The gate of a power MOSFET has an inherent high capacitance (think 2nF) which needs to be charged/discharged when the FET is turned on/off. If the FET is being switched at high frequency (seemingly not in your case), charging a high capacitance in a short time requires a high current.I can't for the life of me imagine an instance where you'd need 6A to turn on a MOSFET, or even a bunch of MOSFETs.
The gate of a power MOSFET has an inherent high capacitance (think 2nF) which needs to be charged/discharged when the FET is turned on/off. If the FET is being switched at high frequency (seemingly not in your case), charging a high capacitance in a short time requires a high current.
Note that the high-side FET driver ICs use a bootstrap arrangement to create a voltage above the drain voltage. This bootstrapping requires the lower end of a bootstrap capacitor to be periodically toggled high/low. If your FET polarity switching is infrequent then you may need some other way to achieve this toggling for the bootstrap arrangement to work correctly.
What switching rate do you anticipate using?
Apart from the 48V, is another supply used to control the switching?
I have a circuit I'm working on in which I need to switch up to 25A @ 48V to a mostly resistive load that will be in the range of 0.1Ω - 10Ω. Every other time I supply power to the load, I need to switch polarity. For the prototype I have been using 2 SPDT relays to accomplish this. But for reasons of cost and reducing the number of moving parts, I'm looking to change this arrangement to an H-bridge.
I've looked around for MOSFETs that meet the operational requirements listed above and also have a very low RDSON, so they don't get too hot. I've found quite a few N-channel MOSFETs that fit the bill, but the P-channels (at least the ones I've found) seem to have a higher RDSON and have fewer options overall. For that reason, I'm going to try to employ the H-bridge arrangement using only N-channel MOSFETs.
The problem is, that the highest voltage I have in the circuit (48V) is also the voltage I'm switching. So, it won't always be possible for the high side MOSFETs to maintain the necessary VGSTH. For this reason, I started looking at using a gate driver IC for the high side MOSFETs.
Now finally to my question:
I've found a few gate driver ICs that have very high output currents. The LTC4441 for example, can output up to 6A. Why would this much current every be necessary to drive the gate of a MOSFET? I can't for the life of me imagine an instance where you'd need 6A to turn on a MOSFET, or even a bunch of MOSFETs. Knowing that the people who design these parts aren't dummies, and knowing that I don't understand why I would ever need to use the full 6A this part can source, makes me think I don't understand something about the proper use for this IC, or worse yet something fundamental about how MOSFETs work.
So, can someone please explain this to me? You can talk to me like I'm 5. You won't hurt my feelings.
Also, if you think I'm going about this H-bridge arrangement the wrong way by trying to do it with all N-channel MOSFETs, please don't be shy.
Hi VNE,This makes a lot of sense. Thanks for the explanation. The datasheet for the part I referred to above does say that the peak current is up to 6A. That implies it can do that for only a short time, or for a small percentage of the time.
I guess that since in my application I won't be switching the load on and off fast enough for a bootstrapping arrangement to work, that I'll need to seek alternate solutions. One other idea ran across was using a voltage doubler to attain the necessary voltage above the drain voltage. Any thoughts on that? Also, have any ideas about where I should look for a suitable P-channel MOSFET, or am I simply constrained by what's available/possible?
Hi VNE,
My advice is not to mess about and use a gate driver for the reasons that Alec says in post #2. Especially as the effective gate capacitor for the type of MOSFETs you will need will likely have an order higher effective gate capacitance (gate charge) of around 20nF.
Even if you do not need a gate driver it makes no sense not to use one. They are cheap, small and freely available.
It is quite true that the performance of PMOSFETs lags NMOSFTs, but all the same there are plenty of PMOSFETs that would do your job. There is a list of PMOSFETs in the image below for you to consider.
spec
View attachment 103909
No problem VNE.Spec,
As you recommend, I'm leaning towards using a gate driver regardless. I'll start going through the list you posted for the PMOSFETs.
Thanks!
I have a circuit I'm working on in which I need to switch up to 25A @ 48V to a mostly resistive load that will be in the range of 0.1Ω - 10Ω. Every other time I supply power to the load, I need to switch polarity. For the prototype I have been using 2 SPDT relays to accomplish this. But for reasons of cost and reducing the number of moving parts, I'm looking to change this arrangement to an H-bridge.
If you want to use all N-MOSFETs, then you could likely use a high-side driver with a charge pump, such as one of those listed here to drive the top FETs.
You could use a h-bridge gate driver such as the **broken link removed**.
Mike.
You're not using this in your (Pipistrel? Tecnam?) airplane, are you?!?
At first glance this seemed like exactly what I needed. The only problem though is that VCC needs to be between 5.5 - 55V, and the voltage I'm looking to switch won't always be above 5.5V. The simplified application diagram on the first page of the datasheet shows VCC of the IC connected to the drains of the high side MOSFETs. I'm not sure if that's a requirement. I'm still looking into it.
If it isn't, I could connect VCC of the IC to a constant 12V source, and the drain of the high side MOSFETs to the variable voltage that would be anywhere between 0 and 48V. Any ideas is that would be a problem?
Spec,
I'm a little confused about how exactly I'd use the 7912 negative voltage regulator. Can you elaborate please?
Thanks.
Unless I am mistaken, the MC3383, that Pommie suggested in post #10, generates a bootstrapped DC supply for the top MOSFETs by using a self oscillating charge pump. It is thus suitable for 'DC' conditions.I'm sure I've heard somewhere about high-side driver ICs that use a photo-electric cell to generate voltage on the isolated side, and so can stay high indefinitely (unlike the bootstrap types we've been discussing here). They're slow as a wet weekend, of course, because they can deliver next -to-no current, but in this application they might just work.
Lunchtime's over now, so I've not got time to try to look up what the parts where called, but it's worth a Google, surely?
input = 0V, ground = 48V, output = 36V
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