AN-7517 Datasheet, PDF (2/8 Pages) Fairchild Semiconductor – Practical Aspects

Application Note 7517
The power dissipated in the device due to UIS will be directly
proportional to the number of times the interruption could
occur per second. If a human provides the interruptions, 5
times per second would probably be sufficient.
All of the losses in the PowerMOS transistor summed, multi-
plied by the total thermal resistance (junction to case, case
to heat sink and heatsink to ambient) gives the rise in junc-
tion temperature above the ambient. From that temperature
the operating r_DS(ON) can be determined and the calcula-
tions iterated. Sometimes several iterations are required.
Example 1
The following example assumes a set of operating condi-
tions and computes the suitability of various Fairchild Power-
MOS devices to operate under those assumed conditions.
The assumed circuit conditions are:
L = 50mH, V_CC = 16V, R_L = 4Ω, IERC PSD1-2U Heat sink.
In addition, the following operational conditions are
assumed:
Rep Rate = 5 pulses/s, T_A = 125^oC, charged current
level ≈ 4A. Sufficient time was allotted for the inductor to
charge; we chose ten time constants (125ms). The inductor
also had to discharge to less than 1% of the charged current
level between pulses, and finally; 10ms of deadtime were
allotted between pulses.
Please note: the number of significant figures in all interme-
diate calculation values were truncated to aid readability.
Check UIS capability and verify junction temperature is less
than 175^oC.
A. Try RFP3055
Assume T_J = 175^oC.
Check to be sure UIS stress is within RFP3055 capability.
t_AV
=


-_R--^L--_L- _
×
In



1^----.^-3^-----×^-----VI^--T--_B^---×-_R^----R-_K^{---L-–-----}V^----_C^----_C^---_{}
+
1
(EQ. 1.5)
(Reference AN-7514.)
t_AV
=


0_----._-40----5_{--}
×
In


1^----.^-3^-----×^-4----6^-×--0^{---4--–-----}1^---6^{--}
+
1
t_AV = 2.9ms
Capability at 4.0A, 175^oC is 0.04ms.
(Unit is not suitable for this application!)
B. Try RFP22N10
Assume T_J = 175^oC.
Check to be sure UIS stress is within RFP22N10 capability.
t_AV
=


-_R--^L--_L- _
× In



1^----.^-3^-----×^-----VI^--T--_B^---×-_R^----R-_K^{---L-–-----}V^----_C^----_C^---_{}
+1
(EQ. 1.5)
t_AV
=


0_----._-40----5_{--}
× In


1^----.^-3^-----×^---4--1^---×0^----04^{-----–----}1^---6^{--}
+
1
t_AV = 1.64ms
Capability at 4.0A, 175^oC is 0.9ms.
(Unit is not suitable for this application!)
C. Try RFP45N06
Assume T_J = 175^oC.
Check to be sure UIS stress is within RFP45N06
capability.
t_AV
=


-_R--^L--_L- _
× In



1^----.^-3^-----×^-----VI^--T--_B^---×-_R^----R-_K^{---L-–-----}V^----_C^----_C^---_{}
+1
(EQ. 1.5)
t_AV
=


0_----._-40----5_{--}
× In


1^----.^-3^-----×^-4----6^-×--0^{---4--–-----}1^---6^{--}
+1
t_AV = 2.9ms
Capability at 4.0A, 175^oC is 3.2ms. OK for UIS.
Check to see if T_J ≤ 175^oC.
r_DS(ON) = 2.1 × 0.028 (See Figure 7, RFP45N06 datasheet.)
r_DS(ON) = 0.059Ω
Dissipation during conduction:
P_T
=



V_{---R--C--L--C--}
2
×
r_DS(ON)
P_T
=


1_----6_-4--._--0_{--}
2
×
0.059
(EQ. 1.3)
P_T = 0.941W
Dissipation due to UIS:
E_T
=



L_-----×_-----I_--T---R--×_---L--V_{----D----S----S--}
×
1 – K × In_1 + _K-¹--_
(EQ. 1.4)
©2002 Fairchild Semiconductor Corporation
Application Note 7517 Rev. A2