SG2525 (SG3525) based PWM and power supply

Discussion in 'The Projects Forum' started by bobdole369, Nov 5, 2009.

  1. bobdole369

    Thread Starter New Member

    Nov 5, 2009
    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?<br>

    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. <br>

    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?
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    The required feedback network is actually dependent on the externals like the output L-C. We used the 3525 on all our power supplies and as I recall the output L-C put a "double pole" at a frequency given by:

    1 / 2 x pi x SQ ROOT (LC)

    You have to put in a lead (zero) to compensate out one of the poles. Ypu also have to have a pole (feedback cap) around the op amp inside the 3525. This was for a voltage mode PWM converter, current mode converters are different. There could be other stuff I forgot, it was 20 years ago.
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    The internal error amp is what controls the PW. You derive feedback fom the output voltage to this amp (it must be in the voltage control loop).

    I think some designs use a fast comparator driving the shutdown pin for a current limit protection.