10
FN9007.8
October 21, 2015
Primary Inductance
The flyback transformer is actually a coupled inductor, acting
as an energy storage unit, as well as performing the usual
transformer functions. Crucial considerations include primary
inductance, working flux density swing, gap length, the
winding scheme and wire diameter. The primary inductance,
LP, for a discontinuous mode flyback converter can be
calculated according to the following relationship:
Lp = n [(V
INmin
- V
DS
)(T
ONmax
)]
2
/ (2)(T)(V
OUT
)(I
OUT
)
Where n is the assumed efficiency of the converter and Iout
is the output current. The ferrite core should have high
saturation, low residual flux density, and low losses. An
EFD15 core material proved to be suitable for this
application.
Current Sense
The ground referenced sense resistor is selected such that
the maximum peak primary current trips the CS pin threshold
when this current is 10% higher than its normal operating
peak value at the minimum input voltage.
This limits the peak primary current in the event of an output
short circuit. This resistor must have a power rating to meet
the (I
2
rms
)(R) requirement, where I
rms
is the root mean
square (rms) primary current. Because this resistor defines
the maximum peak primary current, the input energy to the
transformer is defined and equal to (L
P
)(I
2
PEAK
) / 2. This
defined energy in a fixed frequency discontinuous-mode
flyback results in a fixed output power.
The advantage of current-mode control is that the output
voltage is held constant despite changes in the input voltage,
because the peak-primary current remains constant; the
slope of this inductor current and its pulse width are
adjusted. Leading edge spikes or noise are caused by the
reverse recovery of the rectifier, equivalent capacitive
loading on the secondary, and parasitic circuit inductances.
A small low pass RC filter is added to the current-sense
signal to filter out these spikes, so the comparator does not
assume an overload condition is present during switch turn-
on. To avoid excessive phase lag on the current-sense
signal, the low pass filter corner frequency is selected to be
at least a decade above the switching frequency.
ISL6401
