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I am currently planning to upgrade an existing circuit which charges a lead acid battery using a non-synchronous buck topology. The NMOS which i have used is IRFP4768PbF.Currently my driver is the IR2110 which uses a bootstrap capacitor.

I would like to remove the IR2110 and replace it with an opto isolated gate drive(TLP5701) with a dedicated dc/dc converter. I have drawn up a shematic and i have a few quiries in it ,regarding he dc/dc converter.

enter image description here 1.I am going to use the MEU1S1212ZC(12v/83mA).The input to this module will be referenced to the system groung(solar-) and the output will be referenced to the source of the MOSFET. The the current sufficient enough to grive the MOSFET?

2.What should be the values of R1 and R2.Is 10 ohm(R1) and 3k(R2) sufficient enough?

Edit: I have sketched the DC/DC converter section also to avoid any confusion. enter image description here

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    \$\begingroup\$ In the drawing it says: "referenced to source" You probably mean the secondary side of the isolated DCDC converter, then draw that connection to make it clear ! The DCDC has at least 4 pins, I see only one connected, that makes no sense. \$\endgroup\$ – Bimpelrekkie Aug 29 '16 at 8:57
  • \$\begingroup\$ That NMOS has a gate capacitance of around 11 nF so you'd want to choose R1//R2 such that RC product is much faster than what you need for the switching frequency of the NMOS. From the values of R1 and R2 + some margin you can determine the current drawn from the DCDC converter. Anyway, would it not be easier to just use a PMOS, then you do not need the isolated DCDC converter. \$\endgroup\$ – Bimpelrekkie Aug 29 '16 at 9:02
  • \$\begingroup\$ Edited my origenal post,have added a sketch just showing the dc/dc converter section.Input to it is the lat battery of my battery pack w.r.t system ground(solar-) and output goes to the Vcc of the optocoupler,output ground(Vout-) is shorted to Source of the MOSFET.I prefer it to be an NMOS,(very less switching losses) \$\endgroup\$ – SuperHuman Aug 29 '16 at 10:02
  • \$\begingroup\$ the MOSFET.I prefer it to be an NMOS,(very less switching losses) I would agree with that you choose the optimum NMOS and then tried to find an optimum PMOS. But if you choose a PMOS with a similar 11 nF gate capacitance, switching losses will be similar. My guess is that you can use a less powerfull PMOS and get similar or better switching losses. \$\endgroup\$ – Bimpelrekkie Aug 29 '16 at 10:19
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    \$\begingroup\$ 80 Amps ? Geez, using only one NMOS ? Then I am curious what inductor you're going to use. I hope one with a saturation current of at least 160 A or so. And the flyback diode as well. I hope for you that things will not blow up in your face ! You can calculate the gate current from the gate capacitance and switching frequency you want to use. \$\endgroup\$ – Bimpelrekkie Aug 29 '16 at 10:38
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Energy stored in a cap = QV/2.
Power needed to repeatedly charge a cap from a certain voltage:
P = freq * QV/2 * 2.
Average current needed: I = P/V = freq*Q.

Using the total gate charge Qg = 180nC from the datasheet (even though Qg depends on the actual gate and drain voltage), and 10KHz:
I = 180nC * 10kHz = 1.8mA
which is not much.

With the amount of current being switched at the load, it is easy to guess that minimizing the switching loss of the MOSFET is important. Just guessing a number to get some ideas, say 90ns for the gate charge/discharge time.

The gate drive charge/discharge current needed would be Qg/t (at 10V Vgs, same as the condition given in the datasheet): Igs = Qg/t = 180nC/90ns = 2A.

There is a big difference between the average and the gate-charge current because the duty cycle of gate-charge to total time is 90ns/100us = 0.0009. 100us comes from the inverse of 10kHz.

Some conclusions:

  • A DC/DC converter of 83mA average current should be more than enough.
  • Enough bypass capacitance would be needed to supply the gate-charge switching current of thousand times that of the average.
  • Driving the MOSFET directly with a optocoupler may provide only a fraction of the drive current wanted. The drive current probably should be in the amps range. But the actual required depends on the optimization of the MOSFET switching which takes much more to work out.

Edit to include a way to estimate bypass capacitance on floating supply:

Take the Input Capacitance from the datasheet, e.g. Ciss = 10880pF. If you want the voltage ripple caused by charging the base to be less than 1:100 on the supply, then use a bypass cap that is 100*Ciss*2 (the times 2 is for extra margin for output capacitance of MOSFET), that is 2.2uF in this example. It is a lot less than my impression when I wrote the above. Use ceramic capacitor for low ESR and high current capability.

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  • \$\begingroup\$ thanks for the detailed information.If i use a high power optocoupler say 2 amps,would that be sufficient.And if i have a huge bulk capacitor about 1000uF at the input would that be enough or do i need to have some more there ? \$\endgroup\$ – SuperHuman Aug 30 '16 at 11:27
  • \$\begingroup\$ Without working out the details (which need actual operating conditions plus extrapolation from datasheet values) it is hard to say. My guess is provision for 2A would be good (but it may be good enough even with less). \$\endgroup\$ – rioraxe Aug 30 '16 at 20:15
  • \$\begingroup\$ See my edit above for the capacitance. \$\endgroup\$ – rioraxe Aug 30 '16 at 20:35
  • \$\begingroup\$ Thanks for the capacitor calculation.I will use the TLP352 as the gate driver.It can give an ample 2.5As if required. \$\endgroup\$ – SuperHuman Aug 31 '16 at 7:10
  • \$\begingroup\$ Also should the input capacitor be referenced to system ground or to the source of the mosfet? \$\endgroup\$ – SuperHuman Sep 1 '16 at 6:28

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