Capacitive parameters of MOSFET that affecting to the radiated EMI

[KZM]

Which  capacitance parameters need to be taking care if we compare the mosfet capacitance parameters that affect the radiared EMI performance? (eg, Coer or Cotr and Output capacitance and so on)

[Meghana]

Hello @KZM ,

Typically, its the di/dt and dv/dt of the switch that impacts the EMI performance of the system, and there are several factors that control these di/dt and dv/dt. 

If we just refer to the capacitances of the MOSFET, then we can say that all three capacitances, Cgs, Cgd and Cds have an impact on the dv/dt. Higher are these values, lower will be the dv/dt and better will be the EMI performance. But it will  hamper the efficiency of the system. Hence there is always a tradeoff. 

You may refer to the below application note to understand in detail how the EMI performance can be improved through some best practices related MOSFET and gate drive circuit parameters design. 

https://www.infineon.com/dgdl/Infineon-ApplicationNote_Optimizing_CoolMOSCE_based_power_supplies_for...

Please feel free to reach out to us if you require further support on this topic.

Regards

Meghana

[Ted_Liu]

IPW65R080CFDA，is it normal for Infineon mosfet to appear scratches on the pin, will it affect the use?

 

[KZM]

1. Thanks for your advice, it is a good reference app note for the engineer about how to improve the EMI. Can you pls share the reference part number of the ferrite bead that was used in that application note?

2. Since Mosfet capacitances are affecting the switching/gate drive losses and these affects the efficiency to low but  good result of EMI. is this statement, correct?

3. How about  Qrr or Qr (the lower the better),  which may affect both efficiency and EMI, the smaller the Qrr the better the efficiency and as well asEMI, is it true?

4. Any key parameters that affect the EMI apart from above parameters, in general, How to compare the EMI performance by seeing the datasheet it self?

Really appreciate your support🙏

[Meghana]

Hello @KZM ,

Please find the response below,

1. Thanks for your advice, it is a good reference app note for the engineer about how to improve the EMI. Can you pls share the reference part number of the ferrite bead that was used in that application note?

-- The application note provides the conceptual approach for improving the EMI of any system. Ferrite bead used in the experiment may not be relevant to be used in any application. Hence we refrain from sharing the part number as a reference. Instead we recommend to select one based on your application conditions.

2. Since MOSFET capacitances are affecting the switching/gate drive losses and these affects the efficiency to low but  good result of EMI. is this statement, correct?

- No, we cannot state it that way. MOSFET capacitances impact the dv/dt. But along with capacitances, dv/dt or di/dt in the system is controlled by other parasitics, gate drive design etc., If these transitions are faster, then the switching loss will be reduced but they may result in higher EMI, whereas if the transitions are slower then it could be vice versa i.e, EMI performance will be better but switching loss increases. Hence we cannot draw a direct relation between these capacitances and the system performance.

3. How about  Qrr or Qr (the lower the better),  which may affect both efficiency and EMI, the smaller the Qrr the better the efficiency and as well asEMI, is it true?

- Qrr in real time depends on the If, forward current through the diode and the commutation speed of the diode i.e., dIf/dt. Lower Qrr improves efficiency and lower dIf/dt improves EMI performance of the system. Since diode forward current 'If' also contributes to Qrr, lower Qrr do not directly mean that EMI is lower. 

4. Any key parameters that affect the EMI apart from above parameters, in general, How to compare the EMI performance by seeing the datasheet it self?

- EMI performance is dependent not only on the MOSFET but on the system design. It is not relevant to judge just based on the datasheet parameters. Overall EMI performance of the system is dependent on the MOSFET, gate driver design, layout design and lot more. Its a vast topic and difficult to provide a simple relevance between MOSFET parameters and EMI results. 

I hope above response clarifies your query. Please feel free to share, if you have more questions on this topic.

Regards

Meghana

[KZM]

Thank you for your comprehensive  answer to all of my questions above.  Allow me to ask the last question; Assume:

1. flyback topology 

2. the same PCB with the same controller and the same BOM 

but different MOSFET (with the same footprint),  I'll swap the below 2 MOSFET and measure the effiency and conducted/radiated EMI. which one will give us the better EMI and which one will give us better efficiency. Which parameters from the datasheet will give  us a hint in advance that EMI vs Effciency in general?

1. ISC060N10NM6ATMA1

2. BSC050N10NS5ATMA1

[Meghana · Accepted solution]

Hello @KZM ,

ISC060N10NM6 has lower capacitance values compared to BSC050N10NS5ATMA1, but Rdson of the former is higher. With this reference, we can say that conduction loss would be lower with BSC050N10NS5ATMA1, but switching losses would be comparitively higher. Depending on the switching frequency and duty time of the switch, overall efficiency has to be estimated. Without these details, it is not possible to comment if overall efficiency would increase or decrease. 

Similarly, as the capacitance values are different in BSC050N10NS5ATMA1, the resonance that these MOSFET capacitances would form with parasitic inductances in the circuit will shift and the magnitude of noise at those frequencies will increase or decrease cannot be directly judged based on datasheet parameters. But since capacitance values are higher, the current and voltage transitions will be slower compared to ISC060N10NM6, which may help to an extent in improving the EMI performance but its not possible to quantify the impact just with datasheet parameters. 

Regards

Meghana