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UCC21551-Q1: Questions about UCC21551-Q1

Chase Jeong
Mastermind 7405 points
Part Number: UCC21551-Q1

Tool/software:

Dear TI experts,

My customer considers UCC21551CQDWKRQ1 for their new project.

And my customer has few questiona about it. Could you check the questions below?

1. About DT (Deadtime) pin

In the red area, it is not recommended bypassing this pin with a ceramic capacitor > 1nF.

Is it okay to insert only proper resistor (not insert ceramic cap)? or Do you have a recommended value of ceramic cap?

2. About Bootstrap

2-1. the example of calculated Cboot value is 170uF. In the red area, TI selects 1uF capacitor with margin. and 1uF value is about 6 times of 170uF.

Is it okay to multifly 6 times to final Cboot value? or Do you recommend other multiplied value?

If 6 times is okay, do you have any reference document or formula about it?

2-2. Is △VDDA 0.5V right value? (VDD is 15V in customer's application.)

2-3. About applying Ivdd value, It is okay to apply the value in the blue area?

2-4. About bootstrap resistor, I only found the contents below. Where can I fined detailed description for calculating bootstrap resistor? Do you have any reference or application notes?

3. About calculating external resistor of gate pin

3-1. In the example below, it is okay to use the current value in the red box? or should I use different value which is fit to my customer?

3-2. The values of Ron and Roff which are calculated by the formula below is the minimum values that sre not exceeding gate output current. But I think I should consider switching loss, EMI and other variants.

Do you have any reference or application notes about it? (Ron and Roff value considering switching loss, EMI, and other things)

4. About Rgs

In the datasheet, Rgs is typically sized between 5.1k~20kohm and depending on the vth and ratio of Cgd to Cgs of the power devices.

Do you have any reference or application notes for calculating Rgs value?

5. About external Cge

In the datasheet, It is one of the option adding capacitor Cge to prevent problems due to C*dV/dt.

Do you have any reference or application notes for calculating Cge value in the blue box?

6. About INA/INB Input filter

It is okay to use RIN value between 0~100ohm and CIN value between 0~100pF and corner frequency of approximately 100MHz? any other recommendation or specific reference?

Please check these issues. Thanks.

Best regards,

Chase

  • 0
    TI__Expert 8686 points

    Hi Chase,

    Thank you for the detailed question.

    Chase Jeong said:
    1. About DT (Deadtime) pin

    Yes, it is okay to have only the deadtime resistor connection and no capacitor. There will be no issues there.

    Chase Jeong said:
    2. About Bootstrap

    2-1.) The general statement is that the bootstrap capacitor should be at least 10 times greater than the gate capacitance of the MOSFET.

    2-2.) The 0.5V is referring to the ripple voltage of VDDA. For a more detailed calculation of the minimum bootstrap cap value, refer to equation 3 of 3.1 Bootstrap Capacitor of the bootstrap circuitry selection application note.

    2-3.) Refer to the same area in the application note for the detailed calculation that includes the quiescent current of the supply.

    2-4.) Refer to 3.4 Bootstrap Resistor of the bootstrap circuitry selection application note.

    Chase Jeong said:
    3. About calculating external resistor of gate pin

    3-1.) Different values should be used in the equation to better match the customer's system such as VDD = 15V instead of the 20V used in the example.

    3-2.) 8.2.2.5 Estimate Gate Driver Power Loss in the product's datasheet has all of the calculations for static (quiescent) and dynamic (switching) losses. These can be utilized to have a better idea on the power loss effects of those resistor values.

    Chase Jeong said:
    4. About Rgs

    Refer to 3.5 dv/dt Protection of the application note on the fundamentals of MOSFET and IGBT gate driver circuits:

    Chase Jeong said:
    5. About external Cge

    Here is a useful reference with a detailed response from a colleague:

    Chase Jeong said:
    6. About INA/INB Input filter

    Those resistor and capacitor values are the recommended values that will fit most customer applications. However, tuning of these components should be done to better fit the system. The 100MHz corner frequency is a result of the 51ohm and 33pF RC values in the example. 

    Hope this addresses all of the questions.

    Regards,

    Hiroki

  • 0 in reply to Hiroki Honda
    Mastermind 7405 points

    Dear Hiroki,

    Thank you for your detailed support.

    My customer checked your answers and has some additional questions. Could you check these questions?

    1. About bypass capacitor, You said that no capacitor needed, but my customer wants to add bypass capacitor.

    Could you recommend the value of bypass capacitor under 1nF?

    2. About bootstrap, in the datasheet (not the document SLUA887A), 1uF value is just multifly 6 times of 170nF.

    Is there any reason that multifly 6 times? (formula or any other reference) How about multifly by 3 or 10? Is it not allowed?

    3. About per channel current consumption, I found the figure below;

    3-1. Can I get an exact value of VDD=15V, frequency=15KHz? If I have a formula to caculate, It is best for me.

    3-2. Do you have a figure of no load condition? (Figure 5-10 is the condition of 10nF load.)

    4. About VDD bypass capacitor

    I found the description in the SLUA887A document ;

         

    It says that Cvdd ≥ 10 * Cboot.

    They will user 3 UCC21551-Q1 in one PCB and there are 2 VDD bypass capacitors (in position of C35 and C137) in one UCC21551-Q1.

    So, there are total 6 capacitors in parallel.

    -> if Cboot is 4.7uF, the value of VDD Bypass Capacitor is (4.7*10) / 6 =7.83uF, so that the sum of 6 capacitance in parallel is 47uF, 10 times of 4.7uF.

    Is it right calculaation? or Am I missing something?

    5. About external gate resistor

    I fond the decription below in the document SLUA618A and SLLA385A ;

    5-1. The description in the red box says that it is recommended to start with 0 ohm resistor when calculating external gate resistor value.

    Bu my customer worries about defecting of the device because of the high gate charge current. So, Could you recommend the value of resistor instead of 0 ohm? (starting value of external gate resistor.)

    5-2. In the figure 15, I found that the value of capacitance depends on Vce. -> In the equation (1), calculating Ls, Could you check that Ciss value is under the condition of Vce? (i.e. Ciss value of eq(1) is under the condition of Vce=10V.)

    5.3. Could you check about the equation of calculating value of Roff and series diode? I cannot find the descriotion neither SLUA618A and SLLA385A.

    6. About dv/dt protection (calculating Rgs)

    I found the descriotion below in SLUA618A document ;

    Like I said before, I found that the value of capacitance depends on Vce.

    -> In the equation (20) and (21), Could you check that Cgd value is under the condition of Vce? (i.e. Cgd value of eq(20) is under the condition of Vce=10V.)

    7. About Bootstrap capacitor

    I found the desription below in SLUA887A ;

    If I use the equation above, How can I set the value of I_HBS? I cannot find this value in the datasheet.

     

    8. Finally, My customer will use IGBT with UCC21551-Q1. (not with normal MOSFET, SiC MOSFET) Is there any problem with it?

    Please check these issues. Thanks.

    Best regard,

    Chase

  • 0 in reply to Chase Jeong
    TI__Expert 8686 points

    Hi Chase,

    I will look into these questions and provide an answer early next week.

    Thank you for your patience!

    Regards,

    Hiroki

  • 0 in reply to Chase Jeong
    TI__Expert 8686 points

    Hi Chase,

    Chase Jeong said:

    1. About bypass capacitor, You said that no capacitor needed, but my customer wants to add bypass capacitor.

    Could you recommend the value of bypass capacitor under 1nF?

    If the customer wants to add a capacitor there, I would recommend a 1nF capacitor. This is the value I typically recommend to customers and have had no issues.

    Chase Jeong said:

    2. About bootstrap, in the datasheet (not the document SLUA887A), 1uF value is just multifly 6 times of 170nF.

    Is there any reason that multifly 6 times? (formula or any other reference) How about multifly by 3 or 10? Is it not allowed?

    The reason is to account for external factors that are not part of the calculation such as capacitance shift or load switching. I would recommend the customer to use their own judgment based on their expectations in their system to size this capacitor. 

    Chase Jeong said:

    3. About per channel current consumption, I found the figure below;

    3-1. Can I get an exact value of VDD=15V, frequency=15KHz? If I have a formula to caculate, It is best for me.

    3-2. Do you have a figure of no load condition? (Figure 5-10 is the condition of 10nF load.)

    3-1.) I do not have the exact value for this specific condition. An approximate calculation can be made using the equations in 8.2.2 Detailed Design Procedure of the datasheet.

    3-2.) Fig. 5-8 will have the no load condition:

    Chase Jeong said:

    4. About VDD bypass capacitor

    I found the description in the SLUA887A document ;

         

    It says that Cvdd ≥ 10 * Cboot.

    They will user 3 UCC21551-Q1 in one PCB and there are 2 VDD bypass capacitors (in position of C35 and C137) in one UCC21551-Q1.

    So, there are total 6 capacitors in parallel.

    -> if Cboot is 4.7uF, the value of VDD Bypass Capacitor is (4.7*10) / 6 =7.83uF, so that the sum of 6 capacitance in parallel is 47uF, 10 times of 4.7uF.

    Is it right calculaation? or Am I missing something?

    I believe your calculations apply if there is 1 high-side MOSFET being driven with a bootstrap circuit. 

    If 3 UCC21551-Q1 drivers are being used to drive 6 MOSFETs (3 high-side and 3 low-side), the calculations would be a bit different.

    If the chosen bootstrap is 4.7uF, then the VDD bypass cap for the low-side driver would be 47uF respectively.

    Chase Jeong said:

    5. About external gate resistor

    I fond the decription below in the document SLUA618A and SLLA385A ;

    5-1. The description in the red box says that it is recommended to start with 0 ohm resistor when calculating external gate resistor value.

    Bu my customer worries about defecting of the device because of the high gate charge current. So, Could you recommend the value of resistor instead of 0 ohm? (starting value of external gate resistor.)

    5-2. In the figure 15, I found that the value of capacitance depends on Vce. -> In the equation (1), calculating Ls, Could you check that Ciss value is under the condition of Vce? (i.e. Ciss value of eq(1) is under the condition of Vce=10V.)

    5.3. Could you check about the equation of calculating value of Roff and series diode? I cannot find the descriotion neither SLUA618A and SLLA385A.

    5-1.) The customer can start with a more safe value such as 10-15ohms. The equations in 8.2.2.3 Gate Driver Output Resistor of the driver's datasheet for currents can be used to approximate how much current it will result in through the gate path.

    5-2.) From the plot, it looks to be about 3000pF.

    5-3.) This can be found in 8.2.2.3 Gate Driver Output Resistor in the driver's datasheet to correlate the off resistance with the current.

    Chase Jeong said:

    6. About dv/dt protection (calculating Rgs)

    I found the descriotion below in SLUA618A document ;

    Like I said before, I found that the value of capacitance depends on Vce.

    -> In the equation (20) and (21), Could you check that Cgd value is under the condition of Vce? (i.e. Cgd value of eq(20) is under the condition of Vce=10V.)

    A 10kohm Rgs external pull-down resistor at the gate of the load is sufficient.

    Chase Jeong said:

    7. About Bootstrap capacitor

    I found the desription below in SLUA887A ;

    If I use the equation above, How can I set the value of I_HBS? I cannot find this value in the datasheet.

    This is present in the half-bridge drivers with integrated bootstrap. UCC21551-Q1 does not have an integrated bootstrap, so this value will not be present in the datasheet. This variable is referring to the bootstrap supply leakage current.

    Chase Jeong said:
    8. Finally, My customer will use IGBT with UCC21551-Q1. (not with normal MOSFET, SiC MOSFET) Is there any problem with it?

    UCC21551-Q1 is fully capable of driving IGBTs. 

    Regards,

    Hiroki