I have designed an 1KW AC/DC conversion with PFC using LT1248.
I have observed some weird performance here,
1. First of all, When i connected VCC from another low current AC/DC converter, It's output voltage got dropped to 3.7VDC from 17VDC, which was working fine before connecting the output to LT1248.
2. So, I connected the bench power supply with 17VDC and observed no switching at the transistor, so no 400VDC at output as per my design, instead I am getting 315VDC which is just smoothing of pulsating DC output from rectifier by 560uF capacitor.
3. Now i removed 17VDC connection from Bench power supply and left the connection open, I got a mild current shock at my hand when I touched the wire which was connected to VCC pin, Shockingly, there is same amount of shock I felt when i touched the GND pin too. This observation is made When AC is turned ON. Below schematic will show the circuit in this condition.
I am unable to understand the situation here, Also Please forgive me if my English is bad, as English is not my first language.
[Edit] : I have checked continuity of all pins, no pin is shorted. also I have checked any short externally happening, none are observed. Shock is bothering me, mainly the voltage difference between VCC and GND, when measured potential difference is around 1.7VDC and reduce gradually when AC is turned off.
Thanks and best regards,
WARNING!! Unless you isolate the AC source from earth ground and connect circuit ground to earth ground, circuit "ground" will be at lethal potential: EXTREME SHOCK HAZARD!!!
It is apparent to me that solving your various issues is beyond the scope of this forum. I strongly recommend that you get a consultant with experience in PFC circuits to take-on this project.
After I have connected earth ground with reference ground, the Rs resistor, in my design it is 0.0546 ohms resistor connected between return and ground has burnt.
If you have isolated line and neutral through an isolation transformer, you may connect ONLY signal ground to earth through your oscilloscope. I consider this an advanced application with LETHAL HAZARDS. I would not recommend this undertaking to anyone with less than a few years working with ac line powered converters.
Thanks for the reply.
Please elaborate as I am unable to understand the issue here.
As you said
dkb said:Unless you isolate the AC source from earth ground and connect circuit ground to earth ground, circuit "ground" will be at lethal potential
This is the issue i am facing because I am getting a mild shock from 400VDC return and as well as Vcc pin.
But I have not understood how can we isolate two pins with having same potential for both pins.
dkb said:I suggest an isolation transformer.
As transformer is a 4+ pin device, how do we connect those pins here, as we have only two pins to isolate.
And finally, in reference schematic given in both datasheet and LTSpice tool's schematic had return shorted to ground, this is leading more confusion to me.
Attached the pics from datasheet and LTSpice tool for reference.
Ref sch provided with LTSpice tool.
ref sch provided in datasheet.
This article might be helpful to you.
I have read the article. As you said, I have isolated AC source and connected Line and Neutral using isolation transformer, now the circuit is
Now my fuse is getting burnt, And also I have a question that in the actual field where there is no isolation between AC source and earth, how can i make sure that IC is not turning ON so the output is not 400VDC as per design,
And also the VCC connection, it is having a potential difference, so it may cause damage in the circuits. Because of it, when my circuit is connected on field where there is no isolation transformer.