TOP412/414
General Circuit Operation
In the example circuit, C1 provides local decoupling of the DC
input. ThisisrequiredwhentheDCinputsourceisdistantfrom
this converter. A shottky diode (D2) with low voltage drop
providessecondaryrectificationanddoesnotrequireadditional
heat sinking (PC-board provides adequate heat sinking when
usedwithDPAKdiodepackage). Tantalumcapacitors(C3,C4)
providelowprofileandsmalloutlineforsecondarycapacitance
(electrolytic capacitors can also be used as replacement).
Inductor L1 filters high frequency switching noise forming a π
filterwiththeoutputcapacitors(C3-C6). Thecontrolloopgain
issetbyresistorR2andthestabilityisinfluencedbyR1,C3,C4,
C5andC6. ResistorsR3andR4settheDCregulationpointand
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.
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
waveformisshapedtohavethelowestpossibleRMSandripple
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
maximumcomponentheight. TheTOP414Ghasahighcurrent
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).
Capacitor C2 and resistor R1 form part of the CONTROL pin
feedback circuit. Capacitor CX is used solely to decouple high
frequency noise on the control pin.
D2
MBRD620CT
9, 10
L1
3.3 µH
1
5 V
2.0 A
C6
330 µF
6.3 V
C9
2.2 µF
C3
C4
330 µF
10 V
C5
330 µF
6.3 V
VR1
ZGL41-100
330 µF
10 V
6, 7
RTN
D1
C1
10 µF
100 V
D3
1N4148
MURS120T3
2
36-72 V
DC Input
5
4
U1
C7
100 nF
TOP414G
TOPSwitch
U2
PC317A
R3
10k
D
S
T1
CONTROL
C
R2
150 Ω
CX
R1
15 Ω
100 nF
C8
100 nF
U3
TL431ACD
C2
47 µF
R4
10k
PI-2220-120998
Figure 7. Schematic Diagram of a 5 V, 10 W Isolated DC to DC Converter.
A
4/99
6
POWERINT [ Power Integrations ]
