TFS7704 will not startup
Hi,
At first startup of a new design I have shortcircuit the feedback Optocoupler U1B ( Study DER 368 or fig.30 in the manual )in order study the standby controller without firing the main controller.
Thats is: FB is hold at ground level from start.
BP ramps up and stays at 4.7V with no rise to 5,7V.
If this is a problem for rising BP kindly suggest an alternative approach just starting the standby controller and not the main controller at this initial lab test.
Kindly explain the current path for charging BP/C12 at power-up.
Bias current flow via R1>T2>DSB or ?
Regards,
Tomasf
Comments
Hi,
Ok - got it, thanks.
Standby controller starts up as expected now.
A new question arrives:
Based on the demoboards for PFS-2 ( RD-294 ) and TFS-2 ( DER-368 ) am I allowed to use "12V Bias" from TFS-2 as "Auxiliary Power Supply" to PSF-2 ?
The PFS-2 manual state that S and G must not be tied together.
TFS-2 manual suggest in fact to use the 12Vbias for the PFC boost converter.
This results in a no galvanic separation between S and G. ( S and G will be tied together ).
Kindly sort this out before we combine the demo boards and test further.
Attached I have added extracted pages from both manuals with markings for you.
| Attachment | Size |
|---|---|
| PFS-2 TFS-2 Bias question (622.35 KB) | 622.35 KB |
Hi,
Connect only 12V bias positive from TFS2 stage to PFS2 Vcc.
Do not connect TFS2 G pin to PFS2 G pin.
In that way S and G pin will not be connected together on PFS2.
Regards,
PI-NANO
Hi,
Ok , got it and tried.
At no main output load I got correct regulation.
But at 1A load 12V bias drops below 10,5-10,6V and PFS2 is halted/PG release the RT1 relay.
12V bias rise again and PFS2 is restarted.
Repeated in approx 1s cycles.
Any suggestions ?
BR/Tomasf
Hi,
Standby supply on DER-368 was designed to deliver 0.83A, If you exceed that load level it will enter into auto restart mode.
Do not load the standby more than 0.83A.
Regards,
PI-NANO
Hi,
As I wrote:
Main load = 1A.
Standby = 0A
12V bias drops below 10,5-10,6V and PFS2 is halted/PG release the RT1 relay.
BR/Tomasf
Hi,
Can you try powering the TFS stage with 380V DC input without connecting the PFS stage and see if it works?
Regards,
PI-NANO
Hi,
Same problem when powering TFS stage separately.
It seems that there is a min load requirement for 12V Standby in order for correct regulation of 12V Bias ?
Main load = 1-10A.
Standby >= 10mA
12V bias = 12,30-12,32V @ main load 1-10A
why ?
BR/Tomasf
Hi,
Actually there is no min load requirement on standby supply.
Something might have gone wrong on the board.
What is the voltage that you see on the BP pin when you start loading (from 0A - 1A) the main by keeping the standby at no-load.
Probe the FB pin waveform and enable pin waveform as well. Otherwise request for another board and work on that board.
Regards,
PI-NANO
Hi,
You should not short circuit the feedback pin.
If you don't want to turn on the main, just apply 120VDC at the DC bus capacitor.
It will not turn on the main controller at that voltage. Main controller starts at about 340VDC.
Could you please attach your schematic so that I could review the schematic.
Regards,
PI-NANO