UCC2800: application of UCC2800

Robin,

To try to answer your questions:

1) Yes the single-ended PWM controllers UCC2800 series can also be used to control a boost power stage, just like UC284x family.

2) To drive GaN power devices, you will need a suitable external driver and supply rail for the driver. While negative turn-off voltage is usually not required, the max gate voltage is very important and typically must be tightly regulated to prevent damage of the GaN device. However, the exact gate drive requirement and arrangement will depend upon the chosen GaN device. I would recommend consulting the GaN device supplier for their own specific gate drive recommendations.

3) What is the spec you are trying to achieve? Input voltage range, output voltage, output power?


Thanks,
Bernard

Bernard

thnx much for prompt clarification .

Seems to me that Gansystems devices can be driven "just like " MOSFETs. according to my conversations with them. We need 650V  devices.

VCC to UCC28000 will be from an LDO so it will be quite  well regulated. If the output of the UCC28000 can drive a heavy  Ciss of a MOSFET, it will be no big deal for the GaN. But of course we will follow all other strict design guidelines for appropriate methods of turn ON/OFF.

What we can do in this instance is by no means a "general" rule for  driving GaN devices.

Of course, the sensitivity of the drive will depend upon layout. In small geometry of these devices, this can be well controlled as well.

Now: power being switched is from three phase input 400Hz, max 330W load. So it is relatively "low" power.

I am in process of making a pcb to  figure all this out asap.

robin

Bernard: One more thing.
As in the other UCC28C series, one can use UCC2803 in voltage mode by adding a ramp to the CS input, right?

Brad
Continuing with UCC28000 Boost design, what do you recommend to kick start the circuit in real implementation? I see that it has internal sow start scheme. Is that enough to start a boost stage? Do you not need some artifact from the input to the output to bias at start up? Appreciate if you could find out how start up takes place....can this be modeled in TINA?
I can send you the schematic in a confidential manner; if I select "become a friend" is it possible for you to accept it which allows me to send you the schematic without it going public. Let me know.

Robin,

Yes you are correct, it is possible to run the UCC280x devices in voltage mode by feeding a ramp signal from the osc to the CS pin.

The UCC280x devices include internal 4-ms soft-start ramp. If this is not sufficient for your application, an external soft-start circuit can be added to further slow the ramp up of the COMP level at startup.

The internal soft-start is explained in the datasheet on page 21/22.

There are 3 TINA models available:
www.ti.com/.../toolssoftware

TINA-transient model, TINA-startup reference design, TINA-steady-state reference design.

I expect that the transient and/or startup models would be suitable for you.


If you want to send me your sch to review outside this forum, you can email me directly at bernardkeogh@ti.com.

Thanks,
Bernard

Bernard

thnx really for this information.

I will follow up on the model...

Now , an interesting observation here: and I am stating this with some trepidation because this is a contrarian one. Totally.

In this circuit for input current shaping from a three phase source, boost mode operation does not use a classical PFC chip....surprise surprise.

for a good reason.

If you want to avoid having to deal with inrush current, you want a few things out of the boost arrangement of the switch.

- the controller can operate from very low voltage. In our selection, it is 4v. We generate this off the input ac using a HV LDO

- Although COMP output has a delay but the gate drive output begins even when it is not yet in controlled operation.

So if you are switching  from very low voltage, the output capacitor is being forced to be charged up to boost output in a series of initial start up gate  pulses.  And the current is totally controlled by  the input inductors & ON times.

There is no need for any elaborate inrush current controller. Bear in mind that I have a patent on most elaborate inrush current limiter...LOL...this case shows limit of its application.

Are my observations about early start off of gate drive pulses correct, you think?

r