NCP1013 Ver la hoja de datos (PDF) - ON Semiconductor
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NCP1013 Datasheet PDF : 24 Pages
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NCP1010, NCP1011, NCP1012, NCP1013, NCP1014
MOSFET Protection
As in any Flyback design, it is important to limit the
drain excursion to a safe value, e.g. below the MOSFET
HV
HV
BVDSS which is 700 V. Figure 27 presents possible
implementations:
HV
Rclamp
Cclamp
Dz
+
CVcc
1
8
2
7
3
4
5
NCP101X
+
CVcc
C
D
1
8
2
7
3
4
5
NCP101X
+
CVcc
D
1
8
2
7
3
4
5
NCP101X
A
B
C
Figure 27. Different Options to Clamp the Leakage Spike
Figure 27A: The simple capacitor limits the voltage
according to Equation 15. This option is only valid for low
power applications, e.g. below 5.0 W, otherwise chances
exist to destroy the MOSFET. After evaluating the leakage
inductance, you can compute C with Equation 15. Typical
values are between 100 pF and up to 470 pF. Large
capacitors increase capacitive losses.
Figure 27B: This diagram illustrates the most standard
circuitry called the RCD network. Rclamp and Cclamp are
calculated using the following formulas:
Rclamp
+
2
·
Vclamp
·
(Vclamp
Lleak ·
* (Vout )
Ip2 · Fsw
Vf
sec)
·
N)
(eq. 27)
Cclamp
+
Vripple
Vclamp
· Fsw · Rclamp
(eq. 28)
Vclamp is usually selected 50−80 V above the reflected
value N x (Vout + Vf). The diode needs to be a fast one and
a MUR160 represents a good choice. One major drawback
of the RCD network lies in its dependency upon the peak
current. Worse case occurs when Ip and Vin are maximum
and Vout is close to reach the steady−state value.
Figure 27C: This option is probably the most expensive of
all three but it offers the best protection degree. If you need
a very precise clamping level, you must implement a Zener
diode or a TVS. There are little technology differences
behind a standard Zener diode and a TVS. However, the die
area is far bigger for a transient suppressor than that of Zener.
A 5.0 W Zener diode like the 1N5388B will accept 180 W
peak power if it lasts less than 8.3 ms. If the peak current in
the worse case (e.g. when the PWM circuit maximum
current limit works) multiplied by the nominal Zener
voltage exceeds these 180 W, then the diode will be
destroyed when the supply experiences overloads. A
transient suppressor like the P6KE200 still dissipates 5.0 W
of continuous power but is able to accept surges up to 600 W
@ 1.0 ms. Select the Zener or TVS clamping level between
40 to 80 V above the reflected output voltage when the
supply is heavily loaded.
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