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Posts: 6
Registered: ‎05-06-2017
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Si827x driving speed and ability

1. The source and sink current of isolated driver of Si827x are 1.8A and 4A respectively, but the fall time is larger than the rising time. This phenomenon is quite different from drivers manufactured by TI, Maxim and so on. 

2. The load used to test the rise and fall time of Si827x is only 200pF, while the typical capacitance of MOSFET or GaN are 1nF. Even with 200pF capacitance, the rise and fall time is still larger than the driver of LM5113 from TI whose source and simk current is 1.2A and 5A respectively with load capacitance 1000pF.

     Can anybody help to explain this!

<a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Super Employee</font></font> </a> DKC
Posts: 7
Registered: ‎03-14-2017

Re: Si827x driving speed and ability

This ticket will be supported through Sales Force.

 

Thanks,

Application Support

<a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Super Employee</font></font> </a> DKC
Posts: 7
Registered: ‎03-14-2017

Re: Si827x driving speed and ability

[ Edited ]

Sir,

Tr and Tf depend on load.  Yes we chose 200pF as that was a common load choice for our automatic test system. 

As you've noticed TI only specifies this at one point, 1000pF.  However, if the load is swept it will give a a more complete picture. 

Silicon Labs' driver datasheets generally provide the 1 point guarantee spec like TI.  However, we also provide a picture illustrating how the Tr/Tf vary versus load.  For instance, see the picture below for the Si823x family.   As the plot below shows, either Tr or Tf can be larger depending on load.  A similar plot is in our Si827x datasheet.  In general this relationship will depend on whether PMOS or NMOS type transistors were used on a given driver's output stage.  The exact design details for a driver are usually proprietary. 

Si823x (4A, Tr/Tf versus Load)

See Figure 3.18. Rise/Fall Time vs. Supply Voltage in Si823x datasheet.

 

 

Thanks,

Keith Coffey

Application Support

Posts: 6
Registered: ‎05-06-2017

Re: Si827x driving speed and ability

Thanks very much for you comprehansive explanation!

     In fact, as you have said that either Tr or Tf can be larger depending on load, driving voltage, driver output stage transistor characteristics and so on.  The question is Si827x series driver can be used to drive various power semiconductor device with different gate capacitance at different driving voltage and swiching speed. The Tr/Tf vs load test figure under only one special test conditon can't cover all actual situations as illustrated with Figure2.12 in Si827x datasheet. Maybe it's a problem of Limitation of my knowledge. But  I have no idea how to estimate the Tr/Tf time when adopting Si8271 to drive a GaN device with 5V/0V turning on/off voltage with the informaton from the Si827x datasheet. 

     The driving current may change with the variation of the gate voltage, so the Tr/Tf can only be determined by precise simulation or experiment. I hope that the information of Tr/Tf time of Si827x with 5V voltage and 1000pF capacitance load can be given in the datasheet for the convenience of usage.

 

<a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Super Employee</font></font> </a> DKC
Posts: 7
Registered: ‎03-14-2017

Re: Si827x driving speed and ability

Sir, This information is already included in the Si827x datasheet. See Figure 2.12. Rise/Fall Time vs. Load.
<a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Super Employee</font></font> </a> DKC
Posts: 7
Registered: ‎03-14-2017

Re: Si827x driving speed and ability

Sir, You are correct, we can't test all conditions. I am gathering additional data and hope to provide it to you by the end of this week for the 5V condition with 1000pF load. I will also update that Figure as it is in question. Tr/Tf should not be flat for all load conditions. Stay tuned. Application Support
<a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Super Employee</font></font> </a> DKC
Posts: 7
Registered: ‎03-14-2017

Re: Si827x driving speed and ability

The data at 5V VDDA at 1000pF is Rise 10.2ns Fall 10.1 ns.
Posts: 6
Registered: ‎05-06-2017

Re: Si827x driving speed and ability

Thanks very much.


DKC wrote:
The data at 5V VDDA at 1000pF is Rise 10.2ns Fall 10.1 ns.

 

Posts: 8
Registered: ‎05-26-2016

Re: Si827x driving speed and ability

Better use complementary BJT totem pole external driver for the peace of mind, chips are best to be used as pre drivers.

<a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Super Employee</font></font> </a> DKC
Posts: 7
Registered: ‎03-14-2017

Re: Si827x driving speed and ability

This question has already been answered.

 

Thanks,

Application Support

Posts: 8
Registered: ‎05-26-2016

Re: Si827x driving speed and ability

Even if it is answered, the Capacitive load vs Frequency graph in the datasheet is invalid in real world applications.