ZXGD3103N8
Document number: DS32255 Rev. 3 - 2
June 2016
© Diodes Incorporated
Operation in Typical Application
The operation of the device is described step-by-step with reference to the timing diagram in Figure 3.
1. The detector monitors the MOSFET Drain-Source voltage.
2. When, due to transformer action, the MOSFET body diode is forced to conduct there is approximately -0.8V on the Drain pin.
3. The detector outputs a positive voltage with respect to ground, 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 on-resistance-induced Drain-Source
voltage drop across the MOSFET. Proportional gate drive ensures that MOSFET conducts for majority of the conduction cycle and minimizes body
diode conduction time.
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 is removed 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.
Figure 3 shows typical operating waveforms for ZXGD3103 driving a MOSFET with Q
g(TOT)
= 82nC in a Flyback converter operating in critical
conduction mode.
10%
t
R
t
D1
-10mV
90%
t
F
Body Diode
Conduction
V
D
V
G
I
D
0A
t
D2
1
2
3
4
5
6
90%
10%
Figure 3. Timing Diagram for a Critical Conduction Mode Flyback Converter
