AND8039/D

http://onsemi.com

2

SWITCH

VOLTAGE

SWITCH

CURRENT

MAGNETIZATION

CURRENT

RESET

Figure 2. Power Switch Waveforms

The output rectification and filter section works

identically to the buck converter. The voltage waveform of

secondary looks like an inverted primary winding waveform

except the zero voltage point is the input voltage point on the

primary waveform. The waveform goes positive when the

power switch is conducting. The output rectifier also

conducts during this time. This presents a unipolar, PWM

rectangular voltage signal to the inductor, just as found in a

typical buck converter. The catch diode then operates when

the power switch and the output rectifier are OFF.

Continuous current is then maintained through the output

filter inductor.

Design of the One–Transistor Forward Converter

Please refer to the schematic in Figure 5 when

Component designations are mentioned.

Design Specifications:

Input Voltage Range: +140–+200 VDC

Output Voltage: +28 VDC

Output Current: 0.5 A–4.0 A

Max. Output Ripple Voltage: 30 mV

Predesign Estimates:

Output Power:

Peak Input Current:

Average Input Currents:

Design of the Transformer

One begins with the transformer for every switching

power supply design. All of the needed parameters are now

known and it serves as the backbone for the remainder of the

design.

One must first select a core family that will house the

transformer. This is done first by reviewing various core

styles and their attributes. The most common off–line core

is the E–E core, for which there are several variations. The

standard E–E core is based upon the old 50–60 Hz

lamination core styles, which are very adequate for most

applications. There are some low–profile styles such as the

Philips EFD family which yields a very trim, low profile

appearance, but can cost slightly more for the basic

core–bobbin sets. Selecting an approximate core size is done

by appreciating that first the core must have a sufficient core

crossectional area to contain the needed flux density to

transport the power from the primary to the secondary

winding(s). Secondly, there must be enough winding area to

contain the required turns of the needed wire gauges.

Thirdly, for off–line transformers, the core family must have

the ability to meet the minimum creepage and clearance

dimensions of the safety agencies after the transformer is

finished. To begin, one would use an equation like equation

1 which is an artificial quantity derived from the product of

WaAc [ 0.7 (Pout Wd(pri)

108) fB max (USA) (eq. 1A)

carry the primary current in cm.

In the MKS system (Europe and the rest of the world)

WaAc [ 0.7 (Pout Wd(pri)) fB max

(eq. 1B)

carry the primary current in meters (m).

The core size can then be chosen and should be as large or

larger than this result. For off–line applications, of which

this is not, one should increase the result by about 20

percent to accommodate the added insulating tape needed

for an IEC–qualified transformer. Also, a core and bobbin

set must be used that has sufficient creepage (distance over

a surface) and clearance (distance through air) dimensions.

For 110–220 VAC applications, this is 3.2 mm between

phases, and 8.0 mm between the input and output circuits.

This may be difficult determining the off–line–suitability

of a core and bobbin from its data sheet.

In one–transistor forward converters, the operating flux

which is not used, must be released by the core prior to the

next power switch conduction cycle. This is a point of

on the transformer, thus making the transformer larger than

smaller, but increases the losses related to the core reset

by a factor of 0.04 per 100 kHz above this frequency. One

can then calculate the turns by:

Npri [ (Vin(nom)

108) 4fB max Ac (US)

(eq. 2A)