The UC384X family of current-mode PWM regulators requires a current shunt or some other component to develop a voltage proportional to the output current. This shunt must develop a 1V signal at full load.
For off-line supplies, the 1V requirement is not hard to meet because of the high voltage on the supply's dc bus. However, if you want to use a UC384X regulator in a low-voltage circuit, the 1V drop becomes undesirable. Lowering the shunt's value is not a good idea because the maximum current would rise to an unacceptable value in short-circuit or overload conditions.
The circuit in Fig 1 shows an 11W current-mode flyback converter. An external low-offset comparator, IC2A, overrides the output of regulator IC1's error amplifier. The R4-R5 network reduces the output of IC1's error amplifier and determines the maximum output of the supply. Because of the low offset of IC2A, you can set a threshold as low as 200 mV and then employ shunts of a few tens of millohms.
R2 and C2 filter the output of the shunt R1. The output then goes to the noninverting input of the comparator. In this design, IC1's internal comparator works simply as a switch to reset the internal latch and no longer imposes its threshold value of 1V.
The circuit develops a primary peak current of about 5.2A (2.2A rms). To provide a contrast to the comparator version, we built a version having a 0.22(ohm) shunt. The shunt dissipates more than 1W, dropping efficiency to less than 80%. The efficiency of the comparator version, using a 0.005(ohm) shunt, is more than 88%.
In this application, the low operating frequency of the supply (33 kHz) permits using a slow comparator. In higher-frequency applications, replace IC2 with a faster device. You can also use this comparator technique with a sense FET because the converted voltage delivered by its current mirror does not correspond directly to the internal threshold of the UC384X. (DI #1417)