Part Number:LM5145
For a new project for a customer we want to make a design where we need 3* +24V/15A and 1* +24V /6A out of incoming 230Vac per unit (each machine will contain 42 of these units).
Therefore we are thinking of a Vicor unregulated 1750W converter (BCM6123TD1E5135T00) to create +40Vdc out of +320V dc (=rectified 230Vac in) and using 4* LM5145 to create max +24V/15A max at each output. This is purely based on very worst conditions where we think in practice this will never occur but in theory we have to take account with it. It may also be another converter but I think the LM5145 looks the best solution for us at this moment (also wrt costs)
I already did a simulation with the LM5145 set at +24V output and this showes that the impact of a voltage change at the input has not much effect on the output voltage voltage which is important for us.
Still some questions when using this solution:
1) Do you think this solution will work? Of course the input voltage can vary because the converter is a non regulated version (1:8) so each input change (e.g. ripple) will be visible at the output of the Vicor but it looks the LM5145 could handle over a wide input voltage range and simulation shows not miuch impact at a voltage change from +30V to +50V at the +24V output.
2) The Excel calc sheet shows 350uF cap at the output with an extreme low ESR of a few mOhm. The very best cap I can find is already 13mOhm. Or is it OK when we put a ceramic cap parallel with the big cap at the output to reduce ESR? Or has the ESR much impact on the output ripple? The Excel sheet shows 35* 10uF cap in parallel (I think for the low ESR) but this looks not very practical for the final design...At your eval board I see a lot of 47uF ceramic caps in parallel with the output elco but I guess these will be not cheap for higher voltages (+35V or higher)
3) Are there some special precautions required with a load of 15A at +24V? The Excel sheet shows a power loss of 4W for the upper MOSFET at 15A load. With a thermal resistance of 50C/W this would result in 200C temp rise. We have a 32 degrees coldplate for the converter so maybe we can find out if we can manage to connect the FET to the coldplate? Or are there MOSFETs with lower thermal resistance with similar specs so the temp rise will be less? The dissipation in the coil and lower MOSFET is lower according the datasheet so probably not a problem..
4) We need 4 of these converters at one PCB all connected to the same output of the high power 1750W converter. My idea is to sync one converter with the neighbour to use shifter clock and maybe running the third one at a different clock speed and sync this one to the fourth so we have 4 converters running at different clock edges and frequencies to minimize interference and EMI. We have to deal with EMI Standard class A (we prefer class B when possible)
5) T.I. has an evaluation board with the LM5145 but with +5V output. Do you think we could use this for +24V (out of +40V in) and by changing some component values modify it for +24V output? The inductor (3.3uH) and MOSFETs look the same as I get as suggested values from the Excel calculation sheet at +24V output. So when we could change this by only changing some resistors it would be nice.
6) At your EVAL board I see a large Schottky diode in parallel with the bottom FET. However in webench designer and in the Excel sheet schematics I don't see this diode. Is it required for the design or is there a special reason that it is not shown at the Excel sheet?
I hope you can help us with these questions so we can continue with investigating the final solution
Best regards
Chris van der Aar
Sr Hardware Engineer
NTS Systems Development
Eindhoven, The Netherlands