Features - Bodo's Power

age-mode or PWM technique, and can deliver up to 88% efficiency
along with providing a fast transient response. Output voltage regulation
is achieved by dual over-lapping modulation using both peak primary
current modulation and duty-cycle modulation.
The differential operation of the DCM comparator allows the LT3751
to accurately operate from high-voltage inputs of up to 400V, and
higher. Furthermore, the VOUT comparator and DCM comparator are
needed for lower input voltages down to 4.75V, with the use of a
logic-level external MOSFET. This permits the user to accommodate
an extremely wide range of power sources. Only five external resistors
are needed to operate the LT3751 as a capacitor charger. The
output voltage trip point (VOUT) can be adjusted from 50V to 450V
by using the following equation:
R9 =
0.98 x N______
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VOUT + VDIODES
where N is the turns ratio of the transformer and VDIODES is the
voltage drop across D1 and D2.
The LT3751 stops charging the output capacitor once the programmed
output voltage trip point is reached. The charge cycle is
repeated by toggling the CHARGE pin. The maximum charge/discharge
rate in the output capacitor is limited by the temperature rise
in the transformer and power dissipation in the external MOSFET.
Limiting the transformer surface temperature in figure 1 to 40?C rise
above the ambient temperature with no air flow requires the average
output power to be less than or equal to 40W, as given by:
PAVE = ½ • COUT • freq • ( 2 • VOUT • VRIPPLE - V 2 RIPPLE ) � 40W
Where VOUT is the output trip voltage, VRIPPLE is the output ripple
voltage, and freq is the charge/discharge frequency. The maximum
available output power can be increased by making the transformer
larger and providing force air cooling. For output voltages higher than
450V, the transformer in Figure 1 must be replaced with one having a
higher turns ratio and higher primary inductance. Figure 2 shows the
charging waveform and average input current for a 100?F output
capacitor charged to 400V in less than 100ms.
Figure 2 – Charging Waveform of Figure 1 Circuit
X R8
Another useful capability for the LT3751 is to transform a low voltage
supply to a high-voltage supply in a non-isolated application. This is
accomplished by placing a resistor divider network from the output
voltage to the FB pin and ground, which makes the LT3751 operate
POWER SUPPLY
Ultra-reliable
transformer solutions
. . . reduce premature
power control system failure!
Bicron Electronics specializes in the design
and manufacture of custom high frequency
transformers for critical-use applications
with frequencies up to 1 Mhz.
Rail/marine drive controls
Wind power & solar power controls
Large motor drive controls
High Isolation
Switchmode
Load Leveling
Gate Drives
Signal Conditioning
Pulse
as a voltage regulator. This method provides tighter output voltage
regulation and lower output ripple voltage. This circuit can be converted
to an isolated flyback with direct output voltage sensing by
using an optocoupler to close the feedback loop. Figure 3 shows the
LT3751 as a non-isolated converter and its associated efficiency/regulation
curve. The efficiency ramps up to 88% at full load and it maintains
a 0.25% load regulation from 5mA to 100mA.
Safety and Reliability Features
Large capacitors charged to high voltages can deliver a lethal
amount of energy if handled improperly. It is particularly important to
observe appropriate safety measures when designing with the
LT3751 in any application. The designer must create a discharge circuit
that allows for the safe discharge of the output capacitor. In addition,
adequate space is needed between high voltage nodes from
adjacent traces to satisfy printed circuit board voltage breakdown
requirements. For more information, refer to the printed circuit board
design standards in IPC-2221 (www.ipc.org) and the Underwriters
Laboratory Standard UL60950-1 2nd edition.
The LT3751 has safety and reliability features that include two sets of
under-voltage lockouts (UVLO) and over-voltage lockouts (OVLO) for
the VTRANS and VCC inputs. This allows the user to prevent the
power supply from turning on when the input voltages are in an unsafe
operating range. The FAULT pin goes active when the input voltages
are not within the user programmable safe operating range. In
addition, the LT3751 has over temperature latch off protection and
goes into Burst mode operation during a no load condition. The
www.bodospower.com August 2009 Bodo´s Power Systems ®
www.bodospower.com August 2009 Bodo´s Power Systems ®
www.bodospower.com August 2009 Bodo´s Power Systems ®
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Bicron offers the following transformer types:
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When failure is not an option, choose Bicron.
BICRON
Electronics
www.bicron-magnetics.us
+45.9858.1022 � 1.860.824.5125
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