1N5820, 1N5821, 1N5822
http://onsemi.com
4
NOTE 3 — DETERMINING MAXIMUM RATINGS
Reverse power dissipation and the possibility of thermal
runaway must be considered when operating this rectifier at
reverse voltages above 0.1 V
RWM
. Proper derating may be
accomplished by use of equation (1).
T
A(max)
= T
J(max)
R
q
JA
P
F(AV)
R
q
JA
P
R(AV)
(1)
where T
A(max)
= Maximum allowable ambient temperature
T
J(max)
= Maximum allowable junction temperature
(125°C or the temperature at which thermal
runaway occurs, whichever is lowest)
P
F(AV)
= Average forward power dissipation
P
R(AV)
= Average reverse power dissipation
R
q
JA
= Junction-to-ambient thermal resistance
Figures 1, 2, and 3 permit easier use of equation (1) by
taking reverse power dissipation and thermal runaway into
consideration. The figures solve for a reference temperature
as determined by equation (2).
T
R
= T
J(max)
R
q
JA
P
R(AV)
(2)
Substituting equation (2) into equation (1) yields:
T
A(max)
= T
R
R
q
JA
P
F(AV)
(3)
Inspection of equations (2) and (3) reveals that T
R
is the
ambient temperature at which thermal runaway occurs or
where T
J
= 125°C, when forward power is zero. The
transition from one boundary condition to the other is
evident on the curves of Figures 1, 2, and 3 as a difference
in the rate of change of the slope in the vicinity of 115°C. The
data of Figures 1, 2, and 3 is based upon dc conditions. For
use in common rectifier circuits, Table 1 indicates suggested
factors for an equivalent dc voltage to use for conservative
design, that is:
V
R(equiv)
= V
(FM)
F (4)
The factor F is derived by considering the properties of the
various rectifier circuits and the reverse characteristics of
Schottky diodes.
EXAMPLE: Find T
A(max)
for 1N5821 operated in a
12-volt dc supply using a bridge circuit with capacitive filter
such that I
DC
= 2.0 A (I
F(AV)
= 1.0 A), I
(FM)
/I
(AV)
= 10, Input
Voltage = 10 V
(rms)
, R
q
JA
= 40°C/W.
Step 1. Find V
R(equiv).
Read F = 0.65 from Table 1,
V
R(equiv)
= (1.41) (10) (0.65) = 9.2 V.
Step 2. Find T
R
from Figure 2. Read T
R
= 108°C
@ V
R
= 9.2 V and R
q
JA
= 40°C/W.
Step 3. Find P
F(AV)
from Figure 6. **Read P
F(AV)
= 0.85 W
@
I
(FM)
I
(AV)
10 and I
F(AV)
1.0A.
Step 4. Find T
A(max)
from equation (3).
T
A(max)
= 108 (0.85) (40) = 74°C.
**Values given are for the 1N5821. Power is slightly lower
for the 1N5820 because of its lower forward voltage, and
higher for the 1N5822. Variations will be similar for the
MBR-prefix devices, using P
F(AV)
from Figure 6.
Table 1. Values for Factor F
Circuit Half Wave Full Wave, Bridge
Full Wave,
Center Tapped*†
Load Resistive Capacitive* Resistive Capacitive Resistive Capacitive
Sine Wave 0.5 1.3 0.5 0.65 1.0 1.3
Square Wave 0.75 1.5 0.75 0.75 1.5 1.5
*Note that V
R(PK)
2.0 V
in(PK)
.
†Use line to center tap voltage for V
in.
