LT8364 Don't start during load

Question

Hi. 
 I have designed a SEPIC converter out of your datasheet, page 25, with an input span from 4.5-30 volt and the output at 25 volt will be used to charge a capacitor of 14mF. When I put on the voltage, testing with 12 v to 30v, at the input the output voltage will rise very slowly but it never reaches the level of the input voltage. If i disconnect the 14mF capacitor and don't have any load at the output the output will immediately go to 25 volt. If I disconnect the 14mF capacitor but place a resistor of 100 ohm over the output from the converter to GND the converter will not start at all. Can you please give me any suggestions how to solve the problem.

Giridharan · Accepted Answer

Hello, 
 Thanks for your query. 
 I observe that you are using an AC voltage source, with a diode bridge rectifier and capacitor filters at its output (C2 and C6). For successful start-up of the converter, the input voltage to the SEPIC converter (the voltage across C2, for example) must be above the minimum voltage required by the LT8364. I believe that during power up, the input voltage to the converter may be dropping below the minimum required voltage due to: (1) input impedance of the AC source, (2) voltage drop on the rectifier diodes and common mode choke, (3) low capacitance at the output of the rectifier, and (4) starting off by charging the 14mF as well. Please share the input voltage waveform (across C6) during a power up transition. When you removed the 14mF (or the 100 Ohm load) successful start-up of the converter indicates that a combination of the above reasons is preventing successful start-up with the 14mF in place. 
 We can help you with calculations and suggestions to size a required C2 for successful start-up. We need the following inputs: 
 1. What is the voltage and frequency of the input AC source (RMS, Hz)? Please indicate the minimum and maximum input voltage and frequency range expected. 
 2. What are the components chosen for the SEPIC converter? We can help review of the schematic if you can share the bill of material with component choices (manufacturer part numbers). 
 3. I observe that the SEPIC inductor has two different values (3.3uH and 2.4uH) on the two sides of the coupled inductor. Please confirm if this is chosen purposefully, and what the reasons are. I find that Wurth does not have such a coupled inductor in their published catalogue. 
 4. Apart from charging the 14mF through the 100 Ohm resistor, what is the load current expected during start-up operation? Note that this load current adds to the 14mF charging current. What is the load current expected after the start-up period, after the 14mF is charged to 25V? 
 - Giridharan.)

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LT8364 Don't start during load