UCC28250: Buck converter using UCC28250

Hi Gu,

I will have an Application Engineer respond to your questions as soon as possible.

gu chenjie,

Thank you for your interest in UCC28250. As Brian mentioned above, we have contacted the applications engineering team for this device so they can weigh in on your question.

In the meantime, I have some questions for you. Have you tried running this buck configuration already? If so, please share the results. I would imagine that you could run a non-synchronous buck by leaving OUTB open and controlling the high-side FET using OUTA, but lets wait for the expert engineer to reply/confirm.

Also, are you restricted to using the UCC28250? Or could we suggest a different part if it were to fit better in your application?

Regards,

- Daniel

hi, Daniel,

thanks for your reply. i have not tried the buck converter with UCC28250 yet. the problem is that we have only UCC28250 in our SAP-System and it will take a lot time to permit to use a new component.
i am waiting for your reply.

Hi Chenjie,

Thanks for the interests in UCC28250.

UCC28250 is typically used for dual-ended topologies, such as half-bridge, push-pull and full bridge, but it should have sufficient resources to support a non-synchronous buck (PWM outputs, integrated error-amp, OCP/OVP/OTP features, dead-time programmability, etc).

Since only one PWM output is needed for the non-synchronous buck, you can simply leave the other PWM outputs floating like Daniel said. You will need an external gate driver though for the high-side FET of the buck.

Depending on the mode of control you choose, the RAMP/CS pin can be programmed for feedforward (voltage mode control) operation or current sense input (current mode control).

Note that the switching frequency is half the oscillator frequency programmed by RT pin.

Let me know if you have other questions.

Regards,
Wangxin

Hi, Wangxin,

Thanks for your reply.
In the UCC28250 datasheet there ist no information about the MAX duty cycle of OUTA and OUTB. So is that means both A and B outpouts have MAX duty cycle of 100%?

Chenjie,

Since UCC28250 is typically used for double-ended topologies, the OUTA and OUTB have max duty of 50% (minus the deadtime). It should be fairly sufficient for your application as your duty is around 300V/1200V.

Regards,

Wangxin

my input voltage is between 450V and 1200V, that means it is possible that the duty greater than 50%. Can I connect both outputs by a diode 'OR' to get up to 100%?

Chenjie,

I wasn't aware of the input range is down to 450V. I double checked the datasheet and it seems you might be able to get duty cycle greater than 50% according to (6). Let me check with the definer of this device and get back to you.

Also, what do you mean by a diode 'OR' to get up to 100%? There is a gap between the ON time of the OUTA and OUTB, the OR result would not be continuous.  

i mean that PWM range will be adjusted from 0-100% when tie both OutA and OutB together, is ist right? see www.edaboard.com/thread56183.html

Chenjie,

Ok. I see what you meant now. Then the switching frequency would be equal to the oscillator frequency.

But I want to bring one potential concern to your attention. UCC28250 implements a pulse matching mechanism in order to achieve matched pulses for the two PWM outputs during CBC operation in order to ensure the voltage divider balance for half bridge converter application. Refer to Fig.44 of datasheet for details (www.ti.com/.../ucc28250.pdf).

If you want to OR the two PWM outputs. What you are going to see during CBC operation (basically equivalent to peak current mode control with voltage loop open during CBC) is that the pulse (OUTB) in the next switching cycle is going to be forced to match that of OUTA. It should not cause problems in most cases but I would like to bring it to your attention .

I am still waiting for the response from the definer of this device to see if you can achieve 100% duty without ORing the two PWM outputs.

Regards,

Wangxin 

Chenjie,

I would suggest trying not to OR the two PWM outputs first. You can program the max duty of OUTA based on equation 4-6 shown in the datasheet of ucc28250. You need to use voltage mode control though.

If it doesn't work, try your OR approach but be aware of the concern I mentioned above.

Good luck and please keep us posted.

Regards,

Wangxin

thank you for your suggestion.
do you mean that ucc28250 ist able to get duty cycle greater than 50% ?

Wangxin,

i have a new idea for the duty cycle problem. My buck converter will always operate in DCM Mode so that the the duty of each PWM output will not exceed 50%. but i am not sure if there is any problem by Softstart?

Chenjie,

Duty will never exceed 50% under all conditions including soft start.

Regards,

Wangxin