40
45
50
55
60
65
70
0
6
12
18
24
30
36
MI
N
O
U
T
PU
T
PO
W
ER
(
W
)
REGULATED MIN INPUT CURRENT (mA)
120V
AC
230V
AC
Design Information
www.ti.com
9.6
Hold Current
Dynamic Hold
The LM3450A regulates the minimum input current with a dynamic hold circuit to ensure the triac holding
current requirement is satisfied. The regulated minimum current is set by choosing the sense resistor
(R34||R36):
(21)
The maximum possible holding current (usually occurs during transients when triac fires) is set by
choosing the hold resistor (R12||R14||R15) between the source of the Q1 and HOLD:
(22)
PassFET
The passFET (Q1) is used in its linear region to stand-off the line voltage from the LM3450A controller.
Both the V
CC
startup current and the triac holding current are conducted through the device. Since the
holding current is far larger than the startup current and is dynamically adjusted every cycle, it will
dominate the calculations. Given this, Q1 is chosen to block the maximum peak input voltage and conduct
the maximum holding current. The surge handling capability of Q1 is also important and is evaluated by
looking at the safe operating area (SOA) of the device.
Finally, Q1 needs to be able to dissipate the maximum power. Looking at an absolute worst-case
condition for the Q1 (during open load where the converter draws near-zero power), extremely large
power dissipation is required (many Watts). Designing for this case is unrealistic and costly. Instead,
Figure 15
can be used to find the maximum I
IN-MIN-REG
for the desired minimum output power level. The
minimum output power is defined as the output power that causes the dynamic hold to force
approximately 1W of power dissipation in Q1 (causing approximately 100°C rise in a DPAK). Below the
minimum output power level, Q1 can reach temperatures exceeding 125°C, depending on the conduction
angle, causing potential catastrophic failure.
Figure 15
is only a general guideline based on experimental
testing of this evaluation board. Each application will have a different passFET thermal characteristic,
which suggests thermal protection of the passFET is usually necessary.
Figure 15. Output Power Restrictions
(without thermal protection)
16
AN-2150 LM3450A Evaluation Board
SNVA485B – June 2011 – Revised May 2013
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