Having a large ripple from DCR011205, loaded around 10-20mA. The ripple is some 100mVpp. The datasheet promises
much less than that. Used ceramic X7R capacitors on input/output/recitfier. Any ideas where to look to fix this?
DCR011205 when ceramic capacitors on the out bus at 400kHz ripple has a low output AC component. When 1uf ceramic on the Vrec pin to ground and 0.1uf ceramic Vout pins to ground the DCR011205 has an average low frequency output ripple of 15mV. The switching , high frequency envelope , can be higher depending upon the method used to measure ripple and noise. Close coupling across the output ceramic capacitor at this low amplitude ripple and noise is required for accurate measurment . Additional filtering can be added . See attached to reduce ripple and noise .
The reflected input ripple and noise can be reduced by adding PI filter . See attached filter applications diagram.
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.
Miklos Lammel50528 :
Thanks for the ripple question. I have several questions
1. What is the scope bandwidth ? The ideal bandwidth is 20MHz. The scope probe recommended connection coupling across the ceramic capacitor. The close coupling removes scopeprobe ground lead and clip ( about 7.62cm clip lead) from the ground probe barrel. The probe center pin , after removal of the probe clip ) should be directly connected to the same Vout ceramic as the ground barrel < 0.25cm) wire.
The average output ripple with the 73 material ferrite bead as PI filter inductor and 2 ceramic capacitors ( 1-2,.2uF) is about 5-10mV when measuring with close coupling scope probe.
The frequency above at 2.59us duration is about 386kHZ.
2. On the Sync. pin, is the sync. pin connected to any component or is it floating?It is very sensitive to any capacitance
My e-mail address is firstname.lastname@example.org . Send me your contact information if you want to discuss this application.
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 ?!
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 betweenan inner layer of fl oating copper which overlapsthe logic side and isolated side ground planes. Thisstructure creates two series capacitors, each with approximately0.008" of insulation, supporting the fulldielectric withstand rating of 2500VRMS. The bridgecapacitor provides a low impedance return path forinjected currents due to parasitic capacitances of theLTM2881’s signal and power isolating elements.2. Discrete bridge capacitors (C3, C4) mounted betweenGND2 and GND. The discrete capacitors provide additionalattenuation at frequencies below 400MHz.Capacitors are safety rated type Y2, manufactured byMurata, 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.
The long ground lead and it alligator clip when removed plus the probe center conductor clip when measuring across a ceramic capacitor is considered to be close coupling.
The longer scope ground clip lead which is typically 7cm long when it is attached to any surface or the robe is floating reacts as an e-field antenna. The stray e-field detection when the scope sensitivity is very high and the bandwidth is at 20 to 25MHZ, will detect any e-mil radiation. The is no power in the e-field .
I do not think the capacitance type of filter by itself soes not reqct by attuating the bus noise.
The longer scope ground clip lead which is typically 7cm long when it is attached to any surface or the robe is floating reacts as an e-field antenna. The stray e-field detection when the scope sensitivity is very high and the bandwidth is at 20 to 25MHZ, will detect any e-mil radiation. The is no power in the e-field
The y-capacitance values of <0.5uF at 800kHz switching frequencies may not be its reactive frequency range. 1uF and up including 4.7uf capacitors with X5R tolerance ratings have a low frequency response.
Just when you thought this thread wouldn't die, I have a few questions:
The DCR series or any DCP, DCV series must never be connected to any capacitance.
1. The sync.. pin must be open with no stray or capacitance detected by this pin. orge;
2.The filter design when using the ferrite bead ( inductor fair-rite 2773021447 73 material ( ) in PI filter application with 2x 2.2uf either side to ground) has a attenuation factor of 10-15db the 400kHz to 800kHZ primary switching frequency. I know of no other ferrite bead vendor which has the high permeability and resistance of 75-100 or greater ohms and low frequency range in PI filter application.
If you Google the part number, 2773021447, availablility and contacts are listed.
3. The 0.1uf capacitor connection is between VOUT pin to ground. the tie points are (-)vout to (+) Vout) .
This minimum value for filtering the output bus is 0.1uf.
OK, I've ordered some parts and I think I've got a handle on this but if you could take a quick look at this and provide some feedback I'd appreciate it!
I reviewed the filter and application schematic. The input filter (FB1 ) is I assume the 2773021447 Fair-rite , 73 material. The output capacitor , C9 ceramic ,should be located between pin 14 Vout + and FB2. I suggest adding a 1uf ceramic after FB2 and FB3 on load side for complete PI filtering .
Thanks for the quick reply! Yes, C6, C7, and C9 will be placed as close as possible to pins 1, 2, and 14 of the DCR01. So effectively after FB1 on the input side and before FB2 on the output. Yes, those are the 2773021447 beads.
On the load side, how "bad" would it be to use the same 2.2uF that are on the inputs so as to reduce the number of different components in the BOM?
Just thought I'd post an updated schematic so if I've got everything correct this time you just say "it looks good"!
Is C11 correctly placed in the schematics ? If yes what is its purpose ?
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.
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