LM5121 BOOST CONVERTER IC

Kindly Ignore my previous Post.

The specification is mentioned below.

Vin(min) - 12V
Vin (max) - 36V
Vin (Typ) - 28V
Vout - 65V
Iout - 5.5A
Fsw - 650Khz

We have selected LM5121 for the above mentioned specification. But i am not getting the same chip in WEBENCH. Please let me know whether I can achieve the above mentioned specification using the chip-LM5121.

Waiting for your valuable reply. Expecting your support at the earliest.

Regards,
Prajeesh

Hello...

Kindly give a reply to my above post. Its urgent.
Also i am not able to open the WEBENCH design for the LM5121. Pls help.

Regards,
Prajeesh

I don't directly support the LM5121, but the power management power tree and LM5121 datasheet both suggest that it will support your application.

Sir,

Thanks a lot for the reply.

Pls give the reason why don't support? Pls clarify your statement. Then it will be more clear and helpful to me.
We have assembled the components to achieve this same spec.

Pls have a look to the below mentioned issue and kindly reply me.

During testing it is observed that the Low side switch, High side switch and Free wheeling diode got failed. The input given was 28V. Connected 8mF/125V capacitor at the output. Also a minimum load of 560ohm [130mA approx] at the output. Observed the output using scope and found that the boost voltage peak is going to 85V at the initial time and then came down to 70VDC [constant]. I have designed it for 65VDC, but the peak is going up to 85VDC. Slightly i varied the load and made the output came down to 66V. For 66V the loaded current was 1.36A. Switched off the 28V input line and waited for the capacitor to discharge. Then again switched ON. Observed the output and found that the output is not getting boosted and it is coming only 30V for an input of 28V. After 3 to 4sec the components[As mentioned in first line of the para] got failed. With 8mF/125V capacitor , observed an inrush current of 43A for 6msec. The 2mohm sense resistor is placed near the disconnection gate switch .The failed MOSFETS is having a VDS of 100V and 58A continuous drain current. Pulsed current up to 200A is allowable.

Waiting for your valuable reply. An immediate reply is always appreciated. Since i need to deliver the board with in a couple of days.

Regards,
Prajeesh

The supporting application engineer has been notified of this post. He should respond soon. In tehmean time can you post your schematic and the waveforms you mentioned? I'm sure they will be necessary to analyze your circuit.

Hi

I quickly check your specification. Now, your switching frequency is too fast. The duty cycle might be limited by the forced off-time of LM5121. About 250kHz is recommended.
Since the total power is about 360W and the loss might be around 36W assuming 90% efficiency. At first, you have to check your system's thermal design whether your system is designed enough to handle such a big power loss. My recommendation is to check the temperature of your MOSFETs while increasing the load slowly.
Overshoot at start-up can be caused by loop response of the converter, current limit during soft-start. My recommendation is to increase Css extremely large and then check the overshoot again.

Regards,

Helloo..sir,

Thanks a lot for the valuable reply. Appreciate for the response with minimum time.

I have selected 650Khz switching frequency in order to reduce my Boost Inductor size. Any way in datasheet it is mentioned that LM5121 will support up to 1Mhz Switching frequency. The selected Low side Mosfet and High side Mosfet is having very low rise time(9nsec) and fall time (5nsec) and Rds On - 12mOhm. So that the Switching loss will be minimized. As per the formula given in LM5121 datasheet the Switching loss is coming near 3.7W and Conduction loss is around 1.6W. So that the over all loss in Low side switch is 5.26W. For High side switch overall loss is coming 5.69W.(Including Conduction loss, dead time loss and Reverse recovery loss).In mechanical unit we are giving pillars with thermal pad which will touch the MOSFET body for proper heat transfer.

Can u please suggest me the  approximate Css value . (Atleast greater than what value).

Also, I am not getting LM5121 in WEBENCH for the specification which i mentioned. Kindly give a solution to check whether the compensation network and the feed back value calculated by us can meet the spec and work properly.

I have attached the schematics and waveforms observed during testing. Kindly suggest a solution for the issue.

Waiting for the valuable reply at the earliest. An immediate reply is always appreciated.

Schematic1.pdf

Regards,

Prajeesh

Hello..sir,

Waiting for your valuable reply.

Regards,

Prajeesh

Hi
You can start from 4.7uF and then try 1.0uF. Please check if any improvement.
About the WEBENCH support, I think it is better for you to create one more post in WEBENCH support forum.
Regards,

Hello sir,

Thanks for your reply.

Is there any calculation sheet provided to calculate the  Loop compensation values  of Rcomp, Ccomp, and CHF. If available kindly send it to prajeesh.padmanabhan@industeqsite.com.

Other wise pls let me know what values i have to put for Rcomp, Ccomp, and CHF to achieve my specification.

Waiting for your valuable reply. Pls do the needful.

Regards,

Prajeesh

Hi
Unfortunately, no other calculation tool available except WEBENCH.
What's you total capacitance of your output capacitor?
Regards,

Total output capacitance is 16mF.

Regards,
Prajeesh

Hi
It seems your cross over frequency is low though, your loop response is stable. I checked with a simulation.
Regards,
EL