Effect of di/dt and If on trr
8/17/2015
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Effect of di/dt
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Conditions
•
•
•
•
IF = 10A
VDS = 12V
AMR438N
Sense = 20 m Ohm
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Results
di/dt = 5/40 = 0.125A/ns IRR = 4A ta = 40ns
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Results
di/dt = 5/28 = 0.18A/ns IRR = 6A ta = 33ns
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Results
di/dt = 10/46 = 0.22A/ns IRR = 7A ta = 32ns
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Results
di/dt = 10/34 = 0.3A/ns IRR = 8.5A ta = 32ns
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Results
di/dt = 10/24 = 0.4A/ns IRR = 12A ta = 30ns
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Results
di/dt = 10/2 = 0.5A/ns IRR = 13.5A ta = 28ns
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Results
di/dt = 10/15 = 0.66A/ns IRR = 17A ta = 26ns
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Results
di/dt = 15/19 = 0.8A/ns IRR = 19A ta = 23ns
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Results
di/dt = 20/14 = 1.4A/ns IRR = 25A ta = 21ns
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Results
di/dt = 20/12 = 1.66A/ns IRR = 28A ta = 18ns
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Summary of Results
di/dt A/ns
irr A
ta ns
Qrr: 0.5 x irr x ta
nC
Irr / di/dt
0.125
4
40
80
32
0.18
6
33
99
33.3
0.22
7
32
112
31.8
0.3
8.5
32
136
28.3
0.4
12
30
180
30
0.5
13.5
28
189
27
0.66
17
26
221
25.8
0.8
19
23
218.5
23.8
1.4
25
21
262.5
17.9
1.66
28
18
252
16.9
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Conclusion: di/dt
• Irr increases nearly linearly with di/dt at di/dt <0.5A/ns
• At higher di/dt, Irr / didt decreases
– At very high di/dt Irr is much lower than would be expected using
classic fixed qrr model
• Likewise trr does not decrease as much as expected at
high di/dt
– Therefore Qrr does increase with di/dt for low di/dt (<~0.66A/ns)
– Qrr does not seem to increase significantly with di/dt for dit/dt > 1 ns
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Effect of If on Irr
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Results
If = 6A, irr = 8A
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Results
If = 8A, irr = 11A
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Results
If = 12A, irr = 12.5A
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Results
If = 14A, irr = 13A
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Results
If = 16A, irr = 14A
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Overlay
Di/dt changed as If is changed, magnifying the effect. Increase in irr due to
IF is negligable
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Conclusion
• At typical full load operating currents seen in DC DC
converters, the increase in irr with current is fairly small
• At lower currents, say < half typical operating current, the
irr does reduce as current is reduced
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