
TOP221-227
D
7/01
8
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 200 V and the
corresponding maximum reflected output voltage on the
primary is 135 V. Please see Step 4: AN-16 in the 1996-97
Data Book and Design Guide or on our Web site.
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
in 100 ns
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
current) below 4200 Gauss (420 mT). 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
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.
Replacing
TOPSwitch
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
. 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
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
) 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: 135 V Reflected/200 V 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.