TOP221-227
Key Application Considerations
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.
• 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.
• The transformer should be designed such that the rate of
change of drain current due to transformer saturation is
within the absolute maximum specification (∆I
D
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 I
limit
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.
• 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.
• While performing
TOPSwitch
device tests, do not exceed
maximum CONTROL pin voltage of 9 V or maximum
CONTROL pin current of 100 mA.
• 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 V
C
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.
• 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 I
C
vs. DRAIN Voltage
Characteristic curve).
• 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
completely discharged and the CONTROL pin capacitance
has not discharged below the internal power-up reset
voltage.
• 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.
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 new
TOPSwitch-II
family offers more power capability
than the original
TOPSwitch
family for the same MOSFET
R
DS(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:
• Check the transformer design to make sure that it meets the
∆I
D
specification as outlined in the General Guidelines
section above.
• Thermal: Higher power capability of the
TOPSwitch-II
would in many instances allow use of a smaller MOSFET
device (higher R
DS(ON)
) for reduced cost. This may affect
TOPSwitch
power dissipation and power supply efficiency.
Therefore thermal performance of the power supply must
be verified with the selected
TOPSwitch-II
device.
• Clamp Voltage: Reflected and Clamp voltages should be
verified not to exceed recommended maximums for the
TOP2XX Series: 135V Reflected/200V Clamp. Please see
Step 4: AN-16 in the Data Book and Design Guide and
readme.txt file attached to the transformer design
spreadsheets.
• Agency Approval: Migrating to
TOPSwitch-II
may require
agency re-approval.
8
C
12/97
POWERINT [ POWER INTEGRATIONS, INC. ]
