Datasheet

7
UCC2750
UCC3750
Figure 2. Operating modes.
Figure 3. Circuit waveforms.
UDG-96172-1 UDG-96173
Figure 4a. Mode 1: Forward power transfer, positive
output.
Figure 4b. Mode 2: Reverse power transfer, positive
output.
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UDG-96163
When the reference signal goes from positive to nega
-
tive, a transition is made from mode 2 to mode 3. In
mode 3, the converter once again acts as a DC-DC
flyback converter (with negative output). Similar to mode
1, Q1 is controlled by the PWM output, however, the rec
-
tifying path is now through Q3/DR3 as the output polarity
is reversed. At the mode boundaries, there could be
some distortion which won’t affect the THD too much as
it is near zero crossings. Finally, as the reference signal
starts increasing towards zero, the direction of power
transfer is again reversed and Q2 is PWMed in mode 4.
It should be noted that in modes 2 and 3 when the refer
-
ence is decreasing, the phase of the feedback path is in
-
verted compared to the other two modes. Traditional
PWM methods will result in instability due to this charac
-
teristic. The UCC3750 separates the error signal magni
-
tude and polarity and determines the correct PWM signal
based on a separate mode determination circuit.
APPLICATION INFORMATION (cont.)
Figure 4c. Mode 3: Forward power transfer, negative
output.
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Figure 4d. Mode 4: Reverse power transfer, negative
output.
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