Why use a 6.2V Zener Over Other Available Voltages in a CCS?

Ver 0

In the table below, I marked what I felt were good values in bold and acceptable value in italics. These data points were taken from the Q3/1988 Motorola "Rectifiers and Zener Diodes Data" book.

In the two Families of zeners shown below,  one is a 1/4W family and the other is a 1/2W family.

Nominal Zener voltage Family_1 1/4 watt Family_2 1/2 watt Family_1 Vr test Family_2 Vr test Family_1 uV/sqrt(Hz) at 250 uA (Table) Family_2 uV/sqrt(Hz) at 250 uA (Curve) Family_1 Zzt @ 250 uA (Table) Family_2 Zzt @ 250 uA (Table) Family_2 Zzt @ 1 mA (Curve) Issues
5.1 1N4625 1N5231B 3.0 2.0 2 1.5 1500 1600 200 Poor Regulation
5.6 1N4626 1N5132B 4.0 3.0 4 4 1400 1600 100 Poor Regulation
6.0 N/A 1N5133B N/A 3.5 N/A 50? N/A 1600 15 Unknown Regulation / Availability
6.2 1N4627 1N5234B 5.0 4.0 5 150? 1200 1000 9 Best compromise
6.8 1N4099 1N5235B 5.2 5.0 40 200? 200 750 9 Fam_1 Noisier by 18 dB!
7.5 1N4100 1N5236B 5.7 6.0 40 400? 200 500 12 Noisy
8.2 1N4101 1N5237B 6.3 6.5 40 700 200 500 15 Noisy
8.7 1N4102 1N5238B 6.7 6.5 40 900 200 600 17 Noisy
9.1 1N4103 1N5239B 7.0 7.0 40 1000 200 600 20 Noisy
10.0 1N4104 1N5240B 7.6 8.0 40 1200 200 600 25 Noisy
11.0 1N4105 1N5241B 8.5 8.4 40 1600 200 600 30 Noisy
12.0 1N4106 1N5242B 9.2 9.1 40 2000 200 600 35 Noisy

In the Family_2 noise curves, it was very difficult to read the values from the curve between 6.0 and 7.0V. This is because the curve is changing so fast. Every time I held the calipers against the curve, I got a different value. So I just used the tabulated curves from Family_1. From the Family_1 curves we see that the noise gets bad fast at 6.8V and above. This says we should use a 6.2V zener or lower for noise reasons.

Using a 6.2V zener in a current source we see by at Q1, R1 will have 5.6V across it. 5uV of noise out 5.6V is 121 dB down (very respectable). 150 uV out 5.6V is 91 dB down, still not too shabby.  

If we used 4V drop for the gate of a FET, 150uV out of 2.2V on R2 is 83 dB down. 83 dB down isn't terrible, but it isn't great either. However, we don't use a single 6.2V zener and the gate of a FET for a CCS, we use two in series. We don't use one 6.2V zener because of regulation and output impedance and the gate threshold variation of the FET.

The zener voltage can change +/- 0.31V for a 5% part. The gate voltage can change more than +0.5 volt between power up and stabilizing in voltage and up to +/- 1V from FET to FET (at room temperature.) I'll use +/- 0.5V to be more reasonable for I've never seen +/- 1V on a FET that wasn't damaged. This means the voltage that sets the current can vary from 1.39V to 3.01V a 1:2.16 initial variation and up to 35% temperature variation. This much variation is not acceptable.

With a transistor and one 6.2V zener, the current variation is more like 1:1.12 and the temperature variation will be closer to 4.5% (for a 60C change). With two 6.2V zeners and a FET, the variation is 1:1.30.

  

From On-Semi HBD854/D
"TVS/Zener Theory and Design Considerations"

Below approximately 5.6V, Zener voltage (DC regulation) degrades.
Below 4.7V to 5V the zener is only at it specified voltage in a narrow range of currents.

In the range of 6 to 9V, Zener AC impedances are at a minimum.
Two 7.5V zeners in series will have half the AC impedance as one 15V.

A diode in series with a 6.2V give 6.7V with a near zero temperature coefficient.

At 6.0 to 7.0V zener noise is at a minimum.
Two 6.2V zeners in series has less than 1/3.5 (-11 dB) the noise as one 12V zener.

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First version before 10-Jan-2005, Last change 10-Jan-2005, updated pictures 8-8-2024. 
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