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Topic: Custom H Bridge Design (Read 13 times) previous topic - next topic

BrIDo

So if I tried to use all N types, I wouldn't be able to turn on any of the high sides using that driver? If you're saying the gate voltage is relative to the source of the mosfet, what gate voltage would be required, given that the source of the highside is connected to the drain of the low side?

Actually, this statement seems to make sense regarding that:

Quote

Let's spend some time on this choice for the high-side elements. As said before, N-channel devices would be desirable for this role for their lower losses, but there's a problem: for them to operate properly, their source must be connected to the motor leads and their drain to the power rail. When a P-channel device is used, its source will be connected to the power rail and its drain to the motor leads. Now, the problem is that both devices are controlled by their gate-source voltages. For P-channel devices it means that if the gate is connected to the power supply, the device will be closed (gate-source voltage is 0) and if the gate is grounded the device is opened (provided the power-supply is actually enough to open the device), since gate-source voltage is equal to the power supply voltage.

For an N-channel device however the picture is more complicated. If you connect the gate to ground or to source, the device is closed (gate-source voltage is below or equal to 0). But where to connect it to open the device? The power supply is not enough, since, when the device is open, it's source and drain are roughly at the same potential. Since the drain is connected to power, the source will be at that potential as well, but than gate should be higher than that to keep the device open. In fact at minimum 5V higher for so-called logic-level MOSFETs and 10-15V higher for normal MOSFETs. This is a significant problem, that voltage somehow has to be generated. In most cases some kind of a charge-pump is used for that, either in a stand-alone or a boot-strapped configuration. The latter however is only useful if the bridge is driven in the 'locked anti-phase' mode (see later). In any case, these high-side drivers usually cannot deliver as much current as a regular low-side driver can, which means longer turn-on and -off times for the high-side (lower current takes longer to charge-discharge the gate-capacitance). In high-frequency operation, where switching loss is a significant factor, a P-channel MOSFET might be a better solution because of this. In low-frequency, high-current operation, where switching loss is not a problem, but channel-resistance is, N-channel transistors are usually a better compromise.


I'll go and have a ponder over that for a few hours :-P

RuggedCircuits

Quote
So if I tried to use all N types, I wouldn't be able to turn on any of the high sides using that driver? If you're saying the gate voltage is relative to the source of the mosfet, what gate voltage would be required, given that the source of the highside is connected to the drain of the low side?


Exactly the problem HVIC drivers like the FAN7382 I mentioned are designed to solve: support for a diode-capacitor "bootstrap circuit" that drives the high-side N-type MOSFET with a gate voltage always above the source, whatever the source may be even as it changes. Study the datasheet for that chip carefully, you are likely to learn as much as the explanation you will be pondering for the next few hours (though both are useful).

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MarkT

The tricky part of the design is getting the high-side driver to 'float' with the output signal (which might be a 24V, 50V or whatever switching waveform) AND have it controlled from some circuitry at ground potential.  Typically current-control (or opto isolation) is used.
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zoomkat

The h-bridge in the below schematic is an interesting design.

http://www.openservo.com/moin.cgi/Schematic2
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

BrIDo

Ok. I managed to get a look at Intermediate Robot Building by Robert Cook, which I looked at following this article:
http://www.robotroom.com/HBridge.html
There was alot of good information in the book.


What put me off the P-types was the fact their gate voltage was listed as negative. That made me think you required a negative voltage at the gate and so i though i'd require an inverting mosfet driver. But I realise now that wouldn't be necessary.

If I used the two P-type Mosfets I mentioned earlier on the high side, and the two low N-types on the low side I should (according to the book) be able to use a MIC4426.

I use 2 resistors in the logic lines. Rugged you suggested using 2k2 ohm resistors, any reasons why so high? I'm assuming this is to automatically tie down the inputs and so set the initial state of the h driver. The two high side mosfets are initially closed (i.e. on). Supplying a voltage to them will turn them off. The opposite is true of the N-type. To prevent shoot through (essentially a short circuit) I have to make sure there's a suitable delay between opening and closing each side of the driver.

If i use one MIC4426 chip, and tie the highside and lowside inputs together for the left and right side of the bridge, according to Mr Cook, there will be a brief period of shoot through as the Ptype turns off and the Ntype comes online. This is only a real problem, however, if I'm making use of PWM. (I think that's what Mark was trying to explain?).

I'll post a revised schematic later tonight.

Anyone have any ideas how much current a standard breadboard can handle :P?

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