Electronics > Repair
Ampeg PF350 - IRS20957s no oscillation
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Chris56000:
Evening Dave!

. . .Here are some more circuits for you to refer to – the first is a Behringher B215 that uses the same circuit and the International Rectifier IRS20957s driver IC Reference Design for this device that goes into a lot of detail into its operation!

Chris Williams
Mojoampguy:
Thanks Chris 56000. I’ll check these out.
Chris56000:
Morning Dave!

. . .I have been studying the I.R. Reference Design Books, and in the light of the experiences you have been seeing with your Ampeg and the other similar units also giving you trouble, I can come to no other conclusion that the output  MOSFETs may be fakes or otherwise sub–standard that don't have a sufficiently low dynamic "on" resistance!

It only takes a few microseconds at the most for the internal excess current protection to sense this, which it does by comparing the Vds of the conducting MOSFET with the voltage set on the OCP set pin, and if the Vds is higher than that permitted by the OCP comparator within the '20957, the IC will switch off!

. . .There are two ways to prove whether your MOSFETs are at fault, unfortunately both methods involve some expenditure but that will be repaid by the time saved from an eventual successful resolution!

. . .The first method is to order the recommended MOSFETs used in the I.R. Reference Designs from Mouser, Digi–Key or RS, OR a direct equivalent to them – Ciss, Qgs and Rds (on) MUST all match the specifications given for those used in the I.R. Reference Design ;

. . .The second method is to invest in one of these :–

https://www.ebay.co.uk/itm/275400928273?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=oon53xzNRgC&sssrc=4429486&ssuid=qNpFa7hUTgy&var=&widget_ver=artemis&media=COPY

. . .and set it up using two external supplies from another source, then try fitting the MOSFETs you've already fitted in your Ampeg & other faulty units – if this eBay test board fails to start up on these MOSFETs, you will have proved the replacement MOSFETs you previously ordered were substandard or fake!

Chris Williams
Mojoampguy:
Struggling to appreciate all points in this design note -

One question it did raise for me was, “Maybe the problem is OVer Voltage Protection being triggered”,  as the schematic shows +/-55v dc for supply and I’m reading +/-72v. I’d previously postulated that this was maybe due to Lack of of Mosfet Load???

Maybe to fault is in the SMPS section? I’d convinced myself not, as I thought SMP feedback was derived from +15v supply that is in spec. As you know know much about SMPS maybe you can comment?
Chris56000:
. . .The ± 55 V supplies are of the "semi regulated type" which rely on feedback from the +15 V rail and also on the MOSFETs taking a normal load current as there is no feedback from the + 55 V line directly to control them!

. . .You can make up a load consisting of 2 × 24V 21W HGV type indicator/reverse lamp bulbs wired in series across the +55 V and –55 V supplies, two bulbs in series per supply – if this drops the power supply enough you can try your Ampeg again – if the IRS20957s was turning off solely due to overvoltage it will come on!

. . .If you have a quantity of suitable resistors on hand you can experiment with the values applied to the '20957 used in the "IRAUDAMP6" schematic (page 36 of the "IRAUDAMP6" booklet) – this Reference Design is rated to operate from ± 73.5 V supplies, so you can see from this book that the ±73 V from your Ampeg's power supply isn't too much of a problem if the component values are right!

. . .A suitable alternative SMPS to try your Ampeg with can be obtained from eBay here :–

https://www.ebay.co.uk/itm/272847383409?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=oon53xzNRgC&sssrc=4429486&ssuid=qNpFa7hUTgy&var=&widget_ver=artemis&media=COPY

. . .and if you message the supplier, he can supply it adapted to give ± 55 V supply if desired!

Chris Williams
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