UCC25706 PWM IC 1

[derp]

Hi everybody

I have never used this kind of IC before and have never really fully designed a boost converter ethier. I thought this would be a great learning experience. With the power portion of the converter aside I had a few questions about the UCC25706 chip.

Here is a list of parameters for my circuit:

Vin(min): 36V

Vin(max): 49V

Vin(shutdown): 32V

Converter Frequency: roughly 300kHz

My circuit seems to work ok with the exception that when power is first applied to the circuit it takes almost five minutes for a PWM signal to appear on the OUT pin. There is also no output on the RC pin during this time. If I wait long enough the PWM signal, as well as the triangle wave on the RC pin, will appear. Not sure what is causing this. Any ideas?

VDD on the 25706 is 11.7V. The start up voltage in the spec sheet states: 11.2 to 12.8. So my 11.7V on VDD should be ok.

I also want to know if I can use a 0 to 5V signal on the feedback pin, I have a pot connected to it now, to simulate what the chip will see when the power portion of the boost converter is connected to it?


Attached is the schematic of my current circuit. Please feel free to view and give feedback.

Thanks in advance,

Jesse

 

[derp]

Sorry I thought I uploaded my schematic to the site...here it is. I didn't realize I had to copy/paste the link.

 

[derp]

A little more info.

It seems that the feedback pin on this chip is intended to be interfaced with an optocoupler. So do I need to hard set the feedback voltage and only have it come on if the output voltage falls below a set level? I thought that this pin would use something similar to an error amp that would constantly adjust the feedback voltage. I thought this is what maintained the output voltage. That cant be the case if it is connect to an optocoupler....at least I don't think so. Can anyone please help shed some light on how to use the feedback pin on this IC.

Also attached is the data sheet for the UCC25706

Thanks,

 

[Skogsgurra]

The optocoupler is very well suited for this application.

It is not an on/off device but a rather linear one. The ratio between output current and input current, the Current Transfer Ratio - CTR, is a constant and makes the feedback path quite linear. As you can see, there is a resistor in series with the input. That resistor can be adjusted for the desired output voltage.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[derp]

Great. Thank you. This seems much more simple than setting up an error amp. Any idea why it takes so long for the circuit to output the PWM signal? I can apply power to my circuit and it takes almost five minutes for it to out put the PWM signal.

Again thank you for your reply.

Jesse

 

[derp]

Skogsgurra,

Thanks for the help.

For anyone who wants to know. I found out why the output would not come on right away. In the spec sheet there is a start up threshold. For this chip it is 11.2V to 12.8V. I only had 11.7V on VDD. I increased that to 13.5V (max is 15V) and it works perfect.

 

[Skogsgurra]

Learning by doing.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.