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NCP1652, NCP1652A
the 2 ? stage solution. As a result, potentially higher
leakage inductance induces higher voltage spikes (like
the one shown in Figure 53) on the MOSFET drain.
V in
Active
Clamp
V out
This may require a MOSFET with a higher voltage
rating compared to similar dc ? input flyback
applications.
Figure 56. Active Clamp
The first two methods result in dissipation of the leakage
energy in the clamping circuits – the dissipation is
proportional to LI 2 where L is the leakage inductance of the
transformer and I is the peak of the switch current at
turn ? off. An RDC snubber is simple and has the lowest cost,
but constantly dissipates power. A TVS provides good
voltage clamping at a slightly higher cost and dissipates
power only when the drain voltage exceeds the voltage
Figure 53. Typical Drain Voltage Waveform of a
Flyback Main Switch
There are a few methods to clamp the voltage spike on the
main switch, a resistor ? capacitor ? diode (RCD) clamp, a
transient voltage suppressor (TVS) or an active clamp using
a MOSFET and capacitor can be used as shown in
Figures 54 to 56.
rating of the TVS.
The active clamp circuit provides an intriguing alternative
to the other methods. It requires addition of a MOSFET and
a high voltage capacitor as part of the active clamp circuit,
thus adding complexity, but it results in a complete reuse of
the leakage inductance energy. As a result, the transformer
construction is no longer critical and one can use cheaper
cost solution. Also, the active clamp circuit reduces the
voltage stress on the primary switch and that can lead to
V in
R
C
D
RCD
Clamp
V out
usage of lower cost or lower on resistance (R DS(on) )
MOSFET. Finally, the turn ? on switching losses are
eliminated because the active clamp circuit allows the
discharge of the MOSFET C OSS capacitance prior to the
turn ? on. The energy stored in the leakage inductance is
utilized for this transition.
In many applications, the added complexity of the active
clamp circuit may not be justified. However, the OUTB of
the NCP1652 is also usable for another purpose,
synchronous rectification control. Synchronous
Figure 54. RCD Clamp
rectification for flyback converters is an emerging
requirement for flyback converters. The OUTB signal from
NCP1652 is ideal for interfacing with a secondary side
V in
TVS
TVS
Clamp
V out
synchronous rectifier controller such as NCP4303 as shown
in Figure 57. As shown in Figure 57, using the OUTB
(coupled through pulse transformer or Y ? capacitor) as a
trigger for the NCP4303 allows guaranteed turn ? off of the
secondary side synchronous MOSFET prior to turn ? on of
the primary switch. In any CCM flyback converter, this is a
critical requirement to prevent cross ? conduction and
NCP1652 and NCP4303 combination is the first such
chipset that guarantees the operation without
cross ? conduction.
Figure 55. TVS Clamp
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