12

FN9147.8

September 22, 2005

The RMS current through each primary side FET can be

determined from EQ. 10, substituting 5A of primary current

RMS. Fairchild FDS3672 FETs, rated at 100V and 7.5A

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

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

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.

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

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

The remainder of the switching half-period is the charge

Using Figure 3, the capacitor value appropriate to the

desired oscillator operating frequency of 470kHz can be

appropriate for this frequency. A value of 150pF was

selected.

there is a 10ns propagation delay in the oscillator circuit, it

selected is 10k

Ω.

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

where

If we allow a current ramp,

∆I, of 5% of the rated output

current, the minimum inductance required is

An inductor value of 1.5

µH, 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

π L

lk

+

()

•

2

--------------------------------------------------------------------

≈

s

(EQ. 12)

Cfet

Ichg t

•

V

--------------------

=

F

(EQ. 13)

1

•

–

1

2 235 10

3

•

•

---------------------------------- 45 10

9

–

•

–

2.08

==

=

µs

(EQ. 14)

–

–

()

•

•

------------------------------------------------ 250

≈

==

mV

(EQ. 15)

L

•

∆I

-------------------------

≥

0.25 2.08

•

0.5

-----------------------------

1.04

==

µH

(EQ. 16)

ISL6744