Datasheet

From half bridge
p
C
p
R
s
R
r
C
2
D
1
D
m
L
r
L
TR
s
C
To OC
2
1
4
p
+
=
n n e
Cr _ pk o
n n
jf L Q
V nV
f L
( )
2
2
Cr _ pk
s
Rs
V max
R
P (max)
=
min
10
fR
C
s
s
=
( )
p=
s
p
Cr _ pk
R
R
V max
min
10
fR
C
p
p
=
UCC25600
www.ti.com SLUS846B –SEPTEMBER 2008– REVISED JULY 2011
Over-Current Protection
To prevent power stage failure under excessive load current condition, the UCC25600 includes an over-current
protection function. With a dedicated OC pin, the power stage is shut down when OC pin voltage is above 1 V.
Once the OC pin voltage becomes lower than 0.6 V, the gate driver recovers with soft start. To enhance system
safety, the UCC25600 latches up the whole system when OC pin becomes above 2 V. Bring VCC below UVLO
level to reset the device.
The current can be indirectly sensed through the voltage across resonant capacitor by using the sensing network
shown in Figure 20.
Figure 20. Current Sensing for LLC Resonant Converter
The general concept of this sensing method is that, ac voltage across the resonant capacitor is proportional to
load current.
According to the FHA model, peak voltage of the ac component on the resonant capacitor can be calculated as:
(18)
Therefore, the resonant capacitor voltage reaches its maximum value at the minimum switching frequency and
maximum load. According to this equation, the current sensing network components can be calculated. Due to
the nature of FHA, the final circuit parameters need to be verified through actual hardware test.
Table 2. Calculated Current Sensing Network Components
NAME FUNCTION DESIGN EQUATION
Transfer ac voltage across resonant
R
S
capacitor into current source
(19)
C
S
Blocking dc voltage on resonant capacitor
(20)
R
P
Load resistor of the current source
(21)
C
P
Filter capacitor
(22)
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