Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Class D Switching Power Amplifiers and Power D/A conversion

class d amplifier current output
class d amplifier current output
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th January 2017, 06:31 PM   #1
franticDIY is offline franticDIY  Italy
diyAudio Member
 
Join Date: May 2014
Default class d amplifier current output

Hi,
i'm designing a class d amplifier with current feedback as in figure below.

currsrc.png

with the following features:
Vbus=50V
Iout programmable from 100mA to 1Arms
Vout=35Vrms max

When the load overcomes the programmed current (i.e. 1Arms) the amplifier saturates and the sinusoidal output becomes a square wave output. For example i want to transmit on 1200 ohm a 35Vrms sinusoidal voltage even though the current feedback is not locked.

So my purpose is to mantain a maximum allowable voltage on the output.

Is it possible to obtain that feature? A second feedback is needed?

Could you help me?

Thanks.

Attached Thumbnails
  Reply With Quote
Old 29th January 2017, 05:11 AM   #2
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
The feature you described is nonlinear.

You basically need 2 diodes (or transistors in common base configuration) from Rsense to integrator. (And the forgotten resistor from Vin to integrator.)

In order to vary current limit a variable gain should be inserted after Rsense.

Don't write RMS! This limit current should not be time-dependent (since you wrote square wave output) so RMS is meaningless/misleading.

The characteristics will not be perfect, limit current will not be exact, or limiting will not happen instantly.
  Reply With Quote
Old 29th January 2017, 05:56 AM   #3
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
But yes, a second feedback is needed also...
  Reply With Quote
Old 29th January 2017, 08:14 AM   #4
franticDIY is offline franticDIY  Italy
diyAudio Member
 
Join Date: May 2014
Could you explain me better how to connect 2 diodes? Do you mean OR configuration?
Shouldn't 2 diodes limit the sinusoid to only half wave?

In order to program output current I have thought to do with input signal. So for 1Vpp input sin there will be 1A output current set. I don't know if it is the right way.
Let me know.
Thanks
  Reply With Quote
Old 29th January 2017, 09:12 AM   #5
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
Then I dont know what you want.

If output current is proportional to the input voltage, then no saturation, no square wave until the voltage limit is reached, but you wrote
Quote:
When the load overcomes the programmed current (i.e. 1Arms)
This is meaningless, in case of controlled current load cannot "overcome the current". In case of a voltage controlled current generator the voltage can be limited, but this happens automatically at the power supply voltage.

Quote:
For example i want to transmit on 1200 ohm a 35Vrms sinusoidal voltage
35V/1200ohm=30 mA, which is not inside your specified 100mA and 1A limit.

Think over what you really want!
  Reply With Quote
Old 29th January 2017, 10:08 AM   #6
franticDIY is offline franticDIY  Italy
diyAudio Member
 
Join Date: May 2014
I try to explain my idea.

Vbus=50V
Iout programmable from 100mA to 1Arms
Vout=35Vrms max
PWM switching=200khz
Output frequency 3KHz-20KHz

I want to realize a voltage controlled current source.

In my application the maximum pwm duty is 90%.
So i can generate sinusoids from 50V/sqrt(2)*0.01 to 50V/sqrt(2)*0.9.

Setting gain in the proper way i want that with input signal of 2*0.85Vpp (i forgot that the triangular wave for the comparator is 2Vpp) there is a 1Arms current set on the output. So with a input signal of 2*0.085Vpp there will be a 0.1Arms set.

That approach works for LOAD higher than the maximum set. So i can regulate 1Arms on 30 OHM up to 1 OHM. But if my load becomes 300 OHM the input amplifier (integrator) saturates.

I hope this helps you to understand and advise me on the right way. Perhaps i'm forgetting something in feedback loop scheme.

Let me know.

Last edited by franticDIY; 29th January 2017 at 10:15 AM.
  Reply With Quote
Old 29th January 2017, 12:25 PM   #7
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
OFF: So you call 1 ohm higher than 30. Its OK when you explicitely state this usage of terminology, but should be avoided saying it and expecting others will understand the same way.

And inside the described circuit nothing is programmable, so in connection with it please don't say current is programmed, because it is misleading. Input signal is generated by a programmed device, this is the correct statement.

ON:
Now I understand what you need generally, but some details are missing:
- What is the freq dependancy of load impedance? Constant? Capacitive? Inductive? Variable? Unknown? The required feedbacks depend on this strongly.
- With what accuracy you want to control the current on load at different frequency?
- What ripple current/voltage is allowed?

Depending on the required accuracy and the range of load impedances this task can easily become impossible. The more you limit the possible impedances and the more deviation is allowed (especially at high freq) the more likely that it is possible.

An AC voltage feedback from output is most likely needed. More details can be given only after load specification.

Last edited by Pafi; 29th January 2017 at 12:35 PM.
  Reply With Quote
Old 29th January 2017, 12:28 PM   #8
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
(The load is essential part of the feedback loop, this is why it is very important to know.)
  Reply With Quote
Old 29th January 2017, 12:44 PM   #9
franticDIY is offline franticDIY  Italy
diyAudio Member
 
Join Date: May 2014
Sorry for misunderstanding...
At the momement consider a pure resistive load from 1ohm to open (no load).
There is no particular need of accuracy sai it 10% of error is acceptable.
  Reply With Quote
Old 29th January 2017, 02:40 PM   #10
Pafi is offline Pafi  Hungary
diyAudio Member
 
Join Date: May 2004
Location: Budapest
Unfortunately 10% for 1 ohm to 100 ohm at 20 kHz is already somewhere around (or above) the highest possible accuracy in this topology and with this operating frequency. Only if you could adaptively set control loop for the actual load then you could reach higher.

And even this limit is only possible to reach with relatively high power loss on Rsense in case of low load resistance. Is this acceptable for you?
  Reply With Quote

Reply


class d amplifier current outputHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Class D Vs Class I (Balanced Current Amplifier) rmsaudio Class D 30 23rd February 2018 02:54 PM
High voltage, low current output stage for class D amplifier ionomolo Class D 46 30th December 2017 11:35 PM
class d output current regulation franticDIY Class D 5 11th May 2014 07:16 PM
Bias current through class AB output pair Fusion916 Solid State 22 10th February 2014 09:46 AM


New To Site? Need Help?

All times are GMT. The time now is 05:30 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki