![]() In this case we also are amplifying the PWM ramp signal off of that with a transistor and applying it to the ISENSE input through CRAMP and IRAMP so that the circuit is stable for very long duty cycles. Now, normally we would see a resistor and capacitor, RRT and CCT, connected to the RT/CT pin and providing a time constant for the PWM oscillator. The UC2842 is designed so that if it exceeds 1 V, it shuts the PWM circuit down. The ISENSE input is measuring the voltage drop across RCS, which is to say it’s measuring the current draw through that switching FET. The optoisolator doesn’t have to be very linear for the UC32842 duty cycle to be kept right on the edge so the output voltage is always perfect. It provides a varying voltage to the VFB input to provide feedback to the UC2842 that the voltage is correct and to back off the duty cycle of the output waveform a little bit. The UC2842 provides a small amount of regulated 5 V (made with an internal linear regulator) and that voltage at VREF is used to power the output stage of the optoisolator. So, how does the regulation work? All that other stuff on the secondary causes the LED in the optoisolator to come on when the output voltage exceeds 12 V. U3 provides a reference with which to compare the 5 V line. The optoisolator, U2, is drawn in two individual halves. Notice the Vaux output is referenced to the input ground. Although that 2200 F cap may seem large, if the oscillator is running at 60 kHz, it’s a thousand times as effective as the same value off the 60 Hz line.įigure 2: This schematics shows a typical small switching supply using a 3845 PWM IC. As this happens, current is induced in the transformer secondary, rectified and filtered by DOUT and COUT, and current flows from the output.īecause the PWM oscillator is so fast, the transformer and the filter capacitor on the secondary side can be very small. This turns on the big switching FET, QSW, which pulses the current going through the transformer. ![]() When the PWM oscillator is running, it sends constant pulses from the output pin. But, using the third winding improves efficiency a lot. You might not see that third winding, you might just see all the running power being drawn through a higher power dropping resistor in place of RSTART. RSTART only supplies a small amount of current to start the device, so once the first pulse makes it through the field-effect transistor (FET), current from a third winding on the transformer is used to provide power to run the oscillator. The output charges up a big filter capacitor on the primary side CIN, which provides a filtered (but hardly ripple-free) DC voltage to the primary of the transformer, NP, as well as voltage to start the pulse width modulation (PWM) chip through resistor RSTART. ![]() ![]() This is an important point and you will see this in almost all supplies of any size since isolation from the power line is a primary safety concern.ĪC power comes off the line, and is rectified through the bridge rectifier, DBRIDGE. You can draw a line in your head through the transformer core and through the optocoupler and break the circuit up into two electrically isolated halves. (The datasheet can be found in the Supplementary Materials section of the audioXpress website, see Project Files for the link.) Note that this design, as is typical, has complete isolation between the primary and secondary sides of the circuit. ![]() This comes from the UC2842 datasheet and uses the common UC2842 PWM controller chip. (Original schematic courtesy of Texas Instruments)įigure 1 shows a sample switching supply design (courtesy of Texas Instruments). Figure 1: This sample switching supply design comes from the UC2842 datasheet and uses the common UC2842 PWM controller chip. ![]()
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