AN701 データシートの表示（PDF） - Vishay Semiconductors

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AN701

Designing High-Frequency DC-to-DC Converters With the Si9114A Switchmode Controller

Vishay Semiconductors 

AN701

Vishay Siliconix

The volume of the EF12.6 core is specified as 384 mm3 or

0.384 cm3.

Therefore the core loss will be:

P = 0.742 0.384

P = 0.285 W [ 1/4 W

which represents less than 2% of the total converter losses.

The converter transformer can be calculated by using the

following adapted version of Faraday’s equation:

Vin min x ton max

N + Bmax x Aeff

Where:

Vinmax = minimum input voltage (V)

N = number of turns (Integer)

Bmax = maximum peak flux (Tesla)

Aef = effective area of core (m2)

In the case of the EF12.6, the minimum number of turns will be:

36 V x 1 ms

N + 85 mT x 12.2 mm2

N = 36.64 Turns [ 37 Turns

This is the calculated minimum number of turns that should be

applied. To determine the turns for a given output voltage, first

determine the output voltage (5 V). Then determine all the other

losses that will exist in series with the output: including the

choke, transformer dc losses, and rectifier forward drop.

Assuming these add up to 0.5 V, with a duty cycle of 50%, the

transformer will be required to supply a peak voltage of

Vsec = (Vout+Vloss)/Dmax

Vsec = (5 V + 0.5 V)/0.5 = 11 V

The transformer turns ratio is thus determined: for 36-V input,

11-V output is required. The turns ratio is therefore TR=36/11

= 3.27.

For the 5-V output, the number of turns required is:

NS=Np/TR = 11.01

Therefore, the number of turns selected is 11. The Si9114A will

require a few miliamps of current to power itself and drive the

power MOSFET. This power can be taken from a winding

which peak charges a capacitor through a diode and does not

require an inductor. In this case, 11 turns can also be taken (as

this was calculated to be sufficient for 11 V).

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16

SKIN DEPTH

Conductors carrying high-frequency ac current are subject to

a “skin effect” in which the current has a tendency to flow

predominantly on the surface of the conductor instead through

the whole cross section. The value at which the current falls to

1/e (37%) is called the “skin depth.” Below this depth, very little

usage of copper is made, and multiple strands are required for

applications where higher current is required. In some

instances, it may be necessary to use larger wire than

indicated, due to mechanical assembly constraints and ease

of manufacture. The skin depth can be calculated from:

66

XD + Ǹf mm

Operation at 500 kHz means that the skin depth will be 0.093

mm (or approximately 0.1 mm). Ideally, a conductor with a

diameter just over twice the skin depth is recommended. In this

case, therefore, 0.2 mm or thereabouts will suffice.

LEAKAGE INDUCTANCE

In PWM converters, the coupling factor between windings

should be optimized to minimize the leakage inductance.

Leakage inductance is a parasitic element, storing energy that

will need to be dissipated. This leakage is a measure of the

quality of the coupling; the lower it is, the better the transformer

and its performance. The leakage inductance can be

measured by shorting out the 5-V winding and measuring the

primary inductance. Ideally, this should be zero, but in reality

values of less than 10% of the primary inductance are typical.

This value can be minimized by splitting the primary winding in

two halves, and by sandwiching all secondaries in between the

two primary halves.

In this case, the transformer was wound as follows:

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Winding

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Order

ÁÁÁÁÁÁ Á ÁÁÁÁÁÁ 1

Á 2

ÁÁÁÁ ÁÁ 3

Winding

Name

Half

Primary

Bias

Winding

Secondary

#

Start End

Turns Wire Size Pin Pin

17

1x0.3 mm

1

10

11

3x0.3 mm

3

9

11

3x0 3 mm

4,5

76

4

Half

Primary

18

1x0.3 mm

10

2

Copper wire of 0.3 mm was used to obtain single layer fill

across the surface of the bobbin.

The wound transformer had the following characteristics:

Winding

ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Primary

Inductance

883 µH

DC Resistance

151 mW

Secondary

87 µH

25 mW

The leakage inductance was measured at 2.25 mH, which

represents less than 1/4% of the primary inductance.

Document Number: 70575

16-Jan-01

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