Current mode with SG3524

[cupoftea]

For current mode operation with SG3524...or indeed, the pin4pin UC3524

The comparator is expecting a ramp from 0.5v to 3.8V.....

So i guess a biased up current sense transformer output....
Do you have better ways?

SG3524

https://www.ti.com/lit/ds/symlink/sg3524.pdf?ts=1714417635113&ref_url=https%253A%252F%252Fwww.google.com%252F

UC3524

UC3524 data sheet, product information and support | TI.com

TI’s UC3524 is a 40V VCC, double ended 300KHz voltage mode PWM controller with 45% duty cycle, 0C to 70C. Find parameters, ordering and quality information

www.ti.com

 

[crutschow]

For current mode operation with SG3524

Define "current mode operation".
Does that mean a constant-current output, or something else?

 

[cupoftea]

Thanks, current mode, as opposed to "voltage mode"
Current mode , is where typically, an output voltage is regulated by an error amplifier, which drives the PWM comparator input...the other input of this comparator, is a repeatedly ramping current, from the power stage inductance...as opposed to voltage mode, where the error amp feeds into a pwm comparator again...but the ramp into the other input of the pwm comparator is provided by a ramp generator cct.

 

[Easy peasy]

It's pretty easy to add the isolated CT signal to 1/2 volt.

 

[crutschow]

So what's the advantage to operating it this "current-mode" configuration.

 

[Easy peasy]

much faster dynamics of the power stage as, controlling the peak ( or average ) current turns the buck inductor ( output inductor ) into a current source feeding a capacitor - rather than a double pole LC filter , there is still a Vout error amp stage to control the current level that the current signal ramp goes to to terminate the current ( no pun ) cycle, slope compensation of the current ramp signal is needed above ~ 45% duty cycle to retain stability ( 1 - 2 - 1 pulsing ).

This has been known and used since about 1985

the other benefit is that the peak current is always below a given ref - in the semi's - which removes stress at start up and sudden output short.

 

[cupoftea]

It's pretty easy to add the isolated CT signal to 1/2 volt.

Thanks, yes, i mean would just reference the CT ground to a 0.5Volt gotten from a divider. Thing is, the ramp is expected to be 0.5V to 3,8V....so thats going to put more magnetising current into the CST, and mean a larger CST.
I suppose we can limit the ramp to 1V, (say), and then clamp the error amplifier output to 1.5V...but thats extra cctry, and cost.

Another advantage to current mode, is that as is well known, you dont get the flux walking. And of course, a push pull has no bridge node for us to put a dc blocking cap....unlike the full bridge......(tho' as is known, the blocker is only for use with voltage mode). This is why we dont like voltage mode pushpulls.....they almost are laying the red carpet for the flux walking to come on in.