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LM5109B: Brushed Motor H Bridge Schematic Verification

Rick Bassham
Prodigy 10 points
Part Number: LM5109B

Tool/software:

I'd like some help verifying the schematic for a brushed motor h-bridge using two LM5109Bs.

Part Information:

12V_MOTOR Net is unregulated power
from nominal 12V AGM Battery

Expect voltage from 10-15V

Bootstrap
Diode - 100V 1A; Vf = 0.875V @ 1A
Capacitor - 220nF 50V X7R

Bypass Capacitors
100nF 50V
10uF 25V
100uF 25V

R Gate = 2Ω 250mw
D Gate = Schottky SS210; Vr = 100V; Io = 2A; <500ns

MOSFET Diode Protection
SMA6J18A - Zener TVS
Vbr = 20.0 - 22.1 @ 1mA
Vrwm = 18V
Vrsm = 29.2
Irsm = 20.6A

Q1-Q4 N-Channel
IPT004N03L
Vds = 30V
Rds(on)max = 0.4mΩ
Id = 300A
Qoss = 141nC
Qg (0-10V) = 252nC
Qg max = 336nC
Rise Time = 17ns
Fall Time = 37ns

Thank you for your time!

Rick

  • 0
    TI__Genius 9010 points

    Hello Rick,

    The schematic looks good. Please insure that the unregulated 12V_MOTOR net supplying the gate drivers does not fall below UVLO during a heavy load condition and that the voltage does not exceed the gate driver absolute maximum 18V during a large transient or load dump condition.

    Thanks,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 10 points

    Hi Walter, thanks for taking a look. I tested a similar circuit, but with a 100nF bootstrap capacitor. When running in the direction of MOTOR B High Side On and MOTOR A Low Side On, it ran great. However, when going the opposite direction, the High Side MOSFET was not turning on fully and the High Side MOSFET would overheat. Do you think that is more likely an issue with the board layout, or with the size of the bootstrap capacitor?

    Thank you,

    Rick

  • 0 in reply to Rick Bassham
    TI__Genius 9010 points

    Hi Rick,

    Did both the A side and B side have 100nF bootstrap capacitors? The bootstrap capacitor should be sized to have enough energy to drive the gate capacitance of the high-side MOSFET without being depleted by more than 10%. The Gate capacitance Cg = 252nC / (12V-0.875V) = 22.7nF. We recommend the boot cap be 10X the gate capacitance giving a value of ~220nF which you have in your schematic. More information here: https://www.ti.com/lit/an/slua887a/slua887a.pdf

    If the low side MOSFET does not have enough time to charge up the boot cap, the boot cap is partially discharged, or the boot cap is too small, the mosfet might not turn on fully or UVLO could be triggered. 100nF would be too small with these MOSFETS.

    Thanks,

    Walter

  • 0 in reply to Walter Zollinger
    Prodigy 10 points

    Thank you Walter. Yes, both had the same capacitor, so not sure why one side worked well, when the other did not. I'll try again with the 220nC caps and see how it performs.

    I was running at 25kHz PWM frequency at 98% duty cycle. I'll lower the duty cycle to 95% in the future as well.

    Thanks,

    Rick