understanding MOSFET driver ICs

[j_writer]

I am looking at two MOSFET driver ICs that are similar, the Supertex HT 0440 and the FairchildFOD3181, but some basic information is missing...
1) Do these devices provide output voltages to drive the gates or are they just passive switches?
2) Can anyone point to actual schematics that use these drivers?
3) The outputs are rated at 0.5 A, but again, is this just when they are used as passive switches?
Any help or explanations would be appreciated.

 

[keith1200rs]

The two devies you list are quite different. The Fairchild one is more conventional. It has an opto isolator and MOSFET gate driver. It requires a power supply on the isolated side to generate the gate voltage.

The Supertex one internally generates the gate voltage on the MOSFET side of the isolation barrier so doesn't rely on you providing a suitable gate power supply. See applications note AN-D26 for the HT0440.

They are not passive devices.

The 0.5A rating is the amount of current it can drive into the gate of the MOSFETs.

Keith.

 

[j_writer]

Keith, thanks a lot. This really helps.

a) I just found an app note from Fairchild that shows how to bootstrap a gate voltage (see below). This is new to me and I especially don't understand the use of the HVIC at the bottom. Is this how to use the FOD3180?

b) Regarding the HT0440, it looks like the switching and delay times are huge (see below). Fall time is 3 ms - arg! Can the HT0440 be used in 50-100 kHz applications? Is this what the clock is for, or how do you toggle between A and B at a 50 kHz rate?

Would appreciate any help.

 

[keith1200rs]

The clock on the HT0440 (which can be internally generated) if for generating the floating voltage on the output side. It is slow. The reason for this is that the current to drive the MOSFET gate has to pass through the isolation barrier. There presumably isn't a huge amount of current available (around 2uA sinking current according to my calculation, which sounds wrong, but I checked it twice). So, forget that for anything fast.

The FOD3180 is quite different. You need to supply 10V to 20V for the gate drive, but it can pump 2A into the gate so it will switch a transistor a lot faster.

If you don't need the opto-isolation there are plenty of other devices to choose from.

I will get back to you on the FOD3180 when I have chance to read the apps note - I have to go out for now.

Keith.

 

[j_writer]

I think this device (FAN7382) might provide the voltage for the gate drive. It is from section 6.4 of the bootstrap app note. I still don't understand how it works. It is not optically isolated but it is faster than the HT0440. Maybe you can let me know if this circuit actually works. Thanks.

 

[keith1200rs]

I think this device (FAN7382) might provide the voltage for the gate drive. It is from section 6.4 of the bootstrap app note. I still don't understand how it works. It is not optically isolated but it is faster than the HT0440. Maybe you can let me know if this circuit actually works. Thanks.

Yes, that looks ok, assuming you want a half bridge driver, as show in the diagram (one PMOS, one NMOS) and assuming you don't need opto-isolation.

Keith.

 

[FvM]

I think this device (FAN7382) might provide the voltage for the gate drive. It is from section 6.4 of the bootstrap app note. I still don't understand how it works.

Consult to IRF gate driver datasheets and application notes. IRF invented the high-side bootstrap driver principle. In my opinion, the
operation is straight forward and can be easily understood from the detailed explanations.

In a short, the high side gate driver voltage is supplied by the bootstrap capacitor, which is charged during on-time of the low-side switch.
The "high-voltage" IC aspect is involved by level-shifting control of the high-side driver.

 

[dick_freebird]

The only downside of bootstrapped high-side drivers is their
need for a finite minimum "off" time, to recharge the VH
reservoir capacitor. So unsuitable for a DC application.
Conversely a DC app might be able to stand the limited
power delivery of a charge pump high side gate supply
where a high switching frequency app might bleed it too hard.

 

[j_writer]

Yes, that looks ok, assuming you want a half bridge driver, as show in the diagram (one PMOS, one NMOS) and assuming you don't need opto-isolation.

Minor correction: I think these drivers are designed to allow you to use two N channel devices.

In my opinion, the operation is straight forward and can be easily understood from the detailed explanations.

In a short, the high side gate driver voltage is supplied by the bootstrap capacitor, which is charged during on-time of the low-side switch. The "high-voltage" IC aspect is involved by level-shifting control of the high-side driver.

What do VB and VS typically look like? Are they both swinging up and down, following the load voltage? To me, the data sheets are not clear, as shown in the attached figures. What are the HO and LO voltages, and how do I know if they are within the gate voltage ratings?

The only downside of bootstrapped high-side drivers is their
need for a finite minimum "off" time, to recharge the VH reservoir capacitor. So unsuitable for a DC application.
Conversely a DC app might be able to stand the limited power delivery of a charge pump high side gate supply where a high switching frequency app might bleed it too hard.

Can I assume the 7382 would work at 50 kHz, 50% duty cycle? If the combined propagation and rise time delay is about 250 ns, this is only 1.25% of the total period.

 

[FvM]

I keep my opinion, that the circuit behaviour can be understood from the datasheet description, also with the Fairchildsemi
drivers, although I mainly refered to IRF. To understand the basic operation, assume the driver transistors as ideal switches (possibly
with a finite rDSon). The bootstrap power supply is extern to the chip (diode and capacitor), except for an internal zener diode intended
to protect the chip rather than the connected FET. You'll easily see, that the driver output voltage can't be basically higher than the external
supply, which is typically between 12 and 15 V.

Now addressing some details aspects.

What do VB and VS typically look like? Are they both swinging up and down, following the load voltage? To me, the data sheets are not clear, as shown in the attached figures. What are the HO and LO voltages, and how do I know if they are within the gate voltage ratings?

Part of the question seems ignorant to me, e.g. if VS is swinging with the load voltage. It's connected to the load by a wire.
Do you see a way how it's not swinging with the load voltage?

The other points can be answered by following the above considerations. As said VS isn't actually a question, VB is kept at a constant
difference to Vs (at least in a short time scale) by the bootstrap cap. HO and LO can swing between zero and the respective supply.

Can I assume the 7382 would work at 50 kHz, 50% duty cycle?

Surely.

A special point is about Vs allowed to swing below GND. That't not basic operation, but often required by the non-ideal behaviour of
the connected power stage.

 

[keith1200rs]

Yes, that looks ok, assuming you want a half bridge driver, as show in the diagram (one PMOS, one NMOS) and assuming you don't need opto-isolation.

Minor correction: I think these drivers are designed to allow you to use two N channel devices.

Sorry, my mistake. Do you really want a bootstrapped dual NMOS drive to get a push-pull drive? If all you want is push pull drive it is probably easier to use PMOS/NMOS and a simper drive IC. The bootstrapped ones are not without their problems.

Keith.