AND8099 Datasheet, PDF (2/8 Pages) ON Semiconductor – 5.0 V, 2.0 A Flyback Converter

AND8099/D
The component losses can be evaluated and budgetized with
the following formula:
P_loss = P_in (1 - eff) · P_% where P_% is the percentage loss of
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.
Estimated Power Loss = P_in - P_out = 12.82 - 10 = 2.82 W
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) · ^Ǹ2 + 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%
of the peak rectified voltage at low line, C_bulk can then be
calculated using:
Cbulk
+
fac
·
Pin
(Vpeak(min)2
*
Vin(low)2)
+
60
·
12.82
(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
control input is then sampled continuously during t_on and
has the ability to either turn the power switch circuit on or
off at any time within t_on.
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.
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