Problems with Gate Driver IC

I have been working with the gate drive IC IRS2186 and I have posted the recommended connections from the datasheet, with Vcc at 12V and the MOSFET high side voltage at 30V and 10µF for C2 and 1µF for C1. I can get the low side to work just fine but the high side HO always remains at 12V no matter what input.

As far as I know the diode and C2 are supposed to act as a charge pump, though does this mean that Vb should be at a higher voltage than 30V, because it is always at 12V only? I have now burned out a few of these and am unsure of what I am doing wrong, any help or explanation of what the charge pump output is supposed to look like would be helpful.

Ultimately I would like to increase the voltage to 170V though I thought a lower voltage test setup would help.

Thanks

Hi,

I assume you didn´t connect a load.
--> connect a load.


Klaus

Still probably will not work, even with a load connected.

Vs needs to be pulled right down to ground for C2 to charge, the load is going to prevent that from happening.

Depends on the load. The part looks tailored to a bridge
switching topology and could be unsuitable for a static
application where the load spends all its time "on".

Even if the application is constant switching, bench test
in a static condition will fail to function properly. You have
to test it like you're fixin' to drive it.

No the diode and C2 do don't really form a charge pump. What you see is what you get - there isn't anything hidden in that circuit. The diode can only charge the cap if Vs is pulled to ground.

The circuit is built around a specific assumption that Vs is tied to the drain of the lower mosfet and that the lower mosfet will periodically short Vs to ground, allowing the cap to charge.

Your circuit didn't follow that assumption and hence doesn't work. It's a compromise circuit that provides semi-independent control of mosfets in an H-bridge without needing an isolated supply for the high side.

But if you really need independent control of a mosfet you need an independent isolated supply to drive its gate.

Thank you for your replies, no I had not connected a load, as I thought I would be able to test it first. I have tried connecting Vs to ground, at which point HO then switches from 0V to 12V but since it is now referenced to ground it didn't seem like it would drive the high side mosfet? The application is for a full bridge transformer drive, so in regular operation the voltage of Vs should approach 0V.

Though I had thought that I needed to drive the gate of the upper mosfet higher than the 30V rail, since when switching off the source would be close if not at the 30V then since the gate-source voltage needs to be ~10V to turn on it would need to be 40V? Since it sound like I need an independent supply, what would that consist of, could I just use a transformer from mains and then rectify and supply to Vb as long as the transformer supply voltage is higher than the H-Bridge supply?

Thanks

could I just use a transformer from mains and then rectify and supply to Vb as long as the transformer supply voltage is higher than the H-Bridge supply?

Click to expand...

No. Vb must be held at a constant 12 to 15 V offset (=VCC) above Vs, the diode is achieving this in combination with C2 in a regular half bridge configuration (low side drain connected to high side source). The doubts about the operation of your circuit have been raised because this connection is missing in the schematic.

Vb-Vs must not be higher than 18 V, otherwise driver and MOSFET will be damaged.

Thank you for the information, looking at the circuit of the IRS2186 it seems like the high side output will switch between Vb and Vs, which as you say shouldn't exceed 18V difference, however doesn't Vb have to be at a voltage level higher than than the H-bridge supply, as it needs to drive the high side mosfet?

The reason the schematic doesn't show the high side source connected to the low side drain is because the chip is for one half of a Hbridge, not a half bridge, and both the high and low sides are designed to be turned on at the same time, unlike a half bridge where they are turned on opposite to one another.

For a regular half bridge how is the high side gate driven to a higher voltage than the supply, or does it even need to be using a similar IC?

Hi,

Don't connect Vs directely to GND.
As mentioned before: connect it to the drain of the low side Mosfet.

You have to control both Mosfets. Sometimes the low side is ON and sometimes the high side is ON, but never at the same time.
When the low side is ON, then the bootstrap capacitor gets charged.
When the high side is ON, then this lifts Vs to 30V and the high side gate to 40V (according your example).

Klaus

Ok I will try connecting a load the circuit and connecting Vs to the low side mosfet drain, and hopefully that will help. Though the IRS2186 I have doesn't turn on HO and LO separately, an input to HIN and LIN turns both on at the same time, as it is a part of a full Hbridge not a half bridge.



As far as I am aware I am trying to create S1 and S4, which will turn on in unison. Later I will make S3 and S2 to turn on together, though in opposition to S1 and S4. This will provide full voltage swing across the load.

No, typically a half bridge is S1/S2 or S3/S4. So the nodes you labeled "load" get connected to each other and that forms the S1/S2 connection which is the left side of M.

Just to re-iterate, this bootstrapped gate drive scheme demands that S2 and S4 turn on regularly otherwise C2 deplete itself and leave no more gate drive for S1. The control scheme must guarantee this otherwise you'll lose control and/or S1/S3 may fail.

I notice that the confusing schematic in post #1 has been copied from a IRF datasheet, but the discontinuity between LS drain and HS source node isn't related to any datasheet or application note description, thus I guess it's a drawing artifact.

As far as I am aware I am trying to create S1 and S4, which will turn on in unison. Later I will make S3 and S2 to turn on together, though in opposition to S1 and S4. This will provide full voltage swing across the load.

Click to expand...

As already discussed, highside S1 will never turn on if S2 hasn't been operated before so that the bootstrap capacitor could be charged. In so far "later" doesn't work. It doesn't particularly matter how the IRS2186 low- and high-side drivers are arranged, having S1/S4 and S2/S3 driver combined can work, too, because both halves are electrically independent.

IRS2186 has an undervoltage detect in high-side driver that prevents activation with missing or insufficient bootstrap supply voltage. Correct bootstrap startup is nevertheless not simple and cause all kinds of problems including bridge shoot-through and blowing transistors.

You need to supply external supply voltage to C2.
In order to open the high side Mosfet, there must be a voltage between G and S.

- - - Updated - - -

The drawing is a little bad, but it does.

Hi,

You need to supply external supply voltage to C2.
In order to open the high side Mosfet, there must be a voltage between G and S.

Click to expand...

..."open circuit" I think is considered as "no current flow"
"Open"... If you see it as a mechanical switch...then when the contacts are open --> there is no current flow
"Open" ... if you see it like a water faucet .... then when the faucet is open --> there is water flow
Therefore I don't like the term "open" when one talks about Mosfets.

If you want a Mosfet to be conductive, then you need voltage across G-S.
If you want a mosfet to be isolationg, then you need zero voltage across G-S.

If you have a switching application with the switching frequency always above some 100Hz (depends on used components) then you need no extra power supply. Then the bootstrap circuit (diode, capacitor) delivers/stores the energy for valid V_GS levels.

Klaus