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BQ51025: Vrect unstable

Michael Casey
Prodigy 150 points
Part Number: BQ51025
Other Parts Discussed in Thread: BQ500511, BQ50002, , BQ50002A

Vrect reaches the correct voltage at power up and is stable under no load, however when I connect a load of approximately 200mA Vrect starts to oscillate, period is aprox 2seconds. I am using my own coils for transmitter and receiver based on the wurth 760308111 (transmitter) & wurth 760308103204 (receiver) both as used in the Texas EVM. However with the wurth coils the effect is much worse and the transmitter switches off aprox every 2 seconds and then back on again.

If I disable the CM_ILIM feature by pulling CM_ILIM high and power from an IDT evaluation kit transmitter, Vrect stays stable and the receiver works well. When powered from the Texas transmitter (bq500511 & bq50002) my receiver switches off. It probably expects communications from the receiver; it doesn’t seem from the datasheet that this function can be switched off in the transmitter.

How can I make Vrect stable?

  • 0
    TI__Guru 65706 points
    Hello
    Both coils look good:
    Wurth 760308111 is a WPC A11 TX design, 5V input 5W.
    Wurth 760308103204 is a 38.5mm X 30.5mm RX coil, 16.7uH 300m ohms.

    On RX side what value did you use for C1 (Cs), this is the resonance point set by RX coil and Cs?

    Texas Instruments transmitter - should be the bq50002AEVM-607.

    Also RX bq51025 is a 5W device with 10W mode. But should work on any TI transmitter at 5W.

    Can you send a scope capture of Vrect and output voltage?
  • 0 in reply to Bill Johns
    Prodigy 150 points

    Thanks for the reply Bill, I have

    C1:  0.047uF,

    C2: 0.068uF &

    C3: 0.082uF

    But i see now that in the  bq51025 Evaluation Module (PWR649), C2 is crossed through, ie not fitted.

    Attached is a photo of the scope, sorry don't have a better Image at the Moment.

  • 0 in reply to Michael Casey
    TI__Guru 65706 points
    Try testing with a lower Cs value, 0.047uF + 0.082uF as a start.
    Best way to tune the Cs value is to measure RX coil L with it in mounted in the case and on TX, L value changes with surrounding parts.
    Then use that value to tune Cs for a resonance of 100kHz.
  • 0 in reply to Bill Johns
    Prodigy 150 points

    Thanks Bill, I removed the extra C so that there was just the values you mentioned but there was not a lot of difference.

    I also see that my output voltage drops when i apply a load (problem was there before I removed the extra cap) at around 300mA load the voltage starts to drop, but this is only when I use my power transmitter.

    when I use an evaluation kit power transmiiter my receiver output does not drop as I increase the load. 

    I've attached both my power transmitter and receiver circuits as pdf files.

    Thanks for your help1884.BQ50002_Transmitter.pdf4113.BQ51025YFP_Receiver.pdf

  • 0 in reply to Michael Casey
    TI__Guru 65706 points

    On the TX --- Check R224 and R225, IC201 Pin 4 and 5, they should be no POP, open.

  • 0 in reply to Bill Johns
    Prodigy 150 points

    Thanks Bill,

    I removed R224 & R225 and was able to increase the load current but around 360mA the receiver switched off, however I think because the voltage at the ILIM pin reaches 0.6v
    I changed my R1 (datasheet) to 240 ohms and Rfod to 265 ohms (starting point recommended in evm kit) and started with a 10 ohm load resistor but after a few seconds the receiver voltage drops (see pic of scope).
    I replaced my bq50002/bq500511 my receiver  with the evaluation kit and was able to drive a load of aprox 700mA before Vout starts to drop, the voltage at ILIM was 0.52V with a meter but on the scope there are spikes, maybe upto 0.6v.

    Is the following correct for ilimit of 1A:
    R ilim = 842 / 1.1*1 = 765 ohms,

    R1 = R ilim – R fod
    R1 = 765 - 265 = 500


    If so then my value of 240 ohms is too small and the reason for the output dropping (when driven from the evm)?
    The top trace is V Rect and the bottom V out can you see a reason why my transmitter switches off ?
    thanks

  • 0 in reply to Michael Casey
    TI__Guru 65706 points
    It looks like the TX is backing the power down, V Rect is decreasing. This does look odd.
  • 0 in reply to Bill Johns
    Prodigy 150 points

    Thanks Bill I did not see that you had answered.

    The Problem was caused by the ratio of the Feedback resistors for VIREG and VO_REG, These should have been 130k & 10k but one of the 130k resistors had the wrong value. I imagine that the Output could not converge on the correct voltage because of this, hence the oscillation.

    Another Problem was caused by RFOD and ILIM resistors. I have now set RFOD to 256 ohms which seems to be the starting point for the pot in the kit. I can get 700mA Output current which means with 7v Output I'm around 5w, so I imagine ii is now working correctly. I still have no idea about the setting for RFOD and I don't think the data sheet explains it very well.

    I still have one Problem that when starting with no load, the output Switches on and off about every 4 seonds for the first 5 minutes and then eventually stays on. The Transmitter is switching off so maybe something is being communicated back to the Transmitter, any ideas? 

  • 0 in reply to Michael Casey
    TI__Guru 65706 points
    For info on RX FOD see --
    www.ti.com/lit/zip/sluc577

    A way to check the tuning to determine if it is causing problem at no load is to check the operating freq. Most systems including the bq50002A / 511A are variable freq, max power at 110kHz and minimum power at 205kHz. At start the unit will be at 175kHz and the decrease the operating freq to increase voltage before enabling the output voltage. For a no load condition I would expect operating point to be between 150kHz and 175kHz. If the system has too much gain operating freq could be near 205kHz, this could be causing comm issues.
  • 0 in reply to Bill Johns
    Prodigy 150 points

    Hi Bill,

    I have a frequency Change from aprox 196Khz no load to 204Khz with a load of 460mA (a  small Change)
    I then used the coils from the evm's with my electronics and have a frequency Change from aprox 158Khz & 175Khz under the same load conditions.

    My TX coil gave the same results as the evm coil, but with my RX coil the frequency change was much less.
     
    So maybe the Problem is my RX coil. However the RX evm with my coil does not switch off.
    I will check my coil design but If it is a comms problem on no load is there any way to make it more stable?

  • 0 in reply to Michael Casey
    TI__Guru 65706 points

    200kHz looks high for a no load condition.  What is also happening during no load it the bq51025 will set the voltage at RECT high to help with a load step, this requires a higher operating point.  The coils are good types.

    Some things to check:

    1.) coil to coil distance -- 2mm to 5mm, good coils and too close it could be a problem.

    2.) TX or RX coil tuning--Double check the coil tuning, both coils should be tuned to 100kHz

    3.) Capacitor type used for coil tuning-- should be COG/NPO for TX, X7R or better for RX

    4.) Voltage at RX RECT no load

  • 0 in reply to Bill Johns
    Prodigy 150 points

    Thanks Bill,

    I have a frequency Change from aprox 196Khz no load to 204Khz with a load of 460mA (a  small Change)
    I then used the coils from the evm's with my electronics and have a frequency Change from aprox 158Khz & 175Khz under the same load conditions.

    My TX coil gave the same results as the evm coil, but with my RX coil the frequency change was much less.
     
    I have already had some coils manufactured so hopefully I can retune and change the values of the RX series capacitors.

  • 0 in reply to Michael Casey
    TI__Guru 65706 points
    The 196kHZ no load and 204kHz with 460mA load points to a tuning or coil problem. I suspect at 205kHz the unit is using PWM to reduce power and duty cycle is below 50%.

    The 158kHz no load and 175kHz load sounds better. At no load target voltage for V-RECT is higher and this will be a higher operating point (lower freq). As output current goes up the V-RECT target voltage is lower and operating point is often lower (higher freq).
  • 0 in reply to Bill Johns
    Prodigy 150 points

    Thanks Bill,

    I haven't been able to work full time on this project, just back on it now.

    Both coils look as though they are tuned to 100Khz.

    However I'm not using these devices as a battery charger but to power an isolated data acquisition system that communicates via an optical interface. 

    I can supply dc loads upto the max of 5W without problem but as soon as I turn on the optical interface I have the problem of switching on and off. The pulsed load would seem to be the problem.

    The optical system causes a pulsed load of aprox 80mA and the data acquisition system of aprox 200mA.

    It looks as though the receiver tells the transmitter to switch off and then restart. Is there any way to make the system more stabile or can the comms between the receiver and transmitter be switched off?

    thanks.

  • 0 in reply to Michael Casey
    TI__Guru 65706 points
    A couple of things could be happening:
    1.) The load on the RX could be enough for input voltage to drop below UVLO (2.7V) at RECT
    2.) The noise generated by the optical pulses is occurring often enough to corrupt the communications RX to TX.

    Which TX are you testing with?
  • 0 in reply to Bill Johns
    Prodigy 150 points

    Hi Bill,

    my circuit is based on the bq50002 Wireless Power TX EVM

  • 0 in reply to Michael Casey
    TI__Prodigy 320 points
    Hi Michael,
    I am covering for Bill while he is out of office this week. What is the frequency of the pulsed load to the RX? How long after you turn on the pulsed load does it take the device to shut down. This would give us a hint as to wether the pulse load is interfering with the communication.
  • 0 in reply to Norelis Medina
    Prodigy 150 points

    Thanks for the reply.

    the receiver is sending aprox 1Mbps via IRDA.

    The receiver cycles off and then on every few seconds. However this depends on when the pulsed load is started.

    The elctronics (an MSP) is powered via a 3v3 regulator driven from V-OUT (7V)which starts up long before V-OUT has reached it's final voltage. The leds are driven from another voltage closer to V-OUT.

    If the leds are driven before V-OUT is reached then V-OUT is unstable fluctuating down to 5v and then back up to 6v and switching off in between.

    If I start the leds after V-OUT has reached its final voltage then the receiver is much more stable but occasionaly restarts.

  • 0 in reply to Michael Casey
    Prodigy 150 points

    I haven't hade a reply yet but this will be my last post as I now have a solution.

    (I get the same results with the evm, ie switching on and off every few seconds.)

    It seems that these devices make a great charging unit but cannot cope with pulsed loads. They are too intelligent. Is there any way to disable the communications via the registers, i haven't been able to find anything in the datasheet.

    For now I have a work around for one of my applications. I use the wireless receiver unit to switch on an internal battery supply which then remains stable. The unit is only on for a few minutes so this is not a problem.

    The other design which used the wireless transmitter & receiver is more of a problem but we will change this for a more simpler design.

    Regards.