node and onto the VO node when VR is less than VP provided that VO is less than a second predetermined voltage VO(MAX) and provided that VR is greater than VO, wherein the second predetermined voltage VO(MAX) is determined by a Zener voltage of a Zener diode and a source-to-gate voltage of a depletion mode field effect transistor (dep-FET).

19. The circuit of claim 18, wherein the means comprises a depletion mode field effect transistor through which the charging current flows.

20. The circuit of claim 18, wherein the means comprises a switch through which the charging current flows, and wherein the means further comprises a voltage detector circuit that controls the switch to be open when VR is greater than VP.