LT3798
12
3798fa
OPERATION
to the input capacitor. This is a second-order network
composed of the parasitic capacitance on the switch node
and the magnetizing inductance of the primary winding
of the transformer. The minimum voltage of the switch
node during this discontinuous ring is V
IN
– V
OUT
• N
PS
.
The LT3798 turns the switch back on at this time, during
the discontinuous switch waveform, by sensing when
the slope of the switch waveform goes from negative to
positive using the dv/dt detector. This switching technique
may increase efficiency by 5%.
Sense Resistor Selection
The resistor, R
SENSE
, between the source of the external
N-channel MOSFET and GND should be selected to provide
an adequate switch current to drive the application without
exceeding the current limit threshold.
For applications without power factor correction, select a
resistor according to:
R
SENSE
=
2(1–D)N
PS
I
OUT
• 42
• 95%
where
D =
V
OUT
•N
PS
V
OUT
•N
PS
+ V
IN
For applications with power factor correction, select a
resistor according to:
R
SENSE
=
PS
I
OUT
• 42
• 47.5%
where
D =
V
OUT
•N
PS
V
OUT
•N
PS
+ V
IN
Minimum Current Limit
The LT3798 features a minimum current limit of approxi-
mately 18% of the peak current limit. This is necessary
when operating in critical conduction mode since low
current limits would increase the operating frequency to a
very high frequency. The output voltage sensing circuitry
needs a minimum amount of flyback waveform time to
sense the output voltage on the third winding. The time
needed is 350ns. The minimum current limit allows the
use of smaller transformers since the magnetizing primary
inductance does not need to be as high to allow proper
time to sample the output voltage information.
To help improve crossover distortion of the line input
current, a second minimum current limit of 6% becomes
active when the V
IN_SENSE
current is lower than 27µA. Since
the off-time becomes very short with this lower minimum
current limit, the sample-and-hold is deactivated.
Universal Input
The LT3798 operates over the universal input voltage
range of 90VAC to 265VAC. In the Typical Performance
Characteristics section, the Output Voltage vs V
IN
and the
Output Current vs V
IN
graphs, show the output voltage
and output current line regulation for the first application
picture in the Typical Applications section.
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
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
N
PS
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.
