AND8099/D

http://onsemi.com

2

The component losses can be evaluated and budgetized with

the following formula:

the desired circuit section per total power

supply loss.

Usually, 35% of the losses come from the power

MOSFET, 60% from the output rectifier, 5% from the

magnetics, and 5% from miscellaneous sources.

MOSFET Power Losses = 2.82 · 35% = 0.987 W

Rectifier Power Losses = 2.82 · 60% = 1.692 W

Circuit Description

Input Block

The input block of the power supply consists of a fuse, an

EMI filter, a diode bridge rectifier, and an input bulk

capacitor.

Fuse

The fuse F1 is protecting the circuit from current surges

occurring at turn on. In this application, F1 is rated for 2.0 A,

125 VAC.

EMI Filter

The EMI filter is suppressing common mode and

differential mode noise and is very dependent upon board

layout, component selection, etc. An X capacitor C1 and a

common mode choke L1 are placed across the AC lines to

attenuate differential mode noise, see Figure 1. The EMI

inductor is slowing down any transient voltage surge to

reduce high frequency noise. Both the capacitor and choke

should be placed before the diode bridge and as close to the

AC line input as possible to minimize RFI.

Diode Bridge Rectifier

In order to choose the right diode bridge rectifier, the

values of the forward and surge currents and DC blocking

voltage must be considered. The surge current can reach

values up to five times that of the average input rms current.

It is therefore necessary to select a rectifier capable of

handling such large currents.

DC Blocking Voltage is calculated at high line:

VR w Vpeak(max) + Vin(max) ·2 + 375 VDC

Forward Current:

IF w 1.5 · Iin(avg) + 1.5 · 0.160 + 0.240 A

Surge Current:

IFSM w 5·IF + 5 · 0.240 + 1.2 A

Input Bulk Capacitor

The purpose of the input bulk capacitor C2 is to hold up

the rectified line voltage and also to filter out common mode

noise. It is placed between the bridge rectifier output and

ground. The size of the bulk capacitor depends on peak

rectified input voltage and the ripple voltage magnitude. A

larger capacitor will lower the ripple voltage on the DC input

line, but will induce a larger surge current when the supply

is powered up. Assuming a ripple magnitude of about 32%

calculated using:

Cbulk +

Pin

fac · (Vpeak(min)2 * Vin(low)2)

+

12.82

60 · (1202

* 80.22)

+ 27 mF

Select the closest standard capacitor of 33

mF with low ESR.

Aluminum electrolytics are preferred because of their

sturdiness and high reliability.

Power Stage

At the heart of the power stage is the ON Semiconductor

NCP1055. The NCP1055 is a high voltage switching

regulator that uses a fixed-frequency, duty cycle controlled

oscillator. Rectified AC line voltage is applied to the startup

circuit Pin 5 through the primary winding of the transformer.

The circuit then routes current to the supply capacitor C5

which is typically connected to Pin 1. A switching cycle

begins when the oscillator charges and discharges an on chip

timing capacitor which generates a square wave signal used

to pulse width modulate the power switch circuit. The

control input pin is monitoring source or sink current drawn

by an optocoupler. When the power supply output is greater

than the reference voltage, the optocoupler begins to

conduct pulling on the control input. The output of the

has the ability to either turn the power switch circuit on or

Magnetics Calculations

The next step is the design of the flyback transformer. The

design of the magnetics block is the most important and

delicate part of the whole design process because it will

determine how well the power supply will perform. The

flyback-mode transformer functions by first conducting

current in the primary winding, thus storing energy in the

core of the transformer. The core energy is then transferred

to the secondary winding when the primary side is turned

off. The core and bobbin are standard EFD20 sizes.