Datasheet

Pin
Pout =
h
max
1
2
Pin Ipk Lp Freq
2
= · · ·
Pout 2 Vin
Ipk max tprop
Lp Freq Lp
·
= + ·
· · h
Vin di
Lp tprop
=
LM5023
SNVS961D –APRIL 2013–REVISED JANUARY 2014 www.ti.com
APPLICATION INFORMATION
Line Current Limit Feed Forward
In a peak current mode controlled when the power supply is in an overload, the peak current (measured across
the current sense resistor VCS) is compared to a voltage reference for overload protection. If the peak current
exceeds the reference the LM5023 controller will turn off the primary side Flyback MOSFET on a cycle-by-cycle
basIs. However, the primary switch can’t be turned off instantly, as there are several unavoidable delays. The
first delay is caused by the LEB circuit which provides leading-edge blanking. The second delay is caused by the
propagation delay between the detecting point of VCS and the actual turn off of the power MOSFET. The total
delay time (tprop) refer to Figure 13, includes the current limit comparator, the logic, the gate driver, and the
power MOSFET turning off.
The propagation delay causes the peak primary current to overshoot, the overshoot increase the maximum peak
current beyond the calculated value. The peak current overshoot increase as the AC line voltage increase
because of the increase in the slope of the primary current:
This increase in the peak input current overshoot causes a wide variation of overpower limit in a Flyback
converter. In Figure 4, it can be seen that the overpower limit increases with the input line voltage, because of
Ipkmax increase:
16 Submit Documentation Feedback Copyright © 2013–2014, Texas Instruments Incorporated
Product Folder Links: LM5023