BACKGROUND

[0001] Battery powered devices often include electronic safeguards to prevent damage to the internal electronics in the event of reverse battery installation, accidental short circuiting, or other inappropriate operation. Such electronic safeguards ensure that any reverse current flow and reverse bias voltage is low enough to prevent damage to either the battery itself or the device's internal electronics. One front-end component that manufacturers often employ to effect reverse battery protection is the series diode. In higher power systems (e.g., two amps or higher), the voltage drop across a series diode can cause excessive amounts of power dissipation. More recently, field-effect transistors have been employed to implement reverse battery protection. The most recent metal-oxide-semiconductor field-effect transistors (MOSFETs) have very low resistance and are therefore ideal for providing reverse current protection with minimal loss. However, the use of a FET to protect the battery during reverse current situations involves the use of a controller to regulate the voltage drop across the FET, and the controller itself consumes marginal power.

SUMMARY

[0002] In described examples, a circuit includes a transistor and a controller. The transistor has first, second and third terminals. A voltage level at the first terminal controls in part a current flow from the second terminal to the third terminal. The controller receives a voltage existing across the second and third terminals of the transistor and uses that voltage to power components of the controller. The controller provides a voltage to the first terminal of the transistor. The controller regulates the voltage across the second and third terminals of the transistor by regulating the voltage provided to the first terminal.

[0003] In further examples, a circuit includes a transistor and a controller. The transistor has first, second and third terminals. A voltage level at the first terminal controls in part a current flow from the second terminal to the third terminal. The controller includes a voltage converter and a control circuit. The voltage converter receives a voltage existing across the second and third terminals of the transistor and converts that voltage to a power supply voltage that is a higher voltage than the received voltage. The control circuit is powered by the power supply voltage. The control circuit receives the voltage existing across the second and third terminals of the transistor and provides a control voltage to the first terminal of the transistor. The control circuit regulates the voltage across the second and third terminals of the transistor by regulating the control voltage provided to the first terminal.

[0004] Described examples include a method of controlling a transistor including first, second and third terminals, in which a voltage level at the first terminal controls in part a current flow from the second terminal to the third terminal. According to the method, a voltage existing across the second and third terminals of the transistor is received. The voltage existing across the second and third terminals of the transistor is used to power a control circuit. The control circuit generates a control voltage based on the voltage existing across the second and third terminals of the transistor. The control circuit provides the control voltage to the first terminal of the transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a schematic circuit diagram of a circuit that includes a battery powering a load and that employs a transistor serving as reverse-current protection.

[0006] FIG. 2 is a schematic circuit diagram of an NFET transistor and an associated controller.

[0007] FIG. 3 is a flow chart representing a method of controlling a transistor.

DETAILED DESCRIPTION OF EXAMPLE EMB ODEVIENT S

[0008] Example embodiments are described with reference to the drawings, wherein like reference numerals are used to designate similar or equivalent elements. Some acts or events may occur in different order and/or concurrently with other acts or events. Furthermore, some illustrated acts or events may not be required to implement a methodology in accordance with this disclosure.

[0009] Example embodiments are directed generally toward powering a transistor controller using the voltage existing across the terminals of the transistor being controlled. This general concept is described herein with respect to controlling a series protection FET. Aspects of example embodiments can be applied to and implemented in a wide variety of applications.

[0010] FIG. 1 is a schematic circuit diagram of a circuit 100 that includes a battery 110 powering a load 120 and that employs a transistor 130 serving as reverse-current protection. FIG. 1 shows an n-channel FET (NFET) 130 in the power path of the circuit 100, but a p-channel FET could be employed instead of the NFET, with corresponding changes in circuit operation. The reverse-current protection transistor 130 can also be implemented with other transistor types besides field-effect transistors. In circuit 100, the body diode 135 of NFET 130 is oriented in the direction of normal current flow. The NFET 130 allows current flow in the forward direction, i.e., from the source 140 to the drain 145 of NFET 130 (i.e., from the battery 110 to the load 120), and blocks current in the reverse direction, i.e., from the drain 145 to the source 140 of NFET 130. A controller 160 receives inputs from the source 140 and the drain 145 of the NFET 130. The controller 160 generates a control signal, or gate driver signal, based on the drain-to- source voltage V _d s- The controller 160 provides the gate driver signal to the gate 150 of the NFET 130. In this way, the controller 160 regulates the drain-to-source voltage V _dS in a control loop. In an illustrative aspect of example embodiments, the controller 160 also receives its power from the voltage drop across the NFET 130, i.e., from the drain-to-source voltage V _dS .

[0011] FIG. 2 is a schematic circuit diagram of an NFET transistor 230 and an associated controller 260. In FIG. 2, NFET 230 corresponds to NFET 130 of FIG. 1, and controller 260 corresponds to controller 160 of FIG. 1. The controller 260 includes a voltage conversion circuit 265, an oscillator 270, a reservoir capacitor 275, a differential amplifier 280, and an offset voltage source 285. The controller 260 receives its power from the voltage drop across the NFET 230, i.e., from the drain-to-source voltage V _dS . In an illustrative embodiment, the controller 260 receives all of its power from the drain-to-source voltage V _dS , without receiving any additional power from other power sources. The voltage conversion circuit 265 receives the drain-to-source voltage V _dS of the NFET 230 and converts it to a voltage level appropriate for powering components of the controller, such as differential amplifier 280. In certain illustrative embodiments, the voltage converter 265 includes a capacitive charge pump circuit, while in other illustrative embodiments, the voltage conversion circuit 265 includes an inductive voltage boost circuit. The oscillator 270 receives the drain-to-source voltage V _dS of the NFET 230, generates an oscillating signal, and provides the oscillating signal to the voltage conversion circuit 265 to drive the charge pump circuit. The oscillating signal generated by the oscillator 270 can be a square wave, a pulsed DC signal, or any other type of oscillating signal. The reservoir capacitor 275 stores the supply voltage generated by the voltage conversion circuit 265. The reservoir capacitor 275 can be either internal to the controller 260 or can be external. The controller 260 can be implemented in some cases without the reservoir capacitor. The supply voltage generated by the voltage converter 265 is used to power a control circuit, represented in FIG. 2 by the differential amplifier 280 and the offset voltage source 285, that regulates the drain-to-source voltage V _ds of the NFET 230 by controlling the voltage of a control signal provided to the gate 250 of the NFET 230 in a control loop based on the drain-to-source voltage V _dS . In the illustrative embodiment of FIG. 2, the voltage of the offset voltage source 285 is added to the source voltage of the NFET 230, and the summed voltage signal is provided to the non-inverting input of the differential amplifier 280. The voltage level of the offset voltage source 285 corresponds to a target voltage level of the drain-to-source voltage V _dS . The drain voltage of the NFET 230 is provided to the inverting input of the differential amplifier 280. The output of the differential amplifier 280 is provided to the gate 250 of the NFET 230. In this way, the control circuit represented by the differential amplifier 280 and the offset voltage source 285 works to maintain the drain-to-source voltage V _dS at a voltage level substantially corresponding to the target voltage represented by the offset voltage source 285.

[0012] In operation, at startup, because of the body diode orientation of the N-channel FET 230, there will be initial current flowing through the body diode 235 if the load 120 is on. Therefore, even if the controller 260 is not powered, this will not interrupt the flow of current. However, as the current starts to flow through the body diode 235, a voltage will be generated across the source 240 and drain 245 of the NFET 230, specifically the voltage of a PN junction. This voltage can vary from NFET to NFET, and across temperatures, but nominally may be around 0.5V - 0.7V. This voltage is also seen by the controller 260, across the source terminal 240 and the drain terminal 245 of the NFET 230. Using this voltage, the controller 260 will have enough voltage to turn on and startup its own circuitry, namely the voltage conversion circuit 265 and the oscillator 270.

[0013] The voltage conversion circuit 265 uses the body voltage, i.e., the drain-to-source voltage V _ds (0.5V - 0.7V) to generate an internal supply voltage. In the illustrative embodiment of FIG. 2, this internal supply voltage is shown to be approximately 5 V, but this voltage level is merely illustrative, and other internal supply voltage levels can be used. In an illustrative embodiment, the voltage converter 265 generates anywhere from approximately 0.5 mWatts to 10 mWatts. The controller 260 then enters the regulation mode, in which it varies the gate voltage of the NFET 230 in a controlled loop to keep the drain-to-source voltage V _dS at the desired target level. In the illustrative embodiment of FIG. 2, this target voltage is shown to be 50m V, but this voltage level is merely illustrative, and other target Vd _S levels can be used. In an illustrative embodiment, the controller 260 maintains the gate voltage at a level that is just high enough to give sufficient Vd _S to keep the voltage conversion circuit 265 running and generating the internal supply voltage (5V). As mentioned, this internal supply voltage is used to power the differential amplifier 280 and/or other control circuitry.

[0014] The regulated source-to-drain voltage Vd _S of the FET 230 is dependent on the threshold voltage (Vthreshoid) of the NFET device used. Therefore, the regulation self-adjusts when Vthreshoid fluctuates across temperature by varying the gate voltage to match the Vd _S .

[0015] In the illustrative embodiment of FIG. 2, the controller 260 is shown to include a single voltage conversion circuit 265. In an alternative embodiment, the controller 260 may include two separate voltage conversion circuits to accommodate the two different Vd _S levels that exist at start-up and in the regulation mode. For example, the controller 260 can include a first voltage conversion circuit configured to receive a Vd _S of 0.5V - 0.7V at start-up and convert the Vd _S to the level of the internal supply voltage (e.g., 5V), and a second voltage conversion circuit configured to receive a Vd _S of approximately 50mV in the regulation mode and convert that Vd _S to the level of the internal supply voltage. Alternatively a single voltage conversion circuit 265 can be figured to operate at the Vd _S levels of both the start-up and regulation modes. Similarly, while FIG. 2 shows a single oscillator 270, in an alternative embodiment the controller 260 can include two separate oscillators to accommodate the two different Vd _S levels that exist at start-up and in the regulation mode.

[0016] FIG. 3 is a flow chart representing a method of controlling a transistor including first, second and third terminals, in which a voltage level at the first terminal controls in part a current flow from the second terminal to the third terminal. At step 300, a voltage existing across the second and third terminals of the transistor is received. At step 310, the voltage existing across the second and third terminals of the transistor is used to power a control circuit. At step 320, the control circuit generates a control voltage based on the voltage existing across the second and third terminals of the transistor. At step 330, the control circuit provides the control voltage to the first terminal of the transistor.

[0017] Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.