InnoSwitch3-EP
filtering of the signal at the FEEDBACK pin. Zener VR2 was added for
tighter cross-regulation to limit the 12 V output when it is unloaded.
required to achieve primary sensed OVP. It is recommended that a
Zener diode with a clamping voltage approximately 6 V lower than the
bias winding rectified voltage at which OVP is expected to be
triggered be selected. A forward voltage drop of 1 V can be assumed
for the blocking diode. A small signal standard recovery diode is
recommended. The blocking diode prevents any reverse current
discharging the bias capacitor during start-up. Finally, the value of
the series resistor required can be calculated such that a current
higher than ISD will flow into the PRIMARY BYPASS pin during an
output overvoltage.
Resistors R3 and R4 provide line voltage sensing and provide a current
to U1, which is proportional to the DC voltage across capacitor C3. At
approximately 100 VDC, the current through these resistors exceeds
the line undervoltage threshold, which results in enabling of U1. At
approximately 435 VDC, the current through these resistors exceeds
the line over voltage threshold, which results in disabling of U1.
Key Application Considerations
Reducing No-load Consumption
Output Power Table
The InnoSwitch3-EP IC can start in self-powered mode, drawing
energy from the BYPASS pin capacitor charged through an internal
current source. Use of a bias winding is however required to provide
supply current to the PRIMARY BYPASS pin once the InnoSwitch3-EP
IC has started switching. An auxiliary (bias) winding provided on the
transformer serves this purpose. A bias winding driver supply to the
PRIMARY BYPASS pin enables design of power supplies with no-load
power consumption less than 15 mW. Resistor R6 shown in Figure 9
should be adjusted to achieve the lowest no-load input power.
The data sheet output power table (Table 1) represents the maximum
practical continuous output power level that can be obtained under
the following conditions:
1. The minimum DC input voltage is 90 V or higher for 85 VAC input,
220 V or higher for 230 VAC input or 115 VAC with a voltage-
doubler. Input capacitor voltage should be sized to meet these
criteria for AC input designs.
2. Efficiency assumptions depend on power level. Smallest device
power level assumes efficiency >84% increasing to >89% for the
largest device.
3. Transformer primary inductance tolerance of ±10%.
4. Reflected output voltage (VOR) is set to maintain KP = 0.8 at
minimum input voltage for universal line and KP = 1 for high input
line designs.
Secondary-Side Overvoltage Protection (Auto-Restart Mode)
The secondary-side output overvoltage protection provided by the
InnoSwitch3-EP IC uses an internal auto restart circuit that is
triggered by an input current exceeding a threshold of IBPS(SD) into the
SECONDARY BYPASS pin. The direct output sensed OVP function can
be realized by connecting a Zener diode from the output to the
SECONDARY BYPASS pin. The Zener diode voltage needs to be the
difference between 1.25 × VOUT and 4.4 V − the SECONDARY BYPASS
pin voltage. It is necessary to add a low value resistor in series with
the OVP Zener diode to limit the maximum current into the
SECONDARY BYPASS pin.
5. Maximum conduction losses for adapters is limited to 0.6 W, 0.8 W
for open frame designs.
6. Increased current limit is selected for peak and open frame power
columns and standard current limit for adapter columns.
7. The part is board mounted with SOURCE pins soldered to a
sufficient area of copper and/or a heat sink to keep the SOURCE
pin temperature at or below 110 °C.
Selection of Components
8. Ambient temperature of 50 °C for open frame designs and 40 °C
Components for InnoSwitch3-EP
Primary-Side Circuit
for sealed adapters.
9. Below a value of 1, KP is the ratio of ripple to peak primary
current. To prevent reduced power delivery, due to premature
termination of switching cycles, a transient KP limit of ≥0.25 is
recommended. This prevents the initial current limit (IINT) from
being exceeded at MOSFET turn-on.
BPP Capacitor
A capacitor connected from the PRIMARY BYPASS pin of the
InnoSwitch3-EP IC to GND provides decoupling for the primary-side
controller and also selects current limit. A 0.47 mF or 4.7 mF capacitor
may be used. 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. Their small size also makes it ideal for compact power supplies.
16 V or 25 V rated X5R or X7R dielectric capacitors are recommended
to ensure that minimum capacitance requirements are met.
Primary-Side Overvoltage Protection (Latch-Off Mode)
Primary-side output overvoltage protection provided by the
InnoSwitch3-EP IC uses an internal latch that is triggered by a
threshold current of ISD into the PRIMARY BYPASS pin. In addition to
an internal filter, the PRIMARY BYPASS pin capacitor forms an
external filter helping noise immunity. For the bypass capacitor to be
effective as a high frequency filter, the capacitor should be located as
close as possible to the SOURCE and PRIMARY BYPASS pins of the
device.
Bias Winding and External Bias Circuit
The internal regulator connected from the DRAIN pin of the MOSFET
to the PRIMARY BYPASS pin of the InnoSwitch3-EP primary-side
controller charges the capacitor connected to the PRIMARY BYPASS
pin to achieve start-up. A bias winding should be provided on the
transformer 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.
The primary sensed OVP function can be realized by connecting a
series combination of a Zener diode, a resistor and a blocking diode
from the rectified and filtered bias winding voltage supply to the
PRIMARY BYPASS pin. The rectified and filtered bias winding output
voltage may be higher than expected (up to 1.5X or 2X the desired
value) due to poor coupling of the bias winding with the output
winding and the resulting ringing on the bias winding voltage
waveform. It is therefore recommended that the rectified bias
winding voltage be measured. This measurement should be ideally
done at the lowest input voltage and with highest load on the output.
This measured voltage should be used to select the components
The turns ratio for the bias winding should be selected such that 7 V
is developed across the bias winding at the lowest rated output
voltage of the power supply at the lowest load condition. If the
voltage is lower than this, no-load input power will increase.
10
Rev. D 08/18
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