TIDA-010000: UCC28740 Transformer Design

Rohit Thomas

Part Number: TIDA-010000
Other Parts Discussed in Thread: UCC28600

Hi,

I am trying to redesign the reference design for a Motor Drive application with the following requirements: -

Input voltage	290–1000 VDC
Output voltage	24 V (3 A), Iso. 5 V (2 A)
Vop1	24V	24	V	3	A
Vop2	5V iso	5	V	2	A
Vaux	aux	12	V	0.1	A
Output power	82	W Inclusive of all output rails

I got the following calculations for the Transformer design with Rcs = 0.38 ohm, Ipp_nom = 2A, Iocc_actual = 4A, Lp = 1290uH :-

Transformer Design Requirement Summary
PARAMETER	VALUE	UNIT	COMMENTS
Nps	9.00		Turns ratio for primary to 24-V secondary
Nps_iso	39.00		Turns ratio for primary to 5-V isolated secondary
Nas	16.71		Integer turns reduced Nas from 17.93 to 16.71
Lp	1600.00	µH	Operating frequency at full-load = 32 kHz

Could you help with the Transformer design?
I tried the same with 24V, 4A output in WEBENCH designer (doesn't support multiple output) and got the Core type - E32/16/9.

I am guessing the VS feedback and Line Sense current compensation and Output voltage feedback circuits will remain the same (from the calculations). Would cross regulation be a serious issue in this ?

Could you help to verify the same and also how to proceed ahead ?

Regards,
Rohit.

over 6 years ago

0 Siva Mohan over 6 years ago

TI__Prodigy 640 points

Hi Rohit,

Thanks for the question.

Regarding the cross regulation, I assume you are planning to regulate the 24V, 3A rail. What is the allowable voltage range on the 5V, 2A rail and also what is the minimum load expected on the 5V rail? Multi winding flyback transformers are prone to cross regulation challenges and there are ways to mitigate this - please refer to http://www.ti.com/lit/an/snva562/snva562.pdf and https://training.ti.com/multiple-output-flybacks-how-improve-cross-regulation.

Regards

Siva

0 Rohit Thomas over 6 years ago in reply to Siva Mohan

Prodigy 70 points

Hi Siva,

Thank you for the references, to improve cross regulation. I will go through them, apply the learnings into my requirement and get back.

Yes, I'm planning to regulate the 24V, 3A. About +-10% variation is the allowable range on the 5V, 2A, minimum load expected is about 1 A.

Hope the calculations which I have got, based on the design reference is in the right direction, anything further to be considered ?

Thanks again.

Regards,

Rohit

0 Siva Mohan over 6 years ago in reply to Rohit Thomas

TI__Prodigy 640 points

Hi Rohit,

Since the minimum load on 5V rail is expected to be 50%(1A) of full-load(2A), cross regulation effects on 5V rail should be minimal.

The design guide should be a good reference for the calculations. Also, since the total power is 82W instead of 50W as in the design, please take care to design the snubber circuit to be able to dissipate the higher losses.

Regards
Siva

0 Rohit Thomas over 6 years ago in reply to Siva Mohan

Prodigy 70 points

Hi Siva.

You meant the RCD snubber circuit in the primary along the MOSFET, right ?

I suppose I can refer section 8.2.2.4 Snubber Damping of UCC28600 datasheet for designing this, right ?

Thanks.

Regards,

Rohit.

0 Siva Mohan over 6 years ago in reply to Rohit Thomas

TI__Prodigy 640 points

Hi Rohit,

Yes, I meant the RCD snubber in primary side.

You can use the section you referred to for designing the snubber.

Regards

Siva

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TIDA-010000: UCC28740 Transformer Design