Avoid the use of MTBF

The use of MTBF (Mean Time Between Failures) is almost always applied wrong. As a producer of a product you would want the product to last beyond the warranty period and long enough to be perceived by the customer as a 'Quality' product. One way of quantifying this is that a product will have X% reliability at Y years with C% confidence.

MTBF is the inverse of the failure rate, NOT the life-time of the product. As example a product or assembly might have a MTBF of 400,000 device-hrs. This does not mean the product life is 400,000hrs (45.6 years)! It means the failure rate is 2.5 per million hours.

 Implicit in the concept of MTBF or it's inverse failure rate is the that the failure rate is constant. Think of the MTBF as meaning that in any given hour the possibility of failure is 1/MTBF. This is applicable during the flat portion of the 'Bath Tub' curve showing failure rate versus time (see figure). Quite often in a product there will be components that have a very low failure rate but have a wear out function that forms the right-hand side of the Bath Tub curve. For example, the product with a 400,000 device-hr MTBF might have a element that wears out at 20,000hrs as example.

One way of analyzing a product is to add up all the failure rates of the individual components. Commonly expressed as failures per billion hours this is referred to as the FIT rate (Failures in Time). For our example a product with a MTBF of 400,000 has a FIT rate of 2500.

Back to our example, a product with an MTBF of 400,000 that is operated 24/7 will have a failure rate of about (1 - exp(-t/MTBF)) in a year, or 2.2% annual failre rate (AFR). Because of the uniform distribution of failures a small sample of the failure rate may result in measured values ranging from 1% to 5%, but over a large population will be 2.2%. In 5 years this would be over 10% failures, or less than 90% reliability. In this light the MTBF of 400,000 no longer seems so good.

When designing a product from the start there should be a reliability target and a pro-forma reliability budget based on the target design. If the reliability target looks like it cannot be met then the architecture of the system needs to be re-thought out or the reliability target changed, understanding the business consequences.

I delivered the first phase of a project to a new client. A passive test fixture for HEMT transistors that must operate at 1000Vdc and 200C continuous operation for 1000's of hours. Not super high tech but I did need to learn some new things to make this work.
The next phase is more complex with a custom embedded oven, temperature controller using a Microchip and data logging to boot with a custom Visual C++ GUI. The data logging will record temperature and Drain leakage current as the device is stressed over time.
I'll update the blog as interesting problems arise on this project.

EMI Reduction of Laser Modulation

The topic discussed here involves the solution of an EMI problem caused by modulating a laser at 40Mhz. In EMI testing there were failures at multiple harmonics of the 40Mhz. The root of the problem is that the laser is a two terminal device and the case is electrically tied to the anode. A simplified LTspice circuit model of the laser driver is shown in Figure 1 showing the stray capacitance.

Figure 1 - Original circuit

The stray capcitance, C1,  from the laser case to the chassis ground allows for a high frequncy AC current path in the chassis of the assembly flowing back to the common ground. This current path forms a large loop which radiates EMI. The option of finding a 3-terminal laser with a grounded case was not available. So instead the circuit was re-designed to mitigate the problem. The choice here was to use the principle of a common mode choke (CMC). The modfied circuit is shown in Figure 2 with the choke driving the laser diode.

Figure 2 - Circuit with CM choke

If the current flowing into the diode equals the current flowing out of the diode the flux changes in the CMC cancel and there is no impediment to the current flow. On the other hand if there is a current imbalance due to current flowing through the stray capacitance of the chassis then there is a differeintial current in the CMC. The differential current see's a high impedance of the choke and the amplitude of the current is diminished. There is no effect on the normal operation of the circuit, i.e., no reduction in the 40Mhz modulation of the laser current.

This solution very effectivley reduced the EMI problem and allowed the equipment to pass EMC testing.