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BQ25723EVM: Switching MOSFET Vgs Threshold

Rogelio Colin
Prodigy 20 points
Part Number: BQ25723EVM
Other Parts Discussed in Thread: BQ25713, BQ25723, CSD17308Q3, CSD17578Q3A, , CSD18543Q3A

Initially the power bank design was using the bq25713EVM circuitry . Since the bq25713 was unavailable, the bq25723 IC was dropped into the bq25713EVM  (tidrxq7) reference design. 

For the bq25713EVM, the switching MOSFETs  Q1, Q2, Q3, and Q4 are listed as CSD17308Q3. The Vgs (Threshold) is listed at 1.3V. 

(For the bq25723EVM, the switching MOSFETs  Q1, Q2, Q3, and Q4 are listed as CSD17578Q3A. The Vgs (Threshold) is listed at 1.5V.)

Using a single Li-ion cell configuration, with Vbus = 20V, with charging the single cell, and then discharging at Vbus =20V, Q1 failures occurred for 3/10 units. Replacing MOSFETs and cycling the failed units did not incur additional failures. However, subsequent builds (15 units per build) produced 4 to 5 MOSFET (Q1) failures. The failures occurred at the end of charging and the start of discharging (cell voltage = 4.2V, Vout=20V). 

The MOSFET failures display low impedance. 

Scope shots display a peak voltage of 29V at the drain of Q1. 

Replacing the failed MOSFETs seemed to work but the reworked units seemed to fail at the same rate (30%). 

For a quick fix, a new MOSFET, CSD18543Q3A , was substituted for Q1, Q2, Q3, and Q4.  However, there is an issue with the power bank discharging in a single Li-ion cell configuration.

The battery can discharge starting from a fully charged state down to a completely discharged state (2.8V).

However, if discharging is started at a cell voltage lower than 3.7V, the discharge is unable to start (cell voltage < 3.7V, Vbus =5V).

The circuit is using the bq27513EVM circuit for driving Q2 on (REGN voltage driving Q2 gate high via an NPN with a Schottky turning off Q2 gate).  

If the NPN and Schottky are removed and replaced with a jumper directly connecting Q2 gate to LODRIV1 (Pin 29 of bq25723), the limit is decreased to 3.4V. 

Under 3.4V, the battery is unable to start discharging. 

Replacing Q2 only with CSD17308Q3, the unit is able to discharge starting at any cell voltage (even down to 2.8V). 

Two main questions:

1. What was the possible reason for the original MOSFET failure?

2. What was misunderstood with the IC driving voltage and the replacement MOSFET Vgs threshold ? 

  • 0
    TI__Intellectual 1805 points

    Hi Rogelio,

    The TI employee assigned to this family is out on leave and will get back to you early next week. 

    Regards,

    Jacob

  • 0 in reply to Jacob Rook
    TI__Mastermind 26775 points

    Hi Rogelio,

    It seems you are using a new gate driving circuitry other than our standard EVM. This could be the reason for the issues. 

    I am also confused with the discharge level. Q1-Q4 are switching FETs, the discharge profile is mostly related to the BATFET, instead of the Q1-Q4.

    Could you share the schematic with us?

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 20 points

    No. That is not the issue.

    The circuitry is being used in a USB-Type C power bank application. A  USB power controller IC communicates to the bq27523. The power bank is being charged at an input voltage of 20V, which is bucked down to the single cell voltage of 4.2V at charge termination. The power bank is then discharged at 20V, where the Li-Ion cell voltage is boosted up to 20V.  is The circuit was using the Q2 gate driver circuitry in the bq27513EVM but the driving circuitry was removed.  A direct connection from pin 29 (LODRV1) to Q2  was used. All other MOSFETs are driven directly as per the bq25723EVM. The BATFET is fully on, Vsys = VBat. The application is can discharge (Vout = 20V) when the initial cell voltage is greater than 3.4V. It can continuously  discharge down to a cell voltage of 2.8V. This is good. However, if the initial cell voltage is 3.4V, the bq25723 is unable to drive Q2 on. That is the issue. The cell voltage does not drop or sag at a high/initial current rush. The circuit is identical to the EVM shown below. 

  • 0 in reply to Rogelio Colin
    TI__Mastermind 26775 points

    Hi Rogelio,

    Thanks for your clarification. I now understand you are operating the charger in OTG mode. There are two parameters specifying  the battery operation range. The minimum battery voltage to start the OTG operation is 3.55V typical. If started, the OTG can continue to operate until the battery drops below 2.4V typical.  

    Regards,

    Tiger