Circuit design

Thread Starter

Lolrapa

Joined Sep 19, 2018
21
Hello, I'm struggling to design a circuit.

I want to make a custom feedback control for a FAN7688 LLC Supply

circ.png
These are de design characteristics:
  1. The output voltage can be adjusted from the minimum operational voltage to 18V trough a 1KHz PWM signal decoupled using a optocoupler in open collector configuration. Where 0% duty cycle corresponds to minimum output voltage.
  2. The circuit must sense the output current via a shunt and when the output current exceeds 20A the feedback must be manipulated to lower the output voltage and keep the current caped at 20A. Under 20A the output must behave like a constant voltage source.
  3. The response through the original feedback (from output to feedback pin) must be as fast as possible without incorporating noise.
  4. The response through the open collector must be slow, the FAN7688 does not like abrupt variations in it's feedback pin.
  5. +5V and +12V supply rails are available.

The FAN7688 controls the output in a way that the feedback voltage is 2.4V

So far I have this idea:

Use the open collector with a resistor to generate a square wave to feed into a integrator opamp, that way I obtain a dc voltage representing the duty cycle and I can adjust the response speed using the capacitor of the integrator. That signal must be mixed with one coming from the output to being able to control the feedback voltage.
I don't know how to mix them and how to incorporate the current sensing feature, I would appreciate any ideas.

Thank you!
 

ericgibbs

Joined Jan 29, 2010
12,901
hi L,
A point about that circuit post #3, is that you show a High Side sensor in the circuit of post #1.
How do plan to attach the Integrator Inp and Output to that sensor.?
E
 

Thread Starter

Lolrapa

Joined Sep 19, 2018
21
hi L,
A point about that circuit post #3, is that you show a High Side sensor in the circuit of post #1.
How do plan to attach the Integrator Inp and Output to that sensor.?
E
Hello Eric,

that's what I'm asking, I dont know how to combine the 3 signals to generate the feedback
 
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