SLUS157L − DECEMBER 1999 − REVISED APRIL 2008
UCC1895
UCC2895
UCC3895
APPLICATION INFORMATION
Programming DELAB, DELCD and the Adaptive Delay Set
The UCC2895N allows the user to set the delay between switch commands within each leg of the full-bridge
power circuit according to equations:
t
DELAY
+
(25
10
*12
)
V
DEL
R
DEL
)
25 ns
(5)
From this equation VDEL is determined in conjunction with the desire to use (or not) the adaptive delay set
feature from the following formula:
V
DEL
+
0.75
V
CS
*
V
ADS
)
0.5 V
(6)
The following diagram illustrates the resistors needed to program the delay periods and the adaptive delay set
function.
UCC3895
CS
9
RDELAB
10
DELCD
ADS
11
DELAB
12
RDELCD
Figure 4. Programming Adaptive Delay Set
The adaptive delay set feature (ADS) allows the user to vary the delay times between switch commands
within each of the converter’s two legs. The delay-time modulation is implemented by connecting ADS
(pin 11) to CS, GND, or a resistive divider from CS through ADS to GND to set V
ADS
as shown in Figure 4.
From equation (6) for V
DEL
, if ADS is tied to GND then V
DEL
rises in direct proportion to V
CS
, causing a
decrease in t
DELAY
as the load increases. In this condition, the maximum value of V
DEL
is 2 V.
If ADS is connected to a resistive divider between CS and GND, the term (V
CS
−V
ADS
) becomes smaller,
reducing the level of V
DEL
. This decreases the amount of delay modulation. In the limit of ADS tied to CS,
V
DEL
= 0.5 V and no delay modulation occurs. Figure 5 graphically shows the delay time vs. load for
varying adaptive delay set feature voltages (V
ADS
).
In the case of maximum delay modulation (ADS=GND), when the circuit goes from light load to heavy
load, the variation of V
DEL
is from 0.5 V to 2 V. This causes the delay times to vary by a 4:1 ratio as the
load is changed.
The ability to program an adaptive delay is a desirable feature because the optimum delay time is a
function of the current flowing in the primary winding of the transformer, and can change by a factor of
10:1 or more as circuit loading changes. Reference
[5]
describes the many interrelated factors for choosing
the optimum delay times for the most efficient power conversion, and illustrates an external circuit to
enable adaptive delay set using the UC3879. Implementing this adaptive feature is simplified in the
UCC3895 controller, giving the user the ability to tailor the delay times to suit a particular application with a
minimum of external parts.
12
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