TOP221-227
Key Application Considerations
• Short interruptions of AC power may cause TOPSwitch to
enter the 8-count auto-restart cycle before starting again.
This is because the input energy storage capacitors are not
completelydischargedandtheCONTROLpincapacitance
has not discharged below the internal power-up reset
voltage.
General Guidelines
• Keep the SOURCE pin length very short. Use a Kelvin
connection to the SOURCE pin for the CONTROL pin
bypass capacitor. Use single point grounding techniques at
the SOURCE pin as shown in Figure 9.
• In some cases, minimum loading may be necessary to keep
a lightly loaded or unloaded output voltage within the
desired range due to the minimum ON-time.
• Minimize peak voltage and ringing on the DRAIN voltage
at turn-off. Use a Zener or TVS Zener diode to clamp the
DRAIN voltage below the breakdown voltage rating of
TOPSwitch under all conditions, including start-up and
overload. The maximum recommended clamp Zener
voltage for the TOP2XX series is 200V and the
corresponding maximum reflected output voltage on the
primary is 135V. Please see Step 4: AN-16 in the Data
Book and Design Guide.
ReplacingTOPSwitch with TOPSwitch-II
There is no external latching shutdown function in TOPSwitch-
II. Otherwise, the functionality of the TOPSwitch-II devices is
same as that of the TOPSwitch family. However, before
considering TOPSwitch-II as a 'drop in' replacement in an
existing TOPSwitch design, the design should be verified as
described below.
• The transformer should be designed such that the rate of
change of drain current due to transformer saturation is
within the absolute maximum specification (∆ID in 100ns
before turn off as shown in Figure 13). As a guideline, for
most common transformer cores, this can be achieved by
maintaining the Peak Flux Density (at maximum Ilimit
current) below 4200 Gauss (420mT). The transformer
spreadsheets Rev. 2.1 (or later) for continuous and Rev.1.0
(or later) for discontinuous conduction mode provide the
necessary information.
The new TOPSwitch-II family offers more power capability
than the original TOPSwitch family for the same MOSFET
RDS(ON). Therefore, the original TOPSwitch design must be
reviewed to make sure that the selected TOPSwitch-II
replacement device and other primary components are not over
stressed under abnormal conditions.
The following verification steps are recommended:
• Do not plug TOPSwitch into a “hot” IC socket during test.
External CONTROL pin capacitance may be charged to
excessive voltage and cause TOPSwitch damage.
• Checkthetransformerdesigntomakesurethatitmeetsthe
∆ID specification as outlined in the General Guidelines
section above.
• While performing TOPSwitch device tests, do not exceed
maximum CONTROL pin voltage of 9 V or maximum
CONTROL pin current of 100 mA.
• Thermal: Higher power capability of the TOPSwitch-II
would in many instances allow use of a smaller MOSFET
device (higher RDS(ON)) for reduced cost. This may affect
TOPSwitchpowerdissipationandpowersupplyefficiency.
Therefore thermal performance of the power supply must
be verified with the selected TOPSwitch-II device.
• Under some conditions, externally provided bias or supply
current driven into the CONTROL pin can hold the
TOPSwitch in one of the 8 auto-restart cycles indefinitely
and prevent starting. To avoid this problem when doing
bench evaluations, it is recommended that the VC power
supply be turned on before the DRAIN voltage is applied.
TOPSwitch can also be reset by shorting the CONTROL
pin to the SOURCE pin momentarily.
• Clamp Voltage: Reflected and Clamp voltages should be
verified not to exceed recommended maximums for the
TOP2XXSeries:135VReflected/200VClamp. Pleasesee
Step 4: AN-16 in the Data Book and Design Guide and
readme.txt file attached to the transformer design
spreadsheets.
• CONTROL pin currents during auto-restart operation are
much lower at low input voltages (< 36 V) which increases
the auto-restart cycle time (see the IC vs. DRAIN Voltage
Characteristic curve).
•AgencyApproval:MigratingtoTOPSwitch-IImayrequire
agency re-approval.
C
8
12/97
POWERINT [ Power Integrations ]
