lossless overcurrent protection in UcD scheme

zilog · 14th December 2006, 09:37 AM

I have simulated and also tested this circuit, and it seems to work. Do you have any opinions regarding the distortion from the added gate capacitance versus the distortion added by the regularly employed resistive current sensing elements?

The circuit below is to be used with a UcD style modulator, only the power stage and protection is shown here. On/Off is modulated by pulling the grounding of the CCS in the modulator differential pair.http://zilog.dyndns.org:8080/lagenhe...ssless_Ids.png

The current is sensed by voltage drop across the sensing mosfet. leading and trailing false readings are blanked (the mosfets switch simultaneously so I blank for both mosfet:s leading edges in the final measurement).

zilog · 16th December 2006, 02:45 AM

anyone?

RX5 · 16th December 2006, 11:55 AM

I guess no one wants to delve in the "theoretical" and just wants the easy way... finished AND working schemo...

I feel you too.... keep it up...

Genomerics · 16th December 2006, 03:25 PM

Errrr, well. Don't get me wrong but at first glance it's a bit tricky to figure out what is going on and then it does not seem to do what I thought it might be doing.

It does initially look like what you are trying to do is implement something like the HexSense method provided by IR in a range of their mosfets whereby part of the main mosfet is re-connected to provide a sampled ratio of the overall current. You are adding the extra device plus resistance to do it (but I might be wrong).

Problem is that in order to get the right answer it is recommended that the sampled current sensed through the summing junction of an operational amplifier, if the sensing ratio is to be maintained. If you don't then you get errors. Unfortunately, in your case, you haven't got one of those.

Anyway, that was my first guess. As you have things set up all your additional (sense?) mosfet is doing is sampling the voltage across the main mosfet at the 'correct' time so you are using the RDSon of the main mosfet as your current sense resistor.

The rest of it is blanking and level shifting.

How about this one.......?

limbasic.gif

It still senses the voltage drop across the main mosfet using the Vbe of Q1 and Q3. However Q2 and Q4 are used with their resistors to, sort of, disable sensing when the associated mosfet is not active based on what the main output node is doing.

LIMH and LIML end up being ground referenced, with the diodes saving the day, but will need a little bit of processing. You can do the blanking downstairs as well.

Actually it looks a bit flakey to me, more thinking required. However it's not too different to what you have at the moment?

DNA

zilog · 16th December 2006, 03:39 PM

Genomerics,

Why doesnt my circuit work as intended? Rds(on) of the mosfets increase with temperature, and allow for earlier tripping if the mosfets are too hot.

zilog · 16th December 2006, 04:16 PM

Here is the modulator board to go with the power board, note that the connector is reversed to allow vertical docking.

http://zilog.dyndns.org:8080/lagenheten/modulator.png

Genomerics · 16th December 2006, 05:45 PM

Quote:

Originally posted by zilog
Genomerics,

Why doesnt my circuit work as intended? Rds(on) of the mosfets increase with temperature, and allow for earlier tripping if the mosfets are too hot.

Perhaps I have misunderstood what I thought you were trying to achieve in the first place.

It's just that your description and the circuit looked like an attempt to do something similar to what IR's HexSense mosfets do.... However, looking at the circuit, it doesn't.

If you were not thinking of that in the first place, and you just wanted to use the main mosfet RDSon as a current sense resistor, then mine might do what yours does in a simpler way.

limaddeda.gif

Gets me down to ground and gives a preliminary 'Oh Sh1t' pulse

Maybe....

DNA

TOINO · 16th December 2006, 09:52 PM

Hi Zilog

Here you have a commercial working implementation of your idea.
Is very efficient as far as I know…

Workhorse · 17th December 2006, 04:41 AM

Its a Powersoft Class-d amplifier Toino.....very good illustration....

How about using a Current Sensing transformer Derived from output Filter Inductor....It could also be made to work for Overcurrent protection .....

K a n w a r

TOINO · 17th December 2006, 06:21 AM

Like QSC PLX series power supply... they sense current on the resonant coil.
Same models sense also trough the LF output coil.
It is maybe not very precise but is enough for short circuit detection.

14th December 2006, 09:37 AM	#1
zilog diyAudio Member Join Date: Oct 2005	lossless overcurrent protection in UcD scheme I have simulated and also tested this circuit, and it seems to work. Do you have any opinions regarding the distortion from the added gate capacitance versus the distortion added by the regularly employed resistive current sensing elements? The circuit below is to be used with a UcD style modulator, only the power stage and protection is shown here. On/Off is modulated by pulling the grounding of the CCS in the modulator differential pair.http://zilog.dyndns.org:8080/lagenhe...ssless_Ids.png The current is sensed by voltage drop across the sensing mosfet. leading and trailing false readings are blanked (the mosfets switch simultaneously so I blank for both mosfet:s leading edges in the final measurement).

16th December 2006, 11:55 AM	#3
RX5 diyAudio Member Join Date: Apr 2004 Location: TaC	I guess no one wants to delve in the "theoretical" and just wants the easy way... finished AND working schemo... I feel you too.... keep it up... __________________ RoCk On!!! I love my diy UcD!!

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
xtant X603E - Overcurrent Light	bdtrhc	Car Audio	0	5th May 2009 03:19 AM
Overcurrent, overheat and overvoltage protection for TL494	Janez	Power Supplies	17	21st December 2007 02:36 PM
Short circuit output protection scheme	tool49	Solid State	6	14th November 2005 07:57 AM
output FET overcurrent limiting	zilog	Class D	11	4th November 2005 12:14 PM
Overcurrent sensing using a comparitor	fr0st	Parts	9	18th June 2003 08:49 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

16th December 2006, 02:45 AM	#2
zilog diyAudio Member Join Date: Oct 2005	anyone?

16th December 2006, 03:25 PM	#4
Genomerics Account Disabled Join Date: Oct 2004	Errrr, well. Don't get me wrong but at first glance it's a bit tricky to figure out what is going on and then it does not seem to do what I thought it might be doing. It does initially look like what you are trying to do is implement something like the HexSense method provided by IR in a range of their mosfets whereby part of the main mosfet is re-connected to provide a sampled ratio of the overall current. You are adding the extra device plus resistance to do it (but I might be wrong). Problem is that in order to get the right answer it is recommended that the sampled current sensed through the summing junction of an operational amplifier, if the sensing ratio is to be maintained. If you don't then you get errors. Unfortunately, in your case, you haven't got one of those. Anyway, that was my first guess. As you have things set up all your additional (sense?) mosfet is doing is sampling the voltage across the main mosfet at the 'correct' time so you are using the RDSon of the main mosfet as your current sense resistor. The rest of it is blanking and level shifting. How about this one.......? limbasic.gif It still senses the voltage drop across the main mosfet using the Vbe of Q1 and Q3. However Q2 and Q4 are used with their resistors to, sort of, disable sensing when the associated mosfet is not active based on what the main output node is doing. LIMH and LIML end up being ground referenced, with the diodes saving the day, but will need a little bit of processing. You can do the blanking downstairs as well. Actually it looks a bit flakey to me, more thinking required. However it's not too different to what you have at the moment? DNA

16th December 2006, 03:39 PM	#5
zilog diyAudio Member Join Date: Oct 2005	Genomerics, Why doesnt my circuit work as intended? Rds(on) of the mosfets increase with temperature, and allow for earlier tripping if the mosfets are too hot.

16th December 2006, 04:16 PM	#6
zilog diyAudio Member Join Date: Oct 2005	Here is the modulator board to go with the power board, note that the connector is reversed to allow vertical docking. http://zilog.dyndns.org:8080/lagenheten/modulator.png

17th December 2006, 04:41 AM	#9
Workhorse diyAudio Member Join Date: Dec 2004	Its a Powersoft Class-d amplifier Toino.....very good illustration.... How about using a Current Sensing transformer Derived from output Filter Inductor....It could also be made to work for Overcurrent protection ..... K a n w a r

17th December 2006, 06:21 AM	#10
TOINO diyAudio Member Join Date: Oct 2004 Location: Coimbra	Like QSC PLX series power supply... they sense current on the resonant coil. Same models sense also trough the LF output coil. It is maybe not very precise but is enough for short circuit detection.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)