Author Topic: Searching a high-side driver ic for a p-channel MOSFET, do you know any?  (Read 9455 times)

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Offline Ian.M

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #25 on: August 06, 2019, 02:02:31 am »
@Sourcecharge,
The problem with Zener clamping is excessive junction capacitance.   Its not a major problem if you are directly clamping the MOSFET gate as its almost invariably swamped by the gate capacitance, and its effect is limited by the highish drive currents required to charge/dicharge the gate, but if you need to clamp a higher impedance node earlier in the driver circuit to limit the gate voltage without excessive dissipation in the driver circuit, having what approximately looks like 75pF in parallel when you add a small 12V Zener like the BZX84C12L is *really* not helpful if you want reasonably fast transitions. 

See attached LTspice sim of Zener clamping and of simple limiting by pulse current control through a known resisto, which also has (near) ideal clamping and the same with 75pF in parallel for comparison.    It also shows the vital importance of having a low enough resistor in parallel with Zener to discharge it if the drive is unidirectional.

There are workarounds e.g keep the Zener continuously biassed to provide a clamping rail then use a fast low capacitance diode to clamp to it, but they all add significant complexity.
« Last Edit: August 06, 2019, 02:05:09 am by Ian.M »
 
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Offline JajahoTopic starter

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #26 on: August 06, 2019, 05:27:04 am »
For me here in Germany a new day of work has started and I'm just blown away by your dedication to this issue, thank you all very much.

Spezial thanks to Ian for taking the time to explain his cicuit.
The capacitance of the zener is also a thing to keep in mind.

I will take  every post here into consideration and once my circuit is done I will upload it here.


Best regards.
 

Offline JajahoTopic starter

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #27 on: August 14, 2019, 06:52:23 am »
Hello everybody it's me again, reviving this thread.

I have finfished my schematic aswell as the layout (except for some minor things ofc.)
I would really appreciate it if some of you guys could review it and give me your two Cents about it.

Thank you all in advance.
Greetings from Germany

PS: As I mentioned earlier in this thread this sucker has to switch a maximum of 10A at a maximum PWM frequency of 10kHz. Altough be it for a short duration (say maybe 1 sec), I undoubtably will need some beefy caps to support this switching action. I'm asking if anybody has any experience values he/she can recommend or alternatively an actaul way of determining the required capacitance.
Thank you very much.
« Last Edit: August 14, 2019, 07:49:18 am by Jajaho »
 

Offline JajahoTopic starter

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #28 on: August 22, 2019, 08:33:30 am »
Hello to anybody who still gives a,

I have now ordered the pcb's posted above and I am expecting them to arrive in 2-4 days.
On the other hand I am still intriged by Ian's circuit posted earlier and so I'm giving that one a shot too.
Due to a limited choice in distibuters I am forced to swap the matched 2N3904 pair for something else.
The choice I'm currently planning on going with is the BCM847BS.
I looked through the datasheet and found it to be a fitting replacement.
I wouldn'T mind if anyone could double check that however.

I'll be shure to post my post my results here.

Greetings from Germany.
 

Offline Ian.M

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #29 on: August 22, 2019, 09:35:59 am »
Matching is fairly non-critical, so you don't need a precision matched pair.  All parts in my design were fairly generic so there shouldn't be any problems with substituting broadly similar transistors.  The BCM847BS Vceo and Ic ratings look suitable.  At 32V, in my circuit, it will dissipate 100mW during the PWM on time, which is well within its ratings.   and its certainly preferable to hand selecting individual 2N3904 or similar to match them then having to improvise some sort of thermal coupling for the pair.

N.B. it absolutely needs *GOOD* decoupling, as close as possible to the complimentary driver stage collectors and the output stage emitters.  Remember, its trying to throw sub 1us 800mA current pulses around (if you don't tame it by deleting the output stage).

Edit: *DON'T* attempt to decouple the driver collectors as that would FUBAR the output stage base drive!
« Last Edit: August 23, 2019, 08:34:32 pm by Ian.M »
 
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Offline Dubbie

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #30 on: August 22, 2019, 10:05:03 am »
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!
 
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Offline JajahoTopic starter

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #31 on: August 22, 2019, 11:36:18 am »
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.
 

Offline JajahoTopic starter

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #32 on: August 23, 2019, 06:36:56 am »
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
 

Online IconicPCB

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #33 on: August 23, 2019, 06:49:44 am »
Would someone care to walk me through the schematic (as shown in the PDF file above)?
 

Offline JajahoTopic starter

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #34 on: August 23, 2019, 06:57:40 am »
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. ;)

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.
 

Offline Ian.M

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #35 on: August 24, 2019, 07:15:31 am »
Please note edit to reply #29

All component references in this post refer to the schematic from reply #32, not those from my sim.

It looks basically sane, but I'd tighten up the layout as much as possible to reduce trace lengths and thus stray capacitance and loop area.    I'd also beef up the collector and emitter traces to Q3,Q4 (and out to the gate) even to the point of having to neck them down to transition to the pad.   It would also be beneficial to reduce the length of the gate trace to the MOSFET.   I'd also put a footprint for an optional series gate resistor or ferrite bead in case the MOSFET breaks into HF oscillation while sweeping through its linear region on each transition.  Bridge it with a trace you can cut, or populate with a zero ohm jumper.

Also the large hole in the ground plane is problematic, and likely to seriously increase its radiated emissions.  Maybe re-do with most of the MOSFET driver circuit traces on the other side, but be careful to avoid long slots in the ground plane.  More vias are definitely preferable to long slots.   The only place you may *WANT* a hole in the ground plane is under the node driving the bases of Q1,Q2, where stray capacitance is very detrimental to the rise and fall time.

On the subject of decoupling.  Its somewhat of a black art* unless you can consider the effects of all the parasitics involved (both internal to the caps and trace inductance and resistance) in detail.  Unfortunately I don't have a good reference for a tutorial that will get you up to speed with high current HF and pulse circuit decoupling as I learnt what little I know of the art# well before internet tutorials.   Minimising loop area is key to keeping trace inductance under control.   

Its desirable for all the gate charge required to switch the MOSFET to come from the nearest decoupling capacitors to the emitter of Q4 without significant voltage shift at the emitter.   If the capacitance was numerically equal to the total gate charge, with no other decoupling, you'd get a 1V + I*ESR voltage shift at the emitter.  You probably want far less than that, so try an order of magnitude more capacitance.  You may have to split it into multiple capacitors to reduce ESL.   Minimise track length to the decoupling and if on the opposite side to the ground plane, use two vias for each decoupling cap ground pad to reduce the effect of the via inductance.

* far less so nowadays, as simming the parasitics involved can be done in a few minutes in a SPICE program  on your desktop without deep knowledge of mathematical programming rather than having to write a Fortran program to model the network and submit it as a batch job to a mainframe. 

# Others here are far better equipped to advise on it, as I don't have the benefit of recent hands-on experience in a well equipped lab with a VNA and fast enough DSO to make quantitative measurements of the effects of apparently minor chances to decoupling layout and component choice.  The principles are the same, but I haven't kept up with the effects of ever-shrinking SMD construction on its practical implementation.
« Last Edit: August 25, 2019, 07:53:00 am by Ian.M »
 
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Offline JajahoTopic starter

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Re: Searching a high-side driver ic for a p-channel MOSFET, do you know any?
« Reply #36 on: August 25, 2019, 07:47:56 am »
Thank you very much, Ian. I'll be sure to check it out as soon as I'm back in the lab.
 


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