Electronics > Projects, Designs, and Technical Stuff
Searching a high-side driver ic for a p-channel MOSFET, do you know any?
Dubbie:
I always thoroughly enjoy reading Ian.M’s posts. You can always bet on a clever and well thought through solution to the problem as presented. I have been fortunate enough to have benefited from his patient assistance on more than one occasion. One of this forums hidden treasures. Thanks Ian!
Jajaho:
I can absolutely agree with that.
Thank you very much for your time and effort.
I am currently working on a layout for Ian's proposed design using Pulsonix.
But I'm wondering how good decoupling would look like in this case.
Do I just lay down a couple of 100n's from V+ to GND next to the transistors?
Do I need any bigger caps when powering the circuit from a good laboratory power supply?
Do you, in general, have any kind of application notes regarding this topic I could read through?
Much appreciated.
Jajaho:
Hello,
here is my current layout for Ian's proposed driver design.
I would really appreciate it if you guys could take a look at it.
Thank you very much.
PS: C4 is supposed to be 100uF
IconicPCB:
Would someone care to walk me through the schematic (as shown in the PDF file above)?
Jajaho:
Of course but I am not the one who came up with it so you will find that Ian M. can and has done a much better job at doing so earlier in this thread. ;)
--- Quote from: Ian.M on August 05, 2019, 03:43:33 pm ---I assume one terminal of the valve or clutch coil is commonly grounded to their bodies so you cant simply use low-side switching using a N-MOSFET.
The Q1,Q2 matched pair forms a basic current mirror, applying a current set by R1 and the input logic level to the driver stage pullup resistor R2. The ratio of R1:R2 therefore sets the max gate drive voltage swing. It needs to be a current mirror with a low voltage drop to get as close as possible to rail to rail gate drive so one cant use the more accurate Wilson current mirror there without compromising gate drive at low supply voltages.
D2 speeds up discharging the junction capacitances in the current mirror at the end of the pulse to get a crisper turnoff. However that means that the total of its Vf drop and the logic output '0' voltage must be less than Q1, Q2 Vbe. If not it wont do any good. This shouldn't be a problem for any SSI or MSI CMOS logic or MCU with reasonably robust pin drivers.
Schottky diodes are use for speed, and also in the case of D1, for low Vf drop as it + the Vbe drop of Q3 determine how close the output can get to the supply rail.
The driver stage Q3,Q4 buffers the resulting waveform, with bootstrapping to the top end of R2 (via R3,C1, with D1 letting it momentarily swing above the supply rail) to get a reasonably square rising edge rather than the slowish exponential that a plain resistive pullup would give. R5 helps isolate the driver from the MOSFET gate capacitance to allow the bootstrapping to be more effective. Q5 or Q6 turn on if there's more than about 0.6V across their base pullup/pulldown resistors which only happens if Q3 or Q4 are passing more than 6mA collector current. As the load is capacitive, with negligible DC current, this can only happen during edge transitions, and they act to dump a lot of current into the gate capacitance during each transition to speed it up.
If you want less brutally fast gate drive, simply delete D2, Q5 and Q6 and replace R4 and R6 with short circuits. You can then also increase R5 if you need to 'slug' it even further. ;)
A completely different 'Electronic LEGO' style option that would be viable for a one-off would be an isolated 1W DC-DC converter to get a floating 12V supply from your 5V logic supply, an ordinary NMOS low side gate driver IC running from the floating 12V supply (with *LOTS* of decoupling) and a fast optocoupler to get the pulse signal up to the gate driver input. Tie the positive side of the DC-DC converter output to the main supply if driving a P-MOSFET, or the negative side to the MOSFET Source if driving a N-MOSFET as a high side switch.
In all cases, if you want it to be robust, and you aren't using a 'smart' high side switch with built-in protection, you'll need a fast current sensor on the supply to the MOSFET, and logic to prematurely terminate the drive pulse if the current exceeds a threshold determined from the MOSFET's SOA graph, with a largish safety margin. It should also light an over-current indicator light. You may need to add an air-core inductor in series with the output to slow down the current rise time if its operating into a dead short, just enough for the over-current protection circuit to operate before the MOSFET is at risk. Reset the over-current trip circuit either manually or automatically between pulses.
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