LM5023: What is target design spec for LM5023?

ACGUY

Part Number: LM5023
Other Parts Discussed in Thread: UCC28600

Hello E2E,

Our customer is during H/W debug for isolated DC/DC. That controller is LM5023.
During rated load output, Ton momentarily goes low after 13-usec from the start and then keeps high after that.

I found the parameter tRESTART.
What is tRESTART?
This device can't output FET-High continuously?
How long time the max value of tON ?
Is this device can' t use for High-current power supply? (Low fsw , Heavy Primary inductance transformer , and wide tON)

Regards,
ACGUY

over 7 years ago

0 Kuan Wei Chen (Kevin Chen) over 7 years ago

TI__Genius 9710 points

Dear ACGUY,

Thanks for promoting TI's solution.
I found information from the factory about the tRESTART block.
The tRESTART block is used to allow the IC to start switching if the QR signal stays low for longer than the Trestart time (~12 us).

This situation can occur when the device is first powering up and when waking up from sleep mode (COMP > 120mV + hysteresis). It is not stated in the datasheet, but the tRESTART block delivers a short pulse (about 50 ns wide) to the AND gate every time the QR input signal crosses below 350mV Demag comparator threshold. It is not a continuous High level. Usually the first zero crossing at QR will generate the pulse that will set the OUT flip-flop high. But at start-up and wake-up, there are no zero-crossings, only a 0-V level at QR. So if this state persists for longer than ~12 us, the tRESTART block automatically sets its block output High continuously to allow the PWM comparator to trigger the OUT flip-flop.

I can't find the reason why your OUT pin keep high. When the primary-side flyback MOSFET turns on, the current ramps up until the peak-primary current exceeds the feedback compensation error voltage. So there is no reason it will keep high. Unless, have issue on current feedback CS pin or feedback compensation error voltage COMP pin, can you double check both pins signal?

What is your spec for the design, it should work well around 100W. If is possible, please also attach schematic for review where is the issue.

Best Regards
Kevin

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Hi kevin,

Sorry for bad explanation.
Please see the attached file. During rated load output, Ton momentarily goes low after 13-usec from the start high after that.

Is it normally?

Regards,
ACGUY

LM5023.pdf

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to ACGUY

TI__Genius 9710 points

Dear ACGUY,

Thanks for your waveform and picture for help understand the situation.
Can I know you testing parameter(Vin, Io, Lm)?
It seems the input voltage is quite low, around 60V.
I think the circuit is trigger the tRESTART, cause by the QR signal stays low for longer than 12us.

Best Regards
Kevin

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Hello Kevin,

Design spec is 60-W output (12V / 6A).
Vin is DC24V (DC20V to DC28V).

"cause by the QR signal stays low for longer than 12us."
->In other words, LM5023 can't FET-On continuously more than 12usec. Isn't it?

BR,
ACGUY

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to ACGUY

TI__Genius 9710 points

Dear ACGUY,

It seems like your input voltage is quite low and output current is quite high, which cause MOSFET need to turn on longer to store the energy in the transformer.
Is it possible you can reduce some turns of the transformer?
It will reduce the magnetic inductance, increase the Iprm current slope, reduce the MOSFET turn on time.
I hope this can help you solve the issue here.

Best Regards
Kevin

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Thank you for answer.

About modify the transformer, I will negotiate with customer. May be, I will get angry from them....

Many thanks!
ACGUY

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Hi Kevin,

I'd like to question more.

1:What purpose for the restart function?
2:How much the pulse width of restart signal?
3:Please let us know how to kill the restart function. ( if able ,,,)

Regards,
ACGUY

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to ACGUY

TI__Genius 9710 points

Dear ACGUY,

Is the waveform you capture during startup?
Because the Vqr waveform stay at 0 for a long period.
Or this short turn off issue keep appear during steadt state?

The pulse width of restart signal shoulb be around 50ns.
I will try to find out the way to disable it.

Best Regards
Kevin

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Dear Kevin,

I will request the startup waveform to customer. Please bear with me.

>> Or this short turn off issue keep appear during steadt state?
-> Yes. When FET on time over the 12usec, it surely occurs on steadt state.

>> The pulse width of restart signal shoulb be around 50ns.
-> Thanks. IIs it one pulse only?

>> I will try to find out the way to disable it.
-> Thanks. I looking forward your comment.

Regards,
ACGUY

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Hi Kevin,

I think that tRestart pulse can't to disable. and, if use the LM5023, Ton should be less than 9.4usec at design for transformer.
Could you let me know other device? Replaceable, QR operation, and no tRestart.

Regards,
ACGUY

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to ACGUY

TI__Genius 9710 points

Dear ACGUY,

In TI's portfolio, we only have two QR flyback controller. LM5023 and UCC28600 are the controller we have.
UCC28600 don't have the limitation like tRestart.

Best Regards
Kevin

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to ACGUY

TI__Genius 9710 points

Dear ACGUY,

In TI's portfolio, we only have two QR flyback controller. LM5023 and UCC28600 are the controller we have.
UCC28600 don't have the limitation like tRestart.

Best Regards
Kevin

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Hi kevin,

May I more question?
I guessed that like attached file about Vgate.
Is my opinion correct?

Regards,
ACGUY

Vgate.pdf

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to ACGUY

TI__Genius 9710 points

Dear ACGUY,

Yes, it should look like that.
By the way, what is your Lm value?
If your design have suitable Lm value, the Ton won't longer than 12us.
For example, webench suggested 7uH.

Attached Webench for your reference.
webench.ti.com/.../webench5.cgi

Best Regards
Kevin

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Dear Kevin,

Thank you for answer. I have a question. Maybe It's last.
I traced the internal logic circuit for OUT. OUT is prioritize the PWM-high always by NOT-logic and AND-logic.
OUT is doesn't output the tRestart when during PWM-high.

Is Functional Block Diagram wrong?
or
Could you present realy Functional Block Diagram?

Best regards,
ACGUY

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Dear Kevin,

Please your opinion.

Best regards,
ACGUY

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to ACGUY

TI__Genius 9710 points

Dear ACGUY

Thanks for high light the problem.
I will discuss with the designer, and get back to you soon.

Best Regards
Kevin

0 ACGUY over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

Guru 11170 points

Hi Kevin,

Do you have any update? Please inform me.

Regards,
ACGUY

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to ACGUY

TI__Genius 9710 points

Dear ACGUY,

Already contact the designer, but didn't get reply.
I think the reason is because the 7/4 independence holiday, usually the people will go to long vacation.
I will push it again and keep you posted.

Best Regards
Kevin

0 Kuan Wei Chen (Kevin Chen) over 7 years ago in reply to Kuan Wei Chen (Kevin Chen)

TI__Genius 9710 points

Dear ACGUY,

I got the block diagram from the AE, but I think the question is not solve yet.

I will keep you posted, after I have the answer.

Best Regards

Kevin

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LM5023: What is target design spec for LM5023?