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| 型号： | 750108 |
| 厂商： | Wurth Electronics Inc |
| 文件页数： | 16/30页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR LT3748 |
| 设计资源： | LT3748 Design Schematic/Tips |
| 标准包装： | 1 |
| 主要目的： | DC/DC，步降 |
| 输出及类型： | 1，隔离 |
| 功率 - 输出： | 30W |
| 输出电压： | 12V |
| 电流 - 输出： | 2.5A |
| 输入电压： | 22 ~ 75 V |
| 稳压器拓扑结构： | 回扫 |
| 板类型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | LT3748 |
| 相关产品： | LT3748HMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748HMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748IMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748IMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748EMS#TRPBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP LT3748EMS#PBF-ND - IC REG CTRLR FLYBK ISO CM 16MSOP |
| 其它名称： | 732-3308 |
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LT3748
APPLICATIONS INFORMATION
Although it typically does not decrease efficiency, leakage
inductance energy that would normally have been dis-
sipated in the switch or transformer is also dissipated in
the RC snubber resistor and can be calculated as:
P SNUBBER = f SW ? L LEAK ? I LIM2 /2
An RCD clamp, shown in Figure 7, also prevents the
leakage inductance spike from exceeding the breakdown
voltage of the MOSFET switch. In most applications, there
will be a very fast voltage spike caused by a slow clamp
diode. Once the diode clamps, the leakage inductance
current is absorbed by the clamp capacitor. This period
should not last longer than 200ns so as not to interfere
with the output regulation. The clamp diode turns off after
the leakage inductance energy is absorbed and the switch
voltage is then equal to:
V DS = V IN + N PS ? (V OUT + V F(DIODE) )
Schottky diodes are typically the best choice for use in a
snubber, but some PN diodes can be used if they turn on
fast enough to limit the leakage inductance spike. Figures 8
and 9 show the waveform at the drain of the MOSFET
switch for the 48V output application shown in Figure 17
ring beyond that expected reverse voltage. An RC snubber
or RCD clamp may be implemented to reduce the voltage
spike if it is desirable to use a lower reverse voltage diode.
Secondary Leakage Inductance
In addition to the previously described effects of leakage
inductance in general, leakage inductance on the secondary
in particular exhibits an additional phenomena. It forms an
inductive divider on the transformer secondary that effec-
tively reduces the size of the primary-referred flyback pulse
used for feedback. This will increase the output voltage
target by a similar percentage. Note that, unlike leakage
spike behavior, this phenomena is load independent. To the
extent that the secondary leakage inductance is a constant
percentage of mutual inductance (over manufacturing
200
180
160
140
120
100
80
at maximum rated load and maximum input voltage with
60
V IN = 96V
an RC snubber and RCD clamp, respectively. Both solu-
tions limit the leakage spike to less than 190V, below the
200V V DS(MAX) rating of the Si7464DP MOSFET.
40
20
0
0
50
100
150 200
V OUT = 48V
I OUT = 0.5A
R = 66Ω
C = 150pF
250 300
V IN
C
L LEAK
R
D
GATE
NMOS
V OUT+
–
V OUT
TIME (ns) 3748 F08
Figure 8. Waveform of MOSFET Drain During Normal Operation
of Figure 17 with RC Snubber (as Drawn)
200
180
160
140
3748 F07
Figure 7. RCD Clamp
120
100
Leakage Inductance and Output Diode Stress
The output diode may also see increased reverse voltage
stresses from leakage inductance. While it nominally sees
80
60
40
20
0
V IN = 96V
V OUT = 48V
I OUT = 0.5A
R = 4.99k
C = TDK 0.22μF 250V
D = CMR1U-02M-LTC
a reverse voltage of the input voltage divided by the wind-
ings ratio plus the output voltage when the MOSFET power
switch turns on, the capacitance on the output diode and
the leakage inductance will cause an LC tank which may
0 50 100 150 200 250 300
TIME (ns) 3748 F08
Figure 9. Waveform of MOSFET Drain During Normal Operation
of Figure 17 Using RCD Clamp with Central Semiconductor
CMR1U-02M-LTC Instead of RC Snubber
3748fa
16
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