InnoSwitch-CH
Better load regulation and lower output ripple can be achieved by
matching the time constants of upper and lower feedback divider
network. As shown in Figure 15.
ringing during switching transients that would otherwise create
radiated EMI. The gate of Q1 is turned on by secondary-side
controller inside the InnoSwitch-CH IC based on the winding voltage
sensed via resistor R5 and fed into the FORWARD pin of the IC.
RB CB , RA CA
In continuous conduction mode of operation, Q1 is turned off just
prior to the secondary-side commanding a new switching cycle from
the primary. In discontinuous mode of operation, Q1 is turned off
when the voltage drop across the MOSFET falls below a threshold of
approximately -24 mV [VSR(TH)].
Key application Considerations
Output Power Table
The data sheet output power table (Table 1) represents the minimum
practical continuous output power level that can be obtained under
the following assumed conditions:
As both SR and primary MOSFET control resides on the secondary-
side, any possibility of cross conduction of the two MOSFETs is
eliminated. In turn the time Q1 is on can be maximized for lowest
loss and allows removal of a parallel Schottky diode and/or the use of
a lower cost higher RDS(ON) device for the same efficiency compared to
standalone SR controllers.
1. The minimum DC input voltage is 90 V or higher for 85 VAC input,
or 220 V or higher for 230 VAC input or 115 VAC with a voltage
doubler. The value of the input capacitance should be sized to
meet these criteria for AC input designs.
2. Efficiency of >82%.
3. Minimum data sheet value of I2f.
The secondary-side of the InnoSwitch-CH IC is self-powered from
either the secondary winding forward voltage or the output voltage.
Capacitor C7 connected to the SECONDARY BYPASS pin of
4. Transformer primary inductance tolerance of ±10%.
5. Reflected output voltage (VOR) of 110 V.
6. Voltage only output of 12 V with a synchronous rectifier.
7. Increased current limit is selected for peak and open frame power
columns and standard current limit for adapter columns.
8. The part is board mounted with SOURCE pins soldered to a
sufficient area of copper and/or a heat sink is used to keep the
SOURCE pin temperature at or below 110 °C.
InnoSwitch-CH IC (U1) provides decoupling for the internal circuitry.
During CC (constant current) operation, when the output voltage
falls, the device will power itself from the secondary winding directly.
During the on-time of the primary-side power MOSFET, the forward
voltage that appears across the secondary winding is used to charge
the decoupling capacitor C7 via resistor R5 and an internal regulator.
This allows output current regulation to be maintained down to <2.5 V.
Below this level the unit enters auto-restart until the output load is
reduced.
9. Ambient temperature of 50 °C for open frame designs and 40 °C
for sealed adapters.
*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 (IINIT) from being exceeded at
MOSFET turn-on.
Output current is sensed internally between the ISENSE and
SECONDARY GROUND pins with a threshold of approximately 33 mV
(ISV(TH)) to reduce losses. Once the internal current sense threshold is
exceeded, the device adjusts the number of switch pulses to maintain
a fixed output current.
Overvoltage Protection
The output overvoltage protection provided by the InnoSwitch-CH IC
uses an internal latch that is triggered by a threshold current of
approximately 7.6 mA into the PRIMARY BYPASS pin. In addition to
an internal filter, the PRIMARY BYPASS pin capacitor forms an external
filter providing noise immunity from inadvertent triggering. 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.
Below the CC threshold, the device operates in constant voltage
mode. The output voltage is sensed via resistor divider R8 and R9.
Output voltage is regulated so as to achieve a voltage of 1.265 V on
the FEEDBACK pin. Capacitor C15 provides decoupling to the
FEEDBACK pin that ensure stable operation and prevents switching
noise from coupling into the IC.
The primary sensed OVP function can be realized by connecting
a Zener diode from the rectified and filtered bias winding voltage
supply to the PRIMARY BYPASS pin (parallel to R4 in Figure 14).
Selecting the Zener diode voltage to be approximately 6 V above
the bias winding voltage (28 V for 22 V bias winding) gives good OVP
performance for most designs, but can be adjusted to compensate
for variations in leakage inductance. Adding additional filtering can
be achieved by inserting a low value (10 Ω to 47 Ω) resistor in series
with the bias winding diode and/or the OVP Zener diode. The resistor
in series with the OVP Zener diode also limits the maximum current
into the BYPASS pin.
VOUT
RC
CA
RA
CB
RB
InnoSwitch
Reducing No-load Consumption
The InnoSwitch-CH IC can start in self-powered mode from the
BYPASS pin capacitor charged through the internal current source.
Use of a bias winding is however required to provide supply current to
the PRIMARY BYPASS pin once the InnoSwitch-CH IC has become
operational. Auxiliary or bias winding provided on the transformer is
required for this purpose. The addition of a bias winding that provides
bias supply to the PRIMARY BYPASS pin enables design of power
supplies with no-load power consumption down to <10 mW. Resistor
R4 shown in Figure 14 should be adjusted to achieve the lowest
no-load input power.
IS
RTN
PI-8443-092717
Figure 15. Feedback Network.
9
Rev. J 10/17
www.power.com
POWERINT [ Power Integrations ]
