Circuit and method for producing an average output inductor current indicator

In one implementation, a circuit for producing an average output inductor current indicator in a voltage converter is configured to start a counter when a high side power switch turns on, to sense a sample current through an output inductor of the voltage converter after the high side power switch turns off and when a low side power switch is on, and to register a first count of the counter when the low side power switch turns off. The circuit is further configured to register a second count of the counter when the high side power switch subsequently turns on, and to produce the average output inductor current indicator based on the sample current and the first and second counts of the counter.

BACKGROUND

Background Art

Switched-mode power converters are used in a variety of electronic circuits and systems requiring conversion of a direct current (DC) input to a lower, or higher, DC output. For example, a switched-mode power converter may be implemented as a voltage converter, such as a buck converter, to convert a higher voltage DC input to a lower voltage DC output for use in low voltage applications in which relatively large output currents are required.

In switched-mode voltage converters that include an output inductor coupled between a switch node and the voltage converter output, it is sometimes necessary or desirable to measure the average output inductor current. Conventional approaches to measuring the average output inductor current typically utilize the DC resistance (DCR) of the output inductor itself, or a sampling resistor placed in series with the output inductor. However these conventional approaches are associated with significant disadvantages. For example, the output inductor DCR is not constant, and can vary with temperature as well as from part to part. Moreover, use of a series sampling resistor results in undesirable power losses, as well as typically requiring low pass filtering of the sampled signal. Although a technique for measuring average output inductor current that avoids some of these disadvantages has been developed, that technique is suitable only for switched-mode voltage converters operating in continuous conduction mode (CCM). Thus, there is a need in the art for a solution enabling determination of the average output inductor current that omits reliance on either the output inductor DCR or a series sampling resistor for switched-mode voltage converters configured to operate in discontinuous conduction mode (DCM).

SUMMARY

The present disclosure is directed to a circuit and method for producing an average output inductor current indicator, substantially as shown in and/or described in connection with at least one of the figures, and as set forth more completely in the claims.

DETAILED DESCRIPTION

As stated above, in switched-mode voltage converters that include an output inductor coupled between a switch node and the voltage converter output, it is sometimes necessary or desirable to measure the average output inductor current. Conventional approaches to measuring the average output inductor current typically utilize the direct current (DC) resistance of the output inductor itself (DCR), or a sampling resistor placed in series with the output inductor. However these conventional approaches are associated with significant disadvantages. For example, the output inductor DCR is not constant, and can vary with temperature as well as from part to part. Moreover, use of a series sampling resistor results in undesirable power losses, as well as typically requiring low pass filtering of the sampled signal. As further stated above, although a technique for measuring average output inductor current that avoids some of these disadvantages has been developed for switched-mode voltage converters operating in continuous conduction mode (CCM), that solution is unsuitable for discontinuous conduction mode (DCM). It is noted that the aforementioned solution for measuring average output inductor current for voltage converters operating in CCM is disclosed by U.S. patent application Ser. No. 13/338,013, filed on Dec. 27, 2011 and titled “Power Supply Circuitry and Current Measurement.” This patent application is hereby incorporated fully by reference into the present application.

The present application is directed to a circuit and method designed to overcome the deficiencies in conventional approaches to identifying average output inductor current for DCM operation in a switched-mode voltage converter. The present application discloses a novel and inventive circuit and method for producing an average output inductor current indicator that relies neither on the DCR of the output inductor nor a sampling resistor placed in series with the output inductor. As a result, the present application discloses a solution enabling determination of an average output inductor current in a switched-mode voltage converter operating in DCM that provides improved accuracy, reduced cost, and increased efficiency when compared to conventional solutions.

Referring toFIG. 1,FIG. 1shows a diagram of switched-mode voltage converter100including circuit140for producing an average output inductor current indicator, according to one implementation. Voltage converter100includes high side power switch110(Q1), low side power switch120(Q2), and control/drive block101. High side power switch110and low side power switch120may be implemented as silicon or other group IV based power metal-oxide-semiconductor field-effect transistors (MOSFETs), for example. Accordingly, high side power switch110is shown to include drain112, source114, and gate116, while low side power switch120includes drain122, source124, and gate126. According to the implementation shown inFIG. 1, control/drive block101is coupled to gate116of high side power switch110, and to gate126of low side power switch120. As shown inFIG. 1, control/drive block101may be configured to output high side drive signal106to gate116of high side power switch110, as well as to output low side drive signal108to gate126of low side power switch120As further shown inFIG. 1, circuit140is coupled across drain122and source124of low side power switch120. In other words, circuit140has first current sense input142coupled to drain122of low side power switch120, and second current sense input144coupled to source124of low side power switch120. In addition, circuit140is shown to receive high side drive signal106as high drive input146and low side drive signal108as low drive input148, and to produce average output inductor current indicator180as an output.

Also shown inFIG. 1are voltage converter switch node104connecting source114of high side power switch110to drain122of low side power switch120, voltage converter output132, output inductor102coupled between switch node104and voltage converter output132, and output capacitor104. Voltage converter100may be implemented as a buck converter, for example, configured to receive a DC input voltage VINat drain112of high side power switch110and to provide a stepped down output voltage VOUTat voltage converter output132.

It is noted that although voltage converter100may take the form of a buck converter in some implementations, in other implementations, voltage converter100may be configured as a boost converter, or as a buck-boost converter, for example. It is further noted that although high side power switch110and low side power switch120are depicted as silicon or other group IV FETs in the interests of ease and conciseness of description, that representation is merely exemplary. The inventive principles disclosed herein are broadly applicable to a wide range of applications, including switched-mode voltage converters implemented using other group IV material based, or group III-V semiconductor based, power switches. As used herein, the phrase “group III-V” refers to a compound semiconductor including at least one group III element and at least one group V element. By way of example, a group III-V semiconductor may take the form of a III-Nitride semiconductor that includes nitrogen and at least one group III element, such as gallium.

Thus, although inFIG. 1, MOSFETs are used to represent high side power switch110and low side power switch120, in other implementations, other types of power switches, which may be high voltage (HV) power switches, can be used to provide either or both of high side power switch110and low side power switch120. It is noted that HV, when used in reference to a transistor or switch describes a transistor or switch with a voltage range from approximately two hundred volts to approximately twelve hundred volts (approximately 200V to 1200V), or higher. It is also noted that use of the term midvoltage (MV) refers to a voltage range from approximately fifty volts to approximately two hundred volts (approximately 50V to 200V). Moreover, low voltage (LV), as used herein, refers to a voltage range of up to approximately fifty volts (50V).

The types of switches suitable for use as high side power switch110and low side power switch120may include bipolar junction transistors (BJTs), insulated-gate bipolar transistors (IGBTs), and gallium nitride (GaN) or other III-Nitride or group III-V based high electron mobility transistors (HEMTs), for example.

Continuing toFIG. 2,FIG. 2shows a more detailed diagram of exemplary circuit240for producing an average output inductor current indicator suitable for use in voltage converter100, inFIG. 1, according to one implementation. As shown inFIG. 2, circuit240is configured to receive first current sense input242, second current sense input244, high drive input246, and low drive input248, and to produce average output inductor current indicator280as an output. Circuit240corresponds in general to circuit140, inFIG. 1, and may share any of the characteristics attributed to circuit140, above. In addition, first current sense input242, second current sense input244, high drive input246, low drive input248, and average output inductor current indicator280, inFIG. 2, correspond respectively to first current sense input142, second current sense input144, high drive input146, low drive input148, and average output inductor current indicator180, inFIG. 1, and may share any of the characteristics attributed to those corresponding features, above.

As shown inFIG. 2, circuit240includes differential amplifier256having a negative input terminal coupled to first current sense input242by switch252, and having a positive input terminal coupled to second current sense input244. In addition, according to the exemplary implementation shown inFIG. 2, circuit240further includes sample and hold block262coupled to delay258and receiving output260from differential amplifier256. Exemplary circuit240also includes buffer266receiving output264from sample and hold block262and providing output268to counting and calculation block270.

According to the implementation shown inFIG. 2, counting and calculation block270is configured to produce average output inductor current indicator280. In addition to counter276, counting and calculation block270includes clock272providing clock signal274to counter276. Counter276is further configured to receive high drive input246and low drive input248. Also shown inFIG. 2are protection diodes254coupled to switch252of circuit240. It is noted that switch252is configured to be closed by low drive input248. It is further noted that circuit240may be implemented as an integrated circuit (IC) on a single chip or die.

The operation of circuit140/240inFIG. 1/2will be further described by reference toFIGS. 3, 4, and 5.FIG. 3shows flowchart300outlining an exemplary method for producing an average output inductor current indicator, according to one implementation.FIG. 4shows a timing diagram depicting signal traces corresponding to high drive input246and low drive input248, inFIG. 2, as well as an inductor current trace corresponding to an inductor current of output inductor102, inFIG. 1, according to one implementation. Moreover,FIG. 5shows a diagram depicting a solution for determining a peak output inductor current based on a sensed sample output inductor current, according to one implementation. With respect to the method outlined inFIG. 3, it is noted that certain details and features have been left out of flowchart300in order not to obscure the discussion of the inventive features in the present application.

Referring to flowchart300in combination withFIG. 1andFIG. 2, flowchart300begins with starting counter276when high side power switch110of voltage converter100turns on (action310). Referring toFIG. 4,FIG. 4shows high drive trace446corresponding to high drive input246, low drive trace448corresponding to low drive input248, and inductor current (IL) trace402corresponding to a current through output inductor102. In addition,FIG. 4shows THighcorresponding to the on-time of high side power switch110, TLowcorresponding to the on-time of low side power switch120, and T corresponding to the switching period of high side power switch110, i.e., the time interval from the rising edge of high drive trace446at time410to the next rising edge of high drive trace446at time440. Also shown inFIG. 4are first count N1of counter276registered at time430, second count N2of counter276registered at time440, delay time interval TD, and additional times415and420that will be described in greater detail below.

Action310may be performed by counting and calculation block270of circuit240, using clock272and counter276, in response to high drive input246. As shown inFIG. 4, counter276is started at time410when high drive trace446goes high. It is noted that it is desirable for clock272to have a clock frequency substantially greater than a switching frequency of high side power switch110. In other words, the clock frequency of clock272is substantially greater than (1/T). For example, such a high frequency clock may have a clock frequency greater than or equal to approximately ten times the switching frequency (1/T) of high side switch110.

Flowchart300continues with sensing a sample current through output inductor102of voltage converter100after high side power switch110turns off and when low side power switch120is on (action320). Action320may be performed by circuit240at time420shown inFIG. 4. Referring toFIG. 2in combination withFIG. 4, high drive input246goes low and low drive input248goes high at or about time415, resulting in high side power switch110being turned off after an on-time THigh, and low side power switch120being turned on.

Low drive input248going high also causes switch252to close, resulting in first and second current sense inputs242and244being connected to the respective negative and positive input terminals of differential amplifier256. As shown inFIG. 2, delay258of circuit240also receives low drive input248that is now high. Delay258is configured to elapse the delay time interval TD, which is typically a predetermined delay time interval, before delivering, for example, a rising edge of low drive signal248, to sample and hold block262.

After delay time interval TD, sample and hold block262is activated, resulting in sample and hold block262responding to output260of differential amplifier256beginning at time420. As further shown byFIG. 2, output264of sample and hold block262is provided to buffer266, which, in turn feeds output268corresponding to substantially real-time inductor current ILthrough output inductor102, and depicted by inductor current trace402. Thus, circuit240is configured to elapse delay time interval TDafter low side power switch120turns on before sensing the sample current through output inductor102.

Flowchart300continues with registering first count N1of counter276when low side power switch120turns off (action330). Action330may be performed by counting and calculation block270of circuit240. As shown inFIG. 4, low drive trace448goes low at time430, resulting in low drive input248going low, and low side power switch120turning off after an on-time of TLow. It is noted that counter276driven by clock272continues to run after first count N1is registered.

Flowchart300continues with registering second count N2of counter276when high side power switch110subsequently turns on (action340). Action340may be performed by counting and calculation block270of circuit240. As shown inFIG. 4, high drive trace446next goes high at time440, resulting in high drive input246going high, and high side power switch110turning on. The interval between time410and time440corresponding to second count N2of counter276, and also corresponds to the switching period T of high side switch110. It is noted that counter276driven by clock272may be reset and restarted at time440.

Exemplary flowchart300concludes with producing average output inductor current indicator180/280based on the sample current and the first and second counts N1and N2of counter276(action350). Action350may be performed by counting and calculation block270of circuit240. In one implementation, average output inductor current indicator180/280may be a calculated average current of output inductor102as determined by counting and calculation block270. For example, referring toFIG. 4, the average output inductor current, or IL(average), can be determined in principle from the following equation:
IL(average)=IL(peak)*(THigh+TLow)/(2*T);  Equation 1

Unfortunately, in many applications, THighand/or TLowand/or T may be variable or otherwise indeterminable. According to the solution disclosed by the present application, however, and as shown byFIG. 4, the time interval THigh+TLowmay be substituted in Equation 1 by first count N1of counter276, while the switching period T of high side power switch110may be substituted by second count N2. As a result, Equation 1 may be expressed in terms of N1and N2as:
IL(average)=IL(peak)*N1/(2*N2);  Equation 2
In other words, average output inductor current indicator180/280may be produced based on the peak current IL(peak)of output inductor102and the first and second counts N1and N2of counter276.

However, measurement of the peak current IL(peak)of output inductor102may not be practicable. Nevertheless, referring to diagram500, inFIG. 5, it becomes apparent that the peak current IL(peak)of output inductor102can be expressed in terms of the current through output inductor102sampled after delay time interval TDat time420inFIG. 4(hereinafter “sample current” or IL(sample)) as:
IL(peak)=IL(sample)*NLow/(NLow−ND);  Equation 3
where NLowis the number of clock pulses in TLow, NLow=(1−VOUT/VIN)*N1; and ND=the number of clock pulses in TD

Because the clock frequency of clock272, the delay time interval TD, VOUT, and VINare known or knowable, Equation 3 enables determination of peak current IL(peak)of output inductor102based on sample current IL(sample)through output inductor102. Moreover, substitution of Equation 3 into Equation 2 enables determination of the average output inductor current IL(average)based on known or knowable parameters including the sample current IL(sample)and the first and second counts N1and N2of counter276as:
IL(average)=IL(sample)*NLow/(NLow−ND)*N1/(2*N2);  Equation 4
It is noted that because NLowmay be expressed in terms of N1 and the respective input and output voltages VINand VOUTof voltage converter100, average output inductor current indicator180/280may be produced based on VINand VOUT, as well as the sample current IL(sample)and the first and second counts N1and N2of counter276.

Thus, the present application discloses a novel and inventive circuit and method for producing an average output inductor current indicator that relies neither on the DCR of the output inductor nor a sampling resistor placed in series with the output inductor. As a result, the present application discloses a solution enabling determination of an average output inductor current in a switched-mode voltage converter operating in DCM that provides improved accuracy, reduced cost, and increased efficiency when compared to conventional solutions.