DMOSFET for High Side drive

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I have an application that requires 8 channels of 1v to 12v at about 30 ma max.  This voltage is "settable" and varies slowly over time in response to process conditions.  This must be "high side" drive.

 

My attempt was using a  TLC59116 (Datasheet) to drive a TBD62783AFNG  8-channel DMOSFET gate array (Datasheet) but I discovered that the turn-off spec on the TBD62783AFNG was 2 usec and the (fixed) PWM frequency of the TLC59116 (97-kHz) meant that I had very little usable range of the PWM signal.

 

I selected the TLC59116 because the (ATMega1284P) application firmware already has support for this chip that is used elsewhere on the board.

 

I don't have any pins left over to 'roll my own' PWM.

 

I could switch out the TLC59116 for a PCA9685 (Datasheet) which has a PWM frequency that is settable from 24 Hz to 1526 Hz but I would like to avoid that due to requiring an additional support module in the firmware.

 

I looked for other gate arrays in the 'TBD' sequence but the others were either no faster or were for 'low side' drive applications which won't work here.

 

I would like to keep this to a two-chip solution due to board real-estate.

 

Does anyone know of a faster, high-side gate array I could use and retain the TLC59116?

 

It's ok if it's inverting or not because I can account for that in my PWM setting.

 

Thanks in advance for any insight you might offer ...

 

Chuck

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requires 8 channels of 1v to 12v at about 30 ma max.

 

The first chip you mention is a sink driver (gnd side driven), so it doesn't even seem to fit your statement (it is not a high side driver).  Then you jump in to pwm??/

 

Why not simply use a small 8 output dac?  You can easily wire the outputs to emitter followers & drive hundreds of milliamps per channel (not very efficiently, since it is a linear drive).  But at 30ma, not goona be a lot of heat or wasted power per chan.

 

Of course if the output is moving slowly, you can easily set up a timer irq (say 25KHz) & use some counter variables  to toggle individual i/o pins as a multichannel pwm...then simply RC filter down to  maybe 10Hz   & buffer everything using only 2 quad opamps set for a gain of 3 (to get from 5V to 15v output range).

 

 

I don't have any pins left over to 'roll my own' PWM.

 

 

Not sure what you have free exactly....maybe just stick with trying a DAC. 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Aug 14, 2019 - 02:33 AM
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ChuckH wrote:

I have an application that requires 8 channels of 1v to 12v at about 30 ma max.  This voltage is "settable" and varies slowly over time in response to process conditions.  This must be "high side" drive.

 

Does anyone know of a faster, high-side gate array I could use and retain the TLC59116?

 

That sort of Hi-Side driver, is not designed for speed.

What are you driving with this - does it need very fast PWM ?

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avrcandies wrote:

The first chip you mention is a sink driver (gnd side driven), so it doesn't even seem to fit your statement (it is not a high side driver). 

 

Yes, it's used to drive the DMOSFET from the low side but the DMOSFET output is high side..

 

avrcandies wrote:

Why not simply use a small 8 output dac?  ....

 

Had not considered that - something like a DAC5578SPW has an I2C interface like the LED driver I'm using now so no more pins needed.  

 

One reason I was using the PWM was its efficiency.

 

I may see if I can find an I2C high-side PWM LED driver but I fear they are all low-side, but, then again, I can use the other low-side LED driver I mentioned that has the selectable PWM frequency so that I can live with the slower response of the DMOSFET.

 

Chuck

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Who-me wrote:

That sort of Hi-Side driver, is not designed for speed.

What are you driving with this - does it need very fast PWM ?

 

This is for setting the rail bias voltage for automatic train signals for ride-on railroads (My Web Site).

 

I need to vary the track bias voltage as environmental conditions change.  Wet track has more leakage lowering the track voltage and reducing the signal-to-noise ratio which can lead to "false occupancy" signals.  The track voltage is kept to a minimum for several reasons, among them: reduce rail and tie screw corrosion and keep power consumption down.

 

With my current signal controller this is done by manually selecting a resistor that works in the wettest conditions but this is tedious for the customer and causes excess power usage and corrosion.

 

The new version of my signal controller will do this automatically by monitoring the track voltages and increasing the bias voltage if it falls too low.  It will reduce the bias voltage as the track dries out and the (unoccupied) track voltage rises.

 

So the bias voltage I generate changes very slowly.

 

The PWM speed needs to just be fast enough that it can be integrated by reasonably small capacitors (RC filter) to keep 'ripple' from being detected as a track shunt (occupied rail).  The rail input goes through a conditioning network and then to an ADC input on the ATMega1284P which then uses an algorithm to eliminate the effect of wet/dry track, debris on the track (noise), etc.  There is more to it but that's the thumbnail version ...

 

At the moment it looks like my best move is to change to keep the DMOSFET and switch the FET driver to something like the PCA9685 with a slower PWM frequency but I'm still evaluating alternatives.

 

Chuck

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Yes, it's used to drive the DMOSFET from the low side but the DMOSFET output is high side.

Ok, now makes more sense.  

 

A dac with a buffer (see below) is your best bet...generate any voltage you want on the high side (sourcing).  If you find enough spare pins & slow voltage is ok, you can make multipwms & filter & buffer (basically save a dac chip).    

Assume roughly 12V worst case drop   12x0.03*8= 2.88 Watt...that's a lot of heat.  If Vcc=15V, even worse. This will require some power components at the final stage (if using linear output).  A switcher is more efficient, but 8 of them is "blah"in terms of parts & cost.

 

Could tie the dac into control (adj) pin of 8 small (or 4 dual) linear regulators..the dac then tweaks their output voltages. Let them take the heat & be dac controlled....they also are thermal/short protected.

 

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Aug 14, 2019 - 05:44 AM
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avrcandies wrote:

A dac with a buffer (see below) is your best bet...generate any voltage you want on the high side (sourcing).  If you find enough spare pins & slow voltage is ok, you can make multipwms & filter & buffer (basically save a dac chip).    

Assume roughly 12V worst case drop   12x0.03*8= 2.88 Watt...that's a lot of heat.  If Vcc=15V, even worse. This will require some power components at the final stage (if using linear output).  A switcher is more efficient, but 8 of them is "blah"in terms of parts & cost.

 

Could tie the dac into control (adj) pin of 8 small (or 4 dual) linear regulators..the dac then tweaks their output voltages. Let them take the heat & be dac controlled....they also are thermal/short protected.

 

I do not have any pins for generating my own PWM.  I use the I2C bus to drive the PWM driver.

 

I want to stay with PWM because power consumption is an issue.  One installation currently has power & data bus runs >2,000 feet, 28v @ 1.2A to power all 19 signal controllers, 60 LED signal heads, and 100 sampled tracks.

 

I may have found something that will allow me to stay with the PWM driver that is currently on the board.

 

BSS84 P-CHANNEL ENHANCEMENT MODE MOSFET  (datasheet)

 

in place of the 8-channel MOSFET I have now.

 

Anyone have other P-Channel FETs that have a voltage rating ~50v, a current capacity of about 20 ma and turn-on/turn-off times in the sub-us range?  (I'm new to FET selection ...)

 

The operating voltage is at 12v but I want the margin to (along with other surge suppression components) survive nearby lightning, etc.

 

Not worried about short-circuit protection because there is a 1k ballast resistor in the circuit.

 

Chuck

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BSS84  ---I was looking to see how many milliohms this fet is rated at  (100 milliohms these days is rather high, unless rated for hundreds of volts)...but this fet is rated as 10000 milliohms.....hard to imagine one rated for more!! What a terrible part today, its the equiv of a 16 byte memory.  Spend 10 cents and get one creating less loss.  Of course, its limited drive might mean it won't deliver a punch under short circuits (instead the fet will get hot and act like a 10 ohm resistor).

 

Now if you get fets as part of a packaged chip functionality, those internal fets are often annoyingly high Rds.

 

here's some that might be good for your purpose:   Note that the Rds  will affect the PWM voltage accuracy (not sure how tight is needed)

https://www.renesas.com/us/en/www/doc/datasheet/el7457.pdf

http://ww1.microchip.com/downloads/en/DeviceDoc/MCP14A0303_4_5-Data-Sheet-20006046A.pdf

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Mon. Aug 19, 2019 - 03:14 AM
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avrcandies wrote:

here's some that might be good for your purpose:   Note that the Rds  will affect the PWM voltage accuracy (not sure how tight is needed)

https://www.renesas.com/us/en/www/doc/datasheet/el7457.pdf

http://ww1.microchip.com/downloads/en/DeviceDoc/MCP14A0303_4_5-Data-Sheet-20006046A.pdf

 

(Since I'm new at FET selection) I missed the 10 ohm on resistance.  But, since, in my case, there is a 1k current-limiting resistor between the 'driver' and the load is this really a concern?

 

Assuming 12v and a perfect (0-ohm) FET the max current is 12 ma.  With the 10-ohm FET we are talking max current of 12/1010 = 11.88 ma and 11.88 ma * 10 = 0.1188 volt drop across the FET.  Did I miss something?

 

The PWM voltage accuracy is not a concern in this application and I can adapt to a reasonable fluctuation/inaccuracy.

 

The parts you mention have an 18v rating which does not leave a lot of headroom when there may be shoot-through in the surge suppression circuit between the 1k R and the 'outside world'.  The 50v rating of the part I mentioned gives me more comfort.

 

Also, these parts appear to be pull-up/pull-down where I only want pull-up because, in my application, it basically comes down to the fact that I am measuring the load current/resistance by monitoring the voltage at the junction of the output side of the 1k resistor (also a small cap to Gnd, thus RC) - i.e.: the 'load' voltage.

 

Chuck

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ChuckH wrote:
(Since I'm new at FET selection) I missed the 10 ohm on resistance.  But, since, in my case, there is a 1k current-limiting resistor between the 'driver' and the load is this really a concern?

 

Depends also on how fast you want to slow things down. 470ohm could work between the pin and the FET down to 22ohm..

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18v rating which does not leave a lot of headroom when there may be shoot-through in the surge suppression circuit between

That's true, though the fet will act like a zener if overvoltaged & prob ok if the source of the overvoltage is low powerlimited.    There's plenty of fets that have low ohms, like 0.1 & high voltage...the your pwm accuracy is not mushed.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!