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SG2525 (SG3525) based PWM and power supply

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Nov 2, 2009
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OK, so I just finished building a DC-DC converter based on the SG2525 PWM and some IRFP240s. It works but I don't think I have a very good grasp of the Compensation, purpose of the opamp inputs (inverting and non-inverting inputs), etc.

Lots of circuits I see based on this just enable "shutdown" when voltage gets too high. Others modulate the non-inverting input with the inverting one tied to the Vref, still more use an extensive compensation network.

Why all the difference? Anyone an expert on the SG2525/3525 that cares to speak a bit on how the feedback is "intended" to work on this chip?

My circuit is 70kc, directly drives the gate of the IRFP240, that runs a forward boost converter - 100uh filter, MUR3060 diode (well half of it), into a 1000uF cap. Left to run wild it gets to 80 volts mighty quick.

My comp network is a simple transistor 2n3904 operating in the linear region biased so that 19V is towards the bottom of the compensation voltage, and thus drives the FETS at minimal duty cycle. As the power demand increases (and voltage decreases) the transistor increases the voltage at the comp pin to turn up the duty cycle.

SO why would someone use shutdown pin, and what is the purpose of the error amp? Best I could tell when non-inverting input was less than vref, the thing was on. When less - it wsa off, couldn't adjust duty cycle this way.

Anyone care to offer insight?

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