UCC27712: use UCC27712 high-side switch only

Hi Jay,

Thank you for your interest in UCC27712. 

Yes, it is possible to use the high side driver only to drive a FET. Looking at your schematic, I noticed a couple of things. It looks like you have COM floating, although it is the ground for this device. The inputs of the device are also referenced to COM. Also, since you will not be using the low side, you could also have LI connected to ground and then LO can be unconnected. Out of curiosity, could you use the PWM to control both HI inputs? I see that it is connected to a 12V bus but you could use the PWM to control the inputs if you would like. I also see that you have a bootstrap capacitor for HB but it is connected to ground. Typically, this capacitor would be connected to HS, as HB is the high-side floating supply, and HS is the return for that supply. There is still a need for the HS pin of the driver to transition to ground for there to be a charging path from VDD through the boot diode charging the boot capacitor. Section 8 of the datasheet should also help with selecting component values based on your design requirements.

Also, you will need to make sure that the high side is biased well for your application. For example, if your high side outputs has a long ON time, then you may need to change your biasing methods for the high side. I have attached a link that describes different biasing methods and how to bias the high side of a half-bridge driver: https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1330910/faq-how-to-bias-the-high-side-of-a-half-bridge-gate-driver-and-why The FAQ also covers a charge pump method, which acts like a bidirectional switch. Perhaps this method would be a good option for your battery charger application.

Please let me know if you have any additional questions or if this helped address your question. 

Thanks,

Zihao

Hi Jay,

I'm happy to help!

jay Prabhakar said:

Even though the app note says it is talking about the UCC27712, the examples are all about the UCC27288...

• Since I have already purchased the UCC27712s, can I still use those?

Regarding the app note, something I missed mentioning was that the charge pump configuration shown works on devices that don't have interlock. UCC27712, however, does have interlock, which is why you likely saw it not being used in the examples. You will still be able to use the UCC27712s however. One method is to use a dual channel low side device like UCC27624 and configure the output so that it will be going into the charge pump configuration. So, instead of using LO of UCC27712, you would use the output of UCC27624 instead. This is one way of doing this.

jay Prabhakar said:

I like the external DC-DC converter solution, since it is so much "cleaner"

Looking at your schematic, I prefer an isolated converter solution. One concern I have is during turn on and turn off, there could be transients or ground bounce, for example, that might affect the HB to HS specification of the device, which can lead to failure. With an isolated converter, you are "safer" and have it reference to HS instead of COM for the output of the converter. You could have an isolated converter such as using the SN6501 or the aforementioned UCC27624. I have attached another app note that uses a gate driver as a gate driver transformer. Section 4.5 and Figure 4-3 covers the configuration, but the app note also covers a lot about using a gate driver as a transformer, which might be useful for your DC-DC converter design. 

Also, having just a high-side FET is ok too. We have seen systems with just one high-side FET and also systems with back to back FETs. 

Please let me know if you have any additional questions or if this helped you.

Thanks,

Zihao

Hi Jay,

jay Prabhakar said:

Is there a device like the UCC27712 that can handle more than 100V Vhs?

For UCC27712, HS can handle more than 100V (it can go up to 600V per the datasheet linked here). You will just need to make sure that you do not violate the HB-HS specification because it may cause device failure. 

jay Prabhakar said:

I am confused by your statement, "...works on devices that don't have interlock"

Interlock is a function that prevents both outputs from being turned on simultaneously. Since you can't have both outputs on simultaneously, the previously mentioned document about charge pumps did not use UCC27712 in its examples (you can still have a charge pump designed for UCC27712 though). UCC27288 does not have an interlock feature. 

jay Prabhakar said:

Also, can I use the one DC-DC to power TWO of the UCC27712 devices?

I wouldn't recommend this method. One example concern I have is that the devices' HS are at different voltage levels when tied together and that could cause issues. I would prefer having a supply for each instead. 

jay Prabhakar said:

Can I use that to power Vhb instead of the bootstrap circuitry?

You can most certainly use an isolated DC-DC converter to power the high side of the device! 

I will put the FAQ about biasing the high side here again but it looks like you're on the right track with using an isolated converter to power the device.

Please let me know if this helped answer your questions or if you have any additional questions.

Thanks,

Zihao

Hi Jay,

Yes, you should be able to drive the FETs the way you have set up, and like you mentioned, you will need to account for the gate capacitances and adjust some component values in your system to accommodate if you have this set up. As a small bonus, we do have models for this device on our website that you can use to try to simulate and make sure the values are good, and we also have formulas in our datasheet that should also help!

Please let me know if you have any additional questions or if this helped you.

Thanks,

Zihao