Linear power supplies are often used in downhole applications to avoid generation of noise associated with switching power supplies.  In high temperature linear supplies it is often necessary to use several pass transistors in parallel to reduce the power dissipated in any single pass transistor.  For power MOSFETs, the threshold voltage decreases with increasing temperature. This fact, coupled with the normal variation of threshold voltage from one transistor to another, produces a current hogging phenomenon in parallel transistors at high temperature, where the transistor with lowest threshold voltage, through positive thermal feedback, carries all or most of the load current, and eventually fails. This paper presents a model of such current hogging. The model can be derived from parameters on the transistor manufacturer's data sheet.  It is possible to reduce the risk of current hogging by including a source resistor in series with each transistor. Using a statistical model of threshold voltage variation among transistors, Monte Carlo simulation can be used to select a proper value of the source resistor given the acceptable risk limits of experiencing a current hogging condition.  This paper presents an example design of a high temperature parallel MOSFET linear supply based on the presented theory and design techniques.

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