TOP412/414
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
X
is used solely to decouple high
frequency noise on the control pin.
1
D2
MBRD620CT
9, 10
C3
330
µF
10 V
C4
330
µF
10 V
L1
3.3
µH
C5
330
µF
6.3 V
C6
330
µF
6.3 V
C9
2.2
µF
5V
2.0 A
VR1
ZGL41-100
6, 7
C1
10
µF
100 V
D1
MURS120T3
2
U1
TOP414G
D
RTN
D3
1N4148
36-72 V
DC Input
5
C7
100 nF
T1
4
U2
PC317A
R2
150
Ω
C8
100 nF
R4
10k
R3
10k
TOPSwitch
CONTROL
C
S
C
X
100 nF
C2
47
µF
R1
15
Ω
U3
TL431ACD
PI-2220-120998
Figure 7. Schematic Diagram of a 5 V, 10 W Isolated DC to DC Converter.
6
A
4/99
POWERINT [ POWER INTEGRATIONS, INC. ]
