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LT3799
OPERATION
transformers since the magnetizing primary inductance
does not need to be as high to allow proper time to sample
the output voltage information.
Errors Affecting Current Output Regulation
There are a few factors affecting the regulation of current in
a manufacturing environment along with some systematic
issues. The main manufacturing issues are the winding
turns ratio and the LT3799 control loop accuracy. The
winding turns ratio is well controlled by the transformer
manufacturer’s winding equipment, but most transformers
do not require a tight tolerance on the winding ratio. We
have worked with transformer manufacturers to specify
±1% error for the turns ratio. Just like any other LED driver,
the part is tested and trimmed to eliminate offsets in the
control loop and an error of ±3% is specified at 80% of
the maximum output current. The error grows larger as
the LED current is decreased from the maximum output
current. At half the maximum output current, the error
doubles to ±6%.
There are a number of systematic offsets that may be elimi-
nated by adjusting the control voltage from the ideal voltage.
It is difficult to measure the flyback time with complete
accuracy. If this time is not accurate, the control voltage
needs to be adjusted from the ideal value to eliminate the
offset but this error still causes line regulation errors. If
the supply voltage is lowered, the time error becomes a
smaller portion of the switching cycle period so the offset
becomes smaller and vice versa. This error may be com-
pensated for at the primary supply voltage, but this does
not solve the problem completely for other supply voltages.
Another systematic error is that the current comparator
cannot instantaneously turn off the main power device.
This delay time leads to primary current overshoot. This
overshoot is less of a problem when the output current is
close to its maximum, since the overshoot is only related
to the slope of the primary current and not the current
level. The overshoot is proportional to the supply voltage,
so again this affects the line regulation.
Universal Input
The LT3799 operates over the universal input range of
90V AC to 265V AC . Output current regulation error may
be minimized by using two application circuits for the
wide input range: one optimized for 120V AC and another
optimized for 220V AC . The first application pictured in
the Typical Applications section shows three options:
universal input, 120V AC , and 220V AC . The circuit varies by
three resistors. In the Typical Performance Characteristics
section, the LED Current vs V IN graphs show the output
current line regulation for all three circuits.
Selecting Winding Turns Ratio
Boundary mode operation gives a lot of freedom in selecting
the turns ratio of the transformer. We suggest to keep the
duty cycle low, lower N PS , at the maximum input voltage
since the duty cycle will increase when the AC waveform is
decreases to zero volts. A higher N PS increases the output
current while keeping the primary current limit constant.
Although this seems to be a good idea, it comes at the
expense of a higher RMS current for the secondary-side
diode which might not be desirable because of the primary
side MOSFET’s superior performance as a switch. A higher
NPS does reduce the voltage stress on the secondary-side
diode while increasing the voltage stress on the primary-
side MOSFET. If switching frequency at full output load is
kept constant, the amount of energy delivered per cycle by
the transformer also stays constant regardless of the N PS .
Therefore, the size of the transformer remains the same at
practical N PS ’s. Adjusting the turns ratio is a good way to
find an optimal MOSFET and diode for a given application.
Switch Voltage Clamp Requirement
Leakage inductance of an offline transformer is high due
to the extra isolation requirement. The leakage inductance
energy is not coupled to the secondary and goes into
the drain node of the MOSFET. This is problematic since
400V and higher rated MOSFETs cannot always handle
this energy by avalanching. Therefore the MOSFET needs
protection. A transient voltage suppressor (TVS) and
diode are recommended for all offline application and
connected, as shown in Figure 3. The TVS device needs
a reverse breakdown voltage greater than (V OUT + V f )*N
where V OUT is the output voltage of the flyback converter,
V f is the secondary diode forward voltage, and N is the
turns ratio.
3799fa
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