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Temperature Sensitive MOSFET

A while back I was asked by a company to look into why one of their products failed to operate properly in cold temperatures.  A simplified version of the circuit is shown below. Two n-channel MOSFETs and a few resistors are wired to form a flip/flop latch circuit.  Each time the pushbutton switch is pressed, the circuit changes state and remains in
that state until another activation of the switch.  The circuit is used in a 3v lithium battery coin cell powered product. The company told me that many of their devices, when placed in a freezer to simulate operation in cold weather, would not change state using the pushbutton switch but worked perfectly at room temperature.

Clearly, the only two active parts in the circuit are the two FETs. The resistors are quite stable with temperature and even if the C1 capacitor value did change its value with temperature, it would not have any effect on the circuit operation.   What could change when the FETs get cold?
A quick review of the data sheet for the chosen FETs tells the story.  The TN0201 device has a wide gate-source threshold voltage specification.  The spec says that the threshold voltage could be as high as 3v, which provides no margin when being powered by a 3v lithium coin cell.  Also, when looking at the temperature Vs threshold voltage curve, the threshold voltage can increase by as much as 20% when the part is really cold.  This sensitivity to temperature is common to all FETs but is often overlooked by the circuit designer.  Also, when cold, the internal resistance of a lithium coin cell increases.  This means that when delivering current in cold weather, the cell voltage can plunge below 3v.  All these factors spell problems with this circuit.

TN0201 Data Sheet Link:  http://www.vishay.com/docs/72671/tn0201k.pdf

What is the solution?  A MOSFET with a lower threshold voltage spec would solve the problem.  A quick look around on the www.digikey.com website produces several replacement candidates.  I chose the BSN20 device, which is an inexpensive part and has a threshold voltage which should always be less than 2v.  This should provide the needed margin to take care of part variations, battery voltage droop and the threshold voltage increase with cold temperature.  When the FETs were replaced with the recommended BSN20 part my client reported that all the products tested at -30C worked perfectly.

BSN20 Data Sheet Link:   http://www.nxp.com/documents/data_sheet/BSN20.pdf

December  2010     Issue 14

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