DCM Flyback

9

Design Note DN 2013-01

V1.0 January 2013

Table 6: Recommended Wire Gauge by RMS current

It is important that operating Bmax should not exceed the saturating flux density (Bsat) given on the core's

data sheet. Bsat of ferrite core varies depending on the core material and temperature but most of them has a Bsat

rating closed to 400mT. If there is no further reference data used Bmax= 300mT. Higher Bmax allows for lower

number of primary turns for lower conduction loss but with higher core loss. For optimized design the sum of both

the core loss and the copper loss should be mutually minimized. This usually happened near the point where core

loss is equal to the copper loss.

STEP 6:

Determine the number of turns for the secondary main output (Ns) and other auxiliary turns (Naux):

To get the secondary turns first determine the turns ratio, n

Where: Np and Ns are the primary and secondary turns respectively, Vout is the output voltage and VD is the

secondary diode voltage drop, typically 0.5V for schottky diode at low to moderate current.

For additional number turns such as auxiliary winding for VCC supply the number of turns can be calculated as

follows; Where Vaux is the flyback auxiliary winding , VDaux is the diode voltage drop on this winding.

(Equation 11)

Most Flyback controllers need an auxiliary winding to supply the IC

; this is true of all CoolSET™ types, too.

Use the start up VCC supply, as indicated on the data sheet, to decide the auxiliary number of turns. For non

integer number of turn round off to the next highest integer

STEP 7: Determining the wire size for each output windings: In order to determine the required wire size the RMS

current for each winding should be determined.

Primary winding RMS current:

Secondary Winding RMS current:

: A current density between 150 - 400 circular mil per

Ampere can be used as a starting point to calculate the required

wire gauge. Below is the quick selection for choosing the

appropriate wire gauge using 200CM/A, given the winding's

RMS current. The wire diameter with basic insulation for

different magnet wire gauges are also shown.

STEP 8:

Transformer Construction and Winding Design

Iteration:

Once transformer parameters have been decided, determine

whether the number of turns and the wire size chosen would fit in the given

transformer core size. This step may require several iteration of between the

chosen core, winding gauge and number of turns.

Figure 4 shows the winding area for an EE ferrite core, using the wire diameter

and the number of turns for each winding, we can approximate if the desired

winding will fit given its winding area (w and h). If winding will not fit, either the

number of turns, wire gauge or core size (controlling window area) will need to be

adjusted.

The winding scheme has a considerable influence on the performance and

reliability of the transformer. To reduce leakage inductance, the use of a sandwich

construction, as shown in Figure 5, is recommended. It also needs to meet international safety requirements. A

transformer must have adequate insulation between primary and secondary windings. This can be achieved by

using a margin-wound construction (Figure 5A) or by using triple insulated wire for the secondary winding (Figure

(Equation 9)

(Equation 10)

(Equation 11)

(Equation 12)

(Equation 13)

Figure 4: Ferrite core winding

area