ZXGD3105N8 Datasheet, PDF (9/13 Pages) Diodes Incorporated – Low standby power with quiescent supply current < 1mA

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Functional descriptions
ZXGD3105N8
The operation of the device is described step-by-step with reference to the timing diagram in Figure 2.
1. The detector stage monitors the MOSFET Drain-Source voltage.
2. When, due to transformer action, the MOSFET body diode is forced to conduct there is a negative voltage
on the Drain pin due to the body diode forward voltage.
3. When the negative Drain voltage crosses the turn-off Threshold voltage V_T, the detector stage outputs a
positive voltage with respect to ground after the turn-on delay time t_d(fall). This voltage is then fed to the
MOSFET driver stage and current is sourced out of the GATE pin.
4. The controller goes into proportional gate drive control — the GATE output voltage is proportional to the
MOSFET on-resistance-induced Drain-Source voltage. Proportional gate drive ensures that MOSFET
conducts during majority of the conduction cycle to minimize power loss in the body diode.
5. As the Drain current decays linearly toward zero, proportional gate drive control reduces the Gate voltage
so the MOSFET can be turned off rapidly at zero current crossing. The GATE voltage falls to 1V when the
Drain-Source voltage crosses the detection threshold voltage to minimize reverse current flow.
6. At zero Drain current, the controller GATE output voltage is pulled low to V_G(off) to ensure that the MOSFET
is off.
MOSFET
Drain Voltage
V_D
1
Body Diode
Conduction
MOSFET
Gate Voltage ^VG
2
3
90%
10%
t_r
t_d(rise)
V_T
4
5
90%
6
10%
t_f
t_d(fall)
V_G(off)
MOSFET
Drain Current
I_D
0A
Figure 2 - Timing diagram for a critical conduction mode Flyback converter
ZXGD3105N8
Document Number DS35101 Rev. 1 – 2
9 of 13
www.diodes.com
November 2011
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