The present invention relates to systems and methods for estimating switching supply load current.
The capability to dynamically measure the power consumption of an electronic system is highly desirable. Some of the benefits include: system fault detection (should power exceed a normal range), engineering power savings (during system prototype development), ability to provide accurate battery life estimates (in a battery operated system). Today's modern electronic systems often utilize switching regulators in order to improve the system power efficiency and reduce heat. In applications where large amounts of current are required, external power field effect transistors (FETs) (as opposed to power FETs integrated with the switching regulator) are most economical. One aspect of power estimation requires measuring the output current supplied by these external FETs.
Previous solutions to measuring the output current flowing in external FETs, utilize a sense resistor in series with the output, wherein the voltage drop across the resistor was proportional to the output current. In this circumstance, the output current is measured by sensing this resistor voltage drop. This method works at the expense of lower efficiency, as any voltage across the sense resistor results in a power loss and resulting degradation in the regulator efficiency. Consequently, in order to minimize the power loss, the sense resistor value is kept small which results in poor resolution of low current measurements (due to the low resulting voltage drop across the sense resistor). Also, low value resistors are expensive.
An alternate method for measuring the output current requires the use of specialized output inductors. Most switching regulators utilize external inductors as part of the voltage transformation/regulation loop. It is possible to add a separate set of “turns” around the output inductor which sense the magnitude of the magnetic flux in the inductor. The magnitude of the magnetic flux is proportional to the current in the inductor. Thus, the extra turns provide a means to sense the current flow in the output inductor. While this method does not suffer the efficiency loss of the sense resistor method, it does require the use of a more expensive and non-standard output inductor.
FIG. 1 shows a typical BUCK regulator output power stage. In this regulator design, supply voltage 6 provides power to output power FET Q2 16, which is controlled by “Break Before Make” (BBM) unit 12. Output FET Q2 16 is also connected to output FET Q1 20. Body diodes D1 22 and D2 18 are built into the output power FETs Q1 20 and Q2 16. “FET Control” circuit 10 (shown as a voltage source) generates a switching signal to command the power FETs to turn on and off. BBM units 12-14 insure that under no circumstances are both power FETs 16 and 20 on at the same time, which would cause a large “shoot though” current to flow in the FETs 16 and 20. Thus, for example, if Q2 16 is on and Q1 20 are off then the BBMs 12 and 14 insure that Q2 16 is shut off before Q1 20 is turned on. Because of the BBMs 12 and 14, there is a short period of time, perhaps 20 ns or more, (called the “dead time”) when neither Q1 20 nor Q2 16 is turned on.
The physics of inductor 26 do not allow for instantaneous changes in the current flowing through the inductor. Thus if the power output stage is sourcing current to the load Rload 30 and capacitor C1 28, then even during the short dead time, when both Q1 20 and Q2 16 are off, the load current will continue to flow through inductor 26. During this period of time, the body diode D1 22 will source the current from ground through inductor 26 to the load. When body diode D1 22 is conducting current, the voltage at the PowerSwitch_out 24 will go below ground.