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BQ51025: Vrect unstable

Part Number: BQ51025

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?

  • 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?

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    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.

  • In reply to Michael Casey:

    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.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    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 helpBQ50002_Transmitter.pdf2625.BQ51025YFP_Receiver.pdf

  • In reply to Michael Casey:

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

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    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

  • In reply to Michael Casey:

    It looks like the TX is backing the power down, V Rect is decreasing. This does look odd.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    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? 

  • In reply to Michael Casey:

    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.

    Best Regards,

    Bill Johns,  Applications Engineer

    Texas Instruments Inc

    Dallas, Texas

  • In reply to Bill Johns:

    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?