How to Hurdle Overswing and Noise In Low Power Designs – Industry Articles

Zener diodes generate a relatively stable voltage (V_Z) when operated in reverse bias, a useful feature that makes these devices popular for providing voltage stabilization and clamping in various applications.

Zeners are typically specified for a current (I_Z) of 5 mA but the drive to minimize current consumption in battery powered devices like earbuds (Figure 1) has seen Zeners exhibiting an undesirable phenomenon called voltage “overswing” which occurs when they are pushed to the very limits of their physical operating characteristics.

Figure 1. Low current consumption is a key requirement for battery powered devices like earbuds.

In this article, we’ll explore this phenomenon and the problems it causes, explain the reasons why it occurs and show how Nexperia has overcome this issue in two recently released families of Zener diodes designed for general purpose and low power applications.

What Is Zener Diode Noise And Why Is It a Problem?

For a reverse biased Zener diode to generate a stable V_Z it must first enter avalanche mode. This has two requirements: firstly, the field strength (or reverse voltage) must be sufficiently high; secondly, free charge carriers must be present in order for current to flow (I_Z).

At low reverse bias currents (below 0.5 mA) a Zener diode may no longer conduct with a stable DC-current and at very low average currents (I_Z = 50 µA) conduction can come to a complete stop and the voltage across the device can begin to rise—a phenomenon called Zener noise. Excessive overswing prevents effective clamping and even potentially causes damage to other components in an application.

What Causes Zener Noise?

Noise happens because at very low currents, the diode is no longer in avalanche mode and instead starts to behave like a capacitor whose capacitance depends on the size of the active areas and the width of the depletion area in the reverse mode. The time constant of the rising voltage signal is determined by the resistance of the current drive and the diode capacitance (C_d).

After a statistical delay, a new avalanche breakdown occurs and when the field strength becomes sufficiently high, the diode voltage begins to fall rapidly. C_d is then quickly discharged, conduction stops and the cycle repeats.

Measuring Zener Noise in a Real Zener Diode

The noise phenomenon can be reproduced using the basic circuit shown in Figure 2. The Zener diode under test is placed in series with a resistor R_SERIES and a DC supply voltage (V_TEST), whose value exceeds V_Z and the voltage across the diode is measured using an oscilloscope.
 

Figure 2. Set-up for testing voltage noise in Zener diodes

V_TEST must be chosen to be higher than the sum of the nominal V_Z and the maximum expected overshoot voltage. The value of the series resistor to be used can then be calculated from the following formula:

R_SERIES  =  (V_TEST – V_Z(nom) )  /  I_Z 

Figure 3 shows a scope capture of the overswing pulse from a BZX84-B27 Zener diode tested using a series resistance of 430 kΩ and for I_Z  = 50 µA. Using a high-ohmic value series resistor ensures there is sufficient headroom to measure the maximum overswing value.

Figure 3. Noise testing for a BZX84-B27 diode with I_Z  = 50µA and a 430 kΩ series resistor. (Click on image to enlarge)

The test setup previously described is also referred to as “noise” testing but in reality, the waveforms produced do not correspond to normal Gaussian or pink noise. The overswing voltages have a sawtooth shape with random amplitudes and have a Gaussian distribution, which means that the probability of capturing very high outliers is quite small. Testing must be performed for several minutes in order to generate a more complete picture of the maximum deviation from the nominal Zener diode breakdown voltage.

How Noise Phenomenon Relates to Zener Current

Figure 4 shows the relationship between the maximum overswing voltage level versus reverse bias current I_Z, and it is clear that the maximum noise occurs between 20 µA and 60 µA. For extremely small currents, the rise times of the sawtooth signal increase, giving the Zener diode more time for an avalanche event to occur. Above about 500 µA the noise phenomenon no longer occurs and a small degree of residual normal Gaussian noise is evident in the device’s behavior.

Figure 4. BZX84-C24 noise (overswing) as a function of I_Z

As an alternative to ‘noise’ testing, a pulse clamping test can also be performed. In this set-up no (average) bias current is applied but the Zener diodes are subjected to a pulse with an open loop amplitude which exceeds nominal V_Z. Nexperia performs this test using a typical 100 V voltage source and a 10 kΩ resistor in series with the device under test (DUT). Figure 5 shows the worst case overswing for a BZX84-B33 diode clamping a 100 V pulse.

Figure 5. Pulse clamping overswing testing of the BZX84-B33. (Click on image to enlarge)

New Zener Diodes Overcome Overswing and Noise

Nexperia has designed its new series of 50 µA Zener diodes to completely eliminate the noise phenomenon in low power applications. These devices deliver full Zener functionality even at high voltages (up to 17 V) and are available in small footprint packages including SOT23, SOD323, SOD523, SOD123F as well as DFN1006-2, DFN1006BD- DFN (with or without side-wettable flanks).

Overswing has also been eliminated from Nexperia’s new PZU family of Zener diodes (automotive and standard versions) which are available in SOD323, SOD323F, SOT23 and DFN1006BD-2 (2.4 V to 51 V) packages. Figure 6 shows that both of these device families outperform similar competing devices during testing.

Figure 6. Nexperia’s 50 µA and PZU Zener diodes perform better than competing devices at comparable voltages.

These devices now offer engineers a viable alternative for critical applications where a sharper Zener “knee” is required or where Zeners from other manufacturers can present overswing and noise concerns.

All images used courtesy of Nexperia

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