InnoSwitch-CH
Audible Noise
However a smaller capacitor value <22 mF also can be used as long
as there is enough bias current into the PRIMARY BYPASS pin during
no-load operation at the lowest rated output voltage such that the
internal tap does not turn on.
The cycle skipping mode of operation used in the InnoSwitch-CH IC
can generate audio frequency components in the transformer. To
limit this audible noise generation the transformer should be designed
such that the peak core flux density is below 3000 Gauss (300 mT).
Following this guideline and using the standard transformer produc-
tion technique of dip varnishing practically eliminates audible noise.
Vacuum impregnation of the transformer should not be used due to
the high primary capacitance and increased losses that result. Higher
flux densities are possible, however careful evaluation of the audible
noise performance should be made using production transformer
samples before approving the design. Ceramic capacitors that use
dielectrics such as Z5U, when used in the clamp circuits and especially
the bias supply (C1 and C5 in Figure 14) may also generate audio
noise. If this is the case, try replacing them with a capacitor having
a different dielectric or construction, for example a film type for the
clamp or electrolytic for the bias.
A small resistor ranging from 2.2 Ω to 10 Ω in series with the bias
winding diode is recommended in order to damp the ringing that
could get coupled to FORWARD pin from bias winding. Waveforms
shown in FORWARD Pin Resistor section are the examples for
acceptable and unacceptable waveforms on the FORWARD pin during
secondary rectifier on period.
Primary Sensed OVP (Overvoltage Protection)
The voltage developed across the bias winding output tracks the
power supply output voltage. Though not precise, a reasonably
accurate detection of output voltage condition can be achieved by the
primary-side controller using the bias winding voltage. A Zener diode
connected from the bias winding output to the PRIMARY BYPASS pin
can reliably detect a fault condition that leads to increase in output
voltage beyond the set limits and causes the primary-side controller to
latch off preventing damage of components due to the fault conditions.
Selection of Components
Components for InnoSwitch-CH Primary-Side Circuit
BPP Capacitor
It is recommended that the highest voltage at the output of the bias
winding should be measured for normal steady-state conditions at full
rated load and lowest rated input voltage and also under transient
load conditions. A Zener diode rated for 1.25 times this measured
voltage will typically ensure that OVP protection will not operate
under any normal operating conditions and will only operate in case
of a fault condition.
Capacitor connected from the PRIMARY BYAPSS pin of the
InnoSwitch-CH IC provides decoupling for the primary-side controller
and also selects current limit. A 0.1 mF, 10 mF or 1 mF capacitor may
be used as indicated in the InnoSwitch-CH data sheet. Though
electrolytic capacitors can be used, often surface mount multi-layer
ceramic capacitors are preferred for use on double sided boards as
they enable placement of capacitors close to the IC and design of
compact switching power supplies. 16 V or 25 V rated X5R or X7R
dielectric capacitors are recommended to ensure minimum capaci-
tance requirements are met.
Use of the primary sensed OVP protection is highly recommended.
Primary-Side Snubber Clamp
A snubber circuit should be used on the primary-side as shown in the
example circuit. This prevents excess voltage spikes at the drain of
the MOSFET at the instant of turn-off of the MOSFET during each
switching cycle. Though conventional RCD clamps can be used, RCDZ
calmps offer the highest efficiency. The circuit example shows in
Figure 14 uses RCD clamp with a resistor in series with the clamp
diode . This resistor dampens the ringing at the drain and also limits
the reverse current through the clamp diode during reverse recovery.
Standard recover glass passivated diodes with low junction capaci-
tance are recommended as these enable partial energy recovery from
the clamp thereby improving efficiency.
Bias Winding and External Bias Circuit
The internal regulator connected from the drain pin of the MOSFET to
the PRIMARY BYPASS pin of the InnoSwitch-CH primary-side controller
charges the capacitor connected to the PRIMARY BYPASS pin to
achieve start-up. A bias winding should be provided on the trans-
former with a suitable rectifier and filter capacitor to create a bias
supply that can be used to supply at least 1 mA of current to the
PRIMARY BYPASS pin.
Turns ratio for the bias winding should be selected such that 9 V is
developed across the bias winding at the lowest rated output voltage
of the charger at the lowest (or no-load) load condition. If the
voltage is lower than this, the no-load input power will be higher than
expected.
Components for InnoSwitch-CH Secondary-Side Circuit
SECONDARY BYPASS Pin – Decoupling Capacitor
A 2.2 mF, 25 V multi-layer ceramic capacitor should be used for
decoupling the SECONDARY BYPASS pin of the InnoSwitch-CH IC. A
significantly higher value will lead to output voltage overshoot during
start-up and lower values may lead to unpredictable operation. The
capacitor must be located adjacent to the IC pins. The 25 V rating is
necessary to guarantee the actual value in operation since the capaci-
tance of ceramic capacitors drops with applied voltage. 10 V rated
examples are not recommended for this reason. Capacitors with X5R
or X7R dielectrics should be used for best results.
The bias current from the external circuit should be set to approxi-
mately 300 mA to achieve less than 10 mW no load power consump-
tion when operating the charger at no load and 230 VAC input voltage.
A glass passivated standard recovery rectifier diode with low junction
capacitance is recommended to prevent snapped recovery typical of
fast or ultrafast diodes which typically leads to higher radiated EMI.
A filter capacitor of at least 22 mF with a voltage rating 1.2 times
greater than the highest voltage developed across the capacitor is
recommended. Highest voltage is typically developed across this
capacitor when the supply is operated at the highest rated output
voltage and rated load with the lowest input AC supply voltage.
FORWARD Pin Resistor
A 47 Ω, 5% resistor is recommended to ensure sufficient IC supply
current. A higher or lower resistor value should not be used as it can
affect device operation such as the synchronous rectifier drive timing.
10
Rev. J 10/17
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