Rev. G 08/16

8

TOP221-227

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Key Application Considerations

General Guidelines

• Keep the SOURCE pin length very short. Use a Kelvin

connection to the SOURCE pin for the CONTROL pin by-

pass capacitor. Use single point grounding techniques

at the SOURCE pin as shown in Figure 9.

• Minimize peak voltage and ringing on the DRAIN volt-

age 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

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

The transformer spreadsheets Rev. 2.1 (or later) for con-

tinuous and Rev.1.0 (or later) for discontinuous conduc-

tion mode provide the necessary information.

• Do not plug TOPSwitch into a “hot” IC socket dur-

ing 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 maxi-

mum 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 in-

definitely 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 in-

creases the auto-restart cycle time (see the I

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 capaci-

tors 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' replace-

ment 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

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

Guidelines section above.

• Thermal: Higher power capability of the TOPSwitch-II

would in many instances allow use of a smaller MOS-

FET device (higher R

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.