Diodes ZXGD3105N8 - Manual (Page 7)

ZXGD3105N8
Document Number DS35101 Rev. 3 - 2
7 of 14
www.diodes.com
March 2013
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A Product Line of
Diodes Incorporated
ZXGD3105N8
Application Information
The purpose of the ZXGD3105 is to drive a MOSFET as a low-V
F
Schottky diode replacement in isolated AC/DC converter. When combined with
a low R
DS(ON)
MOSFET, the controller can yield significant power efficiency improvement, whilst maintaining design simplicity and incurring
minimal component count. Figure 1 shows the typical configuration of ZXGD3105 for synchronous rectification in a low output voltage Flyback
converter.
A typical circuit configuration of synchronous rectification with ZXGD3105 for use in resonant converter is shown in Figure 2. Two ZXGD3105
together with two synchronous MOSFETs should be used on the secondary side of the center tapped transformer winding.
Figure 1 Typical Flyback application schematic
Figure 2 Synchronous rectification in resonant converter
Threshold Voltage and Resistor Setting
The correct selection of external resistors R
REF
and R
BIAS
is important for optimum device operation. R
REF
and R
BIAS
supply fixed current into the
I
REF
and I
BIAS
pin of the controller. I
REF
and I
BIAS
combines to set the turn-off threshold voltage level, V
T
. In order to set V
T
to -10mV, the
recommended I
REF
and I
BIAS
are 1.02mA and 0.54mA respectively.
The values for R
REF
and R
BIAS
are selected based on the V
CC
voltage. If the V
CC
pin is connected to the power converter’s output, the resistors
should be selected based on the nominal converter’s output voltage. Table 1 provides the recommended resistor values for different Vcc voltages.
Supply, V
CC
(V)
Bias Resistor, R
BIAS
(kΩ)
Reference Resistor, R
REF
(kΩ)
5 9.6 4.3
10 18 9.1
12 24 11
15 30 15
Table 1 Recommended resistor values for different V
CC
voltages