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TPS65263: Suitability for generating rail for wireless power transmitter

Bryan McLaughlin
Expert 1110 points
Part Number: TPS65263
Other Parts Discussed in Thread: , TPS65400

Tool/software:

Hello - I'm designing a non-standard, 2W inductive power system that will operate in the 100-1000kHz range. I would like to be able to adjust the secondary side voltage during low load to minimize power losses.

One way to do this is to shift the transmitter operating frequency further away from the pair's resonance point, like Qi/WPC.

Another way is to adjust the supply rail that's feeding the transmitter coil's H-bridge driver. For this, I'm considering an integrated-FET DC/DC with I2C control like the TPS65263. 

Can you advise on potential shortfalls regarding operating frequency and control topology? I imagine that a DC/DC set to 250kHz won't be able to "keep up with" an inductive H-bridge load running at 800kHz. I'd expect subharmonics or instability might be an issue without careful sizing of output caps.  I also expect there might be a preference for voltage mode vs current mode converter with a switched load like this.

Thanks!

  • 0
    TI__Mastermind 30671 points

    Hi Bryan,

    The TPS65263 implements a constant frequency, peak current mode control that simplifies external loop compensation. The device operates in fixed 600 kHz. If you want adjustable frequency, you can choose TPS65263-Q1. And for the component selection, you can refer to datasheet "application and implementation" section.

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Expert 1110 points

    Hi Lucia - my mistake, I copied the wrong part number. I'm considering the TPS65400 since it has multiple outputs.

    Are there any rules of thumb to share about setting DC/DC switching frequency for cases like this where the load current is AC near the same frequency as the DC/DC?  

  • 0 in reply to Bryan McLaughlin
    TI__Mastermind 30671 points

    Hi Bryan,

    Let me check and get back to you soon.

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    TI__Mastermind 30671 points

    Hi Bryan,

    It depends on your application requirement. For example, as I know, applications like frequency sensitive RF circuitry where any frequency that would interfere with the Tx/Rx frequency would be considered noise, frequency sensitive data converters and analog circuitry. With adjustable fixed frequency control, it can better control of the switching frequency to avoid interfere in the Transmit/Receive frequency band. Frequency critical applications for example like  radios, test and measurement, medical applications such as ultra sound.

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Expert 1110 points

    That makes sense for preventing interference with other circuits. But the concern I have is about voltage regulation and control stability.  

    Let's say we're starting with the Buck on and Vout stabilized. Now I enable a load with a sawtooth-shape current at 2MHz. The Buck output bulk capacitors will provide current during each load pulse. Let's say for example that each pulse drops the output cap voltage by 30%. (V=Q/C)

    If the Buck is only running at 600kHz (t=1.6us) then after only three 2MHz load pulses (t=1.5us) the output cap is 90% depleted and Vout is collapsed.

    When the Buck HS switch turns on, I would expect high peak switch currents since the Buck has to recharge the output cap AND provide current to the load.  My intuition is that some converter control loops could go unstable in this case, and we may trigger OCP limiting or inductor saturation too.

    But if the Buck was switching at a HIGHER frequency than the load, it would see Vout drop sooner and regulate it before the output cap gets depleted.

    The conclusion I'm coming to is "The Buck should be switching at a higher frequency than the load to minimize the required output capacitance" but I'm sure there are some great app notes or literature that can explain it better. I recall reading about charge balance for converter design, long ago. What do you think?

  • 0 in reply to Bryan McLaughlin
    TI__Mastermind 30671 points

    Hi Bryan,

    Thanks for your good question. Pls kindly let me check and get back to you during these days.

    BRs

    Lucia

  • 0 in reply to Lucia Gao
    Expert 1110 points

    Hi Lucia - checking to see if you have any guidance for this application. Thanks!

  • 0 in reply to Bryan McLaughlin
    TI__Mastermind 30671 points

    Hi Bryan,

    Unluckily I don't find the material related with this topic. But pls kindly let us know how we can better support you if you have further concern. Really thanks!

    BRs

    Lucia