DCR011205 ripple

Dear Tom!

When I started designing my circuit for DCR011205 I came across your post above, and used your schematic for noise reduction. However now after I have soldered everything together, I have measured with an oscilloscope (PicoScope 2205) the output ripple of the DCR011205, and it is much higher than expected.

I have attached my schematic, and the measurement results with 68 Ohm load on output of the DCR011205.

The interesting thing is that if I add a 100nF capacitor between the two isolated ground ( input and output gnd ), then the ripple voltage drops to a fraction (5..10mV which is quite good). Even touching the output with hand ( human body acts kind of like gnd ) the noise drops significantly. This means to me, that the output is kind of  "floating" , but I don't have any idea how to solve this problem. Adding capacitor on output between gnd and vcc did not help. Only thing that has helped, was that I have connected the input gnd and output gnd with a capacitor - but then the hole meaning of a 1000V isolated dc-dc converter is lost.

Please help me with this issue.

Thanks,

Miklós

Hi!

Thanks for the quick answer!

My scope has a 25MHz bandwidth and 200Msample/sec Sampling frequency. I have removed the ground lead, and the clip from the scope probe, and magic has happened, the noise is now around 10mV measured 10 cm away from the capacitor on PCB, and 3mV directly at the output filter.

The frequency is still the same, so I assume that this is the 400kHz switching frequency ( 385kHz is quite close) of the DCR011205.

The synch pin here in this application is used to synchronise with another DCR021205. If there was no other device, where should this pin be connected if not used?

And finally a HUGE question arises in me: what is the reason, and the explanation behind this, that removing GND-lead and the clip from the scope probe had such a huge effect on the measurement ?!

Thanks,

Miklos

Hi!

Meanwhile my friends have pointed out that using a Y2 safety capacitor between the two GND lines is a common design practice to reduce EMI. Actually I found, that Linear Tech suggest to use capacitors with their isolated dc-dc converters.

From the datasheet http://cds.linear.com/docs/Demo%20Board%20Manual/dc1746af.pdf :

"EMI mitigation techniques used include the following:
1. Four layer PCB, allowing for isolated side to logic side
‘bridge’ capacitor. The bridge capacitor is formed between
an inner layer of fl oating copper which overlaps
the logic side and isolated side ground planes. This
structure creates two series capacitors, each with approximately
0.008" of insulation, supporting the full
dielectric withstand rating of 2500VRMS. The bridge
capacitor provides a low impedance return path for
injected currents due to parasitic capacitances of the
LTM2881’s signal and power isolating elements.
2. Discrete bridge capacitors (C3, C4) mounted between
GND2 and GND. The discrete capacitors provide additional
attenuation at frequencies below 400MHz.
Capacitors are safety rated type Y2, manufactured by
Murata, part # GA342QR7GF471KW01L"

So does it mean, that this solution could be used with TI isolated dc-dc converters? Would you recommend it?

And I would be also very thankful, if you could help me to understand why removing the gnd  lead has helped.

Regards,

Miklos

Just when you thought this thread wouldn't die, I have a few questions:

1.  If we aren’t going to be using the SYNC pin should I pull it high or leave it
   disconnected?
2. In your filter design you made up a few years ago you listed a Fair-Rite
   2773021447 part. This part has a max RDC of 0.9mohms which seems extremely low.
   If I wanted to select another part would you have a 2^nd choice? Of
   course 0.9mohm is great for keeping the losses low but a quick search on
   DigiKey (for reference) only comes up with some Wurth parts that meet that
   criteria of 0.9mohm.
3. In your schematic it looks like you’ve got a 0.1uF between Vout and the ERROR pin
   but I think that 0.1uF should be between Vout+ and Vout-, right? And then the
   ERROR pin is just pulled high.

Thanks,

George

Well to be honest I'm not sure either as I'm not a filter expert and didn't look it up. I was just going off of this statement:

" I suggest adding a 1uf ceramic after FB2 and FB3  on load side for complete  PI  filtering ."

which I could very well have mis-interpretted.

I think if you just keep C10 and remove C11 you should be alright (C11 connects gnd to gnd )

Yeah C11 looked odd to me but that's why I put this up so I could get a 2nd pair of eyes on it!

Hello all,

I have couple questions,

1. The spec sheet of the DC01 family suggests the use of a 2.2uF LESR capacitor to reduce the ripple noise at the output. On the pdf called DCR01 "Low noise Power Supply", with the PI filter, they use a 1uF before the ferrites between Vout+ and Vout-. Is there a particular reason to not have used a 2.2uF or was it just for the example ?
2. I am using a DCR010505 and I have some EMI issues on my isolated circuit side which clearly spreads a 400KHz harmonics through my power planes. I have not been using the PI filter configuration but I plan to give a shot to see if I can limit the EMI. I have found recently that I have been using low ESL  caps around the DCR instead of low ESR . Is this a big mistake from a filtering point of view ? If so, I plan to use low ESR caps which have an ESR < 0.01 for the frequency range of 100-1000KHz and an impedance < 0.1ohm for the same frequency range. Is this good enough ?

Thanks !

We'll probably have to wait for Tom to chime in but wouldn't going from 1uF to 2.2uF just help to reduce the ripple? I think you need a certain minimum amount of capacitance and then any additional amount is a matter of cost vs. benefit.

Hi George:

Thanks for the schematic  .C11 is not requried  as it is connected to the same isolated ground as  the ferrite bead FB3. The 2.2uFceramci capacitors can be used on the Vout bus for  filtering .

1. I have  questions  on the TL750m12QKTTRQ1 regulator and 750MA load.

a. Is the DCR011205U   a single stage stage converter  or are there multiple stages with the same layout?

b. What is the  average 12V  current  in this application?

regards

Tom  

Hi Tom:

I hope that you had a good weekend. That's good about not needing C11 as it wasn't making much sense to me.

The board draws only maybe ~150mA and most of that is on the isolated 5V rail for a meter. The input is an unregulated 12V supply so no load voltage is ~17V. The TL750m12QKTTRQ1 really shouldn't be designed in as a standard LDO would work fine but for some reason that's what this design started with. Not really a good excuse so maybe I should swap that out so I'm not asking that same question 6 months from now!

This design just needs a regulated 12V rail and then an isolated 5V rail for the meter. The application is for a MOSFET "tester" which uses a LM234 for a constant current source of 1mA on the 12V rail.

Cedric:

I attached your questions below regarding  teh DCR010505U.

The responses are below the  questions.

1. The spec sheet of the DC01 family suggests the use of a 2.2uF LESR capacitor to reduce the ripple noise at the output. On the pdf called DCR01 "Low noise Power Supply", with the PI filter, they use a 1uF before the ferrites between Vout+ and Vout-. Is there a particular reason to not have used a 2.2uF or was it just for the example ?

1. 1a. The DCR family has any average output ripple and switching pulses  of 20mV at 20MHz bandwidth when using the 1uF ceramci capacitors. 2.2UF capacitor will reduce the  ripple by about10%.

2. I am using a DCR010505 and I have some EMI issues on my isolated circuit side which clearly spreads a 400KHz harmonics through my power planes. I have not been using the PI filter configuration but I plan to give a shot to see if I can limit the EMI. I have found recently that I have been using low ESL  caps around the DCR instead of low ESR . Is this a big mistake from a filtering point of view ? If so, I plan to use low ESR caps which have an ESR < 0.01 for the frequency range of 100-1000KHz and an impedance < 0.1ohm for the same frequency range.

2A.All  ceramic capacitors are required  on both  input  and output buses for the  DCR series and DCP series . PI filter designed with  1-5uH ( ferrite beads 2773021447)  and ceramic capacitors 2.2uF on the input bus attenuates the  reflowed noise  by 10-15db.

Tom

tguerin@ti.com

Hi Tom:

Would you be willing to take a quick look at my board layout? I think you might be able to make some valuable comments and it doesn't have much in teh design so I don't think it would take too much of your time. What would be the best way to provide you the files and in what format? I'm waivering right now with simple trace only layout vs. a few power pours.

Thanks,

George

George Ioakimedes :

I would like to look at your layout.

A pdf file would be best fro viewing.

My e-mail is

tguerin@ti.com

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