InnoSwitch3-CE
SR Disable Protection
Intelligent Quasi-Resonant Mode Switching
In each cycle SR is only engaged if a set cycle was requested by the
secondary controller and the negative edge is detected on the
FORWARD pin. In the event that the voltage on the ISENSE pin
exceeds approximately 3 times the CC threshold, the SR FET drive is
disabled until the surge current has diminished to nominal levels.
In order to improve conversion efficiency and reduce switching
losses, the InnoSwitch3-CE features a means to force switching when
the voltage across the primary switch is near its minimum voltage
when the converter operates in discontinuous conduction mode (DCM).
This mode of operation is automatically engaged in DCM and disabled
once the converter moves to continuous-conduction mode (CCM).
SR Static Pull-Down
To ensure that the SR gate is held low when the secondary is not in
control, the SYNCHRONOUS RECTIFIER DRIVE pin has a nominally
“ON” device to pull the pin low and reduce any voltage on the SR gate
due to capacitive coupling from the FORWARD pin.
Rather than detecting the magnetizing ring valley on the primary-
side, the peak voltage of the FORWARD pin voltage as it rises above
the output voltage level is used to gate secondary requests to initiate
the switch “ON” cycle in the primary controller.
Open SR Protection
The secondary controller detects when the controller enters in
discontinuous-mode and opens secondary cycle request windows
corresponding to minimum switching voltage across the primary
power MOSFET.
In order to protect against an open SYNCHRONOUS RECTIFIER
DRIVE pin system fault the secondary controller has a protection
mode to ensure the SYNCHRONOUS RECTIFIER DRIVE pin is
connected to an external FET. If the external capacitance on the
SYNCHRONOUS RECTIFIER DRIVE pin is below 100 pF, the device
will assume the SYNCHRONOUS RECTIFIER DRIVE pin is “open” and
there is no FET to drive. If the pin capacitance detected is above
100 pF, the controller will assume an SR FET is connected.
Quasi-Resonant (QR) mode is enabled for 20 ms after DCM is detected
or when ring amplitude (pk-pk) >2 V. Afterwards, QR switching is
disabled, at which point switching may occur at any time a secondary
request is initiated.
The secondary controller includes blanking of ~1 ms to prevent false
detection of primary “ON” cycle when the FORWARD pin rings below
ground. See Figure 9.
In the event the SYNCHRONOUS RECTIFIER DRIVE pin is detected to
be open, the secondary controller will stop requesting pulses from
the primary to initiate auto-restart.
If the SYNCHRONOUS RECTIFIER DRIVE pin is tied to ground at
start-up, the SR drive function is disabled and the open
SYNCHRONOUS RECTIFIER DRIVE pin protection mode is also
disabled.
8
Rev. D 08/18
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