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LM74912-Q1: LM74912-Q1 / LM74912QEVM ideal diode is not rectifying AC ripple as expected

Teroa
Prodigy 140 points
Part Number: LM74912-Q1
Other Parts Discussed in Thread: TEST2, , LM74912, LM7472EVM

Tool/software:

Test setup:
EVM LM74912QEVM is connected programmable power supply. Input voltage is 12V superimposed with 2Vp-p 500Hz AC ripple. Input voltage (TP1) and current, DGATE control (TP7) and output voltage (TP2) is measured.

Output load:
Test 1: No load
Test 2: 63 ohm load

EVM jumpers are in default settings:
J5: 1-2
J6: 1-2
J7: 1-2
J8: 2-3
J13:2-3

Ideal diode control (DGATE) is not working as expected and input current reverses. Used test setup should be similar as presented in the datasheet chapter 9.2.2.2 (AC Super Imposed Input Rectification: ISO 16750-2 and LV124 E-06).

Based on datasheet and used EVM circuitry I'd expect different DGATE behavior (similar to datasheet figure 9-3). What might cause this unexpected behavior (DGATE is not turning OFF during reverse current)?

Measurement result Test1 (no load)

DGATE turns OFF briefly but not for all AC cycles

Test2 (63 ohm load)

DGATE does not turn off and input current reverses

  • 0
    TI__Mastermind 22625 points

    Hi Teroa,

    Thanks for the waveforms, can you please plot "DGATE-A" instead of DGATE in the waveforms.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 140 points

    Hi, please find attached measurement with DGATE-A.

    Test1 (no load)

    Test2 (63 ohm load)

  • 0 in reply to Teroa
    TI__Mastermind 22625 points

    Hi Teroa,

    Thanks for the waveforms. The behavior is not expected. Can you increase the ripple and test again? Say 3Vp2p

    Also, do the controller work fine in other aspects? Do you see any damage?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 140 points

    Hi,

    The behavior is same with 3Vp2p input ripple (waveforms attached). The controller works fine in other aspects. There should be no damage as this is the only test that has been performed with EVM. We first saw the similar problem in our own circuitry. Then we tried with EVM and see the same behavior (referred in this conversation).

    Test1 3Vp2p (no load)

    Test2 3Vp2p (63 ohm load)

  • 0 in reply to Teroa
    TI__Mastermind 22625 points

    Hi Teroa,

    DGATE not toggling when load is applied is an expected behavior I would say. For DGATE to toggle, enough reverse current needs to flow.

    Sharing an example. 

    In your case DGATE may not be switching due to very low reverse current flowing, can you test again with higher COUT. 

    Also test with higher frequency.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 140 points

    Hi,

    I've made more testing and I'm still confused regarding the behavior. I've added more capacitance (double and tripled the original EVM capacitance) but all I can see is higher reverse current but DGATE-A behavior is the same.

    Some thoughts:
    I'm referring now to the Application Note - Basics of Ideal Diodes (SLVAE57B). LM74912-Q1 is not listed application note but I assume based on datasheet it is similar to devices listed in appnote Table 6-1 falling to LINEAR REGULATOR CONTROL.

    1) In linear control I would assume that DGATE-A voltage is regulated to control V_A-V_C voltage to 10.5 mV (nom) based datasheet. From the first measurement below I can see that regulation happens only seldomly (see illustration below). Most of the time DGATE-A have constant slope -2 V/ms and not following the V_in/I_in. Any ideas is this normal or not?
    2) What is the bandwidth of the linear control of the device (I've not found the spec from datasheet)?
    3) Without linear control I don't see how this could work. As in datasheet parameter V_AC_REV value is -10.5mV (nom) and if Q3 is in fully conductive state (4.3mOhm) -2.44A current is required to diode to work (in nominal case). I would assume that this relatively low frequency operation (500Hz) should happen in linear mode?

    I've tested also another EVM that I had (LM7472EVM). With that device I can see that DGATE-A is mostly operating in linear operation. Should the LM74912 operation to be fundamentally different compared to LM74720?

  • 0 in reply to Teroa
    TI__Mastermind 22625 points

    Hi Teroa,

    I have some results for 100khz taken in past on LM74912-Q1. It can do the same job as LM7472EVM.

    Can you test the same setup at higher frequency?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Prodigy 140 points

    Hi,

    Unfortunately our current test setup (equipment) is limited to <kHz frequencies. And slight increase from current frequency (500Hz) does not change the operation.

    However from 100kHz measurement you provided I can see that VGS_D is not following Vin/I_supply. Instead VGS_D goes low when high enough negative current (-3A) is reached. I do not see linear operation in this measurement.

    For comparison the measurement below is taken from LM7472EVM with 63ohm load (parallel to 100uF, extra capacitor added to have equal loads in both EVMs) 12V 2Vp2p 500Hz ripple. This device (LM74720) seems to operate in linear mode based on DGATE-A signal.

    1. Is the different behavior due to differences in EMVs (e.g. layout)?

    2. Or is the linear operation bandwidth different in these devices (LM74912-Q1 vs. LM74720)?

    3. What is the bandwidth of linear operation in different devices?