TOP264VG datasheet(18/40 Pages) POWERINT

18 / 40 page

Rev. E 08/12

TOP264-271

www.powerint.com

Clamp Configuration – RZCD vs RCD

•

An RZCD (Zener bleed) was selected over RCD to give higher

light load efficiency and lower no-load consumption.

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

DSS rating of the internal TOPSwitch-JX MOSFET.

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 overload 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

•

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.

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

1.24 V LMV431 to reduce the supply current requirement of this

component from 1 mA to 100

mA.

Output Rectifier Choice

•

Use of high V

OR allows the use of a 60 V Schottky diode for

high efficiency and lower cost.

The higher BV

DSS rating of the TOPSwitch-JX of 725 V

(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

OR). This reduced the output diode

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

•

Inductor L2 used to provide an output soft-finish and eliminate

a capacitor.

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

OR) and efficiency estimates.

4. A 100 VDC minimum DC bus for 85-265 VAC and 250 VDC

minimum for 230 VAC.

5. Sufficient heat sinking to keep device temperature ≤110

°C.

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