Current sense apparatus and method using a combination of a simulation and a real sense for a switching mode power converter

[0001] The present invention relates generally to a switching mode DC-to-DC power converter and, more particularly, to a current sense apparatus and method for a switching mode DC-to-DC power converter.

[0002] Switching mode DC-to-DC power converters are widely used in power supply circuits, in which the currents of each phase are accurately sensed, and it is therefore very important to balance the currents between each phase. When a conventional switching mode DC-to-DC power converter desires to generate a current sense signal for the purpose of current balance thereof, typically it utilizes an external sense resistor, such as a power resistor of low resistance additionally connected in series, the conductive resistance of a power component, or the ESR (Equivalent Series Resistance) of an inductor or capacitor. FIG. 1 shows a conventional current sense apparatus 10 for one phase of a switching mode DC-to-DC power converter that has a high-side MOS 102 connected between a high voltage power supply V_in and an output node 106, a low-side MOS 104 connected between the output node 106 and ground, an inductor 108 conected to the output node 106 to derive the output current I_L and output voltage V_o, and both load capacitor 112 and resistor 114 connected to the converter output 116, and for the purpose of current sensing, a sense resistor 110 is inserted between the inductor 108 and converter output 116 for the output current I_L to flow therethrough, in combination with an operational amplifier 118 to amplify the voltage drop across the sense resistor 110 to generate a current sense signal V_IS. However, a DC value proportional to the output current I_L is generated from the DC value of the voltage drop across the sense resistor 110 and thereby introduces a regulation error to the output voltage. Moreover, due to the switching noise resulted from the parasitic noise element in the system, the SNR (Signal-to-Noise Ratio) is very low for the AC componant of the voltage drop across the sense resistor 110, and the error in the slope of the measured inductor current caused by this noise may result in unstability and failure to the power converter. In addition, the sense resistor 110 consumers electric power and subsequently reduces the efficiency of the converter.

[0003] In order to prevent a switching mode DC-to-DC power converter from the above-mentioned problems, an apparatus and method was proposed by U.S. Pat. No. 6,377,032 issued to Andruzzi et al., which simulates the current sense signal using three current sources to approximate or virtualize the real output current of the power converter. In detail, to generate the ripple of the simulated signal, a first current source proportional to the difference between the input and output voltages is used to charge a current sense capacitor to simulate the rising portion of the real signal, and a second current source proportional to the output voltage is used to discharge the current sense capacitor to simulate the falling portion of the real signal. Also, a third current source proportional to the output voltage is used to charge a ramp capacitor and a switch is used to control the charging and discharging of the ramp capacitor to generate a ramp waveform. The ripple and ramp waveforms are then combined to become the current sense signal that is approximately the inductor current of the power converter. However, this circuit is complicated and the current sense signal generated thereof has no physical meaning since it is a virtual signal or one obtained by way of simulations. It is therefore desired a current sense apparatus and method implemented by simpler circuit to generate the current sense signal almost as real as the output current of a switching mode DC-to-DC power converter.

[0004] An object of the present invention is to provide a current sense apparatus and method which generates a current sense signal nearly the same as the real output current for a switching mode DC-to-DC power converter.

[0005] Another object of the present invention is to provide a current sense apparatus and method using a combination of a simulation and a real sense for a switching mode DC-to-DC power converter which generates a current sense signal having physical meaning.

[0006] In a switching mode DC-to-DC power converter, according to the present invention, a high-side transistor is connected between an input voltage and an output node, a low-side transistor is connected between the output node and a reference potential, an inductor is connected to the output node to derive an output voltage and an output current, and a current sense apparatus and method which employs a DC signal generator to measure the current through the low-side transistor to generate a DC signal proportional to the DC componant of the current through the low-side transistor and a ramp signal generator to generate a ramp signal with a slope proportional to the difference between the input and output voltages. The ramp signal generator comprises a current source to generate a charging current proportional to the difference between the input and output voltages to charge a capacitor during each first half cycle of a clock to generate the ramp signal, and a summing circuit is used to combine the ramp and DC signals to generate the current sense signal.

[0012] FIG. 2 shows a block diagram of a current sense apparatus according to the present invention. In a switching mode DC-to-DC power converter, an output stage 20 is connected between an input voltage V_in and ground and provides an output current I_L and output voltage V_o from the converter output 204 through an inductor 202. In the current sense apparatus 21, a DC signal generator 22 receiving a clock DTC is connected to the output stage 20 to measure the output current I_L from the output stage 20 to thereby generate a DC signal V_ISG(DC) proportional to the DC component of the output current I_L, a ramp signal generator 24 also receiving the clock DTC is connected with the input voltage V_in and output voltage V_o to generate a ramp signal V_ISG(ramp) with a slope proportional to the difference between the input voltage V_in and output voltage V_o during each first half cycle of the clock DTC, and a summing circuit 26 is connected with the DC signal generator 22 and ramp signal generator 24 to combine the ramp signal V_ISG(ramp) and DC signal V_ISG(DC) to thereby generate a current sense signal V_ISG which will be approximating the output or inductor current I_L. The DC signal V_ISG(DC) is the lower portion of the waveform 60 shown in FIG. 4, and is proportional to the DC component of the output current I_L since it is obtained by measuring the output current I_L. Furthermore, it is known that the rising portion of the output current I_L is proportional to the difference between the input voltage V_in and output voltage V_o, and consequently, the rising portion of the output current I_L can be simulated, as shown by the waveform 59 in FIG. 4. For signal control, it is not necessary to simulate the falling portion of the signal, so that there is no simulation circuit for the falling portion of the signal. As a result, both cost and complexity of the circuit are reduced. Moreover, since the DC component of the output current I_L is directly measured, the signal has physical meaning.

[0013] FIG. 3A shows an embodiment circuit of the current sense apparatus of FIG. 2. The circuit comprises a high-side MOS 402 connected to the input voltage V_in and a node 406, a low-side MOS 404 connected to the node 406, an inductor 412 connected between the node 406 and converter output 410, a DC signal generator 42 connected to the low-side MOS 404, a ramp signal generator 44 connected to the input voltage V_in and output voltage V_o, and a summing circuit 46 with two positive inputs 462 and 464 connected to the DC signal generator 42 and ramp signal generator 44, respectively. The ramp signal generator 44 has a summing circuit 442 with a positive input 4422 connected to the input voltage V_in and a negative input 4424 connected to the output voltage V_o to generate the difference therebetween, a transconductive amplifier 444 to transform the difference to a charging current I_r to charge a capacitor 48, a switch 446 arranged between the transconductive amplifier 444 and capacitor 48, and another switch 448 connected in parallel with the capacitor 48. The clock DTC is employed to control the switches 446 and 448, which connects the transconductive amplifier 444 to the capacitor 48 and opens the switch 448 during the first half cycles of the clock DTC for the charging current I_r to charge the capacitor 48 to thereby generate the ramp signal V_ISG(ramp) with a slope proportional to the difference (V_in−V_o) from an output node 450. During the last half cycles of the clock DTC, the clock DTC disconnects the connection between the transconductive amplifier 444 and capacitor 48 to stop charging the capacitor 48 and closes the switch 448 to discharge the capacitor 48 for its voltage down to 0 until the next cycle begines, as shown by the waveform 59 depicted in FIG. 4. From FIG. 3, the ramp signal

V_ISG(ramp) has the slope
SLP=I_r/C_r, and
I_r=g_r(V_in−V_o),
so that
SLP=g_r(V_in−V_o)/C_r,
where C_r is the capacitance of the capacitor 48 and g_r is the gain of the transconductive amplifier 444.

[0014] As shown in FIG. 3A, the DC signal generator 42 includes a measurement resistor 426 connected between the low-side MOS 404 and ground, an operational amplifier 422 serving as a measurement circuit to generate a measurement signal V_ISD which is proportional to the current through the low-side MOS 404 by measuring the voltage drop across the measurement resistor 426, and a sample and hold circuit 424 receiving the clock DTC to record and sample the measurement signal V_ISD at each end of the clock DTC to generate a DC signal V_ISG(DC). FIG. 3B is another embodiment circuit of the DC signal generator 42, which also uses the operational amplifier 422 as the measurement circuit to generate the measurement signal V_ISD for the sample and hold circuit 424 to generate the DC signal V_ISG(DC). However, the circuit in FIG. 3B directly measures the voltage drop across the low-side MOS 404 to generate the measurement signal V_ISD, and in this case, the conductive resistance of the MOS 404 is used as the measurement resistor. Back to FIG. 3A, the DC signal
V_ISG(DC)=I_L(DC)×RS_L×K₁,
where I_L(DC) is the DC component of the waveform 50 shown in FIG. 4, RS_L is the resistance of the measurement resistor 426 in FIG. 3A, and K₁ is the gain of the operational amplifier 422. Finally, the summing circuit 46 combines the ramp signal V_ISG(ramp) and DC signal V_ISG(DC) to generate the current sense signal V_ISG which will be approximating the waveform of the output current I_L.

[0015] In FIG. 4, the waveform 50 represents the output current I_L through the inductor 428 in FIG. 3A, the waveform 52 represents the control signal for the high-side MOS 402 in FIG. 3A, the waveform 54 represents the control signal for the low-side MOS 404 in FIG. 3A, the waveform 56 represents the clock DTC in FIG. 3A, the waveform 57 represents the signal V_ISD outputted from the operation amplifier 422 in FIG. 3A, the waveform 58 represents the DC signal V_ISG(DC) outputted from the sample and hold circuit 424 in FIG. 3A, the waveform 59 represents the ramp signal V_ISG(ramp) in FIG. 3A, and the waveform 60 represents the current sense signal V_ISG generated by the summing circuit 46. As it is described, the current sense signal V_ISG is generated by the combination of the DC signal V_ISG(DC) and ramp signal V_ISG(ramp). The DC signal V_ISG(DC) is obtained by measuring the current through the low-side MOS 404 and is thus a real sensed signal, instead of a simulated or virtual signal, i.e., it has physical meaning. On the other hand, even though the ramp signal V_ISG(ramp) is generated by amplying the difference between the input voltage V_in and output voltage V_o, it can be seen as an almost real signal as the output current I_L, since it is well-known that the slope of the output current I_L is simply proportional to the difference between the input voltage V_in and output voltage V_o. In addition, by the advanced priciple of the present invention, the current sense apparatus and method for a switching mode DC-to-DC power converter becomes much simpler.

[0016] While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.