High Side MOSFET Switching Circuit - How Bootstrap circuits work

Let's look at a simple circuit to allow us to place an N-channel MOSFET on the high side of a load. Then let's compare the results with a purpose build IC.
I hope you enjoy the video, let me know what you think. Thanks for watching!
The component list is:
MOSFET: IRF530
Resistor: 1k, 100R, 47R
Capacitor: 680n
Diode: 1N4004 (picked it randomly, use fast diode for high-f applications!)
IC: UCC27714 from Texas Instruments
PWM circuit: NE555, 1k, 100n
Bulb: 12V incandescent
0:00 Intro
0:27 Circuit on breadboard
0:56 Explanation of the problem
1:35 Bootstrap circuit explanation
2:37 Bootstrap circuit on breadboard
4:45 Half bridge IC
5:18 Testing the IC ircuit
5:52 Feeding with a square wave
6:16 Conclusion
Music: Kevin MacLeod - Space Jazz
#diy #electricalengineering #mosfet

[@__MINT_]

In DIY driver, you can add NPN transistor as an emmitter follower to boost current pumped into the gate, so that rise time decreases without lowering resistor value much. There is one more thing you have to be careful about, when switching on the mosfet discharging the gate, it forces negative G-S voltage on driven mosfet gate, with high drain voltage you can easily exceed max G-S voltage and kill the transistor. Isolated drivers can be built around an optocoupler, but they are not that fast and have a few pitfalls as well. So the best option is just sticking to dedicated ICs.

[@adaminsanoff]

Well explained!

[@big_o1952]

So if i just connected to the bulb to +12 volts and the source to ground will it fully turn on ?

[@leonerduk]

I think it'd be useful also to take a bit of a wider step back and answer why we'd use an n-FET as a high-side switch, given all this extra complexity, rather than use what seems on paper to be a far simpler solution of just using a p-FET instead.

[@5VLogic]

Thanks for the comment. It's true, in fact, actually two assumptions were made in the video: that a P channel FET can't be used and that a low side switch is also not employable. For lower power applications the P FET is the simple solution. Higher performance devices will try to avoid them because of worse characteristics (like Rds) and their overall scarcity which limits choice.

[@jlsmonte]

​@@5VLogic If you don't want us a P-Channel, also you must consider connect the load to the DRAIN and the SOURCE to GND. This way you apply 5V to gate and the switch ON happen!

[@110v4]

@@jlsmonte Of course that is better way to do it. But sometimes we can't do that (for example in half-bridge and 2 switch forward power supply)

[@swapnilkumar9363]

In simple words, it's not very practical to use a MOSFET which will turn itself on when gate is grounded because it's usual practice to keep things pulled to ground by default. Also at voltages more than VGS rating, you need a negative voltage regulation to supply a pmos with safe gate voltage. So for application below 15v, P MOS can easily be used, but not beyond that. Inefficiencies are another reason

[@sunilgokhale612]

Sir what is meant by high side and low side mosfet

[@5VLogic]

High side means that the transistor is placed between the load and the positive supply. Low side instead is when the transistor is connected to ground. In this configuration we have the disadvantage that the load is connected positive even when off.

[@cannapolis9009]

I really don’t see how you get 24 V on the top of the capacitor. Seems to me that you were simply putting ~12 V on both sides when the upper Mosfet turns on.

[@JerrysStuff]

why not just use P mosfet

[@swapnilkumar9363]

P MOSFETs simply suck. They're slower, waste more power and bit expensive. Also they need to be constantly pulled up to keep them off. Driving them requires gate voltage 12-15 v lower than supply voltage, which is again a problem because when you're using them at say, 50v, you need 45v signal on gate to turn them on safely. So P channel MOSFETs are not convenient enough.