Time-optimized Leakage Current Measurement
Original Publication Date: 2003-May-25
Included in the Prior Art Database: 2003-May-25
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The proposal below shows a scheme that could be used to detect a leakage current on an electrical circuit. Without using a second reference node, a leakage current can be detected to any voltage which is lower or higher than the reference value. For example, the scheme could be used to detect leakage a current at the squib of an airbag module. The principle of the measurement is the regulation to a reference voltage (Vref). An example is shown in Fig. 1. In the output stage of the regulator there are two reference current sources (Iref1 and Iref2). For a proper function of the regulator the current source 2 (Iref2) has to be higher than the current source 1 (Iref1). The output transistor can be a MOS (Mean Opinion Score) or a bipolar transistor. There are two possibilities to detect a leakage current with this circuit. On the first hand there is the leakage to a lower voltage than the reference. If the leakage current is higher than the reference current 1 (Iref1) the regulator will switch off the output transistor (M1) of the regulator. As a result the output voltage node VMx will become lower than Vref. The resistance of the leakage to a lower voltage is determined by (Rleak = (Vref – VMx) / Iref1 ). On the second hand there is the leakage to a higher voltage than the reference. If the leakage current on node Mx is higher than the reference current 2 (Iref2) in the output stage, the regulator is not able to regulate the reference voltage Vref anymore (M1 is switched on). The resistance of the leakage to a higher voltage is determined by (Rleak = (VMx – Vref) / Iref2 ). Due to the diode (D1) the circuit is protected against over-voltage at the current source Iref1 . All devices except of diode 1 (D1), transistor 1 (T1) and resistor 1 (R1) can be implemented as low voltage devices. The voltage on pin VMx can be reported to the Analog to Digital Converter (ADC) of a micro-controller via an output buffer, or it can be given as an input to a comparator.