LTC1922-1
18
Selecting the Power Stage Components
Perhaps the most critical part of the overall design of the
converter is selecting the power MOSFETs, transformer,
inductors and filter capacitors. Tremendous gains in effi-
ciency, transient performance and overall operation can
be obtained as long as a few simple guidelines are followed
with the phase shifted full-bridge topology.
Power Transformer
This guide is aimed at selecting readily available standard
“off the shelf” transformers. The basic requirements,
however, apply to custom transformer designs as well.
Switching frequency, core material characteristics, series
resistance and input/output voltages all play an important
role in transformer selection. Close attention also needs to
be paid to leakage and magnetizing inductances as they
play an important role in how well the converter will
achieve ZVS. Planar magnetics are very well suited to
these applications because of their excellent control of
these parameters.
Turns Ratio
The required turns ratio for a current doubler secondary is
given below. Depending on the magnetics selected, this
value may need to be reduced slightly.
Turns ratio formula:
N
VD
V
IN MIN MAX
OUT
= 2•
•
()
where:
V
IN(MIN)
= Minimum V
IN
for operation
D
MAX
= Maximum duty cycle of controller
Magnetizing, Output, and Leakage Inductors
A lower value of magnetizing and output inductance will
improve the ability of the converter to achieve ZVS over the
full range of loads and reduce the size of the external
commutating inductor. One of the trade-offs is increased
primary referred ripple current which has a small negative
impact on efficiency. Other factors to consider are switch-
ing frequency and required maximum duty cycle. A lower
value of magnetizing inductance will require a longer time
to reset the core, cutting into the available duty cycle
range. As switching frequencies increase, this becomes
more significant. In general, the magnetizing inductance
value should be the lowest value required in order to
achieve the necessary maximum duty cycle at the chosen
switching frequency. Output inductor value determines
the magnitude of output ripple current and therefore the
ripple voltage along with the output capacitors. Generally
speaking, the output inductance should be minimized as
much as possible in order to improve transient response.
In addition, output capacitance ESR should be minimized
as much as possible. Using the equations below, plug in
the manufacturers magnetizing inductance value and a
“starting value” of commutating inductance (1% of L
MAG
)
to verify that a sufficient max duty cycle can be achieved
at the desired switching frequency. Next, use equation (2)
to determine what the absolute minimum required L
COM
is
to guarantee ZVT over the entire load range. One or two
iterations may be required in order to arrive at the final
selections.
MAX DC vs L
COM
at f
SW
MAXDC
fT
SW R
≥
2
2
–•
;
(1)
where:
T
R
= transformer reset time (worst case)
=
+
+
IfLVDN
fL N
LL
V
O MAX SW MAG IN
SW MAG
COM L
IN
()
•• •••
••
2
L
COM
vs ZVS vs Load
LL
CL f
D
COM L
OSS MAG SW
+=
43
2
22
2
/••
•
(2)
OPERATIO
U
