Temperature Compensation Techniques for Inductor Current Sensing
Publication Date: 2002-Dec-16
The IP.com Prior Art Database
In power supply applications, it is often necessary to measure the output current for regulation and protection. In the case of a multi-phase converter, information on current through each phase is also needed to distribute the load current equally among phases, in order to reduce thermal stress on components. The power supply for a high-performance microprocessor (such as Pentium® 4) is typically implemented as a multi-phase synchronous buck converter. The requirements on regulation accuracy, ripple voltage and transient responses are extremely demanding. A technique called Adaptive Voltage Positioning (AVP) is employed to enable tighter voltage regulation band, and to reduce the cost of output capacitors. This is done by defining a “load line” for the converter, which means that the regulated voltage will drift lower when the load current goes higher. Hence this is another reason why it is so important to measure the load current accurately during both steady-state and transient conditions. Commonly used current measurement techniques include: 1. Resistor sensing: measure the voltage drop across a current sense resistor 2. Rdson sensing: measure the voltage drop across drain and source of either high- or low-side MOSFET when it is conducting. 3. Inductor current sensing: measure the DC voltage drop across the internal resistance of the inductor winding. Figure 1: Inductor Current Sensing network Resistor sensing is the simplest and most accurate method, but it incurs extra cost and power loss due to the sense resistor. Rdson sensing is loss-less, but the large spread of on-resistance makes this method too inaccurate. Inductor current sensing method is preferred because it is inexpensive, loss-less, and able to achieve good accuracy. The Problem: A problem with inductor current sensing is that the DC resistance of inductor winding increases with temperature. The thermal coefficient of copper wire is 0.4% per degree C. That is, if the inductor winding’s temperature goes up by 100 degree C, its DC resistance increases by 40%, and so does the DC voltage drop measured. This introduces a large DC-error in current measurement during steady-state (due to higher DC resistance), and an AC-error during transient conditions (due to mismatched time constants between L/R and Rs*Cs). This DC measurement error can be compensated by the use of a simple thermistor network, such as the one shown in figure 2. However, this method cannot take care of the AC error during transient caused by mismatched time constants. Assuming a typical inductor value of 500nH and a DC resistance of 2m ohm, the time constant involved is several hundred microseconds long. That means the output voltage (which is a function of output current due to AVP) may not be correctly regulated for hundreds of microseconds after each load transition. The problem is illustrated in figure 3. Therefore, there is a need for a temperature-compensate network for inductor current sensing, such that accurate current measurement can be obtain during both steady-state and transient conditions.