2 pole-2 Zero compensation

[zereshki]

Hello.
I designed a half bridge converter by using tl494 chip. and so it's 2pole - 2zero compensation network.
but this configuration is suitable for inverting error amp configuration not non inverting like tl494. Now I confused how to solve this problem for make circuit to be stable!
Actually using a RC feedback in non inverting configuration same as in ATX power supply(that use tl494 not SG series),just make one pole (at origin) and one zero.It make overall loop -2 slop at cross over frequency not -1. how it is possible to be stable? although there is not any instability in ATX !!!

circuit parameters:
Forward Gain included : [main voltage(for max line Ac=280v) & pwm control ramp=3.2v & transformer ratio=Ns/Np=11/34] =26db
L = 314 uH
C = 1000 uF (for esr:RC=1.6 us)
Osc freq = 55.5 KHz
Fcross = 5.5KHz
Vo=30v
Vref=5v


Regards

 

[mtwieg]

Hello.
I designed a half bridge converter by using tl494 chip. and so it's 2pole - 2zero compensation network.
but this configuration is suitable for inverting error amp configuration not non inverting like tl494. Now I confused how to solve this problem for make circuit to be stable!
Actually using a RC feedback in non inverting configuration same as in ATX power supply(that use tl494 not SG series),just make one pole (at origin) and one zero.It make overall loop -2 slop at cross over frequency not -1. how it is possible to be stable? although there is not any instability in ATX !!!

Yeah this is definitely one reason I never use the tl494. The error amp response always flattens at high frequency, so you have to rely on other parts of the circuit to roll off the overall loop gain. Having a loop gain slope at -20dB/dec at the crossover is still doable if your capacitor ESR is low. Or you could drop an extra pole in the feedback network.

 

[zereshki]

Having a loop gain slope at -20dB/dec at the crossover is still doable if your capacitor ESR is low. Or you could drop an extra pole in the feedback network.

thank you for your worthwhile answer.
1-but Low Esr Or High Esr?!!
as I said because of Low ESR CAP it needs to use 2p-2z, that I used.
2-How it is possible to ADD pole in feedback network? You mean another RC parallel to R4 in attached circuit;am I right? but,are you sure it adds pole, or zero?!!
3-How to change calculations of feedback network in inverting config to non inverting?



Regards

 

[mtwieg]

thank you for your worthwhile answer.
1-but Low Esr Or High Esr?!!
as I said because of Low ESR CAP it needs to use 2p-2z, that I used.

Not sure I understand what you mean here. Having -2 slope requires having two dominant poles at the given frequency, which can be provided by the LC tank alone, if it has high Q (that is if ESR is low). ESR in the LC is always a tradeoff between bandwidth and ripple.

2-How it is possible to ADD pole in feedback network? You mean another RC parallel to R4 in attached circuit;am I right? but,are you sure it adds pole, or zero?!!

Adding a capacitor in parallel with R5 will just add a pole.

3-How to change calculations of feedback network in inverting config to non inverting?

I don't know off the top of my head, but ultimately it's the pole/zero placement that matters, and relating those to component values is just an algebra exercise.