DRV8302 Maximum Frequency with IRFS7530

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Dimitri
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Joined: 07 Dec 2016, 02:25
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DRV8302 Maximum Frequency with IRFS7530

Postby Dimitri » 07 Dec 2016, 03:02

Since the DRV8302 is driving 6 IRFS7530 FETs which have huge Gate capacitances, there has to be a fairly low maximum Frequency limit allowed for driving the IRFS7530 FETs before you damage the DRV8302. Which is a concern to me and i guess most of us here.
So to calculate the Maximum Frequency, we can look at DRV8302 Datasheet, page:22 for an example.

"8.2.2.1 Gate Drive Average Current Load
The gate drive supply (GVDD) of the DRV8302 can deliver up to 30 mA (RMS) of current to the external power
MOSFETs. Use Equation 5 to determine the approximate RMS load on the gate drive supply:
Gate Drive RMS Current = MOSFET Qg × Number of Switching MOSFETs × Switching Frequency - [Eq.5]

Example:
7.83 mA = 29 nC × 6 × 45 kHz
This is a rough approximation only."

Ok cool, that makes sense, since: I = Q x f = Q/T

So using IRFS7530 maximum Gate charge: (IRFS7530 Datasheet, Page:3) Qg max (for Vgs = 10V) = 354 Coulombs

and the same logic as above.. then..
I = Q x f => f = I / Q

So the Maximum PWM Frequency the DRV8302 can drive the 6 x IRFS7530 at, must be:
30mA / (354nC * 6) = 14.124 kHz

Anyone know what is the: Maximum PWM frequency that the DRV8302 drives the FETs at?
Because 14kHz seems quite a low, maximum frequency limit to me..!!
Unless i am making a mistake in this simple calculation.

pf26
Posts: 310
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Re: DRV8302 Maximum Frequency with IRFS7530

Postby pf26 » 07 Dec 2016, 15:39

Typical gate charge is 236nC (as per the Fets datasheet - at Id=100Amps).
So you might be able to reach higher PWM frequency without damage.
But I do agree with your calculations, and I think this puts the DRV8032 at risk in hot environnments or at higher frequencies.

rew
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Joined: 25 Mar 2016, 12:29
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Re: DRV8302 Maximum Frequency with IRFS7530

Postby rew » 08 Dec 2016, 09:08

We (Benjamin and I) think that the "30mA" limit is because gvdd is generated from the input voltage using a linear regulator, and that the package is limited to about 1.5W. 60V-12V = 48V voltage drop. 48V *30mA = 1.5W.

Thus any margin towards 60V gives extra margin that you might use above the 30mA spec.

In practice switching happens at 20kHz, about 50% above the limit you calculated. But you normally get that back from the "typical" gate charge that is also a factor of 1.5 lower. But yes, worst case, at 55V, the DRV will suffer beyond it's rated limits.


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