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    current-mode boost converter UC3845

    hi everyone
    idesign this circuit to bosst voltage from 15v to 36v
    in no load condition everything is ok and output voltage is 36v but when i put 7 ohm load in output voltage drop to about 15.5 volt and current about 2.5 amp.
    where is problem?why voltage drop from 36 volt to 15.5 under load?
    i add 56k resistor beetwen pin3 & pin4 for slope compensation.
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    Re: current-mode boost converter UC3845

    Hi,

    Often made mistake: wrong (saturating) inductance.
    --> So please post a link to the inductor's datasheet.

    Next most often mistake: Unsuitable PCB layout.
    --> Please post your PCB layout, where we can see the complete signal routing, especially the solid GND plane.

    Btw: There are many similar threads here. (Different ICs). Read through them.
    In more than 90% you see one if the above mistakes.

    Klaus
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    Re: current-mode boost converter UC3845

    Quote Originally Posted by KlausST View Post
    Hi,

    Often made mistake: wrong (saturating) inductance.
    --> So please post a link to the inductor's datasheet.

    Next most often mistake: Unsuitable PCB layout.
    --> Please post your PCB layout, where we can see the complete signal routing, especially the solid GND plane.

    Btw: There are many similar threads here. (Different ICs). Read through them.
    In more than 90% you see one if the above mistakes.

    Klaus
    my circuit power is about 400 watt which mean 15v/25A input and 36v/12A output so i use 2 hand wind 55uh inductor in series and 2 parallel diode.pcb is double layer
    inductor uses er28 core(pc40 material)31 turn and 2.5mm gap.i measure inductance by accurate LCR meter and was 55uh
    under 6 ohm load (40 watt due to voltage drop) board make a pretty load noise like old CRT tv

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    Re: current-mode boost converter UC3845

    Hi,

    Now we have:
    * self made inductance.
    We simply can't verify. There is a good chance that they are not suitable.
    Besides the inductance value... the saturation behaviour is very important.

    * No GND plane.
    A couple of big copper areas is no "low impedance GND plane".
    I assume there is no way around a redesign.

    * big resistors:
    Most probably "wire wound" which means "high series inductance".
    If they are in the switching path (what we can't verfy, then they are completely unsuitable.

    All informations for a reliably working SMPS are given in the datasheet and application notes provided by the IC manufacturer.
    Read them and design the SMPS according this informations ... and it will work.

    Btw: the part names in the schematic don't match to the PCB part names. It's a huge ammount of effort for us to find out which part is which part. I don't want to spend that much time...

    Klaus
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    Re: current-mode boost converter UC3845

    Quote Originally Posted by KlausST View Post
    Hi,

    Now we have:
    * self made inductance.
    We simply can't verify. There is a good chance that they are not suitable.
    Besides the inductance value... the saturation behaviour is very important.

    * No GND plane.
    A couple of big copper areas is no "low impedance GND plane".
    I assume there is no way around a redesign.

    * big resistors:
    Most probably "wire wound" which means "high series inductance".
    If they are in the switching path (what we can't verfy, then they are completely unsuitable.

    All informations for a reliably working SMPS are given in the datasheet and application notes provided by the IC manufacturer.
    Read them and design the SMPS according this informations ... and it will work.

    Btw: the part names in the schematic don't match to the PCB part names. It's a huge ammount of effort for us to find out which part is which part. I don't want to spend that much time...

    Klaus
    i change the big resistor and put a film resistor but any change happened
    here part names match to the pcb parts; is there any problem?
    is feedback correct?

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    Re: current-mode boost converter UC3845

    Hi,

    Whatīs the benefit of using 4 diodes in parallel?
    Did you do calculations?

    In best case (Equal current distribution) you get a 18% improvement in power dissipation (which should not be much benefit)...but get the drawback of 4 times the capacitance (huge capacitance!)
    But in reality I expect that the hottest diode will carry the most current and get even hotter. Maybe one diode carries 80% of the total current and the other diodes in total 20%.

    --> Read application notes about paralleling diodes.


    Klaus
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    Re: current-mode boost converter UC3845

    Quote Originally Posted by KlausST View Post
    Hi,

    Whatīs the benefit of using 4 diodes in parallel?
    Did you do calculations?

    In best case (Equal current distribution) you get a 18% improvement in power dissipation (which should not be much benefit)...but get the drawback of 4 times the capacitance (huge capacitance!)
    But in reality I expect that the hottest diode will carry the most current and get even hotter. Maybe one diode carries 80% of the total current and the other diodes in total 20%.

    --> Read application notes about paralleling diodes.


    Klaus
    current can goes up to 35 amp so i use a 40 amp diode with another for overcurrent situation.
    my big question is about voltage feedback.is that correct?



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    Re: current-mode boost converter UC3845

    my big question is about voltage feedback.is that correct?
    As mentioned in your previous thread, I would expect a PI configuration instead of a low gain P controller.


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    Re: current-mode boost converter UC3845

    Hi,

    Feedback:

    I donīt think itīs correct ... but I canīt fully verify because you donīt give the value of "POT".

    I didnīt go through the complete datasheet...but
    * I donīt understand why you use R6 at all. Whatīs your idea behind it?
    * the value of R5 seems to me a bit too low. Other values in the datasheet are 150k as an example.

    The formula of page13, upper circuit should be corrected to: V_O = 2.5 x (R2 / R1 +1)
    In your case
    * R2 = 56k
    * R1 = 10k + pot_value

    Do some caclulations on your own.

    BTW: The part seems to be obsolete. I recommend to choose a more modern one. Maybe with synchronous rectifier.

    Klaus
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    Re: current-mode boost converter UC3845

    Have you calculated the Bmax in the chokes for 25A ... ? if it goes above 0.3 Tesla you are in trouble,

    Can your power source supply all the amps you need? or does it sag in volts due to connecting wires or internal current limit..?

    - - - Updated - - -

    B^2 = Uo. L . I^2 / ( Ae . Lg ) Uo = 1.256 E-6, Ae = core centre leg area in m^2, Lg = gap in centre leg + gap in outer leg in metres. 1mm = 0.001 m.

    - - - Updated - - -

    increase C4 from 100pF to 1nF right on the pins of the controller ...

    - - - Updated - - -

    put 1.0uF ( or bigger ) film/foil cap 100V across the ends of the current sense resistors - this will reduce the inductive effect of these R's ( which is most of your problem )

    - - - Updated - - -

    your chokes are in series - it would be far better to design them to be in parallel - 12.5A each ....

    - - - Updated - - -

    You need some large film/foil 100V caps right after the main diodes to catch the current as it commutates to the output electro - the electro on its own is not enough and you will get very high overvolt spikes when the fet turns off - put at least 10uF 100F film foil from the output diodes to local power ground

    - - - Updated - - -

    also - do your self a favour - and set the gate resistor to 33 ohm for starters - this will reduce RFI and let you get the ckt going a little bit so you can fix the other issues listed above ....


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    Re: current-mode boost converter UC3845

    Simulation of simple boost converter with your spec supply and output, showing expected waveforms.

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    Notice the peak Ampere levels needed to produce 12A at 36V output. Obviously it's much more than 25A from a 15V supply.
    Consider interleaving two or more boost converters.



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    Re: current-mode boost converter UC3845

    total L = 110uH I think - from reading OP's posts, there are two chokes in series ... that will reduce the peak current a bit - but yes - unless the OP's power source can supply the necessary pk currents - testing will be difficult / impossible ...

    - - - Updated - - -

    Oh - why did you simulate 8kHz..? is that what the RC on the IC gave? 50-70 kHz would be a better operating point I'm thinking - lower peak input current again ...

    - - - Updated - - -

    I see that 1nF, 22k on that chip gives about 60kHz ...

    - - - Updated - - -

    the 3845 is 50% PWM drive max - so that limits the o/p to ~ 2x the Vin, so 15 to 30DC out at best ....



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    Re: current-mode boost converter UC3845

    Quote Originally Posted by Easy peasy View Post
    why did you simulate 8kHz..?
    ...
    I see that 1nF, 22k on that chip gives about 60kHz ...
    I adjusted for a slower frequency, in order to bring out the appearance of ramping waveforms, and to help emphasize the massive Ampere levels.

    60 kHz operation may very well be correct, however coil current is smoothed until it's like a DC waveform about 35 A, and the other waveforms resemble pulsed DC square waves. Output V takes many cycles to stabilize. The boost converter action is not as obvious.



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    Re: current-mode boost converter UC3845

    The noise you're hearing is inductor saturation.

    - - - Updated - - -

    Your major issue seems to be your duty cycle. Your duty cycle may be saturating. uc3845 has maximum of 46% or thereabout duty cycle

    - - - Updated - - -

    Without modeling loss elements, your duty cycle is above 50% already.
    -------------
    --Akanimo.



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    Re: current-mode boost converter UC3845

    For ER28 with 2.5mm centre pole gap ( or 1.25mm right thru ) for 55uH and 25Apk the Bpk = 459mT which is too high, the gap needs to increase by 459/300 = 53% on the 2.5mm i.e. total gap needs to be 3.825mm to keep you just this side of saturation for 25A ( the L will drop by the same factor: 300/459 to 36uH, x2 for two chokes in series)

    the current ripple at 50% duty cycle, 15Vin & 70kHz say will be 1.48 amps pk-pk so nothing to worry about - but the duty cycle needed for Vin = 15V, Vo = 36V = 60%

    as Vo/Vin = 1/(1-D) in CCM for a boost, 60% gives 2.5x 15 = 37.5 to allow for diode drop & other losses ...

    So you will need another IC in a DIP8 package ( there are plenty )

    good luck ...

    - - - Updated - - -

    p.s. you still need a power supply or battery and cables that can reliably supply 25A ( and a bit more ) at a solid 15VDC - if the power supply sags your full power testing will be very difficult indeed ...



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    Re: current-mode boost converter UC3845

    always do a representative simulation before doing a power supply. LTspice is free and the LT124X series is the same almost as UC384X.

    - - - Updated - - -

    here is your simulation....in ltspice which is free on the web....you can run this and get your head into it...by the way, you will need a current sense transformer for the uc384x range.,.....i gave it to you in the sim

    - - - Updated - - -

    chnage the .txt to .asc, then open it, then hit running man icon....and you see it.

    Your problems coudl also be due to your not using a current sense transformer...the sense voltage on uc384x is 1v...very high for your high current

    - - - Updated - - -

    If you are trying to design an smps without doing a representative simulation, you are like a climber who attempts everest without taking a food supply....best of luck!

    - - - Updated - - -

    Also, please use my free SMPS course, which is found in this website somewhere
    https://massey276.wixsite.com/electronicsdegree
    Last edited by treez; 9th November 2019 at 22:40.


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    Re: current-mode boost converter UC3845

    thank you all guys
    after those problems i decide to use LT3782 dual phase interleaved boost controller and some changes in specifications like Vin is same 15 Vout is 30volts input current is 14amps.
    after design pcb in altium and build the circuit there is a little problem.
    to set current limit at 14 amps(two 7amps) i use two 0.008ohm resistor but total output current limited to 2.3 amps!i think problem is noises on sense path because when i connetc +sense and -sense pins to ground there is no problem and output current is 7 amps at 30 volts.
    this is input current when senses pins are grounded.average currnet is 14 amp and there is frequently 25amps picks.i thin these pick are cause of current sense resistor limitation problem.am i right?
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    how can i remove these picks?without current sense resistor somtimes right mosfet burn immediately.
    input voltage is from a 15V/30A half bridge switching supply and when i connect it to my board; that supply make a bad noise(probably output inductor)
    https://www.analog.com/media/en/tech...ets/3782fg.pdf

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    Last edited by yoosefheidari; 12th November 2019 at 16:59.



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    Re: current-mode boost converter UC3845

    did you kelvin connect the sense resistor?
    Did you put a gate series resistor to damp the fet switch on?



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    Re: current-mode boost converter UC3845

    Hi,

    I have to repeat what's written in post#2 and post#4 about GND plane.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: current-mode boost converter UC3845

    Quote Originally Posted by treez View Post
    did you kelvin connect the sense resistor?
    Did you put a gate series resistor to damp the fet switch on?
    yes i use kevin sensing method but path length is about 4 centimeter. is that very bad?
    i use a ir4427 driver between lt3782 and mosfet to protect lt3782 because it is very sensitive.i put a 4/7 ohm resistor between ir4427 and mosfet's gate.
    without current sense resistor circuit wors but sometimes the right mosfet burn by unknown reason.

    - - - Updated - - -

    Quote Originally Posted by KlausST View Post
    Hi,

    I have to repeat what's written in post#2 and post#4 about GND plane.

    Klaus
    can you please give me a good source about ground plane in pcb design to read?



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