AND8039/D

http://onsemi.com

4

summary of the MOSFET ratings are:

To further reduce the heat, an MTB8N50E was chosen.

Output Rectifier

The output rectifier will also be a surface mount D2PAK.

This efficiently couples the heat to the copper pad on the

PCB.

The maximum reverse voltage is:

Vr(min) + Vin(max)(N sec Npri) u102 V

The Peak output current is:

Iout(pk) + 2.8 Iout(max) or 11.2 A

The selected rectifier is the MURB1620CT.

Design of the Output Filter Section

As in all forward–mode converters, the output is

converted back to DC by the use of an L–C filter. A

two–stage filter is going to be used which is a much more

efficient output filter than a single stage filter. The

abbreviated schematic is shown in Figure 3.

+

D3

L1

GND

C9

N1

N2

+

L2

C10

+

C11

T1

Figure 3. Schematic of the Two–Stage Output Filter

Below the voltage feedback crossover frequency

be essentially in parallel (i.e., C9, C10 and C11). The first

stage inductor should be calculated such that it does not

enter the discontinuous–mode at light load. The

second–stage filter has its corner frequency at about 22 kHz

and provides an additional 15–20 dB of ripple attenuation

with little additional phase lag and no additional output

capacitance.

The first stage inductor should be sized to allow 20 percent

of the AC ripple current through to the capacitor. This is a

little more than is typically allowed, but the existence of the

second filter provides a more pronounced effect, thus

allowing the first filter to be smaller.

Lo [ (Vsec(min) * Vout)toff(min) 1.4 Iout(min)

(eq. 4)

The resulting minimum inductance is 88 uH. Lets round

this up to 100 uH which will give us a more standard value

off–the–shelf inductor and extend the minimum current

capabilities of the supply. Now one must choose an

inductor whose core can be driven with 4+ amps on its

winding without the fear of core saturation. Coiltronics P/N

CTX100–2–52.

Next the output filter capacitor is calculated. In

forward–mode converters, the roles of the output capacitor

are transient hold–up voltage and output ripple reduction.

The output filter inductor greatly reduces the RMS ripple

current to the output capacitor(s) thus relaxing their ratings

somewhat. The transient load hold–up function is typically

shared with other filter capacitors outside of the power

supply. So the common method of calculating the value of

the output filter capacitance is by the ripple–reduction

function. Assuming a very benign load (resistor) and so that

only the ripple is considered, one then calculates:

Co + Iout(max) (1 * DC max) Vripple

(eq. 5)

output.

This results in a total output capacitance of 533 uF. If one

allocates about one–third of this value to the first–stage

filter and two–thirds to the output, and rounding–up to the

next standard value, one gets C9, C10 and C11 as 220 uF,

50 VDC or Nichicon Part number EVR2E470MPA which

The second–stage filter inductance is determined by

setting its pole above the crossover frequency of the closed

feedback loop so that it will not contribute significant

additional phase shift, but will further reduce the ripple

voltage. If we set the output filter’s filter pole at no more than

25 percent of the switching frequency and at least three times

the filter pole of the first–stage filter, then the nominal corner

frequency of the second–stage filter is around 20–25 kHz.

The second–stage filter inductor can then be found by:

Lo(2) + (2pfp)2 (C10 ) C11)

(eq. 6)

Setting the second–stage filter pole at 22 kHz, the resulting

second–stage inductor value is 0.1 uH. This can easily be

done as an air–core inductor or a spiral PCB inductor,

which is what I will do.