I am building a transfer switch that has two 12v DC inputs and one 12v DC output, current is 10A peak and 4A normally. The switch could comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back prevents reverse flows through the MOSFET's body diode). Should one input fail I need to switch between inputs fast enough such that the output remains on but it is not switching at high frequency. The inherent gate capacitance of MOSFETs causes a current surge on switch on/off. I see the advantages of a gate driver [1] but need help with selection. The surge current is calculated multiplying the total charge by the switch time. The suggestion by diyodemag for high-side P-channel [1] is a TPS2812P [2] which has a 2A peak current. Q: If the MOSFET Qg x dt says more than the peak supplied by the gate driver what happens? A. The gate driver blows up. B. The current is limited and the switching time is extended. If the switch time is extended I assume there is a little more internal heating because it is part-on for longer. Does one add a series resister to the gate drive output to limit current? Microchip Application Note 799 [3] helps and its Table 3 matches devices to gate capacitance. There are devices with higher peak currents, eg, the TC4420/TC4421 [4] [5] deliver 6A/9A. These appear to pull the output between Vdd and 0V but is this source/sink, compatible with N- or P-channel, high/low side switching? The spec sheets talk about peak output current but presumably it is an input current when the output voltage is falling. Can one assume the input current is the same as the output current? Any other suggestions? eg, fancy chips like the LTC4416-1 [6] but I don't need its internal comparators and just the ability to flip on/off is enough. Thank you. 1. https://diyodemag.com/education/mosfet_drivers_why_you_need_them 2. https://www.ti.com/lit/gpn/tps2812 3. https://ww1.microchip.com/downloads/en/Appnotes/00799b.pdf 4. https://ww1.microchip.com/downloads/en/DeviceDoc/21419D.pdf 5. https://ww1.microchip.com/downloads/en/DeviceDoc/20001420F.pdf 6. https://www.analog.com/media/en/technical-documentation/data-sheets/4416fa.pdf
P-channel MOSFET gate driver
Started by ●January 21, 2023
Reply by ●January 21, 20232023-01-21
On Sat, 21 Jan 2023 15:16:17 +0000, James <news@oxdrove.co.uk> wrote:>I am building a transfer switch that has two 12v DC inputs and one 12v >DC output, current is 10A peak and 4A normally. The switch could >comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back >prevents reverse flows through the MOSFET's body diode). Should one >input fail I need to switch between inputs fast enough such that the >output remains on but it is not switching at high frequency. > >The inherent gate capacitance of MOSFETs causes a current surge on >switch on/off. I see the advantages of a gate driver [1] but need help >with selection. The surge current is calculated multiplying the total >charge by the switch time. The suggestion by diyodemag for high-side >P-channel [1] is a TPS2812P [2] which has a 2A peak current. > >Q: If the MOSFET Qg x dt says more than the peak supplied by the gate >driver what happens? >A. The gate driver blows up. >B. The current is limited and the switching time is extended. > >If the switch time is extended I assume there is a little more internal >heating because it is part-on for longer. Does one add a series >resister to the gate drive output to limit current? > >Microchip Application Note 799 [3] helps and its Table 3 matches devices >to gate capacitance. There are devices with higher peak currents, eg, >the TC4420/TC4421 [4] [5] deliver 6A/9A. These appear to pull the >output between Vdd and 0V but is this source/sink, compatible with N- or >P-channel, high/low side switching? > >The spec sheets talk about peak output current but presumably it is an >input current when the output voltage is falling. Can one assume the >input current is the same as the output current? > >Any other suggestions? eg, fancy chips like the LTC4416-1 [6] but I >don't need its internal comparators and just the ability to flip on/off >is enough. > > >Thank you. > > >1. https://diyodemag.com/education/mosfet_drivers_why_you_need_them >2. https://www.ti.com/lit/gpn/tps2812 >3. https://ww1.microchip.com/downloads/en/Appnotes/00799b.pdf >4. https://ww1.microchip.com/downloads/en/DeviceDoc/21419D.pdf >5. https://ww1.microchip.com/downloads/en/DeviceDoc/20001420F.pdf >6. >https://www.analog.com/media/en/technical-documentation/data-sheets/4416fa.pdf >If you post a schematic of your proposed circuit, we could understand the situation better. Why not two schottky diodes? I'm designing some boxes that can run off either USB 5 volts or a wall wart up to 48v, and that turns out to be the most sensible way: diode OR at the 5v level.
Reply by ●January 21, 20232023-01-21
On 21/01/2023 15:56, John Larkin wrote:>>https://www.analog.com/media/en/technical-documentation/data-sheets/4416fa.pdf >> > If you post a schematic of your proposed circuit, we could understand > the situation better.Figure 4 in the LTC4416 documentation is it. V1 and V2 in, Vs out. G1 to one pair of back-to-back N-channel MOSFETS controlling V1, G2 to another pair for V2. E1 and E2 are logic control inputs. I assumed this was a classic arrangement. The Block Diagram of the LTC4416 shows pins are connected to comparators which allows auto control with external resisters. I just need logic inputs and no load sharing. The LTC4416 has 0.5A output. The example MOSFET in figure 4 is an Si7483ADP, 120 / 33 * 2 (charge / dt by 2) exceeds the drive gate current. What gives?> Why not two schottky diodes? I'm designing some boxes that can run off > either USB 5 volts or a wall wart up to 48v, and that turns out to be > the most sensible way: diode OR at the 5v level.Power / voltage drop and lack of control. I want to turn on/off. It is a transfer switch not [just] an ideal diode OR. Thank you.
Reply by ●January 21, 20232023-01-21
On Saturday, January 21, 2023 at 10:16:30 AM UTC-5, James wrote:> I am building a transfer switch that has two 12v DC inputs and one 12v > DC output, current is 10A peak and 4A normally. The switch could > comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back > prevents reverse flows through the MOSFET's body diode).You don't need "back-to-back" P-FETs. If you think about it for even a minute, back-to-back is only necessary to block current flow from the battery to the load. If your load voltage is being maintained by backup, there will be no such current flow because the body diode is reverse biased. In your situation you need only 2x P-FETs, drains to individual power source batt or whatever, and sources joined to the load. One clever trick technique is to pull-up the gate drive of the alternate source P-FET with primary battery voltage. The primary source fet is controlled by a window comparator, you will need hysteresis and immunity to transient voltage dips due to load surge.
Reply by ●January 21, 20232023-01-21
On Sat, 21 Jan 2023 19:37:58 +0000, James <news@oxdrove.co.uk> wrote:>On 21/01/2023 15:56, John Larkin wrote: > >>>https://www.analog.com/media/en/technical-documentation/data-sheets/4416fa.pdf >>> >> If you post a schematic of your proposed circuit, we could understand >> the situation better. > >Figure 4 in the LTC4416 documentation is it. V1 and V2 in, Vs out. G1 >to one pair of back-to-back N-channel MOSFETS controlling V1, G2 to >another pair for V2. E1 and E2 are logic control inputs. I assumed >this was a classic arrangement. > >The Block Diagram of the LTC4416 shows pins are connected to comparators >which allows auto control with external resisters. I just need logic >inputs and no load sharing. > >The LTC4416 has 0.5A output. The example MOSFET in figure 4 is an >Si7483ADP, 120 / 33 * 2 (charge / dt by 2) exceeds the drive gate >current. What gives? > > >> Why not two schottky diodes? I'm designing some boxes that can run off >> either USB 5 volts or a wall wart up to 48v, and that turns out to be >> the most sensible way: diode OR at the 5v level. > >Power / voltage drop and lack of control. I want to turn on/off. It is >a transfer switch not [just] an ideal diode OR. > >Thank you.Two diodes looks easier.
Reply by ●January 22, 20232023-01-22
On 21/01/2023 23:07, John Larkin wrote:> Two diodes looks easier....but I can not control with a micro processor or any logic input. Please, as per my original question if anyone knows about MOSFETs and gate drivers.
Reply by ●January 22, 20232023-01-22
On Saturday, January 21, 2023 at 7:16:30 AM UTC-8, James wrote:> I am building a transfer switch that has two 12v DC inputs and one 12v > DC output, current is 10A peak and 4A normally. The switch could > comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back > prevents reverse flows through the MOSFET's body diode). Should one > input fail I need to switch between inputs fast enough such that the > output remains on but it is not switching at high frequency. > > The inherent gate capacitance of MOSFETs causes a current surge on > switch on/off.So? That current will flow to/from the voltage sources (harmless) or to the load, which is presumably low-impedance (draws lots of current anyhow). If you want the load to stay ON during a switch, it has to have an input capacitor anyhow, which will totally dominate the gate capacitance.
Reply by ●January 23, 20232023-01-23
On 22/01/2023 17:27, whit3rd wrote:> On Saturday, January 21, 2023 at 7:16:30 AM UTC-8, James wrote: >> I am building a transfer switch that has two 12v DC inputs and one 12v >> DC output, current is 10A peak and 4A normally. The switch could >> comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back >> prevents reverse flows through the MOSFET's body diode). Should one >> input fail I need to switch between inputs fast enough such that the >> output remains on but it is not switching at high frequency. >> >> The inherent gate capacitance of MOSFETs causes a current surge on >> switch on/off. > > So? That current will flow to/from the voltage sources (harmless) or to the > load, which is presumably low-impedance (draws lots of current anyhow).I'm asking about the gate current and its driver, not the source-drain.
Reply by ●January 23, 20232023-01-23
On a sunny day (Mon, 23 Jan 2023 09:00:40 +0000) it happened James <news@oxdrove.co.uk> wrote in <tqlibo$3i6lm$1@dont-email.me>:>On 22/01/2023 17:27, whit3rd wrote: >> On Saturday, January 21, 2023 at 7:16:30 AM UTC-8, James wrote: >>> I am building a transfer switch that has two 12v DC inputs and one 12v >>> DC output, current is 10A peak and 4A normally. The switch could >>> comprise 2 pairs of back-to-back P-channel MOSFETs, (the back-to-back >>> prevents reverse flows through the MOSFET's body diode). Should one >>> input fail I need to switch between inputs fast enough such that the >>> output remains on but it is not switching at high frequency. >>> >>> The inherent gate capacitance of MOSFETs causes a current surge on >>> switch on/off. >> >> So? That current will flow to/from the voltage sources (harmless) or to the >> load, which is presumably low-impedance (draws lots of current anyhow). > >I'm asking about the gate current and its driver, not the source-drain.Look up how MOSFETs work The gate electrode is separated from the main current channel by a thin insulating layer So normally no 'gate current' is needed, much like a grid in a electron tube. There is however, due to that construction, a CAPACITANCE, the gate and its insulating layer and main channel form a capacitor. This maybe be a very small value capacitor for for example small signal RF MOSFETs (a few pF) or a rather big one for power MOSFETs (several nF). The drive circuit impedance then determines how fast his capacitor is loaded, so how fast the rise-time of the gate voltage and the controlled current as result of that. There are some other capacitive effects too (drain gate for example). Look up the datasheet of the MOSFET for the value of the Cgs (gate source capacitance). The other important parameter is where the conduction happens versus gate voltage. So I drain versus Vgs (drain current versus gate source voltage). And beware of exceeding maximum allowed gate voltage, exceeding that will break down the gate insulating layer, Also some MOSFETS have a reverse diode in parallel, with its own reverse breakdown characteristics. Not sure if that the question was, but those are things that count when designing.
Reply by ●January 23, 20232023-01-23
On 23/01/2023 09:51, Jan Panteltje wrote:> The gate electrode is separated from the main current channel by a thin insulating layer > So normally no 'gate current' is needed, much like a grid in a electron tube. > There is however, due to that construction, a CAPACITANCE, the gate and its > insulating layer and main channel form a capacitor. > This maybe be a very small value capacitor for for example small signal RF MOSFETs (a few pF) > or a rather big one for power MOSFETs (several nF).[As I understand] There is no gate current when just on/off. The capacitance causes a current flow when going between on and off. The charge is small but the period is short which leads to the values as high as 10A being suggested. Gate drives are deigned to supply this brief transient current. [There is a power requirement if cycling rapidly at high frequency - I am not.]> The drive circuit impedance then determines how fast his capacitor is loaded, so > how fast the rise-time of the gate voltage and the controlled current as result of that.So a 1 Amp gate driver will not fuse if the capacitance asks for more than 1A? It will just fill/dump more slowly and switch on/off more slowly? ie, gate drivers are current limiting devices.> The other important parameter is where the conduction happens versus gate voltage. > So I drain versus Vgs (drain current versus gate source voltage). > And beware of exceeding maximum allowed gate voltage, exceeding that will break down the gate insulating layer,OK, thanks.> Also some MOSFETS have a reverse diode in parallel, with its own reverse breakdown characteristics.Yes, it is why I need the back-to-back arrangement to stop the reverse current flow. Thank you.
