NCP1060, NCP1063
www.onsemi.com
27
P
on
+
I
valley
@
ǒ
V
bulk
) N @ (V
out
) V
f
)
Ǔ
@ t
on
6 @ T
SW
+
0.0464 @ (127 ) 100) @ 10 n
6 @ 16.7 m
+ 2.1 mW
(eq. 7)
It is noted that the overlap of voltage and current seen on
MOSFET during turning on and off duration is dependent on
the snubber and parasitic capacitance seen from drain pin.
Therefore the t
off
and t
on
in Equation 7 and Equation 8 have
to be modified after measuring on the bench.
8. The theoretical total power is then
117 + 15.5 + 2.1 = 127.6 mW
9. If the NCP106X operates at DSS mode, then the
losses caused by DSS mode should be counted as
losses of this device on the following calculation:
P
DSS
+ I
CC1
@ V
in.max
+ 0.8m @ 375 + 300 mW
(eq. 8)
MOSFET Protection
As in any Flyback design, it is important to limit the drain
excursion to a safe value, e.g. below the MOSFET BVdss
which is 700 V. Figure 48 a−b−c present possible
implementations:
Figure 48. a, b, c : Different Options to Clamp the Leakage Spike
Figure 48a: the simple capacitor limits the voltage
according to the lateral MOSFET body−diode shall never be
forward biased, either during start−up (because of a large
leakage inductance) or in normal operation as shown by
Figure 46. This condition sets the maximum voltage that can
be reflected during t
off
. As a result, the flyback voltage
which is reflected on the drain at the switch opening cannot
be larger than the input voltage. When selecting
components, you must adopt a turn ratio which adheres to
the following Equation 3. This option is only valid for low
power applications, e.g. below 5 W, otherwise chances exist
to destroy the MOSFET. After evaluating the leakage
inductance, you can compute C with (Equation 4). Typical
values are between 100 pF and up to 470 pF. Large
capacitors increase capacitive losses...
Figure 48b: the most standard circuitry is called the RCD
network. You calculate R
clamp
and C
clamp
using the
following formulae:
R
clamp
+
2 @ V
clamp
@
ǒ
V
clamp
) N @ (V
out
) V
f
)
Ǔ
L
leak
@ I
leak
2
@ f
sw
(eq. 9)
C
clamp
+
V
clamp
V
ripple
@ f
sw
@ R
clamp
V
clamp
is usually selected 50−80 V above the reflected
value N x (V
out
+ V
f
). 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 I
peak
and V
in
are maximum
and V
out
is close to reach the steady−state value.
Figure 48c: 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 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 W of continuous power
but is able to accept surges up to 600 W @ 1 ms. Select the
zener or TVS clamping level between 40 to 80 volts above the
reflected output voltage when the supply is heavily loaded.
As a good design practice, it is recommended to
implement one of this protection to make sure Drain pin
voltage doesn’t go above 650 V (to have some margin
between Drain pin voltage and BVdss) during most stringent
operating conditions (high Vin and peak power).
