LTC3873_15 Datasheet, PDF (11/16 Pages) Linear Technology – Frequency Current Mode Boost/Flyback/SEPIC DC/DC Controller

LTC3873
APPLICATIONS INFORMATION
plus the secondary-to-primary referred voltage of the
flyback pulse (including leakage spike) must not exceed
the allowed external MOSFET breakdown rating.
Leakage Inductance
Transformer leakage inductance (on either the primary
or secondary) causes a voltage spike to occur after the
output switch (Q1) turn-off. This is increasingly prominent
at higher load currents where more stored energy must
be dissipated. In some cases a “snubber” circuit will be
required to avoid overvoltage breakdown at the MOSFET’s
drain node. Application Note 19 is a good reference on
snubber design. A bifilar or similar winding technique is a
good way to minimize troublesome leakage inductances.
However, remember that this will limit the primary-to-
secondary breakdown voltage, so bifilar winding is not
always practical.
Power MOSFET Selection
The power MOSFET serves two purposes in the LTC3873:
it represents the main switching element in the power path
and its R_DS(ON) represents the current sensing element
for the control loop. Important parameters for the power
MOSFET include the drain-to-source breakdown voltage
(BV_DSS), the threshold voltage (V_GS(TH)), the on-resistance
(R_DS(ON)) versus gate-to-source voltage, the gate-to-source
and gate-to-drain charges (Q_GS and Q_GD, respectively),
the maximum drain current (I_D(MAX)) and the MOSFET’s
thermal resistances (R_TH(JC) and R_TH(JA)).
For boost applications with R_DS(ON) sensing, refer to
the LTC3872 data sheet for the selection of MOSFET
R_DS(ON).
MOSFETs have conduction losses (I²R) and switching
losses. For V_DS < 20V, high current efficiency generally
improves with large MOSFETs with low R_DS(ON), while
for V_DS > 20V the transition losses rapidly increase to the
point that the use of a higher R_DS(ON) device with lower
reverse transfer capacitance, C_RSS, actually provides
higher efficiency.
Output Capacitors
The output capacitor is normally chosen by its effective
series resistance (ESR), which determines output ripple
voltage and affects efficiency. Low ESR ceramic capaci-
tors are often used to minimize the output ripple. Boost
regulators have large RMS ripple current in the output
capacitor that must be rated to handle the current. The
output ripple current (RMS) is:
I_RMS(COUT) ≈ I_OUT(MAX) •
V_OUT – V_IN(MIN)
V_IN(MIN)
Output ripple is then simply:
V_OUT = R_ESR(ΔI_L(RMS))
The output capacitor for flyback converter should have a
ripple current rating greater than:
I_RMS = I_OUT •
D_MAX
1– D_MAX
Input Capacitors
The input capacitor of a boost converter is less critical due
to the fact that the input current waveform is triangular, and
does not contain large square wave currents as found in
the output capacitor. The input voltage source impedance
determines the size of the capacitor that is typically 10μF to
100μF. A low ESR is recommended although not as critical
as the output capacitor can be on the order of 0.3Ω.
The RMS input ripple current for a boost converter is:
I_RMS(CIN)
=
0.3
•
V_IN(MIN)
L•f
• D_MAX
Please note that the input capacitor can see a very high
surge current when a battery is suddenly connected to the
input of the converter and solid tantalum capacitors can
fail catastrophically under these conditions.
3873fa
11