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| Simplifying H-bridge design |
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| cadena:
Hi, thank you all for your replies but I still do not know how to fix this circuit. Please if someone knows a better (and simpler) H-bridge than this one would you posting it, please ? Or at least tell me exactly how to fix this one ? I am still learning how to design circuits and I can't use an IC H-bridge. Please any help would very much appreciated. |
| ajb:
The only bridge that will be both better and simpler (for you anyway) is an integrated one, as everyone else has already told you. In order to avoid shoot-through in the bridge you need to be able to drive the top and bottom transistors separately. You could do that by using separate optocouplers for each transistor, and in turn driving them with four separate PWM signals so that you can insert the required dead time between turning one switch off and the other on. This requires careful configuration and the performance will still suck badly enough that it might still burn up because using a pull-up or pull-down resistor and an optocoupler makes for a horribly slow gate driver. This means that switching takes a long time so at best it limits your switching frequency and at worst the MOSFETs will wind up working outside their safe operating area and die. Sorry, but this is a situation where if you want it to work you really need to go back to the drawing board. |
| sourcecharge:
--- Quote from: cadena on April 07, 2019, 08:28:38 pm ---Hi, thank you all for your replies but I still do not know how to fix this circuit. Please if someone knows a better (and simpler) H-bridge than this one would you posting it, please ? Or at least tell me exactly how to fix this one ? I am still learning how to design circuits and I can't use an IC H-bridge. Please any help would very much appreciated. --- End quote --- You could use the logic from a DFF (CD4013) and 4 NOR (CD4001) gates with a PWM input signal. The PWM input signal would be 2x as fast as the actual frequency of the H bridge, and if you want to use it up to 20V the PWM input would probably need to be stepped up to the logic voltage as function gnerators only usually go up to a 10Vp. The NOR gate outputs can directly gate the H-bridge at any duty cycle your PWM input is at, so if you use 1% duty cycle pulse, there would be a 0.5% duty cycle time between the ON times of the N and P channel mosfets. The clock of the DFF is tied to the PWM, and 2 NOR gate legs. The DFF has Q not tied to the D input and 1 of the NOR gates that are tied with the PWM on it's other input. The other NOR gate that has the PWM on one of it's inputs, has Q tied to it's other input. The outputs of these NOR gates have their signals inverted by the last two NOR gates. The outputs of all four NOR gates can be used directly to their coresponding mosfet of the H bridge. You should be able to see for yourself how this 2 chip 20V circuit works without resistors, optocouplers, boot straps, or caps. It's a simplistic circuit, it's my own creation. It's sort of cheating telling you about it, as it took me alot of thought into making it. You can use any mosfets with it depending on how good you want them to be, because you can decide how much shoot through, if any, you want there to be by manually changing the frequency and duty cycle of the PWM input signal. Old CMOS logic is good up to about 10Mhz Hope that helps.. |
| Mechatrommer:
--- Quote from: cadena on April 07, 2019, 08:28:38 pm ---Hi, thank you all for your replies but I still do not know how to fix this circuit. Please if someone knows a better (and simpler) H-bridge than this one would you posting it, please ? Or at least tell me exactly how to fix this one ? I am still learning how to design circuits and I can't use an IC H-bridge. Please any help would very much appreciated. --- End quote --- you cant fix that youtube circuit in OP easily in SW, you can try fancy hardware RC delay fix etc but you can end up much complexity or poor efficiency... i'm with you for not using ready made H-bridge IC as they are limited to below 10A at affordable price, more than that, its unaffordable or difficult to find i think. once a good member guided me for more efficient minimal bjt count discrete driver.... from https://www.eevblog.com/forum/beginners/is-bjt-power-h-bridge-motor-driver-obsoleted-by-fet-h-bridge/msg759287/#msg759287 now i upgraded my "obsolesenced" bjt h-bridge to fet version (pcb ready to be fabricated). i dont use his version as although 2 bjt per side, it uses 3 diodes, and my smd diodes stock are big foot print i want minimal footprint, so i device my self driver 3 (sot-232 small) bjt, no diode needed... albeit more efficient, his version cannot deal with |Vgs| < |Vds| spec fets, mine will take |Vgs| as half the |Vds|. like my previous bjt h-bridge before, instead of trying to control each side (half of each bridge) from one input (like in youtube circuit in OP), each of my input will control each complete bridge, so deadtime (delay between input) can be programmed easily in mcu. i suspect turning on will be acceptable, since gate charges from turned-off upper p-mosfet will be used to charges its counterpart lower n-mosfet on the other side, but turning off will be a bit inefficient due to 1Kohm switch off resistance (havent build it in reality, only in sim). more efficient discrete h-bridge will requires more than 4 bjt on each side (push pull on each fet at least minimal). from https://www.eevblog.com/forum/projects/h-bridge-motor-control-circut-burns-mcu-after-some-time/msg2320872/#msg2320872 the original circuit from where i got this idea i already forgot where its from, from internet i think, so may God bless the originator and show him the true way, ymmv. |
| Doctorandus_P:
MOSfets in an H-bridge configuration only work well if you switch them properly. Power MOSfets have a pretty high gate charge and this needs respectable current to charge / discharge quickly. Also, P-channel MOSfets's are more expensive / or have worse specs than N-channel Mosfets. Therefore a lot of different chips have been designed to solve this. The IR210x gate drivers are an example of this. They also have a 600V or so isolation barrier built in, and dead time generation to prevent shoot through, even if the software in your uC fails. You can also have a look at the BTS7960. This is an integrated Half bridge. With 2 of these TO220 sized packages you have a complete H-bridge solution. |
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