InnoSwitch-CH
The interaction between the leakage reactance of the secondary and
the MOSFET capacitance (COSS) leads to ringing on the voltage
waveforms at the instance of voltage reversal at the winding due to
the primary MOSFET turn-on. This ringing can be suppressed using a
RC snubber connected across the SR FET. A snubber resistor in the
range of 10 Ω to 47 Ω may be used though a higher resistance value
leads to noticeable drop in efficiency. A capacitance of 1 nF to 1.5 nF
is adequate for most designs.
Recommendations for Circuit Board Layout
See Figure 20 for a recommended circuit board layout for
InnoSwitch-CH IC.
Single-Point Grounding
Use a single-point ground connection from the input filter capacitor
to the area of copper connected to the SOURCE pins.
Bypass Capacitors
Output Capacitor
The PRIMARY BYPASS and SECONDARY BYPASS pin capacitor must
be located directly adjacent to the PRIMARY BYPASS-SOURCE and
SECONDARY BYPASS-SECONDARY GROUND pins respectively and
connections to these capacitors should be routed with short traces.
Low ESR aluminum electrolytic capacitors are suitable for use with
most high frequency flyback switching power supplies though the use
of aluminum-polymer solid capacitors have gained considerable
popularity due to their compact size, stable temperature characteris-
tics, extremely low ESR and simultaneously high RMS ripple current
rating. These capacitors enable design of compact chargers and
adapters.
Primary Loop Area
The area of the primary loop that connects the input filter capacitor,
transformer primary and InnoSwitch-CH IC should be kept as small as
possible.
Typically, 200 mF to 300 mF of aluminum-polymer capacitance is often
adequate for every ampere of output current. The other factor that
influences choice of the capacitance is the output ripple. Care should
be taken to ensure that capacitors with a voltage rating higher than
the highest output voltage with sufficient margin (>20%) be used.
Primary Clamp Circuit
A clamp is used to limit peak voltage on the DRAIN pin at turn-off.
This can be achieved by using an RCD clamp or a Zener diode (~200 V)
and diode clamp across the primary winding. To reduce EMI, minimize
the loop from the clamp components to the transformer and
InnoSwitch-CH IC.
Output Voltage Feedback Circuit
The nominal output voltage feedback pin voltage is 1.265 V [VFB].
A voltage divider network should be connected at the output of the
power supply to divide the output voltage such that the voltage at the
FEEDBACK pin will be 1.265 V when the output voltage is at the set
nominal voltage. The lower feedback divider resistor should be tied
to the SECONDARY GROUND pin. A 300 pF or higher decoupling
capacitor should be connected at the FEEDBACK pin to the SECOND-
ARY GROUND pin of the InnoSwitch-CH IC. This capacitor should be
placed physically close to the InnoSwitch-CH IC. An R-C network
may also need to be connected across the upper divider resistor in
the feedback divider network. Capacitor value should be chosen such
that the time constant matches with lower feedback divider to
achieve better load regulation and lower output ripple. Recommended
value for R is 1 kΩ in order to limit the current flowing through the
FEEDBACK pin in case of a short-circuit at the output.
Thermal Considerations
The SOURCE pin is internally connected to the IC lead frame and
provides the main path to remove heat from the device. Therefore
the SOURCE pin should be connected to a copper area underneath
the InnoSwitch-CH IC to act not only as a single point ground, but
also as a heat sink. As this area is connected to the quiet source
node, this area should be maximized for good heat sinking. Similarly
for output SR MOSFET, maximize the PCB area connected to the pins
on the package through which heat is dissipated in the SR MOSFET.
Sufficient copper area should be provided on the board to keep the
InnoSwitch-CH IC temperature safely below the absolute maximum
limits. It is recommended that the copper area provided for the
copper plane on which the SOURCE pin of the InnoSwitch-CH IC is
soldered is sufficiently large to keep the IC temperature below 85 °C
when operating the charger at full rated load and at the lowest rated
input AC supply voltage. Further de-rating can be applied depending
on any additional specific requirements.
Protection Diode for Secondary Current Shunt
The InnoSwitch-CH IC includes a secondary-side current sense
function which enables a precise CC mode of operation. The power
supply transitions from CV to CC mode automatically when the output
current exceeds the constant current regulation threshold as specified
in the data sheet.
Y Capacitor
The placement of the Y capacitor should be directly from the primary
input filter capacitor positive terminal to the output positive or return
terminal of the transformer secondary. Such a placement will route
high magnitude common mode surge currents away from the
InnoSwitch-CH IC. Note – if an input π (C, L, C) EMI filter is used
then the inductor in the filter should be placed between the negative
terminals of the input filter capacitors.
To sense the output the load current flows from the ISENSE pin
through an internal shunt to the SECONDARY GROUND pin of the IC.
The transition to CC operation occurs when the shunt voltage
exceeds ~33 mV, the very low sensing voltage ensures very low
dissipation.
Output SR MOSFET
During an output short-circuit the output filter capacitor (C10 in
Figure 1) discharges instantaneously through the internal shunt.
Depending on the output voltage, value of the output capacitance
and short circuit impedance the energy dissipated in the shunt can
be very high.
For best performance, the area of the loop connecting the secondary
winding, the output SR MOSFET and the output filter capacitor,
should be minimized. In addition, sufficient copper area should be
provided at the terminals of the SR MOSFET for heat sinking.
ESD
To prevent any damage to the IC, an external 1 A Schottky diode
between the ISENSE and the SECONDARY GROUND pins is recom-
mended for designs with an output voltage above 5 V, where the
power supply can be shorted at the output terminals. When this
diode is used, the anode should be connected to the ISENSE pin and
the cathode should be connected to the SECONDARY GROUND pin.
Sufficient clearance should be maintained (>8 mm) between the
primary-side and secondary-side circuits to enable easy compliance
with any ESD / hi-pot requirements.
12
Rev. J 10/17
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