Part Number: UCC256302
Inquiry about UCC256302_EVL transient load fluctuation operation 2019.10.25 (Thursday)
We purchased EVL for LLC device evaluation from TI, mounted UCC256302, and evaluated output transient characteristics. ∙ During the test that caused the load factor of 12V output to fluctuate rapidly from (0%) to (100%), a phenomenon that the output stopped for 1 second occurred. As a protection function of this device, the output may stop for 1 second in the overcurrent detection protection mode on the LLC primary side. This is understood from the evaluation test results of abnormality protection and the operation explanation of the data sheet. However, in the transient load fluctuation test where the output is the minimum load (0%) ⇔ maximum load (100%), the phenomenon of stopping for 1 second has a problem in reliability for the output voltage stabilization requirement of continuous operation. Please confirm the following operation waveform and answer the questions about the phenomenon that will result in a stop for 1 second.
1. LLC load sudden change waveform 1 (observed 12V output temporary failure).
<Test conditions> ①Input voltage_DC390V, LLC output load sudden change: DC12V 10 [A]⇔0 [A]
① Blue: VCR pin voltage [2V / div], ② Red: LLC input SW current [1A / div], ③ Green: FB pin voltage [1V / div], ④Pink: 12V output voltage [2V / div], H: [400msec / div] |
At this point timing, the 12V output drops and a pause of about 1 second begins. |
2. LLC load sudden change waveform 2 (enlarged waveform in item 1 above) (same as above)
<Test conditions> ①Input voltage_DC390V, LLC output load sudden change: DC12V 10 [A] ⇔0 [A]
① Blue: VCR pin voltage [2V / div], ② Red: LLC input SW current [1A / div], ③ Green: FB pin voltage [1V / div], ④Pink: 12V output voltage [2V / div], H: [2ms(400msec) / div] |
At this point timing, the 12V output drops and a pause of about 1 second begins. |
3. LLC load sudden change waveform 3 (waveform further expanded from the above 1) (same as above)
<Test conditions> ①Input voltage_DC390V, LLC output load sudden change: DC12V 10 [A] ⇔0 [A]
① Blue: VCR pin voltage [2V / div], ② Red: LLC input SW current [1A / div], ③ Green: FB pin voltage [1V / div], ④Pink: 12V output voltage [2V / div], H: [40μs (400msec) / div] |
With this enlarged point timing, the 12V output drops, and a pause of about 1 second begins. |
4. LLC load sudden change waveform 4 (Normal operation timing at the time of 0% → 100% change in the
above 1) _Expanded waveform 1)
<Test conditions> ①Input voltage_DC390V, LLC output load sudden change: DC12V 10 [A] ⇔0 [A]
① Blue: VCR pin voltage [2V / div], ② Red: LLC input SW current [1A /div], ③ Green: FB pin voltage [1V / div], ④Pink: 12V output voltage [2V / div], H: [200μs (400msec) / div] |
The 12V output does not decrease at the timing of this enlarged point. |
5. LLC Load sudden change waveform 5 (Normal operation timing at the time of load 0% → 100% change in
section 1 above_enlarged waveform 2)
<Test conditions> (1) Input voltage_DC390V, LLC output load sudden change: DC12V 10 [A] ⇔0 [A]
① Blue: VCR pin voltage [2V / div], ② Red: LLC input SW current [1A / div], ③ Green: FB pin voltage [1V / div], ④Pink: 12V output voltage [2V / div], H: [200μs (400msec) / div]
|
The 12V output does not decrease at the timing of this enlarged point. |
6. LLC load sudden change waveform 6
Check if there is a change in the input voltage of the BLK terminal voltage when the output drops
abnormally. → Result: No voltage fluctuation.
<Test conditions> ①Input voltage_DC390V, LLC output load sudden change: DC12V 10 [A] ⇔0 [A]
① Blue: VCR terminal voltage [1V / div], ② Red: LLC input SW current [1A / div], ③ Green: BLK terminal voltage[500mV / div], ④Pink: LLC output voltage [2V / div], H: [200μs (400ms) / div] |
At this enlarged point, the 12V output drops, but the BLK pin input voltage does not fluctuate and is stable. At this time, the VCR pin voltage has risen rapidly to about 7V. |
7. LLC load sudden change waveform 7
Confirmation of voltage change of LL / SS terminal voltage when output voltage drop is abnormal. → Result: LL / SS pin voltage is stopped.
<Test conditions> (1) Input voltage_DC390V, LLC output load sudden change: DC12V 10 [A] ⇔0 [A]
① Blue: VCR terminal voltage [2V / div], ② Red: LLC input SW current [1A / div], ③ Green: LL / SS terminal voltage [500mV / div], ④Pink: 12V output voltage [2V / div], H: [200μs (400msec) / div] |
At this expansion point, the 12V output drops and the VCR pin voltage rises rapidly to about 7V. Immediately after that, the LL / SS pin input voltage stops. |
From the above results, output stops due to the following problems.
* 1. After the FB pin and VCR pin voltages suddenly rise, the LL / SS pin voltage suddenly drops and OC
detection occurs.
* 2. The above operation stops output for about 1 second with OC protection.
<Questions for observation results>
(1) When the load suddenly changes from 0% to 100%, two states occur in the VCR pin voltage.
・ When the VCR terminal voltage suddenly rises to about 7V (refer to the waveforms in Sections 2 and 3)
・ When the VCR terminal voltage does not increase rapidly and responds linearly at the control level (see
waveforms in sections 4 and 5)
These two modes of operation were observed. Please explain the mechanism of this difference logically.
(2) Why does the VCR pin voltage suddenly rise due to the FB pin voltage rise?
(3) Is the phenomenon that the VCR terminal voltage suddenly rises to about 7V normal as a control
mechanism?
(4) If items (2) and (3) above are abnormal, are there any measures that can improve this?
(5) If the VCR pin voltage suddenly rises as described in (3), the absolute maximum rating will be slightly
exceeded. Is there stress?