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H bridge MOSFET gate

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There is a load of information (good reading) of application notes on IRF's site. www.irf.com

fabbie, don't feel sad. See what the link below mention..

"Designing safe and reliable automotive motor drive circuits up to 80W can rapidly become a complex task involving a multitude of components and requiring experience in power electronics, controls and thermal design...." which just confirm you must learn to walk before running :)

http://www.irf.com/product-info/auto/autohbridge.html

This said, maybe you should try your circuit as you already bought the components. It will be a good learning experience.
 
Basic question, just what do you want to do? What voltage? Would you use PWM with this circuit? Is it microcontroller-controlled?
 
What I dont understand is why he isnt shorting the P-type and the N-type gates together of one H-bridge leg. Then use a NPN-PNP push-pull gate driver per leg
i'm wondering what that would look like..?
 
this is a start..
any sugestions for the control circuit..
 

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williB said:
i was just wondering .. why couldent you use all n channel Fets..?

You can but the turn-on voltage for the FETs connected to the 5v rail would have to be above 5v+vth.

With only a 5v source, it may be possible to hook the N and P gate together, but I'd recommend against it due to the obvious possibility of them both being on at the same time and shorting out the power supply. The exact behaviour depends on the device, the mfg lot, age, and temperature. It's a good idea to have a short delay between turning off one transistor and turning on another.

Are you planning to use PWM? If so, best thing is to leave on one of the P-channels all the time and PWM modulate only the N-channel's gate signal.
 
i designed IC1A to be a 74LS76 dual JK Flip Flop W/Preset and Clear..
I would Like the 74126 to be a CMOS equivalent of the 126 .. thus giving better rail to rail switching with Tristate output ,hence the enable input..
oh the 126 is a just a tristate buffer..
 

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i just tested the hbridge i posted previously and it works fine. the main reason it works ok might be due to the low voltage applied(12V) and also without any pwm signal.

Forgot to mention this earlier, i will be using PWM from the PIC.

Lastly, i dont get it why the turn on voltage for NPNs connected to the +5v rail has to be 5v + vth. This sounds like a BJT to me.I thought that as long as the gate voltage overcomes the threshold voltage, it would turn on immediately and have all the voltage drop across the g-s.

thx willib for the cool design. but it seems it is operated by switch.
 
fabbie said:
i just tested the hbridge i posted previously and it works fine. the main reason it works ok might be due to the low voltage applied(12V) and also without any pwm signal.

Now see, it wasn't that bad :D
You should always try and evaluate something before throwing in the towel!
 
fabbie said:
Lastly, i dont get it why the turn on voltage for NPNs connected to the +5v rail has to be 5v + vth. This sounds like a BJT to me.I thought that as long as the gate voltage overcomes the threshold voltage, it would turn on immediately and have all the voltage drop across the g-s.

NMOS (not NPN). Say you need vgs=5v for the desired level of turn-on. Your source for a NMOS is connected to the load, and when turned-on as intended the load will be at the same voltage as the +5v rail. So to get vgs=5v, vg must be +10v.
 
hey when my motor was running under the pwm signal, it started to make strange screeching noises. Why is this happening? isit because of the pwm frequency? the frequency im using is 2000Hz.

one more thing. I think you guys were right when u said the p channel might short the entire circuit. Everytime when changing from forward and reverse using the PIC, the current would suddenly rise up from 2A to 3.5A. Lucky thing im using a low supply. Isit a wise decision to add a DELAY subroutine when changing from forward to reverse?
 
no you just need some interlocks between firing the N-type and P-type FET in a leg
 
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