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3575FE

Isolated Flyback Converter without an Opto-Coupler

Linear Technology 

LT3575

APPLICATIONS INFORMATION

Turns Ratio

Note that when using an RFB/RREF resistor ratio to set

output voltage, the user has relative freedom in selecting

a transformer turns ratio to suit a given application.In

contrast, simpler ratios of small integers, e.g., 1:1, 2:1,

3:2, etc., can be employed to provide more freedom in

setting total turns and mutual inductance.

Typically, the transformer turns ratio is chosen to maximize

available output power. For low output voltages (3.3V or 5V),

a N:1 turns ratio can be used with multiple primary windings

relative to the secondary to maximize the transformer’s

current gain (and output power). However, remember that

the SW pin sees a voltage that is equal to the maximum

input supply voltage plus the output voltage multiplied by

the turns ratio. This quantity needs to remain below the

ABS MAX rating of the SW pin to prevent breakdown of

the internal power switch. Together these conditions place

an upper limit on the turns ratio, N, for a given application.

Choose a turns ratio low enough to ensure:

N < 50V – VIN(MAX)

VOUT + VF

For larger N:1 values, a transformer with a larger physical

size is needed to deliver additional current and provide a

large enough inductance value to ensure that the off-time is

long enough to accurately measure the output voltage.

For lower output power levels, a 1:1 or 1:N transformer can

be chosen for the absolute smallest transformer size. A 1:

N transformer will minimize the magnetizing inductance

(and minimize size), but will also limit the available output

power. A higher 1:N turns ratio makes it possible to have

very high output voltages without exceeding the breakdown

voltage of the internal power switch.

Leakage Inductance

Transformer leakage inductance (on either the primary or

secondary) causes a voltage spike to appear at the primary

after the output switch turns off. This spike is increasingly

prominent at higher load currents where more stored energy

must be dissipated. In most cases, a snubber circuit will

be required to avoid overvoltage breakdown at the output

switch node. Transformer leakage inductance should be

minimized.

An RCD (resistor capacitor diode) clamp, shown in

Figure 4, is required for most designs to prevent the

leakage inductance spike from exceeding the breakdown

voltage of the power device. The flyback waveform is

depicted in Figure 5. In most applications, there will be a

very fast voltage spike caused by a slow clamp diode that

may not exceed 60V. Once the diode clamps, the leakage

inductance current is absorbed by the clamp capacitor.

This period should not last longer than 150ns so as not to

interfere with the output regulation, and the voltage during

this clamp period must not exceed 55V. The clamp diode

turns off after the leakage inductance energy is absorbed

and the switch voltage is then equal to:

VSW(MAX) = VIN(MAX) + N(VOUT + VF)

This voltage must not exceed 50V. This same equation

also determines the maximum turns ratio.

When choosing the snubber network diode, careful

attention must be paid to maximum voltage seen by the

SW pin. Schottky diodes are typically the best choice to

be used in the snubber, but some PN diodes can be used

if they turn on fast enough to limit the leakage inductance

spike. The leakage spike must always be kept below 60V.

Figures 6 and 7 show the SW pin waveform for a 24VIN,

5VOUT application at a 1A load current. Notice that the

leakage spike is very high (more than 65V) with the “bad”

diode, while the “good” diode effectively limits the spike

to less than 55V.

An alternative to RC network is a Zener diode clamping.

The Zener diode must be able to handle the voltage rating

and power dissipating during the switch turn-off time.

Application Note 19 has more details on Zener diode

snubber design for flyback converters.

For applications with SW voltage exceeding 50V,

Zener diode clamp must be considered. At higher operating

primary current, the leakage inductance spike can

potentially exceed the breakdown voltage of the internal

power switch.

3575f

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