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Title:
START-UP CIRCUIT
Document Type and Number:
WIPO Patent Application WO/2013/027118
Kind Code:
A1
Abstract:
Aspects of the disclosure provide a circuit. The circuit includes a depletion mode transistor coupled to a power supply and a current path coupled with the depletion mode transistor in series to provide a current to charge a capacitor. The current path has a first resistance during a first stage, such as when the circuit initially receives power, and has a second resistance during a second stage when the capacitor is charged to have a predetermined voltage level.

Inventors:
CHUI SIEW YONG (SG)
LI JUN (SG)
Application Number:
IB2012/001813
Publication Date:
February 28, 2013
Filing Date:
July 25, 2012
Export Citation:
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Assignee:
MARVELL WORLD TRADE LTD (BB)
International Classes:
H02H9/00
Foreign References:
EP0621148A21994-10-26
US20020191359A12002-12-19
US20080061634A12008-03-13
EP0423885A11991-04-24
Other References:
None
Download PDF:
Claims:
WHAT IS CLAIMED IS:

1. A circuit, comprising:

a depletion mode transistor coupled to a power supply; and

a current path coupled with the depletion mode transistor in series to provide a current to charge a capacitor, the current path, having a first resistance during a first stage, and having a second resistance during a second stage when the capacitor is charged to have a predetermined voltage level

2. The circuit of claim 1 , wherein the current path further comprises:

a resistive path configured to have the first resistance; and

a switehabie path coupled in parallel with the resistive path, the switchabie path being in an open state during the first stage, and being in a close state during the second stage when the capacitor is charged to have the predetermined voltage level.

3. The circuit of claim 2, wherein the switchabie path includes a transistor and the circuii includes a control circuit that is powered up t turn on the transistor when the capacitor is charged to have the predetermined voltage level

4. The circuit of claim 1, wherein the first resistance is larger than the second resistance.

5. The circuit of claim 1 , wherein the current path further comprises:

a diode configured to limit a current direction to charge the capacitor.

6. The circuit of claim 1. further comprising:

a passive voltage clamp component to restrict a gate voltage of the depletion mode transistor.

7. The circuit of claim 6, wherein the passi ve voltage clamp component includes a diode configured to clamp the gate voltage of the depletion mode transistor with regard to the capacitor voltage.

8. The circuit of claim L wherein the capacitor voltage powers up a control circuit to control a gate voltage of the depletion mode transistor.

9. The circuit of claim 1, wherein the depletion mode transistor is an N-type metal- oxide-semiconductor-field-effect-transistor ( OSFET) having a negative threshold voltage.

10. The circuit of claim .1 , wherein the circuit is in the first stage when the circuit initiall receives power.

11. A method, comprising:

charging a capacitor through a depletion mode transistor and a current path having a first resistance, and

powering up a control circuit with, a capacitor voitage and then changing the current path to have a second resistance when the capacitor voltage is at a predetermined voltage level

12. The method of claim 1 1 , wherein powering up the control circuit with the capacitor voltage and then changing the current path to have the second resistance when the capacitor voltage is at the predetermined voltage level further comprises:

powering up the control circuit with the capacitor voltage to switch a path from an open state to a close state when the capacitor voltage is at. the predetermined voltage level.

13. The method of claim 12, wherein powering up the control circuit with the capacitor voltage to switch the path .from the open state to the close state when the capacitor voltage is at the predetermined level further comprises:

powering up the control circuit to turn on a transistor in the path when the capacitor voltage is at the predetermined voltage level.

14. The method of claim 1 1 , wherein powering up the control circuit with the capacitor voltage and then changing the curreni path to have the second resistance further comprises:

powering up the control circuit to reduce a resistance of the current path from the first resistance to the second resistance.

15. The method of claim 11. further comprising:

limiting a current direction to charge the capacitor with a diode.

16. The method of claim 11 , further comprising:

clamping a gate voltage of the depletion mode transistor with a passi ve voltage clamp component.

17. The method of claim 16, wherein clamping the gate voitage of the depletion mode transistor with the passive voltage clamp component, further comprises:

clamping the gate voltage of the depletion mode transistor with regard to the capacitor vol tage using a diode.

18. The method of claim 11. further comprising:

powering up the control circuit to control a gate voltage of the depletion mode mi^l-oxide'Semic»ttductor~field-'eS?ect~-rattsistor (MGSFET),

19. The method of claim 1 1 , wherein charging the capacitor through the depletion mode transistor and the current path having the first resistance further comprises:

charging the capacitor through the depletion mode transistor and the current path having the first resistance during an initial power receiving stage.

20. An electronic system, comprising:

a rectifier configured to receive and rectify a power supply from an AC power source; and

a circuit comprising:

a depletion mode transistor configured to receive the rectified power supply; and

a current path coupled with the depletion mode transistor in series to provide a current to charge a capacitor, the current path having a first resistance during a first stage, and having a second resistance during a second stage when the capacitor is charged to have a predetermined capacitor voltage.

21. The electronic system of claim 20, wherein

the current path further comprises:

a resistive path configured to have the first resistance; and

a transistor coupled in parallel with the resistive path; and

the circuit includes a control circuit configured to be powered up to turn on the transistor and control a gate voltage of the depletion mode transistor when the capacitor is charged to have the predetermined capacitor voltage.

22. The electronic system of claim 21, wherein the circuit further comprises:

a first diode configured to limit a current direction to charge the capacitor; and a. second diode configured to clamp a. gate voltage of the depletion mode transistor with regard to the capacitor voltage.

23. The electronic system of claim 21 , wherein the control circuit is coupled to the capacitor directl or is coupled to the capacitor indirectly to be powered up.

Description:
START-UP CIRCUIT

INCORPORATION BY REFERENCE

(0001) This present disclosure claims the benefit of U.S. Provisional Application No. 61/525,631, "Current-Limit Protection Circuits for Use with Depletion MOSFET* filed on August 19, 2011, which is incorporated herein by reference in its entirety.

BACKGROUND

10002] The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently naraed inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.

[0003] Some electrical and electronic devices, such as AC to DC power converters, and the like, need a start-up circuit to power up first and provide an output voltage to the rest of the devices.

SUMMARY

(0004) Aspects of the disclosure provide a circuit. The circuit, includes a depletion mode transistor coupled to a power supply and a current path coupled with the depletion mode transistor in series to provide a current to charge a capacitor. The current path has a first resistance during a first stage, such as when the circuit initially receives power, and has a second resistance during a second stage when the capacitor is charged to have a predetermined voltage level.

(0005) In an embodiment, the current path includes a resistive path configured to have the first resistance, and a switchable path coupled in parallel with the resistive path. The switchable path is in an open state during the first stage, and is in a close state during the second stage, in an example, the switchable path includes a transistor and the capacitor voltage powers up a control circuit to turn on the transistor when the capacitor is charged to have the predetermined voltage level.

(0006] According to an aspect, of the disclosure, the current, path includes a diode configured to limit a current direction to charge the capacitor. Further, in an embodiment, the circuit includes a passive voltage clamp component, such as another diode, to restrict a gate voltage of the depletion mode transistor. [0007] According to an embodiment of the disclosure, the depletion raode transistor is an N-type metal-oxide-semiconductor-field-effect-transistor (MOSFET) having a negative threshold voltage., and the capacitor voltage powers up a control circuit to control a gate voltage of the depletion mode transistor.

[0008] Aspects of the disclosure provide a method. The method includes charging a capacitor through a depletion mode transistor and a current path having a first resistance during a first stage, such as an initial power receiving stage, and powering u a control circuit with the capacitor voltage and then changing the current path to have a second resistance when the capacitor voltage is at a predetermined voltage level.

[0009] Aspects of the disclosure provide an electronic system. The electronic system includes rectifier configured to receive and rectify a power supply from an AC power source and a circuit configured to receive the rectified power supply. The circuit includes a. depletion mode transistor configured to receive the rectified power supply, and a current path coupled with the depletion mode transistor in series to provide a current to charge a capacitor. The current path has a first resistance during a first stage, such as when the circuit initially receives power, and has a second resistance during a second stage when the capacitor is charged to have a predetermined voltage level.

JOOlO j Various embodiments of this disclosure that are proposed as examples will be described in detail with reference to the following figures, wherein like numerals reference like elements, and wherein:

10011 Fig. 1 shows an electronic system 100 according to an embodiment of the disclosure;

[0012] Fig. 2 shows a flowchart, outlining a process 200 according to an embodiment of the disclosure; and

[0013] Figs. 3A-3C show waveforms according to an embodiment of the disclosure.

[0014] Fig. 1 shows an electronic system 100 coupled to an energy source 101 according to an embodiment of the disclosure. The electronic system 100 includes a rectifier 103 and a circuit 1 10 coupled together as shown in Fig. 1. [0015] The energy source 101 provides electric energy to the electronic system 100, In the Fig. 1 example, the energy source 101 is an alternating current (AC) voltage supply to provide art AC voltage., such as 110V AC supply, 220V AC supply, and the like, hi addition,, the energy source 1 1 includes suitable elements to adjust the provided electric energy. For example, the energy source 303 includes a dimmer 302 to suitably adjust the amount of electric energy provided to the electronic system 100,

[0016] In an embodiment, the dimmer 302 is a phase angle based dimmer. In an example, the AC voltage supply has a sine wave shape, and the dimmer 302 is a forward-type triode for alternating current (TRIAC) dimmer 102 having an adjustable dimming angle a within [0, %]. Every 1 time the AC voltage V A crosses zero, the forward-type TRIAC " dimmer 10:2 stops firing charges for a dimming angle a. Thus, in each cycle [0, 2rt}, when the phase of the AC voltage V A C is within {0, a] or [π, π+α], the TRIAC voltage VTRI A output from the forward-type

TRIAC dimmer 1.02 is zero; and when the phase of the AC voltage V AC is within [a, it] or [κ Χ, 2π], the TRIAC voltage VTR C follows the AC voltage VAC- Generally, [0, a] and it, ic+ ] are- referred to as non-conduction angle. Similarly, [«. ] and [π- χ, 2 ] are referred to as

conduction angle.

{0017 j According to another embodiment of the disclosure, the dimmer 102 is

implemented in the electronic system 100. For example, the dimmer 102 in the electronic system 100 receives an AC voltage supply having a sine wave shape. The dimmer 102 in the electronic system 100 then adjusts the amount of electric energy provided to the following components, such as a rectifier, based on a conduction angle.

[0018] According to an embodiment of the disclosure, the electronic system 100 is suitably coupled with the energy source 101. in an example, the electronic system 100 includes a power cord that can be manu lly plugged into a wall outlet (not shown) on a power grid, hi another example, the electronic system 100 is coupled to a power grid via a switch (not shown). When the switch is switched on, the electronic system 100 is coupled to the energy source 103 , and when the switch is switched off, the electronic system 100 is decoupled from the energy source 101. According to an aspect of the discl osure, the electronic system 100 can be coupled to the energy source 101 at any time during an AC cycle, thus a voltage supply to the electronic system 100 may have a step voltage change. The step voltage change causes an inrush current into the electronic system 100. [0019] The rectifier 103 rectifies the received AC voltage to a fixed polarity, such as to be positive. In the Fig. 1 example, the rectifier 103 is a bridge rectifier 103. The bridge rectifier 103 receives the AC voltage, generates a rectified voltage VRE C T, and provides the rectified voltage VKK to other components of the electronic system 100, such as the circuit 110 and the like, to provide electric power to the electronic system 100.

J0020] in an embodiment, the circuit 110 is implemented on a single integrated circuit (!C) chip. In another embodiment, the circuit 1 10 is implemented on multiple 1C chips. The electronic system 100 can include other suitable components (not shown), such as a transformer, a Sight bulb, a plurality of light emitting diodes (LEDs), a fan, another circuit, and the like, thai are suitably coupled with the circuit 1 10. In an example, the circuit 1 10 provides control signals to control the operaticnis of the other components. In another example, the circui 1 10 receives feedback signals from the other components indicative of the operations of the other

components, and provides the control signals to control the operations of the other components based on the feedback signals.

[0021] According to an embodiment of the disclosure, the circuit 10 includes a. start-up circuit 120, and a. control circuit 130. The start-u circuit 120 is configured to receive power supply, setup and maintain a voltage V'Oui, and provide the voltage VO T to other circuits, such as the control circuit 130, to enable the operations of the other circuits. The control circuit .130 is configured to generate control signals to control, for example, the start-up circuit 120 after the Start-up to maintain the voltage Vour-

{0022 j Specifically, in an embodiment, the circuit 1 10 has an initial power recei ving stage and a normal operation stage. In an example, when a power cord of the electronic system 100 is plugged in the wall outlet, the start-up circuit 120 starts to receive power supply, and the circuit 1 1 enters the initial power receiving stage. In another example, when a switch is switched on that couples the electronic system 100 with the energy source 101 , the start-up circuit 120 starts to receive power supply, and the circuit 1 1 enters the initial power receiving stage.

{0023] During the initial power receiving stage, the start-up circuit 120 starts to receive power supply and sets up the voltage Voi.fr. in an example, the start-up circuit 120 includes a capacitor 123, and the voltage Voirr is the voltage on the capacitor 123. During the initial power receiving stage, the start-up circuit 120 charges up the capacitor 123. According to an embodiment of the disclosure, the control circuit 130 requires a supply voltage to be larger than a threshold. Thus, in an example, before the voltage VVxrr on the capacitor 123 is charged up to a certain level, the control circuit 130 is unable to provide suitable control signals to the start-up circuit 120, and the start-up circuit 120 is in a self-control operation mode mat the start-up circuit

120 operates without control from other circuits.

[0024] When the voltage VOUT on the capacitor 123 is charged up to the certain level, the voltage VOUT is large enough to enable the operations of the control circuit 130, and the circuit 110 enters the normal operation stage. During the normal operation stage, the control circuit 130 provides suitable control signals to the start-up circuit 120 to control the start-up circuit 120 to suitably charge the capacitor 123 to maintain the voltage Vou ? on the capacitor 123.

10025] In the Fig. I example, the start-up circuit 120 includes a depletion mode transistor

121 (Ml) coupled in series with a current path 122 to charge the capacitor 123, The current path

122 has different resistivity in the initial power receiving stage and in the normal operation stage. In an example, the current path 122 .has a relatively large resistivity in the initial power receiving stage to limit a current passing the depletion mode transistor 121, and has a relatively low resistivity in the .normal operation stage to support running at a relatively higher load.

J0026] The depletion mode transistor 121 is configured to be conductive when control voltages are not available, such as during the initial power receiving stage, and the like. In the Fig. 1 example, the depletion mode transistor 121 is an N-type depletion mode metal-oxide- semiconductor-field-effect-ttansistor (MOSFET) that has a negative threshold voltage, such as negative three-voltand the like. It is noted that the circuit 1 10 can be suitably modified to use a P-type depletion mode MOSFET as the depletion mode transistor 121. Before the circuit 100 enters the initial power receiving stage or at the time when the circuit 100 enters the initial power receiving stage, the gate-to-source and the gate-to-drain voltages of the N-type depletion mode MOSFET ! 21 are about, zero and are larger than the negative threshold voltage, thus an N-type conductive channel exists between the source and drain of the N-type depletion mode MOSFE T 121. The N-type depletion mode MOSFET 121 allows an inrush current to enter the circuit 100 and charge the capacitor 123 at the time when the circuit 100 enters the initial power receiving stage.

[0027] According to an embodiment of the disclosure, the gate voltage of the depletion mode transistor 121 is suitably controlled during the initial power receiving stage and during the normal operation stage. In the Fig. 1 example, during the initial power receiving stage, the gate voltage of the depletion mode transistor 121 is clamped with regard to the capacitor voltage ouT, and during the normal operation stage, the gate voltage of the depletion mode transistor 121 is controlled by a gate control circuit 131 in the control circuit 130.

j002S j In the Fig. 1 example, the current path 122 includes a diode Dl, a first resistor Rl, a second resistor R2, and a transistor M2. These elements are coupled together as shown in Fig. 1. The diode Dl is configured to limit, a current direction to charge the capacitor 123, and avoid discharging the capacitor 123 when the instantaneous voltage of the rectified voltage VRECT is lower than the capacitor voltage VOTJT, for example,

[0029] In the Fig 1 example, the first resistor Rl forms a resistive path, and the transistor M2 forms a switehable path in parallel with the resistive path. When the circuit 100 is in the initial power receiving stage, the switehable path is an open path, and thus the resistive path (e.g., the first resistor Rl) dominates the resistivity of current path 122; and when the circuit. .100 is in the normal operation stage, the switehable path is a short path and dominates the resistivity of the current path 122. In an example, the transistor M2 is an enhance mode transistor, such as an enhance mode P-type MOSFET, configured to have a. suitable threshold voltage. The gate voltage of the enhance mode P-type MOSFET transistor M2 is collectively controlled by the second resistor R2, and a portion of the control circuit 130, such as a current limit control circuit 132 and a transistor M3.

J003O] During the initial power receiving stage, the current limit control circuit 132 is unable to provide suitable control signal to the transistor M3, and the transistor M3 is off and does not conduct current, for example. Thus, there is substantially no current passing through the second resistor R2, and the gate voltage of M2 (voltage at node 124) is about the same as the source voltage (voltage at node 125). The diode Dl limits the current direction in the first resistor Rl , the drain voltage of M2 (voltage at node 126) is lower or about the same as the source voltage (voltage at node 125). Because the gate-source voltage and gate-drain voltage of the enhance mode P-type MOSFET M2 do not satisfy a threshold voltage requirement, thus the enhance mode P-type MOSFET M2 is turned off.

£0031] During the norma! operation stage, the current limit control circuit 132 generates a gate control signal to turn on/off the transistor 3 to control the current path 122. in an example, whe the transistor M3 is turned on to pass a current, the current also passes the second resistor R2, and causes a voltage drop from node 125 to node 124. In an example, the gate control signal to the transistor M3 is suitable configured such thai the voltaae drop is enough to turn on the transistor M2 to provide a much lower resistance path than the first resistor Rl .

Thus, less current flows through the first resistor Rl , and the power consumption by the first resistor Rl is reduced.

[0032] Further, according to an embodiment of the disclosure, the start-up circuit 120 includes a second diode D2 that couples the gate of the depletion mode N-type MOSFET transistor Ml to node 126 that has the voltage The second diode D2 clamps the gate voltage of the depletion mode transistor Ml not to substantially exceed the voltage Vo r-

[0033] Further, the second diode D2, the first diode Dl , and the first resistor l collectively stable the gate-source voltage (V«s) of the depletion mode transistor Ml , and the drain current ID RAJ of the depletion mode transistor Ml during the initial power receiving stage. Specifically, during the initial power receiving stage, i an. example, when the forward voltage drop of the first diode D l and of the second diode D2 are about the same, the gate-source voltage V GS of the depletion mode transistor Ml is substantially equal to the negative of the voltage drop on the first resistor Rl . The configuration of the second diode D2, the first diode Dl , the first resistor Rl and the depletion mode transistor Ml form a feedback loop to stable the drain current l : ) .\:N

J0034) in an example, when the gate-source voltage Vos increases, the dram current I DRAJN also increases. The increased drain current IDR A I increases the voltage drop on the first resistor Rl, and decreases the gate-source voltage. Similarly, when the gate-source voltage Vos decreases, the drain current IORAIN? also decreases. The decreased drain current t i also decreases the voltage drop o the first resistor Rl, and then increases the gate-source voltage. In an example, the resistance of the first resistor Rl is suitably selected to obtain a desired drain current IDRAJN.

(0035j During operation, in an example, when a powe cord of the electronic system 100 is plugged in the wall outlet or when a switch is switched on that couples the electronic system 100 with the energy source 101 , th start-up circuit 120 starts to receive power supply, and the circuit 1 1 enters the initial power receiving stage. During the initial power receiving stage, the electric current enters the start-up circuit 120 to charge the capacitor 123. Because the output voltage Voi v is Sower than a certain level, the control circuit 130 is unable to provide suitable control signals to the start-up circuit 120. Thus, the transistor M2 is turned off, the first resistor Rl dominates the resisti vity of the current ) 122 and limits the inrush current Further, the gate voltage of the depletion mode transistor M l is clamped by the second diode D2 to stable the drain current IDR A I - Thus, during the initial power receiving stage, the start-up circuit 120 limits the inrush current for charging the capacitor 123, and protects the depletion mode transistor Ml from high power dissipation induced failure,

[0036] When the output voltage VOUT is enough to enable the control circuit 1 30 to provide control signals, the circuit 1 10 enters the normal operation stage. During the normal operation stage, the current limit control circuit 132 provides suitable control signal to turn on the transistor M3, and thus tnm on the transistor M2 to reduce current flowing through the first resistor Rl, In addition, the gate control circuit 131 provides suitable control signals to control the gate voltage of the depletion mode transistor Ml in order to control the charging current to the capacitor 123, and maintain the output voltage V r in a certain range in an example,

[0037] It is noted that the circuit 1 10 can be suitably modified, in the Fig. 1 example, the control circuit 1 30 is coupled to the capacitor S 23 to be powered up. In another example, the control circuit 130 is coupled to the node 125 to b powered up.

[0038] Fig. 2 shows a flowchart outlining a process 200 to start u the circuit 100 according to an embodiment of the disclosure. The process starts at S201 , and proceeds to S210.

[0039] At S210, the circui 100 receives a suppiy voltage rectified from an AC power supply and the circuit 100 enters the initial power receiving stage. In an example, when a power cord of the electronic system 1 00 is plugged in the wall outlet or when a switch is switched on that couples the electronic system 100 with, the energy source 101 , the voltage \½cr becomes nonzero, and provides power to the circuit 100. The start-up circuit 120 starts operation,

[0040] At S220, the start-up circuit 120 charges the capacitor 123 through the depletion mode MOSFET transistor 121 , and the first, resistor Rl that limits the charging current during the initial power receiving stage. The capacitor voltage Vonr is provided to other circuits, such as the control circuit 130, and the like, of the circuit 1 0, Because the output voltage VOUT is lower than a certain level, the control circuit 130 is unable to provide suitable control signals to the start-up circuit 120 during the initial power receiving stage. Thus, the transistor M2 is turned off, the first resistor Rl dominates the resistivity of the current path .1 22 and limits the inrush current. [0041] At S230, the gate voltage of the depletion mode transistor Ml is clamped by the second diode D2 to avoid being too high during the initial power receiving stage, and the gate- source voltage V ' os of the depletion mode transistor M l is stabled to limit and stable the drain current IURAIN- The start-υρ circuit 120 limits and stables the inrush current for charging the capacitor 123, and protects the depletion mode transistor 121 from high power dissipation induced failure.

[0042] At S240, when the output voltage Vorr, which is the capacitor voltage, is high enough, the output voltage Voux enables operations of other circui t of the circuit 110, such as the control circuit 130. Then, the control circuit 130 provides control signals to the start-up circuit 120, and the circuit 1 10 enters the normal operation stage.

[0043] At S250, during the normal operation stage, the current limit control circuit 132 provides a suitable control signal to turn on the transistor M3, and thus turn on the transistor M2 to reduce current flowing through the first resistor l .

[0044] At S260, during the normal operation stage, the gate control circuit 13.1 provides a suitable control signal to control the gate voltage of the depletion mode transistor Ml and thus control the drain current IPRAIN to charge the capacitor 123, and maintain the output voltage Voux in a certain range in an example. Then, the process proceeds to $299 and terminates.

[0045] Figs. 3A-3C show waveforms according to an embodiment of the disclosure.

J0046] Fig. 3 A shows waveforms when the second diode D2„ the first resistor Rl , and second resistor R2 and the transistor M2 are not in the start-up circuit 120, and the depletion mode transistor M l charges the capacitor 123 via the first diode D1.

[0047] In Fig. 3A, at about time 0.005 seconds, a switch is switched on and couples the electronic system 100 with the energy source 101, the rectified voltage VRE C T has a significant change, such as a voltage step change from zero to 150V in Fig. 3 A. The voltage change causes a large inrush current IDR A JK;, such as more than 0.6 A, passing the depletion mode transistor 121 to charge the capacitor 123. The capacitor voltage VOUT increases to about 15V in very short time, such as in less than 0.000.1 seconds, in addition, the gate-source voltage of the depletion mode transistor 121 may temperately become very high, such as over 20V. The high gate-source voltage and large inrush current can cause relatively large power dissipation at the depletion mode transistor 121, and damage the depletion mode transistor 121. [0048] Fig. 3B shows waveforms when the second diode D2 is not in the start-up circuit 1 20, and the depletion mode transistor Ml charges the capacitor 123 via the current path 122.

J0049] in Fig. 3B„ at about time 0,005 seconds, a switch is switched on and couples the electronic system 1 00 with the energy source 101, the rectified voltage VRKCT has a significant change, such as a voltage step change from zero to 1 50V in Fig. 3 B. Because of the first resistor Rl, a relatively small drain current !DRAIN, such as about 0.25 A, passes throug the depletion mode transistor 121 to charge the capacitor 1 23. Because the gate voltage of the depletion mode transistor 21 is not suitably controlled in this example, the gate-source voltage Vos of the depletion mode transistor 121 is not stable. The variation of the gate-source voltage Vos causes significant drain current IDRA! variation. The capacitor voltage Voux increases to about 1 5 V in about 0.0003 seconds.

[OOSOj Fig. 3C shows waveforms when the second diode D2 is used in the start-up circuit 120 to clamp the gate voltage of the depletion mode transistor 1 21 , and the depletion mode transistor M l charges the capacitor 1.23 via the current path 122.

[0051] In Fig 3C, at about time 0.005 second, a switch is switched on and couples the electronic system 1 00 with the energy source 101 , the rectified voltage V m has a significant change, such as a voltage step change from zero to 150V in Fig. 3C. Because of the first resistor R l thai limits the inrush current and because of the second diode D2 that clamps the gate voltage of the depletion mode transistor 121 , the gate-source voltage of the depletion mode transistor 121 is relatively stable, and the drain current IPRAI is relatively small and relatively stable, such as about 0.013 A. The capacitor voltage VOUT increases to about 15V in a relatively long tune, such as in about 0.003 seconds,

(0052| While aspects of the present disclosure have been described in conjunction with the specific embodiments thereof that are proposed as examples, alternatives, modifications, and variations to the examples may be made. Accordingly, embodiments as set forth herein are intended to be .illustrative and not limiting. There are changes that may be made without departing from the scope of the claims set forth below.