Suggestion needed to chose a perfect high side n-MOSFET driver - Page 1
Forum section: electronics / projects

author: Nilesh
author_rank: Contributor
timestamp: March 28, 2019, 01:26:26 pm
content: I'm having trouble in controlling the n-MOSFET (IRF540) I used in my inverting buck-boost converter(the circuit is attached below).I would be thankful if someone suggests me a most suitable high side MOSFET driver for the attached circuit!!RegardsNilesh

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 28, 2019, 05:47:52 pm
content: Maybe give us more details. I suppose you simulated the attached circuit with LTSpice (looks like an LTSpice schematic) and it appeared to work? Then you built it and it doesn't work as well? I'll be assuming so and give a few thoughts.(Just a couple remarks I'm sure you are aware of: it's based on a floating power supply for the NE555, so obviously, your 9V supply here should be floating and not connected to the main ground. Obvious from the schematic, but don't forget that when building it. Another thing, as it's drawn, it should give you a negative output voltage, I suppose this is what you intended.)Just guessing, but the gate charge of an IRF540 is probably way too high to be driven properly with the output of an NE555? As it is, the transistor is probably getting pretty hot.(Regarding the simulation: I'm not sure the NE555 model in LTSpice correctly models its output stage, and second, the IRF530 model you used - probably because you didn't find the IRF540 model - has a lower gate charge than the IRF540.)Before selecting a different transistor, also make sure your 9V supply is able to provide the necessary peak current. Adding a bypass capacitor across it would help.There's a myriad of N-channel power mosfets out there, but at the output current you seem to be targetting (several amps?), I doubt you can properly drive the required transistor directly with an NE555. You'd need a gate driver. There are many gate driver ICs. You could consider the Microchip TC1411/TC1411N which should do the trick here. You could also devise a basic gate driver with additional transistors only.(Then again, if you're going to use an integrated gate driver, you should probably consider using a full-blown integrated buck-boost converter instead of this design. But if it's as a learning exercise, have at it.)

author: Nilesh
author_rank: Contributor
timestamp: March 29, 2019, 08:10:27 am
content: Thanks for your reply!I need to clarify a few things about the circuit and at the same time have a few questions.(i) Yes, I simulated the circuit in LTSpice(I will attach the file below) and it seems to work fine there(probably because in simulation, most of the devices are ideal). Then I built the circuit and everything worked fine in buck mode. But, as soon as the duty cycle goes beyond 50%, things get odd. The 555 timer starts heating up and I'm not able to get boosted voltage out.(ii) Yes, I'm aware of the fact that the 9V supply to the 555 timer is floating. Practically, I'm supplying it by an independent voltage source(capable of supplying the required amount of current) and the power circuit is powered by another source.(iii) And, since it is an inverting buck-boost converter, so yes, I'll get a negative output voltage.(iv) The expected output current at the maximum duty ratio practically possible is around 4A.Question-The microchip you suggested seems to be a low side driver. Will that be fine for my circuit?? since I will need a high side driver I believe!!!The project is just for learning purpose, hence I have not employed the closed loop topology.

author: Zero999
author_rank: Super Contributor
timestamp: March 29, 2019, 09:31:25 am
content: How do you set the 555 to high duty cycles? Have you checked you're not exceeding any of the 555 timer's maximum ratings?

author: Nilesh
author_rank: Contributor
timestamp: March 29, 2019, 11:08:40 am
content: During the charging of 2.2nF capacitor, R4 and R5(in the schematic) are shorted since D2(1N4148) is forward biased. Hence the ON time of the output signal depends only on R2 ,R3 and 2.2nF capacitor and the OFF time depend on R4,R5 and 2.2nF cap. Thus we have the duty cycle to be (R4+R5)/(R2+R3+R4+R5). Now, since R3 and R4 together is from a 10k potentiometer, varying R3 and R4 gives me duty cycle varying from around 5% to around 70% practically, at an almost constant frequency of the output PWM. The two 1k resistors are fixed just to limit the current, in case any of the two(R3 orR4) is zero.Duty cycle = (R4+1k)/(R3+R4+2k) = (R4+1k)/(12k) ----->(R4 varies from 0 to 10K)Yes, I've checked for the maximum ratings of the 555 timer for high duty cycles, but not when it was attached to this circuit.

author: jonroger
author_rank: Regular Contributor
timestamp: March 29, 2019, 01:34:20 pm
content: Adding to the above - put a 50 ohm resistor on the OUT pin.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 29, 2019, 02:23:23 pm
content: Quote from: Nilesh on March 29, 2019, 08:10:27 amYes, since it will be part of your floating PWM controller. Power it with your floating 9V supply.

author: soldar
author_rank: Super Contributor
timestamp: March 29, 2019, 07:35:59 pm
content: I find this way easier to analyze.

author: soldar
author_rank: Super Contributor
timestamp: March 29, 2019, 07:49:33 pm
content: I believe an improvement is theCuk converterandSEPIC.

author: emu_herder
author_rank: Newbie
timestamp: March 30, 2019, 02:17:12 pm
content: Re: High side drivers. The reason they exist is because you need to drive the gate of a FET above its source to turn it on. In a half bridge, the source of the high side FET is above ground, so the high side driver has some way to float its output ground to the same voltage as the source of the FET. Most commonly with a "bootstrap", which is essentially just a capacitor used as a floating supply.In this case, you've already floated your drive circuit with its reference at the source of the FET, so nothing special is needed in whatever drive circuit you choose to use. Any low side driver is fine.