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H-bridge delay question...

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parkerbeder

New Member
Hi, we are controlling an H-Bridge with a lm339 comparator, and are having problems having shoot through when we are changing directions. In the circuit, we're using p-type fets to switch the polarity of the 339, (since it's sinking) now, we need to introduce at least 50ns of delay in the turning on of these p-fets, if we put a capacitor from negative to the gate of the MOSFET, will it delay?

Thanks for any help or ideas!!!

-Parker Bender
 

RCinFLA

Well-Known Member
If you can, try to use an H-bridge high side/low side driver I.C.

They have delay built in to help prevent shoot through.

The delay is variable depending on device and temp but generally about 50 nsec of delay is sufficient. This assumes you have sufficient driver to flush the input capacitance on the H-bridge device which is proper design practice.

Large current and high voltage MOSFET devices can have significant input capacitance and the driver must have low enough drive resistance to flush the cap in time before complimentary device turns on. On top of input cap flush there is some transition time delay of the MOSFET device.

It is not uncommon to need a driver capable of 5 or 10 amps to flush the input capacitance in time. It of course depends on device. Some large devices have 10 nF of effective input capacitance. I = C dv/dt, so 10 volts of gate swing in 20 nsec with 10 nF input capacitance needs a driver capable of 5 amps.
 

parkerbeder

New Member
not going to work

This is a high-performance application, with serious amperage requirements. we're using irfz44 mosfets, and need the thing to work by wednesday afternoon, so we can't hunt an IC by then. we have the control circuit running on relays, but they hunt really bad, so we were switching to mosfets to speed things up so the hunting isn't noticable, and for the weight and size reduction.

Will the capacitor idea work, or is there another way with easily available stuff that we can find at radioshak, or our local EE department store?

Thanks for the quick reply!!!

-Parker
 

RCinFLA

Well-Known Member
It is hard to tell without actual schematic. Since LM339 is open collector so I assume you are relying on a pull up resistor to turn the P-ch device off. This is probably the dominate reason for turn off delay.
 

parkerbeder

New Member
I do have a 10k resistor to pull up the p-fets, I don't know how you guys are drawing your schematics, and i don't have cad on this computer, but this circuit is controlling the steering on a tractor.

The lm339 has the inverting and non-inverting inputs on channels 1 & 2 tied together, and are attatched one to a joystick pot, the other to a linear transducer on a linear actuator, both wired in volume circuits. The outputs of the comparator are tied to the bridges of 2406 p-fets, with 10k pull up resistors to power. The drains are going to an h-bridge of N-type IRFZ44's, with 10-k pull-down resistors going to ground, on the h-bridge circuit, both fet's bridges are tied together, running through one pull-down resistor. The linear actuator is around 26 amps, 12 volts. supply voltage is 13.6ish. will putting a resistor on the p-fet bases slow down turn-on speed? it doesn't have to switch in the nanosecond range, just in the like 100+hz area...
 
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RCinFLA

Well-Known Member
Still can not follow description into a schematic. I think I got that you have a feedback control system where the actuator position transducer matches the joystick setting when position is where you want it.

Do you have some hysteresis on the comparitors to keep them from chattering near the transition point? You need to have some small dead zone at the match point.
 
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parkerbeder

New Member
Nope, the relays worked pretty good, and at center they would hunt for a bit, then calm down, so we figured with a faster switching time, it would just be unnoticable, if we could fix our shoot-through problem. As far as I can tell, all we need to do to fix that is to make our p-fets turn on less quiclky, is there a simple way to trick that into happening?
 

user_88

Member
A standard TTL logic gate has a delay of approximately 10 ns.
If you have a TTL logic level someplace you could insert enough sequential gates ... 4 or 5 and gates maybe ... and get the necessary delay.
Without seeing your circuit layout, it's difficult to see if this suggestion is feasible.
 

MikeMl

Well-Known Member
Most Helpful Member
The usual way of doing this is NOT to add delay; rather AND/NAND gate logic is used to implement a lock-out such that side A will not enable until side B has turned off, and vice-versa. Look at the "An Easier Way to Control It" section on this**broken link removed**.
 
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parkerbeder

New Member
okay, I hunted down a couple hip4080's, and am trying to make one of those guys go, but I'm running into a condition that only the high sides are tripping, which doesn't even exist as a condition on the truth table. Does anyone have any idea what might be happening? I'm not using a bootstrap cap anywhere, as the application notes made it sound like they are only necessary if you are wanting a quick response, which, we're only looking for marginally faster than a relay...

here's the datasheet:
https://www.electro-tech-online.com/custompdfs/2009/12/fn3658.pdf

Thanks for any ideas/insight!!!

-Parker
 

user_88

Member
How do you have IN+ and IN- connected?

One of the data sheet test configurations has IN+ connected to a constant +6 v. Then, apparently, the condition of IN- = 1 or 0 is determined according to whether IN- is greater or less than that 6 volt magnitude.

The relevance here is that an internal comparator connected to IN+ and IN- controls whether the lower fets turn on.

Are you sure that your inputs to IN+ and IN- are correct?

If IN+ is connected to +6 v, then IN- could conveniently be either 0 v, or +12 volts.
 

parkerbeder

New Member
We were using the power off of the joystick pot and the linear transducer as the inputs. Do you think the in+ need to be 6v?

Thanks!

-Parker
 

user_88

Member
Just do a manual test .... +6 v for IN+, and switch IN- between 0 and +12.
You ought to detect actuator motion, if everything else is okay.
Then you can try to see what the pot and transducer are doing ... as inputs.
 
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