Zener Diode Failure in a Clipper Application

This is an application where two Zener diodes were placed in series, in a back to back configuration.  They were placed across the primary winding of a transformer used to apply modulation to an AM transmitter.  The modulation was in the form of a single frequency, and the modulation level was not to exceed 30% by specification. The modulation level showed some variation with temperature, so that the diodes were selected to limit the voltage across the transformer primary to ensure that the 30% modulation limit would not be exceeded.

The drive for the transformer/limiter combination was from a low impedance source, adjusted to provide 30% modulation peaks during final test.  The pole mounted transmitter was required to operate in all weather, unsheltered conditions, at any airport in the United States.

The designer, who was an outside consultant, made some assumptions:

1. The peak voltage for 30% modulation was 12 Volts, so two 11 Volt diodes were placed in back to back series configuration where one would provide an 11 volt drop in the reverse direction, and the other would provide a 1 volt (approximately) drop in the forward direction, meeting the 12 Volt requirement;
2. The diodes operate at low dissipation because they are non-conducting, except for excessive peak conditions;
3. The Reliability and Component Engineering functions of the company could be bypassed because they found too many things wrong, and their input cost too much.

Then there was real life:

1. Field returns with discoloration on the circuit boards under the diodes;
2. Field returns with no modulation;
3. Field returns with modulation intermittently greater than 30%;
4. Field returns with modulation intermittently low;
5. By the time the field returns were on the receiving dock, the consultant was long gone.

The returns were turned over to the Failure Analysis lab of the Components Engineering function, and the design was examined by a Reliability Engineer and a Components Engineer.

The diode characteristics were determined to be:

V_Z = 11V ± 5% @ 23 mA and T_J = 25°C;

PD = 1 Watt, maximum;

Temperature coefficient of voltage = +0.06%/°C typical;

V_F = 1.2 V @ 200 mA, maximum.

Assessment:

Since the diode forward voltage drop would be expected to be considerably lower at low current, the diode forward drop could reasonably be assumed to be approximately 1 volt making the total drop of the diode set approximately 12 volts in either polarity.  At first glance, the initial design assumptions appear to be reasonable.

A simple tolerance analysis begins to show the problem.  The ±5% tolerance on the zener voltage equals 550 mV, placing the zener voltage in the range of 10.45V to 11.55V.  If we assume that the diode forward drop remains constant at 1V, the series combination can have a total voltage drop range of 11.45V to 12.55V.

A further complication is the temperature coefficient of the Zener voltage.  The operating temperature range over all US airports is on the order of -55 °C to +55 °C.  The temperature coefficient applies to the value of the zener voltage at the tolerance extremes, yielding two values of Zener voltage at each temperature extreme.

The calculation for the zener voltage over temperature is straight forward:

V_Z(at temp) =  V_Z  + Tempco * V_Z *  ΔTemp

Calculation at low and high tolerance and low and high temperature yields four values:

The forward biased diode is also affected by temperature.  It has a temperature coefficient of voltage of -2mV/°C, which yields a change in the forward voltage drop that is opposite in polarity to the change in the zener diode.

Combining the forward voltage over temperature with the zener voltage over tolerance and temperature yields the clipping voltage:

Based on this, the new clipping voltage range is 11.108V to 12.698V.

Further conditions that affect the temperature range over which clipping begins are internal temperature rise in the box, and direct heating of the box by sunlight.  Considering these would unnecessarily complicate this discussion.

Production test:

In order to reduce costs, the production boxes were only tested at room temperature.  Two engineering units were successfully temperature tested for type approval, and on that basis, full temperature testing was waived on the production units.

Boxes with diodes that had clipping voltages of 12 V and higher sailed through test without problem, since the modulation voltage could be set to peak at 12 V, meeting the 30% modulation requirement.  In the field, boxes that operated at elevated temperatures operated normally, with no diode failures.  These same boxes, when operated at low temperature, depending upon the exact clipping voltage, frequently failed for low modulation because their temperature coefficients forced the clipping voltage below 12 V.  In a few cases, the self-heating of the diodes due to the dissipation from clipping allowed their clipping voltages to reach thermal equilibrium near 12 V, allowing the boxes to operate satisfactorily.  The modulation level was a function of the ambient temperature, causing intermittent failures on some boxes.

Boxes that contained diodes with clipping voltages below 12 V, for the most part, also sailed through test without problem.  This seems unreasonable, since it should not have been possible for those boxes to be set at 12 V.  Here, the positive temperature coefficient of the zener voltage came to the rescue.  As the test technicians increased the modulation toward 30%,  diodes that broke down below 12 Volts began to conduct, thereby warming up, raising their breakdown voltage.  With enough drive, many of the diodes could be driven hard enough to reach thermal equilibrium at 12 Volts.  Typically, most of these diodes were heavily over dissipated.  When these boxes reached the field, they operated for a time, but eventually, the diodes began to get leaky, leading to increased dissipation and subsequent short.  Some of these boxes also displayed intermittent failure of the modulation level, due to the changes in ambient temperature.

Conclusions:

The basic design approach to limiting the modulation level was flawed.  The diode clipper was intended to limit the temperature variability of the modulation source.  Instead, it induced additional temperature sensitivity and a high failure rate.  This approach was deemed to be less expensive than a design where the modulation level was actively sensed, and feedback applied to control the modulation source.

The initial assumptions did not take into account the zener diode voltage tolerance and temperature coefficient.  This is the root cause of all of the failed units returned from the field.  It is interesting to note that whether the symptom was low modulation, high modulation, or burned boards and shorted diodes, the source of the failure is traceable to the same source.  Inattention to and/or lack of understanding of the basic operating parameters of the zener diode was that source.

Lessons learned:

1. Read and understand the datasheet;
2. Bypassing oversight is more expensive in the end;
3. Using engineering units for type approval carries the risk that the units may not represent the production items;
4. Validation of the qualifications of your consultant is paramount to success.