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    Need advice on Half-Bridge SMPS (Vin:350-400Vdc, Vout:3-35Vdc variable, Iout10A max)

    Hello ALL,
    I am new here on this forum. I hope someone can help me design a Half-Bridge Converter. I want to design a Half-Bridge DC-DC converter with an input voltage varies from 350-400Vdc and output of
    3-35Vdc@10A max. with adjustable output voltage. I would like the SMPS to have current overload protection (short circuit, over current), overvoltage protection, and reverse output voltage protection, soft start (pefer not to use NTC), emergency shut down incase something bad happen due to external source, and any other protection I may have missed...maybe I am asking a little too much of the protection issues (just preparing, incase, I decide to build a higher voltage and current in the future)...LOL :). Can anyone give me a reference schematic so I can make the require changes/addition/modify to meet my requirements?

    I have compose a list of concerns and issues, and hope that someone could also help me address them.

    1- Can anyone help with selecting an operating switching frequency? should I run it at 100KHz? I would like it to run at 200KHz so I can use a smaller transformer, does anyone have any objection to that or see any drawback (emi, fast components availablility)?

    2- Can anyone help with recommending a Hi and Lo side FETs (commonly use and easy to find parts)?

    3- Can anyone help with recommending a IC controller? I am thinking about using LM3524/SG3524/SG3525. I am have conflicting answer when it come to operating Half-Bridge in Current Mode, can it be done using current transformer on the primary side (for short circuit, overload, and components protections, etc)?

    4- I have seen in many cases (circuit samples) that some half-bridge converters use a serie capacitor is insert between the transformer primary and split capacitor to balance the volt-second integral of each switching transistor in order to avoid core saturation (due to DC walking/shifting) and other time they are not used it. Does anyone know why it is use in some half-bridge converters while other half-bridge does not? How do I know when it should be use and when it shouldn't be use? and sometime a low resistor is added in serie with the coupling capacitor.

    5- Which method is the best way to sense the current for short circuit or over current?

    6- Any oscillation concern I should worry about (fault FETs turn ON or OFF)? Maybe placing a series RC snubber across each FETS and output rectification diodes/sync-Fets? Place a filtering capacitor across
    the transformer's secondary?

    7- How to determine the split capacitor's value to hold the charge at 1/2*Vin to deliver the power requirement?

    Thank you,
    ZeroKool

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  2. #2
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    Re: Need advice on Half-Bridge SMPS (Vin:350-400Vdc, Vout:3-35Vdc variable, Iout10A m

    Quote Originally Posted by ZeroKool View Post
    1- Can anyone help with selecting an operating switching frequency? should I run it at 100KHz? I would like it to run at 200KHz so I can use a smaller transformer, does anyone have any objection to that or see any drawback (emi, fast components availablility)?
    The issue is that due to your wide output range, you'll need to operate over a large duty cycle range, which gets difficult at high frequency. Fast FETs will be important, but difficult to find at high voltage ratings. The CoolMOS family is a decent place to start looking.
    3- Can anyone help with recommending a IC controller? I am thinking about using LM3524/SG3524/SG3525. I am have conflicting answer when it come to operating Half-Bridge in Current Mode, can it be done using current transformer on the primary side (for short circuit, overload, and components protections, etc)?
    Yes, current mode control for a half bridge is normally implemented with a CT on the primary.
    4- I have seen in many cases (circuit samples) that some half-bridge converters use a serie capacitor is insert between the transformer primary and split capacitor to balance the volt-second integral of each switching transistor in order to avoid core saturation (due to DC walking/shifting) and other time they are not used it. Does anyone know why it is use in some half-bridge converters while other half-bridge does not? How do I know when it should be use and when it shouldn't be use? and sometime a low resistor is added in serie with the coupling capacitor.
    A blocking capacitor is normally used when the converter operates in voltage mode, but isn't necessary in current mode (because current mode control should inherently prevent asymmetry in the drive waveforms from accumulating errors).
    5- Which method is the best way to sense the current for short circuit or over current?
    This is a complicated issue. The information provided by a primary side CT is not enough to determine average output current (especially over a wide operating range). It is still useful for fault protection because it's a very robust way to limit input power (and thus can be effective for output short circuit protection), but if you actually want to accurately limit secondary current then you need a current sense on the secondary side. Sometimes this is done with a completely independent SMPS on the secondary side, other times a current sense signal if fed back over the isolation barrier to the primary side controller. Either way it makes things more complicated.
    6- Any oscillation concern I should worry about (fault FETs turn ON or OFF)? Maybe placing a series RC snubber across each FETS and output rectification diodes/sync-Fets? Place a filtering capacitor across
    the transformer's secondary?
    A snubber on the half bridge might be useful. A capacitor on the transformer secondary wouldn't make any sense.
    7- How to determine the split capacitor's value to hold the charge at 1/2*Vin to deliver the power requirement?
    This is sort of an arbitrary thing. Usually you just define some maximum allowable ripple on the split bus node (like 1%) and design for that.



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    Re: Need advice on Half-Bridge SMPS (Vin:350-400Vdc, Vout:3-35Vdc variable, Iout10A m

    Quote Originally Posted by ZeroKool View Post
    Hello ALL,
    I am new here on this forum. I hope someone can help me design a Half-Bridge Converter. I want to design a Half-Bridge DC-DC converter with an input voltage varies from 350-400Vdc and output of
    3-35Vdc@10A max. with adjustable output voltage.
    Thank you,
    ZeroKool
    the topology used for variable output is normally flyback which works on energy storage mechanism. so you don't have to worry about very small duty cycles at light load. in other words you would be operating at constant power transfer. also to mention that at low output voltages you have to use variable frequency method and otherwise fixed frequency.



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    Re: Need advice on Half-Bridge SMPS (Vin:350-400Vdc, Vout:3-35Vdc variable, Iout10A m

    Quote Originally Posted by ElectroPhysics View Post
    the topology used for variable output is normally flyback which works on energy storage mechanism. so you don't have to worry about very small duty cycles at light load. in other words you would be operating at constant power transfer. also to mention that at low output voltages you have to use variable frequency method and otherwise fixed frequency.
    Yes, a discontinuous flyback converter is pretty standard for bench supplies. It nicely implements a power input/output limit, but not a constant output current limit.

    But he wants to use a half bridge topology, which seems appropriate given his higher output power. Flyback is possible, but would require a significantly larger transformer.



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    Re: Need advice on Half-Bridge SMPS (Vin:350-400Vdc, Vout:3-35Vdc variable, Iout10A m

    Thank you all for reply. Mtwieg, you are right about the wide duty-cycle due to wide output. I have made some calculation below and few more questions.

    Npri/Nsec=4.5 [Npri=36, Nsec=8 (4turns/secondary C-Tap)]
    @ Vin = 390V
    Vout = 35V @ [DC = 82.8% (41.4%/channel)]
    Vout = 03V @ [DC = 08.5% (4.25%/channel)]

    @ Vin = 350V
    Vout = 35V @ [DC = 92.4% (46.2%/channel)]
    Vout = 03V @ [DC = 09.5% (4.75%/channel)]

    Therefore the duty-cycle varies from 4.24%-46.2% per channel! A maximmum of 3.8% [50-46.2 (lowest delta value is use)] is available, this would mean a maximum of 0.19us [T=5.0us] per channel is available for cross conduction "OFF" time.

    Questions:
    1- Is the minimum and maximum duty-cycle (8.5%-92.4%) acceptable or is it too close to the limits?
    2- Does the 0.19us allow enough time for a generic IC controller to work it cross conduction "OFF" timing?
    3- Does anyone know where I can find a COTS current transformer capable of 10A on its primary?
    4- Can anyone show me (equations) or help me how to select a transformer core that is not too big or too small that will handle 350W and switch at 100KHz? I dont want to take a chance on my own and buy a transformer that is alot bigger than I need or buy a transformer too small that it overheat or won't fit the windings.
    5- Does anyone know where I can find copper foil (5 mil thick) with protective coating? I have seen copper foil sold on ebay but they dont have any kind of protective coating (such as enameled, etc). I don't know if copper foil is typically sold without coating and it's up to users to add in the insulator. I want to use it to wound the transformer's secondary winding since is it high current. I am still not sure how to solder from the foil to the transformer's pin, but will figure that out later when I get there.
    6- I want to use a base-drive transformer (same as pulse transformer?) to drive the Half-Bridge, can any generic 1:1:1 pulse transformer be use or work at Fsw=200KHz? Any recommendation or manufacturer?

    Thanks again,
    ZeroKool



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    Re: Need advice on Half-Bridge SMPS (Vin:350-400Vdc, Vout:3-35Vdc variable, Iout10A m

    i really need help ,, to build 2 half bridge with IC SG3525 a "DC"



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    Re: Need advice on Half-Bridge SMPS (Vin:350-400Vdc, Vout:3-35Vdc variable, Iout10A m

    i really need help ,, to build 2 half bridge with IC SG3525 a "DC"
    Hi William
    I think you'd better to start a new thread , regarding this issue . anywhere you forgot to tell your specifications ( required ) .
    Best Wishes
    Goldsmith
    When you sensed that i can help you , feel free to ask me .
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