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With reference to Mosfet driver ir2301s, why is VS actually powered from a capacitor? The datasheet reads "High side floating supply offset voltage" for pin "VS", not that I really understand what that means, but is that not the high-side voltage?

A comment in the linked thread claims:

In normal operation, where the gate driver controls a MOSFET half bridge, turning the low side transistor on pulls VS low, charging C2 through the diode D1. If you keep the low side transistor off for an extended time, the voltage at VS is likely higher than VCC, causing C2 to slowly discharge and the high side to turn off, even when HIN is high.

I don't know much about high-side drivers. Currently I am using a low-side TC4420CPA driving a HY3912W to disconnect a 57 V, 20 A supply, but I am trying to to work out if I can disconnect the high-side line rather than the low-side in order to keep the ground plane.

I dont really know where to start; currently I am just reading datasheets on high-side driver ICs.

Ideally I would like to find a high-side driver to replace my low-side TC4420CPA that will hold a 57 V, 20 A load continuously on when given a 5 V signal (ideally using HY3912W MOSFETs as I have loads).

I do not want an accompanying low-side MOSFET switch, when in the off state I am expecting an open circuit (output NOT ground), like a mechanical switch really would be open circuit when no power is supplied to the coil. the MOSFET will be HIGH for hours at a time (100% duty cycle) and all the high-side driver ICs I look at do not seem to hold the line high permanently.

ocrdu
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Jay Dee
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    That is a bootstrap capacitor to act as a cheap *temporary* floating voltage supply to supply a source-referenced gate voltage. Do you understand you cannot drive the high side NMOS gate from a ground referenced voltage? Because the NMOS cares about the gate-source voltage *difference*? Notice how the source pin on the high side NMOS is not fixed can change based on how it is conducting, reaching as high as Vcc. – DKNguyen Mar 25 '23 at 00:51
  • https://electronics.stackexchange.com/questions/449562/why-this-h-bridge-with-only-n-channel-fets-didnt-work/449568#449568 – DKNguyen Mar 25 '23 at 00:58
  • https://electronics.stackexchange.com/questions/624470/how-is-a-highside-gate-powered-with-a-bootstrap-circuit-and-mosfet-driver/624499#624499 – DKNguyen Mar 25 '23 at 00:59
  • emmm, i understand that the voltage difference of the gate is relative to the line that it is disconnecting, so if you plan to cut the high side, that is a floating ground plane of its own NOT ground of the entire circuit??? that's what i think i make of it anyway, EG stack 2 pencil batterys, ground of the top battery is 1.5v making its positive 3v. a MOSFET would have its own ground plane relative to BASE, thus the requirement for a MOSFET driver IC to make the potential difference? but then i may just be talking trash as i am no expert – Jay Dee Mar 25 '23 at 01:02
  • "*so if you plan to cut the high side, that is a floating ground plane of its own NOT ground of the entire circuit??*" Correct. Note the thing about 100% duty cycle and refreshing the bootstrap cap in the links. Just turning off the high side NMOS won't refresh the cap. Something else needs to be done which can be natural for some circuits (half-bridges, inverters) but not others (simple high-side switched loads). – DKNguyen Mar 25 '23 at 01:03
  • i was under the impression that... if my 57v supply was to be ON then BASE would need to be greater than 57v + 7v (yet less than 57v + 7v + 20v max) and when the supply is to be in the off state would need to be less than 57v but still greater than 57v - 20v max, and a mosfet driver ic is to handle this shift from high side to ground – Jay Dee Mar 25 '23 at 01:09
  • Correct, except I don't know where 7V is coming from. Normally it's >10V, more often 12V-15V. By the way, base is for BJTs. For MOSFETs it is called the gate. But consider this: When you first apply >57V to the gate the MOSFET is NOT on and the source is floating at some unknown voltage, or ground, and blows it. – DKNguyen Mar 25 '23 at 01:12
  • HY3912W datasheet clames the masfet will turn on above 6.4 volts – Jay Dee Mar 25 '23 at 01:16
  • By "turn on" do you mean barely start to conduct? Or fully conduct? Look closer at the datasheet. You don't care about the Vgs to barely conduct when you intend to use the MOSFET as a switch. The datasheet I found meets my expectations and 6.4V and 7V are nowhere to be seen. – DKNguyen Mar 25 '23 at 01:17
  • you may be right, i would have to check, i think i had the mosfet GATE successfully running at 7v though currently im using a 12v LM2596HVGR-12 and it defiantly turns on. my problem is how to select a high side drive that can do this for me (if that is posible) – Jay Dee Mar 25 '23 at 01:21
  • Yes, if your current is low you might get away with a Vgs higher than the threshold but below that to achieve the rated RDson. Your MOSFET will fry at higher currents though since its source-drain resistance is higher. To know whether a high-side gate driver that uses a bootstrap cap will work for you, you need to determine whether you need 100% duty cycle and whether you have a way to refresh the cap. If either of these answers is no, other approaches are needed (such as replacing the bootstrap cap and diode with an isolated regulator), going PMOS, etc. – DKNguyen Mar 25 '23 at 01:23
  • this is possibly my problem, replacing the bootstrap cap, where to start, i don't i need a duty cycle, i just want it to stay on for extended periods of time (hours), is there not an IC that would handle the isolation internally or would i need to use example ir2301s and an isolated regulator? – Jay Dee Mar 25 '23 at 01:37
  • so if i could use a B1212S-1WR3 or the like connected to BASE with a EL3H7 optocoupler, to drive the base high and that would work fine? (i was expecting to find an IC that did something along these lines) – Jay Dee Mar 25 '23 at 01:48
  • If you do not also a low-side NMOS to drive, there is no point to a typical high-side gate driver IC. These are intended for things like half-bridges. If you only have a high-side NMOS to drive, the only thing you would really be using from the IC is the level shifter to control floating circuitry from a ground referenced signal. You would still need to add floating regulators and such since there is no low-side switch to refresh a bootstrap cap. – DKNguyen Mar 25 '23 at 01:50
  • Discrete solutions are by pairing an isolated gate driver or optocoupler with an isolated regulator floating with the high-side NMOS on the other side. However, there are ICs that contain integrated charge pumps meant for to drive NMOS load switches but these usually only float under 12V. If you do use something like optocouplers, remember that if an optocoupler does not have a push-pull output (most don't...they are just a transistor that conducts or blocks) then they cannot drain the gate-source capacitance to turn off the MOSFET. You need something else to do that (like a pull resistor). – DKNguyen Mar 25 '23 at 01:50
  • ok, thank you for your help, please could you post an answer of a basic circuit diagram with a DC-DC isolator, optocoupler and, NMOS and i will try it – Jay Dee Mar 25 '23 at 01:55
  • Edit your answer to post a representative circuit of what you are doing. It only became clear in the comments that your circuit is a 100% duty cycle high-side load switch without an accompanying low-side MOSFET and not something else. – DKNguyen Mar 25 '23 at 01:56

1 Answers1

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If you do not also a low-side NMOS to drive, there is no point to a typical high-side gate driver IC. These are intended for things like half-bridges. If you only have a high-side NMOS to drive, the only thing you would really be using from the IC is the level shifter to control floating circuitry from a ground referenced signal. You would still need to add floating regulators and such since there is no low-side switch to refresh a bootstrap cap. –

Discrete solutions are by pairing an isolated gate driver or optocoupler with an isolated regulator floating with the high-side NMOS on the other side. However, there are ICs that contain integrated charge pumps meant for to drive NMOS load switches but these usually only float under 12V. If you do use something like optocouplers, remember that if an optocoupler does not have a push-pull output (most don't...they are just a transistor that conducts or blocks) then they cannot drain the gate-source capacitance to turn off the MOSFET. You need something else to do that (like a pull resistor).

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DKNguyen
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