DC-DC Converter for Gate Driver Circuitry (Electric Motorcycle)

Evening all,

I'm trying to pick out a suitable DC-DC Converter for our low power gate driver circuitry and I'm unsure about a number of different things.

First some background on the circuitry and what the DC-DC Converter is needed for.

To date, we've designed, built and tested a low power gate driver circuit similar (almost identical) to the one shown in the figure below. The end goal is to use such a circuit to drive a Power MOSFET Module for an electric motorcycle.



Anyways, we built and tested this circuit on a lab bread board that has its own variable 15V and fixed 5V supplies, and everything seems to work fine. However, the final product must have its own low voltage supplied via a DC-DC converter fed by battery bank of the motorcycle which is ~55V.

Thus we need a DC-DC converter capable of supplying us with an isolated 12V(or 15V) and 5V supplies. The 12V(or 15V) supply is used to power the gate driver IC (IR2110) as well as the power buffer circuitry,(i.e. VCC = 12V or 15V) while the 5V supply will act as the logic supply for the chip. (i.e. VDD = 5V)

One simple option would be to find a DC-DC converter that will provide the 55V to 12V(or 15V) needed and simply feed the 12V(or 15V) into a regulator such as the LM7805 and obtain our 5V logic supply from there.

It seems straightforward enough to find a suitable DC-DC converter that will meet our input/output voltage requirements, but I am uncertain as to what our output current requirements are.

According to the application note for our gate driver, the power buffers in the schematic shown above (i.e. the circuitry shown to the right of the IR2110) are needed to provide a higher gate drive current and lower gate drive impedance than what a typical gate driver IC can provide. (Application note is attached below)

They mention that the one shown in the figure above is capable of providing 8A peak output current, which is sufficient for driving Power Modules. (We are using a MOSFET power module, I've attached its data sheet below)

So does this imply that the 12V(or 15V) output from DC-DC Converter (i.e. VCC = 12V or 15V) must be capable of supplying 8A peak output current? The current needed to the drive the gate must be supplied by VCC, right? Or does the power buffer somehow amplify or boost the current?

Thanks for reading!

jegues said:

...does this imply that the 12V(or 15V) output from DC-DC Converter (i.e. VCC = 12V or 15V) must be capable of supplying 8A peak output current?...

Click to expand...

No, this is achieved with couple large ceramic+electrolytic capacitors, which are able to sink a fast charge to commutation devices.
Note that 8A peak value doesn´t means that DC power supply must be able to continuously supply this value.


+++

Seems to me that you ought to have a natural 12V available
and this is what many commercial gate drivers like. Are you
re-re-re-inventing the wheel?

Hi andre_teprom! :smile:

andre_teprom said:

No, this is achieved with couple large ceramic+electrolytic capacitors, which are able to sink a fast charge to commutation devices.
Note that 8A peak value doesn´t means that DC power supply must be able to continuously supply this value.


+++

Click to expand...

This wasn't obvious for me, as the only context I've seen decoupling caps are in textbook amplifiers where you want allow the small ac signal to pass in order to be amplified, but block the DC by using a capacitor.

What I'm referring to above, are they called coupling capacitors or decoupling capacitors? (Maybe I'm mixing up some terminology here)

On the bottom of page 12 of the application note, they suggest the use of good quality 10mF tantalum or 10mF electrolytic and 0.1mF ceramic capacitors at the output of the buffer, but this is confusing because the values shown in the schematic are 10μF and 0.1μF across the rails of the buffer.

How do I figure out which is the correct value they are referring to?

Also, are both the 10μF and the 0.1μF referred to as decoupling capacitors?

Is the 10μF the decoupling capacitor that provides the necessary current drive while the 0.1μF decoupling capacitor is used for filtering undesired noise?

I realize a lot of what I've said above may sound a little goofy, but I am genuinely confused about decoupling capacitors.

Hopefully you can help me clear things up!

- - - Updated - - -

Hi dick_freebird! :smile:

dick_freebird said:

Seems to me that you ought to have a natural 12V available
and this is what many commercial gate drivers like. Are you
re-re-re-inventing the wheel?

Click to expand...

Could you explain what you mean by "a natural 12V available"?

Are you referring to an auxiliary 12V somewhere off the motorcycle? (such as those used to power the lights etc.) If so, we did explore this option, but we want to use an isolated supply such that our circuit won't effect any other auxiliary circuitry on the bike. (such as the lights)

Decoupling capacitors are called this way when were applied to isolate different DC bias level voltages.
The topology of this use is placed on series of signal.

What I suggested was to apply as Filter Capacitors, and it is placed on parallel of DC supply bus.



+++

Hi andre_teprom! :smile:

andre_teprom said:

Decoupling capacitors are called this way when were applied to isolate different DC bias level voltages.
The topology of this use is placed on series of signal.

What I suggested was to apply as Filter Capacitors, and it is placed on parallel of DC supply bus.



+++

Click to expand...

What distinguishes between the 10uF and 0.1uF decoupling capacitors?

Aren't the 10μF decoupling capacitors are referred to as the bulk capacitors, while the 0.1μF decoupling capacitors are referred to as the local capacitors?

Shouldn't the higher capacitance bulk capacitors be placed closer to the power supply (i.e. the input of the power buffer), while the local capacitors closer to the actual load? (i.e. the output of the power buffer) If so, this seems to be opposite of what we observe in the schematic above.

Thanks again!

jegues said:

What distinguishes between the 10uF and 0.1uF decoupling capacitors?

Click to expand...

I´m sorry for mistake, you are totally right.
Decoupling capacitors refers for the ones placed in parallel.

jegues said:

Aren't the 10μF decoupling capacitors are referred to as the bulk capacitors, while the 0.1μF decoupling capacitors are referred to as the local capacitors?

Click to expand...

This distinguish properly don´t matter, once each part of circuit require different amount of instantaneous charge.

Note that values around 0,1uF not necessarily must be of electrolytic type, but is strongly recommended to be ceramic, due to its smaller equivalent series resistance ( namely ESR ) and also smaller parasitic inductance, if compared to ceramics; so I could suggest you add on above schematic, to place a 0,1uF ceramic in parallel to the 10uF ( it is like ceramic ones placed in parallel are able to potentiate decoupling skills of electrolytic ones ).

jegues said:

...Shouldn't the higher capacitance bulk capacitors be placed closer...

Click to expand...

In fact, placing capacitors closely to switch device, increase in certain way, its capacity to sink higher peak currents.
The final result is a better performance to this circuit to drive the power MOSFET ( that means, decrease losses by heating ).





+++

Hi andre_teprom! :smile:

I have a couple more questions about the schematic I posted in my original post.

Are the decoupling capacitors for the high side switch also functioning as bootstrap capacitors? Our highside switch requires a bootstrap capacitor and I'm wondering if we have to add this in, or if the decoupling capacitors will also serve this purpose.

Also, what is the purpose of the leftmost 100uF capacitor? It seems to be connected between Vcc and Vss.

Thanks again!

jegues said:

...what is the purpose of the leftmost 100uF capacitor? It seems to be connected between Vcc and Vss...

Click to expand...

Seems works as stabilizer of VCC bus once this voltage is generated indirectly.


+++

What about my question relating to the bootstrap cap? Can you clarify that part for me?