CheapoModo: quick and dirty transformer snubber bellringer jig - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

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 5th February 2014, 01:30 AM   #1
diyAudio Member
 
Join Date: May 2011
Location: Silicon Valley
Default CheapoModo: quick and dirty transformer snubber bellringer jig

A high quality test-jig called Quasimodo has been described on diyAudio; it helps you find the optimum value of transformer snubber components without using mathematics.

As presented in that thread, Quasimodo is a no-compromise design using a state of the art vertical MOSFET with Rds(on) < 0.01 ohms, and an insanely powerful gate-driver IC with 6.0 ampere output current. It works best on a 2 layer PCB with a groundplane and careful layout.

But some people might want to slap together a snubber test-jig on their solderless breadboard ("proto board") and not worry about amps of current and milliohms of resistance. Nor do they wish to purchase "exotic" components and pay exorbitant fees for shipping. A snubber jig made of junk-box parts is more to their liking. And so I present CheapoModo, a Quasimodo whose parts and construction are cheap and cheerful.

Instead of a rail-to-rail CMOS oscillator/timer chip, CheapoModo uses the classic old graybeard NE555 chip, perhaps the highest volume IC in the world. Instead of a special Gate Driver IC, Cheapo-Modo uses a pedestrian 3904/3906 junkbox transistor pair. Instead of an exotic output MOSFET, Cheapo-Modo just plunks down 3 dirt cheap junkbox MOSFETs and accepts whatever performance degradation they introduce. The schematic is below. An LTSPICE-ready ".asc" file is also attached (inside a .zip file), making simulation rather simple.

Fool around with it. Change the component values. Put in different MOSFETs. Try some Zetex TO-92 parts. Try some IRF (or NXP!) TO-220 parts. Try different power supply voltages. Does it work at 5 volts? Does it work at 18 volts? Try it and see. What is the power consumption? How quickly will it discharge a battery power pack?

Play with different transformer secondary leakage inductances (Ltrafo). Play with different snubber resistor and capacitor values. It's only simulation, it can't hurt you, it can't hurt your computer. Play with it.

Then: change it! Optimize CheapoModo according to your definition of "optimum". Add or subtract features. Introduce or remove operating modes. Change the speed / power / cost / board_area however you wish.
Attached Images
File Type: png CheapoModo_schematic_V1.png (21.0 KB, 853 views)
File Type: png Cheapo_Waveforms.png (9.0 KB, 823 views)
Attached Files
File Type: zip CheapoModo.zip (28.0 KB, 35 views)
  Reply With Quote
Old 5th February 2014, 08:41 AM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I can see Cx and Cs at the output end.
Where are C1 and C2 and C3 that appear in the Quasimodo?
__________________
regards Andrew T.
  Reply With Quote
Old 5th February 2014, 12:56 PM   #3
diyAudio Member
 
Join Date: May 2011
Location: Silicon Valley
Perhaps the most intuitive way to grasp the similarity of the two circuits, is to work with them visually. Draw both schematics on a single sheet of paper, then encircle matching subcircuits.

To make this easy, I've attached an image file which contains both schematics, re-sized so they're approximately the same width. Open this file with Paint or another graphics editor, and make a red rectangle around the Turbo-Encabulator on each schematic. Draw green ovals around the Frammis on each schematic, and put blue rectangles around the Gaerna-Fjorms. Continue annotating like blocks with like colors.

Not wanting to ruin anybody's fun, I've put my own "solution" (with which you might disagree!) into a .zip file, so that it won't pop up as a thumbnail image and interfere with your own original thinking.

MJ
Attached Images
File Type: png two_schems_v1.png (287.4 KB, 798 views)
Attached Files
File Type: zip matching_arrows.zip (284.1 KB, 40 views)
  Reply With Quote
Old 12th February 2014, 11:40 PM   #4
diyAudio Member
 
Join Date: May 2011
Location: Silicon Valley
the unlock code for post #3's zip file, is the nine character string

diydiydiy
  Reply With Quote
Old 13th February 2014, 08:52 AM   #5
RJM1 is offline RJM1  United States
diyAudio Member
 
Join Date: Jan 2010
Location: Titusville, Fl.
Why not hit it with a pulse, not a square wave?
Attached Images
File Type: jpg schematic.JPG (60.0 KB, 676 views)
File Type: jpg output.JPG (32.7 KB, 645 views)
Attached Files
File Type: asc CheapoModo.asc (3.5 KB, 10 views)
  Reply With Quote
Old 13th February 2014, 12:06 PM   #6
diyAudio Member
 
Join Date: May 2011
Location: Silicon Valley
Quote:
Originally Posted by RJM1 View Post
Why not hit it with a pulse, not a square wave?
RJM1, I think it is not a good idea to eliminate the MOSFET output driver. Its very low Rds(on) clamps the output node to ground very tightly, preventing the active circuit from contributing any excess damping to transformer + snubber resonant circuit. See references 6 & 7 in the Quasimodo design note for more of the theory.

You can watch the post#5 circuit misbehave when resistor Rs is set to 10K, to simulate the situation where the trimmer potentiometer is removed from its socket {the first step of optimizing a CRC snubber with CheapoModo}. Plot the voltage at the collector of Q1: it whangs around in uncontrolled flailing, more than 1000 millivolts away from ground. A BJT output stage is a poor imitation of a low-valued resistor. By contrast, CheapoModo, with its three parallel dirt-cheap (and terrible spec) MOSFETs, clamps this node within 20 millivolts of ground. And Quasimodo, using an optimized low-Rds device, clamps it within 200 microvolts.

I don't think it matters very much whether you DC-couple the output stage to the oscillator (as in CheapoModo), or you AC-couple the output stage to the oscillator (as in your post#5 circuit). Just remember to set the timeconstant of the coupling capacitor(s) long enough, so the output stage doesn't reset itself prematurely. Your circuit appears to reset itself in 0.01 milliseconds (the R4-C5 timeconstant). Unfortunately this is not nearly long enough; reset needs to be delayed at least 1 millisecond after the falling edge, so you can observe phenomena like Figure 12 of the design note, reproduced below. You'll also need to adjust the NE555's duty cycle so that both the output-high and output-low segments of the oscillator waveform, are at least 1 millisecond wide.
Attached Images
File Type: png qm_design_note_fig_12.png (85.2 KB, 131 views)

Last edited by Mark Johnson; 13th February 2014 at 12:15 PM. Reason: typo
  Reply With Quote
Old 19th February 2014, 03:01 PM   #7
diyAudio Member
 
Join Date: May 2011
Location: Silicon Valley
Default Here are three other cheapo gate driver circuits

Post#1 of this thread has a gate driver subcircuit consisting of of 2 junkbox BJTs and 4 resistors. Here are three alternatives that use fewer parts and may have better performance. Simulation will disclose the strengths and weaknesses of each.

ckt#1 is (the empty set); there is no gate driver at all. The NE555 output is directly connected to 3 parallel MOSFETs' gate terminals.

ckt#2 uses a common-emitter PNP to make a damn-fast rising edge at node GATE. A resistor pulldown avoids "crowbar" current but gives leisurely falling edges. Fortunately we don't care.

ckt#3 replaces the PNP in ckt#2 with a discrete Pchannel MOSFET, saving a resistor.

Ckts 2 and 3 include a pullup resistor on the 555's output, to compensate for the fact that the NE555's output stage is a Darlington emitter follower; it doesn't pull its output pin all the way to VCC. And oh, by the way, the 555 simulation macromodel in LTSPICE doesn't model this correctly. The macromodel's output incorrectly swings rail to rail. D'oh!

Have some fun with these.
Attached Images
File Type: png ckt1.png (2.8 KB, 94 views)
File Type: png ckt2.png (7.1 KB, 76 views)
File Type: png ckt3.png (9.0 KB, 92 views)
  Reply With Quote
Old 22nd February 2014, 03:10 PM   #8
gootee is offline gootee  United States
diyAudio Member
 
Join Date: Nov 2006
Location: Indiana
Blog Entries: 1
Great ideas! Thanks!
__________________
The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg
  Reply With Quote
Old 23rd February 2014, 09:56 AM   #9
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
I can get down to 0.7us rise time (on the fast edge) and 1.6us fall time using the 3904/3906
But the very limited currents available from the complementary pair is slowing down the edges.

Omitting the 3904/6 and driving the irf510 from the 555 gets to ~1us for both rise and fall times.

I changed the timing cap from 100nF to 10nF and then to 1nF to allow my analogue scope to see and measure the fast edges.

How fast should this circuit be able to go?
How fast does it need to go?
__________________
regards Andrew T.

Last edited by AndrewT; 23rd February 2014 at 09:59 AM.
  Reply With Quote
Old 23rd February 2014, 04:32 PM   #10
diyAudio Member
 
Join Date: May 2011
Location: Silicon Valley
If anyone prefers gate driver subckt#1 ("the empty set"), but is concerned that Linear Technology doesn't make and sell NE555 chips, so LT are not motivated to make a super-accurate simulation macromodel of the 555, I think you're right to be nervous/skeptical. We've already seen that the LT macromodel swings rail-to-rail but the real NE555 decidedly does not.

Fortunately you can build a simulation of the real 555's internal guts, driving the CheapoModo output stage. Then you can compare results against the LT simulation macromodel of the 555, driving the same CheapoModo output stage. And you can judge for yourself, whether you are satisfied or dissatisfied with the degree of similarity. Your simulation results might resemble the pictures below.
Attached Images
File Type: png NE555_schematic_ST_microelectronics.png (66.2 KB, 152 views)
File Type: png LTSPICE_macromodel_of_555.png (17.1 KB, 146 views)
File Type: png xitor_level_model_of_NE555.png (16.6 KB, 77 views)
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Simple, no-math transformer snubber using Quasimodo test-jig Mark Johnson Power Supplies 476 Yesterday 07:30 AM
ECL86 - quick and dirty beamnet Tubes / Valves 44 1st April 2012 06:54 AM
Quick 'n dirty sub simulation? head_unit Subwoofers 2 29th January 2012 11:34 PM
Quick and Dirty X-over bjackson Solid State 1 16th August 2005 06:32 AM
OTL, quick `n dirty Fuling Tubes / Valves 4 27th December 2004 07:44 AM


New To Site? Need Help?

All times are GMT. The time now is 09:27 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2