12
FN6554.0
October 8, 2007
The RMS current through each primary side FET can be
determined from EQ. 10, substituting 5A of primary current
for I
OUT
(assuming 100% duty cycle). The result is 3.5A
RMS. Fairchild FDS3672 FETs, rated at 100V and 7.5A
(r
DS(ON)
= 22mΩ), were selected for the half-bridge
switches.
The synchronous rectifier FETs must withstand
approximately one half of the input voltage assuming no
switching transients are present. This suggests that a device
capable of withstanding at least 30V is required. Empirical
testing in the circuit revealed switching transients of 20V
were present across the device indicating a rating of at least
60V is required.
The RMS current rating of 7.07A for each SR FET requires a
low r
DS(ON)
to minimize conduction losses, which is difficult to
find in a 60V device. It was decided to use two devices in
parallel to simplify the thermal design. Two Fairchild FDS5670
devices are used in parallel for a total of four SR FETs. The
FDS5670 is rated at 60V and 10A (r
DS(ON)
= 14mΩ).
Oscillator Component Selection
The desired operating frequency of 235kHz for the converter
was established in the Design Criteria section. The
oscillator frequency operates at twice the frequency of the
converter because two clock cycles are required for a
complete converter period.
During each oscillator cycle the timing capacitor, C
T
, must be
charged and discharged. Determining the required
discharge time to achieve zero voltage switching (ZVS) is
the critical design goal in selecting the timing components.
The discharge time sets the deadtime between the two
outputs, and is the same as ZVS transition time. Once the
discharge time is determined, the remainder of the period
becomes the charge time.
The ZVS transition duration is determined by the
transformer’s primary leakage inductance, L
lk
, by the FET
Coss, by the transformer’s parasitic winding capacitance,
and by any other parasitic elements on the node. The
parameters may be determined by measurement,
calculation, estimate, or by some combination of these
methods.
Device output capacitance, Coss, is non-linear with applied
voltage. To find the equivalent discrete capacitance, Cfet, a
charge model is used. Using a known current source, the
time required to charge the MOSFET drain to the desired
operating voltage is determined and the equivalent
capacitance is calculated.
Once the estimated transition time is determined, it must be
verified directly in the application. The transformer leakage
inductance was measured at 125nH and the combined
capacitance was estimated at 2000pF. Calculations indicate
a transition period of ~25ns. Verification of the performance
yielded a value of T
D
closer to 45ns.
The remainder of the switching half-period is the charge
time, T
C
, and can be found from
where F
Sw
is the converter switching frequency.
Using Figure 3, the capacitor value appropriate to the
desired oscillator operating frequency of 470kHz can be
selected. A C
T
value of 100pF, 150pF, or 220pF is
appropriate for this frequency. A value of 150pF was
selected.
To obtain the proper value for R
TD
, EQ. 3 is used. Since
there is a 10ns propagation delay in the oscillator circuit, it
must be included in the calculation. The value of R
TD
selected is 10kΩ.
Output Filter Design
The output filter inductor and capacitor selection is simple
and straightforward. Under steady state operating conditions
the voltage across the inductor is very small due to the large
duty cycle. Voltage is applied across the inductor only during
the switch transition time, about 45ns in this application.
Ignoring the voltage drop across the SR FETs, the voltage
across the inductor during the on time with V
IN
= 48V is
where
V
L
is the inductor voltage
V
S
is the voltage across the secondary winding
V
OUT
is the output voltage
If we allow a current ramp, ΔI, of 5% of the rated output
current, the minimum inductance required is
An inductor value of 1.5μH, rated for 18A was selected.
With a maximum input voltage of 53V, the maximum output
voltage is about 13V. The closest higher voltage rated
capacitor is 16V. Under steady state operating conditions the
ripple current in the capacitor is small, so it would seem
appropriate to have a low ripple current rated capacitor.
However, a high rated ripple current capacitor was selected
t
zvs
π L
lk
2C
oss
C
xfrmr
+()•
2
--------------------------------------------------------------------
≈ s
(EQ. 12)
Cfet
Ichg t•
V
--------------------
= F
(EQ. 13)
T
C
1
2F
Sw
•
--------------------
T
D
–
1
2 235 10
3
••
----------------------------------
45 10
9–
•– 2.08== =μs
(EQ. 14)
V
L
V
S
V
OUT
–
V
IN
N
S
1D–()••
2N
P
----------------------------------------------- -
250≈== mV
(EQ. 15)
L
V
L
T
ON
•
ΔI
-------------------------
≥
0.25 2.08•
0.5
-----------------------------
1.04==μH
(EQ. 16)
ISL6744A
