start-up time for 180 W PFC Front End Using TINY724GN

2 posts / 0 new

Dear PI support team,

we designed AC/DC with PFC using TINY724GN referencing RDR-248. (Please refer to the attached circuit diagram)

1) start-up time limitation
We found Power supply doesn't start-up when Input voltage increases DC=0V to 80V taking 0.5 second.
Is there any limitation for start-up time or possible cause ?

2) Updated line undervoltage current threshold on TinySwitch-III Family datasheet
Refer to "TinySwitch-III Family datasheet", Updated line undervoltage current threshold to 2 mA in Rev.G.on page .23.
But we found .EN/UV Pin Line Undervoltage Threshold is 25uA on page.13.
What is the meaning of different value ?

Your kind and prompt advice is appreciated.

Kozo Nakata
kozo.nakata@futureelectronics.com
TEL 045-224-2145

AttachmentSize
Power Supply Circuit Diagram102.56 KB

Hi Kozo,
Looking at your schematic, no resistor is connected across the positive high voltage rail (10 uF E-cap + pin) and the pin 1 or EN/UV pin of TNY274G. This means that the line undervoltage feature of the device is disabled. If this feature is not use, the flyback should turn-on approximately around 30 VDC. But with or without the line undervoltage functionality, the delay from the input voltage to output voltage regulation is only for few msecs, around 10 - 15 ms (also dependent on the output load) and shouldn't reach 0.5s.
Then Just to give you a brief discussion with regards to RDR-248, as you can see on the schematic of the auxiliary which uses TNY274GN device, R26 and R27 is connected across the positive bulk voltage to the EN/UV pin of the IC, which means it utilizes the Line UV feature of the IC. Using this values the line undervoltage will be around 87 VDC. As you can see on its test data regarding the Brown-In testing, the measured auxiliary supply brown-in threshold is 61.7 VAC, approximately equal to the actual line undervoltage. During this time the current that flows to the EN/UV pin exceeds 25uA (based also on calculation having a line sense resistor equivalent to 3.3M-Ohm, the current that will flow to the EN/UV pin will exceed 25uA at approximately 87 VAC). Once exceeded the threshold current of 25 uA, the BP/M pin voltage will rise to 5.85V from 4.9V, that would initiate the switching of the primary MOSFET and eventually the output.
Without the line undervoltage resistor, as in the case of your schematic, the delay from the input voltage to output regulation should also be in the range that were mentioned above. Except that the output may rise earlier since undervoltage is disabled and output maybe not as monotonic as opposed of having line undervoltage feature.

Maybe if you could help to capture some waveforms, we could analyze it more further.
You can monitor the following points in your circuit at the same time:
Channel 1: DC Input Voltage (across the 10 uF E-cap).
Channel 2: Pin 1 or EN/UV pin of the TNY274G.
Channel 3: Pin 2 or BP/M pin of the TNY274G.
Channel 4: Output Voltage across C7 (output capacitor).
Then next include the measurement of Drain-to-Source of TNY274G. Remove the oscilloscope probe at EN/UV pin and replaced it with the probe monitoring the Drain to Source of the IC (across Pin4 (Drain) and Pins 5,6,7,8 (Source).

Put a oscilloscope probe from each point, then slowly or gradually increase the DC input voltage or apply the DC input voltage from 0 VDC to 80 VDC.

And just have some following questions:
1. What is the highest calculated load of your 18 V output (18 V that supply the IC regulators)?
2. Can you give us the complete part number of the optocoupler and zener diode that you are using?
3. Though already indicated in your schematic that the output voltage or supply of the IC regulators is 18V, what is the actual or measured voltage (the voltage across C9 and C10)?

Or you can also isolate the output voltage across C9 and C10. Isolating, meaning removing the IC regulator IC2 and IC3 and diodes D5 and D6. And just connect the output of C9 and C10 to the E-load. And see if the post regulators have some impact in the operation of your auxiliary or pre-regulator (TNY274G operation).