Datasheet
SLUS168D – APRIL 1999 – REVISED AUGUST 2002
6
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pin descriptions (continued)
NOTE: The oscillator generates a sawtooth waveform on RC. During the RC rise time, the output stages alternate on time, but
both stages are off during the RC fall time. The output stages switch a ½ the oscillator frequency, with guaranteed duty
cycle of < 50% for both outputs.
UDG–97009
Figure 1. Block Diagram for Oscillator
APPLICATION INFORMATION
A 200-kHz push-pull application circuit with a full wave rectifier is shown in Figure 2. The output, V
O
, provides 5 V
at 75 W maximum and is electrically isolated from the input. Since the UCC3808 is a peak current mode controller
the 2N2222A emitter following amplifier (buffers the CT waveform) provides slope compensation which is necessary
for duty ratios greater than 50%. Capacitor decoupling is very important with a single ground IC controller, and a 1
µF is suggested as close to the IC as possible. The controller supply is a series RC for start-up, paralleled with a bias
winding on the output inductor used in steady state operation.
Isolation is provided by an optocoupler with regulation done on the secondary side using the UC3965 Precision
Reference with Low Offset Error Amplifier. Small signal compensation with tight voltage regulation is achieved using
this part on the secondary side. Many choices exist for the output inductor depending on cost, volume, and
mechanicall strength. Several design options are iron powder, molypermalloy (MPP), or a ferrite core with an air gap
as shown here. The main power transformer is a low profile design, EFD size 25, using Magnetics Inc. P material
which is a good choice at this frequency and temperature. The input voltage may range from 36 V dc to 72 V dc.