8.2.2 Detailed Design Procedure

The output, VO, provides 5 V at 50 W maximum and is electrically isolated from the input. Because the UCC3808A is a peak current mode controller, the 2N2907 emitter follower amplifier buffers the oscillator waveform (RC pin) and provides slope compensation to the current sense (CS) input. This is necessary for duty cycle ratios of greater than 50%.

Capacitor decoupling is provided on the VDD pin. TI recommends using a minimum decoupling capacitance of 10-µF electrolytic and 0.1-µF ceramic. The ceramic capacitor must be as close to the VDD pin as possible. The UCC3808A is initially powered up from the 36-V to 72-V input supply . Once the power supply has started, the bias supply is provided by an auxiliary winding on the main power transformer.

Isolation is provided by an optocoupler with regulation done on the secondary side using the TL431 precision programmable reference. The internal error amplifier of the UCC3808A is set up as a unity gain amplifier and the compensation network is provided on the secondary side.

Many choices exist for the output inductor depending on cost and size constraints. Design options are powdered iron, molypermalloy or the ferrite core option used in this design. The power transformer is a low profile design, EFD25 size, using the Magnetics Inc. P material. This material is a good choice for low power loss at high switching frequency.

The switching frequency is set at 210 kHz with the RC network on the RC pin.