ltc4268cdkd-1-trpbf Linear Technology Corporation, ltc4268cdkd-1-trpbf Datasheet - Page 32

• Manufacturer Part Number: ltc4268cdkd-1-trpbf
• Description: High Power Pd With Synchronous Noopto Flyback Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Leakage Inductance
Transformer leakage inductance (on either the primary or
secondary) causes a spike after the primary side switch
turn-off. This is increasingly prominent at higher load
currents, where more stored energy is dissipated. Higher
fl yback voltage may break down the MOSFET switch if it
has too low a BV
spike is to use a snubber circuit to suppress the voltage
excursion. However, suppressing the voltage extends the
fl yback pulse width. If the fl yback pulse extends beyond
the enable delay time, output voltage regulation is affected.
The feedback system has a deliberately limited input range,
roughly ±50mV referred to the FB node. This rejects higher
voltage leakage spikes because once a leakage spike is
several volts in amplitude; a further increase in amplitude
has little effect on the feedback system. Therefore, it is
advisable to arrange the snubber circuit to clamp at as
high a voltage as possible, observing MOSFET breakdown,
such that leakage spike duration is as short as possible.
Application Note 19 provides a good reference on snubber
design.
As a rough guide, leakage inductance of several percent
(of mutual inductance) or less may require a snubber, but
exhibit little to no regulation error due to leakage spike
behavior. Inductances from several percent up to perhaps
ten percent cause increasing regulation error.
Avoid double digit percentage leakage inductances. There
is a potential for abrupt loss of control at high load current.
This curious condition potentially occurs when the leakage
spike becomes such a large portion of the fl yback waveform
that the processing circuitry is fooled into thinking that
the leakage spike itself is the real fl yback signal! It then
reverts to a potentially stable state whereby the top of the
LTC4268-1
32
DSS
rating. One solution to reducing this
leakage spike is the control point, and the trailing edge of
the leakage spike triggers the collapse detect circuitry. This
typically reduces the output voltage abruptly to a fraction,
roughly one-third to two-thirds of its correct value. Once
load current is reduced suffi ciently, the system snaps
back to normal operation. When using transformers with
considerable leakage inductance, exercise this worst-case
check for potential bistability:
1. Operate the prototype supply at maximum expected
2. Temporarily short circuit the output.
3. Observe that normal operation is restored.
If the output voltage is found to hang up at an abnormally
low value, the system has a problem. This is usually evident
by simultaneously viewing the primary side MOSFET drain
voltage to observe fi rsthand the leakage spike behavior.
A fi nal note—the susceptibility of the system to bistable
behavior is somewhat a function of the load current/
voltage characteristics. A load with resistive—i.e., I = V/R
behavior—is the most apt to be bistable. Capacitive loads
that exhibit I = V
Secondary Leakage Inductance
Leakage inductance on the secondary forms an inductive
divider on the transformer secondary, reducing the size
of the fl yback pulse. This increases the output voltage
target by a similar percentage. Note that unlike leakage
spike behavior; this phenomenon is independent of load.
Since the secondary leakage inductance is a constant
percentage of mutual inductance (within manufacturing
variations), the solution is to adjust the feedback resistive
divider ratio to compensate.
load current.
2
/R behavior are less susceptible.
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