Start-up current is far less than running current

Posted by: Bryanmills on 08/18/2008

I have a TOP248Y design that seems to have a start-up issue. The supply is designed to output 24VDC at 7.5Amps. All my tests are done using a pure resistive load on the output. With no resistor connected to the X terminal (X connected to SOURCE) the supply will output 7.7A with a 3 ohm load, great! However, it will not start with a load of more than 6 Ohms (4Amps). I have experimented with various resistor values on the X terminal. Using a 20K ohm resistor on X, the supply will output a constant 4 amps into 6 Ohms while running but will not start with a higher load than 75 Ohms attached (0.31A), bad! My goal is to limit the supply to a maximum of 6 amps while running. Obviously, it also needs to start with that load as well. Have you seen this behavior before? Is this indicative of my needing to use a TOP247 or TOP249 part? Since I can't submit the .uds file generated by PIExpert, I am submitting an exported .doc file.

Files

24V 7.5A (195-265V, 132Khz).doc (170.5 KB)

Comments

hello brynmills, i went through your design sheet. Pls see in the bottom of design sheet there are SEVERAL ERRORS! These errors need to be sorted out. ALso, the sheet has many other drawbacks as the device Voltage is touching 670V+.. I m surprised, for 180W Power supply, u had used 150 uF Input cap? i suggest you to re-check the design completely. moreover, try PI xls (part of PI software) prasun k. prasunkuls@hotmail.com

Hi Prasun, Thank you for reviewing my design. I only see two warnings through PIExpert. Both have been addressed in the design but due to the limitation of PIExpert, I cannot show those changes in the file. The first warning is to check the Zener clamp temperature. I'm using both a zener clamp and a RCD clamp. The design has been tested under full load and that section of the supply runs cool. The second warning is about a possible overvotage condition. I have a surge suppression circuit before the supply. I not concerned about those and I have large caps for reasons dictated by the system in which I am using this supply. My only problem I need to address is the failure of the supply to start under full output load conditions. DO you have any ideas on that topic? Thank you, Bryan Mills Senior design Engineer Fireye, Inc. Derry, NH 03038

FAE, Power integration product line prasunkuls@hotmail.com From your spread sheet i found few drwabacks, but as you said that you had taken care of it, let me tell you, Device drain voltage is very much as per spread sheet. Somehow, pls let us know, 1)is it starting under 50% load? now, pls attach the schematic for same to understand what actually you had implement in circuit. One possible reason i see is that under full load, The device is getting high current spike or exceeding I limit (or internal current limit) & this is why it may be going to auto restart or shutdown mode. This is why i am saying because, THE DEVICE IS SOMEHOW NOT GETTING DAMAGED!!!! REMEDY ACTION TO TRY: Mr bryn, pls turn on the unit under full load, then the device does not works as per you! therefore, in on condition, reduce the load current slowly. if it works, then , on start up, I limit is the issue... try & best luck

Can you post a drain current and voltage waveform. It may be that either the output diode recovery time is too slow, the clamp voltage is too low or the transformer capacitance is too high.

These waveforms will help identify. Take the waveforms at 4 amps into 6 ohms.

Cheers

PI-Chekov

Thank you for all your suggestions but the problem was simple as I thought it would be. I used 1200 uF X 3 capacitors for the output as the design software suggested. I also used a secondary filter with an inductor and another 1200 uF capacitor. It was this extra load of 1200 uF that was causing the start-up issue. I removed one of the 3 primary 1200's and left the secondary filter. Now it starts up with a 6 Amp resistive load with no problem.

Since my project uses multiple external devices each with their own big capacitors, I will need to use a circuit to ramp up the current flowing out of the power supply so as not to overload it during start-up.

Bryan Mills
Senior design Engineer
Fireye, Inc.
Derry, NH 03038