1. Home
  2. Electronics Design
  3. P-channel MOSFET gate driver

P-channel MOSFET gate driver

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. formatting link
    formatting link
    formatting link
    formatting link
    formatting link
    formatting link
Reply to
James
Loading thread data ...

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 to
John Larkin

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?

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 to
James

Two diodes looks easier.

Reply to
John Larkin

...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 to
James

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 to
whit3rd

I'm asking about the gate current and its driver, not the source-drain.

Reply to
James

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.