TOP412/414

A

4/99

6

Figure 7. Schematic Diagram of a 5 V, 10 W Isolated DC to DC Converter.

PI-2220-120998

D2

MBRD620CT

D3

1N4148

C3

330 µF

10 V

C5

330 µF

6.3 V

T1

D1

MURS120T3

VR1

ZGL41-100

U3

TL431ACD

R3

10k

C2

47 µF

U1

TOP414G

D

S

C

CONTROL

TOPSwitch

R1

15 Ω

L1

3.3 µH

C7

100 nF

U2

PC317A

R2

150 Ω

5 V

2.0 A

RTN

1

2

5

4

6, 7

9, 10

36-72 V

DC Input

C1

10 µF

100 V

C4

330 µF

10 V

C9

2.2 µF

C6

330 µF

6.3 V

R4

10k

C8

100 nF

C

X

100 nF

General Circuit Operation

Figure 7 shows a typical DC-DC converter application using

the TOP414G. This supply delivers 5 V at 2 A and works over

a wide input range from 36-72 VDC.

The power supply

operates at an ambient temperature of 0-50

°C.

In order to achieve the highest possible efficiency and smallest

possible circuit board area, the primary and secondary current

waveform is shaped to have the lowest possible RMS and ripple

current. This is achieved by running very continuous and

utilizing the maximum duty cycle available.

For the example shown, the maximum component height is

12 mm. The EFD-20 transformer core was chosen to match this

maximum component height. The TOP414G has a high current

limit, which means that the EF20 core will saturate during

startup, until regulation is achieved. This is acceptable with the

TOP414G and does not cause device stress (provided the

maximum drain voltage is below 250 V peak and provided a

Zener is used for clamping). A Zener diode clamp circuit (VR1

and D1) is used in order to clamp the leakage inductance spike

to a fixed maximum voltage (an RCD, resistor capacitor diode,

clamp circuit would not be acceptable for this application).

In the example circuit, C1 provides local decoupling of the DC

input. This is required when the DC input source is distant from

this converter. A shottky diode (D2) with low voltage drop

provides secondary rectification and does not require additional

heat sinking (PC-board provides adequate heat sinking when

used with DPAK diode package). Tantalum capacitors (C3,C4)

provide low profile and small outline for secondary capacitance

(electrolytic capacitors can also be used as replacement).

Inductor L1 filters high frequency switching noise forming a

π

filter with the output capacitors (C3-C6). The control loop gain

is set by resistor R2 and the stability is influenced by R1, C3 ,C4,

C5 and C6. Resistors R3 and R4 set the DC regulation point and

shunt regulator U3 along with bypass capacitor C8, provide the

drive for the optocoupler U2. Any remaining switching noise

in the system is filtered by ceramic capacitor C9.

Capacitor C2 and resistor R1 form part of the CONTROL pin

feedback circuit. Capacitor C

frequency noise on the control pin.