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DRV8701: How to choose a right Mosfet?

macjan fernandes
Prodigy 115 points
Part Number: DRV8701
Other Parts Discussed in Thread: CSD18532Q5B,

Hello TI Team,

We are facing a problem in mosfet that we selected.

Few years ago we had purchased a mosfet which was on an evalutation board we purchased locally. It had Mosfet NCE40H12K. (D2PAK)

This worked really well for us and we faced no issues.

Due to pandemic and shortage in market of components we decided to buy this mosfet from same vendor but this time this batch was not functioning as we expected.

The pulses on Gate was noticed but the driver was unable to run the motor. We had no idea what was the difference in pulses. It was difficult for us to identify which parameter could had been different here.

We choose another mosfet with similar specifications and that worked. SQR50N04-3m8 We got few samples of these and tested and worked well. One big problem with this was it is Reverse mounted. And our pcb was not meant for it. But we used it for emergency backup during production.

The ti evaluation board uses another mosfet CSD18532Q5B which is also different package and not suitable for us as some of them are already in field and we want to maintain the stock.

Our motor specifications

Motor voltage: 24V

Motor current max: 6A (peak during friction in mechanism), 3A (continuous for 5seconds)

Stall condition: Motor is disabled upon 1second stall at peak current.

Questions:

1. What is the specification to look at when we are selecting the mosfet for DRV8701?

2. I've made comparison chart between mosfets I discussed on top. Most specifications are similar but td(on) and td(off) and rise and fall are different. Does these parameters matter? If so as per question 1, if some reference guide for this selection that will help.

param New selected SQR50N04-3m8 Tested for year NCE40H12K Ti eval CSD18532Q5B
Type Nch Mosfet Nch Mosfet N CH PWR mosfet
VDS(v) 40 40 60
RDS(on) at Vgs 10V 0.0038 0.0036 ~ 0.004 0.0033~0.0043
ID (A) 50 120 172
Package TO-252 rev TO-252 8-VSONP
Vgs(th) (v) 2.5~3.5 1.2 ~ 2.5 1.5 ~ 2.2
td(on) ns 11 ~ 16 15 5.8
td(off) ns 34 ~ 51 52 22
rise time ns 5 ~ 8 18 7.2
fall time ns 9 ~ 14 23 3.1
pulsed current A 200 120 400
forward V VsD 0.9 ~ 1.5 1.2 0.8 ~ 1

3. Based on CSD18532Q5B mosfet specifications, I was browsing on digikey and found similar mosfet which looks quite promising as well but until I know what parameters to look at we will be in doubt.

https://www.digikey.in/en/products/detail/infineon-technologies/IRFS7530TRLPBF/4772499

Best regards,

Macjan

  • 0
    TI__Mastermind 37683 points

    Hi Macjan,

    macjan fernandes said:
    The pulses on Gate was noticed but the driver was unable to run the motor. We had no idea what was the difference in pulses. It was difficult for us to identify which parameter could had been different here.

    Is it possible to provide waveforms of the gate and source signals of the MOSFETs? I want to understand if the driver is functioning properly. 

    macjan fernandes said:
    1. What is the specification to look at when we are selecting the mosfet for DRV8701?

    Section 8.2.1.2 of the datasheet provides detailed guide for choosing the right MOSFET. Double check the parameters of the FETs are acceptable by using the equations given in the datasheet. The main parameter to check is the Qg (total gate charge) since it directly affects how fast MOSFET can open or close.

    macjan fernandes said:
    2. I've made comparison chart between mosfets I discussed on top. Most specifications are similar but td(on) and td(off) and rise and fall are different. Does these parameters matter? If so as per question 1, if some reference guide for this selection that will help.

    rise time and fall time is affected mainly by the total gate charge of the FET (Qg) and the sinking/sourcing gate drive current (IDRIVEx). The relationship is given in equation in the datasheet. Please reference this app-note to learn about how our smart gate drive functions and to understand what MOSFET parameters are important to consider when designing your circuit.

    macjan fernandes said:
    3. Based on CSD18532Q5B mosfet specifications, I was browsing on digikey and found similar mosfet which looks quite promising as well but until I know what parameters to look at we will be in doubt.

    Please refer to my response your question (1). Check that the FET is compatible first.

    Regards,

    Pablo Armet

  • 0 in reply to Pablo Armet
    Prodigy 115 points

    Hello Pablo,

    Thank you for very detailed reply.

    As per 8.2.1.2 I've found following details about our hardware. As per the datasheet 40khz was used. but in my case 8.96khz is used.

    VM (v) (12V ~ 24V) 18 (Nominal)
    Fpwm hz 8960 8.96khz from MCU
    Ivcp as per VM (ma) 0.012  for nominal VM (datasheet)
    as per calc req Qg (c) 1.33929E-06 1339.29nC
    as per calc Qg forced c 6.69643E-07 669.64nC

    Waveform as per SQR50N04-3m8

    As per above details IRFS7530 is having extremely large turn ON/OFF delay compared to other three mosfet. But as per Qg max calculations this is in range. Question is 

    1. This range 1339nC is rightly calculated?

    2. Is 8.96khz ok for driving motor?

    param New selected SQR50N04-3m8 Tested for year NCE40H12K Ti eval CSD18532Q5B new selected IRFS7530TRLPBF
    Type Nch Mosfet Nch Mosfet N CH PWR mosfet

    Strong IRFET
    VDS(v) 40 40 60 60
    RDS(on) at Vgs 10V 0.0038 0.0036 ~ 0.004 0.0033~0.0043 0.00165~0.002
    ID (A) 50 120 172 195
    Package TO-252 rev TO-252 8-VSONP TO-252
    Vgs(th) (v) 2.5~3.5 1.2 ~ 2.5 1.5 ~ 2.2 2.1~3.7
    td(on) ns 11 ~ 16 15 5.8 52
    td(off) ns 34 ~ 51 52 22 172
    rise time ns 5 ~ 8 18 7.2 141
    fall time ns 9 ~ 14 23 3.1 104
    pulsed current A 200 120 400 760
    forward V VsD 0.9 ~ 1.5 1.2 0.8 ~ 1 1.2
    Qg (nC) expected forced decay <669nC 70~105 75 44 274~411
    Qgd (nC) 13 17 6.9 83

    Regards,

    Macjan

  • 0 in reply to macjan fernandes
    TI__Mastermind 37683 points

    Hi Macjan,

    The 40kHz used in the datasheet is the highest frequency the current regulation can switch at. So the DRV8701 can only support FETs with Qg < 200nC.

    macjan fernandes said:
    1. This range 1339nC is rightly calculated?

    The calculation is correct if you plan to operate between drive and brake (slow decay). If you wish to operate in drive and forced fast decay, the range is 1/2 or 670nC. Note that these calculated values are greater than the maximum that our device can support which is 200nC. However, this is the max value and all of your selected FETs fall below this value. Although the new selected FET is above 200nC. I think this may be the reason for the failure. A quick test to do is to set the IDRIVE to the highest value allowed. A higher IDRIVE causes faster rise/fall times. I believe the new FET is taking much longer to fully enhance and turn ON due to the higher gate charge. A faster IDRIVE will cause the FET to go through its miller region quicker resulting in a faster turn ON and rise time. This may help solve your issue. Let me know if it works.

    macjan fernandes said:
    2. Is 8.96khz ok for driving motor?

    It is okay from the driver perspective. The frequency will affect the max gate charge that can be used.

    Recommendations for future debugging:

    1. Increase IDRIVEx to the max value
    2. Select a FET with lower gate charge (if (1) does not lead to successful results

    Regards,

    Pablo Armet

  • 0 in reply to Pablo Armet
    Prodigy 115 points

    Hello Pablo,

    The yellow marked mosfets I've searched on digikey, 4 of them show close by ratings as per old working ones.

    Please let me know if selection of mosfet comply with specifications required as Qg is <200nC

    param New selected SQR50N04-3m8 Tested for year NCE40H12K Ti eval CSD18532Q5B MCU110N06YA-TP FDD86367 DMTH6004SK3-13 IPD90N06S407ATMA2
    Type Nch Mosfet Nch Mosfet N CH PWR mosfet Nch mosfet Power trench Nch mosfet Nch Enhanced Mode Mosfet Optimos3 power mosfet
    VDS(v) 40 40 60 60 80 60 40
    RDS(on) at Vgs 10V 0.0038 0.0036 ~ 0.004 0.0033~0.0043 0.0032~0.0044 0.0033~0.0042 0.003~0.0038 0.0039~0.0049
    ID (A) 50 120 172 110 100 100 90
    Package TO-252 rev TO-252 8-VSONP TO-252-3 TO-252-3 TO-252-3 TO-252-3
    Vgs(th) (v) 2.5~3.5 1.2 ~ 2.5 1.5 ~ 2.2 1.2~2.5 2~4 2~4 1.2~2
    td(on) ns 11 ~ 16 15 5.8 23 20 13.2 9.3
    td(off) ns 34 ~ 51 52 22 80 80 31 37
    rise time ns 5 ~ 8 18 7.2 23 49 11.7 5.4
    fall time ns 9 ~ 14 23 3.1 25 16 12 6
    pulsed current A 200 120 400 450 110 150 400
    forward V VsD 0.9 ~ 1.5 1.2 0.8 ~ 1 0.9~1.2 1.3 0.9~1.2 0.92~1.2
    Qg (nC) expected forced decay <100nC 70~105 75 44 90 88 95.4

    78
    Qgd (nC) 13 17 6.9 13 14 20.4 6.1

    IDRIVE current you mentioned to use max value.

    In our case this is the selection for Idrive resistor.

    regards,

    Macjan

  • 0 in reply to macjan fernandes
    TI__Mastermind 37683 points

    Hi Macjan,

    The FETs comply with the FET requirements.

    macjan fernandes said:

    IDRIVE current you mentioned to use max value.

    In our case this is the selection for Idrive resistor.

    You need to set R_IDRIVE to 68kΩ (150mA) so that the rise time is the fastests.

    regards,

    Pablo Armet