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Thread: Help please with glitch in an half bridge inverter

  1. #1
    diysmps Member* obiwan's Avatar
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    Help please with glitch in an half bridge inverter

    Dear friends, could you please help me solve this glitch in my inverter ?
    Here is the output circuit and oscillograms.
    Attached pictures:

    - A and B signals at de gates
    - C output signal
    - Schematic diagram

    The transformer calculation done with Dimitri's program, output voltage and current are OK as calculated.
    Al signals shown with output loaded with 12V @ 3A output rating.
    Thank you !
    Roberto
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    obiwan

  2. #2
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    Hi, I think you made mistake with point C, it seems to be capacitive centre tap, not output ?
    In any case why does your power supply not have inductor on output ?

  3. #3
    diysmps Member* obiwan's Avatar
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    fourtytwo the output is taken from the mid point in which both MOSFETs connect, and YES, a midpoint between these 2 capacitors where you measure half the +B voltage . This is a very well known half bridge design.
    Anyway I used snubber networks to reduce this glitch and it worked fine. I added some ceramic 470 pf caps that contributed to obtain a very good signal.
    Now the only problem that I have is that the ouput signal is good if I have a minimum power consumption of 1 A. Below this, the MOSFETs work loosely when each of them disconnect, and the signal is bad.
    I really didn't imagen that placing capacitors over the output rectifier diodes could be a good idea, but this worked reducing ringing to much.
    The new drawing includes the output inductor that was missing in the previous schematic circuit.
    MOSFETs are cool but the snubbers dissipate some heat that needs 3W resistors.
    I would still like some advise to improve this circuit.
    Thank you !

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    obiwan

  4. #4
    diysmps Member* obiwan's Avatar
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    BTW I would like to attach some typical well known topologies of SMPS circuits. The picture shows that my circuit is one of these topologies, half bridge.
    Regards
    Roberto
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    Its hard to help with so little information! your gate diodes seem the wrong way around as its normally required to slow down turn-on and speed up turn-off. You say nothing of your transformer construction, I assume given the amount of energy you are dissipating in snubbers it is not good and has very high leakage inductance. You don't mention the inductor value nor the normal load range the psu is designed for, I surmise the input voltage is 110VAC rectified ? I think most people could figure its a half-bridge

  6. #6
    diysmps Member* obiwan's Avatar
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    In the first place, this power supply is needed for a special industrial control application ( a train wagon ) in which the available power source is a DC voltage between 80 V to 150 V. The output of the device should be about 12 VDC to power relays and logic.

    We are not working on the closed loop yet. The circuit shown is just a very well known PWM generator in order to test all the power and driver circuits, and magnetics. We will close the regulation loop once the power section is finished. The circuit is driven with the typical TL494 scheme, working at 60 KHz. The PWM signal is applied to the shown transistor pairs that drive the gate transformers. The gate transformers are toroids, 4300 mu, with 11 turns primary and 8 turns secondary. Two transformers were used (also in the low side MOSFET) to preserve symmetry o both HI side and LO side power MOSFETS, although the transistor BC337/BC327 pair should be enough for the low side MOSFET. We tested this, and best results were obtained using 2 transformers. As shown in pictures before, the obtained gate signal seems to be good enough, with a 12 V amplitude square wave.

    About the switching transformer: attached is a picture of the Dimitri's Excellent program. The transformer has been winded placing the primary in the middle of the secondary winding, winded in two splitted parts. Several transformers have been tested with different known winding techniques, and the result was always the same. E and ER magnetic cores were tested. More or less the same results have been obtained.

    About the output filter inductance: we used this link that has probed to be reliable information:
    http://schmidt-walter-schaltnetzteil...gw_smps_e.html
    As shown in the calculation output, an E20 core was used with 19 turns of 1,5 mm wire.

    The gate diodes and the schematic for the triggering circuit was taken from the following sources: Keith Billings - Switchmode Power Supply Handbook 1St Ed , Marty Brown - Power Supply Cookbook, Half-Bridge Drivers A Transformer or an All-Silicon Drive? (Motorola Slide show)

    >>> One question here is if the picture of the glitch shown in previous original post picture is normal without the use of snubbers <<<

    Placing the snubber network on each MOSFET solved the problem, but there is a certain amount of dissipated energy there.
    Please ask me about any other detail you may need and I will provide the information.

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    obiwan

  7. #7
    C19 has to work very hard with no resistor. Tune this with a resistor to reduce the primary ringing.

    The transformer primary should have a series capacitor to prevent flux walking and transformer saturation.

  8. #8
    diysmps Member* obiwan's Avatar
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    Quote Originally Posted by wally7856 View Post
    C19 has to work very hard with no resistor. Tune this with a resistor to reduce the primary ringing.

    The transformer primary should have a series capacitor to prevent flux walking and transformer saturation.
    wally7856 please let me get this clear: an extra capacitor is needed in series with the primary, no matter that it is actually connected to the midpoint of 2 capacitors C15 and C16 ? What could be a possible value for this new capacitor ? I I do this, could I remove the snubber networks ?

    Here is a corrected version of the schematic, it had some part numbering error.
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    obiwan

  9. #9
    Yes you need a series capacitor in the transformer primary. C15 and C16 are not perfect parts. The voltage will not be split perfectly and with age will only get worse. If you have power to spare you could put resistors across C15 and C16 ( you should have them anyway just for safety discharge). For discharge, size them so a 1/2 watt resistor will dissipate 1/4 watt with highest voltage (my quick rule of thumb). Trying to use higher wattage resistors to keep the voltage balanced may work but i could not be sure on how much wattage this would take. I would start with maybe 2 watt resistors run at 1 watt dissipation. The only problem with this method is it would take a long time to test for long term operation and if it is enough power to balance a production run if that's what you want to do.

    The series capacitor is probably around 4uF, high current film type. of a voltage rating at least as high as your highest input voltage.

    Pressman has a write up on how to calculate it, do you have his book?

    The proper RC of capacitor C17 on transformer secondary will reflect to the primary and help reduce your voltage spikes

    36W half bridge
    12vdc at 3A
    7 x 2.5 uS = primary waveform = 17.5 uS = 57khz
    power source is a DC voltage between 80 V to 150 V
    primary voltage = 80vdc / 2 = 40vdc
    A tolerable droop in the flat-topped primary voltage pulse would
    be a minimum of 5% and no more than 10% or about 4 V.


    Cb capacitor blocking = (Ipft x 0.8T/2) / dV droop voltage


    Ipft peak flat topped current = 3.13Po/Vdc Min
    3.13 x 36W / 40vdc = 2.817A

    Cb capacitor blocking = (Ipft x 0.8T/2) / dV droop voltage
    (2.82A x (0.8T x .0000175 sec) / 2) / 4 dV droop voltage
    (2.82A x (0.000014) / 2) / 4 dV droop voltage
    (2.82A x 0.000007) / 4 dV droop voltage
    0.00001974 / 4 dV droop voltage = 4.94 uF
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  10. #10
    diysmps Member* obiwan's Avatar
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    Thank you wally7856. I am not an SMPS expert as you can see. My skills are in analog and digital design and microprocessors. I am just getting started with this, and this project is not for a large production. This PS is needed for an air conditioning automation contract, and the input voltage is not standard. We should build some 10 or 20 of these.

    I will make the changes and place the primary series capacitor and will add series resistance to de secondary capacitor too.

    My feeling is that I shouldn't place any capacitors in parallel with the output rectifiers, but my oscilloscope is showing that they are reducing ringing.

    Also, the waveforms are fairly good only if I load the output with a minimum 1A. Below that, the MOSFET's "float" when they turn off and the dead time is practically gone.

    Once placing the primary series capacitor you suggested, I will try to remove the snubbers from the MOSFET's, another feeling that this snubbers are not necessary.
    I would like to know your opinion on what elements would you remove (if any) and why...
    I will try to get the Pressman book. The signal picture you attached is what I should get, and if you take a look to my oscillograms, I think that I am close, but I had that glitch, so I started to kill it by adding snubbers.
    Attached pictures are the before (with glitch and no snubber) and after (with all the snubbers and caps)

    Thanks again for the excellent level of your reply.



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