UCC24612: Synch secondary LLC converter

Yivgeni Semidotskih

Part Number: UCC24612
Other Parts Discussed in Thread: UCC24624

I’m currently in a search for synchronous rectification IC for a LLC converter I’m constructing.

The following part number seems to meet the requirement, but one issues is that the drain voltages that I’m encountered in my application are higher than specified in the spec of the datasheet.

In my application the drain voltage can reach to 1kV while the specification is allowing approximately 200V (depends on the part number).

My question is there any external hardware arrangement that can be applied to over-come this issue or as specified in the datasheet the maximum allowed is the maximum allowed and that’s that.

I remind that the problem is that drain voltage is reaching 1kv(as a part of it natural behavior) and drain pin of the IC is rated for lower voltage.

My inquiry is the following, did you technical considered situation like mine? or is there any solution you suggest?

Again I'm interested in Synch rectification in LLC converter such that the drain voltage of the output section is about 1kv.

Also additional recommendation for more fitting IC's are very welcome.

over 3 years ago

0 Jaden Ning over 3 years ago

TI__Expert 7080 points

Hi Yivgeni,

You can find the VD and VCC extension methods in below attached link.

But may I ask why you chose SR in your design and what is maximum output power since you mentioned that the output voltage will up to 1kv, in this way , how much benefits you can get from SR. giving the voltage drop on rectifier diode is 2V , 2V/1000V= 0.2% efficiency improvement you can get if you replace the diode with a SR . and you will pay a high voltage mosfet and a controller.

In my understanding, diode rectifier is much better than SR in a 1kv output voltage application.

https://www.ti.com/lit/pdf/slua860

Thanks.

0 Yivgeni Semidotskih over 3 years ago in reply to Jaden Ning

Prodigy 50 points

Well maybe I was vague in the description, the 1kV is is momentary voltage during turn off of the diode/mosfet the average output voltage is 200V/4kW.

The document you sent seems to fit my request, Fig 10 in particular.

is there any application notes on how to choose the transistor and diode for the hardware extension?

0 Jaden Ning over 3 years ago in reply to Yivgeni Semidotskih

TI__Expert 7080 points

Hi Yivgeni,

Even for 200V/4KW system , I still suggest you use diode for the output rectifier. moreover the transient voltage stress is still up to 1kv. you have to chose > 1kv SR MOSFET , the Rdson and cost will high . you can't get much benefits from the SR.

By the way, UCC24612 is special designed for low power application . it's not recommended to use in high power level design.

Thanks.

0 Yivgeni Semidotskih over 3 years ago in reply to Jaden Ning

Prodigy 50 points

Thank you for you suggestion and concern.

Never the less,

is there any application notes on how to choose the transistor and diode for the hardware extension(Fig 10 from the application note you sent, Q_VD,D_VD)?

Also you say that the IC is "special designed" for low power application.

What do you consider "low power"? and you say I shouldn't apply the external hardware arrangement suggested in the application note Fig 10?

What are the hazards of using this IC at higher power?

Please advice.

0 Yivgeni Semidotskih over 3 years ago in reply to Jaden Ning

Prodigy 50 points

Also can you please advice if the suggested arrangement is also applicable for UCC24624?

0 Jaden Ning over 3 years ago in reply to Yivgeni Semidotskih

TI__Expert 7080 points

Hi Yivgeni,

I believe figure 12 is more suitable compared with figure 11 in your design . since the output voltage is 200V.

If configured as figure 12. You have to chose a >1000V Qvd (as well as in figure 10). the bias voltage before LDO should be <20V , so >30V voltage for Dvd is ok.

Basically , there is no specific boundary for "low power" and "high power", but usually , UCC24612 is used in flyback which <150W, LLC <500W . since there is a proportional gate driver on UCC24612 , the proposed Rdson of SR FET is >4mohm. at high output application , the low Rdson will results in premature proportional gate driver , which is not good for efficiency improvement.

You still need apply all of the external hardware circuits since the purpose of there components to solve the voltage stress problem.

Thanks.