Fairchild FPS service manual

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Application Note AN4140

Transformer Design Consideration for off-line Flyback Converters using Fairchild Power Switch (FPS

1. Introduction

For flyback coverters, the transformer is the most important factor that determines the performance such as the efficiency, output regulation and EMI. Contrary to the normal transformer, the flyback transformer is inherently an inductor that provides energy storage, coupling and isolation for the flyback converter. In the general transformer, the current flo ws in bot h the prim ary and se condary wi nding at the same time. However, in the flyback transformer, the current flows only in the primary winding while the energy in the core is charged and in the secondary windin g while the energy in the core is discharged. Usually gap is introduced between the core to increase the energy storage capacity.

This paper presents practical design considerations of transformers for off-line flyback converters employing Fairchild Power Switch (FPS). In order to give insight to th e reader, practical design examples are also provided.

2. General Transformer design procedure

(1) Choose the proper core Core type

for commercial SMPS (Switchied mode power supply) applications. Various ferrite cores and bobbins are shown in Figure 1. The type of the core should be chosen with regard to system requirements including number of outputs, physical height, cost and so on. Table 1 shows features and typical application of various cores.

: Ferrite is the most widely used core material

Core Features Typical Applications EEEI-Low cost Aux. power

EFD

-Low profile LCD Monitor

EPC EER -Large winding window area

-Various bobbins for multiple output

PQ -Large cross sectional area

-Relatively expensive

Table 1. Features and typical applications of various cores

Core size : Actually, the initial selection of the core is

bound to be crude since there are too many variables. One way to select the proper core is to refer to the manufacture's core selection guide. If there is no proper reference, use the table 2 as a starting point. The core recommended in table 1 is typical for the universal input range, 67kHz switching frequency and 12V singl e outp ut app lication . When th e in put voltage range is 195-265 Vac (European input range) or the switching frequency is higher than 67 kHz, a small er core can be used. For an application with low voltage and/or mul tiple outputs, usually a larger core should be used than recommended in the table.

Output Power

0-10W EI12.5

10-20W EI22 EE19 EPC19 20-30W EI25 EE22 EPC25 EER25.5 30-50W EI28

50-70W EI35 EE30 EER28L 70-100W EI40 EE35 EER35 100-150W EI50 EE40 EER40

150-200W EI60 EE50

EI core EE core EPC core EER core

EE8 EI16 EI19

EI30

EE10

EE13

EE16

EE25 EPC30 EER28

EE60

)

Battery charger

CRT monitor, C-TV DVDP, STB

EPC10 EPC13 EPC17

EER42 EER49

Figure 1. Ferrite core (TDK)

Table 2. Core quick selection table (For universal input range, fs=67kHz and 12V single output)

Rev. 1.0.0

AN4140 APPLICATION NOTE

∆

Once the core type and size are determined, the following variables are obtained from the core data sheet.

- A

: The cross-sectional area of the core (mm2)

- A

: Winding window area (mm2)

- B

: Core saturation flux density (tesla)

sat

Figure 2 shows the Ae and A

of a core. The typical B-H

characteristics of ferrite core from TDK (PC40) are shown in Figure 3. Since the saturation flux density (B

) decreases as

sat

the temperature increases, the high temperature characteristics should be considered. If there is no reference data, use

=0.3~0.35 T.

sat

AwAw

AeAe

(2) Determine the primary side inductance (Lm) of the transformer

In order to determine the primary side inductance, the following variables should be determined first. (For a detailed design procedure, please refer to the application note AN4137.)

- P

: Maximum input power

- f

: Switching frequency of FPS device

min

- V

- D

- K

: Minimum DC link voltage

: Maximum duty cycle

max

: Ripple factor, which is defined at the minimum input

voltage and full load condition, as shown in Figure 4. For DCM opera tion, K

= 1 and for CCM operation KRF < 1.

The ripple factor is closely related with the transformer size and the RMS value of the MOSFET current. Even though the conduction loss in the MOSFET can be reduced through reducing the ripple factor, too small a ripple factor forces an increase in transformer size. Considering both efficiency and core size, it is reasonable to set K universal input range and K

= 0.4-0.8 for the European

= 0.3-0.5 for the

input range. Meanwhile, in the case of low power applications below 5W where size is most critical, a relatively large ripple factor is used in order to minimize the transformer size. In that case, it is typical to set K

0.7 for the universal input range and K

= 1.0 for the

= 0.5-

European input range.

Figure 2. Window Area and Cross Sectional Area

Magnetization Curves (typical)

Material :PC40

500

400

300

200

Flux density B (mT)

100

Figure 3. Typical B-H characteristics of ferrite core

(TDK/PC40)

800 1600

Magnetic field H (A/m)

25 ℃℃℃℃

60 ℃℃℃℃

100 ℃℃℃℃

120 ℃℃℃℃

peak

EDC

peak

EDC

max

Figure 4. MOSFET Drain Current and Ripple Factor (KRF)

CCM operation : KRF < 1

EDC

DCM operation : KRF =1

With the given variables, the primary side inductance, Lm is obtained as

min

()

--------------------------------------------- -= (1) 2PinfsK

⋅

max

APPLICATION NOTE AN4140

where V maximum duty cycle, P the switching frequency of the FPS device and K

min

is the minimum DC input voltage, D

is the maximum input power, fs is

max

is the

ripple factor. Once L

is determined, the maximum peak cur rent and RMS

current of the MOSFET in normal operation are obtained as

peak

rms

where I

and

()

∆

-----+= (2)

EDC

------------------------------------- -=4()

EDC

min

-----------------------------------= (5) Lmf

∆

 

max

-----

------------- -=3()

min

⋅

max

With the chosen core, the minimum number of turns for the transformer primary side to avoid the core saturation is given by

LmI

min

over

-------------------

= (6)

satAe

(turns)

(3) Determine the number of turns for each output

Figure 6 shows the simplified diagram of the transformer, whrere V by the feedback control while V

stands for the reference output th at is regulated

stands for the n-th

o(n)

output. First, determine the turns ratio (n) between the primary side

and the feedback controlled secondary side as a reference.

--------------------------

Vo1VF1+

where N

and Ns1 are the number of turns for primary side

and reference output, respectively, V and V

is the diode (DR1) forward voltage drop of the

---------= = (7) N

is the output voltage

reference output that is regulated by the feedback control. Then, determine the proper integer for N

resulting Np is larger than N

min

obtained from equation (6).

so that the

The number of turns for the other output (n-th output) is determined as

sn()

on()VFn()

---------------------------------=N V

o1VF1

⋅

turns() 8()

The number of turns for Vcc winding is determined as

where L

is the primary side inductance, I

pulse-by-pulse current limit level, A area of the core and B

is the saturation flux density in

sat

is the cross-sectional

is the FPS

over

tesla. If the pulse-by-pulse current limit lev el of FPS is larger than

the peak drain current of the power supply design, it may result in excessive transformer size since I

is used in

over

determining the minimum primary side turns as shown in equation (6). Therefore, it is required to choose a FPS with proper current limit specifications or to adjust the peak drain current close to I in Figure 5. It is reasonable to design I

considering the transient response and tolerance of I

over

by increasing the ripple factor as shown

over

peak

to be 70-80% of

over

Pulse-by-pulse current limit of FPS (I

70-80% of I

∆

Increasing ripple

factor (K

)

over

peak

Decreasing pirmary

side Inductance (L

)

over

∆

EDC

)

Vcc*V

--------------------------- -=N

where V FPS device, and V

* is the nominal value of the supply voltage of the

is the forward voltage drop of Da as

defined in Figure 6. Since V increases, it is proper to set V

o1VF1

⋅

turns() 9()

increases as the output load

* as Vcc start voltage (refer to

the data sheet) to avoid trigge ring the ov er voltage pro tectio n during normal operation.

S(n)

F(n)

R(n)

+ V

- V

O(n)

Figure 5. Adjustment peak drain current

Figure 6. Simplified diagram of the transformer

AN4140 APPLICATION NOTE

other windings can act as Faraday shields. When the primary Once the number of turns on the primary side have been determined, the gap length of the core is obtained through approximation as

=mm() 10()

G40



πA

-------------------- -



1000L



–

-----A

side winding has more than t wo layers, the innermost layer

winding should start from th e drain pin of FPS as show n in

Figure 7. This allows the winding driven by the highest

voltage to be shielded by other windings, thereby

maximizing the shielding effect.

where A

is the AL-value with no gap in nH/turns2, Ae is the

cross sectional area of the core as shown in Figure 2, L specified in equation (1) and N

is the number of turns for

the primary side of the transformer

(4) Determine the wire diameter for each winding

The wire diameter is determined based on the rms current through the wire. The current density is typically 5A/mm when the wire is long (>1m). When the wire is short with a small number of turns, a current density of 6-10 A/mm also acceptable. A void using wire with a diameter larger than 1 mm to avoid severe eddy current losses as well as to make winding easier. For high current output, it is better to use parallel windings with multiple strands of thinner wire to minimize skin effect.

3. Transformer Construction Method.

(1) Winding Sequence

(b) Vcc winding

In general, the voltage of each winding is influenced by the

voltage of the adjacent winding. The optimum placement of

the Vcc winding is determined by the over voltage pro tection

(OVP) sensitivity, the Vcc operating range and control

scheme.

-Over voltage protection (OVP) sensitivity : When the

output voltage goes above its normal operation value due to

some abnormal situation, Vcc voltage also increases. FPS

uses Vcc voltage to indirectly monitor the over voltage

situation in the secondary side. However, a RCD snubber

network acts as an another output as shown in Figure 8 and

Vcc voltage is also influenced by the snubber capacitor

voltage. Because the snubber voltage increases as the drain

current increases, OVP of FPS can be triggered not only by

the output over voltage condition, but also by the over load

condition.

The sensitivity of over voltage protection is closely related to

the physical distance between windings. If the Vcc winding

is close to the secondary side output winding, Vcc voltage

will change sensitively to the variation of th e output voltage.

Meanwhile, if the Vcc winding is placed close to the primar y

side winding, Vcc voltage will vary sensitively as the

snubber capacitor voltage changes.

(a) Primary winding

3mm 3mm

Barrier tape

Insulation tape

...... Na ...

.......... Ns .........

....... Np/2 .....

........ Np/2 ..... .

Bobbin

To FPS Drain pin

Figure 7. Primary side winding

It is typical to place all the primary winding or a portion of the primary winding innermost on the bobbin. This minimizes the length of wire, reducing the conduction loss in the wire. The EMI noise radiation can be reduced, since the

Figure 8. Primary side winding

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