Bootstrap diode

Seems to imply that such a configuration cannot work unless the low side is used often? Probably just as often as the high side?

Pretty much, no exercise -> no voltage. How often depends on the size of the cap and the hunger of your high side circuitry. Bootstraps are meant for complementary circuits where the whole chebang runs at some reasonable frequency and both high- and low-side are switched.

Yes - ideally, it'd be symmetrical, to avoid current imbalances and stuff.

I kinda redrew the circuit in my head, and the "bootstrap" portion is just a bent half-wave voltage doubler. :-)

Cheers! Rich

I guess then I can't use this for a full bridge since the low side drain and high side source must connected directly. I guess with a motor inbetween the cap will not be charged properly?

The bootstrap cap is normally referenced to the high-side source. As long as the source is pulled low, either by the low side fet or the load, the bootstrap cap will charge.

In ~self-oscillating full-bridge situations, where the drive signals depend on the output state, care must be taken to ensure that the bottom fet does in fact turn on at least once before drive to the upper fet is required.

This may complicate start-up and limiting situations.

RL

But with a motor the cap will not be pulled to ground or even close.

My confusion is actually how it accomplishes. In the pdf link it has a diagram of a basic driver. It shows a level shifter and some logic then an omp amp.

My guess it that it has some logic to level shift the signal to whatever floating hi and low it uses. The op amp is for drive capabilities. The op amp is floating as the cap supplies power to it. Essentially as Rich as mentioned it is similar to a voltage doubler. The cap is charged on the low side then "lifted" to the high side giving effectively 2Vcc from ground but puts Vcc across the op amp to power it and drive the high side gate.

If a load is between the high side source and low side drain then the cap will not be charged up properly.

I imagine I could create some additional circuitry to charge the cap up on the low side. Basically disconnect the cap and connect it to ground during the low side. This is so that the cap can be charged during the load side but is separate from the load. Of course this creates more problems than it's worth.

If the way you use it prevents it's normal function, you are misapplying the part. I think this is one of the reasons why app notes are published.

In a full bridge there are four switches. The low side switches in each case are responsible for refreshing the bootstrap cap specific to the high-side switch on it's own side of the bridge. The load is applied between the switch pair junctions on each side of the bridge.

Your reference app note describes a half-bridge driver; one side of a bridge is used.

An asymmetrical bridge replaces two diagonally situated switches of the full bridge with rectifiers - it is capable of producing load current in only one direction.

A 'leak' resistor between the source and the negative rail could serve this function, prior to the first high-side gate drive signal. Once any motor load current is established, its own flyback energy will ensure that the source is pulled low in the freewheeling interval.

RL

It can be, depends on the load. If the low side FET conducts then its drain pulls towards zero. Then the load tugs on the source of the high side FET and pulls that to ground.

Of course if the load is sluggish this falls apart.

In that case I'd use a real motor driver or something wit a separate charge pump.

For what its worth..

I see the circuit as a simple method of supplying the HS line, which is the Vee of the driver of the high side, a "common" point as long as the charge in the cap sustains.. When the low side output is on, this will then provide the "COMMON" point for the HS line and also the cap how ever, the high side isn't on at this time so the only thing that takes place is the recharging of the cap.

The diode is there only to prevent back drain of this cap and supply voltage for charging.

For larger caps with slower switching times, an external larger diode should be used to relieve the strain from the internal one, which could actually short it. This is something you do not want because it will then most likely take out the high side output due to insufficient drive at the gate which will put it into linear state.

These types of drivers are only good for PWM system or some kind of pulsed system.. Most of the time, at idle, you keep the low side on to keep the cap charged. I guess the ideal thing would be to run it into an inductive load or have a inductor in series and have the circuit generate a very narrow PWM to maintain the cap. With the inductor in line, you shouldn't see much DC at the other end.

If you were to combine two of these for a full bridge, you'd keep both low side outputs on at idle. Using the load isn't going to help much here since it may be reactive or a some what higher level of R>

That's my take of it for what its worth!..

I just see it as using the cap for temporary power to the op amp. It charges the cap on the low side(obvious when the low side mosfet is shorted) and supplies power to the op amp on when the low side is off.

I have a similar one, the MAX5062(I think). It says it can be used for motor control but all the circuits show it as a half bridge. I've tried using it once and could run a small motor but trying to run a large one caused the IC's to burn up. Not sure why the motor size had anything to do with the IC's burning up... maybe inductive kickback somehow screwed it up?

if you use a resistive load I don't see how this can happen except at the moment of switching. If the low side switch is on and the high side is off then the cap will charge up through the resistor which means it is not grounded... but if the cap is small enough it might approach ground. The higher the load the better as it will approach ground more. But then it also depends on the switching times and requires a dead time.

I could use a small low side mosfet or maybe even a bjt that alternates with the high side. This allows the cap to charge every time the high side is used. Then just use the drivers for high side and use something else for a low side. This might cause some problems with inductive loads. Also if the low-side small mosfet is ever turned on without the motor's low side then it will take all the current and burn up rather quickly. (this shouldn't happen if everything is consistent though)

They seem to be hard to find compared to these "half-bridges".

Well, of course you can't leave both switches on for too long. Then you are better off with a real motor driver.

Not really:

But I don't know what it exactly is that you want to do with it. This is an example for high power, there's cheaper ones for less amps.

Ok? For bi-directional application you use two asymmetrical half-bridges... I have done that with the MAX5062 but with a much larger motor it causes them to burn up. The MAX5062 seems to be designed similar to the app note. In the datasheet for the MAX5062 it says it can be used for Motor control. I can't think of any other way than to use it asymetrically. It uses a boot cap and high side sensing and shows circuits similar to other half bridge drivers. it does not show one for a motor though.

It works fine on a small 12V DC motor(

This might cause some problems with the gate drive since the HS source is no longer floating. One would need to match the resistance to the capacitance so that it can be charged fast enough yet not pull the source down to ground to much reducing the HS gate drive.

was just looking at

which shows what I did but my load was a motor. I used two of them configured "asymmetrically" as to get bi-dir. I think I even tried disconnecting one side with similar results.

But these have built in mosfets ;/ I've seen a few of these. I need more than 10A ;/ Why can't they just have pins for external mosfets? This one has the 2 low side gate drivers but has built in high side. I've thought about trying to parallel such devices because they have everything else I could want. But I think paralleling is probably a bad idea. If only I could use my own high side drivers then I'd have all my problems solved ;/

Maybe if I can find those in 20A+ from 15-30V then I could use them too. Of course I would still like to use my own outboard mosfets or at least have the option.

They do have

Which I used two MAX5062's to get the same effect but the drivers burned up on a large motor(everything worked fine for a small

Thanks though, I've seen some of hte "pre-drivers" which is what I'm basically looking for. Again I've done this with the MAX5062's and can't see why such a device would burn up just because I'm using a larger motor. The external mosfets are the only ones that should care about the side of the motor and they were just fine(even after the drivers burned up).

I'm going to try some of those full-bridge pre-drivers though and see what I can get.

I never use Maxim so I can't say. One scenario for a burn-up is stray spikes finding their way into the chip. Traditionally such spikes will have 10 times the gusto when you go from a 1A motor to a 10A motor :-)

sure you can, in a full bridge the motor doesnt go between the H- source and L-drain on the same side. it goes between the connected H-source and L-drain on one side of the bridge and the connected H-source and L-drain on the other side of the bridge

when 2 is on boost cap for 1 is charging, when 4 is on boostcap for 3 is charging.

right side is driven inverted from left side. Maximum dutycycle, minimum switching frequency, and the current consumption of the high side driver must all fit together with boost caps size and charging current to avoid the high side drive voltage drooping too much during high side on time and/or not being fully charged for the minium lowside on time

-Lasse

In addition to this, if there was current in the motor inductance when the top FET turned OFF, the the corresponding bottom FET doesn't even have to come on in order for the boost cap to charge..... There will be a path to ground through the bottom FET's intrinsic, or additional, Drain-Source diode.

boB