TPS54628: TPS54628DDAR inter FET body diode question

Hello Zong, 

Thank you for posting your question here. 

The reverse current should be avoided as possible, i would not recomend anything more that few mA, otherwise it may cause long term damage to the device. Is the current flow due to an over voltage / pre-bias condition at the OUT put? under what condition their is a concern of backwards current will be created for our device? The only case that I can think of is when the VOUT is higher the VIN and Body diode will start conduct and possibly creating a negative voltage at the VIN. This device implement an output discharge to prevent some of that.  There some work around by adding Sotkey or diode that prevent diode, see section 5 on this application note. 

https://www.ti.com/lit/eb/slyy151a/slyy151a.pdf?ts=1693337974514&ref_url=https%253A%252F%252Fwww.google.com%252F

Thanks! 

Tahar 

Hi Zong,

Can you elaborate the conditions under which the reverse current can exist? If this is during normal operation and output is being regulated, this device cannot sink current and will likely result in damage. You need a device that can sink current like TPS54627 which is the FPWM counterpart of TPS54628. However, there is no negative current limit specified and likely this feature is not implemented either.

If the TPS54628 will see a VOUT when VIN is not present, then reverse current flows through the HSFET body diode and you need to ensure the reverse current is less than the IOUT rating of the part which is 6A in this case. All pin voltage levels should be kept below abs max ratings specified in the datasheet. You can refer to https://www.ti.com/lit/an/slyt689/slyt689.pdf for all the careabouts during such a case.

Thanks,

Amod

Hi Zong,

With EN low and Vin < Vout, the HSFET body diode will conduct on both TPS54628 and TPS54627. The max current will be a function of the body diode loss and junction temp limit on the IC. At 45C,  max reverse current = (150-45)/(0.7*43.5) ~3.4A. Above 3.4A, the IC Tj recommended limit will be exceeded and would not recommend exceeding this current level.

Thanks,
Amod

Hi Zong,

During EN=LOW if reverse current exceeds the max 3.4A limit I specified, it can cause burnout. This is because the device is disabled and thermal shutdown feature as a result will also be disabled so the package can heat up and cause burnout.

Thanks,

Amod

Hello Zong, 

Amod is off today, so i am backing him off. 

Based on the math Amod shared above, anything above ~3.4A will damage the device as the TJ will be exceeded. The thermal shutdown of the device is rate at 165C. if the device is ON and the junction temp goes above this level the device will shut down. during shutdown, if VIN is less than VOUT and the reverse current keeps heating up the device, there will be a point of damage due to thermal. 

Thanks

Tahar

Hello Zong, 

Yes HSFET body diode will conduct and will act almost like a short with VF, the current will be a function of the body diode and how much bias voltage is. 

Thanks! 

Tahar

Zong,

VOUT .> VIN situation when EN=LO is when the HSFET body diode will conduct. This situation should not cause damage to the IC unless the reverse current through the body diode exceeds ~3.4A. You can recreate the condition for your case and ensure that the case temp exceeds the recommended max by taking thermal measurement of the device.

On the TPS54628, any kind of reverse current when EN=HI or device is active will cause IC damage most likely due to internal FET damage. So, you need to ensure no reverse current happens during active device operation.

Thanks,

Amod

Zong Wu said:

Previously answered me that it is 6A. Way the difference between 3.4A?

Yes the maximum current the HSFET diode can sustain can be 6A but it leads to higher junction temp than recommended. So, limit to 3.4A.

Zong Wu said:

Is forward current Imax=6A, reverse current Imax=3.4A?

Yes, correct. This is applicable only when the IC is turned off or EN=LO. When EN=HI or IC is turned on, please do not use TPS54628 with reverse current.

Zong Wu said:

TPS54628 EN=HI , Vin=5V , Vout=7V , this will create reverse current (below 3.4A / above 3.4A).

Can you provide your application details and when this situation occurs?

Zong Wu said:

What FET is reverse current Imax?  Will this damage the TPS54628?

TPS54628 cannot sustain any negative current when EN=HI and Vin < Vout. It can only sustain reverse current when IC is turned off or EN=LO as mentioned above.

Zong Wu said:

Is the following description correct?

Body diode: forward current Imax=3.4A (IC reverse current)

FET: forward current Imax=6A , reverse current =3.4A

When IC reverse current over 3.4A damage, Vin short Vout? Vin and Vout open Vout no output? Vin & Vout short GND? 

Yes, please note reverse current 3.4A only applies when EN=LO. for EN=HI, TPS54628 cannot sink current and will result in FET damage. So likely a VIN-SW short or SW-GND short.

Thanks,
Amod

Hi Zong,

Yes if there is no switching activity and SW is at 5V with no VIN, the drop is across the HSFET body diode. The OVP feature is activated only after VIN is present and a complete soft-start is achieved. This function is enabled after approximately 1.7 times the soft-start time but since there is no switching activity on your case, OVP will not be active.

Thanks,

Amod

Hi Zong,

Both EN and VIN need to be present for the soft-start to be achieved. Normally in conditions where VIN < VOUT it would be recommended to turn off the IC by pulling EN=LO. OVP feature will not be active during such times and you will need to just control the applied VOUT at SW to below the abs max ratings of pins. I would recommend testing it with the exact conditions you plan on using.

Thanks,

Amod