UCC25600: wide range output

Ras

To adjust the output voltage of UCC25600, the reference voltage out the outer voltage loop needs to be adjusted. In the typical schematic the reference is set by the TL431, which has an internal voltage reference. In order to turn this fixed reference to a variable voltage the feedback needs to be modified. A common implementation technique is to use an op amp as the following app note shows www.ti.com/.../slva662.pdf. Note that the Vref in Figure 1 of this app note is the control voltage that will determine what the output voltage will be.

I'd like to highlight that the performance of UCC25600 will be very limited with such a wide output voltage range. Resonant LLC converters generally do not work well with wide input or output voltage ranges, so a large range of 400V will be very challenging to meet and will have limited performance. A Phase Shift Full Bridge with UCC28951 would be a better topology to use since it does not have the same voltage range limitation. You can adjust the output voltage of this controller/topology using the same scheme that's described earlier.

Best Regards,
Eric

Ras

For your question on UCC2813, the internal error amplifier and reference can be bypassed. PMP2187 (schematic: www.ti.com/.../slur196.pdf) is a good example of it configured with an external error amplifier and reference. The same techniques discussed earlier can be used to adjust the voltage.

Best Regards,
Eric

Ras

The following training video should help you with the feedback compensation design

training.ti.com/power-tips-compensating-isolated-power-supplies

Best Regards,
Eric

Hello Ras

If you need to change the output voltage from 0V to 400V then the LLC is the wrong topology to use. The reason is that the input to output voltage conversion ratio of the LLC depends on the switching frequency - typically a 3:1 change in switching frequency will give you about a 2:1 variation in output voltage - take a look at the curves on page 16 of the UCC25600 DS for some examples. A 400:1 ratio is not possible - irrespective of the controller you use.

There are some advanced techniques which will extend this conversion ratio by modulating the duty cycle as well as the frequency but you still won't get to the range you want.

Your application is for a 400W converter running off a 400V DC bus. In this case the best solution would be a PWM stage running at a fixed frequency. The topology could one of several but here are the ones I would consider.
1/ Half-Bridge using for example a LM5035, you will have to use external High/Low side drivers with this part to cope with the 400V input
2/ Hard Switched Full Bridge using a TL494, again, external High/Low side drives will be needed
3/ Phase Shifted Full Bridge (PSFB) using UCC28951, external High/Low side drivers will be needed


When operating off a 400V dc input one would normally use a zero voltage switching topology like the PSFB and if your output voltage was always going to be high then the PSFB could maintain ZVS over most of the power range - down to about 20% of load or maybe less with careful design but in your case the output power will go down to zero so it's more important that the power stage function over the entire power range than maintaining low switching losses. What I'm suggesting is that in your case maintaining ZVS isn't a key consideration and the increased switching losses can be compensated by switching at a lower frequency.
This means that you sould consider a hard switched topology running at a relatively low frequency - maybe about 35kHz as well as a PSFB.


1) non isolated power supply - easiest if you don't need an isolation barrier:cg -
2) using high linearity optocouplers or isolation amplifiers. - both of these will work but will be expensive:cg
3) primary side regulation. - Works well for some topologies - used a lot for low power flybacks but Vo accuracy is never going to be very good. Could be used on a Buck derived converter like the Half Bridge, Full Bridge or PSFB but accuracy will be worse than in the flyback case.:cg
4) secondary side controller. - Works really well, you will need a small auxilliary supply to get the controller running during the start-up phase - and you will have to use isolated gate drivers like the UCC21520: cg

It is possible to use 'ordinary' photo-transistor opto-couplers to form part of an isolated feedback path providing you understand the device degradation mechanism and how it compares to the expected PSU lifetime.


Regards
Colin

Hello Ras

That's fine - please open a new post when you have a question and we'll do our best to answer it. In the meanwhile, I'm going to close this post.

Please note I'm going to be away until 7th Jan for Christmas Holidays.

Regards

Colin