Apparatus and method for high-bandwidth opto-coupler interface

The invention is related to opto-couplers, and in particular, to an apparatus and method for an opto-coupler interface in which the photodetector current is provided to a current mirror.

Certain applications may require galvanic isolation. For example, for safety reasons, AC-to-DC switch-mode power supplies typically require galvanic isolation between the AC line voltage and the outputs of the power supply. For the feedback path of the switch-mode power supply, galvanic isolation is typically accomplished by employing an opto-coupler in the signal path. Additionally, opto-couplers may be used for galvanic isolation in applications other than switch-mode power supplies.

An opto-coupler typically includes a light-emitting diode (LED) and a photodetector. A signal may be transmitted optically from the light-emitting diode to the photodetector. The light-emitting diode provides an optical signal (e.g. visible or infrared light) which, in turn, is received by the photodetector.

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings, in which:

FIG. 1 illustrates a block diagram of an embodiment of a power supply;

FIG. 2 shows a block diagram of an embodiment of an opto-coupler and an opto-coupler interface; and

FIG. 3 illustrates a block diagram of an embodiment of the power supply of FIG. 1 , arranged in accordance with aspects of the present invention.

Various embodiments of the present invention will be described in detail with reference to the drawings, where like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the invention, which is limited only by the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the claimed invention.

Throughout the specification and claims, the following terms take at least the meanings explicitly associated herein, unless the context dictates otherwise. The meanings identified below do not necessarily limit the terms, but merely provide illustrative examples for the terms. The meaning of “a,” “an,” and “the” includes plural reference, and the meaning of “in” includes “in” and “on.” The phrase “in one embodiment,” as used herein does not necessarily refer to the same embodiment, although it may. The term “coupled” means at least either a direct electrical connection between the items connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means at least either a single component or a multiplicity of components, either active and/or passive, that are coupled together to provide a desired function. The term “signal” means at least one current, voltage, charge, temperature, data, or other signal. Where either a field effect transistor (FET) or a bipolar junction transistor (BJT) may be employed as an embodiment of a transistor, the scope of the words “gate”, “drain”, and “source” includes “base”, “collector”, and “emitter”, respectively, and vice versa.

Briefly stated, the invention is related to an opto-coupler interface that includes a current mirror and a resistor. The opto-coupler interface is arranged such that a relatively fixed voltage is provided across the photodetector. At one end of the photodetector, the voltage is relatively fixed because the photodetector is coupled to the input of a current mirror. Because the voltage across the opto-coupler is relatively fixed, the opto-coupler interface is a current interface rather than a voltage interface. Current from the photodetector is mirrored by the current mirror to provide an output mirror current. The output mirror current is provided to a resistor such that the resistor provides a voltage that is based on the output mirror current.

FIG. 1 illustrates a block diagram of an embodiment of power supply 100 . Power supply 100 may include opto-coupler interface circuit 120 , pulse modulation circuit 130 , power stage 140 , resistors R 1 and R 2 , error circuit 150 , and opto-coupler 110 . Opto-coupler 110 includes LED D 1 and photodetector Q 1 .

In operation, power stage 140 provides output voltage VOUT from input voltage VIN, based in part on pulse modulation output signal PO.

In one embodiment, voltage divided R 1 /R 2 provides feedback signal FB from output voltage VOUT. Although FIG. 1 illustrates an embodiment of voltage-mode regulation in which signal FB is a feedback voltage that is provided based on output voltage VOUT, in another embodiment, current-mode regulation may be employed in addition to or instead of the voltage-mode regulation.

Error circuit 150 may be arranged to provide error current Ierr based, in part, on feedback signal FB. In one embodiment, current Ierr is substantially proportional to a difference between feedback voltage FB and a reference voltage VREF (not shown).

LED D 1 may be arranged to provide optical signal OPT 1 based, in part, on error current Ierr. Additionally, photodetector Q 1 is arranged to receive optical signal OPT 1 at a base of photodetector Q 1 .

Opto-coupler interface circuit 120 is coupled to photodetector Q 1 at node N 1 and node N 2 . Opto-coupler interface circuit 120 is arranged to provide a fixed voltage drop across photodetector Q 1 (between nodes N 1 and N 2 ) so that the interface is essentially a current interface. Further, opto-coupler interface circuit 120 is arranged to receive a photodetector output current (not shown in FIG. 1 ), which is provided (by opto-coupler 110 ) at either node N 1 or node N 2 . In one embodiment, opto-coupler interface circuit 120 is further arranged to provide a first voltage (not shown in FIG. 1 ) that is based, in part, on the photodetector output current. In this embodiment, pulse modulation input signal PI is based, at least in part, on the first voltage.

Pulse modulation circuit 130 is arranged to provide pulse modulation output signal PO such that pulse modulation output signal PO is modulated based on pulse modulation input signal PI.

FIG. 2 shows a block diagram of an embodiment of opto-coupler 210 and opto-coupler interface 220 . An embodiment of opto-coupler 210 and opto-coupler interface 220 may be employed in an embodiment of power supply 100 of FIG. 1 . However, the invention is not limited to power supplies, and embodiments of opto-coupler interface 220 may be employed in virtually any application in which an opto-coupler is used. Opto-coupler interface 220 may include impedance circuit 260 , and a current mirror including transistors Q 2 and Q 3 . Although FIG. 2 shows transistors Q 2 and Q 3 as n-type transistors with emitters coupled to ground, in other embodiments, transistors Q 1 and Q 2 may be p-type transistors with emitters coupled to VDD, and/or the like.

In operation, opto-coupler interface 220 provides voltage V 2 at node N 1 such that voltage V 2 is a relatively fixed voltage. Voltage V 2 may be a reference voltage, VDD, ground, and/or the like. Also, because transistor Q 2 is arranged in a diode configuration, node N 2 is held at a relatively fixed voltage (e.g. the base-emitter voltage of transistor Q 2 ).

Current mirror Q 2 /Q 3 is arranged to receive photodetector output current IP and to provide output mirror current IM. In one embodiment, current mirror Q 2 /Q 3 is a 1:1 current mirror such that current IM is substantially equal to current IP. In other embodiments, current mirror Q 2 /Q 3 may have a ratio other than 1:1 to provide current gain.

In one embodiment, impedance circuit 260 is arranged to provide first voltage V 1 responsive to current IM. Impedance circuit 260 may be coupled to voltage V 3 . In one embodiment, voltage V 3 is voltage V 2 . In another embodiment, voltage V 2 and voltage V 3 are separate voltages.

If opto-coupler 210 is used in a power supply, control current Ictl may be an error current. However, opto-coupler 210 may be used in other applications, so that control current Ictl may represent any current that is received by LED D 1 .

Although one embodiment of opto-coupler interface 220 is illustrated in FIG. 2 for illustrative purposes, various other embodiment of opto-coupler interface 220 are within the scope and spirit of the invention. For example, although BJTs are illustrated in FIG. 2 , in other embodiments, one or more of the BJTs may be replaced with a different type of transistor, such as a FET, or the like. Also, as previously discussed, in another embodiment, the circuit may be turned “upside down” so that transistor Q 2 and Q 3 are p-type transistors rather than n-type transistors. Additionally, if a current output is desired rather than a voltage output, impedance circuit 260 may be excluded from opto-coupler interface 220 . These embodiments and others are within the scope and spirit of the invention.

FIG. 3 illustrates a block diagram of an embodiment of power supply 300 , which may be employed as an embodiment of power supply 100 of FIG. 1 . Also, opto-coupler interface 320 includes an embodiment of opto-coupler interface 220 of FIG. 2 . Impedance circuit 360 includes resistor R 3 . Pulse modulation circuit 330 includes pulse width modulation (PWM) comparator 331 . Error circuit 350 includes shunt regulator 351 . Power supply 300 further includes voltage source 370 , resistor R 4 -R 5 , a REF pin at node N 1 , and a COMP pin at node N 2 .

In operation, shunt regulator 350 provides error current Ierr based on a difference between feedback voltage FB and a reference voltage. In one embodiment, the reference voltage is 2.5V, and resistors R 1 and R 2 are substantially equal, so that output voltage VOUT is regulated to about 5.0V. In other embodiments, different sets of values may be employed.

In the embodiment illustrated in FIG. 3 , resistor R 3 is arranged to provide voltage V 1 such that voltage V 1 may be substantially given by V 2 −(IM*R 3 ). In one embodiment, resistor R 3 is about 5 kiloOhms.

PWM comparator 331 is arranged to provide signal PO such that the pulse width of signal PO is modulated based on signal PI. Also, the polarity of PWM comparator 331 is such that zero current into pin COMP produces a maximum duty cycle at the gate driver output.

Power supply 300 has a relatively high bandwidth since the limiting pole normally associated with the opto-coupler interface is at a relatively high frequency. The bandwidth-limiting effect normally encountered from the significant capacitance of the opto-coupler is substantially negated. The capacitance at the collector of transistor Q 3 is much less than the capacitance at the emitter of photodetector Q 1 .

In one embodiment, opto-coupler interface 320 is arranged to provide voltage V 2 such that voltage V 2 is a reference voltage of about 5V that is provided to pin REF. In other embodiments, a different voltage level may be provided at pin REF. Pin COMP is coupled to the emitter of photodetector Q 1 to receive photodetector output current IP.

Although one embodiment of power supply 300 is illustrated in FIG. 3 for illustrative purposes, various other embodimenst of power supply 300 are within the scope and spirit of the invention. For example, although BJTs are illustrated in FIG. 3 , in other embodiments, one or more of the BJTs may be replaced with a different type of transistor, such as a FET, or the like. Also, as previously discussed, in one embodiment, the circuit may be turned “upside down” so that transistor Q 2 and Q 3 are p-type transistors rather than n-type transistors, and voltage V 2 is ground. Additionally, if a current output is desired rather than a voltage output, resistor R 3 may be excluded from opto-coupler interface 220 .

Further, although in one embodiment shunt regulator 351 is employed to provide an error current to LED D 1 , in other embodiments, a different type of error circuitry may be employed for providing error current Ierr. Similarly, although an embodiment of circuitry for providing output voltage VOUT from voltage V 1 is illustrated in FIG. 3 for illustrative purposes, in other embodiments, different circuitry for providing output voltage VOUT from voltage V 1 may be employed. Further, as previously discussed, although a power supply circuit is discussed herein for illustrative purposes, the invention is not so limited. Opto-coupler interface 320 may be employed in virtually in application in which an opto-coupler is employed. These embodiments and others are within the scope and spirit of the invention.

The above specification, examples and data provide a description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention also resides in the claims hereinafter appended.