Level shift

[jeppson]

Hi. I'm having trouble getting the level shift to work for my 30W amplifier. I get 0-5V output from the comparator and I want to level shift it to 0-15V for the upper transistor and -15-0V for the lower one. I don't want to use an IC driver. Thanks in advance.

[stepheno]

Hello jeppson,
If you post a circuit, then I can maybe help you.

[hilli_billi]

Hi jeppson,
take a look at Phillips UM10155 (just google for it).
It's descrete with level shift, driver stage etc.

[jeppson]

Here is the circuit, the bottom transistor turns on and off but there is some loss of voltage at the totem poles, the bjts are probably not fast enough? Or what else could it be. What about the top level shift, can't get that one to fully work.

ImageShack® - Online Photo and Video Hosting

[jeppson]

Originally Posted by hilli_billi

Hi jeppson,
take a look at Phillips UM10155 (just google for it).
It's descrete with level shift, driver stage etc.

Thanks but I want to try to fix my own solution, got any tips?

[Pafi]

3 stages of BJT are totally unneccessary, use only 1!

6N138 is specified up to 7 V (absolute maximum for Vo), not 15!

BJTs are fast enough, but that optocoupler isn't.

R5 is connected to the wrong place, it should be on the GND pin of the opto!

[jeppson]

Originally Posted by Pafi

3 stages of BJT are totally unneccessary, use only 1!

6N138 is specified up to 7 V (absolute maximum for Vo), not 15!

BJTs are fast enough, but that optocoupler isn't.

R5 is connected to the wrong place, it should be on the GND pin of the opto!

I'm sorry the lower should deliver -15V to 0V. The upper transistor never turns on. That is not the optocoupler I'm using, pspice don't have 6N137. Ok I'll remove the unnecessary BJT's. So what should I do to turn on the upper transistor? Thanks a lot.


http://img4.imageshack.us/i/jeppson.png/

The upper one is after the lower mosfet, the lower after the lower optocoupler. Regarding the 7V maximum, I'm just gonna use this opto temporarily.

[Pafi]

Originally Posted by jeppson

I'm sorry the lower should deliver -15V to 0V.

It was clear already. But Vo absoulte maximum is 7 V referenced to GND pin of optocoupler.

The upper transistor never turns on. That is not the optocoupler I'm using, pspice don't have 6N137.

Then what R5,R6 do? Are they there only for simulation?

Ok I'll remove the unnecessary BJT's. So what should I do to turn on the upper transistor?

Post a real schematic, and please don't use that imageshack shít! You can attach pictures directly to the posts. And you can attach a higher resolution picture.

Thanks a lot.

The upper one is after the lower mosfet, the lower after the lower optocoupler. Regarding the 7V maximum, I'm just gonna use this opto temporarily.

After the lower mosfet? Are you shure? And 5 V/div? How could this possible? The informations you provided are inconsistent! And I can't figure out which zero line corresponds to which signal.

[jeppson]

Originally Posted by Pafi

It was clear already. But Vo absoulte maximum is 7 V referenced to GND pin of optocoupler.



Then what R5,R6 do? Are they there only for simulation?



Post a real schematic, and please don't use that imageshack shít! You can attach pictures directly to the posts. And you can attach a higher resolution picture.



After the lower mosfet? Are you shure? And 5 V/div? How could this possible? The informations you provided are inconsistent! And I can't figure out which zero line corresponds to which signal.

Hi, sorry I've not been able to reply to this thread. Yes R5 and R6 are there only for simulation, since pspice doesn't have the right component. Regarding the picture of the oscilloscope, I forgot to add that we only used +/-5V for that picture.

So what I want to know is how I'm gonna get 0 to (-15)V after the optocoupler, I've read that a schmitt trigger can be used but I can't get that to work. Should I look for an opto that has 15V output, or what do you think is the best way to go. Here is the picture of our circuit, but I can't replace the 6N138 with the right one, it's not in the pspice library.

[hilli_billi]

hi jeppson,
you need a voltage supply of +12..15V with reference to Vss.
That's V3, or Q5...
This is the supply for the lower driver section.
The higside driver has a bootstrap condensator C5/C6.
When U4 is switched on, it is loaded via D3.
When U4 is switched off, the upside driver and its bootsrap-C float.
D4, R15 help at startup, DZ1 limits the driver volatage to 12..15V
they are not required.
I don't know how to connect the optos right, but you have done that already.
Like your inverter-sage and dead time generation.
If the opto coupler is to slow, maybe you use an ADUM.