I am designing a Full Bridge DC to DC convertor using SG1525(SG3525).
I'm doing the simulations in OrCAD Capture(PSpice).
Actually, I've successfully simulated the Power section of the converter using four IRF540 in two bridges and igniting their V_GS 's with two Pulse Generators that are isolated(virtually) via VCVS's.
For the control section, OrCAD's PSpice says:

* SG1525 *
* The following model for the 1525 was obtained by consulting the data sheets
* and corresponding with Silicon General. A number of simplifications were
* made to speed up the model, among these we have:
* (a) replaced th oscillator with ideal voltage sources,
* (b) simplified the output stage (only two bipolars per output driver),
* (c) the sync pin is ignored (can drive clock directly if needs be),
* (d) made the shutdown pin respond to digital stimulus, and
* (e) used digital simulation for the internal logic.
* The impact of that these simplifications must be considered in the context
* of the parameters of the circuit, and the circuit being examined. The
* above list might change as we get feedback. .subckt SG1525 ; note: the node numbers are equivalent to chip pinout
+ 1 ; - input of error amp
+ 2 ; + input of error amp
+ 4 ; oscillator output
+ 5 ; ramp output
+ 8 ; soft start pin
+ 9 ; compensation pin
+ 10 ; shutdown pin
+ 11 ; output A
+ 12 ; ground
+ 13 ; collector of the output drivers
+ 14 ; output B
+ 15 ; vin
+ 16 ; vref
+ params:
+ period = 25us ; internal clock period
+ deadtime = 0.5us ; internal clock deadtime
* Pin 3 (sync) NOT USED due to (a) above.
* Pin 6 (oscillator time resistor) NOT INCLUDED due to (a) above.
* Pin 7 (discharge resistor pin) NOT INCLUDED due to (a) above.

I have the following questions:

1. R_T is ignored in simulation, so the period must be set by the user. In model editor it is claimed that period is 25us, but in Schematic Editor there is also a period. Which one is used in simulation?

2. What should be connected to the pin OSC? Actually I don't even know what it is used for?

3. As I know, each of the output pins should ignite two MOSFETS... So where should we get feedback from? Since the output is between the two bridges...

4. Where is the feedback pin? Is it C(pin 13) or COMP(9)(I think it is the controller)
OrCAD PSpice model of SG1525

ANY help is greatly appreciated :(


Well, I've also built a circuit that uses SG1525. I connected the pin 4 (OSC) to pin 5 (CT) and they altogether connect to a pwm source. Like this:enter image description here

The pulse will be nice as you want it to be. However, the only problem is that both pin 11 and 14, which is OUTA and OUTB gave out the same pulse but not complementary. I am trying to figure it out. Hope this help! :)


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