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LT1683

1683f

APPLICATIO S I FOR ATIO

Continuous operation occurs when the current in the

inductor never goes to zero. Discontinuous operation

occurs when the inductor current drops to zero before the

start of the next cycle and can occur with small inductors

and light loads. There is nothing inherently bad about

discontinuous operation, however, converter control and

operation are somewhat different. The inductor is smaller

for discontinuous operation but the peak currents in the

switch, the transformer, the diodes, inductor and capaci-

tor will be higher which may produce greater losses.

For continuous operation the inductor ripple current must

be less than twice the output current. The worst case for

this is at maximum input (lowest duty cycle, DCMIN) but in

the following we will evaluate at nominal input since the

IOUT/4 is somewhat arbitrary.

Note when both inputs are off, the inductor current splits

between both secondary outputs and the diode common

goes to 0V.

Looking at the inductor current during off time, output

ripple current is:

∆=

() −

()

∆

OUT

OUT MIN

OUT MAX

OUT MIN

OUT

•

••

•

()

(

)

()

The inductance of the transformer primary should be such

that L, when reflected into the primary, dominates the

input current. In other words, we want the magnetizing

current of the transformer small with respect to the

current going through the transformer to L. In general,

then, the inductance of the primary should be at least five

times that of L reflected to the input. This ensures that

most of the power will be passed through the transformer

to the load. It also increases the power capability of the

converter and reduces the peak currents that the switch

will see.

PRI =

•

If the magnetizing current is small, say below 100mA, then

a smaller L can be used with a higher percentage of the

switch current generated by the magnetizing current.

With the value of L set, the ripple in the inductor is:

∆=

() −

()

OUT

F ••

•

However, the peak inductor current is evaluated at maxi-

mum load and maximum input voltage (minimum DC).

LMAX

()

∆

The magnetizing ripple current can be shown to be:

∆=

MAG

OUT

PRI

••

and the peak current in the switch is:

ISW(PEAK) = N • IL(MAX) + ∆IMAG

This current should be less than the current limit.

Worst-case switch ripple is:

∆ISW(PEAK) = N • ∆IL(MAX) + ∆IMAG

In the push-pull converter the maximum switch voltage

will be 2 • VIN. Because voltage is slew-controlled, the

leakage spikes are small. So, the MOSFET should have a

maximum rated switch voltage at least 20% higher than

2 • VIN.

So, given the turns ratio, primary inductance and current,

the transformer can be designed. The design of the trans-

former will require analyzing the power losses of the

transformer and making necessary adjustments.

Most transformer companies can assist you with design-

ing an optimal solution. For instance Midcom, Inc. (1-800-

643-2661). Linear Technology’s application group can

also help.

As an example say we are designing a 48V

±20% to 5V

100kHz converter with 2A output and 500mA ripple. Then

starting with a guess for the on voltage of the MOSFET plus

sense resistor of 0.5V and VF of 0.5V:

−

()

50 5

48 80

0 5

%•

•

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