INN2604 Auto-Restart Problem
Hi.
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After an exhaustive investigation for the reason of my auto-restart problems, I think they only can be due to one of these mechanisms:
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1) Voltage on FEEDBACK PIN below "Feedback Pin Auto-Restart Threshold – VFB(AR)", i. e., Vout - 2 V;
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2) Voltage on FORWARD PIN below 10 V during primary mosfet ON time.
(See following text, extracted from the page 9 of datasheet relating to the application example in the previous page: "... This allows output current regulation to be maintained down to ~10 V. Below this level the unit enters auto-restart until the output load is reduced.")
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Relating to the possibility 1, is there any timing for the "VFB(AR)" on start up, like tSS(RAMP) of InnoSwitch3?
Besides, how does the controller "know" the set output voltage to apply the related "VOUT-2 V" nominal threshold?
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Relating to the possibility 2, Is the information on the page 9 accurate? If so, isn't there any further information about this related cause of auto-restart (timing, values etc.)?
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Regards,
Christos
Comments
Thank you, PI-Kryten.
"Possibility 1": I'd already read the paragraph of datasheet you mentioned. My doubt is specifically about the existence of a timer for the "Secondary Bypass Pin Undervoltage Threshold" — "VFB(AR)" — on start-up.
Regarding how the controller knows the set point: yes, I knew about the resistor divider tied to FB pin. I`d understood how the internal comparator could detect a proportional difference but not an absolut one... But ok, it can do that becouse Vout is also directy connected to the controller through VO pin.
"Possibility 2": I think I`d misunderstood the text on page 9. The voltage which level below 10 V makes the InnoSwitch enter in auto-restart seems to be that on the OUTPUT pin (and not on the FWD pin). I'm reading the text this way becouse the related example output nominal voltage is 12 V, and so, the referred minimum level is, again, "Vout - 2 V". This leads us to the same case of the "possibility 1". Is it right?
All this discussion is due to my big difficulty to find out the reason of the start-up problem in my project. Even after succesfully fixing a previous "VFB(AR)" problem, the power supply continues entering in auto-restart in some conditions, but this time I can't relate it to any of the reasons exposed in the datasheet.
At start-up, it is clear that the InnoSwith primary controller switches without feedback for a few milliseconds and then the secondary one gets control and doesn't request any switching cycle. Further, the problem seems to be related to the FWD pin, but its both voltage waveforms (when the circuit starts properly and when not) look the same...
Hi Christos,
Can you kindly share your schematic, PCB layout details and any transformer documentation that you have such as the one obtained from the PI-Expert tool on our website?
Best Regards,
PI-Kryten
Hello,
Thank you for considering Power Integrations for your power supply application
Looking at possibility 1,
In the event the FEEDBACK pin voltage is below the VFB(OFF) threshold at start-up, the secondary will complete the primary/secondary handshake and will stop requesting pulses to initiate an auto-restart. The secondary will stop requesting cycles for tAR(SK) (Typ: 1s) , to begin primary-side auto-restart of tAR(OFF)SH (Typ: 0.5s) . In this condition, the total apparent AR off-time is tAR(SK) + tAR(OFF)SH. During normal operation, the secondary will stop requesting pulses from the primary to initiate an auto-restart cycle when the FEEDBACK pin voltage falls below VFB(OFF) threshold. The deglitch filter on the VFB(OFF) is less than 10 msec.
Regarding how the controller knows the set point:
The mid-point of an external resistor divider network
between the VOUT and SECONDARY GROUND pins is tied to the FEEDBACK pin to
regulate the output voltage. The internal voltage comparator reference voltage
is VREF (1.265 V). In other words, one must choose the resistor network such
that the FB pin “sees” 1.265V when operating at the required Output Voltage.
Possibility 2:
Yes, the information on Page 9 is accurate. No additional
timing information is available related to this.
Auto Restart can also be triggered due to external
component/pin short.
Best Regards,
PI-Kryten