AN-5841 Datasheet, PDF (5/8 Pages) Fairchild Semiconductor – Applying SG5841 to Control

AN-5841
Output Driver / Soft Driving
The output stage is a fast totem-pole gate driver capable
of directly driving external MOSFETs. An internal Zener
diode clamps the driver voltage under 18V to protect
MOSFET’s against over voltage. The maximum duty
cycle is around 65%. By integrating special circuits to
control the slew rate of switch-on rising time, the external
resistor R_G may not be necessary to reduce switching
noise, improving EMI performance.
Figure 11. Gate Drive
APPLICATION NOTE
Constant Output Power Limit
The maximum output power of a flyback converter can
generally be determined from the current-sense resistor
RS. When the load increases, the peak inductor current
increases accordingly. Once the output current arrives at
the protection value, the OCP comparator dominates the
current control loop. OCP occurs when the current-sense
voltage reaches the threshold value. The output GATE
driver is turned off after a small propagation delay, t_PD.
The delay time results in unequal power-limit level under
universal input. A sawtooth power-limiter (saw limiter) is
designed to solve the unequal power-limit problem. As
shown in Figure 12, the saw limiter is designed as a
positive ramp signal (V_{LIMIT_RAMP}) and is fed to the
inverting input of the OCP comparator. This results in a
lower current limit at high-line inputs than at low-line
inputs. However, with fixed propagation delay, tPD, the
peak primary current would be the same for various line
input voltages; therefore, the maximum output power can
practically be limited to a constant value within a wide
input voltage range without adding any external circuitry.
Figure 12. Constant Power Limit Compensation
© 2006 Fairchild Semiconductor Corporation
Rev. 1.1.1 • 4/24/08
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