High Side MOSFET Switching Circuit - How Bootstrap circuits work - YouTube
https://www.youtube.com/watch?v=izUDxwgpzt8

Transcript:
(00:01) here I have a very simple circuit consisting of an nend Channel mosfet and a resistor between source and ground now what do you think happens when I connect the gate to 12 volts is it a the mosfit turns on and the output voltage is 12 or is it B the mosfit doesn’t turn on or is it C something in between or D the mosfit blows up actually we don’t even have to guess we can just wire it up and see what happens here I have the circuit on a breadboard and as as a load I’m going to use a light bulb this way we can see
(00:33) visually what’s happening to the voltage on it as a reference I can attach it directly to the power rails and we can see that it turns on quite brightly now we connect the gate to 12 volts and we can see that the light bulb turns on just faintly and this means that answer C something in the middle was correct now let’s try to understand what’s going on and why this is happening for a mosfit to be fully on we need the gate Source voltage to be 12 volts or at least higher than the threshold voltage now initially this is the case
(01:08) because I apply 12 volts to the gate and the source is zero but as the mosfet turns on the output voltage which is also the source voltage Rises to the point where the gate Source voltage is just enough to keep the mosfit slightly on but not completely on so this means the output voltage is also somewhere in between 12 and 0 volts at this point it’s obvious that we need the vol voltage on the gate to be higher than 12 volts for the output load to get the full voltage to achieve this we have different options there’s a very simple
(01:38) circuit you can find online or you can use an integrated circuit that takes care of the whole thing let’s first take a look at this very simple circuit that you can find online it just consists of a diode a capacitor and a pull-up resistor and then we have another transistor to pull the Gate of the main mosfet down to zero so when our input is high the first mosfit is on and so this means that the Gate of the main mosfit is zero so the source is also zero and the top leg of the capacitor is 12 VT because
(02:11) the diode can conduct when our input goes low the First mosfit turns off and this means that the pull-up resistor brings the Gate of the main mosfit to 12 and while this happens The Source Also Rises the top leg of the capacitor Rises as well and the mosfet will fully turn on because the capacitor is fully charged and its lower leg can go up to 12 while the top leg is 24 now let’s build the circuit on a breadboard and see if it actually works here I built it using two identical mosfets just for Simplicity
(02:45) and we can see that when I connect the Gate of the first one to zero the light bulb turns on completely and when I pull the gate High the output is zero now let’s look at the output voltage on the oscilloscope we can see here that it rise es quickly but then it gradually Falls and this is mainly due to the impedance of the oscilloscope probe but aside from that we can take this as an indication that we shouldn’t use this circuit to permanently turn on the power of a load instead we can use the circuit
(03:16) for fast switching now instead let’s probe the gate Source voltage and see how that behaves we can see that it rises pretty slowly it takes about 6 micros seconds for it to go up completely now some of you might be saying that 6 micros seconds is incredibly fast and that’s true in general but we have to consider that in switch mode power supplies we can have multiple hundreds of kilohertz as switching frequencies and if we have the full load on the mosfet while it’s switching and the switching time is 6
(03:49) micros seconds we’re going to burn up an incredible amount of power as an obvious solution I tried decreasing the value of the pull-up resistor from 1 kiloohm I went to 100 ohms and we can see that the time has gone down to about 600 NS which is what we were expecting because we decreased the resistance by 10 which means that the time to charge the capacitor should also decrease by a factor of 10 unfortunately though for a switch mode power supplies this time is still pretty bad so I used a 47 Ohm resistor this time which is about half
(04:22) of what we had before and we can see that the time also got cut in half although I can’t decrease the resistance any more than this because it’s already heating up quite a bit so the power that we’re saving from Quick switching we’re losing on the pull-up resistor this means that this circuit is not too great when we want high frequency performance devices we’re going to have to turn to something different so the next thing I want to do is take a look at this IC because it’s made exactly for solving
(04:50) this problem in fact it includes two gate drivers for mosfets both capable of giving 4 amps to the gate to charge it super quickly and and reduce the switching time the circuit I’m going to build is the one shown in the data sheet but I’m going to leave out the parts referred to the lows side switch and just go with the high side one unfortunately I had this IC only in a small SMD package so I came up with this solution to make it a little bit more breadboard friendly here you can see that it’s working but the mosfit only turns on for
(05:26) a brief moment and then turns back off this is because the this IC is made for fast switching and not for keeping the mosfet on all the time if we want to increase the time that the switch can stay on we can put a bigger capacitor like I’m doing here and you see that it improves it a little bit if you want to put an electrolytic capacitor you can but remember that they’re only good for low frequency applications so in the case of a switch mode power supply I would avoid them another test that we can do is supply the IC with a square
(05:58) wave and see what happens s we can see that the output appears to stay high but it’s a little bit less bright than it was when we’re turning the light bulb on in fact this is because the duty cycle is a little bit under 60% so the brightness is about 60% as well okay well I guess this just about wraps it up the last thing I want to say is that some people have asked me what the value of all the components I use are so I’m going to write them all in the description in case you’re interested so
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