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BQ51050B: CLAMP capacitors

Thomas Fanous
Prodigy 45 points

Part Number: BQ51050B

Hi,

The data sheet states that the CLAMP capacitors are used to aid the clamping process to protect against overvoltage. Choosing a 0.47-µF capacitor with a 25V rating is appropriate for most applications. What applications would this not be appropriate for? Using a bq51050BEVM-764 with 25V CLAMP capacitor I used a transmitter coil to transmit 10W at various frequencies and noticed that the bq51050B got extremely hot and eventually stopped working. However, I replaced the 25V capacitors with 50V and did not see the problem. I would like to understand this, but there is not much information about how this CLAMP circuit works. 

Thanks,

Thomas Fanous

  • 0
    TI__Guru* 79526 points
    The clamp circuit will be active when voltage at RECT pin reaches V-OVP, 15V typical. At that point CLAMP1 / 2 mosfets turn on and load AC1/2 to reduced input voltage. Clamp capacitors will only trip during an V-OVP event and are not active during normal operation.
    Also see Data Sheet Section 8.3.4.5.

    Thermal rise of the device with 25V vs 50V capacitors is unusual. Check the voltage of RECT pin behavior of CLAMP1/2.
  • 0 in reply to Bill Johns
    Prodigy 45 points
    This is not during normal operation, I am trying to simulate overpower conditions in which V-OVP does exceed 15V. In this case having a 50V capacitor gives very little thermal rise compared to 25V capacitor. Could it be that in this case voltage on CLAMP 1,2 capacitors is derating capacitors to much? What is the minimum capacitance needed in this case? What happens if the capacitance is too small?

    Thanks,
    Thomas Fanous
  • 0 in reply to Thomas Fanous
    TI__Guru* 79526 points
    What is probably happening in this case is the 25V capacitors are overloading, the 50V capacitors have more margin.
    This could be confirmed by comparing current in the clamp capacitor. Suspect it is not the capacitor value (470nF) but behavior of the capacitor at high load current.