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AN701

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

Vishay Semiconductors 

AN701

Vishay Siliconix

Appendix A

FORWARD CONVERTER TRANSFORMER

DESIGN

The forward converter transformer is operated as a voltage

transformer. An ac source applies voltage across the primary,

which is then transformed (up or down) by the turns ratio. For

correct operation, the transformer must be correctly sized (in

order to avoid core saturation) and large enough to accept the

number of turns required. Since these two requirements may

often compete with one another, compromise may be

necessary to complete a design.

In operation, the transformer is driven through the magnetic

B/H loop. In a forward converter, the core is only driven in

quadrant 1. Care must be taken not to saturate it, since only a

“minor loop” is used. The core is driven in the +H direction, and

when the MOSFET turns off, the core is allowed to “float back”

to the H = 0 position. As there is no negative drive, the core

cannot be driven into quadrant 3 (as in the case of a push-pull

converter), and so the core always returns to the BREM

(remnant) position.

B

4

Bsat

Brem1

Brem2

1

H

be reduced. once this is done, the problem becomes

insignificant, very small core excursions are used.

In selecting the core type and size, the transformer losses

need to be divided more or less evenly between core and

copper losses. Most manufacturers will now supply formulas

extracted from core loss tables that allow precise core loss

calculations to be made. It is important to remember that most

ferrite manufacturers use peak-to-peak flux for their

calculations, while for single quadrant converters, the core

losses will be halved compared to the data published. Most

ferrite manufacturers will likewise recommend levels no higher

than 200 mT for the highest flux swing, to cover the complete

temperature and operating conditions. Most ferrites have the

lowest core losses at between 75 and 95_C. As the

temperature in the transformer gradually rises, the efficiency of

the transformer will increase.

To design a forward converter transformer, the following data

is necessary:

FSW

dmax

h

Vinmin

Pout

Vout

Operating Switching Frequency, usually 20 to

500 kHz

Maximum duty cycle, usually 50% in

Vishay Siliconix products

Target efficiency, usually 0.75 to 0.85

Minimum input voltage used

Total output power

Output Voltage(s) required

3

2

Figure 28

In some cases, a small air gap is placed in the core, which

causes the B/H curve to be skewed out. The remnant position

can thus be lowered from the Brem1 to the Brem2 position,

allowing a larger area of operation. In all joined cores (such as

RM or EE), this small gap actually exists in the form of the

interface between the two cores and causes the BH curve to

skew over. In high-frequency applications, this gap has a

significant beneficial effect on the operation of the transformer.

The number of turns is already low, and the gap prevents core

saturation.

Other converters (such as half-bridge and push-pull) use all

four quadrants and make better use of the whole core. These,

however, require additional power switching devices and

windings and are therefore not used. At high frequencies, the

core losses of ferrite materials are high, so the used flux must

Document Number: 70575

16-Jan-01

In this case, a design for a 48-V (38- to 60-V), 5-V, 4-A (20-W)

device operating at 50% duty cycle with 500-kHz switching

frequency will be demonstrated. The period of conversion will be

2 ms, and the maximum on-time thus Tonmax = 1 ms.

The EF12.6, or the EPC13, which have similar characteristics,

will be used as the core. The core operating flux has been

selected to 85 mT as a first pass design. The loss in Philips 3F4

material is calculated from (source Philips Components):

Pv = 12x10–2@ƒ1.75@B29@(0.95x10–4@T2–1.1x10–2@T + 1.15)

Where:

PV = power loss in W/m3

B = operating flux density in Tesla

ƒ = operating frequency in Hertz

T = core temperature in _C

With 500 kHz, 85 mT, and 50_C, the core loss can be

calculated as:

Pv = 742.5 x 103 W/m3 or 0.742 W/cm3

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