A704
Component Selection
This application note discusses the design of a buck-based LED driver using A704 with the help of an off-line
application example. . In this example, Vin= 90~240Vac & LED string voltage= 12V are used. Anyway, the same
procedure can be used to design LED drivers with wide AC voltage input & DC voltage output.
1. Maximum output voltage consideration (Vo,max
)
The first design criterion to meet is that the maximum LED string voltage Vo,max is should be less than half the
minimum input voltage to avoid having to implement a special loop compensation technique. For this example, the
minimum input voltage
should be:
Vin,dc
V
= 2 ×V
in,dc _ min
o,max
Where, in,dc _ min is the minimum voltage of bridge diodes
V
For example, the V
in the circuit of this application note is about 100V. So the Vo,max (LED string
in,dc _ min
voltage) should < 50V.
2. Choose the Input Diode Bridge (DBR
)
The voltage rating of the bridge diode bridge will depend on the maximum value of the input voltage. The
V
current rating will depend on the maximum average current drawn by the converter.
…(1)
)
max,ac
V
=1.25×( 2 ×V
bridge
Where, Vmax,ac is the maximum input voltage RMS value.
The 1.25 factor in equation (1) is used for a safety margin.
For this design, theVbridge should be exceed than (1.25* 1.414* Vmax,ac )= (1.25*1.414* 240)=425 V. A 600V,
0.5A bridge diode is chosen in the example.
3. Choose the Input inductor and input Capacitors
Placing an inductor (L1) in series with input bridge rectifier reduces input current rejection to input source. At
this times, utilizing the equivalents series resistance (ESR) of L1 to limit the inrush current charge to input bulk
capacitor C1. The ESR should limit the inrush current not more than the Maximum Peak forward Surge Current (IFSM
)
of the bridge rectifier specification as given by equation (2), assuming maximum voltage is applied. The required
resistance is:
Vbridge
ESR ≥
(2)
IFSM
Contack Info:Samsun Zhang 13556850583 IC9898@163.com
Copyright © 2008 ADDtek Corp.
8
A704_V0.6 -- AUGUST 2008
ADDTEK [ ADDTEK CORP ]
