AVR-Chat

Hello guys, I hope to get inputs. i am doing a a power supply card. 
input is 80V, output is variable from 0-76V (up to 500mA). i want my 
AVR to set the ouput voltage. i came accross high voltage variable 
regulators such as LM5008(national) and TL783 (TI). But outputs are 
controlled by variable resistors. Still thinking how to adjust it 
using MCU. Also im thinking of using a FET. Then using avr, adc, and 
opamp to set bias voltage on the gate terminal. But again problem 
arise when for example the output is set to 10V, means 70V drop across 
the FET. This will generate much heat. Anyone?

On Nov 18, 2008, at 8:01 PM, magzky02 wrote:

>
> Hello guys, I hope to get inputs. i am doing a a power supply card.
> input is 80V, output is variable from 0-76V (up to 500mA). i want my
> AVR to set the ouput voltage. i came accross high voltage variable
> regulators such as LM5008(national) and TL783 (TI). But outputs are
> controlled by variable resistors. Still thinking how to adjust it
> using MCU. Also im thinking of using a FET. Then using avr, adc, and
> opamp to set bias voltage on the gate terminal. But again problem
> arise when for example the output is set to 10V, means 70V drop across
> the FET. This will generate much heat. Anyone?
>
>
> 
That means LOTs  of heat! Suppose the input is 80V, output 50V, load  
current 500ma. Voltage difference is 30V. Power dissipation in the  
linear pass device is 30V*0.5A=15W. That, friend, is a LOT of power  
and a LOT of heat.  Suppose output is 20V and current is 500ma.  
Voltage drop is 60V. Dissipated power in a linear pass device is 30W.  
That is even more than a LOT of heat.

Either you will need a VERY big heat sink or you will need a high  
efficiency switch-mode regulator.

Jim



[Non-text portions of this message have been removed]

Yes, Thanks for response. switch regulator seems to be a good solution. i googled and came to the switch regulator LM5008. I have no 1st hand experience in switch regulator. This 8 pin SMT  IC is capable handling 70V drop at 300mA? Datasheet says efficiency is about 82% at 80Vin 300mA. Will order the part and test and see what happens:)
 
regards

--- On Wed, 11/19/08, Jim Wagner <wagnerj@proaxis.com> wrote:

From: Jim Wagner <wagnerj@proaxis.com>
Subject: Re: [AVR-Chat] MCU controlled ouput voltage
To: AVR-Chat@yahoogroups.com
Date: Wednesday, November 19, 2008, 1:36 PM







On Nov 18, 2008, at 8:01 PM, magzky02 wrote:

>
> Hello guys, I hope to get inputs. i am doing a a power supply card.
> input is 80V, output is variable from 0-76V (up to 500mA). i want my
> AVR to set the ouput voltage. i came accross high voltage variable
> regulators such as LM5008(national) and TL783 (TI). But outputs are
> controlled by variable resistors. Still thinking how to adjust it
> using MCU. Also im thinking of using a FET. Then using avr, adc, and
> opamp to set bias voltage on the gate terminal. But again problem
> arise when for example the output is set to 10V, means 70V drop across
> the FET. This will generate much heat. Anyone?
>
>
> 
That means LOTs of heat! Suppose the input is 80V, output 50V, load 
current 500ma. Voltage difference is 30V. Power dissipation in the 
linear pass device is 30V*0.5A=15W. That, friend, is a LOT of power 
and a LOT of heat. Suppose output is 20V and current is 500ma. 
Voltage drop is 60V. Dissipated power in a linear pass device is 30W. 
That is even more than a LOT of heat.

Either you will need a VERY big heat sink or you will need a high 
efficiency switch-mode regulator.

Jim

[Non-text portions of this message have been removed]

 














      

[Non-text portions of this message have been removed]

On Wed, Nov 19, 2008 at 4:30 AM, mago Umandam <magzky02@yahoo.com> wrote:
> Yes, Thanks for response. switch regulator seems to be a good solution. i googled and came to the switch regulator LM5008. I have no 1st hand experience in switch regulator. This 8 pin SMT  IC is capable handling 70V drop at 300mA? Datasheet says efficiency is about 82% at 80Vin 300mA. Will order the part and test and see what happens:)

That's not the only solution..

You could implement a switching pre-regulator, to a volt or so above
the linear reg's dropout, and use the linear to de-noise the SMPS
output.
You can select the SMPS operating frequency to fall at the point of
your linear reg's maximum point.

In a lot of bench supplies, they use a relay to switch the line
transformer between two or more output voltages as you change the
voltage setting.

Same idea, keep the dissipation reasonable, but use linear regulation
for easy low noise output.

> On Wed, Nov 19, 2008 at 4:30 AM, mago Umandam <magzky02@yahoo.com> wrote:
>> Yes, Thanks for response. switch regulator seems to be a good solution.
>> i googled and came to the switch regulator LM5008. I have no 1st hand
>> experience in switch regulator. This 8 pin SMT  IC is capable handling
>> 70V drop at 300mA? Datasheet says efficiency is about 82% at 80Vin
>> 300mA. Will order the part and test and see what happens:)
>
> That's not the only solution..
>
> You could implement a switching pre-regulator, to a volt or so above
> the linear reg's dropout, and use the linear to de-noise the SMPS
> output.
> You can select the SMPS operating frequency to fall at the point of
> your linear reg's maximum point.
>
> In a lot of bench supplies, they use a relay to switch the line
> transformer between two or more output voltages as you change the
> voltage setting.
>
> Same idea, keep the dissipation reasonable, but use linear regulation
> for easy low noise output.
>

Be aware that NO regulator, linear or switcher, that I know of, will
regulate down to zero volts. Almost all are limited by the reference
voltage as the minimum output value.

Dave's suggestion of a switcher followed by a linear is a good one if you
want to minimize noise. It  certainly adds complexity and keeping the the
output of the pre-regulator within the "right" range above the output
regulator is a challenge. Whether or not it is worth the effort is up to
you to decide.

Controlling a regulator with a micro is not simple. There are some schemes
that treat the regulator like an op-amp and the feedback input like the
inverting input of the op-amp. You can inject current through a resistor
to that node to shift the output voltage and that current can come from a
DAC. Or, if you design things correctly, the "resistor" can be THE DAC.

Jim Wagner
Oregon Research Electronics

I gave up the idea of setting the voltage using the MCU. I decided to use a potentiometer and monitor the output voltage. the linear regulator after the SMPS maybe good idea but it may not be applicable to high voltage settings. Maybe only use Inductors and capacitors and maybe ferrite bead to denoise the output voltage.

--- On Thu, 11/20/08, wagnerj@proaxis.com <wagnerj@proaxis.com> wrote:

From: wagnerj@proaxis.com <wagnerj@proaxis.com>
Subject: Re: [AVR-Chat] MCU controlled ouput voltage
To: AVR-Chat@yahoogroups.com
Date: Thursday, November 20, 2008, 8:52 AM






> On Wed, Nov 19, 2008 at 4:30 AM, mago Umandam <magzky02@yahoo. com> wrote:
>> Yes, Thanks for response. switch regulator seems to be a good solution.
>> i googled and came to the switch regulator LM5008. I have no 1st hand
>> experience in switch regulator. This 8 pin SMT IC is capable handling
>> 70V drop at 300mA? Datasheet says efficiency is about 82% at 80Vin
>> 300mA. Will order the part and test and see what happens:)
>
> That's not the only solution..
>
> You could implement a switching pre-regulator, to a volt or so above
> the linear reg's dropout, and use the linear to de-noise the SMPS
> output.
> You can select the SMPS operating frequency to fall at the point of
> your linear reg's maximum point.
>
> In a lot of bench supplies, they use a relay to switch the line
> transformer between two or more output voltages as you change the
> voltage setting.
>
> Same idea, keep the dissipation reasonable, but use linear regulation
> for easy low noise output.
>

Be aware that NO regulator, linear or switcher, that I know of, will
regulate down to zero volts. Almost all are limited by the reference
voltage as the minimum output value.

Dave's suggestion of a switcher followed by a linear is a good one if you
want to minimize noise. It certainly adds complexity and keeping the the
output of the pre-regulator within the "right" range above the output
regulator is a challenge. Whether or not it is worth the effort is up to
you to decide.

Controlling a regulator with a micro is not simple. There are some schemes
that treat the regulator like an op-amp and the feedback input like the
inverting input of the op-amp. You can inject current through a resistor
to that node to shift the output voltage and that current can come from a
DAC. Or, if you design things correctly, the "resistor" can be THE DAC.

Jim Wagner
Oregon Research Electronics

 














      

[Non-text portions of this message have been removed]