FAST and SIMPLE driver for HEXFETS (linear use). - diyAudio
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Old 7th February 2011, 06:38 AM   #1
ontoaba is offline ontoaba  Indonesia
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Default FAST and SIMPLE driver for HEXFETS (linear use).

This only in simulation of course, but this may the simplest way if we need fast driver for strong error correction in hexfet amps.
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Old 7th February 2011, 07:03 AM   #2
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20KHz picture, and THD simulated is 0.5%
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Old 7th February 2011, 08:00 AM   #3
ontoaba is offline ontoaba  Indonesia
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I am trying with lower power. Better in linearity (now 0.35%THD) and still easily reach 600KHz.
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Old 7th February 2011, 09:44 AM   #4
CBS240 is offline CBS240  United States
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Hi

One thing about Hexfets, and other similar vertical fets, they are fast. The question is how much of that speed can you turn into usable BW? And how can you combat the inherent non-linear characteristics within? Simplicity is relative with these types of devices, that is if your goal is low distortion and good stability. It is certainly possible to ascertain these goals. Here is a quick example sketch I just made of a circuit similar to what I'm currently using for vertical fets. There will be some minor upgrades in the future.

First thing to get right is stability. These devices may not show in simulation their unstable nature when inserted into an actual PCB as well as their destruction due to not having a protection Zener from gate to source. A great way to keep these devices stable is with a gate LP filter instead of just a gate stopper resistor as close to the device as possible. This allows you to increase the roll off slope and gain extra BW with less phase shift. The gate stopper alone creates a LP filter with the internal capacitances of the mosfet and can dampen the parasitic but capacitance value and Fc becomes somewhat dependent on the operating point of the fet. A decoupling capacitor at the drain as close to the device as possible is also a must. At MHz frequencies in the range of resonance for these devices, the inductance effects of the device pins can become significant.
In order to drive a vertical fet at a fast rate of change in conductance, it requires charge to increase the conductance as well as to turn the device off. A suitable driver stage should bias about 20-30mA. For a source follower configuration, a drive voltage of Vd + Vgs + delta Vgs is required. The pre-driver stage shown here uses a few mA of current and the driver stage is bootstrapped from the output. Since it is impossible to find two complementary vertical fets that match in both Gm and in gate charge, the two devices will require different amounts of charge and discharge to be moved into and out of each gate so two separate paths are used here. +/- 10V is supplied by the VAS. The error amplifier transistors as well as the next driver stage use small signal RF transistors. Cascode is used because RF devices are typically <20Vceo. Thermal compensation is done by allowing the error amp transistors (the ones with the star) to be in contact with the drain pin of the respective output device. A clamp prevents overcharge of the gates when the output devices are saturated.
It is important to keep PCB traces as short as possible for the RF paths within the EC loops. This will make the EC more effective. I think Hexfets make great power output stages. They are fast and very tough if you can fix the non-linear nature of the device. And I have also found the planer stripe type vertical fets to be able to take significant amounts of abuse.
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Last edited by CBS240; 7th February 2011 at 09:46 AM.
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Old 7th February 2011, 11:05 AM   #5
ontoaba is offline ontoaba  Indonesia
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Thanks, CBS.
Your sketch is already with error correction input. I am yet pace them, may be soon.
I know about gate ringing and sometimes spice models are just unfair.

This driver are two connected, not common gate as usual to minimize gate charge/discharge when operated at high speed. I am also using this in my qiasi driver.
Gate source voltages are highly damped with Rgs.
It draw 50mA at idle for low power and more than 100mA for high power, that is why I use floating supply.
I like to use IRF540 instead of IRFP240, because I feel something wrong with the model, and not the same as its real condition. I like to compare my real results with simulation for IRF540 and always small differences in compensator. And it act different in higher voltage, that not happened in simulator.
I never trying vertical fet.

Here I post IRFP result:
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Old 7th February 2011, 01:47 PM   #6
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Hi CBS, is that you mean with vertical mosfet is DMOS?, not VMOS? but planar stripe fet is still expensive.

Simulation of its performance is finished. I am using 500uH supply inductors and 14KHz 30V input. Supply ringing very bad, and it ringing whole the circuits. This driver is very stable, not far from an emitter follower.
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Old 7th February 2011, 02:36 PM   #7
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Hello ontoaba

Would you mind telling me what software you're using. Looks quite good.

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Mike
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Old 7th February 2011, 10:18 PM   #8
ontoaba is offline ontoaba  Indonesia
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hello Mike,

It's Tina-TI 7, free version from texas instrument. You may download it at their website, or search it with google.

Regards
OA
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Old 8th February 2011, 12:52 AM   #9
CBS240 is offline CBS240  United States
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Quote:
Originally Posted by ontoaba View Post
Hi CBS, is that you mean with vertical mosfet is DMOS?, not VMOS? but planar stripe fet is still expensive.
Vertical mosfets tend to have an exponential relationship between Vgs and Gm while lateral types tend to adhere to the Square law for Vgs vs Gm function. Planer stripe diffusion mosfets such as Fairchild's Q-fet line are like this in charactoristics, and like all verticals they are designed for switching. Also they are fairly cheap, at least in the US. For example. For DC operation this TO-220 device is capable of about 25W @ 150C. Cdg and Cds is a bit greater for the planer stripe types though.

The picture illustrates the major difference in die structure between planer stripe and multi-well devices such as IR's Hexfet.
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Old 8th February 2011, 01:19 AM   #10
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Quote:
Originally Posted by ontoaba View Post
hello Mike,

It's Tina-TI 7, free version from texas instrument. You may download it at their website, or search it with google.

Regards
OA
Thanks, OA.

Still using my ancient B2SPICE. Will try out TINA.

Mike
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