TOP232-234
using a Zener clamp, 165 V when using an RCD clamp and
185 V when using RCD clamp with current limit feed forward.
sink attached to the tab should not be electrically tied to any
nodes on the PC board.
For designs where operating current is significantly lower than
the default current limit, it is recommended to use an externally
set current limit close to the operating peak current to reduce
peak flux density and peak power (see Figure 17). In most
applications,thetightercurrentlimittolerance,higherswitching
frequency and soft-start features of TOPSwitch-FX contribute
to a smaller transformer when compared to TOPSwitch-II.
When using P (DIP-8) or G (SMD-8) packages, a copper area
underneaththepackageconnectedtotheSOURCEpinswillact
as an effective heat sink.
In addition, sufficient copper area should be provided at the
anode and cathode leads of the output diode(s) for heat sinking.
Quick Design Checklist
Standby Consumption
Cycleskippingcansignificantlyreducepowerlossatzeroload,
especially when a Zener clamp is used. For very low secondary
powerconsumptionuseaTL431regulatorforfeedbackcontrol.
Alternately, switching losses can be significantly reduced by
switching from 132 kHz in normal operation to 66 kHz under
light load conditions.
As with any power supply design, all TOPSwitch-FX designs
should be verified on the bench to make sure that components
specifications are not exceeded under worst case conditions.
The following minimum set of tests is strongly recommended:
1. Maximum drain voltage – Verify that peak VDS does not
exceed675Vathighestinputvoltageandmaximumoverload
output power. Maximum overload output power occurs
whentheouputisoverloadedtoaleveljustbeforethepower
supply goes into auto-restart (loss of regulation).
TOPSwitch-FX Layout Considerations
Primary Side Connections
Use a single point (Kelvin) connection at the negative terminal
of the input filter capacitor for TOPSwitch-FX SOURCE pin
and bias winding return. This improves surge capabilities by
returning surge currents from the bias winding directly to the
input filter capacitor.
2. Maximumdraincurrent–Atmaximumambienttemperature,
maximum input voltage and maximum output load, verify
drain current waveforms at start-up for any signs of
transformer saturation and excessive leading edge current
spikes. TOPSwitch-FX has a leading edge blanking time of
200 ns to prevent premature termination of the on-cycle.
Verify that the leading edge current spike is below the
allowed current limit envelope (see Figure 33) for the drain
current waveform at the end of the 200 ns blanking period.
TheCONTROLpinbypasscapacitorshouldbelocatedasclose
as possible to the SOURCE and CONTROL pins and its
SOURCE connection trace should not be shared by the main
MOSFET switching currents.
All SOURCE pin referenced components connected to the
MULTI-FUNCTION pin should also be located close to
SOURCEandMULTI-FUNCTIONpinswithdedicatedSOURCE
pin connection. The MULTI-FUNCTION pin's trace should be
kept as short as possible and away from the DRAIN trace to
prevent noise coupling. Line sense resistor (R1 in Figures 29 and
30) should be located close to the MULTI-FUNCTION pin to
minimize the trace length on the MULTI-FUNCTION pin side.
3. Thermal check – At maximum output power, minimum
input voltage and maximum ambient temperature, verify
that temperature specifications are not exceeded for
TOPSwitch-FX, transformer, output diodes and output
capacitors. Enough thermal margin should be allowed for
the part-to-part variation of the RDS(ON) of TOPSwitch-FX as
specified in the data sheet. The margin required can either
be calculated from the tolerances or it can be accounted for
by connecting an external resistance in series with the
DRAIN pin and attached to the same heatsink, having a
resistance value that is equal to the difference between the
measured RDS(ON) of the device under test and the worst case
maximum specification.
Inadditiontothe47µFCONTROLpincapacitor,ahighfrequency
bypasscapacitorinparallelmaybeusedforbetternoiseimmunity.
The feedback optocoupler output should also be located close to
the CONTROL and SOURCE pins of TOPSwitch-FX.
Y-Capacitor
Design Tools
The Y-capacitor should be connected close to the secondary
output return pin(s) and the primary DC input pin of the
transformer (see Figures 29 and 30).
1. Technical literature: Data Sheet, Application Notes,
Design Ideas, etc.
2. Transformer design spreadsheet.
3. Engineering prototype boards.
Heat Sinking
The tab of the Y package (TO-220) is internally electrically
tied to the SOURCE pin. To avoid circulating currents, a heat
Up to date information on design tools can be found at Power
Integrations Web site: www.powerint.com
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PAM [ POWER ANALOG MICOELECTRONICS ]
