I am using TL431 to build a regulator with max. average load current of 0.5Adc and transient start-up peak current of 2Apk. This regulator is used for power of SiC mosfet driver.
I notice there's a 0.01uF aluminum capacitor recommended in application notes. Normally compensation is necessary when using TL431, but how to evaluate this capacitor?
Rookie
TINA is not modeled for exact stability of the capacitor.
If you analyze it's small signal model then you can see VO is connected to cathode with 30 ohm + 2/gm ohm resistor. That will be considered as ESR and you can refer the stability plot for your load capacitor.
(Internal gain * 0.01uF) capacitor will come in parallel to that.
Your circuit should be stable. You can also refer to app note slva482a for capacitor selection.
Regards
Trailokya
Hi, Rai:
I download PSpice model of TL431 from TI website and make a transient and AC simulation in OrCAD PSpice. I add small perturbation (Ufed_purtb) in feedback loop(below voltage divider resistor) to do an AC sweep. Then I get gain-phase bode plot, this helps me to evaluate stability of my design.
According to the AC sweep simulation, It seems that cut-off frequency move left (get smaller) if we enlarge miller compensation capacitor:
If I choose 10nF for miller compensator capacitor, I get a cut-off frequency of around 3.7kHz and phase margin of around 151degree:
Do you have any comment. Is this simulation right and advisable?
Thanks for your attention and hope to see your reply.
Ral, thanks for your prompt reply. the simulation was created Rookie but not me, i just the guy who interested with this topic also.
However, I was measure phase gain in real case and find out when I opened miller cap, phase gain result is good but Vo start oscillation.
It's makes me confused, did you have any idea on this?
| Desc | Phase | Gain | cutoff freq |
| With miller cap 10nF | 83.97 deg | -6 db | 6.21kHz |
| Without miller cap | 72.96 deg | -18.58 dB | 6.32kHz |
I forget to attached the waveforms. The frequency is about 27k~28khz, measured with C1 open condition
Ch3 output ripple
Ch4 VkA
Hi, Hugo.Yang:
Below is my AC Sweep loop stability simulation considering C1 (miller cap) = 10nF. The simulation shows when using 10nF, cut-off frequency is around 4.535kHz and phase margin is around 99.63degree, a little different from your first loop measurement.
And if C1 (miller cap) is removed, I get transient simulation below, which is similar to your waveform measurement.
But AC Sweep loop stability simulation below is quite different from what you have mentioned.
It shows that there's no cut-off frequency and relevant phase margin as well. And it seems that there's a pole around frequency 100kHz.
Is this AC Sweep loop stability simulatoin can explain the oscillation you have seen?
