The amount and type of information found on datasheets for zener diodes (or any category of electronic device) varies from one type of diode to the next. The datasheet for some zeners contains more information than for others. Figure 3–7 gives an example of the type of information you have studied that can be found on a typical datasheet. This particular information is for a zener series, the 1N4728A–1N4764A.
A certain zener diode has a maximum power rating of 400 mW at 508C and a derating factor of 3.2 mW/8C. Determine the maximum power the zener can dissipate at a tem- perature of 908C.
Solution PD(derated) =PD-(mW/8C)DT
=400 mW-(3.2 mW/8C)(908C-508C)
=400 mW-128 mW =272 mW
Related Problem A certain 50 W zener diode must be derated with a derating factor of 0.5W/8C above 758C. Determine the maximum power it can dissipate at 1608C.
EXAMPLE 3–3
An 8.2 V zener diode (8.2 V at 258C) has a positive temperature coefficient of 0.05%/8C. What is the zener voltage at 608C?
EXAMPLE 3–2
Solution The change in zener voltage is
DVZ=VZ*TC * DT=(8.2 V)(0.05%/8C)(608C -258C)
=(8.2 V)(0.0005/8C)(358C)=144 mV
Notice that 0.05%/8C was converted to 0.0005/8C. The zener voltage at 608C is VZ+ DVZ=8.2 V +144 mV=8.34 V
Related Problem A 12 V zener has a positive temperature coefficient of 0.075%/8C. How much will the zener voltage change when the junction temperature decreases 50 degrees Celsius?
Notes:
1. Zener Voltage (VZ)
The zener voltage is measured with the device junction in the thermal equilibrium at the lead temperature TL) at 308C ± 18C and 3/8" lead length.
1N4748A 1N4749A 1N4750A 1N4751A 1N4752A
20.9 22.8 25.65 28.5 31.35
22 24 27 30 33
23.1 25.2 28.35
31.5 34.65
11.5 10.5 9.5 8.5 7.5
23 25 35 40 45
750 750 750 1000 1000
0.25 0.25 0.25 0.25 0.25
5 5 5 5 5
16.7 18.2 20.6 22.8 25.1 1N4753A
1N4754A 1N4755A 1N4756A 1N4757A
34.2 37.05 40.85 44.65 48.45
36 39 43 47 51
37.8 40.95 45.15 49.35 53.55
7 6.5
6 5.5
5
50 60 70 80 95
1000 1000 1500 1500 1500
0.25 0.25 0.25 0.25 0.25
5 5 5 5 5
27.4 29.7 32.7 35.8 38.8 1N4758A
1N4759A 1N4760A 1N4761A 1N4762A
53.2 58.9 64.6 71.25 77.9
56 62 68 75 82
58.8 65.1 71.4 78.75
86.1
4.5 4 3.7 3.3 3
110 125 150 175 200
2000 2000 2000 2000 3000
0.25 0.25 0.25 0.25 0.25
5 5 5 5 5
42.6 47.1 51.7 56 62.2 1N4763A
1N4764A 86.45
95 91 100
95.55 105
2.8 2.5
250 350
3000 3000
0.25 0.25
5 5
69.2 76
1N4728A - 1N4764A
Zeners
Absolute Maximum Ratings *
* These ratings are limiting values above which the serviceability of the diode may be impaired.
Electrical Characteristics Ta = 258C unless otherwise noted Ta = 258C unless otherwise noted
Symbol Parameter Value Units
PD Power Dissipation
@ TL < 508C, Lead Length = 3/8" 1.0 W
Derate above 508C 6.67 mW/8C
8C TJ, TSTG Operating and Storage Temperature Range -65 to +200
Device
VZ (V) @ IZ(Note 1) Test Current IZ (mA)
Max. Zener Impedance Leakage Current
Min. Typ. Max. ZZ @ IZ
(V) ZZK(V) @ IZK
IZK (mA) IR
(mA) VR (V) 1N4728A
1N4729A 1N4730A 1N4731A 1N4732A
3.315 3.42 3.705 4.085 4.465
3.3 3.6 3.9 4.3 4.7
3.465 3.78 4.095 4.515 4.935
76 69 64 58 53
10 10 9 9 8
400 400 400 400 500
1 1 1 1 1
100 100 50 10 10
1 1 1 1 1 1N4733A
1N4734A 1N4735A 1N4736A 1N4737A
4.845 5.32 5.89 6.46 7.125
5.1 5.6 6.2 6.8 7.5
5.355 5.88 6.51 7.14 7.875
49 45 41 37 34
7 5 2 3.5
4
550 600 700 700 700
1 1 1 1 0.5
10 10 10 10 10
1 2 3 4 5 1N4738A
1N4739A 1N4740A 1N4741A 1N4742A
7.79 8.645 9.5 10.45
11.4 8.2 9.1 10 11 12
8.61 9.555
10.5 11.55 12.6
31 28 25 23 21
4.5 5 7 8 9
700 700 700 700 700
0.5 0.5 0.25 0.25 0.25
10 10 10 5 5
6 7 7.6 8.4 9.1 1N4743A
1N4744A 1N4745A 1N4746A 1N4747A
12.35 14.25 15.2 17.1 19
13 15 16 18 20
13.65 15.75 16.8 18.9 21
19 17 15.5
14 12.5
10 14 16 20 22
700 700 700 750 750
0.25 0.25 0.25 0.25 0.25
5 5 5 5 5
9.9 11.4 12.2 13.7 15.2 DO-41 Glass case
COLOR BAND DENOTES CATHODE
(
▲ FIGURE 3–7
Partial datasheet for the 1N4728A–1N4764A series 1 W zener diodes. Copyright Fairchild
Semiconductor Corporation. Used by permission. Datasheets are available at www.fairchildsemi.com.
Absolute Maximum Ratings The absolute maximum power dissipation, PD, is speci- fied as 1.0 W up to 508C on the data sheet in Figure 3–7. Generally, the zener diode should be operated at least 20% below this maximum to assure reliability and longer life. The power dissipation is derated as shown on the datasheet at 6.67 mW for each degree above 508C. For example, using the procedure illustrated in Example 3–3, the maximum derated power dissipation at 608C is
PD(derated) =1 W -108C(6.67 mW/8C)=1 W -66.7 mW=0.9933 W At 1258C, the maximum power dissipation is
PD(derated)=1 W -758C(6.67 mW/8C)=1 W -500.25 mW=0.4998 W
Notice that a maximum reverse current is not specified but can be determined from the maximum derated power dissipation for a given value of VZ. For example, at 508C, the maximum zener current for a zener voltage of 3.3 V is
IZM= PD
VZ = 1 W
3.3 V =303 mA
The operating junction temperature, TJ, and the storage temperature, TSTG, have a range of from -658C to 2008C.
Electrical Characteristics The first column in the datasheet lists the zener type num- bers, 1N4728A through 1N4764A.
Zener voltage, VZ, and zener test current, IZ For each device type, the minimum, typi- cal, and maximum zener voltages are listed. VZ is measured at the specified zener test cur- rent, IZ. For example, the zener voltage for a 1N4728A can range from 3.315 V to 3.465 V with a typical value of 3.3 V at a test current of 76 mA.
Maximum zener impedance ZZ is the maximum zener impedance at the specified test current, IZ. For example, for a 1N4728A, ZZ is 10 V at 76 mA. The maximum zener imped- ance, ZZK, at the knee of the characteristic curve is specified at IZK, which is the current at the knee of the curve. For example, ZZK is 400 V at 1 mA for a 1N4728A.
Leakage current Reverse leakage current is specified for a reverse voltage that is less than the knee voltage. This means that the zener is not in reverse breakdown for these mea- surements. For example IR is 100 mA for a reverse voltage of 1 V in a 1N4728A.
From the datasheet in Figure 3–7, a 1N4736A zener diode has a ZZ of 3.5V. The datasheet gives VZ=6.8 V at a test current, IZ, of 37 mA. What is the voltage across the zener terminals when the current is 50 mA? When the current is 25 mA? Figure 3–8 represents the zener diode.
EXAMPLE 3–4
VZ
DVZ
IZ DIZ
+ –
VZ
– +
▲ FIGURE 3–8
Solution For IZ =50 mA: The 50 mA current is a 13 mA increase above the test current, IZ, of 37 mA.
DIZ =IZ -37 mA= 50 mA-37 mA = +13 mA DVZ = DIZZZ=(13 mA)(3.5 V)= +45.5 mV
The change in voltage due to the increase in current above the IZ value causes the zener terminal voltage to increase. The zener voltage for IZ =50 mA is
VZ=6.8 V + DVZ =6.8 V+45.5 mV =6.85 V
For IZ =25 mA: The 25 mA current is a 12 mA decrease below the test current, IZ, of 37 mA.
DIZ= -12 mA
DVZ= DIZZZ=(-12 mA)(3.5 V)= -42 mV
The change in voltage due to the decrease in current below the test current causes the zener terminal voltage to decrease. The zener voltage for IZ=25 mA is
VZ =6.8 V- DVZ=6.8 V -42 mV=6.76 V
Related Problem Repeat the analysis for IZ=10 mA and for IZ=30 mA using a 1N4742A zener with VZ= 12 V at IZ=21 mA and ZZ=9 V.
1. In what region of their characteristic curve are zener diodes operated?
2. At what value of zener current is the zener voltage normally specified?
3. How does the zener impedance affect the voltage across the terminals of the device?
4. What does a positive temperature coefficient of 0.05%/°C mean?
5. Explain power derating.
SECTION 3–1 CHECKUP
Answers can be found at www .pearsonglobaleditions.com/Floyd.