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F4 Beast with hundreds of amperes
F4 Beast with hundreds of amperes
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Old 4th March 2018, 04:31 PM   #1
woofertester is offline woofertester  United States
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Default F4 Beast with hundreds of amperes

Since I am going in a different direction than the other F4 beast builders, and I don't want to pollute what L H Quam is doing with the bias analysis, here is the front end of my "Beast with hundreds of amperes". I will journal the project in this thread.

The inputs are matched K170/J74 from Punkydawgs.

Rail voltages are +/- 24V.

There are 200 ohm pots in the drains of the input Jfets. Idss of the Jfets is about 8 mA.

The LATFETS require about 0.495v bias each for 50mA of current with the LATFETS at operating temperature. Cold, the bias voltage needed is 0.693v each. The Vgs of the LATFETS are identical at 50mA.

This initial plug-in arrangement is to learn how to set the bias voltages for proper operation.

There are some solder perfboard proto boards on order to make this mountable to the massive heat sink that holds the pucks.

There is a 1-ohm resistor between the LATFET drains to monitor current. The bias spreader for the pucks will go here in the final build.
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Last edited by woofertester; 4th March 2018 at 04:36 PM.
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Old 4th March 2018, 07:19 PM   #2
woofertester is offline woofertester  United States
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Here are some observations and measurements and changes made.

I was biasing both LATFETS at around 0.495v.
The first observation was that the K1058 was getting much hotter than the J162. Measuring voltages, almost all of the 48v of combined power supplies voltage was across the K1058.

I pulled the LATFETs and ran the Vgs test again. I tied the drain to gate and then forced 50mA through the device. Both LATFETs measured 0.693 Vgs with 50mA forced through them.

Next I performed a curve trace test biasing the D-S at 20V and then adjusting the G-S voltage for 50mA of D-S current.

The J162 requires -0.27V Vgs for -50mA at -20V and the K1058 requires 0.62V Vgs for 50mA at 20V.

Armed with that info, I adjusted the 200 ohm pots for -0.27v Vgs on the J162 and 0.62V on the K1058. This causes very close to 50 mA to flow and the K1058 is no longer hogging all of the 48v.

I was able to make a fine adjustment to the 0.62V bias pot to balance the voltages across the LATFETs.

My assumption is that the Vgs test does not bias the D-S channel into the region where it will be used. You have to test the device at the D-S voltage where it will be working to know what bias voltage to use.

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Old 4th March 2018, 09:08 PM   #3
gl is offline gl  United States
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Location: Sierra Foothills - California
A schematic would be helpful in understanding your narrative.

Graeme
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Old 4th March 2018, 10:33 PM   #4
woofertester is offline woofertester  United States
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Good point. Here it is.
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Old 4th March 2018, 11:07 PM   #5
woofertester is offline woofertester  United States
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I drew in it LTSpice. First iteration. Never done this before.
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Old 4th March 2018, 11:10 PM   #6
woofertester is offline woofertester  United States
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Visual
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Old 5th March 2018, 12:29 AM   #7
gl is offline gl  United States
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Thank you.

gl
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Old 5th March 2018, 07:59 AM   #8
Ian Macmillan is offline Ian Macmillan
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Just a small point but M1 and J2 should be flipped vertically if you wish to follow normal convention and/or wish to simulate the circuit. I appreciate that you have made the drain and source connection explicit for M1/M2 but it is usually easier and less confusing to follow convention...
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Old 5th March 2018, 12:21 PM   #9
woofertester is offline woofertester  United States
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Quote:
Originally Posted by Ian Macmillan View Post
Just a small point but M1 and J2 should be flipped vertically if you wish to follow normal convention and/or wish to simulate the circuit. I appreciate that you have made the drain and source connection explicit for M1/M2 but it is usually easier and less confusing to follow convention...
Thank you for pointing out the mistakes. This version should be correct.
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Old 5th March 2018, 01:32 PM   #10
Ian Macmillan is offline Ian Macmillan
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That looks better
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