TOP266KG Power Integrations, TOP266KG Datasheet - Page 18

• Manufacturer Part Number: TOP266KG
• Description: IC OFFLINE SWITCHER 39W 58W
• Manufacturer: Power Integrations
• Series: TOPSwitch®-JXr

Specifications of TOP266KG

• Output Isolation: Isolated
• Frequency Range: 66 ~ 132kHz
• Voltage - Output: 725V
• Power (watts): 39W
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 12-BSOP (0.350", 8.89mm Width) Exposed Pad
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 596-1399

Clamp Configuration – RZCD vs RCD
•
The clamp network is formed by VR1, C4, R5 and D5. It limits
the peak drain voltage spike caused by leakage inductance to
below the BV
This arrangement was selected over a standard RCD clamp to
improve light load efficiency and no-load input power.
In a standard RCD clamp C4 would be discharged by a parallel
resistor rather than a resistor and series Zener. In an RCD
clamp the resistor value of R5 is selected to limit the peak drain
voltage under full load and over-load conditions. However
under light or no-load conditions this resistor value now causes
the capacitor voltage to discharge significantly as both the
leakage inductance energy and switching frequency are lower.
As the capacitor has to be recharged to above the reflected
output voltage each switching cycle the lower capacitor voltage
represents wasted energy. It has the effect of making the
clamp dissipation appear as a significant load just as if it were
connected to the output of the power supply.
The RZCD arrangement solves this problem by preventing the
voltage across the capacitor discharging below a minimum
value (defined by the voltage rating of VR1) and therefore
minimizing clamp dissipation under light and no-load conditions.
Zener VR1 is shown as a high peak dissipation capable TVS
however a standard lower cost Zener may also be used due to
the low peak current that component experiences.
In many designs a resistor value of less than 50 W may be used
in series with C4 to damp out high frequency ringing and
improve EMI but this was not necessary in this case.
Feedback Configuration
•
•
•
Typically the feedback current into the CONTROL pin at high
line is ~3 mA. This current is both sourced from the bias
winding (voltage across C10) and directly from the output. Both
of these represent a load on the output of the power supply.
To minimize the dissipation from the bias winding under no-load
conditions the number of bias winding turns and value of C7
was adjusted to give a minimum voltage across C7 of ~9 V.
This is the minimum required to keep the optocoupler biased
and the output in regulation.
To minimize the dissipation of the secondary side feedback
circuit a high CTR (CTR of 300 – 600%) optocoupler type was
used. This reduces the secondary side opto-led current from
~3 mA to <~1 mA and therefore the effective load on the output.
A standard 2.5 V TL431 voltage reference was replaced with the
Rev. C 11/10
An RZCD (Zener bleed) was selected over RCD to give higher
light load efficiency and lower no-load consumption
A high CTR optocoupler was used to reduce secondary bias
currents and no-load input power
Low voltage, low current voltage reference IC used on
secondary side to reduce secondary side feedback current
and no-load input power
Bias winding voltage tuned to ~9 V at no-load, high line to
reduce no-load input power
18
TOP264-271
DSS
rating of the internal TOPSwitch-JX MOSFET.
1.24 V LMV431 to reduce the supply current requirement of this
component from 1 mA to 100 mA.
Output Rectifier Choice
•
The higher BV
(compared to 600 V or 650 V rating of typical power MOSFETs)
allowed a higher transformer primary to secondary turns ratio
(reflected output voltage or V
voltage stress and allowed the use of cheaper and more efficient
60 V (vs 80 V or 100 V) Schottky diodes. The efficiency
improvement occurs due the lower forward voltage drop of the
lower voltage diodes. Two parallel connected axial 5 A, 60 V
Schottky rectifier diodes were selected for both low cost and
high efficiency. This allowed PCB heat sinking of the diode for
low cost while maintaining efficiency compared to a single
higher current TO-220 packaged diode mounted on a heat sink.
For this configuration the recommendation is that each diode is
rated at twice the output current and that the diodes share a
common cathode PCB area for heat sinking so that their
temperatures track. In practice the diodes current share quite
effectively as can be demonstrated by monitoring their
individual temperatures.
Output Inductor Post Filter Soft-Finish
•
To prevent output overshoot during start-up the voltage
appearing across L2 is used to provide a soft-finish function.
When the voltage across L2 exceeds the forward drop of U2A
and D10 current flows though the optocoupler LED and
provides feedback to the primary. This arrangement acts to
limit the rate of rise of the output voltage until it reaches
regulation and eliminates the capacitor that is typically placed
across U3 to provide the same function.
Key Application Considerations
TOPSwitch-JX vs. TOPSwitch-HX
Table 3 compares the features and performance differences
between TOPSwitch-JX and TOPSwitch-HX. Many of the new
features eliminate the need for additional discrete components.
Other features increase the robustness of design, allowing cost
savings in the transformer and other power components.
TOP264-271 Design Considerations
Power Table
The data sheet power table (Table 1) represents the maximum
practical continuous output power based on the following
conditions:
1. 12 V output.
2. Schottky or high efficiency output diode.
3. 135 V reflected voltage (V
4. A 100 VDC minimum DC bus for 85-265 VAC and 250 VDC
5. Sufficient heat sinking to keep device temperature ≤110 °C.
Use of high V
high efficiency and lower cost
Inductor L2 used to provide an output soft-finish and eliminate
a capacitor
minimum for 230 VAC.
DSS
OR
rating of the TOPSwitch-JX of 725 V
allows the use of a 60 V Schottky diode for
OR
OR
). This reduced the output diode
) and efficiency estimates.
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