LT3825EFE-PBF LINER [Linear Technology], LT3825EFE-PBF Datasheet - Page 14

• Manufacturer Part Number: LT3825EFE-PBF
• Description: Isolated No-Opto Synchronous Flyback Controller with Wide Input Supply Range
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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 as 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 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/volt-
age characteristics. A load with resistive—i.e., I = V/R
behavior—is the most apt to be bistable. Capacitive loads
that exhibit I = V
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load current.
2
/R behavior are less susceptible.
Secondary Leakage Inductance
Leakage inductance on the secondary forms an inductive
divider on the transformer secondary, reducing the size
of the feedback 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 in-
ductance is a constant percentage of mutual inductance
(within manufacturing variations), the solution is to adjust
the feedback resistive divider ratio to compensate.
Winding Resistance Effects
Primary or secondary winding resistance acts to reduce
overall effi ciency (P
increases effective output impedance degrading load regu-
lation. Load compensation can mitigate this to some extent
but a good design keeps parasitic resistances low.
Bifi lar Winding
A bifi lar or similar winding is a good way to minimize
troublesome leakage inductances. Bifi lar windings also
improve coupling coeffi cients and thus improve cross
regulation in multiple winding transformers. However,
tight coupling usually increases primary-to-secondary
capacitance and limits the primary-to-secondary break-
down voltage, so it isn’t always practical.
Primary Inductance
The transformer primary inductance, L
on the peak-to-peak ripple current ratio (X) in the trans-
former relative to its maximum value. As a general rule,
keep X in the range of 20% to 40% ripple current (i.e., X =
0.2 to 0.4). Higher values of ripple will increase conduction
losses, while lower values will require larger cores.
OUT
/P
IN
). Secondary winding resistance
P
, is selected based
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