| Electronics > Projects, Designs, and Technical Stuff |
| Searching a high-side driver ic for a p-channel MOSFET, do you know any? |
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| Ian.M:
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. |
| langwadt:
--- Quote from: Jajaho on August 05, 2019, 02:21:10 pm ---Hello guys, in order to shine some light on the task at hand I want to explain what I am actually doing. The pcb I'm designing is for use in a testing/evaluation laboritory. We are testing valves/clutches. In this context they use a technique called "peak and hold" where first you drive the valve at full power (which could be up to 10A) in order to bring it in position. After this first phase the duty cycle is reduced to say 30% -40% in order to hold it in place. --- End quote --- with PWM and those current you'll need a beefy flyback diode, I'd consider a halfbridge instead |
| sourcecharge:
--- Quote from: Jajaho on August 05, 2019, 08:48:46 am ---Thank you all very much for taking time out of your day to answer my question. The MOSFET I was planning on using and that I already have on hand is this AUIRF4905 (IRF not Infineon as I preveously stated ofcourse :palm:). I also drew up this driver cicuit with a push pull configuration. The Problem with that is my wide range of supply voltages (from 8 - 32V) So when I set the turn on gate voltage to be 1/2 Vcc in order to not damage the gate at a supply voltage of 32V this comes around and bites me in the ass when I only have a Vcc of 8V as it wouldn't turn on the MOSFET all the way. I guess I could use a zener to clamp the gate voltage to the suppply, but I'm not so sure about that. I will now look into smart switches (those are these Infineon thingies, right?) and using a N-channel FET. Again thank you all for your suggestions so far. If you have any other Ideas be sure to let me know. Greetings from Germany --- End quote --- The only thing you have to worry about is the current through the zener. The above schematic works fine up to about 50khz. The design has only the limitations of the components. You don't have to use the irf9530 pch mosfet or the 2n7000 nch mosfets..any mosfets will work, but their limits determine the limit of your voltage output. The two resistors can be decreased to speed up the signal to about 10 Mhz, if your mosfets can go that high. The only thing is that the current through the resistors and the zener is increased, so approprate wattage resistance above the nch mosfet is required. Lastly, zeners can be paralleled to increase their overall current capacity. There is nothing as simple and as variable as this design, good luck though, and I hope you find what you are looking for, but I'm not sure why everyone has such a hesitation with using zeners. |
| AndersJ:
I’m on a iPad, no LTspice in sight. Can someone attach a picture of Ians circuit? |
| Mechatrommer:
while Ian.M circuit is more featurefull, attached can be a simpler bjt push pull driver that can maintain -5V Vgs down to 6Vs using 15V zener clamp (shown Vgs dark red traces for Vs 6V and 50V). suggestion welcomed i may have missed something, its a 10 minutes sketch, zener current @ 50Vs should be around 50mA. ymmv. --- Quote from: AndersJ on August 05, 2019, 11:35:15 pm ---I’m on a iPad, no LTspice in sight. Can someone attach a picture of Ians circuit? --- End quote --- see last attachment. |
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