TOP250F-TL datasheet(22/52 Pages) POWERINT

22 / 52 page

TOP242-250

12/04

A High Efﬁciency, 250 W, 250-380 VDC Input Power Supply

The circuit shown in Figure 43 delivers 250 W (48 V @

5.2 A) at 84% efﬁciency using a TOP249 from a 250 VDC to

380 VDC input. DC input is shown, as typically at this power

levelap.f.c.booststagewouldpreceedthissupply,providingthe

DC input (C1 is included to provide local decoupling). Flyback

topology is still usable at this power level due to the high output

voltage, keeping the secondary peak currents low enough so

that the output diode and capacitors are reasonably sized.

In this example, the TOP249 is at the upper limit of its power

capability and the current limit is set to the internal maximum

by connecting the X pin to SOURCE. However, line sensing

is implemented by connecting a 2 MΩ resistor from the L pin

to the DC rail. If the DC input rail rises above 450 VDC, then

TOPSwitch-GX

will stop switching until the voltage returns to

normal, preventing device damage.

Due to the high primary current, a low leakage inductance

transformer is essential. Therefore, a sandwich winding with

a copper foil secondary was used. Even with this technique,

the leakage inductance energy is beyond the power capability

of a simple Zener clamp. Therefore, R2, R3 and C6 are added

in parallel to VR1. These have been sized such that during

normal operation, very little power is dissipated by VR1,

the leakage energy instead being dissipated by R2 and R3.

However, VR1 is essential to limit the peak drain voltage

during start-up and/or overload conditions to below the 700 V

rating of the TOPSwitch-GX MOSFET.

The secondary is rectifed and smoothed by D2 and C9, C10 and

C11. Three capacitors are used to meet the secondary ripple

current requirement. Inductor L2 and C12 provide switching

noise ﬁltering.

A simple Zener sensing chain regulates the output voltage.

The sum of the voltage drop of VR2, VR3 and VR4 plus the

LED drop of U2 gives the desired output voltage. Resistor R6

limits LED current and sets overall control loop DC gain.

Diode D4 and C14 provide secondary soft-ﬁnish, feeding

current into the CONTROL pin prior to output regulation and

thus ensuring that the output voltage reaches regulation at start-

up under low line, full load conditions. Resistor R9 provides a

discharge path for C14. Capacitor C13 and R8 provide control

loop compensation and are required due to the gain associated

with such a high output voltage.

Sufﬁcient heat sinking is required to keep the TOPSwitch-GX

device below 110 °C when operating under full load, low line

and maximum ambient temperature. Airﬂow may also be

required if a large heatsink area is not acceptable.

48 V@

5.2 A

+250-380

VDC

0 V

RTN

PI-2692-081204

All resistor 1/8 W 5% unless

otherwise stated.

CONTROL

TOPSwitch-GX

µF

400 V

0.1

µF

50 V

6.8

Ω

100

Ω

10 k

Ω

µF

10 V

4.7 nF

1 kV

C13

150 nF

63 V

µF

50 V

C14

µF

63 V

560

µF

63 V

C10

560

µF

63 V

C11

560

µF

63 V

C12

µF

63 V

2.2 nF Y1

µH 8A

MUR1640CT

1N4148

LTV817A

BYV26C

2 M

Ω

1/2 W

68 k

Ω

2 W

68 k

Ω

2 W

VR1

P6KE200

VR2 22 V

BZX79B22

VR3 12 V

BZX79B12

VR4 12 V

BZX79B12

CONTROL

1N4148

PERFORMANCE SUMMARY

Output Power:

250 W

Line Regulation:

± 1%

Load Regulation:

± 5%

Efficiency:

≥ 85%

Ripple:

< 100 mV pk-pk

No Load Consumption:

≤ 1.4 W (300 VDC)

TOP249Y

Figure 43. 250 W, 48 V Power Supply using TOP249.

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